2204 to Dirty Shirley conversion?

[ledvedder]

I followed this schematic and changed to all the proper values. First I was getting barely any volume, and minimal gain. So I tried removing the 560k into the MV input. It got a tiny bit louder, but still barely any gain. So I tried removing the 220k to ground from both the MV pot and the preamp gain pot. There's still barely any volume or gain with both the preamp and master at full. Any ideas?

 

[ledvedder]

This might be a dumb question, but is one side of the 47k-220k/4n7 depth circuit supposed to be ground?

 

[burger]

Same question posted in the DIY group. Your problem is not those component values, you have something more fundamentally wrong.

And no, your NFB line should not be grounded, but if you are testing it on the side of the OT you meter will fool you into thinking it’s ground because the OT secondaries are very low resistance.

 

[ledvedder]

Same question posted in the DIY group. Your problem is not those component values, you have something more fundamentally wrong.

And no, your NFB line should not be grounded, but if you are testing it on the side of the OT you meter will fool you into thinking it’s ground because the OT secondaries are very low resistance.

I was testing it from chassis ground.

 

[burger]

As I said, your meter will fool you. You’re reading straight thru the OT secondaries to ground.

 

[ledvedder]

Thanks for everyone's help. I spent a good 4 hours today going through everything. Resistor and capacitor readings, signal tracing, voltages, tubes. I'm completely stumped. I have sound, but it's no where close to the volume I should be getting. And barely any gain with the gain pot all the way up. Actually a clean tone. I even tried putting it back to the 800 mod spec. Still the same results.

I should have just left it alone. It sounded good with the Plexi mod. I even gigged with it a few weeks ago. But since I already have a plexi, I wanted to try something else. And now it's all screwed up.

 

[ledvedder]

I'm just posting some pics in case anyone notices any obvious issues.

 

[glpg80]

There’s a lot of potential problems in this photo. The relay is potentially compromised with the solder iron. The cap is overheated/deformed and likely internally shorted. The solder joints on the pins need attention and the resistor up top needs resoldered.

Suspect any solder joint with clip on pins. They aren’t made to constantly connect and re-connect so it’s better to just solder them together or at least solder one side permanently and keep the other a clip.

Check for shorts between signal and ground with shielded cables.

 

[ledvedder]

I spent the afternoon going through every resistor and capacitor and measuring them. All seems well. I also pulled every component that I've modded, cleaned up the board, and resoldered them. It's still not functioning correctly. This video is with the master and preamp gain dimed. Towards the end I pull the preamp gain back to about half. I'm hoping someone that's much more knowledgeable than me can give me any clues where I could be having issues.

 

[ledvedder]

There’s a lot of potential problems in this photo. The relay is potentially compromised with the solder iron. The cap is overheated/deformed and likely internally shorted. The solder joints on the pins need attention and the resistor up top needs resoldered.

Suspect any solder joint with clip on pins. They aren’t made to constantly connect and re-connect so it’s better to just solder them together or at least solder one side permanently and keep the other a clip.

Check for shorts between signal and ground with shielded cables.

View attachment 149536

I checked C51, and it reads correct at 470n. I'm not using the relay, so I'm not too concerned about it. In the stock configuration, it activates a ground on C37. I've resoldered all suspect joints. I checked all the cables for both shorts and continuity. All my voltages seem fine. Are there any specific areas that I should test, that I'm not aware of at this point in my learning?

 

[glpg80]

I checked C51, and it reads correct at 470n. I'm not using the relay, so I'm not too concerned about it. In the stock configuration, it activates a ground on C37. I've resoldered all suspect joints. I checked all the cables for both shorts and continuity. All my voltages seem fine. Are there any specific areas that I should test, that I'm not aware of at this point in my learning?

Just because the relay isn’t used doesn’t mean the coil couldn’t be shorted causing issues with other relays that are in use. If you don’t need it remove it completely since it’s compromised.

First rule of thumb for stages of gain is verifying all dc bias points for all stages. Verify all grounds are actually ground and not open circuit from a bad connection or solder joint. Verify B+ is in the correct range.

You should probably fill out another one of those headfirst preamp stage voltage charts for all pins so all of us can help out. You should continue to look for questionable solder joints.

 

[ledvedder]

Here are the voltages.

 

[CrazyNutz]

Maybe one of your preamp tubes took a dump. Not out of the question, I've had it happen before.

Also is the first coupling cap 0.0022uf, or 0.022uf ?

 

[ledvedder]

Maybe one of your preamp tubes took a dump. Not out of the question, I've had it happen before.

Also is the first coupling cap 0.0022uf, or 0.022uf ?

I've swapped out all 3 preamp and both power tubes. Right now, it's 0.022.

 

[glpg80]

I need all voltages including heaters and grids

 

[CrazyNutz]

I've swapped out all 3 preamp and both power tubes. Right now, it's 0.022.

Then I'd start looking for lifted pads/broken traces, and poke the board with a chopstick in different areas, and listen for changes.

 

[ledvedder]

I need all voltages including heaters and grids

Here you go.

 

[ledvedder]

First rule of thumb for stages of gain is verifying all dc bias points for all stages. Verify all grounds are actually ground and not open circuit from a bad connection or solder joint. Verify B+ is in the correct range.

What would be the best way to verify this? Particularly, actual ground vs open circuit? For example, the bottom of that 10k cathode resistor on v1b tests as ground. But, I tried alligator clipping it to chassis ground and the volume increased. It still didn't sound great, but it definitely changed. I tried the same for the 2k7/0.68 on v1a and the 1k on v2b, but nothing changed with those.

 

[glpg80]

What would be the best way to verify this? Particularly, actual ground vs open circuit? For example, the bottom of that 10k cathode resistor on v1b tests as ground. But, I tried alligator clipping it to chassis ground and the volume increased. It still didn't sound great, but it definitely changed. I tried the same for the 2k7/0.68 on v1a and the 1k on v2b, but nothing changed with those.

You need a meter that has a continuity capability. You test each ground node for continuity one by one. This is done with the amp off and unplugged.

Sounds like you have a bad solder joint somewhere involving the cold clipper stage. You may want to look at those solder joints. Let us know when you’ve got the other voltages in the voltage chart.

We will figure it out one way or another.

 

[ledvedder]

You need a meter that has a continuity capability. You test each ground node for continuity one by one. This is done with the amp off and unplugged.

Sounds like you have a bad solder joint somewhere involving the cold clipper stage. You may want to look at those solder joints. Let us know when you’ve got the other voltages in the voltage chart.

We will figure it out one way or another.

I posted the additional voltages a few posts up.