Mesa Subway Blues Wiring/Mod Question

[cvjanick]

Hi Folks,

I have an early Mesa/Boogie Subway Blues SN#108 and have a question if another owner can help me. I removed the chassis to replace a broken power switch and noticed a short gray wire that is soldered near what I assume is the V1 preamp tube labeled 2.3 gry and the other end is snipped and dangling. Any ideas where on the board this is supposed to go. I'm still scanning the schematics trying to figure that one out. I've attached a photo of the dangling wire located to the mid-right of the circuit board on the output transformer side and the view is from the back panel.

I think I'll also replace the .01 cap for as recommended for hum. Would also be interested if anyone tried Chuck's mods from the amperage archive and how the tone of the amp changed before I think about making those mods also.

Thanks.

[mikeboone]

I cant really say what the dangling wire is, but if you are having an issue with hum I wouldn't discount that as the source.

Tube amps are generally sensitive to lead dress and you have an unterminated antenna sticking up that could "possibly" be transmitting or receiving noise.

I would make a grand effort to figure this one out.

[Chuck H]

Mesa Engineering is typically very good about customer support. The schematic for the Subway Blues is available online at schematic heaven. That could help with the ID of that lead.

When I had a problem with my early Subway amp I called Mesa. They told me that a lot of those amps had hum issues because of the effects loop (silicooties) board. My amp was no longer under warranty and they replaced the board with a new one (amp was over 10 years old and I got new filters...Yay). No charge, all I paid was shipping. It can't hurt to give them a call.

FWIW that amp has since been extensively modified and sounds much better for it.

Chuck

[cvjanick]

Mesa Engineering is typically very good about customer support. The schematic for the Subway Blues is available online at schematic heaven. That could help with the ID of that lead.

When I had a problem with my early Subway amp I called Mesa. They told me that a lot of those amps had hum issues because of the effects loop (silicooties) board. My amp was no longer under warranty and they replaced the board with a new one (amp was over 10 years old and I got new filters...Yay). No charge, all I paid was shipping. It can't hurt to give them a call.

FWIW that amp has since been extensively modified and sounds much better for it.

Chuck

Thanks Chuck. I'll give the folks at Mesa a try prior to trying any mods. I assume your mods got rid of what looks like treble peaking stuff at the beginning of the circuit and you did some other mods to change the voicing a bit in the tone stack? How would you characterize the changes and a fuller more even tone?

[Chuck H]

I didn't use the term "extensively" lightly. As you guessed I did change the input arrangement. I also removed a bunch of locl NFB and top end bleeder circuits as well as raising the preamp voltage and changing the power tube bias and grid feed arrangement. The tonestack is about the only thing I didn't change. Or did I...Hmmm. I may have raised the value of the treble cap but I can't remember. I'll look through an old box of misc. schematics tomorrow. I have the schem with mods noted somewhere. If I find it I'll post it. Anyone with a SB would be well off to do these mods to it. I no longer own this amp, but it's partly because of this amp that I recently had an opportunity to do some design work for a big cootie in the biz. He heard the amp and asked about the builder .

Chuck

[drewl]

I've seen them do this to help prevent oscillation/noise on that stage...the wire usually is just bent over the socket. sometimes it's a flat piece of copper tape.

[Chuck H]

Ok...FWIW the Subway Blues schematic is redily available online. When I first got mine you had to call Mesa with a sob story . I don't see any reference to a dangling wire or copper tape. I also never found any such circuit in my amp. It's most likely a loosed connection. You will need to study the schem to find it. But as I said, it's redily available online at several sites.

Here are the modifications I told you about. The amp is improved dramatically with these mods. This is not just proud speak. If you do these mods the amp will sound much, much better.

Alterations are indicated in red. Either save the file or open it in a new window and then magnify it. Otherwise you won't be able to read it.

Chuck

[Euthymia]

I know that it's usually bad form to reanimate a thread from 5 years ago, but since this forum is the Sum of Wisdom for people working on amps, I thought I would add my .02 to clear up the mystery of the dangling grey wire.

The one I'm working on now has that, which is how I found this thread.

I think drewl has it right, it's some kind of Mesa band-aid. Mesas are notorious for sketchy layout leading to parasitic oscillation, so it's likely.

Also, the Hum Reduction Mod, where you swap out a .01uF for a .68uF (per the readily available schematic floating about the www) is really really necessary and reasonably effective. I took the thing halfway apart trying to find what the hummmmm was about, then spotted the mod. I find it rather baffling that a product could make it all the way out the door with such a design flaw, but hey.

[J M Fahey]

Agree.
The unterminated wire is an antenna and is there on purpose

Quite popular Mesa bandaid.

There you got one, many amps same nominal circuit won't have any because they don't need it, and very high gain ones (MBDR and later) may have up to 3 or 4

Some of them are plain antennas, others are laid parallel and very close to a track, or parallel and close to some plastic cap body, in that case I think they work as very low value compensation caps or trimmers, think values such as 1.7pF , etc. (which in a high gain high impedance circuit are significative).

It's an old trick which used to be very popular with Radio Amateurs, they called it (literally) a "gimmick capacitor".
Nowadays rescued by high bandwidth Op Amp designers, go figure.



Note that Ci and Cf are not "component" capacitors but the stray capacitance of tracks.
Actual values are a couple pF .