Epiphone Valve Junior Mod Thread

[Rusty]

Let's collect all the mods and put them here in this one thread with a brief description for each. That way people won't have to search forever through the other thread. Try not to post any replies on this one; Just the Mods and what they do. But keep the other thread alive of course.

Notice: Harmful or even fatal levels of voltage can be present in

amplifiers, even when they are unplugged from the AC outlet. You

should be familiar with safety precautions and electricity principals

before opening up your amp. You should drain any stored charges and

check for them with a multi-meter BEFORE putting you hands inside the

amp. There is a wealth of safety information on the Internet such as

this: http://www.geofex.com/tubeampfaq/tube_amp.htm#Section0 Perform

a Google search for "tube amp safety" and read up!

The following information is in no way warranted to be accurate or

assure any desirable results for its users. Your amp's warranty will

be voided if you open it up and start modifying it. Performing any

changes to your amp are solely your responsibility. Proceed at your

own risk!

[Rusty]

The only one I have to contribute right now is this simple kit I bought for $25 off of ebay. Took about 45 minutes to do the whole thing including drilling the holes. It includes a Standby Switch, Bright Switch, and Hi/Lo Gain Switch. I highly recommend this one. Also changed the stock tubes to JJ's.

http://cgi.ebay.com/V-Mods-Epiphone-...QQcmdZViewItem

TONE CLIPS HERE:

Bridge Humbucker w/ volume rollback:
http://www.fileden.com/files/2006/1...%20Rollback.mp3

Both Humbuckers w/ Volume rollback
http://www.fileden.com/files/2006/1...Roll%20Back.mp3

Middle Position w/ coil split & volume rollback
http://www.fileden.com/files/2006/1...plit_Volume.mp3

Neck position coil split w/ volume rollback
http://www.fileden.com/files/2006/1..._Volume%20R.mp3

Bridge position coil split w/ volume rollback
http://www.fileden.com/files/2006/1...plit_Volume.mp3

I tried to summarize everything I ordered into a single place. I have put together a MS Excel spreadsheet that looks like this:

NOTE: 11.13.06 07:00 C3&C4 stock numbers above have been revised from 647-UVR1H3R3MDA to 647-UVR1H3R3MDD because of a catalog change at Mouser.com.

That's a snapshot so you cannot cut and paste from it, however the spreadsheet available for download. Click here to download parts.xls.zip

This is the mod to the EL84 pin 9: I used a 1K ohm 1 watt resistor. The power ratings can be 1 to 3 watts. Drill two holes in the trace for the leads, cut the trace between the holes and solder in the resistor.

^^^About this photo: I cut the trace with a Dremel tool and small grinding stone. You can see a white mark just below the right end of the resistor body. To the left of the resistor I scraped some of the clear coating off the trace so I could probe with my DMM and verify I had a good solder; it is much brighter than the rest of the trace line in this photo. You can mount the resistor on the top of the PCB if you like.

Standby switch: There is a single red wire with a blade connector on the end. The connector is pressed down onto a flat post terminal on the PCB labeled T2. I just snipped the blade terminal off the wire, soldered it to the switch, then ran a piece of wire from the switch to the T2 terminal on the PCB.

500k pot for gain control (or 250k~300k). If you install one, here is how it's wired in. I got this pic from the 18watt thread, and the location of the R7 ground point on the PCB is actually at the bottom, not to the right. I drew in the actual location and an arrow pointing to it. Remove R7, solder the pot's wire-leads in the resistor's place.

[Big Mike]

Very cool guys. Let me know when you're done, and I'll copy it to the archive.

[Greg Jones]

I'll simply add some theory behind what each modified section of the circuit does and the reasons for the chosen values. I've already deviated quite a bit because once I did the base gain and voicing mods I found I didn't really like it.

R1/R2 - These set the input level to the first stage of the 12AX7 preamp tube. The stock values for R1/R2 are both 68K. They form a voltage divider that cuts the signal from the guitar in half. That's right, IN HALF!!! NOT GOOD! The suggested changes of 10K for R2 and 1M for R1 alter this voltage divider and results in 99% of your guitar signal hitting the grid of the first preamp stage instead of 50%. For some this may be the only gain increase you want. (I am probably in that camp!)

