Author Topic: EL84 push-pull kit with 12AT7 split load phase inverter  (Read 1118 times)

Offline Mars

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EL84 push-pull kit with 12AT7 split load phase inverter
« on: April 16, 2018, 05:14:52 PM »
Hi all valve amp lovers

I am building an EL84 push-pull amplifier for a workshop this weekend. I think I have the layout nailed this time. What do you think?

Here is the chassis with the basic parts mounted:

IMG_20180412_120810 by Karel Mars, on Flickr

Here is a close up of the action around the two EL84's

IMG_20180412_120748 by Karel Mars, on Flickr
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Offline fredeb

Re: EL84 push-pull kit with 12AT7 split load phase inverter
« Reply #1 on: April 16, 2018, 05:51:06 PM »
Looks poetic Karel - super clean . ;)

Love the simplicity !
« Last Edit: April 16, 2018, 05:55:42 PM by fredeb »
...evolution is the gradual development and stratification of progressive series of wholes, stretching from the inorganic beginnings to the highest level of spiritual creation.
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Offline fredeb

Re: EL84 push-pull kit with 12AT7 split load phase inverter
« Reply #2 on: April 16, 2018, 09:52:39 PM »


Source : https://www.tubecad.com/2010/12/blog0196.htm

This looks super interesting Karel . Is it going to work like schematic above ?

...evolution is the gradual development and stratification of progressive series of wholes, stretching from the inorganic beginnings to the highest level of spiritual creation.
Jan Smuts

Offline Mars

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Re: EL84 push-pull kit with 12AT7 split load phase inverter
« Reply #3 on: April 17, 2018, 09:35:28 AM »
Yes, the basic circuit is like this:



I'll post a schematic a.s.a.p.

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Offline El Sid

Re: EL84 push-pull kit with 12AT7 split load phase inverter
« Reply #4 on: April 17, 2018, 11:13:52 AM »
Karel - any particular reason you moved away from the 6U8 triode-pentode in the Mars EL84 kit? And the 12AT7 instead of the more common 12AX7? Just curious

Offline Mars

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Re: EL84 push-pull kit with 12AT7 split load phase inverter
« Reply #5 on: April 17, 2018, 07:22:14 PM »
El Sid - I personally prefer the 6U8A tube, but it is not as easily sourced as the 12AT7. The EL84 is readily available in guitar amp shops, as well as the 12AT7. I have many requests for amp kits with common tubes.

I have recently built an amp with a 12AX7 PI , and the results were good, but the 12AT7 sounds better i.m.o. I have used the 12AT7 before in an amp for Sweetwater (was that his name?), and they were very positive about the sound.

This time I did some simulations on the TUBECAD program, and the correct load resistors are now in place.
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Offline fredeb

Re: EL84 push-pull kit with 12AT7 split load phase inverter
« Reply #6 on: April 18, 2018, 02:14:26 AM »
El Sid - I personally prefer the 6U8A tube, but it is not as easily sourced as the 12AT7. The EL84 is readily available in guitar amp shops, as well as the 12AT7. I have many requests for amp kits with common tubes.

I have recently built an amp with a 12AX7 PI , and the results were good, but the 12AT7 sounds better i.m.o. I have used the 12AT7 before in an amp for Sweetwater (was that his name?), and they were very positive about the sound.

This time I did some simulations on the TUBECAD program, and the correct load resistors are now in place.

It was for Sweetsound ( Stef ) . It has EL821's at the outputs ( you can't buy those at your local guitar shop , but you can buy a quad of original Mullards at good prices ) :)

http://i1171.photobucket.com/albums/r555/Stefsound/Picture863small.jpg

http://i1171.photobucket.com/albums/r555/Stefsound/Picture861small.jpg

https://www.avforums.co.za/index.php/topic,17235.msg225716.html#msg225716
...evolution is the gradual development and stratification of progressive series of wholes, stretching from the inorganic beginnings to the highest level of spiritual creation.
Jan Smuts

Offline vinyljan

Re: EL84 push-pull kit with 12AT7 split load phase inverter
« Reply #7 on: April 18, 2018, 07:42:18 AM »

I'll post a schematic a.s.a.p.


