Magnatone MP1 trouble

[pontiacpete]

this cathode resistor is good now, I did have to change once from the burning. But it measures fine now.

[g1]

Are you hearing anything at the output (like hum etc.) ?

[The Dude]

......Even more astonishing that this happens together with the bias voltage being 15V more negative than normal......

Where did you see that? I must have missed something.

[pontiacpete]

I’m hearing nothing on the output. There was sound at the beginning of this.

[g1]

Where did you see that? I must have missed something.

The cathode voltage was reported as 20V rather than the 4.5V shown on schem. So grid is -37V relative to cathode.

[The Dude]

Gotcha. I thought Helmholtz was talking about the bias supply only.

[pontiacpete]

Heres the rest of the story. I did try a 250ohm cathode resistor and it red plated right away, which didn’t make sense to me. I’m going to go through all the work I’ve done, because I had sound when it came to me and when I was using the tubes it had in place, an old pair of GEs. The volume was low but it sounded pretty good. I’ll attach the YouTube clip I made.

https://youtu.be/5FfN4egGc1I?si=i6fXTSw3Ta-se6NL

That’s full volume. There was a pretty bad hiss it’s hard to hear in the video but it’s there. The owner wanted it as quiet as possible. Replaced a couple plate resistors in the pre. but it still hissed and volume was low. Then, not having a pair of 7189s I tried el84s and the cathode resistor smoked. Finally got the 7189s and it still smoked. The new caps I put in were in a bundle, I had disconnected the old cap can.. This bundle caps was kind of tight where I had put them, there’s not a lot of room, so I got a CE distribution can 40/40/30/30 and installed. That’s when I lost sound and I also discovered that one of the GEs was not conducting. So I think it was running fine with the GEs because only one was conducting. With 2 good tubes the cathode resistor smokes.
I have to go through all of what I’ve done and get the sound back and bias it as Juan suggests. With the 100r cathode resistor or not.

[Helmholtz]

With an effective bias of -37V the tubes should be in cutoff (no cathode current).
I don't think changing the bias supply will help.

I suspect a socket wiring issue.
Also note that with a 7189A pins 1+2 and pins 6+9 are internally connected, not so with a 7189 (without the"A").
So the original socket wiring may not be compatible with 7189s.

BTW, what plate and screen voltages did you originally measure?

Pictures might help.

[pontiacpete]

Here are 2 pictures: one is the original wiring, untouched by me except for the new 100r resistor and the blue wire for the screens. Now I can see why one socket wasn’t conducting, no screen connection on the right one. Anyway

[Helmholtz]

Exactly what I suspected: Grid pins (No.2) are not connected.
That wiring only works with a 7189A having an internal connection between pin 1 and pin 2.

Jumper No.1 and No.2 socket lugs.

[pontiacpete]

I see what you’re saying, with the new plain old 7189s I need to connector their grids to pin 2.
I’m getting sound now with the 7189As GEs.

[Helmholtz]

Remember posts 8 and 10?
Look at the datasheet I posted and compare internal wirings of 7189 and 7189A.

What's the cathode voltage now?

[pontiacpete]

Ok I had a massive hum, reversed the OT leads and hum is gone. Voltages are Screens 435v, plates 443v, grid -17v and 4.2v on cathode

[Helmholtz]

Of course a tube having suffered a plate dissipation of around 300% might be bad now.

Again: What is cathode voltage?

[pontiacpete]

4.2v on the cathodes. It’s up and running. It still has the hiss in the back round. I have to wait for new 7189s to come in.