R8/C4 - These are both in series with the cathode of the first gain stage (12AX7). R8 sets the bias and C4 in conjunction effects the low frequency cut off point. The stock R value of 2.2K biases the preamp tube rather cold and can result in fizzy sounding distortion. The stock capacitor/resistor value results in a -3db point of approximately 7.8 Hz. This is why in stock form they have so much low frequency content and sound dark. The recommended value for R8 is between 1K and 1.5K. C4 1uf – 3.3uf. The lower value capacitor the higher the -3db point will be I mentioned earlier. The formula to calculate this cut off frequency is 1 / 2*pi*R*C. Here is a link to a page showing Rk and Ck values and the resulting cut off frequency for the 12AX7 triode using a 100K plate resistor.

http://aikenamps.com/RC-FHz.htm

R9/C3 – This is just a repeat of R8/C4 above. Same recommended values apply. In practice most people have used the same value for C3 as C4 but R9 is typically being set to 620-680 ohms with R8 between 1K – 1.5K.

R6/R7 – In stock form both these resistors are 1M and form yet another voltage divider that cuts the signal from the first gain stage in half before it reaches the second stage’s grid. Recommended mod is to remove R6 and replace it with a jumper. R7 is typically replaced with an audio taper potentiometer between 250K – 500K. With R6 jumpered and a 91K resistor for R7 the gain will be the same as stock but with less high end loss. Raising the value of R7 increases gain. The latest update I read on 18watt.com now has a recommendation for R6 to be around 47K and the pot. (R7) to be 250K. I am currently running 10K for R6 and will be installing a 100K audio taper pot. I may increase R6 to 20K though to further reduce the gain. I’m thinking about setting R6 to 50K and using a 250K pot for R7. Again, season these values to taste.

Screen grid resistor - See Jake's photo/instructions above about adding a 1K 1W resistor between the EL84 pin 9 and B2. This will help increase the life of the power tube. It simply limits current flow through the screen grid. Without this additional resistor the screen grid will be red hot when the amp is cranked and tube life will suffer.

Standby switch is pretty simple. Jake has talked about how to do this. This switch just removes the B+ voltage from the plates but leaves the heater voltage so the tubes are always warm and ready.

Rusty has added a kit that adds bypass capacitors across R2 and R6. These act like a high frequency boost or a bright switch. I’m not sure what values his kit came with.

Upgrading the output transformer is highly recommended. The stock OT has a primary impedance of 7.5K. EL84’s prefer 5K. This mismatch results in less than perfect power transfer (not as loud as it could be) as well as poor frequency response. The recommended upgrade path is the Hammond SE series. 125DSE is probably the overall best choice. However the 125CSE is adequate or the 125ESE has a higher power rating and some say sounds even bigger/better than the DSE.

I think I covered most everything. Feel free to fill in any holes I may have left guys.

Happy modifying!

Installing the Hammond 125DSE Output Transformer (the following borrowed from Pepi's post #25 in the long thread)

Primary:
On the 125DSE are the Brown and Blue primary wires. These go to to the output from the EL84 tube. (in place of the yellow and white wires of the stock OT seen here)

Some will say it makes a difference which one goes where but it doesn't. I tried both ways and got the same results. Now, on a push-pull amp it does make a difference.

Secondary:
For the 16 ohm jack wire the White wire on the 125 to the PC board where the 16 ohm jack is.

For the 8 ohm jack wire the Yellow wire on the 125 to the PC board where the 8 ohm jack is.

For the 4 ohm jack wire the Greem wire on the 125 to the PC board where the 4 ohm jack is.

Black wire goes to the jack PC board where the exsisting black wire is.

Orange wire is unused. Suggest capping it off with some shrink wrap.

[IIIBoomerIII]

[quote=Greg Jones]

Rusty has added a kit that adds bypass capacitors across R2 and R6. These act like a high frequency boost or a bright switch. I’m not sure what values his kit came with.

QUOTE]

Rusty, can you get us the bypass capacitor values for R2 and R6???

BTW guys thanks for all the effort for this project.

[Greg Jones]

[quote=IIIBoomerIII]

Originally Posted by Greg Jones

Rusty has added a kit that adds bypass capacitors across R2 and R6. These act like a high frequency boost or a bright switch. I’m not sure what values his kit came with.

QUOTE]

Rusty, can you get us the bypass capacitor values for R2 and R6???

BTW guys thanks for all the effort for this project.

I goofed on my description about Rusty's kit initially. One switch puts a capacitor in parallel with R6 for treble boost. The other switch simply switches out R2, which in Rusty's case is the stock 68K. This would increase the gain. Rusty posted a link to the ebay seller in the other Epi VJ thread. Maybe the seller's ebay ad shows the value.

There is a freeware program here called Electronics Assistant. It runs in Windows and does some cool stuff. Could be useful for folks (like me) who are trying to understand all of the values of these bits

How to reduce brightness:

After you do your mods if the tone is a bit too bright, try replacing R15 with a 10k ohm resistor. (The stock value 1.5k)

I made this change after playing through my modified VJr. for a couple of weeks and it took the sharp edge out of the tone.