Still modding a Valve Jr in 2025? Yes. It's my only amp!
I've done a lot of work in this thing since I got it in 2015. Simple circuit, easy to work on , tons of space in the chassis. My version (V2 Head) has the self draining resistor, but I still always check for high voltage before working in there.
Previous Repairs:
-Replaced dead power transformer
-Replace dead rectifier
-Replaced scratchy volume pot
-Replaces a few power capacitors
Previous Mods:
-JP1, but quickly undid it.
-New OT (Hammond 125DSE)
-Bitmo Trio Mod kit (I kept the tone pot section, and the voicing on the first stage cathode, removed the rest)
-Added a Reverb PCB circuit before the amp circuit. 1776 Rub-a-dub deluxe with Push-pull on the pot to bypass it. Mix knob on the front panel, Decay and tone knobs on the back of the amp (used these 2 pots to mount the pcb).
This round of mods:
-Add Master Volume
-Add BOOST pcb before the amp circuit. Push-pull the M.VOL knob to bypass or activate. Boost level and Tilt Eq knobs are on the back of the amp, and used to mount the pcb
-Add Framus Mids knob (with push-pull bypass) from the Rob Robinette 5E3 mods page.
This round of mods was inspired by looking through all the Rob Robinette 5E3 mods, and considering a 5E3 type build with several mods. I thought I'd try them out on the VJr to see if they seemed worthwhile. I really like all of them.
When I first put in the M.Vol, I realized I was getting ZERO pre-amp gain with a telecaster (Volume knob maxed, M.Vol turned down). This was surprising to me, and I realized I must have only ever heard this amp with Power Tube distortion.
There are a few things I could do to get more gain out of the preamp (1M input pull-down resistor, completely bypass the first cathode with 22uF, jump R6 R6 & remove R7). But I like the sound of the amp as it was before this round and didn't want to lose that. So, I added the Boost pcb.
The boost pcb is actually a Tilt EQ I had designed. Basically a simple Bass Treble Baxandall circuit, but I used a dual gang pot to make is a tilt. And it happens to have a ton of volume boost, without clipping. I set the EQ just a bit to the treble side since I'm driving the VJr preamp into distortion. Then, I can completely control the amount of preamp gain (using the original amp volume knob), and the output volume (using the M.Vol). It sounds really great.
The Valve Jr has no BMT tone stack, and nothing else that scoops out mids. I've always considered this a positive, making it a mid-strong amp, but it can be somewhat congested. I didn't want to permanently modify it, just give it another flavor option.
I modified some of the resistors and capacitors, using LT Spice to try to match the EQ to the Fender Super Reverb tone stack. The phase plot is not exact but I think similar enough.
The result is a really nice clean tone with the MIDS knob about halfway. The bypass pull-out is good because at max on the mid knob, you are still losing some volume compared to bypass.
Anyone else working on these, or similar simple amps?
I've done a lot of work in this thing since I got it in 2015. Simple circuit, easy to work on , tons of space in the chassis. My version (V2 Head) has the self draining resistor, but I still always check for high voltage before working in there.
Previous Repairs:
-Replaced dead power transformer
-Replace dead rectifier
-Replaced scratchy volume pot
-Replaces a few power capacitors
Previous Mods:
-JP1, but quickly undid it.
-New OT (Hammond 125DSE)
-Bitmo Trio Mod kit (I kept the tone pot section, and the voicing on the first stage cathode, removed the rest)
-Added a Reverb PCB circuit before the amp circuit. 1776 Rub-a-dub deluxe with Push-pull on the pot to bypass it. Mix knob on the front panel, Decay and tone knobs on the back of the amp (used these 2 pots to mount the pcb).
This round of mods:
-Add Master Volume
-Add BOOST pcb before the amp circuit. Push-pull the M.VOL knob to bypass or activate. Boost level and Tilt Eq knobs are on the back of the amp, and used to mount the pcb
-Add Framus Mids knob (with push-pull bypass) from the Rob Robinette 5E3 mods page.
This round of mods was inspired by looking through all the Rob Robinette 5E3 mods, and considering a 5E3 type build with several mods. I thought I'd try them out on the VJr to see if they seemed worthwhile. I really like all of them.
When I first put in the M.Vol, I realized I was getting ZERO pre-amp gain with a telecaster (Volume knob maxed, M.Vol turned down). This was surprising to me, and I realized I must have only ever heard this amp with Power Tube distortion.
There are a few things I could do to get more gain out of the preamp (1M input pull-down resistor, completely bypass the first cathode with 22uF, jump R6 R6 & remove R7). But I like the sound of the amp as it was before this round and didn't want to lose that. So, I added the Boost pcb.
The boost pcb is actually a Tilt EQ I had designed. Basically a simple Bass Treble Baxandall circuit, but I used a dual gang pot to make is a tilt. And it happens to have a ton of volume boost, without clipping. I set the EQ just a bit to the treble side since I'm driving the VJr preamp into distortion. Then, I can completely control the amount of preamp gain (using the original amp volume knob), and the output volume (using the M.Vol). It sounds really great.
The Valve Jr has no BMT tone stack, and nothing else that scoops out mids. I've always considered this a positive, making it a mid-strong amp, but it can be somewhat congested. I didn't want to permanently modify it, just give it another flavor option.
I modified some of the resistors and capacitors, using LT Spice to try to match the EQ to the Fender Super Reverb tone stack. The phase plot is not exact but I think similar enough.
The result is a really nice clean tone with the MIDS knob about halfway. The bypass pull-out is good because at max on the mid knob, you are still losing some volume compared to bypass.
Anyone else working on these, or similar simple amps?
