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PostPosted: Mon Jan 01, 2018 7:27 am 
Bottle Rocket

Joined: Fri Feb 27, 2015 5:02 pm
Posts: 14
(I edited my post a bit, it was a tad "messy")

So I got my second Subway Blues a while ago (I sold the first one a year back). I bougth it for about 60 bucks since it clearly was dead and it was missing the speaker and cabinet (just the chassis and reverb pan), but I figured at least one of the transformers should be fine so that I could harvest parts for a build. But then I replaced some components in the output stage and retubed and the amp is working again. Yay! But theres some not so nice oscillation going on when its turned up, especially when the middle and/or treble is turned up past noon. The middle potmeter is also a bit scratchy, I'm measuring about 8V DC on all eq pots but I'm not sure if that's the problem. How much DC is within limits?

Okay, so I bypassed the tone stack and put a 0.1uf as coupling cap between V1a and V1b and it seems to be working okay. Then I soldered out the caps in the tone stack, but they all measure what I would think is within specs on my LCR meter (I actually really dont know what the specs are except for cap values, but the dissipation is below 0.05 on all caps). Anyway - after I put the caps back in i goofed around measuring some voltages here and there, but at some point I noticed that the ground bus at the pre-amp is not directly grounded to the rest of the amp. Theres about 55k ohm resistance between input jack sleeve (or gain and middle potmeter "ground lug") and the power input ground (or first filter caps ground). There's 0R "inside" the two sections of the amp, so I guess theres a connection or whatever missing (or trace acting up?). I have never seen this before, and the schematic doesnt give any clues (I'll attatch the schematics to this post). The first section that measures 0R is the pre-amp fropm inout jack up to the 180pf cap at the "end" of the pre-amp schematic. Can anyone help me out here? Is the PCB done for? (it does look okay) I could always build something else in this chassis with these transformers I guess :)

I think this is the one:

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