How hard would it be to bypass/remove the Pentode/Triode Switch on a JCM900 MKIII

[SixStringSting628]

As some of you may have seen in one of my posts, my JCM900 MKIII 2100 has the board burnt near the High/Low power switch and the board has been drilled out. When looking at the bottom of the amp, the two prongs on the left of the High/Low switch are each wired to pin 3 of the middle power tube sockets (one wire from the switch to power tube 2, and one to power tube 3). I am not sure if this repair is making the switch operate backwards as the Low mode is louder than High mode. I only desire to use 100w mode so I am wondering how difficult it would be to either bypass the switch and make it just pentode operation or to remove the switch?

 

[thetragichero]

it connects the screens to the plates so if you remove the switch and wires from it to socket it should no longer be there
i personally would not feel comfortable with all that carbon still there

 

[fitz]

As some of you may have seen in one of my posts, my JCM900 MKIII 2100 has the board burnt near the High/Low power switch and the board has been drilled out. When looking at the bottom of the amp, the two prongs on the left of the High/Low switch are each wired to pin 3 of the middle power tube sockets (one wire from the switch to power tube 2, and one to power tube 3). I am not sure if this repair is making the switch operate backwards as the Low mode is louder than High mode. I only desire to use 100w mode so I am wondering how difficult it would be to either bypass the switch and make it just pentode operation or to remove the switch?

If your going to fix it, how hard would it be to just make it work correctly?

 

[SixStringSting628]

it connects the screens to the plates so if you remove the switch and wires from it to socket it should no longer be there
i personally would not feel comfortable with all that carbon still there

Given I can clean off some of the carbon, if I remove the switch from the board and the hard wires from the switch to the tube sockets, would the amp still work and be in pentode mode? Or does the switch need to be there for it to work at all?

 

[KraftyBob]

Just make sure the screen grid is not tied into the same DC supply as the plate and you'll be in Pentode mode. You don't need the switch for the amp to work.

 

[SixStringSting628]

Just make sure the screen grid is not tied into the same DC supply as the plate and you'll be in Pentode mode. You don't need the switch for the amp to work.

I removed the switch and the wires going from the switch to the tube sockets and got no sound. So I put the switch back in and just left the low side of the switch disconnected , as I saw another member did this on their amp with a very similar problem. Now I have sound and all seems to be working properly. I haven’t gotten to crank it up yet, just plugged a guitar in at low volume and strummed a few times and I have sound now. I’ll give an update tomorrow when I get a chance to play. Thanks for your input, I really appreciate it.

 

[KraftyBob]

Sorry, I should have been more clear. While you don't need the switch, you do need to connect the DC supply to the screen grid to complete the circuit.

Here's the Pentode/Triode circuit for my DSL40C. The two green circles are the switch in circuit. Right now it's showing in Pentode mode as the plate and screen grids each have their own DC supply. Both are high voltage, however, the plate will be slightly higher than the screen grid. When switched to triode mode, the screen grid is then tied into the plate which results in reduced power. In the bottom circle I show what needs to be done - which is permanently connecting the screen voltage to the screen grid (pin 4).

Hope this helps.

 

[SixStringSting628]

Sorry, I should have been more clear. While you don't need the switch, you do need to connect the DC supply to the screen grid to complete the circuit.

Here's the Pentode/Triode circuit for my DSL40C. The two green circles are the switch in circuit. Right now it's showing in Pentode mode as the plate and screen grids each have their own DC supply. Both are high voltage, however, the plate will be slightly higher than the screen grid. When switched to triode mode, the screen grid is the tied into the plate which results in reduced power. In the bottom circle I show what needs to be done - which is permanently connecting the screen voltage to the screen grid (pin 4).

Hope this helps.

View attachment 87777

Yeah that makes sense, I figured the circuit wasn’t completed with the switch removed. So yeah I went ahead and hooked up the switch and left the 2 prongs on the low side not connected to anything (the ones that had those wires going to the plates of the 2 middle power tubes). It seems to be working properly now and should indeed be Pentode operation only. I’m just not going to touch the switch. Thanks for the info and diagram!

 

[KraftyBob]

Sounds like a plan. You may have already done this but the wires from the plates going to the switch are carrying high DC voltage so you want to make sure they are removed if possible. If you can’t remove them just make sure they are well insulated so they don’t inadvertently short to something. 400VDC +/- hitting something inside the amp would not be a pretty sight

 

[SixStringSting628]

Sounds like a plan. You may have already done this but the wires from the plates going to the switch are carrying high DC voltage so you want to make sure they are removed if possible. If you can’t remove them just make sure they are well insulated so they don’t inadvertently short to something. 400VDC +/- hitting something inside the amp would not be a pretty sight

Yes I disconnected the wires on both ends haha. I can only imagine what arcing would happen if they grounded out on something lol

 

[KraftyBob]

Yes I disconnected the wires on both ends haha. I can only imagine what arcing would happen if they grounded out on something lol

I figured as much, and it was not my intention to imply you didn’t know. But figured it was better to be safe than sorry. Glad you got it running how you like!

 

[SixStringSting628]

I figured as much, and it was not my intention to imply you didn’t know. But figured it was better to be safe than sorry. Glad you got it running how you like!

Yeah it’s always good to remind people about tube amp safety even if they probably already know! And it sounds great so far! I have a Boss Super Overdrive that I just got to use with it! I traded my TS9 for it, because everyone in the 80s would use a Super Overdrive with their Marshall’s haha

 

[Pete Farrington]

Any discoloured areas of pcb substrate should be assumed to have compromised insulation. Grind it back to clean material. Left like that it seems something of a time bomb to me.