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Trying to recreate this from memory as I seem to have lost track of the original. Would appreciate any comments on if it looks useable as is or what might be missing. Sorry about the sideways photo, but for some reason it always comes up side ways no matter how many times I rotate it in my files.Thanks.
Follow Ups:
Hot diggity dam! Got BOTH the 845 and the 813 amps up and running this morning per the attached schems. I took blackplate's advice and put a capacitor in front of the output tubes and was then able to get things running well. Both amps now sound really good, with the triode connected 813 being particular pleasing. The operating points for 845 are pretty close to those I was trying to hit that Mike Lafevre said were a favorite in Japan. Once I had the 845 running well, I basically just dropped the 813 in it's place with a few changes to accommodate that tube. I have no idea if the operating points of the 813 are considered to be good ones, but when you look at the plate curves for the 813 it seems to be in a pretty linear area.
If anyone has any suggestions on other operating points for the 813, I would like to hear them. These are both easy to build, good sounding amps. The only hassle is the 10-volt filaments. Even then, by using the resistors off of the filament pins to balance out the hum, I am able to use AC voltage on all the filaments with so little hum I have to have my ear right next to a speaker to hear it.
What king of wattage are you getting with this setup
I do not know for sure, but reading about it I got the idea it would be around 4 watts. I do know it is more than enough to drive my Altec Granada's. Their efficiency is about 95.
Edits: 02/17/19
With all that free time, connect a scope, let it clip into an 8ohm load and read the voltage.
Inquiring minds would like to know!
Why is it everyone assumes that anyone who is retired has nothing but free time? I have more than when I wasn't retired, but it is not unlimited. I still have all my chores around the place and doing things to help my with her art work and studio among others.Anyway, you need to be more specific. Connect a scope to where? How do I know when it is clipping? Anything else that pertains?
Edits: 02/17/19
1khz sine wave at input
Connect scope to output with an 8ohm load(load resistor best)
Turn up input until sine wave just starts to flatten at the peaks and read the voltage, its subtle on a tube amp but obvious.
Its crude but gets you close to output wattage. I did it this way for 30 years until i invested in an analyzer.
Well I gave that a try but I must have done something wrong because I came up with a voltage of 48vdc. If I were to use the formula P = V*V/R I would get a ridiculously high figure. Is there another way to calculate it that would give a much lower value? If not then I am doing the measurement incorrectly.
Edits: 02/18/19
"Well I gave that a try but I must have done something wrong because I came up with a voltage of 48vdc."First of all, the voltage you should be measuring across the 8 ohm load resistor is ac, not dc. (You can't get dc out of a transformer.) Trying to guess what you actually meant to type, one possibility is that you meant 48v pp (that is, 48 volts peak-to-peak of the 1KHz sine wave). That would correspond to 48/( 2 * sqrt2) volts RMS, which is about 17 VRMS. This would imply a power of 36 watts. That seems implausibly high, though.
However, there are too many uncertainties and guesses involved, with trying to estimate what you meant to type when you wrote 48vdc. Perhaps you could clarify?
Edits: 02/18/19
I meant peak to peak. In the morning I will see if I can set the scope to measure AC instead.
"In the morning I will see if I can set the scope to measure AC instead."
I shan't even try guessing what you mean by that.
There's nothing wrong with measuring peak-to-peak voltage of the sine wave; just divide by (2 * sqrt2) (which is about 2.83) to get the RMS voltage, like I said.
But the answer you got seems to be coming out to be much too large, implying 36 watts into 8 ohms, which just isn't believable for a single-ended amplifier like you have built. So there is evidently something wrong with your interpretation of what you are seeing on the scope. (Misreading the volts per division setting?) Or else something wrong with the scope?
You could practice by checking the output of your signal generator on the scope, and comparing the peak-to-peak voltage (divided by 2 * sqrt2) with the rms voltage you measure with a digital voltmeter. Do those two results agree, within expected experimental errors?
Then what exactly did you mean by the voltage I should be measuring is AC not DC? That would imply that either could be measured. Please add a little clarification.
By the way; you did have an 8 ohm dummy load connected across the OT secondary when you made the measurement, right? Because first of all, it could be bad for the output transformer and/or output tube if you run it without a load. And also, who knows what voltage the secondary would develop if you run the amplifier to clipping without a load. (And one really does not want to find out!)And, incidentally, if it really had been giving 48v peak-to-peak across an 8 ohm load, that resistor would have been getting very hot very quickly, with 36 watts being dissipated in it. I assume it wasn't getting very hot?
