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All,I'm new to this forum. I've posted numerous times on the Bottlehead forum in the past, but am just now getting back into the hobby after a 4 year break.
I'm interested in building a small EL84-based amp using iron that I I salvaged from an old console. I measured the OPTs to have a 4.5k primary and am thinking of building this circuit:
http://db.audioasylum.com/cgi/m.mpl?forum=tubediy&n=9930&highlight=SE+pentode+EL84&r=&session=
I need to figure out if my 4.5k OPTs will work also. Based on a quick scan of the data sheets for an EL84, 4.5k should be fine for a class A1 amplifier using a plate voltage of ~250V.
I like the idea of SE pentode since I'm fairly certain that's what the original console amp was (it had an EZ81, 12AX7, and 2 EL84s). I've never designed or built an amp using feedback, and the whole concept is very foreign (and baffling) to me at this point. I have many questions about negative feedback - how much do you apply, how do you design for it, etc. I'm hoping that I will learn more about it as a result of this project.
It seems that I'm in the same boat as the original designer (Erik) who posted this schematic!
Does anyone have experience with this circuit?
I've also come across this second schematic which looks interesting as well (some of the values are incorrect, as noted in the post):
http://www.diytube.com/phpBB2/viewtopic.php?p=1261&sid=6693fd10a1e28cb49ee75552b709a6a7
I know this topic has been discussed many times (I've searched and read many posts over the past few days) but I would appreciate feedback (no pun intended) before I begin.
Thanks!
-Matt
Follow Ups:
You could easily sub a 12ax7 for the 6EU7 if you prefer. A picture of one of these amps is in my inmate gallery if you'd like to take a look. These are sweet sounding amps just the way they are, I sometimes use them as tweeter amps in a tri-amped system. Good luck, wpod
Yup, the triodes in a 6EU7 and a 12AX7 are equivalent.Notice the piezoelectric phono cart. and tone control circuitry. The mu 100 triode was needed to deal with that. If a CDP is the source and tone controls are not used, mu 100 is too much.
BTW, the bad sound some people hear in an amp with loop NFB is at least partially due to slew limiting caused by a large HF error correction signal. A high gm triode is protection against slew limiting. The 'X7 triode is LOW gm. :~(
Eli D.
is the *cl86 in this circuit - two envelopes to do the job.The triode section is an 'x7 equivalent, and the pentode is an el84 equivalent...
Should make life a little easier...
Owen
Hi all,
I've not posted here before, but I do love tube audio.
So I have a question on the schematic that was posted earlier (http://www.tubeaudio.8m.com/RH84/rh84.html). In that schematic there is no cap on the screen. Is this a misprint do you think, or was it intentional? Not having any bypass on that screen would mean that you would get a gain reduction due to the negative feedback you get at the screen due to voltage swinging when the current swings up and down.(I'm the co-worker that Matt mentioned earlier)
-nate
Yet another good reason for using a separate regulated screen B+ supply. The screen grids are at ground potential as far as AC is concerned.
Eli D.
On the point of having a well regulated supply for the screen, I've considered making something like this to be a super stiff screen supply. I know that there are some people who would find the use of solid state things to be bad, but I'm open to all possibilities.By changing around R2 and R1 in this, you could make the screen voltage whatever you wanted, and it would be pretty much independent of the currents involved (so long as they weren't TOO high). Also, you could not use a BJT there, you could use a FET, though you might have to use a 10V zener depending on how much Vt+Vdsat+(regulator headroom) you needed. Just a thought.
So after rereading some of the posts, I realized that I had drawn the Maida regulator setup that was mentioned elsewhere (I didn't remember that as the name of it). Something I didn't put in it was the capacitor on the output of the lm317. I don't know if you need that for stability or if it will be OK without it.
just to confuse you a little more ;-)http://www.tubeaudio.8m.com/RH84/rh84.html
I built it and it was very good. The only change I made was I increased the value of the g2 resistor as the 2.2K was allowing the voltage to be a bit high. Used junk box parts and 125ese hammonds. I was so impressed I ordered a pair of Lundahl opt's.
I am rebuilding it with "good" parts and will also regulate the screens to compare with the resistor.The regulation "should" improve it but the thing is it sounded so good as is I don't know if the regulation would really improve it. I will find out I guess.
Thanks for the link - I hadn't seen that schematic yet.I think I'm going to build one of these using unregulated screens first (I like the Maida supply, so I'll probably be placing an order for the necessary parts).
The RH designs circuit is very straightforward and could probably be built in no time.
-Matt
Matt,Zobsky is correct. For maximum linearity in pentode mode, screen grid B+ should be regulated at a fraction of the plate B+. Also, it is desireable to maximize (consistent with proper operation) the voltage differential between plate and screen grid. With those facts in mind, I suggest you use the Russian 6p14p-ev/EL84M, which is a 7189 equivalent that's rated for 400 V. on the plate. Look at the linked 7189 data sheet. Please notice that the screen grid should be 250 V. above the cathode.
In order to get the B+ rail voltage up, SS rectification is indicated. Use series connected pairs of UF4007s. Scan the Bottlehead Forum archives for RRSF. You SHOULD use a RRSF. CLC filter the rectifier O/P. 15 muF. is enough in the 1st position. The power trafo will run cool and you can squeeze a little more current out when the 1st filter cap. is relatively small. Follow with a Hammond 159Q choke. The bulk of the energy storage goes in the 2nd cap. position.
Google search for Maida regulator. That type of circuit will allow you to power both the 7189 screen grids and the driver triodes from the same supply.
Speaking of driver triodes, the 'X7 has too much gain and a gm that's too low. IMO, a BETTER choice is the 5965.
Have you given consideration to the NFB ratio that will be used?
Eli D.
