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Old 25th October 2008, 12:54 PM   #1
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Default Looking to build a 25-30 WPC Push Pull amp - Need Help

I need suggestions for a stereo push-pull amp producing about 25-30 WPC using something like a 6L6 or el34 (or something similar, I am flexible)

This will be my first amp build and I was looking for something relatively straightforward, not overly complex, and not a bank breaker.

I was looking at the Simple P-P, and would still love to build that, but I want something for this Fall and the Simple P-P looks like it's design may be delayed for a while.

Any help would be appreciated.

ps. One small feature I would like to add are VU meters for each channel, but not sure how that's accomplished.
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Old 25th October 2008, 02:30 PM   #2
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Do you already have a preamp or is an "integrated" amp the thing you have in mind? If you have a preamp, does it have gain?

You said simple. IMO, that means 7591 "finals", as they are as easy to drive as "12" W. types (EL84s and 6V6s).

BTW, 7591s work into the same 6.6 KOhm primary trafos that 6L6s work into.
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Old 25th October 2008, 06:48 PM   #3
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The only equipment I currently have is a Lafayette 224a integrated amp. Don't have a pre-amp.
Can you use the 224a as a preamp or is that not a good idea?

I suppose if I had to build both a preamp and an amp, then maybe building and integrated amp is the way to go. Makes it more complex, I imagine.

I am willing to hear advice. Nothing is set in stone.
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Old 25th October 2008, 07:03 PM   #4
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hey-Hey!!!,
Try an E-Linear circuit. No loop NFB used. It is a long-tail-par input stage of pentodes with the plate loads connected to the screen taps of a U-L output TX instead of a separate, decoupled supply. 6AU6 work well, as do many other small-signal pentodes. I attached a pdf drawing of the amplifier stage.
cheers,
Douglas
Attached Files
File Type: pdf 6au6_st70.pdf (60.3 KB, 1296 views)
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Old 26th October 2008, 03:15 AM   #5
83stang is offline 83stang  Canada
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That's a sweet easy circuit how does that preform? Does it use the same power supply at the ST70? or is it different? I'm in the same boat as you SigloOne glad you started this thread!
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Old 26th October 2008, 10:47 AM   #6
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Quote:
Originally posted by 83stang
That's a sweet easy circuit how does that preform? Does it use the same power supply at the ST70? or is it different? I'm in the same boat as you SigloOne glad you started this thread!
hey-Hey!!!,
It can( and should ) use the rest of the St.70 power supply and chassis. As to 'how it sounds?', the answer is, GREAT!. I built a Mk.III pair up with this circuit using the 6AC7 and most recently my pair of W6m using nearly the same parts. The 6AU6 is also excellent, but the 6AC7 delivers a more sensitive amp.

On the power supply, put a small cap right after the rectifier, then the choke, then parallel everything else after the choke as all the power and signal current goes to a single node( the OPT's center tap ).

The dropping R to the screen grids is about 100k, and you can reduce the g2 bypass cap to about 0.1 uF( it's PP so one increases; the other decreases, it is the difference that needs bypassing ).
cheers,
Douglas
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Old 26th October 2008, 01:24 PM   #7
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Doug's "exolinear" connection is a clever implementation of "plate to plate" short loop NFB.

Budget O/P "iron", such as Edcor and Hammond, badly needs (IMO) the linearizing help a few dB. of global NFB provide. Expensive, "boutique", trafos, with superior bandwidth and linearity, are in order, for circuitry that omits a global NFB loop. Keep in mind the fact that David Hafler expected the now well regarded O/P trafos he designed at Dynaco to be used in combination with global NFB. Dyna "iron", both OEM and clone, is better than Hammond/Edcor.

For net HD spectrum reasons, I like the 12AT7 in PP amps. Doug's circuit can be implemented using cascodes, with 'T7 sections "downstairs" and DN2540N3 MOSFETs "upstairs", instead of pentodes, in the LTP. Set the tail CCS for 6 mA.

Adding global NFB is easy. Lift the non-inverting grid from ground and apply the global feedback to that grid. You end up with an "exolinear" "El Cheapo". Say Doug, would peaking inductance as part of the LTP loads upset operation of the "plate to plate" NFB?