Edits: 02/19/19
"Then what exactly did you mean by the voltage I should be measuring is AC not DC? That would imply that either could be measured. Please add a little clarification."What I mean is that you cannot be getting a steady DC voltage from the output transformer secondary. You must have been seeing an AC voltage on your scope. And indeed, you then confirmed that when you had said 48vdc, you had meant to say 48v peak-to-peak AC voltage. Which would be fine, except that 48v peak-to-peak is way too high to be getting from that single-ended amplifier you have breadboarded. So something must be amiss with your measurement.
I am just suggesting that you try playing around with measuring the RMS output voltage from your signal generator using a dvm, and then comparing it with the peak-to-peak voltage that you can easily read off from the scope. The two should be related as I said before. If they are not, then it would provide concrete evidence that something has gone astray with your interpretation of the peak-to-peak reading from the scope. (Or that something is wrong with the scope.)
There isn't really an issue of "setting the scope to measure AC or DC." It is just showing a trace of the waveform you feed into it. And you are feeding in an AC waveform from the output transformer. (It is true one can usually set an AC or DC input coupling on the scope, but that is not relevant here, since your signal you are feeding it will be pure AC, with no superimposed DC.)
But since the peak-to-peak voltage you are reporting from the amplifier seems to be implausibly high, the obvious first thing to try would be testing things out on a convenient AC voltage such as from the signal generator itself, and comparing with the RMS voltage measured with the dvm.
Edits: 02/19/19
Yes, I was using an 8 ohm resistor across the opt output leads, and no the resistor does not get warm, and yes I realize that voltage is way too high.
However;I went ahead and tried your test and got a 0.12 volts reading with the dvm on the signal generator output and 0.13 volts using the scope. I was very careful to follow the directions in the scope manual for measuring voltages. That seems like a pretty good match to me. HOWEVER, when I switched over to the amplifier everything went gunny sack. I did not change any of the settings, but I had to go clear up to 50 volts/division to get a sine wave that would stay within the screen of the scope, and that was with only 1/3 volume on the amp. I think the scope is working ok because it read the signal generator ok. I do not have a clue why the voltage is so high when I switch to the amp. I follow the instructions in the scope manual again, but obviously something is wrong somewhere. The final thing is the amp doesn't play any loader than my SET 2A3, so I know the voltage has to be wrong. I am just going to build the other channel for now and forget about trying to figure the watts unless someone has a eureka moment from reading these posts. Maybe deaf by khorns can add something if he checks back in. Thanks for the effort.
Edits: 02/19/19
Dont use the scope to read voltage
Clip in your RMS meter and set to AC
Turn up the volume until you see the sine wave start to flatten at its peaks and read the voltage.
E2/R= watts
I would say you want to adjust the level to where the amp is just below visible clipping.Turn up the volume until you see the sine wave start to flatten (clip) at it's peaks then turn down the volume just a little until the sine wave is not flattening at its peaks and read the voltage.
If clipping is visible on a scope, the distortion is quite high. That's not usable power.
"..."full power" for the purposes of measuring distortion in amplifiers is usually taken as a few percent below clipping..."
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 02/22/19
It works slick as a whistle when you use the right procedure. Now the only problem is deciding where the peaks start to flatten out. It can make quite a bit of difference in the voltage reading. Can you post a picture of what you consider it to be? Right now I can come up with anywhere between 2 and 4 watts output for the 845 depending on where I say the flattening starts.
It is usually pretty unambiguous when the clipping starts, in my experience. The scope trace looks like a nice sine wave, and as you crank up the level you reach a stage where the tops (and or bottoms) of the sine wave start to flatten off. The maximum level before clipping is the level just before any sign of flattening of the positive and/or negative peaks begins.
This is what 2.4% distortion looks like, you can see one half of the cycle is pretty flat. If you adjust the volume down just enough to make the flat part go away, your at about 1-1.5% distortion which is acceptable for a tube amp. This should get you pretty close without using an analyzer.
I think I have now tweaked the 845 about as much as I can. It sounds pretty good and the op points are close to target values.
