8417 Power Tube Conversion

[garytoosweet]

Amplifier is an SG Systems 2x12.

From the previous owner, it is running Sovtek 5881/6L6WGC at 580 plate volts! Data sheet for this tube specifies max 250V. Who knows how long it's been like this. The amp actually seems to running fine, sounds decent.

Screen Voltage is 290V. Bias supply is 25V.

Originally this amp was running 8417's, I'm attaching the schematic of the power amp.

My question is what power tube would be a good replacement for the 8417's in this amp? What would need to be done for a proper conversion?

I am decent at repairing/troubleshooting, but converting an amp for use with a different power tube would be a first for me.

[garytoosweet]

Looking like KT88's are my answer. Would love to get another opinion.

Anything that needs to be done besides swap tubes and rebias?

[sluckey]

The maximum bias voltage you have available is only -35vdc. Might not be enough for KT88s.

[garytoosweet]

Thank you Sluckey. I could add a simple Voltage Double if need be, I think. Seems like others have done that for bias supply with no issues.

Or is there a better option?

[sluckey]

Probably gonna need an auxiliary transformer for bias. The bias winding on this PT already has a doubler, although it's ground reference is in the middle so it produces +35 and -35.

[martin manning]

I'd never heard of these amps before. Anybody know what that PI circuit is called?

In any case, the bias supply can be modified as shown to use KT88's.

edit: Rev 1 has a change to the negative terminal connection on the second added 10u cap. It now sees a lower voltage, from raw negative bias to -35V. Otherwise, all the same.

[pompeiisneaks]

I'm sure I'm missing what you mean, but isn't that a cathodyne PI?

~Phil

[Stevem]

A 6550 looks like it would be nice and happy in class A with 600 on the plate , 300 on the screen and -31 on the grid with a 5K OT.

Since it's really the screen that sets the operating point, I would go with those and if need be drop the screen down to 270 or 260.

[martin manning]

I'm sure I'm missing what you mean, but isn't that a cathodyne PI?

No, a cathodyne has approximately equal loads on plate and cathode. This is something like a cascode, in that the two triodes are stacked.

[martin manning]

Looking like KT88's are my answer. Would love to get another opinion.

KT88 and 6550 are essentially interchangeable, so either would work. Pin 1 may or may not be connected to the base shell, so check that.

Another option would be to use EL34's, and miss-match the speaker load to double the primary impedance. I don't know what the Zpri is, but I'd guess 4k8. With EL34's you'd be close to Music Man specs, and they would be cheaper and easier to find. You'd lose some output power, though.

Can you post the whole schematic?

[garytoosweet]

Martin, thank you for the schematic. That was what I was thinking and you made it too easy for me.

I've revised the amp to that circuit, except I kept the stock 22k resistor and stock electrolytic value (changed to 47uf 160V).

Bias voltage now ranges from -37 to -68V.

I have ordered a set of KT88's and should get them next week. I will check on Pin 1.

Attaching the full schematic here as requested.

re: OT, 8471's specify 4200 effective load resistance, KT88's specify 4500. Will work fine.

[martin manning]

Thanks for the schematic. You might have a bit too much negative voltage now, but you can increase R80 when you have tubes to install. I'll be interested to hear how it turns out.

[garytoosweet]

Yes, will adjust R80 if needed for correct bias.

Looking at the datasheet for JJ KT88 I've purchased, it looks like Pin 1 is G3. So I should tie pin 1 and 8 together, correct?

Right now Pin 1 on the socket is not wired to anything.

https://www.thetubestore.com/lib/thetub ... J-KT88.pdf

Edit: On the diagram it looks like G3 is already connected to K internally. What's standard practice here, wire together Pins 1 & 8 on the socket, or leave Pin 1 disconnected?

[martin manning]

The g3 label on pin 1 in the linked diagram looks to be incorrect. Here's a link to the data sheet on JJ's site: https://www.jj-electronic.com/en/kt88
Pin 1 has no connection, and g3 and k are connected internally. With pin 1 on your sockets unused, you are safe all around, and no other connection is required.

[toddgw]

I would consider checking the voltage potential across C61 and it's leakage. It would be prudent to replace it with a 600V version. Especially if the bias voltage is going to be even more negative. At this age these amps are subject to severe bias drift and a leaky coupling caps are commonly the cause.