6550 SE Amplifier

[Paul Barker]

So Ive been thinking of “this EL34 migration circuit requires a challenge” Can it really be this good.

But actually progress would come from starting the GM 70 build. Just a quick diversion there and more detail about Eric Barbour’s concept circuit. Eric has cap coupled to the A2 output stage which is supposed to cause “blocking distortion” in A2. Though the bias point is only minus 3.15 volts.

Certainly for the Gm 70 I would power it from EL84 direct coupled cathode follow, floated with a resistive divider on a new +/- power supply just for that stage.

I looked again at an intermediate project where I build the EL84 driver stage in this amp and drive the 801a as the output valve. I drew up the curves and remarcably the A2 element mirrors almost perfectly the right hand most curves with regard to 2nd harmonic distortion. Which I calculated to provide 6 watts at as little as 1.7% 2nd harmonic. Surely that is the way to go to show the EL34 the door.

But its not all about 2nd harmonic. There is a snag. The odd order distortions reportedly sound worse to the human ear. Only building it would answer the conundrum…

So just maybe I’d not divert to that yet.

I think to get similar power out into this 4k7 opt would require a dht not operated into A2. The ONE which might sound better than the EL34 in this circuit is PX25.

[Paul Barker]

After which I may never need to build the GM70.

So, lets just look at the PX25 and then look at any other ideas, to make sure we aren’t in error.

From the Vade Mecum there are two SE operating point options.h

Vade mecum option 1/ 400v -31 62.5mA into 3k2 (but I’d be into 4k7) gets 6 watts @ 6% 2nd harmonic. Very similar to the EL34. Which is 8.2 watts power out at 8.7% 2nd harmonic.
Option 2/ is 500v -45 50mA into 5.5k (to which my 4k7 is close enough) for 8.5 watts @ 7% 2nd harmonic.

The px 25 can cheat a bit here as it can operate at 500v.

But I think we are starting to see why the triode EL34 sounds so good. It is pretty close to the PX25.


It would only come down to whether the dht sounds better than the idht. I cant say that matters when the distortion profiles are this close. My PX25’s would probably cost me Platinum prices and these lovely sounding EL34’s can be had from Derek Rocco @ no more than £50 each.

Is it worth even trying …..? Probably better preserving my PX25’s to sell to a madly wealthy foreigner, in the past we’d have said Japanese. But maybe elsewhere these days. Maybe try sell them at Vintage fair down the road…..

What does make sense is a parallel pair of cheapish Sovtec 2a3’s (which Ive always recommended) with which I can use my 2k opt’s to get 7.5 watts at 5% 2nd harmonic. If I neede to prove anything. I may build that inly because I have the opt’s. But there is no pressing need. The EL34 is good enough.

[Nick]

Eric has cap coupled to the A2 output stage which is supposed to cause “blocking distortion” in A2

Yep, it is a bit ugly that output stage. No need for quotes around blocking distortion, its a real thing and it will happen in that amp. he has tried to avoid it by using as big a coupling cap as practical, so he can use a 47k grid resistor hoping the grid current is going to be low enough to avoid problems. Should be ok at audio, but the 1.7Hz time constant would make me expect some LF instability in the output op point driven by the rectified signal level and any even order harmonics in the drive signal (will be ok with girl and a guitar, EDM maybe not so much). And then that's forced a high current driver stage that's probably working harder driving the 47k than it is the grid.

But it avoids an additional power supply or an interstage.

[Nick]

This may help as a A2 driver:

http://www.pmillett.com/A2CF.html

https://www.ebay.co.uk/itm/166653910355

[Paul Barker]

Eric has cap coupled to the A2 output stage which is supposed to cause “blocking distortion” in A2

Yep, it is a bit ugly that output stage. No need for quotes around blocking distortion, its a real thing and it will happen in that amp. he has tried to avoid it by using as big a coupling cap as practical, so he can use a 47k grid resistor hoping the grid current is going to be low enough to avoid problems. Should be ok at audio, but the 1.7Hz time constant would make me expect some LF instability in the output op point driven by the rectified signal level and any even order harmonics in the drive signal (will be ok with girl and a guitar, EDM maybe not so much). And then that's forced a high current driver stage that's probably working harder driving the 47k than it is the grid.

But it avoids an additional power supply or an interstage.

Great review.

[Paul Barker]

Meanwhile b+ delay fully automated.