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Epiphone Valve Junior to Bass! A thread of tiny modding and major sound

Discussion in 'Amps and Cabs [BG]' started by BassJunkie730, Oct 2, 2008.

  1. BassJunkie730


    Feb 3, 2005
    So I'm going to open this thread as a beginning to my modding of my Epiphone Valve Junior into a bass amp.

    I bought one two years ago (when they were $99 YEAH!) and I have since recorded countless times on it.

    But now I want a bass amp out of it.

    In fact - it's my plan to actually gig with this thing through a homemade 115 with a speaker DI and a mic.

    I combed the net and found a news group post by Gary Gerhart of Gerhart amps.

    So I sent him an email. One that i would also like any of your thoughs on.

    I think when the dust settles I'll have a kicking little bass amp, with a fender tone stack and a super wide output section. And if the plans for gigging fail- we'll hey at least I have the knowledge and a great piece.

    I'll keep the information coming in.

    Please contribute to the discussion, thoughts, observations with the same goal.

    Feel free to be as top line or technical as you want in this thread as I'll be openly brainstorming and posting pretty technical info (well no more technical than voltages and eq curves - that reminds me I need a mac version of SPICE if there is such a program.)

    Bass V likes this.
  2. scotch

    scotch It's not rocket science! Supporting Member

    Nov 12, 2006
    Albany, NY USA
    Please see Profile for Endorsement disclosures
    subscribed.... I tried out the Valve Jr w/ bass at the local GC. It was intriguing - definitely needed some modifications to make it useful!
  3. just be careful, modding a tube amp can be deadly. Am very interested in how this turns out.
  4. thejohnkim


    Sep 30, 2003
    You must have the first board revision of the Valve Jr like me. have you done the standard mods that have been recommended for it by teh g**tar brethren?

    1. Improve grounding on the PCB by adding a jumper wire between two of the traces. This corrects a grounding design deficiency.
    2. Replace the stock input jack with a nylon insulated jack, to isolate it from the chassis.
    3. Add a larger filtering capacitor (100uf/450V) to the main high voltage supply stage, to supplement the smaller stock caps.
    4. Convert the voltage supply to the tube heater filaments from AC to DC, requiring the addition of a small bridge rectifier and 1000uf/25V capacitor.
    5. Boost the amount of input signal going to the preamp stage and reduce the signal bleed to ground by changing two resistors on the PCB. Specifically, R1 is increased to 1M and R2 is lowered to 22K. These resistor changes also cause a net increase in the input impedance of the amp, which has the effect of boosting the highs a bit.
    6. Add a switchable tone capacitor to an open position on the PCB (C3), i.e. install a "brightness switch".

    i've done everything except for #6 since i was too lazy to find a metal bit to drill the hole in the front panel for the switch, but doing the rest of teh mods were a definite improvement for the elimination of most of the hum, and overall gain and tone. there is still some noise that you would expect from any class A tube amp though of course.

    Further mods include the hols for the output impedance leads, and switching out the Output transformer to a Hammond unit which apparently increases the tone, clarity and power output, but i dont have enough interest or time to do that mod since it also involved drilling out one hole through the chassis.
  5. Stunned by the TBers again....I should've joined years ago! I had the same thought a couple of months ago. While I like the idea of the head version modded for bass, a super-tiny combo with speaker outs would be really really nifty.

    I've eagerly subscribed.

  6. Thangfish

    Thangfish ...overly qualified for janitorical deployment...

    I'd like to do some tweaking also, but I'd be wanting:
    1. A little more clean headroom if possible
    2. Maybe a high CUT
    I can't imagine boosting the highs on this thing, much less adding
    a brightness switch.

    Mostly #1. #2 is not really necessary since there are other ways to accomplish that, I think.

    The one I've been messing with is very quiet, stock.

    Also would need to build a small, low power, super-efficient cab to pair it up with.
  7. BassJunkie730


    Feb 3, 2005
    I'm not sure of what revision or board I have in the amp, but I am confident that this is such a simple circuit and such a simple mod (not simple in skill - but simple in scope) that I'll end up with something really special.

    It will basically be a fender showman with a stiff output section of maybe 15 watts (Kt88)

    I am not beyond relaying out then entire inside of the amp. Or replacing every component. i know for a fact the power and output trannies are going to need to be switched - but as for quality level - I just need something that will get the job done.

    What's great is I already have the tubes I want to use (or at least know exactly which brand and type.)

    Now i just need the circuit to reflect their implementation.

    Still waiting for Gary's reply.

    I'm trying to get on www.sewatt.com wo access all their mod pages.

    Basically I'm looking to do an octal output tube mod with a power section mod to support a KT88. And output transformer mod to "overengineer" the power coming out (I'm thinking doberman amps Heyboer 20w SE output tranny)

    then I need to rework the tonestack for a classic fender stack using the Showman pre as my guide.

