Epiphone Valve Junior Mods

Discussion in 'The Squawk Box' started by Batman, Mar 3, 2012.

  1. TWANG

    TWANG Senior Member

    Messages:
    2,783
    Likes Received:
    749
    Joined:
    Feb 6, 2009
    Pin 7 12AX7... cut the trace to the little white box that goes to
    the volume control,
    put in a 100K. or higher.. 1W resistor.
    cleans up .. and by that I mean less fizzi like.. less ugly.. distortion tones esp.
    when cranked.

    consider a cathode bypass switch... 1/2 power so less volume when driven hard.

    the thing is they are made to put out 5Watts Class A.. and that's pretty loud watts when you push it.
    It's not like a solid state amp where you can get whatever the freak tones they make at any volume.

    so 'low volume' winds up meaning different things to different people
    My champ puts out 4W, cranked it shakes the living room.
    But it's my damn house, so there.


    VVR can help a lot, but even then some complain they aren't getting the same exact tones as loud. ears is diabolical!
     
  2. Batman

    Batman Senior Member

    Messages:
    6,709
    Likes Received:
    6,986
    Joined:
    Nov 10, 2011
    Here is the schematic for clarity.

    [​IMG]

    The master volume is a simple volume control between the pre-amp and the power amp.

    What it's doing is shunting a variable amount of the pre-amp signal to ground. With the volume at zero, all of the signal is sent to ground. This gives you zero output. With the volume all the way up, you are passing all of the pre-amp signal to the power amp.

    A single ended amp does not have a phase inverter.

    With some of the other mods, you can increase the gain in the pre-amp. This may give you the saturation you are looking for at lower volume. I went the other way and removed some of the available gain so I could have a cleaner pedal platform. I found I could achieve better tone at lower volume by reducing the amps gain and using an overdrive pedal.
     
    1 person likes this.
  3. bilbarstow

    bilbarstow Premium Member

    Messages:
    2,412
    Likes Received:
    3,003
    Joined:
    Apr 1, 2011
    Edwardsforest,

    Batman is exactly right, you will need to do some of the other mods to get the gain where you want it for the Master volume to give you a good sound at low volume levels.

    And I really recommend that you use the schematic that he presented : the use of the 500 K pot + 470 K resistor not only gives you a better MV control, it provides a "Safety" for your tube - if your 250 K pot goes bad, you can get over-voltage on your EL84 tube, say goodbye ! (also possible OT damage !!).

    I also suggest the addition of the NFB / Bright Cap lift switch. This gives a good way to select fixed tones that work well with Single-coils or Humbuckers.

    ALWAYS listen to what TWANG says ... he is the Yoda of the VJr.s. I didn't like the cathode bypass switch ("triode") in mine, but I did listen to him and put it in and tried it to determine how I felt about it first.
     
  4. edwardsforest

    edwardsforest Junior Member

    Messages:
    7
    Likes Received:
    0
    Joined:
    Apr 4, 2013
    Thanks for the replies guys! With the 500k pot, does split or solid matter? Also, will a 470k 1/2 watt resistor work? Or rather, does the wattage of the resistor matter?

    Also, where do you guys purchase your parts (wire, pots, components) online?
     
  5. Batman

    Batman Senior Member

    Messages:
    6,709
    Likes Received:
    6,986
    Joined:
    Nov 10, 2011
    Split or solid shaft pots don't make much difference. . .mine were all solid shaft. I prefer solid shafts as the knobs on amps are usually held in place with set screws and you run the risk of bending a split shaft if you over tighten the set screw.

    Wattage matters greatly in a tube amp. . .run too low a wattage resistor and you run the risk of overheating and cooking the resistor (lets out the magic smoke and you run the risk of expensive collateral damage)

    A 470K 1-watt resistor was recommended when I did this mod.

    I buy all of my supplies from an electronics supplier in my local business park. Some of the items are more expensive than I can find on-line, but I don't have to worry about shipping so it works out to about the same.
     
