[sdiy] JFET question - Sound leak in Mesa Road King II amplifier

Thu Sep 7 16:42:44 CEST 2017

Yeah, I have one of those, what would I be looking for when testing the
JFETs?

Do you think they might be leaking the soubd around the circuit like i
described above? (Signal chain -> JFET 1 drain -> JFET 1 Gate -> JFET 2
Gate -> JFET 2 Drain -> Signal chain later)

On Thu, 7 Sep 2017 09:10 Barry Klein <barryklein at cox.net> wrote:

> I don't think jfets have a body diode, but there is a max. gate current
> spec
> of 50mA so a series resistor with each jfet gate might help out.  Could be
> an unexpected surge current on the drain too given so many things are being
> switched...
> There are these cool component testers like for 20 bucks on ebay that will
> test these parts quickly out of circuit.  I bought the American first
> version, called the m3, and it has saved me a heck of a lot of time on
> things like this.
>
> Barry
>
>
> -----Original Message-----
> From: cheater00 cheater00
> Sent: Wednesday, September 06, 2017 6:15 PM
> To: synth-diy mailing list
> Subject: [sdiy] JFET question - Sound leak in Mesa Road King II amplifier
>
> Hi guys,
> I've been trying to figure this one out for a year or so, but it's been
> elusive.
>
> I have a Mesa Road King II tube amp. When I turn the gain all the way
> down, there is still sound coming out. It's a slightly high-passed
> version of the input. I even put a cable into the effect loop return,
> with the other end of the cable disconnected - and sound was still
> coming through. The cable should have made sure no audio comes through
> from the peramp and input. Yet I still get room level output when the
> head is connected to a Mesa oversized 4x12.
>
> At first I thought it might have been the tubes coupling via their
> voltage rails, but I haven't really tested that idea, because
> decoupling each tube would be expensive (in part cost) and also
> technically complex and a huge butcher job on the PCB.
>
> I thought maybe the capacitors have dried out, but they seem ok-ish,
> maybe a wee bit swollen, but they just might have been manufactured
> that way.
>
> I stumbled upon this video today, with a guy debugging an issue in a
> Roadster, which is a scaled down Road King:
> https://www.youtube.com/watch?v=4kNMtOmT76U
>
> here he works on the JFETs used for muting the sound. Turns out he
> shorted some pads on one of them and that made the whole amp silent.
>
> Here are the relevant sections of the RKII schematics:
> https://imgur.com/a/astMS
>
> You will see some J175's connecting from the signal chain to ground,
> with the gate seemingly disconnected. All those gates are tied
> together and connected to the pop reduction circuit ("mute circuit",
> seen in the third diagram on the bottom right). The RKII uses a lot of
> relays, and when they switch, normally there would be a loud pop in
> the output - so during that time, the J175's momentarily short the
> signal to ground, to make sure the pop doesn't propagate (and possibly
> sustain due to built in spring reverb or outboard). The J175's are
> controlled by the 6426 in the lower right corner of the third diagram,
> but I'm not sure how that works exactly.
>
> If you look at the comments of the video, you will see user mrkv4k
> comments on the way the J175s are used. He says they are bound to be
> damaged because the internal (parasitic) body diode gets biased too
> much in this design. As a consequence, those JFETs die often and often
> need to be replaced. And it is true, they often get replaced by the
> techs. I didn't fully understand his explanation of using N-channel
> transistors instead of P-channel. Could someone explain this to me?
> Here are the comments:
>
> """
> mrkv4k:
> > This is the weirdest design with JFETs I have ever seen. IMHO they are
> > literary boud to malfunction at some point, because the method they
> > choosed to switch them On is just wrong!
> > You should never use low impedance to ground to switch p-chanel JFETs,
> > because this way, they are gonna be damaged every time it's drain gets
> > above 1V.
> > If you wanna ensure that they won't be damaged, and you don't wanna
> > rebuild the mute circuit with PNP instead of 6426, add one 4k7 resistor
> > between R128 and colector of NPN 6426 (on the transistor side).
>
> The Guitologist:
> > Maybe there's a job at Mesa in your future.
>
> mrkv4k:
> > I doubt that :D I am thousands of miles away and I design HW for acces
> > control systems (mostly RFID time-clocks). Building Tube amps is just a
> > hobby.
> > I think that the problem is that more and more engineers design with some