Target values
plate volts 430 vdc
plate current 62mA
grid volts -51 vdc
As breadboarded now
plate volts 487 vdc
plate current 62.2 mA
grid volts -52.2 vdc
I like the sound of it in mono, so now it's time to breadboard the other channel and see how it sounds in stereo. After that I will take another shot at the 813. Wish I had some low voltage target op points for it, but I have not found any so far.
It may be my equipment or the operator, but I am having a hard time getting a sine wave like that. Here's what I have been able to get so far. It's kind of weird because it starts to flatten out at about 1/2 volume (2.3 vac) and the flattens more as you increase volume until you get up to almost full volume. That's the top pic (9.8 vac). The interesting part to me is I don't really hear any increasing distortion in the music until almost full volume. So what do you guys think?
Edits: 02/23/19 02/23/19 02/23/19 02/23/19 02/23/19
That top pic is pretty ugly, probably your driver giving up
The sine wave should be symmetrical, whats your driver tube?
1/2 of a 6sn7
Edits: 02/24/19
According to my simulation, your 845 is starting to pull grid current at about 3 volts output (about 1 watt) using your operating points. Your 6SN7 driver is not going to do its job alone. Your sine wave isn't clipping but it sure is producing harmonics (on paper). It would be interesting to see what the real harmonics would be using your OPT. Depending on your music source, your probably not picking up on the harmonics but they are definitely there according to your scope. What is your plate and cathode resistor values.
You should look into a UMC204HD and the free ARTA Software. You get a scope, THD, FFT along with many other features. Its easy to implement and setup, I set it up in an hour. It's not too bad and reads very close to my HP Distortion analyzer.
Edits: 02/24/19
What would happen if I used both halves of the 6sn7 in srpp?
I'm not an expert in SRPP but I don't think so. It may help the output impedance and distortion but it wont provide grid current.
An IT transformer or a mosfet (grid drive) would be better suited for this or run the voltage up a little higher.
With your current driver and voltage, not sure you will get more than a couple watts on a low voltage 845. Those transmitting tubes start pulling grid current early.
You can do better using a 300B and its easier to drive. I would use some DHT's to drive it as well.
Actually, I can live with a couple of good watts with my Altec's. I am playing one channel in mono right now, and I do believe it would play more than loud enough for my tastes with a stereo pair. Sooooooo, before I spend any more effort on this project I am going to breadboard the other channel and see how it compares to my SET 2A3. That is the amp it has to beat to stay in my permanent setup. I already determined a while back that I like the SET 2A3 better than the 300B model 91 clone it replaced.
Edits: 02/25/19
If your are referring to the WE 91 you probably didn't like it because of the pentode drivers.
That sounds like a good candidate for another breadboard project one of these days. Would you happen to have a schematic of a good one you would be willing to share?
You don't really need much of a schematic, simply run any DHT at its op point and couple it with a cap.
Here's one I did back in the day, i got about 6-8 watts depending on transformer
nt
.
Have Fun and Enjoy the Music
"Still Working the Problem"
I got the following values with the amp turned on, no signal.
plate volts pin 2 to pin 3 103.7 vdc
grid volts pin 1 to pin 3 1.31 vdc
bias volts pin 3 to pin 1 minus pin 1 to grnd 1.31 vdc
how much plate current?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
It's got a 1.5vdc nmh battery from the cathode to ground. How do you figure the plate current with that arrangement?
Measure the voltage across the plate resistor.
.
Have Fun and Enjoy the Music
"Still Working the Problem"
Well there is part of my problem, I thought the flattening would occur on both the positive and negative sections of the sine wave. Thanks, I will try it again.
"I thought the flattening would occur on both the positive and negative sections of the sine wave"
It would if the tube was 1. very linear and 2. biased to idle right at the half way point between cutoff and saturation.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
"The maximum level before clipping is the level just before any sign of flattening of the positive and/or negative peaks begins."
I agree. Once clipping is reached, the HD is high and that power is not of much use.
It's a available power BEFORE clipping that vinnie wants to know.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Yeah, that is what def said too, but I am finding very difficult to find a spot where it seems to be clear cut. I mean there is quite a bit difference in voltage between where it is a rounded top and where it is a flattened top. I was hoping to get a better estimate of the power than just 2 to 4 watts.
