Schematic needed please for 70's Randall Alpha II RG150-2 with two 8419 tubes. Thank you.

[BStringThumper]

Schematic needed please for 70's Randall Alpha II RG150-2 with two 8417 tubes. OOPS! 8417 not 8419 tubes.
It has a solid state preamp section and one phase inverter tube and two 8417 output tubes.
Thank you.

[BStringThumper]

Hi, Everyone-

I hope everyone is staying safe and healthy out there in the outside world. I've been safe on the bench (short of threat of electrocution and solder fumes) puzzling away on this 1970's Randall Alpha II power amp. From the limited information out there on this amp, I can't find a schematic, it did use two 8417 output tubes. I found mixed answers on the phase inverter tube. I installed 6550 tubes and piggybacked a bias resistor and got the tubes biased at 660Vdc plate @ 30mA. I was using a 12AX7 Phase Inverter tube. The sound was distorted and no power. Another suggestion was a 12AT7 tube. The sound was cleaner but still very low power/output. After some research I found that the 6550 has half the transductance as the 8417 tube. Which then made me wonder if a 12A?7 was even the right tube. Looking at the P.I. tube socket it has wires from the heater circuit going to pin 6 & 9. Then a wire from pin 9 runs up onto the board through a 100 ohm resistor to ground. So that's to reduce hum by elevating the ground, right? What is the other heater wire doing going to the plate/pin 6 of the P. I. ? It also has a potentiometer connecting pin 2 to the plates 1 & 6. I've never seen a circuit like this. Here is a picture of the circuit board. This is the before picture. You can see a burnt up 1 watt CC resistor. I took a guess that it was probably a 10k resistor when I replaced it. If any one suspects this was not the right value please let me know. I also replaced the EL caps and upgraded the bias cap to 100uf-100V. Also, I ran the preamp signal into a solid state amp and the sound was clean. So questions are- Is a 12A_7 even the right tube? If not enough drive for the 6550 is the problem- what would be the best approach?? Cathodes each are going through 270 ohm resistors. I will get voltages, but didn't want to run the amp too long with the possible wrong tube. Thank you in advance for reading through this, any thought are greatly appreciated.

[g1]

I don't think the 12A*7 is the right tube, at least not as circuit described. Could you draw out that part of the circuit? (and double check your pin numbering)

[Leo_Gnardo]

I don't think the 12A*7 is the right tube, at least not as circuit described. Could you draw out that part of the circuit? (and double check your pin numbering)

I'm with g1: 12A_7 filaments are pins 4, 5 and 9. I can't think of a tube that has filaments going to pins 2 or 1 or 6. You have a mystery there, I'd sure like to find out the answer.

In amps I've encountered with 8417, I've managed to get those amps working - somewhat - OK by substituting 6L6GC and making an adjustment to the bias voltage. All that, and had to accept a power figure quite a bit lower than expected. 8417 were a magical tube, promising lots of extra power in a package the same size as a 6L6GC, otoh nearly every one I've found still working is microphonic & rattly to the point of being unacceptable. Although a couple of today's manufacturers have gone to the trouble of making their versions of other oddball power tubes like 6973 and 7591, nobody's taken on the 8417. I can only imagine there's not enough of a market for them, what a pity. Competently made new 8417's would be a miracle.

[glebert]

I'm finding a few references to Randall amps with 8417s and they all say 12AT7 PI. This thread seems pretty informative about switching to other tubes, the power amp looks the same as yours. https://el34world.com/Forum/index.php?topic=19435.0

[BStringThumper]

Hi, I tried to draw out the schematic. I've never done this before.... and it shows. Correction last post--- the heater is not going to pin 6, it is going to 4&5 as it should. The other filament is going to pin 9 and then a wire off of pin through a 100 ohm to ground. I did see the post about using a 12AT7, I tried it and it wasn't as distorted but very low output volume. The circuit uses 47K plate load resistors but check out that potentiometer. I did see that some older circuits used a 6AN8 tube. I will look up the pinout. So maybe it's just the low transconductance of the 6550, the 12AT7 lacks the drive to get more power out of the 6550 tubes.....? Sorry for the poorly drawn circuit. It's really hard to get any light under there and actually see where everything is going. Thanks for looking at this mess.

[g1]

Somewhere there should be a 100ohm going from the 'other side' of the filaments to ground.
You said you replaced the burnt resistor in the supply section with a 10K. What did the burnt one measure? Can you draw up the supply showing where that burnt resistor was located?

[BStringThumper]

Hi g1, good to chat with you again. Thanks again for your help on this one. I like your bottom line quote. I don't like SMD either.
I looked at pins 4&5 and sure enough they are going through a 100 ohm resistor to ground. So this is the elevated/artificial ground to reduce hum. If you look at the first pictures I posted you can see the resistor between the first two black capacitors on the left. The resistor was burnt totally open, no reading. You can see that the body of the resistor is all black. By comparing other power supply schematics I "guesstimated" the 10K resistor value. When I tested the circuit with no tubes (unloaded) I got 700Vdc on the plate, -23.76V on G1 grid, and 345.5V on screen G2. I will try to draw the schematic for that one tomorrow. I found a neat software program called TinyCad. I hope it helps show the circuit better. 700Vdc is a record for high voltage I've seen on a tube amp!!!

[g1]

That is high, practice your high voltage distancing!
They get away with it mostly by running screens at around 1/2 plate voltage like that.

[Justin Thomas]

The few 8417 amps I was able to fibd schematics for had 12AU7 phase inverters. I didn't look at what particular PI configuration, though, but Cathodyne seems to come to mind.

I know AU7s are sometimes used as power tubes in tiny PP amps.

Come to think of it, a LOT of BIG amps I've seen using 6550s, KT88s, etc use a cathpdyne with a driver after or before it. Look at the big HiWatts, Ampegs, Orange, & the GEC design book...

Justin

[BStringThumper]

Hi, Justin- thanks, I'll check out the schematic on those amps. I did try a 12AU7, still distorted and low output power. The 12AT7 seemed the cleanest but again no power.

I took some voltages today with the tubes in place. g1, yes, the plate voltage on this amp makes me want both hands in my pockets... thanks for word of caution.

Thanks for any suggestions. I will keep researching. Happy Easter.

[g1]

What is the supply voltage going to the other side of the PI tube plate resistors?
That pot connected to the PI plates seems to be some type of 'symmetry' adjustment. Have you played with it for the various PI tubes you've tried? Ideally you would want to look at it with a scope to make the adjustment, but you might have to do it by ear if you have no scope.

[BStringThumper]

Hi g1,
The voltage supply side of the 47K plate resistors are 294 volts. Plates with a different tube (12AU7) pin 1. 76, pin 6. 74, pin 3. 1.1, pin 8. 1.09. I did find that I have 313v on that 10K 1W dropping resistor I replaced. That's dissipating 9.79 watts! I think I guesstimated too low of a value. I looked at other schematics, but none match this amp. I'm still researching. Thanks for the questions and suggestions.

[g1]

I would be nice to see a drawing of the supply nodes & voltages.
It could also be that the 10K is too high a value.
In any case, the plate voltages at the PI are too low and costing headroom.

That 313V at the 10K is across it? Or is it 313V at one end and 294V at the other?

[glebert]

I looked at other schematics, but none match this amp. I'm still researching. Thanks for the questions and suggestions.

Did you look at the schematic for the RR 1-100 from the thread I linked to above? It looks exactly like your amp, I would be surprised if that isn't very similar.