Epiphone Valve Junior to HiFi Conversion

[riverty]

Admittedly, I'm using the term HiFi loosely here.

I purchased an Epiphone Valve Junior guitar amplifier with the intent to convert it to a cheap, general use, EL84 SE monoblock audio amplifier. I found a schematic called the RH 84 SE (thanks RH Design) and used it to modify the Valve Jr. The result works and sounds much better than it ever has for HiFi use. It uses line-level inputs nicely and seems to have plenty of gain. All good so far.

This is not my first amp project BUT I'm still very green with tube technology and I lack experience with these types of amplifiers. If you are interested, I invite you to stop by my blog and read up on the project. I have pictures and schematics that I simply could not list here.

Here is the final schematic with my edits. If you see anything that is wrong or could be better (besides the cheap Hammond output transformer - I know) please let me know. I'm counting on more experienced amp techs to help guide me to getting this amp to sound nice(er) and not overdrive the tubes for long service life. The output transformer will be replaced at some point in the future.



Thank you very much for reading! It is appreciated.

[VictoriaGuy]

Interesting project!

I've built a few RH84s- it's a good simple design. Also fixed a few VJs which I bought broken.

Your schematic is a bit different than the RH84, as you know.
Some of the differences may be typos (?).

Questions:
Are you using the 12AX7 preamp tube?
Is R3 really 1 meg ohm?
C1 is .022 uF?
You don't seem to have the Rfb 100k, which I understand is a pretty important part of the RH design

Did your VJr come with a Hammond OT? I thought most of the VJs came with 'no name' Chinese iron.

I wonder how the PT would do if you tried to get another 40 mA out of it for a second EL84? (stereo)

[costis_n]

Has no resemblance to RH84, what did you copy off that design, really?

[riverty]

Originally Posted by VictoriaGuy

Interesting project!

I've built a few RH84s- it's a good simple design. Also fixed a few VJs which I bought broken.

Your schematic is a bit different than the RH84, as you know.
Some of the differences may be typos (?).

Aside from R3 not being 1M, no typos that I'm aware of. My changes are in red of course.

Originally Posted by VictoriaGuy

Questions:
Are you using the 12AX7 preamp tube?
Is R3 really 1 meg ohm?
C1 is .022 uF?

I'm using a Sovtek 12AX7 right now, though I've tried a few other 12AX7's in the amp just for audio quality. The Sovtek sounds the best from my 12AX7 stock.

R3 is NOT 1M! I didn't catch that on the original schematic and thanks for pointing that out. R3 is 100K. C1 is correct at .022uF.

Originally Posted by VictoriaGuy

You don't seem to have the Rfb 100k, which I understand is a pretty important part of the RH design.

I do NOT have the Rfb 100K installed but will add it later today and see.

Originally Posted by VictoriaGuy

Did your VJr come with a Hammond OT? I thought most of the VJs came with 'no name' Chinese iron.

You are correct. Mine did NOT come with the Hammond OT, it came with the "no name" OT. I had this Hammond that matched very well and just knew it would sound better.

Originally Posted by VictoriaGuy

I wonder how the PT would do if you tried to get another 40 mA out of it for a second EL84? (stereo)

Don't know how the PT would handle it but for my application, mono is preferred!

[riverty]

Originally Posted by costis_n

Has no resemblance to RH84, what did you copy off that design, really?

I'm sorry you don't see the similarities. Guess you can't help then. Thanks for your comment though!

[VictoriaGuy]

Originally Posted by costis_n

Has no resemblance to RH84, what did you copy off that design, really?

I agree.

Well there were a few things copied from RH84 (Rk for EL84), but most of the important features were not imitated.

Wrong preamp tube & preamp wiring.
No feedback
Wrong coupling cap
etc....

I think the OP is using the VJ PCB.
It really would be simpler to just build something on a new chassis, and the PT and PSU in the VJ is not worth tearing up a guitar amp, IMO.

A couple of tube sockets, a handful of caps/resistors and a new PT and you could start a proper RH84 build. But everyone has their own ideas for a project....

[riverty]

You are both right, I failed to add some background information that explains how I got to here. I also have an updated and accurate schematic that will explain more.

Check out the post: EL84 SE advice'n'schematics needed

And the updated schematic:



Thank you again!

[preludefan]

please excuse me, i have a small question- if i was to make my own circuit similar to that of the VJ, would it be possible to use i without an output transformer with say a 600 om resistor in series with an 8 ohm speaker grounded at the other end? im building a small guitar amp (about 3 to 8 watts) out of bis i have, using an el84- thanks in advance

[costis_n]

Originally Posted by preludefan

please excuse me, i have a small question- if i was to make my own circuit similar to that of the VJ, would it be possible to use i without an output transformer with say a 600 om resistor in series with an 8 ohm speaker grounded at the other end? im building a small guitar amp (about 3 to 8 watts) out of bis i have, using an el84- thanks in advance

You mean without an output transformer? In short, no. EL84 is high impedance, and wants to see around 5000 Ohms connected to it. So you could in theory have a 4K8 or 5K6 10 Watt resistor connected between HT and the anode, and take the output through a capacitor connected between anode and loudspeaker, but a lot of power will be dissipated on the resistor, and your output will be low.