Edits: 02/22/19
Just noticed that formula for power is different than what I saw else where. Do you mean E*2/R? The other one I saw was V*V/R where V was the rms voltage.
Edits: 02/21/19
You measure 5 volts out when it begins to clip5*5/8ohms=3.1 watts
If you want to check freq response doing manual plots, you can do a frequency sweep starting at 5hz ending at 20K and plot it on paper.
Your meter needs to have a good freq response to do this. Simply plot the voltage as it drops across the sweep. You could simply sweep 5hz, 10hz, 20hz, 30hz, 40hz, and then start at around 12KHZ up to 20KHZ
Edits: 02/21/19
He means the square of the RMS voltage, divided by the resistance.
Ohhhhhhhhhhhhh! Now that sounds much different. I will give it a try on the 845 I have breadboard now in the morning. I know that it is supposed to be close to 4 watts. If that works I will try the 813 again. Thanks!
That sounds like something even I might be able to do. I will give it a try and report back when done. Actually, I am kind of curious too.
Edits: 02/18/19
Decrease R(g) to 150k and increase the size of your coupling cap to 0.22 uf.
You said decrease Rg to 150k, but wouldn't that be an increase? And do you mean the grid stopper or the grid leak resistor?
Edits: 02/16/19
Vinnie:
My apologies, I misread your grid-to-ground resistor as being 600k (not 60k).
Please ignore my suggestion.
FWIW: 0.1 uf and 60 kohms yields a first-order rolloff at 26.5 Hz. This will produce phase shifts below 106 Hz.
Tre' suggested I try a 0.47uf cap to get rid of that problem, so I will give that a try.
Decided to try an 845 first as I have a schematic for it as posted. Amazing what a difference that made. I had it up and running in about 20 minutes on the new bread board. Now this is a long ways from a finished design, I just wanted to see if I could get something working. It actually sounds pretty good. I just did the one channel for now and used a mono source. I took some measurements just to see where I am and I know things will need adjustment. You can read some of them on the schem, but just in case you can't...
B+ 500vdc
plate volts (filament to plate 370 vdc)
cath to grnd - grid to grnd (117-98= 19vdc)
plate current 64 mA (115vdc/1800 ohms)
Now I know I still would have to do a lot of work on this to get it where it should be (62mA and -51vdc grid volts), but my real question for now is should I be able to replace the 845 with an 813 an get music out the other end? I think that is what I did before, but I have no record of what I changed to get it to work.
Many moons ago Mike Lafevre posted some operating points for a low voltage 845 tube amp that he had seen in stereo sound magazine and had heard a version of himself.
The operating points were plate volts 430vdc, plate current 62mA, grid volts -51vdc, with a 5k loading. He said it sounded great to him. I liked the idea the idea of the low voltage so I breadboarded one, and that is where the above schem came from. The only problem was I never hit all his operating points, and putting it together again yesterday I found I still haven't. The strange part is it sounds pretty good, but I can't help but wonder how it will sound with Mike's numbers. As it stands now I have plate volts 370vdc, plate current 64mA, grid volts -19vdc and loading is 5k. I can adjust the B+ voltage, but it is the grid volts that have me puzzled. If you look at the plate curves for the 845, at 370 vdc plate volts and 64mA current, the grid volts should be somewhere in the area of -40 vdc. I want to try and hit Mike's magic numbers, but I know this is a can of worms because when you change one thing the others change with it. Does anyone have a system they use for zeroing in on a set of operating points, or is it all trial and error?
Edits: 02/13/19
You have three variables: P/K voltage, grid bias voltage and plate current. Only two of those variables can be independent. That means that you can fix two of them but the third will vary from tube to tube. No two tubes are exactly the same and no tube will stay the same as it ages. If you want to get close to a published OP I have used a simple test jig. You would need a current source that can sink the desired plate current and a variable B+ supply. The current source does not need to be anything special. A single DN2540 MOSFET, 1K carbon comp. grid stopper and 50R or 100R trim pot would do fine. Ground the grid of the tube to be tested, the current source goes between the cathode and ground along with a 1R series current sense resistor. Connect the variable PS to the plate. Use the current sense resistor to measure plate current and adjust the current source as necessary. Vary the B+ supply until the P/K voltage is where you want it. Then measure the grid bis voltage. Or set any other two variables and measure the third.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
"You have three variables:..."
