Looking for Supro Thunderbolt 6420 Build info

[mikepukmel]

Hi Helmholtz, thanks YES I misunderstood your original. OK understand now.

[mikepukmel]

So i have the overall idea down pretty well I think, but don't have my sea legs for specific numbers/ranges. I understand no load, and load and its affect on voltage a bit. But how do you estimate voltage sag for a given circuit? Or is it something you have to measure?

I see many schematics with voltages written on them at various points in the circuit. Im assuming that most of these numbers, someone opened up the amp, plugged it in, turned it on and did some measurements, with no input signal applied. Most of the time, these numbers are very far from the spec on the transformer. E.g one good schematic around has 405v, 395v, 315v measured at the first three power supply nodes, for this amp, for a 325v ac rms tranformer. From purely naive calculations, the first node should be closer to 460v dc. With a solid state rectified amp, Im guessing that it didn't drop 50v (sag), its instead variation transformer to transformer. Or, is it a theoretical no load rating where the 325v number came from, and what you see in practice, when the transformer is hooked up and running in a real circuit?


***

And re your previous explanation, yes, Id like to be able to run with the amp dimed, that seems to be where most people say it emits the most 'mojo'.

[Helmholtz]

Generally PT regulation depends on primary and secondary wire resistances. You can expect equally sized PTs to have similar regulation expressed in percents of voltage. The voltage drop of the rectifier adds to this. Silicon diodes only drop about 0.7V, so 1.4V with a bridge rectifier.
Yes, it's safest to measure the amplifier but using numbers from a similar amp with same size PT and same rectifier should give good orientation.

If the question is still about the screen dropper I wouldn't worry about +/-10% difference. The 2W rating I suggested has some margin.

[mikepukmel]

Hi sorry to bother you again, Re:

"Example: max. screen dissipation = 5W, screen node dropper = 1k, assumed screen voltage at full power = 425VDC. This gives 2x5W/425V = 23.5mA max. screen current for 2 tubes. Adding a few mA for the preamp tubes, we can assume that the dropper will not see more than 30mA, resulting in 0.9W."


10W/425V=23.5, ok, but I don't understand where the 425v drop is occurring. This is from the screen grid to cathode? So, estimate current flow through the screen grids, then use that currrent to calculate the power dissipated/consumed by the 1k dropping resistor?

Thanks,
MP

[Helmholtz]

but I don't understand where the 425v drop is occurring.

The 425V was just an example assumption for screen voltage at full power. No voltage drop but voltage measured directly at the screens (wrt ground). As said +/-10% won't change much regarding the screen dropper resistor. And it never hurts to choose power rating of resistors a little higher. Makes them run cooler.

So, estimate current flow through the screen grids, then use that currrent to calculate the power dissipated/consumed by the 1k dropping resistor?

Correct.

[mikepukmel]

so, the repurposed Deluxe Amp AB763 eyelet board was so ugly, parts all crooked, that I fished around and found the link to DIYLayoutCreator on Doug Hoffman's Amp site. A lot easier to use than I thought, even a banana like me picked it up. Took a few rounds, but I got a board layout nice and straight, triple checked the measurements, all parts will fit. Sent it off to Hoffman to make a turret board for me. (Shoulda done this with the first attempt build, since the resistors these days just don't span the old board widths, had I known. Same with the JTM45 but REALLY should have known by then).

This is the image spit out by DIYLayoutCreator.



More stuff. Mix of parts and symbols, will make 2 versions

[mikepukmel]

DIY Layout Creator is pretty good and very easy to use. A few things not possible to do, like bend leads on parts; rotate tube sockets, but the workarounds are good enough. At least, better than my horrible hand drawings.

[mikepukmel]

need re-engineering or leave well enough alone? -- grid stops on input jacks

Im assuming the 2 47k resistors on the input jacks are functioning as "grid stops". The grid leak is on the other side of the 0.005 cap.

Is it better to
- to leave well enough alone,
- move the grid stop to pin2 of V1
- leave everything as is but add a grid stop on pin2 of v1.





For the other end of the amp, someone recommended to move the output tube grid stops right to pin 1-5 like Fender did, so I'll do that.

Thanks, and happy SUnday. Hope its cooler and less humid wherever you are.

[Leo_Gnardo]

Im assuming the 2 47k resistors on the input jacks are functioning as "grid stops". The grid leak is on the other side of the 0.005 cap.

Is it better to
- to leave well enough alone,
- move the grid stop to pin2 of V1
- leave everything as is but add a grid stop on pin2 of v1.

The 47K's there are combined grid stops & mixers. BUT a grid stop located at that remoteness isn't doing much as a grid stop. Even worse, on the other side of the DC blocking cap .005. On this schematic, neither input has a shorting bar to ground, so the 47K's act as mixers only and the 2nd input is not attenuated 6 dB as in the Fender style input circuit.

In fact that .005 cap isn't really necessary circuit wise - the triode has a cathode resistor, so its grid will tend to stay at 0 volts anyway as long as there's a grid leak R. However the .005 does act as a filter for very low frequencies, and that has something to do with the tone of the amp.

