Crosstalk between gain stages of Mesa Road King II tube amp

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Crosstalk between gain stages of Mesa Road King II tube amp

Postby cheater » Fri Feb 05, 2016 10:05 am

Hi guys,
I'm new to this forum 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%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 occurring?
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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Re: Crosstalk between gain stages of Mesa Road King II tube

Postby Geek » Fri Feb 05, 2016 1:24 pm

Common problemI found in old AND new amps. Seems even new amps will use top-notch everything, except electrolytics.... which likely fell off a reject truck headed for the landfill :/

Your PDF - Page 4, parts C71, C7, 8, 16, C5, 6, C3 and C9. Replace them with known good caps - Cornell-Dubilier, Nichicon CS or GU series, Panasonic ED or EE, etc. Don't be afraid to put two 350V ones in series with 220K/1W equalizing resistors across each.

What you are trying to accomplish is getting the ESR (resistance value only) and impedance (broadband resistance +/-reactance) down to eliminate the crosstalk through the supply rail. The cap values may measure OK, but their impedance may be through the roof. Why I immediately trash brands like Illinois Capacitor PSU caps upon sight... fine cap for some things, but c'mon, 22 ohms of ESR fresh off the factory floor? =:o

Cheers!
-= Gregg =-
Fine wine comes in glass bottles, not plastic sacks. Therefore the finer electrons are also found in glass bottles.
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Re: Crosstalk between gain stages of Mesa Road King II tube

Postby cheater » Fri Feb 05, 2016 3:58 pm

Hi Gregg, thanks a lot for your reply! Yeah - I'll try improving upon the voltage rail.

I didn't quite understand the part with the equalizing resistors. Is it something like the attached image?

If not, would you mind whipping up a quick schematic in circuitjs? http://lushprojects.com/circuitjs/circuitjs.html

What ESR value do you think I should settle for?

Thanks
Attachments
capacitor ladder with equalizing resistors.jpg
capacitor ladder with equalizing resistors.jpg (10.91 KiB) Viewed 1958 times
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Re: Crosstalk between gain stages of Mesa Road King II tube

Postby Geek » Sat Feb 06, 2016 12:03 am

Yeah. A stack like C72 and C73 on the same page of the scematic is fine.

I targer an ESR under 2 ohms if possible. If you have room, you can use caps in the milliohm range.

Digikey part number 493-5670-ND, a Nichicon PS series is a good cap to use. Non-inductive and low ESR well out to almost a MHz.

Cheers!
-= Gregg =-
Fine wine comes in glass bottles, not plastic sacks. Therefore the finer electrons are also found in glass bottles.
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Re: Crosstalk between gain stages of Mesa Road King II tube

Postby cheater » Sat Feb 06, 2016 6:44 am

Thanks, I'll check out the Nichicon. Thank you for the part number, that'll save a lot of time!

Here's one thing you might try that I find useful. Rather than make the caps for the whole rail huge and super low esr, make sure whatever draws rapidly changing/noisy current is decoupled locally. Say it's a single chip, it should have a capacitor across Vdd/GND and the cap+chip should be fed by a resistor that's an order of magnitude higher than the capacitor's ESR. This way you can use small capacitors, and things actually work better than with huge caps and no such local decoupling. I did that with the capacitors I could get locally, I can only imagine how good they were, and how much better the Nichicon you suggest might be. I remember when denoising one circuit used "ok" caps that I believe had ~5 Ohm ESR with a resistor that was 100 ohms. That was LED matrix switching noise and it was getting into a high gain audio generator circuit which was then also put into a distortion box. There was a very audible buzz beforehand, and nothing at all after. I first used several "C" cell sized capacitors first, and then replaced them with a tiny electrolytic cap + resistor.
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