Looking for Supro Thunderbolt 6420 Build info

[mikepukmel]

Dirty girl amps used to have some Supro schematics, not sure if 6420 was there.

Cool, thanks Mozz, will check it out. Dirty Girl amps

[mikepukmel]

I found this layout in the travels around the web. As far as I can tell, the connections look correct. A couple of things I picked up (think Im right here?):

(3) grid stop not on the input tube,
(7),(4), (4) connections probably should not zig zag across the board,
(1) , (1) put the output tube grid stops on the socket pins,
(2) put the screen grid resistors on the socket pins and use separate resistors for each tube (don't need such a honking big resistor and better balance).

I have a board from another project (got the wrong one), that has a bunch of turrets installed. I'll scan and put this layout on top of that and see how it goes. The board is bigger than this schematic, so I can probably fit 2 of the filter caps on the board as well.

[Helmholtz]

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.

[soundmasterg]

The schematics that Jazz P Bass shared are from the schematic section at the Hoffman amps web page, which also has an excellent forum. https://hoffmanamps.com/

There are also several threads there and at the AX84 forum where people did Supro Thunderbolt builds back in the day when Sean Weatherford first posted the schematic. The newest schematic for the tube rectified version and for the solid state version will be the correct ones to go by as there were several revisions.http://ax84.com/

You would have to search for the threads as it has been close to a decade since the guys were doing these builds.

Those readable schematics were put together by Sean Weatherford, and I and some others helped him with these schematics. The TR in the file name indicates tube rectifier. The early amps had a tube rectifier and the later ones had a solid state one. I prefer the solid state one as the amps are already pretty loose and saggy on the bottom end in stock form. I based my input to the schematics on three original Supro Thunderbolts from a friend of mine that I overhauled. On the schematic it lists the correct Heyboer power transformer to use for the solid state version. The original amps were true PTP, with terminal strips to connect all the parts. The OT was secured under the chassis in the middle. There is a odd value for a filter cap stage in the preamp, if I recall correctly it is 0.01uF, which seems very small. The original amps also used large ceramic coupling caps, which would contribute to the sound of the amps. The T-bolt uses an unbalanced paraphase inverter, and very hot biased 6L6's. The Sovtek 5881 actually works well and sounds good with this amp, and is a good choice because this amp will chew through power tubes pretty quickly and those are reliable and cheap.

I'll see if I can post some pics that I took of the originals a bit later this evening for you. They aren't the best pixel quality as they were taken when digital cameras weren't the greatest, but they should give you an idea of how they were made originally.

Greg

[mikepukmel]

Cool info, thanks everyone.

[mikepukmel]

I can make out most of the parts. But not at all sure what those 0.05uf (that's what they should be) paper caps are. Are those electrolytic caps? They're banded, so are they polarized?



Also, what are those black caps used on the cathodes. They look newer than the other stuff in this amp.

[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.

Its part of what is so cool about this whole thing. The designers/builders of many of these amps surely thought 'no one would put the volume on 8!", but the rock guitar guys loved the sound of some of these units dimed all the way up. Im wondering how many went pouf because they were run all the way open for long recording sessions!

[Leo_Gnardo]

I can make out most of the parts. But not at all sure what those 0.05uf (that's what they should be) paper caps are. Are those electrolytic caps? They're banded, so are they polarized?

Also, what are those black caps used on the cathodes. They look newer than the other stuff in this amp.

No those 0.05 uF are not electrolytic. (Who EVER made el caps of that value ???) The stripe probably indicates "outer wrap" for those rare instances it may matter. (Oh my, let the arguments begin. Again... Some believe it matters, some don't. It goes on, and on, and on.)

The black electrolytic bypass caps look original to me. They were perhaps a bit modern looking for the time, sleek black plastic instead of old tan/yellow parchment inscribed by medieval monks.

[mikepukmel]

Thanks Leo. Sorry, lack of electronics experience here, re 0.05's. So, if I use, say, orange drop caps there, those dollar and a half ones, they probably work similarly? Is there something similar to those, but modern construction and not 18.00 a piece?

[Enzo]

They were paper dielectric, modern equivalents are film. ANY film cap will be fine if it meets voltage needs. Hint 0.05uf is no longer a standard value, today we get 0.047uf caps instead. orange drops, or Mallorys, or whatever.

They are just caps, not a darn thing special about them.

[g1]

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.

