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

[sgelectric]

Quicksilver Audio (hi-fi) had a mono block design that used 8417 tubes.
Many were converted to EL34s as 8417 tubes became unavailable/problematic.
An internet search has much discussion of these conversions/modifications.
Might want to check it out..............

[g1]

The PI you drew up (and the pic of the board) don't look anything like the PI in the SG100 schematic.
So although there may be some similarities, I don't think we should be considering that SG100 as THE schematic.
Once you get that supply resistor sorted out, DC voltages for all the tubes and supply nodes would be very helpful.

[Helmholtz]

The PI in the SG100 schematic looks rather odd. V1A is a gain stage and V1B is a cathodyne splitter. Strange thing is that V1A doesn't have its own power supply connection and seems to be supplied by the cathode of V1B. I would have expected to see an additional V1A plate resistor connected to a PS node.


Regarding the OP's PI, it would be essential to see where the second 0.22µ coupling cap connects.

[BStringThumper]

Good morning,

g1, you're right, the SG100 schematic is not the correct one for this Randall Alpha II amp, but it has some similarities. I can't find the correct one. I've looked at other Randalls, Standel, Norlin/SG, CMI, schematics. This is an oddball amp.

sgelectric, thanks for the info.

Hi Helmholtz, here is my horribly hand drawn schematic and some new voltages from yesterday with the R77 47K resistor in place.

Thanks everyone for checking them out.

[Helmholtz]

Ok, this looks like a paraphase PI with a pot for balancing outputs.
(http://www.valvewizard.co.uk/paraphase.html)

To increase power amp gain you may increase the 6.8k NFB resistor.

[BStringThumper]

Thanks Helmholtz! I think it's closer to the floating paraphase since the potentiometer is connected to both plates 1 & 6. I will research the local negative feedback circuit. Thanks for the suggestions.

[BStringThumper]

My first attempt at CAD. I hope this helps.

[Helmholtz]

I will research the local negative feedback circuit.

What do you mean with "local"?
I was speaking about reducing global NFB.

[vintagekiki]

The schematic (cathodyne) phase inverter is wrong drawn.
The correct schematic is shown in red color.

[BStringThumper]

That was the paragraph I was reading at Valve Wizard---- Triodes with Local Negative Feedback

[BStringThumper]

Hi vintagekiki, that schematic is not the correct schematic for my amp. The handwritten or the CAD schematic is the one on my amp. I can't find the correct schematic. Thanks for looking.

[dmeek]

Would it be possible to flip the board over and post a photo of the foil side?

[BStringThumper]

Hi dmeek,
I will try to get a picture the next time I change some components around the PI. Thanks for following along.

[g1]

Be sure to document any changes you make.

[BStringThumper]

UPDATE: AMP IS WORKING-

Thank you The Dude for asking about the 4 ohm speaker jack. The amp is a lot louder through that jack and an old Peavey 4 ohm 12" speaker. There must be a short in the winding somewhere in the 8 ohm tap..... I'm still investigating the jack. It is the 4 ohm jack that has the switch. Continuity was good through the switch section but I'll test and check it over again. This is the first time I've ever seen this happen. glebert, thanks for the link. I finally found the "refit" schematic you were referring to. I can see they turned the PI into a Long-tail pair on that amp. I was doing load line calculations to try to get more drive out of this PI and it wasn't even the problem. This was a frustrating project. A big thanks to all the people who helped me out again. Be safe and stay well.