Epiphone Valve Jr Head tweaks

Hello, everyone,

I have a Epiphone Valve Jr amp head that I have had sitting around for years. I recently tried to fire it up to use on a current project and to no surprise it did not pass audio, probably due to whatever is ratting around inside. So I already need to open it up and see what is the matter. While I have it open I was curious if there were any mods or components that could be swapped to alter (read "improve") the stock sound, which if I remember when it did work was a bit lack luster, but my memory regarding this amp when working is foggy at best.

I'm in no rush to getting it working as I just another amp and to keep moving on, but I am curious if anyone else here has worked on one of these and if they did any component changes that yielded pleasing results.

Thanks!

Paul

[Referencing this schematic: https://elektrotanya.com/gibson_epiphone_valve_jr.pdf/download.html ]

Since there's not really much going on in there to begin with, maybe reducing the value of those two 22u cathode bypass caps could be a starting point? To roll off some of the low end (and thus relatively "brighten up" the sound)...

PS: If this is to be used on guitar, you'll probably want to up the value of R1 from the stock 68k to 1meg or so (unless you like your guitar with the tone very much rolled down all the way ;D ).

Reducing the value of R6 (and/or removing R7) will drive more signal into the second half of the 12AX7.
Maybe add a 1nF "treble bleed" cap across the gain pot (top to wiper)?

That is, assuming you're unwilling to "go all in" and add another 12AX7 in there, and/or a tone control... :

But then again, you should perhaps define (at least to yourself) what exactly "lackluster" might mean

Thank you for posting the schematic.

Since it had been a long time since it was working, I couldn't give you really accurate description of what I was meaning by "lack luster" except for the fact that did give that feeling of excitement you get when using a fine audio circuit. Probably it was related to the transient response and/or the way the tube was driven/loaded to make it sound "tubey." This will indeed be used for guitar.

I had seen some mod kits for this amp that added things like tone control, etc, but I don't mind doing this myself. And so far during my stay around these here parts I haven't yet been afraid (although sometimes not successful) to go all in.

I think your suggestions are a great place to start.

Thanks!

Paul

Sure, no probs, good luck in your endeavour ;D

I'm by no means an expert, but i've been messing around with JFET "recreations" of tube guitar amps, and i picked up a thing or two on the way

I am not sure what could be done with that amp as it is extremely minimalistic (other than correcting the possible mistakes already mentioned). If it could take another preamp tube the obvious option might be to add a 'proper' preamp?  Or, if Khron has experience with jfet preamp substitutions this might be a good place to try that?

Michael

I would definitely welcome experimenting with a JFET preamp substitute when the time comes. Hopefully will get a chance to look at this in a couple of weeks.

Thanks!

Paul

Worth keeping in mind that, "obviously", JFETs won't be a drop-in replacement ;D

On a more practical note though, ages ago i stumbled upon this site:
http://www.runoffgroove.com/fetzervalve.html

And their Projects page has quite a few... practical applications, let's call them

PS: Granted, it's a bit of a "one-sided" example, but here's a taste of what JFETs can do :
https://www.dropbox.com/s/nzs53d4ucc1jg1b/BoMBer_pad_2.mp4?dl=0
(Disclaimer: this was recorded on a phone, so... ;D )

Potato Cakes said:

I would definitely welcome experimenting with a JFET preamp substitute when the time comes. Hopefully will get a chance to look at this in a couple of weeks.

Thanks!

Paul

Click to expand...

Get it working before you mess it up.

It was a $99 unit--- how "fine" can you expect?

IIRC, the major "upgrades" were a better speaker (on the Combo) (this may cost as much as the amp did) and a better output transformer.

The circuit is hasty and begs for re-think, but the PCB will not stand much tampering.

Alternative is to chisel it all out and install turretboard--- a valid approach but not an overnight project.

Get it working and loan it to your nephew.

Actually, I have even less money than that. Someone had left it at TV taping and never could get anyone to tell me who it belonged to nor did anyone even ask about it. I took it home so it wouldn't just be left on site (the taping was in an actual prison) and no one ever claimed it. I'm not expecting it to from budget level to master crafted boutique with a couple of parts, but I would like to learn a little bit about working with tube amps after I've spent so much time building studio gear. Completely gutting it and installing a turret board with a different circuit is also an option. I'm looking at this as an educational pursuit not an economical. And if the end result is a great sounding amp then I'll consider that a bonus.

Thanks!

