Author Topic: 100 watt push-pull fixed bias amplifier  (Read 461 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

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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

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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

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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

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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

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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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