This was my understanding as well. Sometimes it's just a dropping resistor for the supply node and not really a 'shared screen' resistor. But I guess it depends on the filter cap you have on the node.
Once I asked a manufacturer why their screen node filter cap was so small (4uF I think?). They responded they wanted a major drop in the voltage when there was any substantial screen current. But normally there is a bigger filter cap at the screen node, and not so much screen current limiting.
Some amps (YBA1 Traynor I think?) have the screen node type dropper resistor, but then a common screen resistor for both output tubes.

[Leo_Gnardo]

Thanks Leo. Sorry, lack of electronics experience here, re 0.05's. So, if I use, say, orange drop caps there, those dollar and a half ones, they probably work similarly? Is there something similar to those, but modern construction and not 18.00 a piece?

What Enzo said. Any competent film cap will work. I happen to like Mallory 150 (now owned by Cornell Dubilier), and Panasonic polyethylene or polypropylene caps from Mouser or any other distributor. Used to use Xicon MPP's - Xicon as it turns out was a Mouser house brand and they phased them out a couple years ago. OD's work fine also.

No need to spend big bucks on caps that rhyme with Bozo, Russian teflon, or expensive caps aimed at the snooty hoi-foi crowd made with five-nines pure OFHC copper or silver foil and unicorn hair dielectric.

Despite that ^^^ I aim to try Mojo's Brazilian-made PIO caps sometime, just for a lark. Not terribly expensive, and just exotic enough maybe to attract boo-teek attention. Caiparinha, anyone? mmm... Yeh!

[Helmholtz]

This was my understanding as well. Sometimes it's just a dropping resistor for the supply node and not really a 'shared screen' resistor. But I guess it depends on the filter cap you have on the node.
Once I asked a manufacturer why their screen node filter cap was so small (4uF I think?). They responded they wanted a major drop in the voltage when there was any substantial screen current. But normally there is a bigger filter cap at the screen node, and not so much screen current limiting.
Some amps (YBA1 Traynor I think?) have the screen node type dropper resistor, but then a common screen resistor for both output tubes.

Connecting the screens to a filter cap supresses all signal frequency voltage (e.g. 1k dropper and 20µ filter cap suppress above 8Hz). But to limit high instantaneous screen current peaks, current dependent inverse signal voltage at the screens is necessary.
The voltage at the filter can only change slowly but this effect can be used for good as it produces (desirable?) screen compression. A smaller cap value enhances noticeable compression. Higher cap values make the amp react stiffer. But too small values may lead to motorboating.

[mikepukmel]

What Enzo said. Any competent film cap will work. I happen to like Mallory 150 (now owned by Cornell Dubilier), and Panasonic polyethylene or polypropylene caps from Mouser or any other distributor. Used to use Xicon MPP's - Xicon as it turns out was a Mouser house brand and they phased them out a couple years ago. OD's work fine also.

No need to spend big bucks on caps that rhyme with Bozo, Russian teflon, or expensive caps aimed at the snooty hoi-foi crowd made with five-nines pure OFHC copper or silver foil and unicorn hair dielectric.

Despite that ^^^ I aim to try Mojo's Brazilian-made PIO caps sometime, just for a lark. Not terribly expensive, and just exotic enough maybe to attract boo-teek attention. Caiparinha, anyone? mmm... Yeh!

Ooo, paper in oil, Im in! Ive been looking for *genuine* unicorn hair dielectric, but could only find faux unicorn hair, that rumor has it, is from rodents in Camden NJ. No way Im putting that in my amp.

[mikepukmel]

Connecting the screens to a filter cap supresses all signal frequency voltage (e.g. 1k dropper and 20µ filter cap suppress above 8Hz). But to limit high instantaneous screen current peaks, current dependent inverse signal voltage at the screens is necessary.
The voltage at the filter can only change slowly but this effect can be used for good as it produces (desirable?) screen compression. A smaller cap value enhances noticeable compression. Higher cap values make the amp react stiffer. But too small values may lead to motorboating.

Interesting, thanks Helmholtz. The schematic Im working from, other guys mentioned from Weatherford, http://www.laurensbuijs.nl/wp-conten...1864129669.jpg have two caps at the screen node, comment says the extra cap could be 20 or 30uf, in addition to the 10uf in the "power supply" strip. In the photo I found, the second cap was mounted right on the tube socket pins. The way those amps were laid out, it looked like an afterthought, or maybe a repair, as I didn't think caps in those days were sturdy enough to take the heat of having the lead soldered right to the socket pin like that. Does this cap's intended use require mounting that close to the screen pin?