Problem with Supro Thunderbolt Build

[johno9]

I built a Thunderbolt clone, based on the Sean Weatherford schematic and with a cascade input of the unused section of the input tube. The amp sounds great. However, if the volume knob goes past 3 o'clock-ish, striking the low E or A guitar string will cause a momentary drop out of sound. Normal operation returns right away. This effect occurs with both the normal and cascaded inputs.
B+ voltages seem close to expected (407, 398, 352, 269 measured vs 425, 415, 316,247 estimated). There are no screen resistors.
Any thoughts on a cause or place to troubleshoot would be most appreciated?

[PRR]

> if the volume knob goes past 3 o'clock-ish,

So don't do that.

This thing has a LOT of gain. A hot guitar or guitarist can easily over-drive the finals into grid-blocking. The immediate fit is Don't Do That. You can try removing one or both cathode bypass caps- early Fender Champ controlled blocking this way.

[sluckey]

I built a Thunderbolt clone, based on the Sean Weatherford schematic and with a cascade input of the unused section of the input tube.

Show us a schematic of YOUR mod. The problem likely lies with your mod. The stock schematic you posted should work just fine. It's not a sin to have an unused triode in an amp.

[johno9]

Here’s the schematic. I’m thinking lowering the gain of V2a by reducing/eliminating the cathode cap may be the fix. Note, the problem exists with both the normal and cascade inputs.
Thanks.

[scstill]

I built a Thunderbolt clone, based on the Sean Weatherford schematic and with a cascade input of the unused section of the input tube.

Show us a schematic of YOUR mod. The problem likely lies with your mod. The stock schematic you posted should work just fine. It's not a sin to have an unused triode in an amp.

would it be problem to apply HV to plate of an unused triode section?
Just in case its implemeted for additional gain in the future...

[66Strat]

How hot are the guitar pickups? With hot pickups, it may be a matter of not turning the volume knob past the 3 o'clock position, as suggested by PRR. Another option may be to substitute a 5751 for the V1 position.

[shooter]

Just in case its implemeted for additional gain in the future...

why not ducktape the resistor to the back of amp til you need the extra triode.
It's not a problem, but not really "best practice"

[sluckey]

I would try a voltage divider between V1B and V1A.

[SoundmasterG]

I would try a voltage divider between V1B and V1A.

Out of the three vintage T-bolts that I worked on and used for my contribution to Sean's schematics you built your amp from, two were completely stock. The third was modified and was using the un-used triode as a gain stage. There was a voltage divider in between the two stages, and that and the second stage gain setup was something the owner came up with by trial and error. It is a great suggestion Sluckey made and I highly suggest it when people are going to use that second stage as a gain stage and are building the T-bolt circuit.


The fact you have the problem with the normal input also suggests that you may have problems elsewhere in the amp. Check all your connections; do things to reduce gain and see if you can get the amp to behave with less gain, then start adding in the gain you want to see what is causing it to break the circuit. Process of elimination and you should be able to find the culprit. Sometimes layout can be the issue too, so pics of your layout would help.

Greg

[johno9]

Thanks for the input all. I added a 1M resistor from V2A grid to ground, and the loss of output with low notes at high volume levels is gone. This fix was suggested by another person, who noted the grid doesn't have a direct resistive path to ground (I'm not sure I fully understand that).

[shooter]

I'm not sure I fully understand that

guessing;
look at schematic, V1B has no grid resistor, or a coupling cap.  look at V1A