Thunderbolt Build Pt1

[johno9]

Four years ago, after successfully building a Fender Princeton and two 18w watt amplifiers, I was looking to build something with more power. I came across the Weatherford schematic for the Supro Thunderbolt and thought that was interesting. I thought the PS multi-section can caps might be difficult to find, but found a two section 20+20/500V and a 10+10/500V at Just Radios (Canada). Purchased them, and there the project sat until this past spring, where I had time on my hands.
I decided to build the Thunderbolt like the original. There isn't a lot of details available about the cabinet, but there are a lot of photos of it, especially with units for sale. So using lots of proportions from my computer screen I came up with a drawing and got the cabinet made. The blue covering did not seem available, so I went with a wine tolex and wheat grill. I installed the Weber 15F150B speaker.

[pompeiisneaks]

Wow that looks great! Well done.

~Phil

[johno9]

Again, with the schematic and studying photos, I came up with a chassis drawing. I had one made in cold rolled steel, thinking of chrome plating and silk-screening it. Just the cost of plating turned out to be more expensive than to have a new chassis made in stainless steel with laser etching the panel markings included.
Though I hand drew original parts locations in the two sections of the chassis, I deviated a bit in locating parts around the input tube and used the unused section for another gain stage for a cascade input. Other changes included a phone jack for the speaker out, off/stby/on power switch, and an IEC power cable jack.
No problems on turn on. So far I'm very happy with how it turned out. I'll report later as I play more with it (I am not a guitarist, I just like building stuff).
It's a heavy weight to move around!

[pompeiisneaks]

I think it might make more sense to keep these in the same thread instead of new threads? No? You can just post more to the same thread.

If you're fine with that I can merge these two threads.
~Phil

[statorvane]

Very cool build! I see you used the Classic Tone transformers. I used the same for a Supro 1688TN clone, 15" speaker, and yes it's heavy. Housed it in a narrow panel-like cabinet.

[fred.violleau]

Cab looks stunning! Congrats!

Fred.

[johno9]

One issue has come up. With the volume knob around 3 o'clock, a low E or A string on the guitar will cause a momentary loss of output. Then output returns to normal right away. Thinking it might be an output stage problem, I note I did not include output tube screen resistors (100R) as they were not on the Weatherford schematic, though I have seen them on later variations of the drawing.
Could that cause the issue? Any other thoughts?
B+ voltages are close to expected (407, 398,352,269)
Thanks.

[mhuss]

That is a very nice build.

I'm not familiar with the Weatherford schematic, but I've attached a schematic of the last real S6420 I had here.

Does the tone control have any effect on this phenomenon? It might be blocking distortion on the second stage. I've seen this in preamps before when a prior stage overloads the next stage that has a fully bypassed cathode. When you really smack it. the next stage blocks for a second then comes back.

SuproThunderbolt.pdf

[johno9]

Hi Mark,

I followed a lot of your work on 18wattt.

Attached is the Weatherford schematic, thought to be definitive, with the cascade input added. Note, I did not include the 100R output tube screen resistors.

So maybe if I remove/lower the value of the cathode cap on V2a after the tone section it may reduce the gain?

Thanks,
John

Thunderbolt Schematic - Cascaded PreAmp v7.jpg

[johno9]

I added a 1M resistor from V2A grid to ground, and the loss of output at high volume levels is gone. This fix was suggested, as the grid doesn't have a direct resistive path to ground.

[Colossal]

This fix was suggested, as the grid doesn't have a direct resistive path to ground.

Yep, you gotta have that. Grids accumulate electrons and they need a place to go. Great looking amp! Nice job!