A couple refinements: A/ As on Fenders, a signal entered at the #1 input passes thru both (typically) 68K resistors, so the effective value is really 34K. On your amp, 23.5K. So you would be justified in using a 22K, close enough. and... 2: The grid leak resistor in the schemo is 270K. If you want to sound brighter, not loading your guitar pickup as much (assuming you will plug your guitar into the amp without intervening effects/preamps/buffers) you could increase that to the Fender standard of 1M, or even more.

Another mild mannered mod, if you choose to go with one input, you could use the hole for the 2nd input to mount a "brown tone" pot, where you could vary the value of that grid leak/pickup load resistor. Or just forget about the whole thing, and stick with 270K because it's what Supro used. Gotta try to get that Page tone, y'know! Nothing wrong with that.

[Helmholtz]

I think the primary purpose of the two 47k resistors is to avoid mutual downloading when 2 instruments are connected. A second effect is that they constitute a low pass filter together with the input (Miller) capacitance of around 100pF, resulting in a HF roll-off above audio frequencies - a typical grid stopper effect. This is slightly mitigated by the 270k grid leak resistor but not influenced by the 0.005µ coupling cap as this can be considered an AC short at high frequencies.

Mounting a grid stopper directly to the grid pin only makes a differency in the MHz range. Reason is that any wire length between grid stopper and grid inserts some tiny series inductance which may (in rare cases) give rise to MHz oscillation. If you are afraid of this, just add a 2k to 5k grid stopper at the socket.

As already mentioned I also recommend to increase the grid leak resistor to 470k or 1M. This might slightly increase noise level, though.

[mikepukmel]

Thanks!!!

[mikepukmel]

THinking about downloading the source code, and seeing what makes DIYLayoutCreator tick. Nire program, would be nice to have discrete parts that you can bend the leads into place, like the cathode cap/resistor; would be nice to have rotatable tube sockets, and maybe a little fancier hookup wire routing.

Its not a pro layout, but can read it ok!



Doesn't yet take into account all your suggestions, working on that tonight: bigger grid leak, some work on the grid stops, fixing the input wiring.

[mikepukmel]

Board is tighter fit than anticipated. By dumb luck, just fits under the input jacks and pilot light. Had to drill out all the holes except switches and IEC. didn't butcher them too badly. Holding off gluing and soldering parts on the board until. Still have to drill for transformers (will be here in a few weeks) and drill a few other screw holes.


By some stroke of good luck, ended up with a powder coat chassis, liking it better than bare aluminum due to my greasy fingers.

Thanks for all the tips and pointers to great info.

[mikepukmel]

Not quite. In the 'daisy chain' system Merlin describes, all the grounds are daisy-chained together into one ground buss/wire, but only the input stage end of this daisy chain ground wire is attached to the chassis. The output tube end of the ground wire is floating (because it it weren't, you'd end up with a ground loop. Similarly, if you split the daisy chain up into components, and grounded each section separately, you'd be more likely to get ground loop hum, depending on whereabouts the chassis mains ground was situated w.r.t. each of the other signal stage grounds in the split-up ground).

The idea behind the daisy chain is that the most sensitive pre-amp stages get a ground that is as-close-as-possible to chassis ground potential, and the other higher-current signal ground returns are further away from this, so that the daisy chain acts like one long (albeit low-resistance) voltage divider, and therefore any micro-rises in ground potential along this voltage divider caused by the high-current ground returns, will be smaller nearest to where the chain is connected to the chassis, so that there will be less interference (through filter cap reverse-shunting noise in) to the power supply rail of the most noise-sensitive ground returns in the input end of the pre-amp.

Hi Tubeswell,
Getting close to the point of having to decide where the ground wires would go. This board (completely by accident) is laid out so that the daisy chain scheme would be easy to implement except for the ground side of the cathode bias resistor for the output tubes. I could run a wire under the board to a reasonable place in the ground bus.
Ive never seen an amp wired like that before, e.g. the merlin diagram daisy chain. Is there an danger of ruining something, or would I just get more noise/hum if I didn't do it correctly? If the single ground point came loose, would the amo smoke it self? With more standard commercial grounding schemes, there are lots of chassis tie points so if one came loose, the others would still be there. Damned if I understand what that means.

[tubeswell]

Hi Tubeswell,
Getting close to the point of having to decide where the ground wires would go. This board (completely by accident) is laid out so that the daisy chain scheme would be easy to implement except for the ground side of the cathode bias resistor for the output tubes. I could run a wire under the board to a reasonable place in the ground bus.
Ive never seen an amp wired like that before, e.g. the merlin diagram daisy chain. Is there an danger of ruining something, or would I just get more noise/hum if I didn't do it correctly? If the single ground point came loose, would the amo smoke itself? With more standard commercial grounding schemes, there are lots of chassis tie points so if one came loose, the others would still be there. Damned if I understand what that means.

Doesn’t matter if you run the output tube cathode ground return to the daisy chain end with its own wire. Just make sure you follow Merlin’s ground layout. The amp won’t smoke itself if the PT second-side ground return gets lifted. Commercial amps with multiple ground returns for the signal path use elevating resistance for all but one return, and only that single return gets a direct chassis connection.

But you don’t have to follow Merlin’s grounding scheme. It up to you