> > simulation programs and they don't test extreme conditions on the real
> > thing. And some engineers are not aware of the fail-modes of basic
> > components, like with those JFETs, you learn how it works under standard
> > conditions (applying positive voltage on gate of p-JFET is going to
> > "shrink" the conductive channel), but ommit the non-standard conditions
> > (if you look at the schematic of JFET, there is an arrow on the gate,
> > which is actual "parasitic"/body diode that shouldn't be ever polarized
> in
> > conducting direction - and that is what can happen in this design).
>
> The Guitologist:
> > You would know better than I. I appreciate you sharing your expertise
> > here. Something to chew on for sure. So your suggested mod for this Mesa
> > Boogie Mute Circuit would be to add one 4k7 resistor between R128 and
> > colector of NPN 6426 (on the transistor side)? And this should lengthen
> > the life of the JFETs?
>
> mrkv4k:
> > Yes, if the problem is what I think it is, than it will help protect
> those
> > JFETs. Also, I checked  the numbers once more and I think 470R would be
> > better, since that is sufficient to supress that effect and it won't slow
> > down shutdown of  RM2. So I would check this solution, to see if the mute
> > circuit works fine. Other way would be to put those resistors to each
> > gate.
> > What I think happends is that when you try to mute while there is high
> > signal (like when you switch on effect while playing), gates of JFETs
> are
> > grounded via  low resistance of that NPN transitor. Since drain is at
> high
> > voltage, you will get spike of current going throught the body diode
> (just
> > until it's muted). Those body diodes are small and can handle only pulse
> > up to 50mA.
> """
>
>
> My question is: could those JFETs - even if they are in OK-ish
> condition (remember that's a really old amp with old, possibly crappy,
> silicon in them), could they be leaking sound from the input (see
> first schematic diagram, just next to guitar input jack), through the
> gate, to the common gate "rail", and then to the gate of the J175MU on
> the very right of the second schematic diagram and subsequently
> through that chip to the power amp (V6B grid, part of the power amp's
> input pair)? This is all in the JFETs' "off" state, where they are
> *not* shorting the signal path to ground.
>
> If I were to replace those JFETs, how could I limit this behavior?
>
> Here are a few ideas - please let me know if they are sound:
> - add 470 to 4k7 resistors in series with each JFET's gate. This
> resistor would be placed between each gate and the bus that is common
> to all gates.
> - in addition to that resistor, add a capacitor from the junction of
> the resistor and the JFET's gate to ground to make sure no sound makes
> it into the gate rail. The gate itself is mostly a DC signal so that
> should be blocked by the capacitor, consequently arriving at the gate
> pin of the JFET, with a rounded leading edge. (please correct me if
> I'm wrong).
> - use a different transistor, maybe N-channel JFET instead of
> P-channel, but I don't know how I would do that
>
> I don't really know what characteristics of a JFET I would be looking
> for. I had a stroll through the J175's data sheet (eg this one:
> http://www.jameco.com/Jameco/Products/ProdDS/992409.pdf ) but I'm not
> sure what number I would be looking to improve upon over what the J175
> provides. Does anyone have tips?
>
> Another user commented on possible FET replacements:
>
> """
> Evil Gremlin:
> > Seen same JFET issue. Replaced with more powerful MOSFETs and adjusted
> > gate voltage, so little static wouldn't be an issue in future. It's just
> > pull-downs, so basically you can put anything here. JFET, MOSFET,
> bipolar,
> > IGBT - whatever perverted fantasies you have :) It's just p-channels
> don't
> > require any other components.
>
> The Guitologist:
> > Did you have to adjust the values of the timing capacitors in the mute
> > circuit also? If you're changing voltage, it seems you might need to in
> > order for the mute not to last too long or too short.
>
> Evil Gremlin:
> > Nope, got lucky. Well, in theory mute time became slightly longer,
> because
> > of higher gate capacitance, but not noticeably :)
> """
>
> Not sure how to select a transistor like this. How would I make sure
> the transistor is a drop-in replacement and doesn't need me to change
> the timing capacitors? Would I look for the same gate capacitance as
> J175? Some other thing? What did The Guitologist mean with changing
> voltage?
>
> Thanks a lot. Any tips would be highly appreciated!
>
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