Paul

I modded up a Peavey Nanovalve which is quite simlar to a Valve Jr ,
I added simple passive tone control ,
I rectified and passively RC smoothed the Ac heater after the EL84 ,so my 12ax7 pre stage ended up with a little less hum  .
I used a two pole switch to simultaneously adjust triode/pentode connection with the extra pole adjusting the bias resistor in triode mode , it provides only around 1.5 watts in triode mode  but with a different distortion spectrum to Pentode ,

The 5 watt single ended make a nice  practice amp ,very sweet clean tones at low volume ,it  doesnt really crunch in the same way you expect from a push pull tube amp, it turns to mush  quickly when you open up the volume .Its fairly usefull for giving magnetic or piezo pickups and other sources a little more colour than you'd get with a typical active modern DI box , output can be tapped off across the speaker or a suitable load resistor/attenuator for silent mode .

The Peavey  wasnt the easiest due to layout, using the stock board and chassis space is very limited , Valve Jr looks  a little better in that respect .

repeating some of what was posted in this thread

I would first repair the stock amp.

Then try another speaker if you have an external cab or can borrow an external cab to test how the circuit sounds.
If it does not improve a lot I would use it as it is.

If it sounds better search for the single ended amp schematics
Fender Champ, Silvertones, etc.

Note the cathode and plate resistor values,  cap coupling values and cathode bypass values in the different schematics

Looking at the schematic it uses a el84 so you might want to try the 1st and 2nd gain stages without bypass caps

Orange Tiny Terror, sounds fantastic!  but it is a different amp, nevertheless, there might be some stuff you could try on your amp,

those Champ amps are tube sensitive, at least according to Gerald Weber who says he had to try about 10 different 6V6 tubes before he found one with the sauce.

we had a champ in the shop with one of those Celestion alnico 300 dollar speakers in it,  wow, what a sound, but you get used to any sound after a while, then you want something else, that's why i take the SG and the Tele to the bar gigs.  having to play metal and country to keep the beer bottles from flying means pack two guitars.  last saturday the singer says "play Jackson", i go wtf, the Cash/Carter ditty are you kiddin me? whatever, i start chunkin out the C cord with alternating bass roots as fast as i can, the money was flying into that tip jar so fast i thought the IRS was gonna show up, bunch of drunken necks  drivin sand rails to the club, i mean out back.  "oh, it's in Elma,WA. yeah right. back in there anything within a 40 mile radius is considered Elma,  i had to use  the barb wire fence to split the wires on a wall wart so i could flip it to Tip - for the H2O Chorus/Delay that i borrowed from the shop,  nobody brought a knife, or a screwdriver, for now on i'm packin a swiss army and a roll  of gorilla tape,

you can add reverb to that amp, we had a Swart in the shop and wow, what a heck of a reverb circuit,  what you do is run the input jack into two triodes, one is the main amp, the other is the reverb sidechain,  and go from there,

you want total sickness overdrive then this is your boy right here>

(need a phd in diy to build this but there could be some mods you like)

I was actually thinking about circuit designs outside the standard fare amp brands like the Orange or Dr. Z for component considerations. I had found a single ended tube amp DIY circuit some guy posted a while back that looks pretty interesting as it uses a single 12ax7 for volume and tone, but I may borrow some component values from the Orange schematic you posted.

I have a fair amount of really fun pedals for effects/overdrive, so adding effects to the amp itself is not something that I am considering at this point. But the spring reverb would be pretty nice.

Thanks!

Paul

Ax84 has some really good sounding single ended dot guitar amp designs. Could build one of those on turret board inside the valve or chassis pretty easily.

The  lost cost limiting part might be the output transformer and speaker

That is why I posted about getting it to work then trying it with a different speaker(s). If it sounds good then keep the output transformer.

A good experiment is to use a inexpensive 10watt etc solid state practice combo and plug a good speaker(s) cab into it instead of the stock speaker. Try a 2X12 4x12 etc.

CJ post makes a  good point I have a an old silvertone SE guitar amp and I tried a number of 6V6s and that does make a difference.

some links
note how similar the 6V6 EL84 SE guitar circuits can be

http://www.prowessamplifiers.com/schematics/post/kendrick_ruffneck.pdf

https://tubeamplifierparts.com/schematics/fender_princeton_5d2_schematic.gif

http://schematicheaven.net/gibsonamps/skylarkga5t.pdf

http://schematicheaven.net/gibsonamps/ga5.pdf

http://schematicheaven.net/bargainbin.html