Crosstalk between gain stages of Mesa Road King II guitar amplifier

[cheater]

Hi guys,
I'm new to this forum so if I'm breaking any rules please let me know! This post is about a Mesa amp, so it might be a bit off topic - but one of the users assured me I should come here and register and ask.


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. 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!

[jjasilli]

We are not afraid of Mesa Boogie amps.  I suggest that you think through, simplify and untangle your presentation: 


"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. . . 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."


This is too much inconsistent info.  E.g.:  Crosstalk between gain stages inside the amp is different from the rest of the things you present.  Signal cannot be going through the effects loop if there are empty plugs in the jacks.    Palm muting the guitar and use of an effects box are not relevant to crosstalk inside the amp.


Detach all outside gear from the amp.  Put signal (guitar, or preferably a signal generator, cd player, radio) into one channel at a time of the amp.  Does that signal come out of other channels?

[cheater]

Hi jjasilli! Thanks for your reply!

I understand what I described is confusing. Perhaps it makes more sense when I tell you that the G-String II is inserted in the effects loop, and that's where it attenuates the signal.

In my configuration, the signal *is* going to loop 2. When the gate is at 100% attenuation, nothing should be coming back into its return jack, aka "return 2". In order to exclude the gate as a source of issues, I just inserted an empty jack into send 2 and return 2, to simulate "full attenuation".

Regarding your question about leaking between channels. There aren't really separate channels in the amp. It uses a switching system. You can see that on the second page of the schematics diagram. There is only one input, so it's impossible to plug the guitar to one channel and listen to another channel as you say. My issue is that signal from V1, V2, and V3 is leaking into V6, even if the audio path between the first three and V6 is interrupted. This means that the headroom is reduced by a lot, meaning you can't get the desired dynamics a gate would normally deliver.

[cheater]

If you look at the drawing in the upper right of the first page here, that's the way I'm hooking up the gate to my amp.

http://www.isptechnologies.com/manuals/Decimator%20II%20G%20String%20Manual.pdf

but to make things simpler during debugging, I leave out the gate completely, and insert an "open" into the effects loop (that is, empty plugs). The guitar might as well be connected directly to the guitar input, but I use a boost box to exacerbate the issue, because the amp needs to perform its duty with the boost in place.

[birt]

so the problem you have is that when you interrupt the signal at the FX loop (either 1 or 2 or both) you still hear the signal in your speakers. so a part of it is present in the power amp.




the problem is you don't understand the working of the FX loop circuit in this amp so you can't find out why it is behaving this way.
(i don't understand it either, too much silicon going on in there for my knowledge. i just wanted to make sure i understand your question)

[sluckey]

I don't suspect the tube circuits or "C" rail as causing the symptom you describe. I think it's a switching issue (FETs) or switch control issue. I see a couple FETs directly across the send/return jacks that could leak signal around open jacks.

But, I'm not convinced that the loops are not operating as designed. There is a user's manual available on Mesa's website. If you don't have a hard copy, it would be worth downloading and reading. This is not a plug 'n play amp. You do need the instruction manual. It could be that you don't know what to expect from all the switching. You may just be a switch flip away from solving the issue, or at least understanding it.

[cheater]

Slucky, the switches are set in the right way. Each effects loop is either switched in or not, and this one is switched in. The JFETs are not incorrectly switching. They are switching fine. One could think they are leaking, but no: if I remove V5 completely, the same leak exists. This means the JFETs on the effects chains are not involved in this.