  6. bilbarstow

    bilbarstow Premium Member

    Messages:
    2,412
    Likes Received:
    3,003
    Joined:
    Apr 1, 2011
    Shaft type doesn't really have any impact other than what types of knobs will fit. Quality and tolerance will have a bigger impact on the sound. Alpha and CTS are some of the better brands. Stay away from the minis, unless you really have to, they are generally not up to the quality level. Brand choice is somewhat driven by which suppliers you choose to use.

    I have used a lot of the online sources. Particularly Antique Electronic Supply, Watts Tube Audio, Allied, Amplified parts, and I've heard a lot about Mouser, but have never ordered from them.

    But I have also been known to make-do with parts I've obtained with last minute runs to Fry's Electronics or Radio Shack. Sometimes you use what is available and upgrade the component quality later.
     
  7. Batman

    Batman Senior Member

    Messages:
    6,709
    Likes Received:
    6,986
    Joined:
    Nov 10, 2011
    Heard good things about Small Bear as well but have not used them. . .
     
  8. TWANG

    TWANG Senior Member

    Messages:
    2,783
    Likes Received:
    749
    Joined:
    Feb 6, 2009
    Tayda has some good stuff.. really cheap and FAST from china with low low shipping.
    but they have some stuff not suitable, too. Still, gotta mention!

    Mouser for me, most times. But Small Bear, Pedal Parts Plus, Dawg chassis, Ken Watts at turretboards, tubesandmore, thetubestore,
    all dot coms
    probably more... Ukraine ebay guys.. stuff like that.

    all above are good comments!

    I spent the day picking up a new SS amp to gut... and making some cabinetry for some SS amps I already gutted.. gonna donate 'em to the Guitars For Vets things when I get those done.

    only one thing bothers me.. people being nice to me?????

    [ame=http://www.youtube.com/watch?v=5K-iAt_uXyE]Powder Blues Band - What Have I Been Drinking - YouTube[/ame]

    *g*
     
  9. edwardsforest

    edwardsforest Junior Member

    Messages:
    7
    Likes Received:
    0
    Joined:
    Apr 4, 2013
    Thanks for the responses guys.So far i have added that master, wired a switch for the jp1 mod, and removed r7 and replaced r6 with a different value.


    Question, in a vid on youtube a guy gets rid of both r6 and r7. I tries doing this but get no sound aftet. Is there somethibg else i need to adjust when doing this?


    Also, the guy removes r2. I tried doing this as well and no sound. Any ideas?


    Thanks! This is so fun ans rewarding - when it works! Ha
     
  10. bilbarstow

    bilbarstow Premium Member

    Messages:
    2,412
    Likes Received:
    3,003
    Joined:
    Apr 1, 2011
    Edward,

    I believe (from memory) if you remove R6, you need to replace it with a jumper (piece of wire), or you have "broken" a necessary connection to the gain (volume) pot.

    You can remove R7 without issue, as it simply shunts to ground.

    You need R2 - it is input screen resistor to the first pre-amp stage.

    Best way to understand these is to reference the circuit diagram(s).

    If you don't read schematics, at least give them a look anyway, as you will ultimately begin to figure it out and it will make a HUGE difference in your understanding. You can do this "paint-by-number", but it really helps if you understand why #13 = bright blue.
     
  11. Batman

    Batman Senior Member

    Messages:
    6,709
    Likes Received:
    6,986
    Joined:
    Nov 10, 2011
    Review this post for R1 & R2. . .

    http://www.mylespaul.com/forums/squawk-box/186850-epiphone-valve-junior-mods.html#post3595759

    This is the schematic I used for most of my mods. . .if you want to remove both R6 and R7, you need to put a jumper in place of R6.

