Author Topic: 100 watt push-pull fixed bias amplifier  (Read 1106 times)

Offline Mars

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100 watt push-pull fixed bias amplifier
« on: June 13, 2018, 12:08:34 PM »
I have dabbling with fixed bias output stages in an attempt to get more power output for less sensitive speakers. The EICO HF60 caught my attention, since it has a 6SN7 phase inverter, as opposed to the high gain Mullard 5-20. Here is a schematic of the circuit:



This article (http://tronola.com/html/hf-60_with_6550s.html) discusses possible improvements and modifying ideas. I used some of them in this build. The power supply is my own design (regular bridge/cap/inductor/cap) PI filter.

Mars 100 by Karel Mars, on Flickr

After a bit of to and fro', the sound is settling beautifully. The EF37's were chosen in stead of EF86, since I recently acquired a whole hoard of EF37's that begged to be used. KT66's were handy, since the 6550's are still on order. A very appealing sound. If you like music loud, this may be the way to go.
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Offline John.Perry

Re: 100 watt push-pull fixed bias amplifier
« Reply #1 on: June 13, 2018, 04:56:16 PM »
Damn nice  :thumbs:

Offline JonnyP

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Re: 100 watt push-pull fixed bias amplifier
« Reply #2 on: June 14, 2018, 06:24:39 PM »
Is that 100wpc or 50wpc? Add that it looks magnificent
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Offline Den123

Re: 100 watt push-pull fixed bias amplifier
« Reply #3 on: June 14, 2018, 08:22:09 PM »
Now that is an amp I like! I can understand the circuit. No nonsense.

Offline fredeb

Re: 100 watt push-pull fixed bias amplifier
« Reply #4 on: June 14, 2018, 08:26:20 PM »
Looking good Karel !

@JonnyP - The improved design ( with 6550's that Karel is waiting for ) delivers 60wpc .

From link - http://tronola.com/html/hf-60_with_6550s.html

" Power output is now easily 60 watts RMS from 20Hz to 20kHz, with all distortions significantly reduced over that of the original design as previously noted. Frequency response is slightly and intentionally reduced to 70kHz +0/-0.5 dB (down from the original spec of 90 or 100kHz ± 0.5 dB depending on which design you had), but in return, low and high frequency stability is now rock solid: Pulse conditions settle virtually instantly, while no value of capacitance across the unloaded 16 ohm secondary causes any tendency towards oscillation. The original design would fail this test miserably. With the new design, the effect of the improved stability is very noticeable in the presentation of detail. Overall, the modification produces a sense of effortless power, and great clarity. "

Methinks you would probably need parallel push-pull to do the 100wpc with ease . 100wpc is possible with a pair of KT88's or even a pair of EL34's , but the tubes would probably being run at their absolute limit .

Still - 60wpc ( perhaps heading on 70 wpc ) of wideband undistorted tube power is quite a lot already .

Here's a copy of the revised schematic of David Gillespie :

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Offline Mars

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Re: 100 watt push-pull fixed bias amplifier
« Reply #5 on: June 17, 2018, 11:28:49 AM »
Update: the amp has been playing for a week now. The high frequency response was extreme and I had to listen to music at whisper levels to not get a head ache. This is an amp that needs taming. And the bass response is lacking.

I tackled the bass response first. I tried more capacitance at each PSU node; no improvement. Enlarging the coupling caps to from 0.22 to 0.47uF did give me the bass I needed.

I did the aikido trick and coupled the PSU rail to the phase inverter grid via a 0.22uF cap. I also bypassed the all the PSU electrolytics with polypropylene caps. Lastly I enlarged the 220pF NFB cap to 1000pF.



Listening to music is much more fun now. I might remove the 1000pF NFB bypass cap next.

The sound is very clear and powerful.
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Offline Mars

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Re: 100 watt push-pull fixed bias amplifier
« Reply #6 on: June 20, 2018, 09:51:22 AM »
Update: I was talking to fredeb about the amp, and he mentioned C11 being peculiar, since it is only on one anode. I realised that I have left C11 out! We'll that was a mistake. I added a 390pF on both channels, and the sound shifted dramatically. Top edge gone, bass more prominent.  At the moment I have C7 as 220pF. I might go back to 68pF as suggested by Gillespie. Then to try out the negative rail on the phase inverter. :rubhands:
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Offline Ampdog

Re: 100 watt push-pull fixed bias amplifier
« Reply #7 on: June 20, 2018, 05:37:33 PM »
A few thoughts from me:

One needs to be careful here. C11, C12, C13 (in connection with R22, R23) form supersonic phase correction, from about 40 KHz to over 120 KHz. Likewise C7 curtails open-loop frequency response above 26 KHz in order to keep NFB stable. Making C7 = 220pF starts roll-off at some 9 KHz.