Eli/Zobosky -Thank you for the excellent information!
My original goal was to keep the circuit as close to the original (at least using the original components) as possible. As a result, I already punched and assembled the chassis assuming tube rectification (which I've never used before - another learning experiment). I thought about switching to SS rectification, but wanted to try tube since I've done it. Perhaps I should have gone with SS . . . . (maybe on the next rev).
I still have some learning to do regarding pentodes as well. I was not aware that the screen grid was as important as it is. My understanding was that the screen grid in a typical pentode could further increase or decrease the current through the tube, but that it had a relatively small impact compared to the control grid. I need to do some reading.
Regardless, many of the simple SEP circuits that I've been looking at (like the two in the original post) just use a resistor from B+ to provide the screen grid voltage, which is usually a few volts less than the plate. I don't quite understand why this approach won't also work? The voltage won't be as solid as it will be using zeners, but it should still work? I need to research this more too . . . .
Lastly, I have not thought much about the amount of feedback to be used. I was hoping to find and build a simple circuit and learn about feedback in the process. At this point it is a totally foreign concept to me. One of my co-workers (who is much more knowledgeable about these things) tried to explain in to me, but I still don't quite get it.
I need to better understand how much feedback is needed for optimal fidelity and how to design that feedback into the circuit. And just when I had it all figured out for SET circuits without feedback . . . .
Hello,Never mind the tube rectifier! It always adds a natural warmth and a balanced tone to the sound, insted of the edgyness of solid state. When solid state rectification is done right, it does sound really good, but a tube can still help it. Simply parallel the tube rectifier with solid state diodes! That way, you will get instant high voltage, and you can put no matter what size cap for the first position, as the tube will not be strained: the SS conducts the bulk of the current (due to its much lower internal resistance), and when the SS diode gets cornered (around the recovery point) the tube takes over.
The voltage drop of paralleled tube / SS diodes is the same as of SS only. In addition, you can toy around a LOT with rectifier tube rolling! When the amp is on, simply unplug the rectifier. You won't even hear a pop, the amp is not affected, but you'll be able to hear the sonic difference. I've made a preamp with such rectification, and the combination of tube + solid is better than any of the two alone.Good luck!
Janos
Matt,Many console amps provided power to a separate tuner. If that was the situation in the amp you acquired, there is "juice" to spare in the power trafo. If extra "juice" is available and you spaced things out well, you can enlarge the EZ81 socket hole to take an Octal socket and use a 6BY5 as the rectifier. The forward drop in the 6BY5 is lower. Check TDSL for tube data.
The instantaneous plate voltage in a pentode varies a LOT. When the screen grid is connected to the "same" B+ as the plate, the instantaneous plate voltage fall below the screen voltage and screen current is higher than plate current. Therefore, NON-LINEARITY occurs.
NFB works BEST when the open loop circuit is reasonably linear to begin with. Regulate the screen grids at 250 V.! The sound is better when NFB is working primarily to lower O/P impedance, not correct distortion. EVERTHING hangs together well when the NFB error correction signal is small.
Eli D.
Thank you for the excellent advice guys. I'm still reading about this, but I have one additional question. If I simply regulate the B+ supply, then I can just use a resistor to drop a few volts to the screen supply, correct? I guess my point is, why not just regulate the whole HV supply instead of only the screen supply?I think I have some 1W zeners around somewhere . . . . . just have to find them.
This is probably in the tube data sheets, but what's a good design point for the screen if the plate is running at 275V?
-Matt
> > This is probably in the tube data sheets, but what's a good design point for the screen if the plate is running at 275V? < <Matt,
Zobsky gave you a good hint in mentioning the 6p15p/SV83/EL84N's requirements. 150 V. will do nicely. That puts a 0A2 gas discharge regulator "on the map". If you wire your sockets for the 6p15p, 3 decent and affordable tubes are available for "rolling": the 6p15p, the 6p14p-ev, and the JJ EL84.
In any event, use "fixed", as opposed to cathode, bias on your O/P tubes. IMO, you can't afford the voltage drop across a RC bias network.
BTW, you've yet to indicate the heater current capability of your power trafo. Did your console amp support an external tuner?
Eli D.
Eli,Yes, I believe the amp also supported a tuner (it's been a while, but I think it also had a tuner).
That being said, I probably do have enough extra Watts in the power transformer to play around with some of these things.
I think I'm going to start with one of the simple circuits. I'll build it bone stock, and then swap in different elements as recommended by you and Zobsky. I think I'll learn the most this way.
-Matt
I just answered one of my own questions - just regulating the B+ supply won't do much for the screen supply, as the current through will still vary thus varying the voltage across the screen resistor.-Matt
Regardless, many of the simple SEP circuits that I've been looking at (like the two in the original post) just use a resistor from B+ to provide the screen grid voltage, which is usually a few volts less than the plate. I don't quite understand why this approach won't also work? The voltage won't be as solid as it will be using zeners, but it should still work? I need to research this more too . . . .
For one, the B+ can sag too, depending on the load conditions, which would cause the screen voltage to vary too.
You could go overboard and implement a separate screen supply ( a la drlowmu) but I suspect that's more than what you want to do for a beginner project.
Do try a regulated screen supply. Use a cheap easy series string of low voltage zeners (and not a high voltage zener) or look up Maida regulators on diyaudio.
See the version on my web page:
Please excuse my ignorance . . . .What exactly do you mean by clamp down the screen voltage? It is being fed directly from the B+ supply.
Thanks,
-Matt
I meant regulate the screen voltage (take a tap from the B+). You can either use a VR tube or a string of zeners. Quite easy to do.
Google up for more info.
Here's a link of how to do this for an SV83 (related tube, but with lower screen voltage limits) .. although the text is related to a push pull application. SE is similar.
Hope that helped
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