Epiphone Valve Junior Mods

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

  1. Shady_Grove

    Shady_Grove Senior Member

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    I thought about the MV as well. I just wasn't sure as it didn't seem to be rated well here through the pages. I am going to sit down tonight and write everything up. I appreciate your help bilbarstow. Hopefully in the next couple of weeks I can get a little review of the build and sound to update the pages with.
     
  2. bilbarstow

    bilbarstow Premium Member

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    Jeff,

    The EVjr is a very simple and straight forward circuit design. The advantage to that is that the various mods are easy to add and try out to see if you like it. The Master Volume is possibly the one that most people overlook because they are concerned with drilling extra holes in the faceplate to install the pot. Don't be - a hole is not that big of a deal, and if you don't like the MV, just take it back out.

    I tried the Pentode/triode switch, and also adding the "tweed" tone circuit. Neither of those mods did it for me, so I yanked them back out. But the MV is really straight forward, and because it is a single-ended EL-84 power tube, doesn't really change the tone very much (no interaction with a phase inverter). I like having that extra control over the ultimate volume, as well as being able to control the pre-amp gain.

    I also like the NFB mod - particularly if you are going for a clean platform.

    Try things out. It's the best way to really determine what works best for YOU.
     
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  3. Shady_Grove

    Shady_Grove Senior Member

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    Thanks again Bil,

    I am having trouble tracking down the NFB mod, or well what it does and how to go about it. Tonight I had it all apart. Switched it up cosmetically and I started with the Design Flaw fix and then the R1 to the input jack. It does sound a bit cleaner with the R1 to the input jack, however I am ready for more, just waiting on parts and funds. I was definitely worried at first, she took a bit longer than normal to warm up but she did spark on. I definitely like the cosmetic switch so the tube can be seen glowing.

    Again I appreciate everything here in this thread. And your help keeping an old ass one going.
     
  4. bilbarstow

    bilbarstow Premium Member

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    Jeff,

    The Negative Feedback mod is documented in the Fender Schematic from the Sewatt pages regarding the VJr mods.

    What makes this even better is if you put it on a 3-pole switch, wiring it so that the NFB is on 1 side, center is "normal" (no NFB), and the other side is Bright Cap (C3) lift. I got this hint from TWANG, and it provides a really nice set of "preset" tones. If you follow the Fender Schematic, there is no C3 (always lifted), if you follow the Marshall Schematic, C3 is between pin 8 of the 12AX7 and ground.

    So to do this, take your 3-pole switch and tie the center pole to the pin 8 side of R9 (before the C3 + connection). Choose your value for your NFB resistor (22 - 68 K ohm) and put 1 side of that resistor to one pole of the switch, and the other side to the + side of your speaker output. On your other switch pole run it to your ground (circumventing or "lifting" C3). Just remember that the switch throw is the opposite direction of the connections (up on the switch connects the "bottom" pole, etc.).

    You could think of this as a Marshall/Fender switch, but other components really effect those tones more. I actually think of it in use as more of a Humbucker/unmodified/single coil switch, because it sort of provides that effect without other tone circuits involved. Give it a try, I think you will like it.
     
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  5. Shady_Grove

    Shady_Grove Senior Member

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    Beautiful. I appreciate your help. I will try and maybe get some pictures this next time. Lucky for me there is an electronic store that carries everything but the OT and its just down the street. Seems like the local tax vs the shipping is also about the same.
     
  6. Shady_Grove

    Shady_Grove Senior Member

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    After a little bit of tinkering I finally decided which direction I would like to take everything. I have been documenting it so far. Still waiting of everything to get here. I decided to finally settle on an 18w Lite II conversion.

    Took me about the last 3 weeks but I have finally ordered everything I needed. Tracking down everything seemed pretty simple, but like most of us this is a hobby as it is for the people supplying the parts, other than AES and Mouser, it took a bit to get responses and orders placed.

    Since I have been documenting everything and taking pictures I am going to do a thread about the conversion.
     
  7. Dangerbird

    Dangerbird Junior Member

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    I've done the first mods up to but not including the orange drop caps (they only just arrived in the mail) and I've lost volume to about 1/10th of what it was. It had been sounding great. I haven't changed the OT though, could that be the reason?
    Also wanted to know about pots and their wattage rating. From what I can find most guitar pots are 0.5watt rated. Do I need 1W rated pots? If so where can I find these.
    Thanks for the great info. I'm really enjoying this as my first amp mod.
     
