Tube Amp Archive

Hi oggiedog! The bright switch can be any of a whole bunch of miniature toggles available from Mouser. The Express/Liverpools used an SPDT switch with a center off position for this job. I've had good results with the 633-M201301, though you might also want to consider using a DPDT switch, not becau...

I've got the SLO PCB Layout .pdfs, but the forum software won't allow me to post them... I can e-mail the three files, should anyone want to take a look.

Matt

Hi Charliewired! The University of Victoria's McPherson Library has a copy of the Radiotron Designer's Handbook that's due back on the 30th of April, 2005, and I'd bet they've got a few other great books on tube design and theory there as well. A person not affiliated with the University can get a l...

Hey James! That's a very good question! I wasn't really sure which models of the small Mesa amps have the two parallel rectifier tubes, but I guess in theory that it might have something to do with Boogie's propensity to use large plate filter capacitance (110uF, usually). It might also be just to c...

Hi Billy! Since the problem you're describing goes away when you ground the second input of the PI across the presence control, you can be pretty sure the fault has something to do with the negative feedback. I'd suggest checking the routing of that feedback wire first (try to get it as far away fro...

Hey Omar! My mistake, it became pretty obvious what you meant when I re-read the question ;) You're right on the money about the use of an insulated terminal for that job in most military equipment. I don't have any shots of these standoff terminals still in place any military gear (as they're one o...

Hey Mark! I'm not sure what common format I could use that has that feature (Quicktime maybe?), but if you've got any ideas I'd be glad to give it a shot! It's pretty easy to render any specific point of view, make a part transparent or anything else along those lines, so if there's a specific detai...

Hey Omar! Here's the way I do it, ripped from MIL-HNBK-2000 Soldering handbook. http://img88.exs.cx/img88/436/soldersocket2fo.gif Just tacking it through the hole isn't really something I'd use on someone elses amp, but it's fine for trying something out in a pinch. The solder shouldn't have to prov...

Hi Eric! Thanks for the photos! Those voltages do seem a little low to me, as normally the 600Vct. specified for the B+ winding is at the rated current draw while in this case you're getting somewhere closer to 545Vct. at whatever fraction of the total load normal operation is. A good way to check i...

Hey Eric! I'm glad to hear it worked out well. To bias your amp, and in this I'm assuming your plate voltage is about 410V with EL34s, you can adjust the bias to be anywhere up to about 42mA per tube (the ~70% plate dissipation limit). What I'd suggest would be to start at about 30mA and play a litt...

Hi again, Peter!

That's a great idea when using the JJ's! I formed my personal opinion on that particular trick when using the somewhat bright SED's, but I can really see how that might be just the ticket with the darker tubes. I'm really looking forward to trying that out. Thanks!

Matt

Hi again! I'm also surprised this is the first time anyone's mentioned having any problems with the bias circuit in A1a as well! Once you get a nice solid bias voltage and it'll fire up safely (start with the most negative voltage setting), you can check the feedback issue by just shorting the outer...

Hey Pete! Thanks for the correction, that's a very good point you make about tying the suppressor to the bias supply. It does provide better reliability in the EL34, but at the same time it does change the sound a bit, so I guess it's really something a person should try for themselves to see if the...

Hi Eric! I'm fairly sure I know what your bias problem is, and I really should have brought this up sooner, but I think the flaw is in the schematic diagram itself. I don't calculate anywhere near an adequate bias voltage from the circuit in the A1a schematic. Has anyone used this and been sucessful...

Hi Eric! That's not a dumb question! The screen grid is on pin 4, so that connects to B+2, the control grid is on pin 5 (which you've wired right), and the suppressor grid is on pin 1. Connect a jumper between pin 1 and the cathode at pin 8, ground the two of them together, and you're all set. Good ...