Two-Stroke Silent after hours of use

[deansouthpaw]

So I built the original Two-Stroke per Dave Hunters plans (2 6V6s). For awhile, I only had a 10″ Celestion Gold using one 6V6. Everything was working great. Finally, I ordered a 8″ Weber 8F150 Ceramic. I added this speaker and plugged in the additional 6V6, switched the impedance switch for 2-6V6s at 8ohms, and it sounded incredible. Only some squeal when the tone knob was maxed. I figured I could look at this later…

That brings us to today. I turned on the amp to play a few riffs, and it’s silent. I’m getting hum from the speakers, but absolutely nothing from the input jack. I did take the cabinet apart yesterday, but again, it worked after I added the 8″ speaker.

Any ideas where I should start? I figured I’d ask here, since I’m training with the FAA right now and I don’t have a ton of time to poke around here without truly knowing what I’m doing. All the tubes appear to be healthy, though I was thinking of switching out the 12ax7. Any ideas of where else there could be a problem? I briefly poked around with some chop sticks but didn’t find anything.

thanks,

[Robin]

Confirm that the hum is actually coming for the speaker(s) and not just transformer hum. If you know for sure that the signal from the OT is getting to the speaker (hum from the speakers), then check the input jack and connection to pin 2 of V1 (the shielded cable from the input tip lug to lug 2 on the 12AX7 socket), be sure the the input has not shorted to ground. Confirm that the speaker(s) is wired correctly from the OT through the switch, if you accidentally the two secondary taps from the OT together, it will reduce the output to next to nothing.
This can also happen if the feedback loop is wired incorrectly to the speaker jack.

It’s possible that there is an unintended ground somewhere, or that the output tube socket(s) is not making good connection at one of the lugs. (use the chopstick test to poke around and see if you can find a loose or intermittent connection).

Regarding the tone control squeal: use the chopstick test to see if you can reduce or eliminate the noise by moving the leads around, it could be parasitic oscillation.

[deansouthpaw]

Thanks for your reply Robin! It turns out the fix was really simple–12ax7 had gone bad. I’ve replaced it with an old EHX tube and she sounds good.

Question, I feel like the output on this amp is a bit lower than I expected. Are there any quick ways too increase the max dB output without doing a crazy rebuild? Again, I built this to Dave Hunter’s original specs with his suggested mojotone tranny.

[Robin]

You mean 8 watts isn’t cutting it? I guess output is in the ear of the beholder, I added a 1/2 power switch to mine so the sound guys would stop complaining about it being too loud when mic-ed into a house system. A different speaker might be louder and if you are still using the 6V6s, changing to a good 6L6 might be louder too. Certainly changing to an EL-34, KT-66 or even a 6550 would increase the output noticeably. We just did another round of testing/listening to different power tubes in a Two Stroke. The Fender fans liked the 6L6, the Marshall guys like the 6550. In the end, most everyone agreed that a good 6L6 is tough to beat.

[deansouthpaw]

Thanks Robin! The amp is plenty loud, but it’s only when we have band practice I don’t have it mic’d. I’ll have some fun swapping out tubes and see what happens. Thanks for the help!

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