Hi,

I have a Marshall Mosfet Lead 100 guitar amp (3210), and when I switch to the boost channel while playing, there's an audible pop (also if connected directly to a mixer from the DI in the back). The other way (boost->normal) there's no pop if I switch while playing. Also, normal->boost there's no pop if I'm not playing.

What I want to ask is, is this expected behaviour?

So, I did some breadboarding today to find collector resistors for my BC337's. I've found a set of 4 that bias very close to 4.5 VDC with a 22k in the collector.
I didn't bother to look for the 5th one because it's an emitter follower. Should I do it? Also, should I increase the value of the buffer stage's emitter resistor from 10k to 22k?
Next step is to put the whole thing together on the breadboard.

Just for the record, I'm attaching the datasheet for the TIS58.  It's pinout does not match the interfet 2N5484, nor the on-semi or centralab 2N5484.  So that's 3 possible pinouts for the Randall Fet's.   

From the voltages you listed, it would seem that T4 collector is connected to T6 base, as you have the same voltage listed for them.
But T4 collector should go to T6 emitter, so once again the voltages listed are questionable.

Thanks so much Loudthud!  I swapped those JFET leads and we now have audio!  Thanks to everyone who contributed to this thread!

Quote from: Umlaut on May 21, 2024, 03:18:58 AMGK schematics are an extremely useful resource on how to bias and design jFet gain stages, btw. Most if not all of their guitar amps from the late 70s on (and a lot of their bass amps) feature jfets in one form or another.

So much to learn, so little time... 

Yes, bipolar supply. Look for Gallien-Krueger 200RG model (and similar very early 80s models) schematics to see where that particular gain stage originates : ) they use J113, but for these particular applications, depletion mosfets behave like jfets.
That stage CAN be built to single supply, but then we'd have to tinker with an additional Vref, and since I am running the whole preamp on bipolar, there's absolutely no need to go down that rabbit hole.
GK schematics are an extremely useful resource on how to bias and design jFet gain stages, btw. Most if not all of their guitar amps from the late 70s on (and a lot of their bass amps) feature jfets in one form or another.

That's an interesting idea you've got there. Say, are you running those LND150's at +/- 15V? Am I reading the circuit diagram correctly?

Thanks a lot. I really appreciate your help and support. I'll keep you posted on my progress.
M.

Yes, I did use 2N5089. I tried 2N3904 but it did not work as well
BC550 should give you a similar result to 89s though.
Bear in mind that I tinkered with it for less than 1h, so it was a bit of a summary test.
As for the voicing, I'd say marshall-ish with a raw, unfiltered edge to it. Into the right amp it can sound quite nice (think Black Sabbath, Pentagram, etc), but I prefer a bit more filtering and tone-shaping. While it sounds quite full and "amp-like" (not a term that I like, but...), and it has a nicer feel than your standard boss disto-box, I would still be partial to other multi-stage pedals like the AMT Legend ones, for example (check on FSB for schematics).
Oh, I tested it on the clean channel of my Peavey Bandit silver stripe, low input and bright switch off, which is an amazing pedal platform IMO.
Yesterday I was fiddling with a distortion circuit of mine which is quite similar in its topology and voicing, but using LND150 depletion mosfets. To my ears, they sound much more pleasant than BJTs in this kind of cascading gain stage circuits. I will probably post it at some point in the coming days, as I am pleased enough with it to share it 
Again, breadboard and see for yourself! lotsa fun can be had with this circuit, whether you tinker with the values or not.