Looking for Supro Thunderbolt 6420 Build info

[mikepukmel]

Hi All,

Has anyone out there built a copy of a Supro Thunderbolt 6420 amp? If so, any tips on parts, layout? The only schematics I have found are fairly awful, look like hand drawn by someone who had one of the amps, and one schematic looks like a photocopy of a manual that has the top section cut off.
Found a good web page with some info, but no idea as to construction details, is it point to point or turret/eyelet, and what kind of parts did they use.

Thanks
Happy New Year
MP

[nosaj]

Hi All,

Has anyone out there built a copy of a Supro Thunderbolt 6420 amp? If so, any tips on parts, layout? The only schematics I have found are fairly awful, look like hand drawn by someone who had one of the amps, and one schematic looks like a photocopy of a manual that has the top section cut off.
Found a good web page with some info, but no idea as to construction details, is it point to point or turret/eyelet, and what kind of parts did they use.

Thanks
Happy New Year
MP

Check this thread has a pdf schem in it.

Layout should be preyy straight forward. Power supply on one side , input, preamp on the opposite side. If your getting better with schematics you can build it anyway you want as far as PCB, PtP, or turret eyelet. If not build a champ true point to point following just the schematic, You WILL get a better understanding after that.

https://music-electronics-forum.com/...t=35300&page=2
Search function on this board I haven't quite figured out. Using Google search for this site is much better I think.

nosaj

[Jazz P Bass]

Here is what I have, schematic wise:
Thunderbolt.zip
Sorry. No layouts or parts list.

[mikepukmel]

Thanks!

[mikepukmel]

Ive gone through the schematics, some photos found on the web. Found transformers from ClassicTone. Most other parts.

Question about the schematic: there are no screen resistors on this amp. Is this because its cathode biased and such puts put less power? It does use 6L6's bu only I think 22 watts. Still, I don't think Ive seen a schematic for a tube amp with no screen grid resistors. Would it be advisable to put some in?

[Jazz P Bass]

The screen supply is already 1K below the B+.

[g1]

Yes, individual screen resistors is always good practice, especially with modern tubes.

[nosaj]

The screen supply is already 1K below the B+.

Yes, individual screen resistors is always good practice, especially with modern tubes.

Can someone clarify this for me? It seems to be in conflict in the way I'm reading it.
Thanks,
nosaj

[Jazz P Bass]

My thoughts where that the screens are already held below the B+ so why make them lower yet.
To me, that would mean less power.
Although G1 makes an excellent point about new tubes.

Use 470 ohm/ 1 watt if you will.

I also noted that the output cathode resistor is not bypassed with a capacitor.
Another item to try.
25uf/ 50V.

[mikepukmel]

Oo, thanks, I think I got it now (mostly). In the Fenders, e.g. there isn't much resistance across the choke, the plates on the OT's are fed before the choke, and the screen grids after. So, they're close to the same potential. The supro has no choke, but has a 1K resistor. In the Fenders, the screen grid is 470R + a little for the choke, below. In the supro's, its 1k. Is this right?



I don't know enough about the guts of the tubes to understand why the screen grids need to be kept at a lower potential, and by how much. Back in the day, they must have gotten some recommendations from tube MFR's, but today, how is this done?

(oops, sorry, image clip is from another question, please ignore the red/green lines)

[Leo_Gnardo]

I don't know enough about the guts of the tubes to understand why the screen grids need to be kept at a lower potential, and by how much.

Mostly, so electrons will be more attracted to the plate than to the screen grids. Of course there are some exceptions - mostly 50+ year old amps with 6V6's - where screen grid voltage is higher than plate voltage. In those, the tough RCA & Sylvania 6V6's of the day seemed to put up with it. And it's always an option to change that situation when we run across amps like that. Certainly not a good idea to design & build an amp these days with that built-in flaw.

[Enzo]

Repeat after me, "It is only a guitar amp."

There is best practice, there is good enough practice. This isn't built to mil-spec. Is it better to have individual screen resistors? Probably. DO amps work perfectly fine with a common screen resistor? Yes. It isn't a big deal.

[mozz]

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

[mikepukmel]

Mostly, so electrons will be more attracted to the plate than to the screen grids. Of course there are some exceptions - mostly 50+ year old amps with 6V6's - where screen grid voltage is higher than plate voltage. In those, the tough RCA & Sylvania 6V6's of the day seemed to put up with it. And it's always an option to change that situation when we run across amps like that. Certainly not a good idea to design & build an amp these days with that built-in flaw.

Thanks Leo. Oh man, for years and years, back in the late 80's early 90's, before the internet, I had a few old amps, and called all over the place looking for some tube, can't remember what it was. I don't know how many hundreds on long distance phone calls, every one was the inevitable "Well they don't make those tubes anymore and you CAN'T get them. You can try substituting ... but it won't work right and ...". little did we know that there were all those warehouses with 'new old stock' but no way to get to them. At least for me anyway. So, sounds like, even with this single resistor for two tubes, 1k is enough?

[mikepukmel]

Repeat after me, "It is only a guitar amp."

There is best practice, there is good enough practice. This isn't built to mil-spec. Is it better to have individual screen resistors? Probably. DO amps work perfectly fine with a common screen resistor? Yes. It isn't a big deal.

Thanks Enzo! Yeah, as mentioned many times, even though I don't know much, always worried about little things . But since this is a scratch build, about as scratchiest as ive done so far in my nearly 2 amp build career I can put a screen resistor on each tube, socket pins 6 to 4, and put the grid stop right on the tube, pins 1 to 5 like the fenders?

Also, found a layout someone did, on one of the many amp blogs. Going through it even with my tendency to read things backwards, looks ok, kinda 'fender-ish' aside from a few things that look like they can be improved. Will post and ask...