Subway Blues troubleshooting

The Boogie Board

Help Support The Boogie Board:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

goldenGeek

Member
Joined
Feb 27, 2015
Messages
16
Reaction score
0
Hello :)
I am currently trying to figure out a Subway Blues thats humming too much. I have read some posts about the .01uf mod (replace with .68uf), but I think the problem with this amp is earlier in the path. I will replace the cap anyway, but heres the problem:

There is a loud static that gets louder with the volume knob. If I pull tube V1 the hum goes away so I think the problem must be in the first two gainstages - probably between input and before V1b. The volume pot is also scratchy. Theres also some trouble with te reverb but I'll fix that later since the noise goes away with reverb at 0. Does anybod have any clues what to check first? Replace the volume potmeter? Capacitors? Resistors? I have replaced the V1 tube with a couple of other known working ones but they were actually worse than the one originally present when I got it. I can of course replace the whole circuit up to V1b, but if anyone has any experience with this amp I'll be glad to hear from you :) Oh, by the way, this is probably a newer one, it doesnt have the "mojo module" installed, all the components are visible. Serial number in the 17xx-range if I recall.

I'm going to trade this amp for a MK3 black stripe thats absolutely amazing so I really need to get this amp in better shape ASAP 8)
 
So I tried replacing the .01 with a higer value, I didnt have a .68 so I put in a .1, it should have made some improvements if that was the problem but the noise is just the same. I also tried to replace the volume pot with a new one but it did not improve anything.

I recorded the noise, starting with volume at 0 going to 10 and wiggling the volume at the end. Does anybody have a clue here?

https://dl.dropboxusercontent.com/u/43679524/Noise.mp3

Thanks!
 
I'm obviously not very patient. I had to fiddle with the amp again while waiting for someone to chime in :)

Okay, so I chopsticked a bit and some capacitors in the input section is really microphonic. There is a loud tapping in the speaker when I tap the 20, 120 and 250pf before V1a. Is this normal or should I start here? I have never seen that particular input before, usually its just 33 or 68k into V1a, but here its multiple caps and resistors. If thats not the case I guess the next suspect is the tonestack and the caps there? The scratching on the volume, is that DC which in turn points to leaky caps in the tonestack?
 
Back
Top