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Advice on refining SE EL84 pentode Amp
Advice on refining SE EL84 pentode Amp
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Old 28th December 2017, 05:15 PM   #1
LeftHandFool is offline LeftHandFool  United Kingdom
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Default Advice on refining SE EL84 pentode Amp

Afternoon all,

Some of you may have been following (or perhaps avoiding) my attempts to design and build a SET amp:

Designing a Single-Ended Triode EL34 Power Amp

a project I've shelved for the time being.

Instead, I started a new project building an amp using pentode strapped EL84's and an ECC81 driver. Limited cashflow (and know how) forced the supsension of the previous project, so this one has been done on a strict budget, mainly from spares scavenged from the parts drawer and the darkest recesses of my workshop, where there be spiders...

The only significant outlay was 100 on a pair of Hammond 135ESE's and half as much again in sundries, which represents a considerable saving when compared to the previous project.

I applied power for the first time this afternoon, and was more than a little surprised that not only did it refrain from instantaneous and catastrophic self destruction but also passed a signal remarkably quietly.
Whilst there's still plenty of time for something to go wrong, I'm cautiously optimistic.

Circuit below, with measured voltages:

Single-Ended Pentode Power Amp.png

I scavenged the mains transformer from an old guitar amp, using a pair of diodes into an EZ81 for hybrid rectification. I've currently tapped the 190V secondary, but may brave it and switch to the 275V winding, although I may have to shuffle things around a little to get the voltages correct.

I'm playing it through my old Mission 752 Freedom speakers, which I'm aware are far from an ideal choice, but it's what I'm stuck with.

Predicatably, the mid-range is more than a little strident (listening to Kraftwerks 'Neon Lights', my wife described the upper register synth sounds as 'lemon for the ears' ).

Any suggestions on ways to tame the mid range would be most welcome. Before I started this whole project, I took the time to post a thread in the speaker forum, where the consensus was that this would happen, so I know I'll need to refine the response of the speakers (the phrase 'you can't polish a t**d' springs to mind).

Is there anything in the circuit I can change to even things out? There's currently no negative feedback, so presumably that could help?

As usual, all suggestions are welcome, and feel free to tell me I'm an idiot for trying. It won't be the first time, or I suspect the last...

Matt.

Last edited by LeftHandFool; 29th December 2017 at 11:03 AM.
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Old 28th December 2017, 05:56 PM   #2
kstagger is offline kstagger  United States
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off the top of my head:

Regulate the EL84 screens and the front end
negative feedback, which will cost you some gain.

more advanced? a pentode driver with plate-to-plate feedback

As an aside I've always wanted to build an EL84 SE amplifier using an EL84 driver, ie an EL84 being driven by another EL84.
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Old 28th December 2017, 07:05 PM   #3
ArcticBrew is online now ArcticBrew  United States
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I agree with KStagger.

What you do not like about the sound is probably because of the pentode output stage operating without feedback.

I would switch to an ECC81/12AT7 for the driver and apply Plate to Grid feedback. The 12AT7 will provide the additional gain that you need so you can apply the feedback.

Steve
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Old 28th December 2017, 07:37 PM   #4
LeftHandFool is offline LeftHandFool  United Kingdom
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Quote:
Originally Posted by ArcticBrew View Post
I agree with KStagger.

What you do not like about the sound is probably because of the pentode output stage operating without feedback.
Thanks.

I'm a little hazy as to how negative feedback actually works, and exactly I should implement it. I'll have a re-read of Morgan Jones and see if I can get my head around it.
Do I need global or local feedback, or both? Looking at other circuits, global feedback seems to take the form of a resistor and capacitor in parallel, connected between the output transformer and the cathode of the input valve. Are there any combinations which work in most situations, or will I need to calculate the component values specifically for this circuit?

Quote:
Originally Posted by ArcticBrew View Post
I would switch to an ECC81/12AT7 for the driver and apply Plate to Grid feedback. The 12AT7 will provide the additional gain that you need so you can apply the feedback.

Steve
Actually, I'm one step ahead of you there. Looking back at my loadlines, I did in fact design the circuit for an ECC81, but somewhere along the way I convinced myself I was using an ECC82, to the point where there's actually one plugged in right now . Whoops.
Naturally, a check through the parts drawer reveals no ECC81's, so that's one thing to sort out tomorrow. I'm surprised the two are so interchangeable, given the ECC82's relatively low mu.
Logically, you'd expect the ECC81 to have a lower mu than the ECC82, with the ECC83 being hgher again (which is probably what confused me), but that would make things too simple .

