Hey, Psabin8951,
I'm no expert, but a couple of things popped in my mind.
Which fuses does it blow - primary or secondary?
As I understand Mesa Dual Rectifiers, it should have a rectifier selector switch. Does the amp do that with solid-state rectifier selected?
Could it be a short on the primary side of the power transformer? 🤔
Like DrGonz78 said, a schematic and voltage readings would help.

You need to be super careful and you must know what you're doing if you ever decide to tinker with it yourself. As you probably know, there are lethal voltages inside that thing. If uncertain, take it to a tech.

Quote from: Umlaut on Today at 04:18:37 PMOh, and ref. biasing, nothing wrong with using 2 resistors in series to get to the right bias either.

The 337's I tried today were very consistent, or maybe that's just my limited experience. Out of the 15-ish pieces I tested, one biased at 4.44 V, the five I selected are between 4.51 and 4.55 V, and the rest gave a reading of around 4.6 V. All that with 1k emitter resistor, 22k collector resistor, and the 4.7M / 1N4148 / 1M network connected to the base, as per Joe Davisson's schematic.

Yeah, I meant the last stage, the emitter follower. I'll leave the load resistor at 10k then.
Speaking of the 3-stage BJT version, I kinda know how that thing sounds because there is a video of it on YouTube and, indeed, it does display the quality you described earlier. Still, I'd like to (and am going to) try the 4-stage preamp because I'm curious what it can do. There are no clips that I was able to find so I might become the first one to upload some
I started putting the circuit together on my breadboard, but I needed to take it apart halfway through because a lot of the connectors were overly loose, so I figured I'd retension them. Should have checked that first...
I might have some time to get back to working on it this weekend.

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.

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... 

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.

Hey, Umlaut.
It's great that you've been able to breadboard and play it. Thanks a lot for all your input. It's going to save me, and possibly others, some time.
My parts came in this afternoon and I'm looking forward to breadboarding it myself, but it's going to have to wait a bit. Hopefully, no longer than two weeks.
Oh, and I found some BC550's the other day. I might give them a whirl too.

I also have a couple of questions:
1. Did you use the 2N5089 transistors?
2. Now that you have some hands-on (and ears-on) experience, how to would you describe the voicing of this preamp?

M.

Thanks for all the info and hints. I'm about to order the parts that I don't have in my stash and as soon as they arrive, I'll start breadboarding. Provided nothing unexpected gets in the way, that is.
As far as the higher voltage goes, I happen to have an LT1054-based voltage doubler put together, so I'll give it a try at roughly 18V too.
Have a good one and stay tuned!
M

Thanks for the reply, Umlaut.
Looking at the schematic, I also expect it to sound like a Marshall on steroids. Like a DSL or a Bogner Ecstasy perhaps?
Speaking of snubber caps, do you mean in parallel with collector resistors?

Hi, everyone, first build-related post of mine   
Soon, I'm going to build Joe Davisson's 4-stage Vulcan preamp using the schematic posted on his website and I have a few questions before I embark on this project.
1. Has anyone tried it?
2. If so, how does it sound? Judging by the number of gain stages, it's a hard rock / heavy metal machine.
3. I'm going to use BC337 transistors because I have a lot of them (not much of anything else tbh). Apart from pinout and biasing, is there anything I should specifically pay attention to?
I'm going to breadboard it first and, of course, I'll keep the forum updated on the progress. That, however, may take a while because adult life is the way it is
Stay tuned.