  8. Dangerbird

    Dangerbird Junior Member

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    So I've added the orange drop caps and still no increase in volume. I'm really not sure what might have gone wrong. The only thing I've done differently that I've used a 100K carbon film resistor in R4 not the carbon composite which is still coming in the mail. I'm a little reluctant to poke around with a multi-meter but I think I'll just have to do it. I'll just make sure I'm in plain sight. I've just received my Classic Tone OT in the mail so I'll add that in while I'm at it.
    Anyone still following this thread?
     
  9. Kevin Murray

    Kevin Murray Junior Member

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    I have a question with regard to post # 19 ... ie: Power Tube voltage mods.

    Here are my voltages stock and after replacing with a 1.633 K ohm resistor.

    EL 84
    P9 322 ... 271
    P7 345 ... 290
    P5 6.88 ... 6.96
    P3 10.7 ... 8.99

    12AX7
    P8 1.57 ... 1.33
    P6 201 ... 166
    P5 6.85 ... 6.97
    P4 6.82 ... 6.99
    P3 1.55 ... 1.31


    Are the Voltages now too low ?

    Thanks in advance,
    Kevin
     
  10. Batman

    Batman Senior Member

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    You look to be really close. Personally I would try something closer to 1K and see what that yields you. 290V is not a bad number but you want it as close to 300V as possible.

    Of more concern are the values for pins 4 and 5 on the 12AX7. When the R10 resistor was increased in value in my build, I got a slight increase in heater voltages. I was advised that these need to be kept under 6.9V (Heaters are usually 6.3V +/- 10% which puts my 6.88V at a high but acceptable value. . .they should be less than 6.9V.) Yours are over that value but lowering the value of R10 should bring these voltages down slightly.
     
  11. Kevin Murray

    Kevin Murray Junior Member

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    I don't have any other appropreate resistors other than a a 1.006 Ohm.
    Looks like something between 1.6 K Ohm and 1 K ohm will be good.
    Unfortunatly the heater voltages are not improved and the desored 10 volt differential between pin 9 and pin 7 is worse, ... went from 19 volts to 22 volts.

    Here is what I have now:


    EL84
    P 9 296
    P 7 318
    P 5 6.96
    P 3 9.86

    12ax7
    P 8 1.46
    P 6 184
    P 5 6.97
    P 4 6.97
    P 3 1.45


    Any further tweeking will require a parts reorder.

    What would you recomend doing Batman ?


    Thanks,
    Kevin
     
  12. noNameUsername

    noNameUsername Senior Member

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    I've just tried this mod but I'm having issues... With the pot turned all the way up, the amp operates as normal, but when I turn it down, I lose the gain and the sound not only becomes quieter but Also cleaner... When I turn down a little bit all the gain is pretty much gone.. And it just acts as a volume control.. Any ideas where I mightve gone wrong?
     
  13. noNameUsername

    noNameUsername Senior Member

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    SORRY!! I must've skimmed over the part where someone else asked the exact same as me! My bad.. I'm gonna try removing R7 and see where that gets me :)

    Also, wanted to say what a great and informative thread this is!! Excellent stuff, really really helpful. Thanks to Batman and TWANG, who seem to be the VJR Gods :') and also to everyone else who's contributed their knowledge, I think it's great that there's guys out here who not only know all of this stuff but also are wiling to share their knowledge and help and teach others.

    Thanks guys :thumb:
     
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  14. spunky99

    spunky99 Junior Member

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    Hi all,
    I bought my Valve Junior a while back so I have an old Rev 2. I modded it a bit but my master volume wasn't done right and it sat in my closet for many years until last week when I saw this thread. First off I want to say thanks to Batman and extension to Twang as his posts got a lot of the gremlins sorted out in the amp.
    Here is where I'm at so far. I haven't done the Negative Feedback yet or the JP1 snip.

    I'm a hack playing guitar and have carpal tunnel in both wrists and worse, cubital tunnel in both elbows. The good news is that I will be getting surgery soon on one arm and then the next so eventually I might be able to play guitar.