No, there are at least five of consequence. Number four is the condition of the tube. The need to reduce grid bias in order to create predicted current levels implies the tube is worn out. Variable number five goes to the competence and experience of the project manager. Past threads on similar topics indicate a significant problem in this area. Don't take for granted that any aspect of the test is as described.
--------------------------
Buy Chinese. Bury freedom.
" No two tubes are exactly the same and no tube will stay the same as it ages."
I think that I covered #4. #5? I was discussing physical variables.
This can be a useful test jig when setting up direct coupling of stages. Several tubes can be tested and ballpark values can be established so that appropriate cathode resistors etc. can be chosen.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
Thanks for the explanation!
If you have 370V on the plate and 19V on the cathode, your plate to cathode voltage is 351.
If you want 51V of bias and 64mA of plate current, that is a 796 ohm cathode bias resistor.
If you want 430V of plate voltage, then you need 481V showing at the plate.
To try and get a handle on all this I decided to bracket the operating points by using several different cathode resistors. The results I got are as follows:
Cathode resistor plt volts plt current grid volts bias B+1.2 k 367 90mA -13 13 492
1.8k 376 65mA -20 20 509
2.7k 375 46mA -24 24 515
To make sure we are all on the same sheet of music, the definitions I used for the above values are as follows:
Plate voltage - voltage difference between the cathode and the plate
Plate current - voltage across the cathode resistor divided by the value of the cathode resistor
Grid volts - voltage difference between the grid and the cathode
Bias voltage - cathode to ground voltage - grid to ground voltage
To my ear the best sounding combination was using the 1.8k resistor. This gave very close to the desired plate current of 62mA, but the grid volts are way off of the desired -51vdc.
I was feeling bummed about the grid volts until I remembered a posting that Jim McShane made a while back called "tube problems that are not tube problems". In it he says "DO NOT TRY TO SET THE VOLTAGE AT THE GRID TO A SPEC"! He says it should be whatever it needs to be to set the cathode current to spec. I am not sure if he meant even at 30 volt difference, but I hope so. I find it interesting that that is also the setting at which it sounds best to me.
I occurred to me that I should probably try another tube to see what I got with it. Unfortunately, I do not have any nos 845's only new stock Chinese units. I tried another one that is from a different maker (at least the markings are different) and got virtually the same results using the 1.8k resistor on the cathode. I wish I had an old stock tube to compare with. It is my guess at this point that perhaps the grid voltage given in Mike's numbers may have been from an old stock tube and they are quite a bit different. If any one can shed any light on this or anything else in my results, I would appreciate it.One other point. I don't think my psu has enough ompha for this as 375 as 515vdc is the most B+ I get at line voltage. I am not sure how much difference it would make to all these readings I I was able to get the plate volts up to the desired 430vdc.
I think tomorrow I am going to take the plunge and try substituting an 813 for the 845 and see what happens.
Edits: 02/13/19 02/13/19 02/13/19 02/13/19
As I posted before: You can only pin down 2 variables. In this case plate voltage and plate current. The grid bias voltage will go where it needs to go. The fact that it is so much lower than expected would indicate a weak tube (requires less negative voltage to control the plate current).
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
I just found a site this afternoon that had the formula for calculating the value of the cathode resistor. I am going to give it a try this evening. It seems like that should effect the plate voltage too, but it wasn't in the formula. It will be interesting to see what happens. I must be missing something else though because the 370 plate volts was measured from cathode to grid. Is there an additional voltage in there I have not taken into consideration?
The formula for a cathode resistor is Ohm's Law (if you know bias voltage and idle current).
Based on the schematic you posted at the start of this thread, do be sure to include a coupling capacitor and grid leak resistor between the driver and the 845.
I guess my question would be why? I am hoping to learn something. I have run it for a while without either of those and have noted no problems.
Edits: 02/13/19
Why what?
Why a coupling capacitor. I have built other direct coupled amps that did not have a coupling capacitor.
It will be a lot less confusing if you have a coupling cap. The grid of the output tube will be at 0V DC, then you can bias up the cathode accordingly.
Direct coupling means that you are relying on the plate voltage of the driver stage to be predictable so that the output stage operates properly. This isn't always the easiest thing to do. You're also going to run the same output tube current through a much higher value biasing resistor, so heat will become quite an issue.
