Epiphone Valve Junior to HiFi Conversion - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 17th August 2011, 05:27 AM   #1
riverty is offline riverty  United States
diyAudio Member
 
Join Date: Jun 2007
Default Epiphone Valve Junior to HiFi Conversion

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.

Click the image to open in full size.

Thank you very much for reading! It is appreciated.
  Reply With Quote
Old 17th August 2011, 07:45 AM   #2
diyAudio Member
 
Join Date: Dec 2008
Location: Victoria, BC
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)
  Reply With Quote
Old 17th August 2011, 01:57 PM   #3
diyAudio Member
 
Join Date: Aug 2008
Has no resemblance to RH84, what did you copy off that design, really?
  Reply With Quote
Old 17th August 2011, 03:56 PM   #4
riverty is offline riverty  United States
diyAudio Member
 
Join Date: Jun 2007
Quote:
Originally Posted by VictoriaGuy View Post
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.

Quote:
Originally Posted by VictoriaGuy View Post
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.

Quote:
Originally Posted by VictoriaGuy View Post
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.

Quote:
Originally Posted by VictoriaGuy View Post
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.

Quote:
Originally Posted by VictoriaGuy View Post
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!
  Reply With Quote
Old 17th August 2011, 04:00 PM   #5
riverty is offline riverty  United States
diyAudio Member
 
Join Date: Jun 2007
Quote:
Originally Posted by costis_n View Post
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!
  Reply With Quote
Old 17th August 2011, 05:16 PM   #6
diyAudio Member
 
Join Date: Dec 2008
Location: Victoria, BC
Quote:
Originally Posted by costis_n View Post
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....
  Reply With Quote
Old 17th August 2011, 09:06 PM   #7
riverty is offline riverty  United States
diyAudio Member
 
Join Date: Jun 2007
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:

Click the image to open in full size.

Thank you again!
  Reply With Quote
Old 18th August 2011, 06:59 AM   #8
diyAudio Member
 
Join Date: Jul 2011
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
  Reply With Quote
Old 18th August 2011, 08:08 AM   #9
diyAudio Member
 
Join Date: Aug 2008
Quote:
Originally Posted by preludefan View Post
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.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Epiphone Galaxy 25 transformer problem Monty123 Tubes / Valves 0 26th January 2009 09:50 PM
Worlds smallest HiFi valve amp fergs1 Tubes / Valves 51 17th September 2008 11:32 AM
epiphone pacemaker 50 t uliami Tubes / Valves 1 14th July 2008 03:56 AM
Epiphone Galaxie 25 schematic TRguy Instruments and Amps 1 8th January 2008 04:41 PM


New To Site? Need Help?

All times are GMT. The time now is 12:03 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2