    Here is my one issue on the amp. I think my 12AX7 tubes 1st and 2nd triodes are a bit low on voltage. Check out pin 8 and 3.

    Here are the changes in values from stock

    1. R1, (68K) change to 1M 1W Metal Film (reduces attenuation of guitar signal)
    2. R2, (68K) change to 10k 1W Metal Film (reduces noise)
    3. R3, (100K) leave stock 100K. (Change to 220K and R8 to 1.5K for more gain)
    4. R4, (100K) 90K to 100K Carbon Comp
    5. R5, Master Volume Mod, remove R5, lift Cap , (Use shielded wires and 500K pot with 470K ½ watt )
    6. R6, (1M) change to 100K for more gain, less hiss and no loss of high frequencies
    7. R7, (1M) remove R7 (no jumper)
    8. R8, (2K2) change to1k Metal Film (can be 1.5K)
    9. R9, (2K2) change to 820 ohm Metal Film (can be 1.5K and Carbon Comp)
    10. R10 (100) change to 1K 3W (first check your voltage on pin7 of the EL84...mine was 318VDC and the change reduced it to 288VDC)
    11. R11, (100)
    12. R12, 4K7)
    13, R13, (10K)
    14, R14 (220) change to 220 ohms
    15, R15, (1K5)
    16. Add a 10K/10 watt resistor and a 100uf 450v filter cap to tighten up the bottom end
    17. C1 and C2 (.022) change to .022 Sozo
    18. C3 and C4 (22) change to 2.2uf, 35v
    19. C5, (22) change to 470uF 50v
    20. C6, (22) change to 47uF 450v
    21. C7, (22)
    22. C8, (22)
    13. Screen Grid Mod EL84, add a 1K 3W resistor between pin 9, cut the trace.
    14. TungSol 12AX7, TAD EL84
    15. Swap the diodes for FREDS
    16. Replace the Output Transformer with a Hammond 125ESE
    17. Replace Volume pot with Alpha 1M

    Mods
    Screen Grid Resistor
    Replace OT with Hammond 125ESE
    Master Volume (Thanks Batman)
    Add additional filter cap mod
    Tweed Tone Control (Thanks Batman)
    Line Out (connect dummy load for silent playing)

    Readings after all mods done for base BIAS readings
    EL84
    Pin 9 – 318 V
    Pin 7 – 349 V
    Pin 5 – 7.04 V
    Pin 3 – 10.9 V
    12AX7
    Pin 8 – 1.01 V
    Pin 6 – 168 V
    Pin 5 – 7.04 V
    Pin 4 – 7.02 V
    Pin 3 – 1.18 V

    The Plate voltage (pin 7) of 349 Volts was too high. I used a 1 K 3W resistor

    After replacing R10 (1K 3W)
    EL84
    Pin 9 – 288 V
    Pin 7 – 314 V
    Pin 5 – 7.07 V
    Pin 3 – 9.68 V
    12AX7
    Pin 8 – .909 V
    Pin 6 – 153.1 V
    Pin 5 – 7.03 V
    Pin 4 – 7.03 V
    Pin 3 – 1.065 V

    Any help would be greatly appreciated. The amp sounds great and has lots of headroom. With the gain it distorts nicely until I move the master volume about 3/4 full and then it makes a shrieking whistle sound. (horrible) Most of the buzz went away after I biased the EL84 but I lost some of the crunch. Should I do the JP1 mod and then check voltages and see how it sounds? Is my 12AX7 cathode voltage too low? If so what will bring it up to a proper level?

    Thanks again,
    John
    Here are a couple pics of the closed up head. The line out was a tight fit and I'd change location if I did it again.
    [​IMG]
    [​IMG]
     
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  15. Tailgate77478

    Tailgate77478 Junior Member

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    Sorry to resurrect an old thread...but I am getting very frustrated with my EVJ mod. It's been going GREAT, thanks to Batman and Twang's posts, and I've learned a lot. However, on this MV Mod, Batman says to connect the 470k resistor to the wiper and the ground posts of the pot...but the photograph shows the resistor is connected to the wiper and the input of R5. The schematic diagram looks like it says wiper and ground. Which is it???