You also don't seem to have enough B+ to properly run a directly coupled circuit, at least with what you've been discussing.
Learned a new trick today I guess. I did like you suggested and put a .1uf Russkie cap and 1k grid leak resistor in front to the output tube and a 60k grid stop resistor going to ground from the cap. The plate volts jumped way up to 480, the grid volts went up to 48.1 and the plate current went to 65mA. Looks like I need to do some tweaking now, but at least I am in the ball park now. It's sounding better too. Can't wait to hear it when I get things dialed in. Thanks for the tip; this one will go in my notebook.
Vinnie, you have the terms reversed. The 1K in series with the grid is the grid stopper (it's there to damp or "stop" oscillations) while the 60K to ground is the grid "leak" ( it leaks off any charge that may accumulate on the grid).
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
Opps! I believe you are correct. Thanks for the heads up.
That .1uf coupling cap will not let all the bass through to the output tube's grid.
(.1uf into the 60k ohm grid resistor creates a high pass filter with a -3db of 26Hz.)
Try a 1uf cap.
(1uf into the 60k ohm grid resistor creates a high pass filter with a -3db point of 2.6Hz)
A .5uf (.47uf) would also be just fine with a -3db point of just over 5Hz.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
You know that is one of the more interesting things I have run into. JC Morrison, in a article in SP magazine a while back, said that he had come to the conclusion that the smallest coupling capacitor he could use was the best. Just for the heck of it I have tried using the 0.1uf Russian silver mica on each amp I have breadboarded over the last couple of years. I have a pretty good collection of other types so I would have my own little shoot out each time. Invariably I liked the sound of the 0.1 silver mica best each time. I don't know if it's my particular taste or if there is something else going on there but I am now of the opinion that JC had it right. I can't tell you why, but it sure works for me. You can still get them on the 'bay, you ought to give it a try.
Forgot to mention that I did try a 0.5 cap and did not like it as well. The other thing is I am getting really good bass, especially on the 813. I haven't tried the low organ notes yet, but I hardly listen to that music anyway.
Had to add an edit.... went back and tried the 0.5uf cap again after reading your post since I have made a few changes to the amp since I tried it the first time. It sounded pretty good this time, and I noticed the bass seemed to be a bit tighter than with the 0.1uf cap. I will have to do some more listening before I know for sure, but it's showing promise.
Edits: 02/15/19 02/15/19 02/15/19 02/15/19 02/15/19 02/15/19
Please understand it not the value of the cap alone.
If you had a 270k ohm grid resistor (don't try that with the 813, it won't be happy with that large of a grid to ground resistor) then you could use a .1uf cap without causing frequency response problems.
One more time, the coupling cap, in conjunction with the value of the grid resistor, forms a high pass (blocks the lows at some point) filter.
Here's the math. The 3db down point of the low pass filter equals 159155 divided by (the value of the cap (in microfarads) times the resistance value of the grid resistor)
159155/(.1x60,000)= 26.52Hz
159155/(1x60,000)=2.65Hz
To make sure the frequency response is flat all the way down to 20Hz and to make sure the phase response is not altered all the way down to 20Hz we want the -3db point of the filter to be 2Hz. (it takes a full decade from the -3db point for the frequency response to get to flat and the phase shift to go away)
Most people say this it not real critical and settle for a -3db point of 5Hz or a little less.
BTW, I agree with you. Smaller coupling caps sound better. So I make my grid resistor values as large as I can without pissing off the tube.
The following is a subject for a different day,
All tubes exhibit what is called "grid leakage current". Some more than others. It is the grid leakage current that dictates the "max permissible grid resistor value".
With the 813 in triode mode, I don't think you would get away with a higher value grid resistor than the 60k one you are using so you have to use a larger value (1uf or .47uf) coupling cap if you want the low bass to not be filtered out and/or phase shifted.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Thanks for the explanation. This one goes in my notebook!
Ok, thanks. I think I understand.
The amount of drive you need for the 845 is way more than for an 813.
Are you sure? What's the mu of the 813 in triode mode?
--------------------------
Buy Chinese. Bury freedom.
But in pentode the manual says, "Because of its high power sensitivity, the 813 requires very little driving power to give full power output."
Although, i recall running across somebody drew load lines for triode connected. I did not save that link. It might have been on this forum.
