Mesa Subway Blues problem

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goldenGeek
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Mesa Subway Blues problem

Post by goldenGeek »

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 from input 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 :)
Mesa Boogie Subway Blues schematic.pdf
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Stevem
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Re: Mesa Subway Blues problem

Post by Stevem »

If a plate blocking cap or tube stack cap is passing more then .250 D.C. It's bad!
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martin manning
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Re: Mesa Subway Blues problem

Post by martin manning »

Funny the schematic doesn't show any ground return from the filters. How about the FX loop jacks, Rec Out jacks, and Reverb circuit? Are they grounded to the chassis? Which of the filters are chassis-grounded? What about the 0.01u (or 0.68u) cap in the hand-drawn sketch? is one end of that grounded?
goldenGeek
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Re: Mesa Subway Blues problem

Post by goldenGeek »

@Stevem: I guess I'll have to replace the tone caps then, the oscillation will most likely go away. Thanks.

@martin manning: I got one thing wrong, all the filter caps are grounded to the chassis. The 0.01 is replaced with a 0.68uf and is not grounded (which it shouldn't be). I poked a bit more and it seems like everything is "grounded"/tied together (including fx loop, reverb, rec out) all the way up to the ground point right before the phase inverter. But still about 55k between this "ground" and the chassis ground.
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martin manning
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Re: Mesa Subway Blues problem

Post by martin manning »

Is there any voltage difference between the preamp grounds and chassis ground with power on? Can you find some deliberate path where the preamp ground goes to the chassis ground? Leaky coupling or tone caps aren't likely to cause oscillation, but if you find DC on them, replace them as Steve says.
goldenGeek
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Re: Mesa Subway Blues problem

Post by goldenGeek »

Thanks, I will check that tomorrow. The caps do make some noise when i chopstick them (it was like this in the other Subway Blues I had as well). Another thing I noticed (which I just recalled) is that when the reverb tube is pulled the amp sounds a bit more quiet. From the schematic it shouldn't be much of a difference with the tube in and the reverb mix is set to 0, or am I misreading that part?
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TUBEDUDE
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Re: Mesa Subway Blues problem

Post by TUBEDUDE »

If the noise increase is minimal, it might just be current noiise from the additional current draw of the verb driver. That is if it's and old high current design with 2 stages in parallel cless a SE. If so it will likely go away wi the grounding issue resolution.
Some circuits float sensitive circuit grounds above chassis ground to get rid of noise currents. That is usually only 10 to 200 ohms though. 55k is mighty high. Have you tried paralleling this with a short?
Tube junkie that aspires to become a tri-state bidirectional buss driver.
goldenGeek
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Re: Mesa Subway Blues problem

Post by goldenGeek »

Theres about 13-14V DC between the preamp ground and the chassis ground.

So I tried to replace the tone stack caps and the eq got a bit better I think, but there' still squealing going on. However, when I bridged the two "grounds" the squeal disappeared. A side effect is that the sound was silenced quite a bit as well. The HV dropped also, especially in the pre-amp. After running the amp for a while now the Power transformer gets a bit hot as well (i haven't measured, but I would say 40-50 degrees Celsius). The PCB is also cooking a bit around the tube sockets as well. I don't think I will bother wasting any more time on this amp so I guess I'm gonna re-use the transformers and chassis and build something else.
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martin manning
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Re: Mesa Subway Blues problem

Post by martin manning »

Clearly there is something in there that is not on the schematic, intentionally or otherwise ;^)
goldenGeek
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Re: Mesa Subway Blues problem

Post by goldenGeek »

Hello all :) This is an old post, but I decided that I'd give this amp a last try before gutting the parts. After an hour or two of pointless poking around I decided to give up and I decided to write down the voltages and pinout for using the power transformer in another project. The transformer has a 10-pin connector and upon measuring the matching connector/pins on the PCB for finding which pins go to ground I found that both HV CT and 6.3V CT had a really inconsistent connection. After reflowing the pins on the connector - viola! It works. It's still a bit noisy and there's some oscillation on full volume, but that's part of the design I guess, the other Subway Blues I had also had this problem. If I recall, the noise went down when the other amp was in the cabinet with a shielding plate/foil over the chassis. I will try to make a plate and see how it goes. I'm really pleased with todays work :D
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