Frankenstein'd Pro Junior build

[stereomonostereo]

Hey guys,

I'm currently looking at a Pro Junior that a somewhat known builder decided to hand wire and 'hot rod'. It came to me powering up but with zero output. The build is decent quality. Eyelet board, CTS pots, F&T caps, etc. I sorted that there's no output due to the B+ coming disconnected. This is where the fun begins.

The circuit seems to be stock Pro Junior, but the transformers have been replaced with a Princeton Reverb power tranny, and a Bandmaster output tranny. The Princeton transformer is delivering a whopping 462vdc to the EL84s. My first inclination was to try to get the B+ down to around 320 or so like the schematic calls for, but with the Bandmaster output tranny calling for 440 i'd be starving it.

So here's the dilemma, i don't trust running EL84s at 462vdc. I could install 7189As, which handle 440vdc. I could punch the chassis for octals and install 6V6s or 6L6s. Aside from that it looks like the other option is replacing the transformers.

What would you do if this happened to you?

[Jazz P Bass]

Instead of redesigning it, why not simply repair it.

It's not only the voltage on the EL84 that matters.
The bias current & the screen voltage/ current also come in to play.

"Bandmaster output tranny calling for 440 i'd be starving it"
Not at all sure what you mean by this.

[stereomonostereo]

If possible i'd much rather repair it than have to change the whole thing. My screen is reading 435vdc and bias is at -15vdc.

[Justin Thomas]

Are those voltages with tubes in, or out?

Justin

[stereomonostereo]

These are with all of the tubes in

[g1]

You found "the B+ coming disconnected". Is it possible you connected it back to the wrong spot, maybe there was a dropper or something that is now bypassed?

[stereomonostereo]

It was a leg of a resistor connecting the first filter capacitor to the second. Everything looks to be in the right place

[g1]

How much current are the EL84's idling at?

[stereomonostereo]

47mV and 50mV

[stereomonostereo]

Putting in a different pair of EL84s brings my current draw to 55mV and 445vdc plate voltage. I don't see the tubes surviving too long.

[The Dude]

............What would you do if this happened to you?

I'd fix it, lower the B+ with zener(s) and get the bias in reasonable range. Done. No need to change transformers, punch new holes, or do a complete redesign.

The only question I have, since it's been modded: The builder didn't add a cathode resistor (cathode bias) did he?

[stereomonostereo]

It's still fixed bias. He added an adjustable bias pot but wired it wrong so that it had no affect on the voltage. I've since sorted that out.

If i lower the B+ to say, 320vdc, will that affect the Bandmaster OT that usually receives around 400vdc B+?

Also, what would be a safe wattage of Zener to run?

[stereomonostereo]

Say something like this: Solid State Manufacturing - 1N3009B - 1300 Ohm DO-4 10 W 19 mA 130 V Standard Zener Diode - Allied Electronics

130v 10w Zener. Should put the voltage down around 330vdc, and i believe 10w would be enough dissipation

[The Dude]

The actual B+ won't make a lick of difference to the OT, unless it would be over the rated voltage of the transformer. I will let more experienced builders comment on the zener. I know that Chuck H has experience using zeners to drop B+, but there are certainly others.

[TimmyP1955]

55mV across what value of cathode resistor(s)? Shared resistor?