Lil’ Dawg about to sleep outside

[fdesalvo]

Hi, all. Hope this note finds you well!

I’ve got a Wonder Dawg on the bench with a reverb issue. The amp arrived with a strange condition that would cause the fuse to blow once the reverb level was increased, sending sparks flying within the rectifier tube. I found some birds-nesting on the reverb transformer, which was shorting it to ground.

The remaining issue is a very loud buzz (~721hz) which increases
in intensity and volume with the reverb level pot. This is a BF fender with a few tweaks and layout liberties and the voltages are within spec at the tube sockets. I’ve done the following to no avail:

- preamp tube swap
- removed reverb pan/cables
- grounded both driver and recovery grids
- removed reverb level pot
- removed a ground loop in preamp
- removed microphonic, under board grid wire
- used shielded cable on all reverb circuit grids

Of note:

- the 220k grid leak on the recovery grid/jack is bypassed with a .002uf cap, as I’ve seen in other amps. The original cap had a fracture with intermittent connection. With the cap off, there is massive oscillation in the amp, which makes me suspect layout.

- The designer placed the OT at the preamp end of the chassis, with its loom routed inside the chassis, passing beneath the RCA jacks.

- this particular PT must be spec’d for use in a few of his designs. In this one get hot quickly.

- the reverb transformer has splices/extensions on almost every lead. 2 were broken internally.

- buzz is much worse with the pan in place

Any help would be appreciated. This dog is going to the pound soon! TIA!

[TomCarlos]

Some years ago, I met Jim Nickelson, the owner of Lil' Dawg amps. He lives in the next town over from me. Jim is a nice guy, gave me a tour of his assembly area, and we talked shop for a bit. There was something I noticed very quickly - other than a DVM, I did not see any other test equipment in his shop. I asked Jim, so if you build an amp, how do you test it? The reply was, "I turn it on, take DC measurements, if they look ok, I plug in a guitar and if the amp sounds ok, I am done." I then asked him, do you do any type of repairs or what happens if you have an amp that has an oscillation or other problem? His reply was "no, I am not in the repair business and I build amps based on a proven design and layout similar to the original." So I wonder if that meant he did not repair his own amps!!!! Jim's business has always centered around building Fender clones. So I took that to mean he is not a a design engineer; he just stuffs turret boards, assembles the cab and speaker, and that is it. So it makes me wonder about the amp you have. Was it properly designed?

From reviews I have read on the net, Jim is well liked and very willing to help you. I found him to be very personable. So look him up, give him a call, and see if he can give you some hints on this amp.

https://www.littledawgamps.com/

Good luck!!

[fdesalvo]

TC thank you for the reply. He does a great job of assembling an amp. Clean layout and good soldering skills for sure. There’s also the probability that the owner of this amp tinkered with it more than he led me to believe.

[TomCarlos]

Yes, Jim does nice work. See if your amp looks like this one...

[fdesalvo]

Yes, Jim does nice work. See if your amp looks like this one...

No sir. Completely diff beast.

[tedmich]

This reminds me of when self taught "street chemists" try to reproduce something by the late A.Shulgin or a similarly talented organic chemist. They can kind of sort of do what's been done, can't at all extend any work or ideas and if something goes wrong (and say, they give people, including themselves, Parkinsons) they have zero clue as to why or how to fix it.

In these instances, KISS should be their central philosophy. Granted the stakes are much lower with amps, but thats still someones hard earned money!

[Mick Bailey]

With the driven and recovery grids grounded I'd expect the oscillation to stop if the problem was a straightforward feedback loop within the usual signal path. I would disconnect the recovery tube plate connection at the tube (be sure to insulate it) to see if the oscillation is via the node supply.

EDIT: I have a couple of test 12AX7s for testing - one with pin 1 clipped and the other with pin 6 clipped. I use these where I want to effectively disconnect the plate of one section without affecting the function of the other.

[fdesalvo]

Mick, good suggestions and thank you. I’m gonna use your preamp tube suggestions for testing. Brilliant! I did replace the plate and screen filter caps (IC) due to excessive hum. I paralleled caps in the pi and preamp positions without effect.