Matt.
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Old 28th December 2017, 07:39 PM   #5
LeftHandFool is offline LeftHandFool  United Kingdom
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Quote:
Originally Posted by kstagger View Post
off the top of my head:

Regulate the EL84 screens and the front end
negative feedback, which will cost you some gain.
Thanks,

Regulation seems to come up a lot here, but it's not something I've tried, or indeed know much about. Is there a simple way of achieving it without too much extra circuitry?

Matt.
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Old 28th December 2017, 07:56 PM   #6
ArcticBrew is online now ArcticBrew  United States
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If you want to try local feedback look at RH84 designs. Connect a 100-150k resistor from the plate of the EL84 to the plate of the 12AT7.

If you have an oscilloscope you could use a potentiometer to find the resistor value that achieves your goal. And then replace it with a 2Watt resistor of the same value.

I followed your EL34 SE thread. I purchased a used BEZ 6SL7 EL34 SE amp on the cheap because the owner did not like the sound quality. It was probably a factory wiring error because the tube was biased very low and distorted heavily after 1 watt output.

I changed the cathode resistor and that upped the power output and improved the sound.

However, the amp was pentode wired without any feedback. I calculated that I should use ~300k for the local feedback resistor. I put in a 500k pot and adjusted it while I watched the result on my scope and 270-350k range looked best. I installed 3 100k 2watt resistors in series per channel. The amp sounds excellent and is a keeper.

That is the easiest thing to try first.

This type of local feedback will clean up the pentode but not the output transformer. You need global feedback from one of the output transformer windings to do that. I can't advise how to do that but, perhaps another AK member will

Steve

P.S. If you decide to revisit your EL34 plans let me know and I will post the working schematic.
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Old 28th December 2017, 08:10 PM   #7
Shef is offline Shef
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100-150K is quite low, I would start with the conservative 470K and work it towards but not less than 220K.
With 10K load you do not need the GNFB to fix the OPT, to my ear the GNFB always makes the amp sounding dull.
Adding the el cap to 84's G2 will make it operating better though not as good as with being regulated. The UL would be better alternative, does OPT have the UL tap?
What you probably need more is the proven schematics, with both stages' operating points selected deliberately to mutually cancel the distortions. The B+ will matter in such.
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Old 29th December 2017, 02:50 AM   #8
ArcticBrew is online now ArcticBrew  United States
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Thanks Shef. I did not pay attention to how far he deviated on OT impedance and other factors from the RH84 design.

I did say that I calculated the effective impedances for the SRPP 6SN7 EL34 design and computed that a 300k feed back resistor was in range. And then wired in a 500k potentionmeter and watched the results on my oscilloscope.

After you do the calculations that is still my recommendation for the next step.

Steve
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Old 29th December 2017, 04:31 AM   #9
Eli Duttman is offline Eli Duttman  United States
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Full pentode mode without loop NFB of some type = inadequate damping factor. Low damping factor = poor voice coil control = crappy sound.

JMO, the distortion spectrum of the 12AT7/ECC81 makes it good driver of PP "finals", but OTT with SE. The 12AV7/5965 or (perhaps) 6GK5 would be "better".

The Hammond 125ESE is very marginal. It's not a HIFI piece, but a "universal" replacement for old radios and TV sets. In this case, no GNFB makes sense, as the core will likely saturate.
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Old 29th December 2017, 05:36 AM   #10
6A3sUMMER is offline 6A3sUMMER  United States
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A few quick and easy things to try:

1. Up the B+ to as much as 300V.
Can also reduce the EL34 cathode resistor to 220 Ohms for a little more current.

2. The ECC82 plate is starved for voltage at only 77V. Can try reducing the ECC82 plate resistor from 47k to either 39k or 33k Ohms.

3. Less power but more damping factor, and smoother frequency response
across the low, mid, and high frequencies when driving a loudspeaker: Remove the 1.5k Ohm EL84 screen resistor, and connect the EL84 screen to the plate through a 100 Ohm resistor. (Triode Mode).
And distortion at the lower power will be less than the same power in pentode mode.

The RH amplifier ideas listed in earlier posts in this thread are good too. But it will take more changes than in 1. 2. and 3. above.

It may be more of a task to apply global negative feedback properly (make it stable).

Last edited by 6A3sUMMER; 29th December 2017 at 05:39 AM.
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