Pentode sensitivity is irrelevant. I thought maybe you knew what it was in triode, based on your comment. The two tubes are probably similar when both are triodes.
--------------------------
Buy Chinese. Bury freedom.
Edits: 02/13/19
Looks like a mu of about 7 to me.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Honestly, using hand-drawn curves like those to design an amp is worse than just picking some random schematic off the 'Net and building it.
That graph is from Andrea Ciuffoli's site. There are two more; one highlights amplification factor (7.7), and the other shows gain with a 10K load (6.6). Again, I'm skeptical of the accuracy of things like this, even when they come with reasonably good credentials. In this case, I'm particularly bothered by what appear to be several mis-starts in the curves. There's just no substitute for data taken in a laboratory environment using calibrated equipment.
--------------------------
Buy Chinese. Bury freedom.
I need to check my notebooks. I think I have a drawing of plate curves for a triode connected 813 I found a while back.
.
.
Thanks Buffy! I think that is the same one I have someplace, but now I won't have to dig it out.
Guess I will just have to do the substitution and see what happens.
Since you expressed interest in the Tubelab site, you might like to investigate Powerdrive which is relatively easy to implement and would fit in better with your lowish B+. You might remember my GK-71 amp which uses Powerrdrive. It also uses high frequency AC heating that was the subject of a long thread. That a look here to refresh your memory.
http://rh-amps.blogspot.com/2014/06/rh813-flagship-project.html
also
https://db.audioasylum.com/mhtml/m.html?forum=tubediy&n=251288&highlight=halogen&r=&search_url=%2Fcgi%2Fsearch.mpl%3Fauthor%3DAmpzilla
ray
I will have to take a look at it. Thanks!
Ok, let's look at your schematic and see what it says.
The 6SN7 looks like it is running around 2mA.
This would put 132 volts on the plate and through direct coupling, 132 volts on the grid of the 813.
To bias at 300mA at the 300 volt level you need -20 volts cathode to grid. So the cathode has to sit at -152 volts. This runs the 813 at a 300 volt plate to cathode at 300ma or 90 watts on the plate. This is close to the max rating of 100 watts. And it is very non-linear part of the tube curve.
560 ohm cathode resistor at 300mA would produce 168 volts which is in the ball park. The cathode resistor would have to withstand 168 * .3 = 50 watts or 100 watts for safety margin.
However, RCA says for 813 audio, cathode bias should not be used. The direct coupling may offset some of the problem, but they were pretty smart dudes.
This will play and sound very unique, however this is really not how you should run this tube. Get a hammond 126b interstage and you will have a 1000% improvement by providing the 813 another 160 volts and not rip the tube apart at turn on and you won't need 100 watt cathode resistors and you can run the 6SN7 better
There are plenty of actual schematics out there with an interstage, this is for a reason.
Now I really wish I could find that schem! I do not think I used a an interstage the last time I did this, but darned if I can remember exactly what I did. May have to do some more research before I start bread boarding or maybe start with a 845 first. I till have a schematic for it. Maybe that will help me remember.
Your schematic seems to show direct coupling which requires a higher starting B+, so that must be a mistake. That said, you may want to check the maximum grid resistance for 813, because it's way lower than most, which is why Altec used a CT choke to feed the bias to the grids of their PP 813 amp. Therefore if you used a resistor your coupling cap would have to be quite large. Same with a grid choke. An IT may be useful here.
Beginning to look like I need to do an 845 first to try and refresh my memory. I do have the schem for that one. I don't think I used a choke, but I don't know for sure.
I got a couple 240mh 5A filament chokes, free, if you pay the shipping.
They'd be good for 813, 845, 211, etc... big and heavy but well made and good.
I really appreciate the offer, but let me see what I can do with what I have around here and see if I get serious about the amp first. Thanks!
.
They are getting down close to the knees, but they are not in them. The sound was quite good on the last one I built, and that is what matters to me.
;-)
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
nt
Putting my AA "hall monitor/board police" cap on...
We can see you both disagree with how to implement this tube. Contend on how to the meet end goals. There is no need to add personal insults to the equation.
Please limit comments to audio related experiences.
Thanks!
Okay off the soap box. Removed special police cap.
Whew!
