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Old 30th December 2009, 09:39 AM   #1
diyAudio Member
Join Date: Dec 2009
Default Help needed with my Epi Valve Jr

I know this is not a guitar amp forum, but all you knowledgable and experienced DIY'ers are bound to be able to answer some questions I have ...

I have a sweet little valve amp (5W provided by a 12AX7 and an EL84) from Epiphone. It's their Vavle Junior head.

I've been modding it to get better tones. All mods so far came from a book, and presented no problem at all. (including adding a Negative Feedback Switch) to allow switching between default (pin 8 of 12AX7 goes to ground via a resistor (R9) and via an electrolytic cap (C3)) and NFB mode : pin 8 goes to ground via 1.5K resistor (R9) and goes, via NFB switch, to a 47K resistor and then straight to the output jack's pin.

Worked fine.

A mod not described was adding a tone stack. I found a sweet little tonestack (body and presence) from I wanted to be able to have or the raw amp sound, or the sound through the tone stack. So I cut a trace running from the wiper of the volume pot meter (1M) to pin 7 on the 12AX7. I wired in a 2pdt switch, of which one end uses a jumper to implement a bypass. This is my raw amp sound position. (without the tonestack that I still need to build).

I was stupid enough to swap out the standard 1M clearly oxidized volume pot meter for a new one at the same time ... 2 changes at once = dumb dumb dumb

Switchin the amp on, I seem to hear more hum/noise on the signal. In bypass (the new 2pdt) there is guitar sound...but if I flip the NFB switch, it totally breaks up a squeels... this never happened before!

Also, if I measure the voltage at pin 7 of the EL84, I get a reading that depends on the (variable) resistance of the new pot meter I put in... I'm quite sure that this voltage was a constant 305V before .... I've measured it multiple times before, and regardless of pot meter, it was always 305V ... now it seems to fluctuate .... I've had readings of 160V, 200V, 305V (when I open the volume pot completely)

I used 26 gauge wires for the wiring of the switches and the new pot meter. I read somewhere of this guy who describes some similar his case, it seems to have been fixed by using shielded wire instead of normal wire....

Is there someone who can explain this to me? How can using normal wire instead of shielded wire have such an effect on the amp? Is 26 gauge (which I use for effect pedal running at a mere 9V) even usable for amps? I can dig that shielded wire can have an effect on the noise on a signal, but how about the voltage readings on EL84???

The schematics for the Epi Valve Jr can be found at page 3 here:

Mind you, mine is a bit different (new version) with improvements in caps and resistors (e.g. R9 is at 1.5K, and I swapped out C3 for a 1uF).

I really hope one of you more experienced DIYers can at least give me some clues/explanation ... I'm fairly new at this
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Old 30th December 2009, 11:04 PM   #2
Cassiel is offline Cassiel  Libya
Account disabled at member's request
Join Date: Sep 2004
Location: Madrid
The circuit is prone to oscillation. A change in the layout is giving you problems. You have paid the price for messing around...if you can't fix it leave it as it was.

but if I flip the NFB switch, it totally breaks up a squeels... this never happened before!
Reverse the transformer secondary leads.
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Old 31st December 2009, 04:02 AM   #3
diyAudio Member
Join Date: Dec 2009
I've taken out completely the original volume pot including the PCB mounted connector, resoldered a pot using shielded wire. Since I cut the signal trace, I soldered the signal lead directly on pin 7 of the 12AX7. Works like a charm again.

I think it was a combination of oscillation and a bad solder joint ... the two extra solder point I created don't have that much copper around them and were hard to solder...

Thanks for the input, I'm going to dig a little deeper into "oscillation" since I don't know too much about that.
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