Updated Components Continued

The amp was way too quiet for my tastes, so I made two more changes. First, I took out R7. This value was originally 1 Mega ohm, but I changed it to 100 kilo ohm. Now it is nonexistent.

Originally, R2 was 68 kilo ohm, but I lowered it to 47 kilo ohm originally. Next, I lowered the value of R2 down to 10 kilo ohm.

Below is a schematic of the Valve Jr, Version 2. Ignore those arrows, that isn't my work. As you can see R7 was used to bring the signal down, as it goes straight to ground. This is useless for what I want, so it is gone.

R1 and R2 are in parallel on the guitar input jack. Since I changed R1 from 68 kilo ohm up to 1 Mega ohm yesterday (HUGE INCREASE), I decreased R2 much more. Since R2 is actually before R1 in the circuit, it creates a voltage divider which decreases the signal right when you plug in your guitar. Making R2 smaller will give less signal loss. One modification option I have in the future is to move R1 in front of R2 in the circuit, which would decrease the voltage divider even more. 

Un-modded Valve Jr Version 2.

The results? Sounds incredible. The amp can get loud again, which is what I wanted, and the mellow sounds when you turn the tone pot down on the guitar and use a neck pickup is incredible. I love this amp and the way it sounds so far. I have been working on my pedal board at home, and I'll bring that in next week to play some pedal through the amp. I haven't played anything but clean on it so far.

Changed Components Successfully

This morning I changed the following components:

• R1 to 1 Mega ohm. Was originally 68 kilo ohm.
• R2 to 47 kilo ohm. Was originally 68 kilo ohm.
• R6 to 100 kilo ohm. Was originally 1 Mega ohm.
• R7 to 100 kilo ohm. Was originally 1 Mega ohm.

To get to the bottom of the board, I temporarily removed the tubes to have more access. I also removed all the cables to the Transformers, as those limited the mobility of the main board. This was my first time using de-soldering braid, and it is probably the greatest invention ever. I don't know how I lived before knowing about it. When I was done, I made sure to put the tubes in, and then connect all of the Transformer wires to their proper pin. The connector for the T3 broke off while I was pulling it from the board, so I stripped the end of the wire and re-soldered the tip to the connector. Now it's better than new.

Anyways, these modifications are intended to make the potentiometer change, or volume change, more linear. Before the change, it would be really quiet, and then really loud, with not much in between. Now, the change is noticeably more linear, with the volume ranging evenly from quiet to loud. This does have some trade-offs though. The amp is almost silent until about 10 o'clock on the dial. Maybe it's because I'm almost deaf because of all those years of rock n' roll.

Also, those DPDT from Hong Kong never came in, so they're sending me two more. Maybe those will actually arrive.

Here's some pictures:

With all components changed.

R1 and R2 successfully soldered in.

The back of R6, before clipping the excess resistor wire.

Change Components Plan

After figuring out I can't solder on miniature switches today, I've decided to go a new route and change some components on the main board. I will change R6 and R7 to 100 kilo ohm resistors. This will bring R6 up and R7 down. I will use the 1 Mega ohm resistor from R7 and put that in R1, and change R2 to 47 kilo ohm. R1 and R2 directly affect how much the volume knob changes the output volume. I discovered today that any resistor connected to ground will change the gain, and a capacitor connected to ground will affect the tone. I also discovered de-soldering braid, and how to use it, so these components will be changed in a flash.

Tonight, I ordered the Bitmo Trio mod, which contains a secret list of parts (I could not find a part list anywhere). Anyways, it will add a Push/Pull Tone knob (potentiometer) and some sort of 3-way switch. I'll find out soon enough. I also plan to inverse the orientation of my amp (turn it upside down, permanently). The heat dissipates better this way, and I also want to change the look of the head.

Installing Bright Switch

I'm at the lab currently, trying to install a "Bright" tone modification switch, using a DPDT switch. This should put a capacitor in parallel with the potentiometer, but it is just making my amp really noisy so far. I double-checked my wiring, and I think I have 1a bad design.

I checked to see if the DPDT switch was causing all the noise by taking the wiring out of the pot, and the switch circuit was causing all of the noise. Now to debug.

I think a root cause could be the type of capacitor I am using could be wrong. The modification called for a 120pF silver mica capacitor, which I used earlier to replace a broken cap. I am currently using a 100pF ceramic disk cap, which seems to handle higher voltages worse. Amps seem to use these silver mica capacitors exclusively because of their reliability and ability to handle high voltages.

I tried changing the capacitor to an electrolytic capacitor, with the same negative results as before.
I tried using the SPST for a few things. I strapped a 1Megaohm capacitor across the terminals, and then hooked that up to the input and ground of the cap. More buzz.

In other words, today, I tried a bunch of things, and they all just created a bad hum for the output. I have reverted it all back to the way it was at the beginning of the day, and I'll start again tomorrow.

Tube Updates

Right before I left for break, I had installed the new JJ Tubes. Well, after a day, they did not work anymore. There was no sound output from the amp, and the tubes were not lighting up. So I pulled the tubes and put back in the stock tubes, which I knew were good. Still no lights from the tubes, and no sound output. I pulled all three fuses for the tubes and replaced them. Then I got lights in the tube filaments and sound output from the speaker. Success.

So then, I changed the tubes back to the new JJ Tubes. Well, I thought I did. You have to unscrew 9 screws to get to the tubes, then re-screw them all in to make sure the main board is stable. I did that, then realized that I had only changed the power tube to JJ, and taken the pre-amp old tube out, and put the old pre-amp tube back in. So I unscrewed everything and replaced the pre-amp tube with the new tube. Phew. Success.

I was tired of waiting on this Hong Kong seller to send me my DPDT switches for the "Fat" and "Bright" switch, so I drove to Radio Shack and bought some. They're small, so I need to drill new holes for them. I should have the new tone modifications in by the end of the weekend.

Also, I got the washer and bolt in for my potentiometer this week, so now the pot doesn't move around inside the chassis. I'll order some cool looking cover for it soon, or maybe 3D print one in the SPaRC lab.

I also drilled holes for and temporarily mounted my new DPDT switches. Looks fancy. 

New Tubes Installed

I was doing research on how to bias tube amps, and then I discovered that my Epiphone Valve Jr is self biasing. So just put the new tubes in and play. I replaced the Russian Sovtek tubes with some new JJ Tubes. I bought the JJ Tubes set from Amazon as a matched set for my amp. The new tubes are the same types of vacuum tubes as stock: an EL84 as the power tube and a 12AX7 as the pre-amp tube. With the JP1 bypass, the amp makes a substantial amount of hum. I think this is due to the tubes not being warmed up. I tested the amp immediately after turning on, and tubes need a few minutes to warm up before they sound good.

Tubes taken out.

Old Sovtek tubes.

New tubes (left) and old tubes (right).