Yet Another EL84 Amp

[sorenj07]

Stereo chassis. Building it for a friend. EL84's driven by a 6922 per channel, in true negative-voltage supply LTP. If B+ is too high, I'll just sub in a 5U4G. Should bring some parts together with the next couple of weeks. The OPT's are Edcor's CXPP25-8-7.6K (pretty self-explanatory). My only question is where to connect NFB since it's direct-input a la Musical Machine?

[zobsky]

I'd like to know how those edcor PP outputs turn out. Please post your impressions when done.

Thanks

[Brian Beck]

Feedback would go to the grid of the lower 6922. Put a resistor from OPT secondary to this grid, and a resistor from the grid to ground (removing the short). This is your feedback voltage divider ratio. Make sure OPT phase is correct; otherwise you’ll have positive feedback. You may need to play with frequency compensation caps, or maybe not (check for ringing on square waves).

BTW, the LTP cathode voltage will be positive relative to the ground-referenced grids, not -3.65V.

[sorenj07]

Originally posted by Brian Beck
Feedback would go to the grid of the lower 6922. Put a resistor from OPT secondary to this grid, and a resistor from the grid to ground (removing the short). This is your feedback voltage divider ratio. Make sure OPT phase is correct; otherwise you’ll have positive feedback. You may need to play with frequency compensation caps, or maybe not (check for ringing on square waves).

BTW, the LTP cathode voltage will be positive relative to the ground-referenced grids, not -3.65V.

OK, thanks! I'll definitely post pics and tell you how it sounds once it's done.

[alexistheo]

1. negative feedback can be injected at the grid of the non-inverting triode in the long-tailed pair

2. i'm not sure how you came about those voltages for the input tube, but you have the grids at ground and the cathodes at -3.6v, which means positive bias - not what you want. is that a typo? perhaps +3.6v is what you mean?

3. any reason for a choke-input on the negative supply? never tried it but it seems to me that too little current will be drawn for it to work correctly. i think a CRC filter would suffice here

[sorenj07]

Originally posted by alexistheo
2. i'm not sure how you came about those voltages for the input tube, but you have the grids at ground and the cathodes at -3.6v, which means positive bias - not what you want. is that a typo? perhaps +3.6v is what you mean?

3. any reason for a choke-input on the negative supply? never tried it but it seems to me that too little current will be drawn for it to work correctly. i think a CRC filter would suffice here

2. sorry, it was indeed a typo. bias voltage is indeed a relatively standard -3.65V.

3. no particular reason other than that i don't have small electrolytic capacitors rated at close to 500V. i'm pretty sure choke input should work due to the relatively high resistance and low current capacity. if worst comes to worst, the negative supply is too great, and little current runs, and i'll just replace it with a resistor...

[gingertube]

My 2c worth:
EL84 cathode bias resitors - increase to 270R at 2W. 220R will run the tube too hot unless using a rail of not more than 250V.

Rg1 resistors can be much higher than 100K. Specs sheets say 470K max for cathode bias. Take them up to at least 270K - higher if you like. This will lighten the load on the diff amp and reduce distortion a little.

Equalise the diffamp anode loads and put a CCS in the Cathodes - Cascode BJT will do nicely.

Cheers
Ian

[sorenj07]

Originally posted by gingertube
My 2c worth:
EL84 cathode bias resitors - increase to 270R at 2W. 220R will run the tube too hot unless using a rail of not more than 250V.

Rg1 resistors can be much higher than 100K. Specs sheets say 470K max for cathode bias. Take them up to at least 270K - higher if you like. This will lighten the load on the diff amp and reduce distortion a little.

Equalise the diffamp anode loads and put a CCS in the Cathodes - Cascode BJT will do nicely.

Cheers
Ian

Hi. I'll readily take 2/3 of your 2c but will have to ask you what you mean for a CCS. I'd taken a fancy to my little B- supply, but I'm curious as to what your solution might be.

[gingertube]

Sorry,
Forgot to look at the actual value of your B- supply.

-280V is not what I'd call a "little" B- supply and it means that you can (and have) used a fairly respectable size tail resistor in the diffamp. Try it as you've drawn it first. If you think its "gloriously warm" - leave it like that, if you think it sounds a bit "covered" then post again and we'll can add a Current Source circuit suitable for replacing that tail resistor.

I'm a little worried about 550V or perhaps a little more across the ECC88 tube just after switch on - the 5AR4 has a fairly quick warm up time and you don't want it delivering full B+ before the ECC88 starts to conduct. Try it and see what happens.
I'm also concerned that your "little" B- rail comes on straight away (Solid State rectifier) and so you may run into heater to cathode breakdown problems during this time. Might I suggest that you don't use your best ECC88 for set to work tests until we see if these things are a problem or not.

If they are a problem them the best solution is a CCS in the diffamp tail to a "small" B- ("small" as in say between -6 and -20V).


Cheers,
Ian

[sorenj07]

could I tame the b- rise (or would it be a dive?) by putting a CL-90 in series with the 1k resistor in the CRC filter? I was considering trying a 6C4 tube but they max out at 20mA... either that or I put a switch on the HT secondary...

edit: i might increase the first and/or 2nd 22uF cap in the - filter to something big like 150-200uF. it'd take a while to charge up and reach max voltage, right?