Point: One needs to rather check operation involving these capacitors with an oscilloscope and a say 5 - 10 Khz square wave to watch for over/undershoots/instability. By the time you can hear a difference with music, case might be that the 'poor sounding' side is already suffering from instability, as all of the above except C7 has effect only well above the audio band (guesstimate some 40 Khz to 120 KHz, also directly dependant on OPT  h.f. characteristics.).

This in connection with the OPT internal capacitance and leakage inductance; unknown(?) in Dr Gillespie's case.  It can be simulated; sadly my SPICE programme does not work at present (seems to age even faster than its owner!)
« Last Edit: June 20, 2018, 05:40:04 PM by Ampdog »
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Offline bbe22

Re: 100 watt push-pull fixed bias amplifier
« Reply #8 on: June 20, 2018, 08:31:17 PM »
post in error ignore
« Last Edit: June 20, 2018, 08:35:24 PM by bbe22 »

Offline Mars

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Re: 100 watt push-pull fixed bias amplifier
« Reply #9 on: June 25, 2018, 12:30:46 PM »
Thanks Ampdog

I will check the ringing at 5 to 10k on the scope and report back.

The OPT's do not have 16ohmm output taps, so the C12/13 network will have to adjusted. I have 4 ohm and 8 ohm though. Do you have any idea how to adjust R22 and R23 accordingly?
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Offline Mars

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Re: 100 watt push-pull fixed bias amplifier
« Reply #10 on: August 14, 2018, 06:51:14 PM »
The amplifier has passed listening tests, and has been delivered to the customer. I took a photo just before delivery.

IMG_20180813_134358 by Karel Mars, on Flickr

IMG_20180813_134418 by Karel Mars, on Flickr
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Offline fredeb

Re: 100 watt push-pull fixed bias amplifier
« Reply #11 on: August 15, 2018, 04:45:27 AM »
Damn ! I wish I'd have made a turn to hear it before it left - must be quite a beast . :)
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Offline Ampdog

Re: 100 watt push-pull fixed bias amplifier
« Reply #12 on: August 22, 2018, 12:47:32 AM »
The OPT's do not have 16ohmm output taps, so the C12/13 network will have to adjusted. I have 4 ohm and 8 ohm though. Do you have any idea how to adjust R22 and R23 accordingly?

Karel,

Apology! I seem to have missed this question.

The winding ratio between 16 ohm and 8 ohm is 1.414, thus R22 + R23 will have to be decreased  to 70,7%, giving 4,2K total resistance.  If one stays with the feedback via C12 to the 4 ohm output winding, then R23 becomes about 2,2K, with R22 remaining at 3K. (R23 now connected to your 8 ohm output.)

But the values of C12, C13  are entirely a function of the OPT characteristics. They work in the supersonic region, to compensate for OPT internal capacitance and leakage reactance (thus phase correctors.)  Their effect for that particular OPT is somewhere >60 KHz and certainly cannot be trimmed by hearing.  Thus I cannot recommend values for your OPT; to get those right one will need a square wave generator and oscilloscope to check. 

Also the necessity of C12 might indicate some inequality between secondary sections of the particular OPT; ???  I have always just used a single RC feedback combination from the whole secondary, but that is with using all secondary windings for both 8 ohm and 4 ohm outputs (for three secondary sections the usual 30% - 40% - 30%  secondary winding segmentation).


P.S:  Despite Dr Gillespie's remark that his design is stable, I would still add a 680 ohm 5W resistor over the secondaries just to assure stability without a load. (I do this in all my valve designs. Stray capacitance for point-to-point wiring will always differ from model to model.)
« Last Edit: August 22, 2018, 12:55:28 AM by Ampdog »
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Offline Mars

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Re: 100 watt push-pull fixed bias amplifier
« Reply #13 on: August 24, 2018, 10:04:23 AM »
Thanks for the input, Ampdog. My testing of the transformers I roll, have shown the best results for square wave test with a 47pF cap paralleling the feedback resistor  (one resistor instead of R22 and R23). This is also working well in this circuit.

 
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Offline Ampdog

Re: 100 watt push-pull fixed bias amplifier
« Reply #14 on: August 24, 2018, 11:43:49 PM »
Hi Karel,

That is good!

If I may be a  mite tutorial also for the benefit of others reading here: That compensating capacitor and others are all working together - to at the 'other' side', compensate for the phase shifting effects of small capacitances, mainly at the input stage but also all over the show, trying to keep control, until the amplifier loop gain has fallen to below zero at some supersonic frequency, at which point instability is no longer possible.  (Do read up on the internet!)  This phase shifting also includes OPT leakage reactance. That C is thus determined for every separate circuit on its own.

In Karel's circuit the OPT already seems to be well within control. Let us for now forget about the contribution of the 4 ohm tap in the Gillespie circuit, and the 'funny' divided feedback circuit. Other circuits are suggestions, not prescriptions.   End of tutorial!

So, Mr Mars, good fortune to you; you know what to do!

 
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)