    All in all I think this will be attainable.
  8. Mojo-Man


    Feb 11, 2003

    I was also interested in a Valve Jr. for bass.
    But, after some research.
    I came to conclusion, that 5-10 watts, after mods.
    Would not produce enough "Headroom" for bass. (anything live)
    It, would work for recording.

    I came across "BearPaw Amplifiers"
    James Barrett make class A style amps by hand. (1-to-40 watts)
    This guy knows his s**t.
    And now is making me a 25 watt Bass Head.
    With the ability to cut wattage in half.(12.5 watts, for recording)
    I will run it at 25 watts live for small gigs. ( with a 2x15 cab)

    Check Jim out.
    I think many TBers. would like a low wattage tube head.
  9. CrackBass


    Aug 10, 2004
    just to put in my .02, what you're talking about doing isn't really modding the amp, it's more like replacing the amp. unless you're really attatched to the enclosure i might suggest buying a chasis for way less than the cost of the amp you are about to destroy. the total cost would be very close but you would have two amps instead of one.

    oh, and i can vouch for the heyboer's they are top quallity.
  10. BassJunkie730


    Feb 3, 2005
    Thanks for the info.

    I started the mod today by opening up the basic fender tonestack (the one in the Alembic pre's and such)

    I'm always the tweaker - and I thought "how can I improve the stack?" So I changed all kinds of values to see what values change what in terms of the curve, the midrange center point - the treble and bass center points. After all the changes - i simply changed the value of the midrange pot (from 10K to 50K) and the midrange cap (from .047u to .005u) and got pretty dramatic results without changing the overall character of the circuit. I basically cinched up the belt on it. now it looks like it sits somewhere between a classic fender and classic mashall stack.

    Here's the stock - stack


    here's my modded Fender Stack


    The classic marshall stack


    And for reference - here's a B15 tonestack


    and a Big muff tone stack


    The only thing I worry about is introducing too much gain into the circuit.

    but then again - with a big overengineered single ended output section - perhaps that's a good thing.

    Thoughts anyone?
  11. BassJunkie730


    Feb 3, 2005
    With all respect CB, I don't think I'll be able to find a custom chassis and cabinet for the same price of my depreciated valve junior.

    If anything - I'll just hold on to the old transformers and parts for another build.

    i don't care about the value of the Junior intrinsically. The company is making thousands of them. It's not like I'm about to hack into an old bassman head or something.

    Also there has already been so much discussed about the amp - i think i can pull this off with the existing layout and board with modifications grafted or added in.
  12. your not gonna be able to fit big enough transformers in a epiphone valve junior to run kt88s maybe kt77s but still i doubt the transformer would fit on the chassis. i think you'd be be better off starting from scratch if you intend to build something you could use live
  13. Greyvagabond

    Greyvagabond Supporting Member

    Aug 17, 2007
    Los Angeles
    Man, I'd love a sweet little 10 watt tone-machine...eager to hear the results!
  14. BassJunkie730


    Feb 3, 2005
    Well the special design heyboer output tranny works in terms of fit - and I have an email into Doberman Amps as to the feasability of their tranny in this application.

    here's a pic

    But I am worried about a power tranny replacement and where the hell am i going to fit the KT88?
  15. Tomass


    Nov 1, 2005
    Sounds like a sweet project, might have to embark on this for my summer break. It would reinforce the knowledge that im getting from my first year of electronic engineering. Let us know how it goes for you.
  16. BassJunkie730


    Feb 3, 2005

    I think i'm going to go for a 6L6-GC instead of a KT88 - because the transformers are just going to get REALLY big and fitting a 6L6 will work better.

    I thought about keeping the EL84 - but the transconductance of the tube would prohibit it from getting sqeaky clean easily. that's why marshall 18watters have SOOO much gain.

    So 6L6 it is- for now.
  17. Crockettnj


    Sep 2, 2005
    North NJ
  18. Interesting.
    I was thinking about picking up one of these for guitar. How do they sound?

    I'm in the negative Nancy camp on this.. I think if you want to use it live you might be better trying a Bassman or B15 build. You're pretty much building a new amp in an existing cabinet- and one that will limit output power bc of its limiting size.

    I try and play my B15 miked up live but its often a bit dissappointing. If stage volume gets too loud you end up relying totally on stage monitors and will have you wishing you had a bigger amp so you could control your level. .... and bands I play in do not play loud live.
  19. Saetia


    Mar 27, 2003
    This is sweet, subscribed!

    I really want to pick up one of these for guitar, I've heard these live in a 3 guitar group with loud drummer, bassist played an ampeg fridge as well as keyboard into mac book and it kept up through an orange 112. There are quite a few sites out there which have mods on them, I'm really intersted.

  20. Crabby


    Dec 22, 2004
    I had my uncle do some mods to my VJ and it turned out great. he added a standby switch, a tone control that can be switched out of the circuit and an impedance switch. The amp is a monster to play guitar through and it holds its own quite nicely for bass depending on the cab.

    My favorite tone was running it with my Ampeg 810 believe it or not.

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