Fender Pro Junior with el34 tubes

[Tauruspain]

What changes would need to be made to the basic schematic of a Fender Pro Junior to be able to use it with EL34 tubes and ensure the phase inverter supplies enough voltage for them to deliver their power?
The power transformer I'll be using has a 325-0-325 secondary and 300mA. The transformer will be a Hammond 1760J.
I think if I copy the Pro Junior schematic directly, the amp will sound thin because the EL34s won't have enough signal to function properly, even though the specifications for both types of power tubes are around 11-12 MU.

Thanks in advance for your help.

[martin manning]

The PI is not the problem. Looks like in the Pro Jr the signal is attenuated by the 470k feeding the 250k volume control. Your PT needs to supply more heater current 1.5A/EL34 vs 0.76A/EL84, and of course you will need to roughly double the high voltage current and output transformer power capability, and modify the bias supply to suit the EL34's. The transformers will need to be 50W Marshall specifications.

[Stevem]

I will assume you’re just using the chassis from the Fender amp for this build and not the cabinet ?

If you are using the cabinet also then this phrase comes into my mind, “ 10lbs of crap stuffed in a 5 lb bag “

[nuke]

You might want a 1750N or 1750P, 3.2k-3.4k)depending on the load line you want for your EL34's, but that's a bigger transformer than the one you selected (Vibrolux/Pro-Reverb 4k).

You can tweek the PI section a bit with the tail resistors. You'll need different screen resistors of course for the EL34. Look at 50-watt Marshall 2x EL34 for examples.

The point about R5 and R6 ahead of the volume control is a good one. The 470k and the 56k form a divider to attenuate the signal going into V1A. The other part of that is that the tone control needs a fairly high source resistance to get the same effects.

If you want more gain to drive the rest of the amp you build, you could add bypass capacitors to the preamp tube cathodes. The 1.5K cathode resistors are not bypassed with an electrolytic capacitor. I'd probably suggest adding a bypass capacitor to R4 (cathode of the first stage preamp) to get substantially more gain. If that's not enough, maybe R11 as well.

25uf@25v would be a good start. Or if you found it getting flabby, perhaps a 0.68uf and a bigger higher cathode resistor in the first stage. (get rid of bass early).

But, overall, just a few changes would do the trick. Mostly you'll need to tweek the power supply voltage divider to suit the new transformer and loading from the EL34. Maybe add an inductor to the power supply as well.

(again, look at the EL34 50-watt Marshall, especially the older ones, for some inspiration).

[Tauruspain]

Hello, thanks for the answers. I don't think the preamp is the biggest problem, but rather the phase inverter. The Junior Pro hits the phase inverter hard, hence its characteristic distortion. All the necessary changes to provide the correct amperage and voltage are taken into account. My problem lies in obtaining sufficient output from the inverter to properly excite the EL34s. If I limit myself to only increasing the preamp gain, I won't be able to extract the necessary power from the EL34s.

[nuke]

Hello, thanks for the answers. I don't think the preamp is the biggest problem, but rather the phase inverter. The Junior Pro hits the phase inverter hard, hence its characteristic distortion. All the necessary changes to provide the correct amperage and voltage are taken into account. My problem lies in obtaining sufficient output from the inverter to properly excite the EL34s. If I limit myself to only increasing the preamp gain, I won't be able to extract the necessary power from the EL34s.

Ok, well work backwards from the power section then.

How much power do you want at the speaker terminals, what is the plate-to-plate impedance of the OPT, what voltages are you running plates and screens on? (presuming fixed bias, and conventional pentode, AB1 operation)

Do the math for the operating points, that will reveal how much drive you need from the PI stage.

Then do the calculation to setup the PI for g1-g1 swing for a given input voltage to the long-tail-pair.

Then we work back to the preamp, if it can provide sufficient signal to get the PI working.

[martin manning]

Hello, thanks for the answers. I don't think the preamp is the biggest problem, but rather the phase inverter. The Junior Pro hits the phase inverter hard, hence its characteristic distortion. All the necessary changes to provide the correct amperage and voltage are taken into account. My problem lies in obtaining sufficient output from the inverter to properly excite the EL34s. If I limit myself to only increasing the preamp gain, I won't be able to extract the necessary power from the EL34s.

The Pro Jr's PI is basically the same as any other LTP.

[nuke]

The Pro Jr's PI is basically the same as any other LTP.

Indeed it is, and the tail and cathode resistors can be dialed in a little, but it doesn't affect the gain very much.

The Pro Jr values provide about voltage gain of about 26x (28.38db).

The Marshall 2204/JCM800 PI adjusts the values of the tail resistors a little bit, and it works out to about 25.6x voltage gain (25.6db)

Which says the Pro Jr. has more gain in the PI.

The big difference is the voltage supplied to the PI section. The Pro Jr. is run on 270v, while the Marshall runs around 330v supply.

Given sufficient power supply voltage, the PI as is, should work with EL34.

[Tauruspain]

Hello, thanks for the answers. I don't think the preamp is the biggest problem, but rather the phase inverter. The Junior Pro hits the phase inverter hard, hence its characteristic distortion. All the necessary changes to provide the correct amperage and voltage are taken into account. My problem lies in obtaining sufficient output from the inverter to properly excite the EL34s. If I limit myself to only increasing the preamp gain, I won't be able to extract the necessary power from the EL34s.

Ok, well work backwards from the power section then.

How much power do you want at the speaker terminals, what is the plate-to-plate impedance of the OPT, what voltages are you running plates and screens on? (presuming fixed bias, and conventional pentode, AB1 operation)

Do the math for the operating points, that will reveal how much drive you need from the PI stage.

Then do the calculation to setup the PI for g1-g1 swing for a given input voltage to the long-tail-pair.

Then we work back to the preamp, if it can provide sufficient signal to get the PI working.

About 430V plate voltage for the el34
The OPT is the hammond 1760j with a reflected plate to plate impedance of 4k ohms.

[nuke]

Do you have a proposed schematic?

I think you'll have plenty of gain in the PI.