8^)
I replied to what is obviously a derogatory comment that added nothing to the discussion and was totally unnecessary. What would you have said in reply to a comment like that? Why didn't that comment catch your attention as a monitor when it first came up?
Edits: 02/12/19
Than, continue a fruitless diatribe. No?
Well, I guess from now on I will just bring it to a monitor's attention and let it go at that. I'm usually not one to ask someone else to fight my battles, but maybe in these cases it is best.
Edits: 02/12/19
I usually let things go, unless threads get to 1/2 the computer's page or more --- of tit-for-tat, between two ppl. Then, I'll look at whether I need to put in a word or two.
Believe me, it's a lot better than before.
8^)
Thanks for letting me know.
If you are running less than 500 volts there is no benefit for running this tube. Parallel a couple of 300b or do a KT150 or something like that. You will get more power and a lot fewer headaches.
It's the sound of this tube I want. The one I built before had a really nice depth to it. Damn, I wish I still had the schematic!
YAWRRRN!
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
You seem to like taking pot shots at other peoples work Bubba. How about posting some of the things you are working on so we can return the favor? Or are pot shots all you can manage?
And, I can read and understand a given valve's 'needs.'
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
So you are not doing any diy yourself, but feel compelled to take shots at other peoples work? That's pathetic.
Edits: 02/11/19
So that, on this board, I comment, validly. As in this case.
If you can't cope with that, that's your problem, not mine.
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
If you were making constructive comments I would be fine with that. What you have been doing is being a smart ass, and I am not fine with that.
And others have been making the same points, for a few years, now!
You take exception to me doing that.
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
"Like Vinnie knows that or gives a shit if he does."
Yeah, real valid and real helpful. That's your opinion and nothing more.
"YAWRRRN!"
Can't get more helpful or valid than that.What I take exception to is your attitude that you know everything and anyone who doesn't agree with you is wrong.
Edits: 02/12/19 02/12/19
Actually, the OPTs shown in the schematic will muddy everything. Might as well run a dozen 12AX7s in parallel. Sow's ear and all that...
--------------------------
Buy Chinese. Bury freedom.
That's my considered view, over several years. And after learning valid things and applying them.
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
Without a coupling cap and grid leak, you will blow up the cathode resistor you've selected.
The filament supply may or may not also explode, and it will depend enormously on the parts you've selected and the implementation. You'll also want a 10V/10A transformer for that duty. The 0.2 ohm resistor will drop a full volt, which you probably won't have to drop. You'll also want to make that 0.2 ohm resistor a 20W part. It will get very hot.
A Hammond 195P5 choke for each filament supply would be a nice idea. Drop the first cap in your supply and use the choke instead of the resistor. The AC current drawn through your filament transformer will drop by a ridiculous amount and my experience tells me that a bridge of very high current rated Schottky diodes feeding this choke, then a big cap after will be adequately quiet. A 12V/7A power transformer should get you really close to 10V in this situation, but I suggest modeling it in PSUD, then buy the parts and test them out on your bench.
You can do a lot better than the 125ESE...
Already have heavy duty filament trannies from when I built it before. I know it worked before but since I lost the schem I can't be sure of the values. I guess I will have to experiment. Also, I already have the 125ese's and I like their sound.
Edits: 02/10/19
Can't drive a power triode with anything but a 12AX7, running less than .7 mA idle current. Direct coupled of course. Anything else is not going to work. AC heaters too...:)
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
12AX7 has now been replaced by a 12BZ7 for budget SE DC 2A3 builders.
sez who? I have not seen a post about it. last I heard was the high plate resistance, low current, high gain was the Only Way.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Link posted.
Be enlightened by the master of SE DC 2A3's.
At the rate he's going it may be that we will see the 6H30Pi driver running comparable idle current to his 2A3...LOL
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
DF is still using the 7B4 driver for his SS 2A3 amps, I doubt that will ever change.
His departed friend is actually working on a PP design - of course the details are forbidden to be released to those outside an elite circle of contacts.
PP can't possibly work. I read it here. It has to be so.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
I have built one previously with these very tubes and it worked fine. Problem is I lost the bloomin' schematic!
That would be a very noisy filament supply.
Start with more voltage and use a critical inductance input choke after the bridge rectifier and before the first cap.
With the high current draw the choke will only need to be 20 or so mH.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I will try it first as is and then try your suggestion for comparison. Should be educational. Thanks!
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