Opening up my Epi VJr

honyock · Sun Jul 04, 2021 9:35 pm

I have had an issue with a microphonic tube since I had this Amp, I just sold my Monoprice 15w and I took out the upgraded tubes I had before doing so.

If I turned this VJ up above 12 o'clock it would squeal a little.

The tube sockets are hard to see with chassis in the cabinet so I pulled it out.

I found that the OT has been upgraded to a Hammond. Nothing fancy, just the 125CSE, but still an upgrade. I need to see if there have been any other upgrades, nothing looks obvious at first glance.

Anyway, now I can crank the volume all the way to max, no squealing.

The old Russian tube sounds great, and I put a Sovtek 12AX7 since I don't know if the JJ was the same age as the JJ EL84.

I wish I still had access to the old forum, Twang had given me a couple other mods that are simple improvements, but significantly improved the amp.

Guess I'll have to see if Google can help, maybe he gave similar advice over at MyLesPaul forums.

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nomadh · Mon Jul 05, 2021 3:46 am

So much list knowledge because of that mofo

toomanycats · Mon Jul 05, 2021 8:17 am

honyock wrote: ↑Sun Jul 04, 2021 9:35 pm I wish I still had access to the old forum, Twang had given me a couple other mods that are simple improvements, but significantly improved the amp.

What was lost when the old forum was purloined sickens me when I think about it. So much collective knowledge now inaccessible.

honyock · Mon Jul 05, 2021 11:35 am

I found old pictures of the VJ I modded, but I am going to have to do some reverse engineering to see why I did what I took pictures of, such as this missing capacitor at C3 and C4 and resistor R1 (which looks like I moved it to the jack).

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honyock · Mon Jul 05, 2021 12:00 pm

Well, there is a little hope.

This thread has most of the mods from what I can see.

https://www.mylespaul.com/threads/epiph ... ds.186850/

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andrewsrea · Mon Jul 05, 2021 5:18 pm

Lol - I speak Twang. What would you like to know?

Looks like a lot of what you did reduced gain. The resistor at the jack was simply moving the 1M load resistor from the board , in an attempt to lower RF noise, but unnecessary. C3 & C4 are the 22uF cathode by-pass resistors of the 12AX7 gain stage 1 & stage 2, respectively. Removing them makes the signal more linear, but greatly reduces gain.

I read the mods and schematics. Some are legit, depending on what you are trying to make the amp do. Some are old-wives tales.

Let me know what you are looking to make this amp do.

honyock · Mon Jul 05, 2021 5:42 pm

andrewsrea wrote: ↑Mon Jul 05, 2021 5:18 pm Lol - I speak Twang. What would you like to know?

Looks like a lot of what you did reduced gain. The resistor at the jack was simply moving the 1M load resistor from the board , in an attempt to lower RF noise, but unnecessary. C3 & C4 are the 22uF cathode by-pass resistors of the 12AX7 gain stage 1 & stage 2, respectively. Removing them makes the signal more linear, but greatly reduces gain.

I read the mods and schematics. Some are legit, depending on what you are trying to make the amp do. Some are old-wives tales.

Let me know what you are looking to make this amp do.

honestly not sure. it sounds good as is, so i may just button it basck up and call it a day. i already have too much to do and not enough time

andrewsrea · Mon Jul 05, 2021 8:22 pm

^ that's what counts!

honyock · Mon Jul 05, 2021 10:14 pm

andrewsrea wrote:^ that's what counts!

I think a decent cab and speaker did more for this head than all the mods and money I spent on an OT with the last one through the stock speaker/cab.

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andrewsrea · Tue Jul 06, 2021 1:54 pm

IMHO, your observation is spot-on.

Considering the cheapest ouput transformer vs. the nicest (highly conductive carefully placed coil wire, big with lots of iron), the latter may afford more volume efficiency (transconductance), more headroom and a greater frequency range (lower-lows) - it will be ultimately limited by the speaker in the combo cab.

This is the basic RCA tube manual amp circuit, very akin to the Fender Champ and countless others. If a particular amp of this design is lacking to my taste, the first thing I do is check and adjust the cathode bias (yes, against conventional wisdom). I've often found the output tube is running at or over 100% of max tube wattage and robbing the amp of its dynamics. I find the EL84's sound best at 95% and 6V6s between 88% & 92%. You need a lot of 7W to 10W resistors on hand to increase the resistance in small increments.

Next, I'll try running into a good speaker cab. Then, I'll try a higher headroom output tube and then see how a 5751 or 12AT7 (less preamp gain) sounds for the preamp tube. If the distortion sounds too loose, I may insert a 10K variable resistor (trimmer) between C4 and ground. Adding resistance will begin to decouple the bypass cap, adding negative feedback in relation to the input signal strength, for the 2nd gain stage.

My neighbor begged me to mod his Soldano Special 6. After I checked over the schematic and the cathode bias (it was perfect), I plugged into a Fender Bandmaster 2x12 Celestion G30H cab and he said "Oh."