Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: brewdude on August 05, 2017, 10:09:08 pm
-
Hello all,
I recently bought a t-shirt from AES that had an early schematic printed on it. I went looking to find what circuit it might be and found a copy of it in "The Tube Amp Book, Volume 3" by Aspen Pittman--page 380. However, the diagram's only title is a hand written "Old Bassman".
I went looking through the web via google to see if I could find any reference to an early Fender Bassman that used this peculiar circuit--5U4, choke input PS, pair of 6L6's, 6N7 paraphase PI(?), two 6SJ7 preamp channels (one with 2 inputs and a tone control).
Does anyone have any insights to offer regarding this schematic?
Thanks
Ps: I tried to atatch a photo but my phone seems to take to large a file to submit... I'll work on it.
-
5A6 http://vintagefenderamprepair.com/wp-content/uploads/2012/03/Bassman-5A6-Schematic.pdf (http://vintagefenderamprepair.com/wp-content/uploads/2012/03/Bassman-5A6-Schematic.pdf)
-
Wow, thanks for the quick response.
I don't suppose you know any details or have any experience with this amp, do you?
I would assume this to be the original Fender Bassman?
-
I'm guessing that "50M ohm" is really a 50k resistor? And, similarly for "250M" and "100M"?
What do you suppose the transformer spec's to be? And, how about the choke?
What voltages are likely to be at the plates and screens of both the 6L6's and the 6SJ7's?
I apologize for all the questions, but the schematic gives very few details to satisfy my cuoriosity. By the way, I have been infatuated with choke input PS since reading Merlin Blencowes first book on tube amp power supplies (I have built a couple with great success), but this is one of the few schematics I've seen that utilizes a true choke input PS--most have a cap before the choke, more like a pi-filter. I also have a fondness for 6SJ7's.
-
I have been infatuated with choke input PS since reading Merlin
I'm curious, why?
-
I don't really have a rational explanation. I know of nobody else who is using a similar circuit. However, I am very pleased with the amps I've built with the big ass choke input PS. I know that it's a very uneconomical strategy.
My go to amp sounds great and is fun to play. It's a parallel 12ax7 w/ dissimilarly biased triodes upfront, an early Bandmaster style TB stack, into a 6sj7 pentode with a VVR controlling the screen voltage, feeding a paralleled 12au7 cathodyne PI driving a pair of 6v6's, true choke input PS and a global VVR. It is more of a clean'ish blusey kind of amp, but it's easy for me to get fun overdriven tones at any volume, the controls are intuitive and responsive. It has amazing bass response--in fact bass guitar sounds pretty damn good through it (I generally keep the bass knob around "1")--and never farts out. Plus, it takes pedals well.
I've got another choke input amp which can run either 4x 6v6, or 2x 6L6 and a much more complicated preamp design that I've been working on, but have put aside for a while. It works but has too much gain. I started bypassing gain stages and the active passive fx loop as well as removing cathode bypass caps etc. to get it under control... it works, but it's not quite what I want it to be yet. One of these days I will get back to it.
I've got another project on the bench right now--an all 7 pin tube single channel amp w/out tremolo inspired by an early Gibson GA40 schematic squeezed into a Valve Jr. size chassis.
-
I don't really have a rational explanation. I know of nobody else who is using a similar circuit. However, I am very pleased with the amps I've built with the big ass choke input PS. I know that it's a very uneconomical strategy.
That's good enough for me. I like using octal preamp tubes partly for the way they sound, and mostly because they're easier to wire :icon_biggrin: and the tubes look cooler than the 12__7's. I have gone back to using 12au7 for the SS reverb buffer tube though, the 6SL7 just injects too much noise into the reverb circuit. Or maybe my design has a flaw. ;)
-
That's an oldie
I added to our schematic library
http://el34world.com/charts/TubeAmpSchematics.htm (http://el34world.com/charts/TubeAmpSchematics.htm)
-
> transformer spec's to be?
It's South California, 1949? The LA basin is full of electronics shops that did radios through the Depression, Mil-stuff through WWII, and again radio etc post-war.
Big OTs suited for 2-6L6 are less common and expensive. Little OTs for two 6F6/6V6 are everywhere in abundance.
I think Leo saw $19 for a 2-6L6 or $4 for 2-6V6 iron. For a small guy with no capital, easy choice.
Likewise for the 6N7 which was going out of style.
I'd think 10KCT winding because it fits the page 2 250V condition (http://www.mif.pg.gda.pl/homepages/frank/sheets/021/6/6L6.pdf). But Leo may not have been fussy.
There's way too much gain in that first stage, for modern guitar pickups. It was a different world back then, and bass. Aside from AmPeg and a few others, bass was mostly dynamic mike on the bridge.
-
That's called the "Bassman (old)" in Aspen Pittman's schematics
-
Regarding the choice of transformers:
PT: I presume that this would be something like 300-0-300v? Maybe 200mA? With a choke input PS one might expect about 270v at the first node?
Choke: It would need to be able to handle the entire current, so also 200mA? How about inductance, maybe 10H?
OT's: So, two 10k/8R in parallel on the primary side, each feeding it's own 8R speaker? Would this be equivalent to 5k a-a?
Regarding the inputs/preamp:
The first (top) input has no grid stopper?
The second has two inputs each with 50k grid stoppers and no grid leak? I suppose that if you plugged into only one of these inputs the other's grid stopper would act as a grid leak and the combo would be a 50% voltage divider? If one was to plug into both inputs, there would be no reference to ground?