Crosstalk between gain stages of Mesa Road King II guitar amplifier

cheater

Member
2004-08-14 5:19 pm
Hi guys,
I haven't been to this forum for a very long time so if I'm breaking
any rules please let me know! :)

I have a Mesa Road King II here. It's basically a very souped up Dual
Rectifier. It's not doing what I would like it to be doing.

You can find a schematic here:
http://milas.spb.ru/~kmg/files/schematics/Mesa Boogie/Mesa Boogie Road King II.pdf

The amp is in pretty good condition, however the stages have a lot of
crosstalk. What I mean by this is: if I make the signal run through an
effects loop, and put empty plugs into the send/return, the amp should
be totally quiet, but a lot of sound does come through.
If you look at the schematic, the (two) loops are between V5a and V5b
which is a 12ax7. The sound that comes through is fairly loud
especially compared to normal practice-at-home levels; it's a bit
bypassed. Either way, it means to me that V6 is getting the signal
somehow through a voltage rail or some other way.

What this results in is that when my nice Decimator G-String II gate
cuts out because I've palm muted the guitar, i still get a lot of
string rattle coming through, which pretty much makes the gating
useless. The gate gets its control signal from unprocessed guitar, but
it mutes at the very end, in the last effects chain.

Here's how the leakage responds to the various controls on the amp:

chan 1/2 master: at 0, there is no output. turning up to max makes the
output loud.
chan 3/4 master: at 0, there is noticeable output. turning up to max
adds bass to the leaked signal or seems to change the resonance of
whatever "send level" is doing.
gain: at 0, no leaking. at max, noticeable output.
presence, bass, mid, treble, raw/vintage/modern switch, clean/fat/brit
or clean/fat/tweed switch: no control of leaked signal at all
output: at the lowest setting, there's noticeable output. then as you
go 5% up the output becomes duller and a bit quieter(but there's still
a lot of low signal leaking through, just the high freqs are
attenuated), then as you turn output up it becomes really, really,
really loud.

signal leaks even with tuner mute engaged, but a bit quieter. BTW,
engaging tuner mute results in a really loud popping noise.
send level (on the back) seems to not control the loudness of the leak
at all, but there is a resonance, like a 4th order resonant peaking
EQ, and as you turn the knob, the resonance sweeps frequency. So it's
like the freq knob of a really nasty peaking EQ.

taking out V4 or V5 changes nothing, the leak persists. removing
either V1, V2, V3 or V6 turns off the sound completely, including the
leak, and the usual expected hum/buzz that you get even when the
guitar is perfectly quiet.


I have several questions. I would appreciate it if anyone could help
me with this as it's really making the amp terrible! :(
1. Has anyone come across this kind of issue?
2. How do I fix this?
3. Where would the crosstalk be coming from? Could it be coming from
the preamp tubes' "C" voltage rail? That's the HV rail going to the preamp
tubes. Are there any other places the leak might be occuring?
4. Could this be due to dual tubes being used rather than singles? AKA
could two halves of a tube be leaking to one another?
5. Could this be fixed by re-biasing the tubes in the preamp?
Especially what I assume is a differential push-pull pair made out of
V6a and V6b.
6. Can I decouple the C rail? The idea was to do something like this:
Lushprojects.com - Circuit Simulator.
however I've never worked with tube circuits and don't know how the
tubes might react to this kind of thing. Naturally the values are
wrong but that's the topology.
7. Does anyone know of a good mailing list or forum that talks only
about servicing and modifying Mesa/Boogie amplifiers? Or just in
general only tube guitar amps?

Thanks everyone!
 
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