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

[BStringThumper]

Hi glebert,

Thanks for asking if I'd seen that link. I totally missed it. That's the one! Thanks. I'll do some more research on that RRM 1-100 and that schematic. Thanks!!

[BStringThumper]

Hi g1,

That poor resistor is seeing 313v on one side and 0V on the other. I think the value was probably 47K or 56K. glebert found a link that looks like the right amp. I'm going to go check it out right now. Thanks!

[BStringThumper]

Hi glebert,

The schematic was very different from this amp. It looks like upgrades with different values and higher wattage components.
Here are some more pictures to check out. Thanks everyone for reading along.

[glebert]

Hi glebert,

The schematic was very different from this amp. It looks like upgrades with different values and higher wattage components.
Here are some more pictures to check out. Thanks everyone for reading along.

That schematic is labelled as RR 1-100 refit (aka it isn't the original), and has KT88s, so maybe that is what you want to be moving towards. I will admit I did not follow every back and forth in that thread, so ymmv.

[BStringThumper]

Hi glebert,

Thank you for contributing. This gets me one step closer.

[g1]

That poor resistor is seeing 313v on one side and 0V on the other.

Is the zero side connected to ground? If not, something is shorted to ground.
You really need to draw out the power supply section.

[BStringThumper]

Moderators?? Question, please.

Moderators,

Should this thread be moved to repair and head scratching?? I don't know if I should have continued this under the schematic search post.

Thank you.

[BStringThumper]

The 10K resistor is going to ground. I'm thinking I was shunting too much voltage to ground. Doing some wattage calculations at 313v a 47k or higher would get me under the 1W. But I did see on the schematic glebert found is some components have been upgraded to higher wattage. I'm not sure yet if this was one of them but it did burn up the first time. You can see the black under the new resistor. I'm learning something new every day.

[BStringThumper]

SG-100 Schematic that is close to this amp. It's really hard to read.

[The Dude]

I see what might be a switching jack on the output- maybe like the typical Fender jack configuration. Could it be that you're not using the correct speaker jack and the OT is still shorted by the switching jack?

[g1]

The 10K resistor is going to ground. I'm thinking I was shunting too much voltage to ground. Doing some wattage calculations at 313v a 47k or higher would get me under the 1W. But I did see on the schematic glebert found is some components have been upgraded to higher wattage. I'm not sure yet if this was one of them but it did burn up the first time. You can see the black under the new resistor. I'm learning something new every day.

Look at the 47K resistor at the standby switch on the SG100. Maybe that's it. They've drawn the switch a bit wrong, when not in 'play' mode, the resistor is connected to the switch. So it discharges C59 when in 'stand by' mode.

[BStringThumper]

Hi The Dude ( the Dude Abides ),
I've been using the 8 ohm output jack. The switching jack is clean and making good contact. I am getting sound out of the output transformer but it's distorted and low. It distorts more when trying to turn the volume up more but no real volume change. I had to change a 1W resistor that was totally burned open, so I guesstimated the value. I think it needs to be higher. R77 on the schematic says 47K and I think that is the correct component on this amp. The schematic uses zener diodes on the PI cathodes. I'm researching the changes from my amp and the schematic. Please everyone keep the suggestions and ideas coming. Thanks!

[Enzo]

I think it is drawn right. They use the same standby method as Peavey - they remove screen voltage. In the SG amp, flipping into standby disconnects the screens from B+, and also discharges the screen node cap through the 47k resistor. That hastens the muting.

[The Dude]

Gotcha. The schematic doesn't show a switching jack at all, yet your pictures (from what little is visible) seem to show a switching jack. The schematic shows the 8 ohm jack as standard (non-switching) and as the external speaker jack, so............ I'm wondering if the 4 ohm jack is switching. Can you tell us which jack is the switching jack and is that the one you are using for testing?

On the resistor: Yes, I'd change that to 47K. From what I can tell following traces on your pic in post #18, that is the resistor across C59. Too low value will load the supply and drop the voltage at that supply node.

[BStringThumper]

Good morning,

Thanks everyone for the confirmation of needing the 47K resistor in the R77 position. I will replace it.
I haven't tried the 4 ohm output jack but I will to do the test. I don't suspect that's the problem. I think once I have the 47K resistor replaced the rest of the voltages will be in line. If I had the correct 8417 tubes with the higher transconductance (input sensitivity) it would probably be working. But the 6550 tubes have less than half the transconductance. I'll replace R77 with 47K and report back some voltages. If I need more AF drive from the PI tube what would be the best way to get it?? My amp has 47K plate load resistors on the PI, the schematic shows 100K. I'm still learning about Cathodyne, Paraphase , PI circuits. Again, any thoughts are appreciated. Have a great day.