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Goosing a Supro Thunderbolt S6420

A Supro S6420 with volume issues.

Hi Jeff,
I have a Supro S6420 that I recently had serviced by a local tech. I brought in the amp because the bottom end sounded like mush. It broke up at low volume, and for a 35-watt amp, it was not very loud. In fact, my ’66 Princeton Reverb has a lot more volume and headroom.

So, my tech replaced the filter caps, removed a non-original output cathode bypass cap, checked all power and plate-load resistors, checked the transformers, and adjusted the bias (VP 425, 45 mA bias). All preamp and output tubes are good and the speaker is good. Still, it has no volume. My Princeton blows it away. What’s causing this lack of power?

Townsend, Tennessee

Hi Bob,
Thanks for writing. Well, it sounds like your very cool Supro S6420 is not living up to your expectations, and from your explanation of the servicing— most particularly removing a non-stock output cathode bypass cap—it seems likely that its previous owner may have felt the same way. However, I’m not necessarily agreeing there is something “wrong” with the amp. If you plug in a Tele, crank it up, and you suddenly sound like the first Led Zeppelin album, the amp may indeed be functioning properly! Comparing it to a Fender Princeton Reverb is quite honestly an apples-to-oranges scenario, but let’s see if there may be an explanation for your findings and better yet, a fix.

Regarding the Fender having a better bottom end, this may be due to a couple of factors. First: Most Fender amps, due to the type and position of the tone-stack circuitry, have a substantially reduced mid content. This results in the amps’ top and bottom end being more pronounced. Coupled with the substantially more closed-back design of the Fender cabinet, this may very well explain why the Princeton’s bottom end sounds fuller.

Even though the Supro has a 15" speaker, the fact that the baffle board and cabinet are not much larger than the speaker itself and the rear of the cabinet is virtually wide open, the enclosure is basically a baffle with sides. There’s not much low end being developed by the enclosure itself. If you’ve ever compared a 10" speaker mounted in a nice enclosure with a 15" speaker mounted on a piece of plywood, you’d know which one would win the battle of the bass.

Another consideration would be the speaker itself. Just because it “works” or doesn’t have any buzzing or tears in the cone, doesn’t necessarily mean it sounds good. Speakers get old and tired, and I can’t tell you how much a new speaker can completely change an amp for the better. A worn-out speaker along with the cabinet design could easily be responsible for the fact that the amp is not as loud as you believe a 35-watt amp should be.

You mentioned that the tubes are “good,” but didn’t explain how you determined this. Just because a tube checks well, does not necessarily mean that it sounds good. Substitution is always the best way to confirm how the tubes are holding up, so if you haven’t already done so, swap out the tubes for a quick check.

You also mentioned that the amp breaks up early and does not have much headroom. This too could be the result of a tired speaker, but it could also be design-related. Looking at the schematic, I see the final stage of the power supply— which supplies power for the first preamp stage—is fed by a 100k resistor. This is a far larger resistor than is typically used in such basic amps, and it would substantially reduce the voltage supplied to this stage. Though this is great for producing compression and overdrive, it reduces headroom. Decreasing the value of this resistor to 27k, or even 10k, should increase the headroom in this stage.

There is one other aspect of the comparison to your Princeton that I’ve not yet addressed: The Princeton has an extra gain stage in its basic amplification section. By contrast, this could easily make the Supro sound anemic. The good news is there’s an unused half of a 12AX7 in this Supro that you could press into service. Your tech would know how to turn this into a working gain stage. Placing this extra stage ahead of the existing first stage would yield more gain and saturation, while placing it after the volume control would yield more sensitivity.

If you’re not willing to go that far at this point, there are a couple of quick changes you can implement to make the amp more “full sounding.” Just be aware that the fuller the amp becomes, the more difficult it may be for it to stay clean at higher volumes. If you’re willing to give it a try, I’d suggest changing the input cap and resistor of the first gain stage. These are located immediately after the input-jack resistors. Currently, the capacitor has a .005 μF value and the resistor measures 270k Ω. Try changing the cap to a .01 μF or even a .047 μF, and the resistor to a 1M Ω. This should give the amp a fuller response, and the mod is easily reversible if you don’t like the result.

If all this fails to bring the amp to what you believe is an acceptable performance level, I’d recommend a quick substitution of a similar output transformer. Again, something may appear to be good, but a quick, temporary substitution is the best way to know for sure. I hope you get your Thunderbolt thundering!

Jeff Bober is one of the godfathers of the low-wattage amp revolution, co-founded and was the principal designer for Budda Amplification. Jeff recently launched EAST Amplification, and he can be reached at