converting Pro Jr to 6V6

[bluemonk]

Hello, I'd like to build a Pro Jr. It is an EL84 amp but I got a PT better suited for 6V6 (about 360V B+).
What mods are needed to Pro Jr design for 6V6 conversion?

Thanks in advance

[tubeswell]

For that sort of B+ voltage in a cathode-bias amp (which will be grungier-sooner), take a look at the 5E3 circuit (with a cathodyne inverter), or the 5E9A (with a paraphrase inverter). Or for a fixed-bias amp (which will be a bit cleaner) look at the 5G9 (with an LTP inverter). Either will work with a 5k-8k primary on the OT. You might have to play around with the rectifier etc to get the voltage close to where you want it. JM2CW

[bluemonk]

You're right, different rectifiers will produce different output voltages. I believe that 6V6 are driven with higher plate voltage than EL 84.

I'm thinkink to replace EL84 with 6V6 in Pro Jr schematic.
Are there changes needed in components and/or voltages?

Thanks

[tubeswell]

What are your PT specs? (and has it got a 5V winding?)

[bluemonk]

It's a salvaged old PT. It has 5V winding for rectifiers, 3,15v-0-3,15v and about 0-320v high voltage.

[Steve Conner]

You might need more bias voltage than the stock Pro Jr. circuit provides, and/or less screen voltage. (Increase R32.)

If you lowered the screen voltage, it would probably work fine with EL84s too.

Another way of burning up some of that extra B+ is to use cathode bias instead of fixed.

[bluemonk]

I'd add a bias pot to stock Pro Jr schematic to get correct voltages. I already got a suitable schematic.

and/or less screen voltage. (Increase R32.)

How much the screen voltage should be for 6V6?
Original schematic (for EL84) says:
plates= 319v
screens= 294v
pre= 232v

Thanks

[Steve Conner]

The "294V" quoted on the schematic should be fine for 6V6s too. However, since your PT gives more voltage than the Pro Jr. one, you will need to use a larger value of R32 to get that ~300V. You'll need to get the right voltage at this node anyway, or the preamp supply voltage will be wrong too.

Trial and error once the amp is finished, is the best way to find the correct value. Three times the value on the schematic would be a reasonable starting point: you need to drop 60V instead of 20.

Alternatively, you could just let the voltage at that node be 340V, and increase the screen resistors to about 1k.

[bluemonk]

Trial and error... that's right. Now I have some concepts for my aims

Thanks!