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In Reply to: RE: update.... posted by vinnie2 on February 12, 2019 at 14:57:38
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: 02/13/19Follow Ups:
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.
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