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Designing a Single-Ended Triode EL34 Power Amp
Designing a Single-Ended Triode EL34 Power Amp
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Old 7th November 2017, 06:59 PM   #1
LeftHandFool is offline LeftHandFool  United Kingdom
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Default Designing a Single-Ended Triode EL34 Power Amp

Evening all,

I'm getting back into hi-fi after a few years of making do with whatever was to hand, but (I suspect) like a lot of us here my ears have expensive taste, whilst my pockets are relatively shallow. Thus, I've been compelled to dust off my brain cells and am planning a winter project to design and (hopefully) build a single-ended triode amp.

Money and availability of parts/accessories have forced a couple of design limitations:

- My current speakers are 8 ohm. Since I don't intend to replace them any time soon, I'm designing for an 8 ohm load.
- As a long time guitarist, I have a lot of spare valves commonly associated with guitar amps, including some NOS examples which caould be put to better use.
- I'm very new to all of this, so the circuit must be initially simple - firstly to increase my chances of making somethng that not only works but sounds good too, and so as not to over-stress my lazy brain cells, which I've been subduing with strong ale and wine.
- The amp itself needs to be physically quite small, as space is at a premium.

After dabbling with EL84s (both pentode and triode), I finally found a promising output stage using a triode strapped EL34, fed from an EF86 driver stage. Here's a tentative first go at a circuit:

Single-Ended Power Amp.jpgEL34 Loadlines.jpg

I'm estimating 4.1W per channel, with 2.63% 2nd harmonic distortion. Feel free to laugh if my figures are way off, although when you're quite done I'd appreciate it if you could tell me where I've gone wrong.

Assuming it's not a complete non-starter, there are a couple of questions I'd appreciate some advice on, namely:

- Will the driver stage be powerful enough to drive the output?
- Will I need a seperate pre-amp to feed the driver?

I'm currently mostly listening to vinyl, through an old Project Phono Box MM, who's output is a suggested 500mV @1kHz.

I can show my working, if that'd help, but I won't ask you to check my errors just yet. Any more info you'd like to see I'll do my best to provide.

Thanks,

Matt.
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Old 7th November 2017, 07:57 PM   #2
audiowize is offline audiowize  United States
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Quote:
Originally Posted by FlemingsLeftHandFool View Post

- Will the driver stage be powerful enough to drive the output?
- Will I need a seperate pre-amp to feed the driver?
-
The EL34 is biased at 20V/94mA. If anything, the EF86 is way too sensitive to drive the EL34, especially wired as a pentode. Based on your phono preamp having 500mV of output (kinda low, but adequate), something like a 12AT7, 5965, 6DJ8, or 5670 will provide adequate but not over the top gain.

A 220 ohm cathode bias resistor should be just fine for what you're doing. A 5W part is probably adequate, but a 10W resistor will provide lots of thermal margin. I don't think it would hurt to go to a 470uF bypass cap on the output stage.
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Old 7th November 2017, 08:20 PM   #3
Ketje is offline Ketje  Belgium
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Try someting like this.
Mona
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File Type: jpg SE-Amp.jpg (36.1 KB, 1071 views)
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Old 7th November 2017, 08:45 PM   #4
Sodacose is offline Sodacose  United States
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I like the suggestions to triode strap the EF86 if you find it too noisy or too sensitive as is. That's an easy change, so I say build it as you drew it out and then tweak it to hear the difference first hand.

FWIW, I like the EL34 as a single ended output stage. I have such an amp with a 5965 driver and it's very musical.
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Old 7th November 2017, 09:17 PM   #5
gingertube is offline gingertube  Australia
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Various threads here on this topic.
For Example:
Simple EL34 SET; advice?

Many suggestions in that thread including this one:
Tubelab SSE | Tubelab

Cheers,
Ian
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Old 7th November 2017, 10:55 PM   #6
LeftHandFool is offline LeftHandFool  United Kingdom
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Quote:
Originally Posted by audiowize View Post
The EL34 is biased at 20V/94mA. If anything, the EF86 is way too sensitive to drive the EL34, especially wired as a pentode. Based on your phono preamp having 500mV of output (kinda low, but adequate), something like a 12AT7, 5965, 6DJ8, or 5670 will provide adequate but not over the top gain.

A 220 ohm cathode bias resistor should be just fine for what you're doing. A 5W part is probably adequate, but a 10W resistor will provide lots of thermal margin. I don't think it would hurt to go to a 470uF bypass cap on the output stage.
Thanks for the advice. I was worried the EF86 wouldn't be powerful enough, turns out I'd gone too far the other way! Oh well, back to the drawing board. Would you suggest using the EF86 as a triode (as Ketje ably illustrates below), or opting for something different?
What about the 12AT7 (or ECC81, in Brit speak)? I think I've a few of those, although they might be ECC82's. Would you prefer the 12AT7 to the triode strapped EF86?

Re the output cathode bias, if memory srves I was aiming for a 1Hz cut off frequency with the 130uF cap. Assuming I've got my calculations correct (which I may well have not), changing up to a 470uF is presumably going to raise that frequency? Is designing for 1Hz overkill in this situation? I'm learning most of what I know from Morgan Jones 'Designing Valve Amplifiers'. He certainly recomends a 1Hz cut off in small signal stages, does that concept not transfer to the output stage?

Matt.
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Old 7th November 2017, 11:02 PM   #7
LeftHandFool is offline LeftHandFool  United Kingdom
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Quote:
Originally Posted by Ketje View Post
Try someting like this.
Mona
Thank you, I'll give that some serious consideraion. One question - what effect will the change of value in the grid stopper resistor from 1M to 330K have?
Is it there to increase the gain or alter the input impedence?

Just one more thing (doing my best Columbo impression), is 0u1 the same as 0.1uF? I don't think I've seeen it written that way before.

Matt.
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Old 7th November 2017, 11:17 PM   #8
LeftHandFool is offline LeftHandFool  United Kingdom
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Quote:
Originally Posted by gingertube View Post
Various threads here on this topic.
For Example:
Simple EL34 SET; advice?

Many suggestions in that thread including this one:
Tubelab SSE | Tubelab

Cheers,
Ian
Thanks for the links. I've been browsing several of these threads and garnering what information I can understand - there's a lot to take in!

Given my build is designed from the ground up, I thought I might risk a new thread to ask some specific advice about the circuit I've designed. Reinventing the wheel, to be sure, but it'll keep me busy on the dark winter nights. Assuming I'm not at the pub.

Matt.
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Old 8th November 2017, 08:26 AM   #9
LeftHandFool is offline LeftHandFool  United Kingdom
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Quote:
Originally Posted by audiowize View Post
I don't think it would hurt to go to a 470uF bypass cap on the output stage.
Just re-read the section by Mr Jones on power amp design. 1Hz is indeed a bad choice for a power amp, I'll take your advice and investigate some different values. Or more likely just change the value to 470uF...

Matt.
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Old 8th November 2017, 11:48 AM   #10
Ketje is offline Ketje  Belgium
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Quote:
Originally Posted by FlemingsLeftHandFool View Post
Thank you, I'll give that some serious consideraion. One question - what effect will the change of value in the grid stopper resistor from 1M to 330K have?
Is it there to increase the gain or alter the input impedence?
It's not a stopper, but a path to ground for the grid to get it on zero voltage.
Mullard specifies 500k as a max. , with 330k we are on the safe side.
Quote:
Just one more thing (doing my best Columbo impression), is 0u1 the same as 0.1uF? I don't think I've seeen it written that way before.
Yes same thing. 100nF or 0,1F or 01
Is like 2,2kΩ writen as 2k2.
Mona
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