    R6 (1W 1 Meg Attenuating Resistor) - Replace R6 with a 1W 100K resistor. R6 and R7 create another voltage divider which reduces the signal to the second pre-amp stage by 50%. These resistors are not necessary strictly speaking but they help limit the amount of signal going into the second triode of the preamp tube. This gives the amp some clean headroom and emphasizes power tube overdrive over preamp overdrive. Reducing the value of these resistors will increase pre-amp gain and somewhat reduce the clean headrooom. As a note; the lower the values of R6 and R7, the brighter the tone.

    R7 (1W 1 Meg Attenuating Resistor) - Replace R7 with a 1W 100K resistor. This resistor dumps signal to ground; the higher the resistor the greater the gain loss. R7 is actually wired in parallel witht he volume pot which effectivly makes the 1 Meg volume pot a 500k pot.

    I used several values for both R6 and R7 and in the end opted to remove R7 all together and leave R6 100K
     
    1 person likes this.
  12. rjonp

    rjonp Junior Member

    Messages:
    1
    Likes Received:
    0
    Joined:
    Apr 1, 2013
    My first post here. I'm about to start work on my V3 -- actually the board from a combo that I have installed in a custom cabinet. I've been reading everything I can find on mods. This forum is the best I've found -- clean and straight forward. I do have a question about a mod suggested by Dirk Wacker in the April 2007 issue of Premier Guitar. He lists this as the first mod to make:

    Connect a 20-ohm/20-watt power resistor between the IEC mains input socket and one of the primaries of the power transformer. This will bring the plate voltage down to approximately 300VDC and will extend tube life dramatically. This will result in a better ("browner") overdriven tone.

    Read more: Rags to Riches, Cont. - Premier Guitar

    So, is this a different way (a big hammer) to get the voltages under control than the R/C circuit tweaks discussed in this forum? Is there a disadvantage to using this "big hammer?" Any advantages?

    Also, I haven't seen anyone mention Dave Hunter's book The Guitar Amp Handbook. I found this a great place to learn about amps and amp circuits. He walks the reader through the basics of tube amps and identifies components and the role they play in sound. I'd recommend it to anyone who wants to dig a bit deeper and understand what is going on inside these little beasts.

    Love the posts here. I'm inspired.
     
  13. Batman

    Batman Senior Member

    Messages:
    6,709
    Likes Received:
    6,986
    Joined:
    Nov 10, 2011
    Hey Dude,

    Welcome to MLP! :thumb:

    I have the Dave Hunter book and it is just awesome. . .written in a very easy to understand style that was perfect for this noob!

    My two resources for these mods were TWANG and the folks over at SEWATT.com

    It was suggested by them to fix the voltages with R10 (Page 1 post #19 of this thread) so you are using the PCB and its existing components to tweak the circuit. When I approached these mods, I wanted to learn about the circuit and what was going on so I chose to tweak the existing components rather than adding to the circuit.

    The notable exceptions to this were moving R1 to before R2 (post #15) and adding a SGR (post #10)
     
  14. bilbarstow

    bilbarstow Premium Member

    Messages:
    2,412
    Likes Received:
    3,003
    Joined:
    Apr 1, 2011
    Rjonp,

    Welcome to the group. While the Hunter book may not have been specifically recommended in this thread, I and many others have recommended it many times over in various "how do I get started in tube amp" threads on this board. Yes, it has inspired many to set sail on these oceans of depravity :laugh2:.

    As you guessed, Wacker's resistor is indeed a "big hammer" solution, while TWANG/Batman/SEWATT et. al.'s correction at R10 is the preferred method, and actually test voltages and modify R value until voltages are optimized. The result may be the same, but making the circuit work correctly is the more "elegant" solution. Plus, it educates you on how "Wrong" values may provide the sound you are looking for.

    Testing after every change is something that Batman has advocated all along through this thread. It is the biggest learning tool you can have, to let your ears tell you (subjectively) what the results of a specific change made and whether it is what you are looking for. Theory is great, but some of the things that "Shouldn't work" actually provide that perfect result.