Hi Karel, please do, as a noob this looks SE to me :thinking:.   Is there/should there be a difference in the amount of gain at the different outputs
In a maximum bank turn in a normal category airplane, like a typical Cessna, everything is fine until you turn across your own wake

Offline El Sid

Re: EL84 push-pull kit with 12AT7 split load phase inverter
« Reply #8 on: April 18, 2018, 10:24:19 AM »
Hi Karel, please do, as a noob this looks SE to me :thinking:.   Is there/should there be a difference in the amount of gain at the different outputs

If I may answer for Karel, that is only the pre/splitter stage of the amp. The inverting/non-inverting outputs are the same amplitude but opposite phases - each one will drive one of the two power valves in the push-pull power stage.

Offline vinyljan

Re: EL84 push-pull kit with 12AT7 split load phase inverter
« Reply #9 on: April 18, 2018, 03:04:55 PM »
If I may answer for Karel, that is only the pre/splitter stage of the amp. The inverting/non-inverting outputs are the same amplitude but opposite phases - each one will drive one of the two power valves in the push-pull power stage.

OK, let me rephrase: as this part of the schematic reminds me of a SE/triode,  I thought the amplitude of the non-inverting signal would be higher than that of the  inverting :thinking:
In a maximum bank turn in a normal category airplane, like a typical Cessna, everything is fine until you turn across your own wake

Offline Ampdog

Re: EL84 push-pull kit with 12AT7 split load phase inverter
« Reply #10 on: April 19, 2018, 02:41:05 AM »
Carefully replying, as I have no desire to take Karel's narrative onto a shunt path:

Firstly, it should be clear that the same ac and dc current flows through both Ra and Rk. If these are equal (meaning including all external circuitry). the signal from both outputs must be the same.

Looking at the Brosky circuit adapted for low ripple, I cannot but see a very real extra cost in components. Though it can guarantee zero ripple output signal ... is it really the only way to achieve that?

In a reasonally designed circuit there is little excuse not to attenuate ripple sufficiently by simple R.C decoupling circuits. Thus while generally agreeing with Mr Brosky's circuit, is it really necessarry in a particular application?  In the simple circuit Karel is suggesting, there is no extra triode available; one can certainly do a lot of hum filtering for the cost of an extra triode!

But these comments not to detract from Karel's design. This is his thread, I wish him well.
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

Offline Mars

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Re: EL84 push-pull kit with 12AT7 split load phase inverter
« Reply #11 on: April 19, 2018, 05:33:01 PM »
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Offline Mars

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Re: EL84 push-pull kit with 12AT7 split load phase inverter
« Reply #12 on: April 19, 2018, 05:48:52 PM »
Here is a photo of the stereo audio circuit

IMG_20180419_162318 by Karel Mars, on Flickr

and one channel:

IMG_20180419_162327 by Karel Mars, on Flickr

detail around ECC81 socket:

IMG_20180419_162343 by Karel Mars, on Flickr
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Offline El Sid

Re: EL84 push-pull kit with 12AT7 split load phase inverter
« Reply #13 on: April 19, 2018, 08:43:05 PM »
Nice work Karel! Interesting to note the differences and similarities between the 6U8 version i have and the 12AT7 shown here. What are the supply voltages - I see the resistors are slightly different?

Offline Mars

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Re: EL84 push-pull kit with 12AT7 split load phase inverter
« Reply #14 on: May 24, 2018, 12:34:34 PM »
Here is an aside around the above design:

Baxandall's 5 Watt Design

In March 1957, Baxandall returned to the field of audio amplifier design with a simplified, relatively low power (5W) design, because he felt that there was a niche for an inexpensive design intended for use with relatively efficient loudspeaker systems (Baxandall, P.J., Wireless World, pp. 108-113, March 1957). In this design, of which I have shown the circuit in FIG. 9, Baxandall retained both the use of output pentodes and a fairly high level of NFB to keep the residual hum level low, and the total harmonic distortion (THD) below the 0.1% target level at 400Hz and 5 watts output, though he now used the split load phase-splitter system (V2) proposed by Williamson.

FIG. 9 Baxandall's simple amp



The use of high levels of NFB with an output transformer of relatively simple construction brought with it the probability of HF instability - which, if it occurred while handling a speech or music signal, would lead to a greater degree of impairment in the audible quality than any possible improvement due to reduced steady-state THD. To provide the very necessary correction to the poor HF loop stability, Baxandall added a further output Zobel network (R22,C10) across the whole secondary of the transformer, in addition to the networks (C8, R20, C10, R21) retained across the output transformer half primaries. Further circuit refinements for the purposes of HF NFB loop compensation were the lag-lead networks (C2, R8 + C3, R9) interposed in the signal line between V1 and V2.

article here http://www.gammaelectronics.xyz/vtaa_6.html
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