[sdiy] [AH] Crosstalk between stages of Mesa Road King II guitar tube amplifier

[cheater00 .]

Thanks, I've posted there.

On Fri, Feb 5, 2016 at 2:42 AM, KHeck73 <kheck73 at aol.com> wrote:
> You might also try the audiokarma.org forums. A very helpful and civilized
> list. If you register, they'll even wish you a Happy Birthday every year!
> -Karl.
>
>
>
> -----Original Message-----
> From: cheater00 . <cheater00 at gmail.com>
> To: funwithtubes <funwithtubes at yahoogroups.com>; Analogue Heaven
> <analogue at hyperreal.org>; synth-diy <synth-diy at dropmix.xs4all.nl>
> Sent: Thu, Feb 4, 2016 6:14 pm
> Subject: [AH] Crosstalk between stages of Mesa Road King II guitar tube
> amplifier
>
> Hi guys,
> 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%20Boogie/Mesa%20Boogie%20Road%20King%20II.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:
> 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? Are there any other places?
> 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:
> http://tinyurl.com/j35c7dq
> 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