    Glad to hear that you are inspired, as many of us have been by those who went before. The guys will be along shortly to measure you for your straight-jacket. ;)
     
    1 person likes this.
  15. steinros

    steinros Junior Member

    Messages:
    2
    Likes Received:
    0
    Joined:
    Aug 11, 2012
    I just did some mods in my VJ:
    C6 22- to 47-450v
    C5 22- to 470-50v
    C3 and 4 to 4,7-35v
    R10, R4, R8 and R9 changed according to Batman's instructions.
    The amp to works normally some minutes. Suddenly the sound becomes fizzy and very weak before it dies totally. If I remove the jack from the guitar, the amp seems to work again, but it dies as soon as it gets a signal from the guitar. If I let it rest for some time, I can play it some minutes before the same thing happens again. The speaker hisses as normal, so I think that the problem lies in tha pre-amp.

    Suggestions? :(
     
  16. steinros

    steinros Junior Member

    Messages:
    2
    Likes Received:
    0
    Joined:
    Aug 11, 2012
    Three cold solder joints!:applause:
     
  17. bilbarstow

    bilbarstow Premium Member

    Messages:
    2,412
    Likes Received:
    3,003
    Joined:
    Apr 1, 2011
    Steinros,

    Glad that you caught those. Sorry I was away for a few days, as that would have been my suggestion on what to look for, as when you have a problem like yours, where something is working until the amp warms up under use, that is a very common source for the problem. Or sometimes heat causes the circuit board to flex / deform which can make a connection that works intermittently. With heat, comes stress on the weakest parts of the system.

    Congrats on solving your own problems. We are usually more help than that around here !
     
  18. Batman

    Batman Senior Member

    Messages:
    6,709
    Likes Received:
    6,986
    Joined:
    Nov 10, 2011
    Yeah man. . .sorry for the lag in answering. My work schedule blows chunks right now!

    I'm really glad you worked out the issue!
     
  19. Shady_Grove

    Shady_Grove Senior Member

    Messages:
    143
    Likes Received:
    113
    Joined:
    Dec 2, 2013
    I hate to beat on an old horse but I want to thank you guys (Batman & TWANG) for this thread, I must have been through it a dozen times so far. I bought an old EVJ from a pawn shop for 75 bucks. I figured why not. When I got it home to play on I found out why it was sitting in the the shop, it sounded terrible. After a little research apparently you can turn this thing into quite a beast. Several different kinds at that. Right now I was just trying to make it less of a mud box and more of something I can practice with.

    I think I have read over everything a few times now and what I have concluded for making something along the lines of a clean stomp box machine. I enjoy pedals and I believe I read on here that a clean application can be had without some of the gain mods.

    With the clean application in mind I would be changing:
    (After starting with post #8)
    Post #10 - Design Correction
    Post #11 - OT Replacement
    Post #14 - Filtering and Bass Response
    Post # 19 - Voltage

    Edit* After looking inside and tinkering whilst leaving everything together I wonder if a pot upgrade for the volume would be beneficial as well?

    Are/Is there anything I have left out to achieve what I am after?

    I appreciate your help!
    Jeff
     
  20. bilbarstow

    bilbarstow Premium Member

    Messages:
    2,412
    Likes Received:
    3,003
    Joined:
    Apr 1, 2011
    Jeff (Shady Grove),

    I would suggest that a Pot upgrade is a good idea, as the cheap mini pots that Epiphone used are frequently a source of crackle when turned, etc. Be sure to use shielded wire to avoid possible noise induction.

    I would also recommend that you consider the master volume addition (post #32), since you are wanting it to be a pedal machine. This would allow you to set lower pre-amp gain (current volume knob) while controlling output volume (power tube output). This also makes it friendlier for low volume practice, while still allowing you to get "cranked" tones if you want.

    Be safe ! Have fun, it is a great way to get into the amp mod world.
     

Share This Page