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Simple PP Amp with +ve & -ve FB



 
 
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  #1  
Old November 23rd 15, 10:33 PM
John L Stewart John L Stewart is offline
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Smile Simple PP Amp with +ve & -ve FB

John Stewart Nov 2015

This circuit is based on an amplifier I built for a friend to be used with a guitar & mike. The date on the schematic is Dec 7, 1958, almost 57 years ago. The entire project was built on a sub chassis & installed inside an 8x12x3 Hammond chassis. Only the power amp is covered here.

The original amp used PP 25L6s driven by the same cct as shewn here. There are no pentodes in Electronic Workbench software so here we have used the low mu power section of 6EM7s. That still manages to demonstrate the effectiveness of +ve FB on the performance of the cct.

The power supply used a 170-0-170 HT transformer, something that was plentiful at a good price at one of the surplus shops adjacent to ElectroSonic when they were downtown at 543 Yonge St. The 25L6s & 6SL7 heaters ran directly off the line in series with a 50Y6GT rectifier.

Oddly, this transformer had no 6.3V winding but rather a very useful 25V at 150 mA. That ran the preamp, a 6BH6 for the mike & a 12AX7 for the guitar inputs & tone cct driver.

In the original cct there was not enough gain. To get around that problem I decided to try a simple +ve FB cct. The full loop NFB would control the overall gain & get distortion down to a reasonable number. The extra gain here is possible by returning the –ve lead of the split load phase inverter to the previous cathode rather than the amps common buss. An added bonus is the much improved Damping Factor.

The OPT used was a Hammond 125D, the 125E had yet to be developed. My notes show the amp was down one db at one watt, 20 Hz to 28 KHz. And down one db at 6 watts, 30 Hz to 15 KHz. D = 0.5 % at 6 watts, 7 watts clipping.

The no FB version still has a very respectable DF, typical of power triodes. For the 6EM7 mu is 5.5 while rp is 750R, very good for driving a loudspeaker. The OPT loading could be increased resulting in more audio power.

On the schematic & spreadsheet switches labeled Space, z & x control the feedback & loading conditions.
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  #2  
Old November 24th 15, 08:55 PM posted to rec.audio.tubes
Peter Wieck
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Default Simple PP Amp with +ve & -ve FB

On Monday, November 23, 2015 at 7:37:07 PM UTC-5, John L Stewart wrote:
> John Stewart Nov 2015
>
> This circuit is based on an amplifier I built for a friend to be used
> with a guitar & mike. The date on the schematic is Dec 7, 1958, almost
> 57 years ago. The entire project was built on a sub chassis & installed
> inside an 8x12x3 Hammond chassis. Only the power amp is covered here.
>

Thank you! A very nice, simple circuit. And further support of my contention that with tube amps, there has been nothing new since perhaps 1970, or so (very probably 1960) but for a few tube variants. We try to make a better mousetrap, but soon realize that the good old Cat still does it best.

This is not a bad analogy, as it happens. Tubes can be troublesome, are entirely incompatible with some, require careful attention and understanding, yet can be very rewarding. Much as a cat. I also find, coincidentally, that of my acquaintances and friends that keep tube audio, more than half of them also are kept by a cat. Murphy & Seamus spend a good deal of time with me when I am either listening or working in my Radio Room. Supervision and companionship.

Peter Wieck
Melrose Park, PA
  #3  
Old February 3rd 16, 04:03 PM
John L Stewart John L Stewart is offline
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First recorded activity by AudioBanter: Jan 2011
Location: Toronto
Posts: 301
Smile

very nice, simple circuit. And further support of my contention that with tube amps, there has been nothing new since perhaps 1970, or so (very probably 1960) but for a few tube variants. We try to make a better mousetrap, but soon realize that the good old Cat still does it best.

Peter Wieck
Melrose Park, PA[/quote]

--------------------------------------------------------------

Here is something definitely new since 1960 AFAIK. I did this one in the late 1990s. It was published in Glass Audio & Electronics World (UK). No one ever came forward to say it had already been done.

The cct is has a simple mod to make driving very low mu power triodes easy. In a following article I shewed a another way not requiring a UL OPT.

This amp makes an easy 30 Watts. The distortion plot shows THD to be down 44 db (0.6%) at 25 Watts. Some guy in SE Asia is building the amp commercially without permission. One of the buyers contacted me since his copy did not perform well. I could see the biasing circuit was screwed up.

I've put all the amplifier information into a PDF of 1850 KB. Just ask-

johnnhelen4-------gmail

You know the drill!

Cheers to all, John L Stewart PEng
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