Looking for Supro Thunderbolt 6420 Build info

[mikepukmel]

Pulling my hair out to try to figure out how this amp would work without shorting the PT secondary. Going back and forth between the various versions of this amp schematic, I think the CT should not be grounded. Wouldn't it provide a path to a dead short across one diode, depending on which half cycle?




"It must be right, is on the internet"

[soundmasterg]

Don't use that schematic. The ones from Sean Weatherford are known to be the most correct. That schematic you posted, and a couple unreadable others were why Sean created his in the first place.

Greg

[nickb]

Pulling my hair out to try to figure out how this amp would work without shorting the PT secondary. Going back and forth between the various versions of this amp schematic, I think the CT should not be grounded. Wouldn't it provide a path to a dead short across one diode, depending on which half cycle?

[ATTACH=CONFIG]51928[/ATTACH]


"It must be right, is on the internet"

You are correct. It's wrong.

[g1]

Don't use that schematic. The ones from Sean Weatherford are known to be the most correct.

And here's the link:
http://www.angelfire.com/mech/beansamps/supro/supro.htm

[mikepukmel]

Thanks again for the angelfire link. Great info in the Thunderbolt there.
Maybe CJ Poulos put that in there for anyone who breaks his copyright.

[mikepukmel]

I don't want my epitaph to read "Build an amp. Lasted exactly one complete AC cycle before the fire."

[mikepukmel]

The 1k voltage dropper resistor in the PS chain is no (common) screen resistor. It does not allow the screen voltage to change with signal/screen current. The primary purpose of power tube screen resistors is to limit screen current at low instantaneous plate voltages/high plate currents. This way average screen dissipation is kept low.
They might not have been necessary in this design at least as long as the power stage is not driven into clipping, which was probably considered abuse in the old days.
Apart from the protection effect screen resistors reduce tube distortion as well as gain by local NFB. Also available clean power will be a little lower.

Reading and re-reading all the supro posts, thanks again Helmholtz. Its the funny thing about these amps: original design was not intended for overdrive, but just about all usages Ive seen use these amps for their unique overdrive character. So (with my limited electronics knowledge) have gone in circles a few times. First, original amps had no screen resistors (reasons listed above by several posters), but those days, the tubes were built like cars were: heavy duty, could take much abuse. Todays tubes, meh at best. Goal is to abuse this amp well into overdrive, to see what the supro overdrive sounds like. So, protect the amp circuit so it won't blow up i my face would be nice, but not so that it changes sound character to a different amp. If it *is* driven well into clipping, then, it should have some resistor on the screens?

[Helmholtz]

it *is* driven well into clipping, then, it should have some resistor on the screens?

Definitey. Screen resistors preserve the screens from excessive dissipation and will extend tube life. Most great clipping amps have them: '59 Bassman, JTM45.......
I don't understand why you didn't try a couple 470R in the meantime and see what sound difference they make. You can always short them for comparison. JUst watch for "red screening".

[mikepukmel]

Definitey. Screen resistors preserve the screens from excessive dissipation and will extend tube life. Most great clipping amps have them: '59 Bassman, JTM45.......
I don't understand why you didn't try a couple 470R in the meantime and see what sound difference they make. You can always short them for comparison. JUst watch for "red screening".

Hi Helmholtz, thanks!! Well, its not built yet. I started collecting info last year, than ran into a whole cascade of household repairs to do (still not finished). But its next on my list. Will definitely try with and without 470R.

[mikepukmel]

Just one thought I had (late in the week, running low on thought power) is to make sure the lead to V2's first grid is shielded - pretty low level on that signal wire.

edit: OK, a couple more thoughts. Don't be afraid to put a 10k grid stopper on the first couple stages. And related to that, what's the function of the 47k resistor from the input? If it's a grid stop, then put it right on the socket. The cap after it confuses me as to its intended purpose.

Re the 0.05 cap after the two 47k resistors on the inputs, I don't even know enough to ask why its there. I did check a few build photos that i could get and there are one or two other schematics floating around, might be from the same source, but they all have it. So, there are things that affect tone, and then stuff that doesn't affect tone, but noise, rf interference etc. And, maybe something to do with input matching to certain pickups, etc. Am I wrong in guessing that the components on the inpt side of the input tube are mostly not related to tone? Grid stops (even if put in the wrong place), grid leak, and that odd cap?

[soundmasterg]

Re the 0.05 cap after the two 47k resistors on the inputs, I don't even know enough to ask why its there. I did check a few build photos that i could get and there are one or two other schematics floating around, might be from the same source, but they all have it. So, there are things that affect tone, and then stuff that doesn't affect tone, but noise, rf interference etc. And, maybe something to do with input matching to certain pickups, etc. Am I wrong in guessing that the components on the inpt side of the input tube are mostly not related to tone? Grid stops (even if put in the wrong place), grid leak, and that odd cap?

I would just duplicate the parts that they used if you want that sound. A large part of the sound of those amps is the unbalanced paraphase inverter, but even something as simple as using ceramic caps like the originals vs PIO caps in their place would change the sound of the amp. We noticed when compiling the schematic that using a 240 ohm cathode resistor instead of the stock 200 ohm made the tubes happier and would likely help them to last longer, but the sound of the amp was less rich and colder sounding with less harmonic richness. The owner of the amps I worked on insisted on the stock 200 ohm value in all three amps.

Greg

P.S. These amps sound better with 10 inch speakers than the stock 15 inch speaker. I'd experiment if I were you, but that is what I found.

[mikepukmel]

Cool, thanks Greg, and thanks for the tips on speaker and stock parts. Can't wait to get done with some house repairs and get this amp running.

[tubeswell]

I've built a few tbolt 6420 amp circuits. I like 'em with a B+ around 400 and 5881s running off a shared 270R cathode resistor that is fully bypassed. The spare triode is useful for goosing the front end up, like a switchable single-parallel input stage. (see schematic). The cathode-bias vibe is cool, but they are not the loudest of amps (you can get more oomph from the same number of tubes if you build a 1-channel AB763 circuit sans reverb and tremolo).

The James tonestack in the attached schematic works well. The treble and bass controls have a wide range and a decent mid scoop with the controls at full and relatively flat with both controls at about mid rotation.

Or build it how you want. YMMV

[Helmholtz]

a shared 270R cathode resistor that is fully bypassed

Your circuit doesn't show the bypass cap

[tubeswell]

Your circuit doesn't show the bypass cap

Fixed :-)