807 Audio Amp Progress Report

[tubelab.com]

Miles:
I have been through at least a dozen hurricanes, and they are all different. Most don't get real dark though. This one was unique that a cold front came through and actually pushed the hurricane offshore faster. The TV weather people claim that was the catalyst for the destruction. It did have the classic eye, where the wind and rain stopped, and everyone came outside to check out the damage. Things were not too bad. 10 minutes later, someone yelled that halftime was over, and all hell broke loose. Most of the damage happened in the next 10 minutes.

The good thing was that the weather cleared up real quick, and we have had excellent weather for the past three days. This is good, since I have been working outside since the storm. You can see the clear blue sky in the later pictures.

I have been told that the tubes are OK, but I haven't been able to check on them myself. Maybe tomorrow.

When I used to build HF transmitters with 807's or 6146's I used a 10 to 100 ohm carbon composition (important that its non-inductive) resistor with 10 turns of wire wrapped arround it (in parallel). The wire forms an inductor that provides a low DC resistance, but a high impedance to parasitic oscillations. The resistor de Q's the inductor to avoid building a tank circuit.

[Miles Prower]

I have been told that the tubes are OK, but I haven't been able to check on them myself. Maybe tomorrow.

When I used to build HF transmitters with 807's or 6146's I used a 10 to 100 ohm carbon composition (important that its non-inductive) resistor with 10 turns of wire wrapped arround it (in parallel). The wire forms an inductor that provides a low DC resistance, but a high impedance to parasitic oscillations. The resistor de Q's the inductor to avoid building a tank circuit.

Sounds like a good idea. I'm not looking for this to take off and make like a HF xmtr. I've had some sand state audio amps go and do that. Although the fix is quite different.

I have been through at least a dozen hurricanes...

So far, that's something I've missed. Been to Florida to visit, but never had a hurricane hit while there. Although out West, I've seen some hellacious winds come down out of the Sierra Nevadas and blow off roofs.

[tubelab.com]

When a sand state amp oscillates, you can usually tell by the smell of burnt parts. I can remember a Dynaco Stereo 120 that blew up at least 5 times. I couldn't get to oscillate in the shop but in the customers home it would blow up. After a few calls to Dynaco they sent us some special driver transistors that solved the problem.

When a tube amp oscillates, you might not even know it. Sometimes it causes distortion, rarely you get the red glow of death.

I always test my new design by placing it next to an AM radio and a TV set. Try all the channels on the TV while receiving a signal. Then operate the amp normally. Try different volume and bias settings. If there is any interference, the amp is oscillating.

[jane]

Looking at your schematics: I love that DC-couplet driver design. And it also scares me to death to think that the bias of the output tubes is depending on another set of tubes. (I’ve got the same kind of design in one of my amps).
I can see that you have a 2.2M resistor connected from the wiper to the negative side on the bias pots. That’s in case of any kind of glitches in the pots?

How low is Vkk -- about –250v?
Is it your plan to drive the 807s into AB2?

[Miles Prower]

"And it also scares me to death to think that the bias of the output tubes is depending on another set of tubes. (I've got the same kind of design in one of my amps)."

Not too worried about this. In the vast majority of cases, VTs fail open, not shorted. If the 6SN7 fails open, the 807's are biased way into cutoff. Of course, the only way to be 100% sure would be to isolate every stage with capacitor coupling. That's something I wanted to avoid as much as possible. "I've got the same kind of design in one of my amps": then it really can't scare you all that much, now can it?

"I can see that you have a 2.2M resistor connected from the wiper to the negative side on the bias pots. That's in case of any kind of glitches in the pots?"

Yuppers, the 2.2M resistors are in case the bias adjust pot fails open (which they are wont to do).

"How low is Vkk -- about -250v?"

-300Vdc, actually.

"Is it your plan to drive the 807s into AB2?"

This is a Class AB1 design. The 6SN7 driver is included to get the slew rate up. At 30KHz, the 807's require ~0.6mA of instantaneous current to charge up the input + Miller + stray capacitance. That'd be asking a bit too much of the 6SL7's. Class AB2 807's would be power overkill, I think. Anyway, I'd go with a different driver for Class AB2 operation. Something like an SRPP, perhaps, although my preference would be to use a MOSFET source follower for that. I've done quite a few solid state designs, and so am not "sand-o-phobic", so that wouldn't be a problem.

[tubelab.com]

I went to the warehouse today, all of the tubes are safe and warm. There is a fine dust made of powdered roof material (particle board and tar paper) all over everything, but the tubes are all in closed boxes. I can't figure out how that stuff got in the closed warehouse bay.

Still no power. FPL came through the area and fixed the easy stuff. Said that the rest would come later.

[Miles Prower]

I went to the warehouse today, all of the tubes are safe and warm. There is a fine dust made of powdered roof material (particle board and tar paper) all over everything, but the tubes are all in closed boxes. I can't figure out how that stuff got in the closed warehouse bay

Good deal. Now the experiments can go on.

"Still no power. FPL came through the area and fixed the easy stuff. Said that the rest would come later."

Oh well, hopefully they'll get it done before the next hurricane blows through.

[Miles Prower]

Finally have the second preamp/phase inverter done. (see attached) So far, so good: no oscillation, just some residual AM from the 50KW station up the road. Seems particularly bad today. No distortion evident with 40Vp-p output (as high as the o'scope goes) triangle wave input. The Sovtek 6SL7's are measuring a bit on the low side:

Ip: 0.575mA (calculated)
Ip: 0.5mA (measured)

Av: 26.75 (calculated)
Av: 25.0 (measured)

May be a Sovtek thing since the plate curves came from an RCA data sheet. Still not getting any second or third harmonics out of the noise. Nothing blew up, and the performance is looking quite good.

The tail CCS is built with Fairchild MPSA42 high voltage (Vceo= 300V) small signal transistors. These came with a 15-pack of "transistors anonymous" from Rat Shack. The bias diodes are still the basic 1N941's, also from Rat Shack. (Helpful hint here: check them before installing. There might be a bad one in there. ) Just about any small signal NPN's with a Vceo of 45V or more should work here, though the extra margin that the MPSA42's provide is quite nice since without the load of the finals, the PS overvolts by about 40Vdc, or so. That put the cathodes of the 6SL7's at 82.5Vdc instead of the expected 56Vdc.

[tubelab.com]

Glad to hear of your success. I can't speak about Sovtek 6SL7's but I have tried Sovtek 6SN7's. They appear to be relabled (after the fact) Russian tubes since you can see faint Cyrillic characters under the red Sovtek logo on some of them. They don't look like any American 6SN7 and don't exactly work like them either. I don't remember the details but they did exhibit lower gain than the American tubes, and they weren't as microphonic as the American tubes either.

Our electricity was restored last weekend. I am slowly cleaning out my lab (it was used as a storage shed during the storm). Hopefully experiments will resume in a few weeks. There is still a bag full of 807's waiting to be fired up!

[Miles Prower]

The Sovtek 6SL7s and 6SN7s don't have any red logos, and I'm not seeing any wiped out Cyrillic characters either. Just the usual type identifiers, "Made in Russia", and "Sovtek". As for how they compare to US NOS, I can't say as I've never seen them before. (Although the 6SN7's have substantial grid cooler "wings".) I'll have to wait to see how the 6SN7s perform when I get the driver stage complete. These operate as followers to drive the 807s. Hopefully that will work as well as everything has so far.

Good going on getting the power back ahead of schedule.

"There is still a bag full of 807's waiting to be fired up!"

All at once That'd be one helluvan amp!