7591 vs 6V6

[Gibsonman63]

I have a PANaramic 250R (same as Sano 250R) accordian amp on my bench. The schematic calls for 7591 power tubes. The amplifier has 6V6 power tubes installed. Looking at the tube datasheets, the only difference I see is pin 6, but I am not sure what to do with it. The partial rewiring doesn't make sense if you look at the tube datasheets, so I need to figure out if I can use 6V6 power tubes (which I have) or if I need to order some 7591s.

Are these tubes interchangable? If so, what do I need to do with pin 6?

[Albert Kreuzer]

No, they are not interchangeable. The 7591 is a stronger tube than a 6V6 (19W dissipation, more like a 5881) and has a different pinout.
Did it ever work with the 6V6s in there? Can you post a schematic? Which connections have been rewired?

[Gibsonman63]

Someone had rewired one power tube, but not the other. I changed the pinout to match a 6V6 type tube, but it is not very loud and runs too hot, so I would think that it is not biased properly. I would rather run a 6L6 if the amp can handle it.

Schematic attached.

[Gibsonman63]

I put in a pair of tubes labled 5881/6L6 and they seem to survive better, but are biased too hot still. Around 100mA at 370V. The volume is more reasonable, but no where near where what my 50W Fenders would put out.

To be honest, I do not fully understand the biasing scheme on this amp. Suggestions?

[Albert Kreuzer]

It is a cathode-biased amp according to the schematic. To bias it cooler, put a higher resistor in place of the 125 Ohm from cathodes to ground. Try 220 or 270 as a start. Also make sure the 25µF cathode cap is good. Well, while you're in there, you could change all the electrolytics. Preventive maintenance
The amp should take 6L6 without problems, heater current is 0.9A vs 0.8 (7591) per tube. Screen resistors (470 Ohm) are a good idea also IMO.

[Gibsonman63]

Thanks for the help. I already changed out all the electrolytics and added a grounded power cable to the amp. There is no cathode cap. I'll add that and see what I have. The 125 Ohm resistor is 10Watts. I have some 5 Watt 470 Ohm resistors I could parallel, so that should put me in the ballpark.

I remember perusing several screen resistor threads on the forum. I will have another read and add those as well.

[soundmasterg]

You may find that the amp doesn't have as much gain as it should, as the 7591 has a lot of gain for a power tube and doesn't need as hot of a preamp in front of it as a 6L6 would. 7591's sound really good, and JJ and EH make good modern replacements. The EH is a taller bottle than vintage while the JJ is the correct size as vintage 7591's. Power-wise, the 7591 and 7868 are in the middle of the 6V6 and 6L6GC, with a pair of 7591's making 44 watts RMS with 450v plate and 400 screen fixed bias in the RCA manual. Cathode bias with those voltages is around 30 watts.

Greg

[Gibsonman63]

Thanks for all the info and help guys. Unfortunatly, my client wants to do it on the cheap. I got it working with the 6L6s. It seems to break up fairly decently, but doesn't have a lot of volume compared to my Fenders with a pair of 6L6s. Maybe an accordian supplies a hotter signal than a guitar and I would think that you wouldn't want a lot of breakup with an accordian. The tubes are all old RCA and Phillips tubes. They work, but I don't know how much life is left in them. I would suspect that they have been in there forever.

The wires to the tubes are all run together, so I would suspect there would be some problems if the gain was increased without rerouting some of the wiring. As it is now, it has very little hum. Not a lot of volume, but decent break up. It has a Gibbs three spring reverb tank and the reverb sounds really unique. Almost like a delay.

[Bruce / Mission Amps]

Most of us probably won't chase all over this website finding other posts about this project so I'll comment here.
Stick to one thread so it will be simple for us all.

Power tube biasing;
two 470 ohm resistors in parallel should help quite a bit, but you'll have to measure the actual plate to cathode voltage to determine if it is good.

Because of the sensitivity of the 7591s and the low drive level in this amp, I wouldn't be surprised to learn that the design was leaning at Class A push pull... that limits the actual output power to less then what it would be if Class AB1. The 12AU7 triode, used as a predriver, might not be supplying enough peak to peak drive to the cathodyne triode to make the 6L6 power tubes come to full power now.
The 6V6s and 6L6s will need a bit larger voltage swing on their grids to be driven into full power vs the 7951s.
You can probably make the amp operate more in the push pull Class AB1 range if it was supposed to be more Class A'ish.
After cooling down the cathode idle current (bias of the power tubes), try replacing that 12AU7 (it is the combination pre-amp, cathodyne driver phase inverter) with a higher mu tube like a 12AY7, 12AT7 or 12AX7.

Sometimes the cathode bypass cap for the power tubes is in a multisection cap mounted to the chassis.
Look and see if a wire comes from the cathodes of the power tube sockets over to the main filter cap section, or from the actual + end of the biasing resistor.
Regardless, from the looks of the schem, I think this amp ought to be able to hit at least 25-30 watts output with a little tweaking.
50 watts? Hmmm... I don't think so.
But, 25 watts is not half as loud as 50 watts if you test using the same speaker. So, if you are hearing a major difference between this amp and 50 watter into the same speaker, I think the 6L6s and the suggestions above will get you started down the right path.