BTW, it's a very good idea for LTP B- to rise after B+ rises. If B- is applied before B+ the grid will be positive with respect to the cathode and tube damage can be expected.
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Old 26th October 2008, 01:44 PM   #8
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Quote:
Originally posted by Eli Duttman
Doug's "exolinear" connection is a clever implementation of "plate to plate" short loop NFB.

Budget O/P "iron", such as Edcor and Hammond, badly needs (IMO) the linearizing help a few dB. of global NFB provide. Expensive, "boutique", trafos, with superior bandwidth and linearity, are in order, for circuitry that omits a global NFB loop. Keep in mind the fact that David Hafler expected the now well regarded O/P trafos he designed at Dynaco to be used in combination with global NFB. Dyna "iron", both OEM and clone, is better than Hammond/Edcor.

For net HD spectrum reasons, I like the 12AT7 in PP amps. Doug's circuit can be implemented using cascodes, with 'T7 sections "downstairs" and DN2540N3 MOSFETs "upstairs", instead of pentodes, in the LTP. Set the tail CCS for 6 mA.

Adding global NFB is easy. Lift the non-inverting grid from ground and apply the global feedback to that grid. You end up with an "exolinear" "El Cheapo". Say Doug, would peaking inductance as part of the LTP loads upset operation of the "plate to plate" NFB?

BTW, it's a very good idea for LTP B- to rise after B+ rises. If B- is applied before B+ the grid will be positive with respect to the cathode and tube damage can be expected.
couple of things...
12AT7 is too high plate Z to go cascode; it will eat up too much headroom. Type 5687 is very good, as is type 6H6Pi.

It is not a good idea to have B+ before or after the negative rail. If it comes up first, it pulls current from the B+ caps( OK if they're film, not a good idea if they're 'lytics ). Same issue if it comes up after, B+ charges the negative rail reverse polarity. BAD for 'lytics. Put a 18V, 500 mW zener from ground to cathode( banded end to cathode). Then the cathode never goes much below 0.7V negative and poses little risk to other lytics.

E-Linear, E for voltage, and Emotive Audio where it was first developed and named....and it is plate-to-grid around the output stage.
cheers,
Douglas
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Old 26th October 2008, 02:56 PM   #9
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Quote:
Originally posted by Bandersnatch
E-Linear, E for voltage, and Emotive Audio where it was first developed and named....and it is plate-to-grid around the output stage.
I know I'm being anal and evil here, but the Pièce de résistance of the "E-Linear" circuit, being anode to grid feedback in the output stage, is described in detail on pp. 332-334 of Langford-Smith's Radiotron Designer's Handbook, 4th edition. Incidentally, that was published in 1952. It is further described in more approachable language in John Broskie's TubeCad Journal in March 2001 - including walk-throughs of how to calculate values and providing practical examples of its use. It seems the innovation is that feedback to the grid is taken as a from a tap on the output transformer as a ratio of the anode voltage rather than from the anode directly.

It's not a personal attack on anyone, just saying that there's very little new under the sun.

As an aside, I would be concerned about the circuit posted in post no. 4 being able to operate in anything but strict and complete class A. The differential pair phase splitter is dependent on identical loading of both valves. However, as the output stage enters class B, one of the output valves is driven into cutoff, where its gain is approximately zero (further negative grid voltage doesn't generate additional positive anode voltage, and in UL, the screen is at a constant ratio of anode voltage).

As I see it, in this state, the loading of the half of the differential pair driving the output valve in cutoff would see the full 'open loop' load impedance looking into the output stage grid, while the other half driving the output into saturation will be seeing the intended load impedance, being reduced by the feedback. In this situation the loading of the differential pair will differ by a factor of approximately the gain of the output stage, which is significant, and would lead to severely asymmetrical drive and premature clipping.
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Old 26th October 2008, 03:17 PM   #10
83stang is offline 83stang  Canada
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Bandersnatch I'm a little confused with the bias one, two and negative bias tap. are they connected together to the bias tap of the power supply? and the t1 and t2 taps are they connected to the transformer? or to somewhere else? what kind of power can you derive from this circuit? sorry about the questions, just intrigued that's all!
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