    My frustration is I am getting really high pitched squealing sounds. Funny enough, these were my last mods, and the amp has sounded clean and beautiful up until now. I have re-checked all of my solder connections (re-did ALL of them) on both the MV and the "gain" (old Volume...I replaced the pot) knobs. I even replaced the tubes, but it shows up anyway. When the gain is about 70% and volume is getting up there, it's like a dog whistle or old TV set, and the sound coming out of the cab (I have a head and separate cab) is very scratchy and cuts in and out rapidly. Talk about distortion!!! Ha. I even Ohmed both pots to make sure neither were bad. They were fine. I even thought maybe it was the lead dress...but I've got them all twisted up as well.

    One more question...is it necessary to use the output of the C2 cap in the MV mod? I see that most people online simply just put it to R5.

    Thanks in advance...I can't seem to PM Batman, so that's why I resurrected this excellent post.
     
  16. Tailgate77478

    Tailgate77478 Junior Member

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    Update: I said "what the heck" and put C2 back in place and just used R5. I also put the resistor across the wiper and ground. Reinstalled...still squealing...except it was OUTRAGEOUS because I accidentally had both the gain and volume all the way up, so when the tubes heated up, YOWZA!!!! I have no idea why it'd be doing this. I have been very gentle with the board, components, careful with quick solders (and only a 25w gun), etc. Again, when I replaced both tubes, it didn't solve the problem.

    Hmmmm......
     
  17. Batman

    Batman Senior Member

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    It's been awhile and I don't have the amp at home right now so I can't confirm with the amp but I think I had installed the 470k resistor on the wrong lug in this picture. It was noticed by someone and corrected.

    Yes, the correct way to do this is to raise one leg of C2 and remove R5.

    The squealing may be caused by reversed Output Transformer leads; I had that happen to mine at some point when I was doing all of the mods. Try reversing the leads and see if that eliminates the squealing.
     
  18. Tailgate77478

    Tailgate77478 Junior Member

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    Apologies for the noob response, but not sure what your asking me to do. Do you mean reverse the wires for T1 and T2? I have them labeled (THANKS to your instructions) and have not installed them incorrectly at any point.

    FWIW, here are my mods and values:

    Pin9 V2 Grid Resistor Metal 1K 1W (actually saved my bacon and had to replace early on!!!)
    R1 Carbon 1M 1W
    R2 Metal 10k 1W put on Input Jack
    R3 stock
    R4 Carbon 100k 1W
    R5 Removed, 500M Pot w/ 470k resistor (see Volume below)
    R6 Carbon 470k 1W
    R7 REMOVED
    R8 Carbon 1.5K 1W
    R9 Carbon 1.5K 1W
    R10 1K 5W
    R14 stock
    C1 Sprague 715 Orange Drop .022uF 600v
    C2 Sprague 715 Orange Drop .022uF 600v
    C3 2.2uF 35V radial
    C4 2.2uF 35V radial
    C5 470uF 50v radial
    C6 47uF 450V radial
    J1 Cut with On/Off Switch for “Bright”
    OT ClassicTone 40-18031 15w
    Gain Pot Audio Taper 500k
    Volume Pot Audio Taper 500k connected to R5 and C2 output
    EL84 stock
    12AX7 stock
    1N4007 Replaced all four with UF4007
     
  19. Batman

    Batman Senior Member

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    Yes, just try reversing them to see if it cures the squeal.
     
  20. Tailgate77478

    Tailgate77478 Junior Member

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    OK, reversed T1 and T2...was worse. For reference, when I ohm the tubes while there is power to them, the 12AX7 pin #9 is 295, and the EL84 pin #6 is 180. Was the same when I reversed the leads. I have literally spent about 6-8 hours of reading / researching this problem, re-soldering / tweaking, etc. before posting here, and nothing seems to work. I'm at a loss.

    I did turn the JP1 "bright switch" off (essentially returning it to the jumpered stock configuration) and it seemed to help quite a bit, but at high volumes, it's as if the head is overdriving the speaker. This sucker is SMOKING loud after adding the gain knob mod (and replacing the OEM volume pot). But the speaker has been perfect until now (it's the matching Epiphone 2x12 cab with Eminence speaker). I tell you, hardly anyone would believe this is a 5 watt amp.

    But it's driving me crazy. I want to get that break-up of the tube at low volumes, but this latest mod is not working right. Arrrrgh!
     

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