Supro Thunderbolt Clone

[passfan]

Here is a newly completed Supro Thunderbolt Clone based on this schematic:

https://tubeamparchive.com/files/s6420_789.pdf

The problem I am having is when a note is struck I get a very diminished 60 hz hum with my original note for about 3 seconds and then it stops and all you can hear is the original note. It is as if a noise gate opened and closed, it is that sudden. You have to listen to hear it but it annoys me because it will be out of tune depending on the note struck. I have tried a series of mods including tube recto, prl. the V1 triodes, swapping out bypass cap values, it doesn't matter what I do the only way to get rid of it is to turn down the gain. Otherwise the amp plays and sounds great and my tweaks have taken me through a whole range of sounds and textures with this amp so I pretty much know how I want it set up, I just have to get it to stop. Here are some pics of the chassis for those who like to look.

[tubeswell]

Have you tried different pre-amp tubes (esp in the inverter)?

Another possibility that occurs to me is the paraphase PI not quite being balanced enough. In this regard, you could try changing the output voltage value in the voltage divider feeding the signal into the PI's second stage. E.g.; try a 10k or 15k resistor in place of the 12k (which is in the leg of that voltage divider.)

A more refined approach to experimenting with this would be to wire a 5k (or 10K) pot as a variable resistor and put this in series with a 10k resistor, and put that whole gizmo in place of the 12k resistor.
2CW

[passfan]

I've swapped out the tubes a few times to no avail. Tried a different guitar, moved the two 100 ohm hum resistors from the power tubes to the 1st preamp tube. I've noticed at idle I have 4mv dc on my volume pot but when I strike a note it runs up almost to 1v dc before dropping back down. Has anyone ever looked at what was going on around the pots when you play and is this normal ? Could a bad coupling cap be causing this ?

[passfan]

Another possibility that occurs to me is the paraphase PI not quite being balanced enough. In this regard, you could try changing the output voltage value in the voltage divider feeding the signal into the PI's second stage. E.g.; try a 10k or 15k resistor in place of the 12k (which is in the leg of that voltage divider.)

A more refined approach to experimenting with this would be to wire a 5k (or 10K) pot as a variable resistor and put this in series with a 10k resistor, and put that whole gizmo in place of the 12k resistor.
2CW

This is pretty much a classic Supro circuit used in a lot of their stuff with a proven track record. I think is has to be a bad component, or my layout is bad, something I did. I listen to these amps on you tube and they don't make this noise.

[Structo]

Your symptom sounds like ghosting.

That usually has to do with the power filtering.

So perhaps you either have a bad filter cap or one is leaking too much.

Some times the easiest method to find it is to jumper a similar valued cap on top of the suspect cap.

Be careful of the charged cap!

If you clip it on the bad one the problem should stop.

[jjman]

The orange grid wire to one of the output tubes is line up with some other wires. I would re-route it or even go with shielded to rule out that vicinity. I had a similar problem with both a '74 VC and '71 SFDR after removing their anti-oscillation grid caps. Shielded wires resolved it.

[passfan]

Well after further review, I'm almost too embarassed to fess up, but here goes:

I mis-wired the PI bias resistors between the turret board and socket, Crossed them up.
I then swapped pins on the second PI triode between the cathode and grid. Had the grid wire to the cathode and the bias wire to the grid.
I placed the last 100k in the power string after the last cap instead of before.

I have received my comeupance. Too many builds have gone too well for me and started right up and played beautifully. Therefore I was due

It does sound tons better however, and my noise problem disappeared. Tubeswell was right in that the PI wasn't balanced enough.

[jon]

I like your build a lot. I also just finished a Thunderbolt build. I built the later solid state rectified version. I used a Hotrod Deluxe PT and a JTM45 OT. I found with modern tubes that a 220ohm or 250ohm cathode resistor yielded about 90% plate dissipation on the power tubes. I also added a pair of 100ohm screen resistors.

[IMG:800:600]http://i82.photobucket.com/albums/j250/ ... G_0280.jpg[/img]
[IMG:800:600]http://i82.photobucket.com/albums/j250/ ... G_0278.jpg[/img]

I am totally pleased with the amp. I use it for both guitar and bass (its original design intent) through a EV TL-606 cabinet. It really sounds great.

[dynaman]

I like your build a lot. I also just finished a Thunderbolt build. I built the later solid state rectified version. I used a Hotrod Deluxe PT and a JTM45 OT. I found with modern tubes that a 220ohm or 250ohm cathode resistor yielded about 90% plate dissipation on the power tubes. I also added a pair of 100ohm screen resistors.

[IMG:800:600]http://i82.photobucket.com/albums/j250/ ... G_0280.jpg[/img]
[IMG:800:600]http://i82.photobucket.com/albums/j250/ ... G_0278.jpg[/img]

I am totally pleased with the amp. I use it for both guitar and bass (its original design intent) through a EV TL-606 cabinet. It really sounds great.

+1 on the screens and higher cathode resistor. That's exactly what I did in mine too.

[passfan]

I like your build a lot. I also just finished a Thunderbolt build. I built the later solid state rectified version. I used a Hotrod Deluxe PT and a JTM45 OT. I found with modern tubes that a 220ohm or 250ohm cathode resistor yielded about 90% plate dissipation on the power tubes. I also added a pair of 100ohm screen resistors.

[IMG:800:600]http://i82.photobucket.com/albums/j250/ ... G_0280.jpg[/img]
[IMG:800:600]http://i82.photobucket.com/albums/j250/ ... G_0278.jpg[/img]

I am totally pleased with the amp. I use it for both guitar and bass (its original design intent) through a EV TL-606 cabinet. It really sounds great.

Thanks Jon, it's a simple design and I like the point to point you executed as well. You made it look ....simple and in the case of the thunderbolt, simple works very well. I'm still running the 200 ohm cathode because of the tubes I'm using (a pair of Mesa STR 6L6GC's). Compared to a regular 6L6GC they're huge and their taking it in stride. Now that it's working properly I may unparallel the preamp triode to see what my gain does. It's a tad too much at full throttle. +2 on the 100 ohm screen resistors as well. Thanks for sharing your porn.

[Structo]

Nice, glad you got it sorted out.
What's kind of neat about tube amps is, we can wire them wrong, have a brain fart and really screw them up and when we correct the error(s), they still work most of the time.

Try that with a box full of discrete semi-conductors.

[rkscott7]

I have built from this weatherford schematic one older version of their schematic leaves out a grid stopper that makes it redplate

[sluckey]

[quote=rkscott7 post_id=408430 time=1581615257 user_id=19720]
After reading this post again I realize I didn't understand what you were saying. Sorry.

[thetragichero]

Nice, glad you got it sorted out.
What's kind of neat about tube amps is, we can wire them wrong, have a brain fart and really screw them up and when we correct the error(s), they still work most of the time.

Try that with a box full of discrete semi-conductors.

I'm guessing this is why most guys won't even attempt to repair solid state amps, because they probably had a similar first experience to mine: replaced the trashed output and driver transistors on my 1969 kustom k200 and didn't bother bringing it back up on a current limiting device, so when the outputs shorted again i had to replace 40 bucks worth of transistors again. meanwhile I've done some pretty boneheaded stuff on tube amps and all i did was today a couple of fuses

[rkscott7]

I have built two of these Supro Thunder bolts this one is in a Mojo Tone cabinet