Mystery Amp

[hossymandias]

I just picked up a mystery amp from a pawn shop. It is obviously a diy job. It looks like a 5f1 head from a kit, but I can't find any trace of the kit online. The chassis has a faceplate that says "Class of '57" with no other markings. The transformers have no identifiers that I can figure out.

I've built a couple Weber kits and modded said kits to my preferences (both amps sound amazing) but I am still very much reliant on schematics. I thought I could open this guy up and find the matching schematic online but I quickly realized this amp doesn't match any schematics I could find.

It runs a 5y3 rectifier, 12ax7 preamp, and a KT66 power.

Then I saw four 33uf filter caps and a choke.

Also it has a tone control.

Any idea what this thing is? Is it just a modded 5f1?

Thanks for any input!

[Phil_S]

I assume you have one volume control in addition to the one tone control? Based the amp's name and the tube compliment, it seems like it is a souped up early Champ-like thing. I don't think the 5F1 had a tone control. The 5F2/5F2A had a primitive tone control. I don't think either had a choke. I'd venture a guess that these could be the amp on which yours was modeled.

[dorrisant]

Post Pics.

[Stevem]

With a choke and 4 stages of filtering it's would not seem to be a champ clone of any kind!

[hossymandias]

Here you go. I was thinking 5f2a as well. The 5f2 has a choke and a tone control. Look like it could be the one.

One of the many parts of tube amp operation that I'm still foggy about is the power supply filtering. Does four 33uf filter capacitors seem like overdoing it? Does the KT66 make a difference here? Thanks for the information!

[pompeiisneaks]

anytime I see wire nuts inside an amp I cringe...

~Phil

[Phil_S]

anytime I see wire nuts inside an amp I cringe...

And I'm sure we all love the budget strain relief, too. I'd sooner see a couple of zip ties inside and out. Anything but a big ole knot! How about the location of the socket for the 12AX7? Plenty of fun in this chassis. Still, it is a decent platform for a low wattage amp and all the inappropriate stuff is easily corrected.

I'm doubtful the KT66 does much of anything over a 6V6 other than give it a little different flavor. Maybe it's strawberry instead of vanilla. In terms of output, maybe 7-8W instead of 4.5W from a 6V6. It would not be significantly louder. To double the perceived volume from 4.5W you'd need 45W. Also, I'm not convinced the output transformer was anything more than a typical Champ type.

If 33uf was what the builder had on hand, I suppose that's OK. Four filter caps, including a CLC filter at the first stage, in this amp its just silly, but I'm doubtful it is harmful in any way. The 5F2 used 3x 8uF and it was OK. Caps are better and cheaper than in the '50's. If I were building this, I might use a pair of 10uf each side of the choke and 10uf for the 12AX7. Performance-wise, the power supply probably offers no sag at all, making the tube rectifier an unnecessary cost.

[hossymandias]

This is great feedback. I can fix the strain relief and wire nut no problem.

What other fixes or modifications would you do? Move the 12ax7 socket?

My hope is to use this amp for recording. I'm not interested in volume, but reducing noise and hum is a priority.

[Phil_S]

What other fixes or modifications would you do? Move the 12ax7 socket?

There are some things that are just wrong (wire nuts, strain relief) that don't have anything to do with the audio signal and clearly need to be done for safety and good workmanship. Do all of those things.

Anything that has to do with the signal chain, fix those one at a time. I'd only move that socket if you have noise. There are times when we expect something won't be right, yet, there is nothing wrong. This could easily be one of those things.

[Stevem]

That location of the line out / speaker jack next to the input jack is begging for oscillation's to take place boy!

[Phil_S]

I feel a need to make a minor retraction. I was confused on the location of the 12AX7. Somehow, I missed the socket near the input and output jacks. I was disoriented and thought it was under the choke and hidden. It is probably OK there, except as Steve says, the output next to the input...

[pompeiisneaks]

if I'm looking at the wiring right, the speaker output is really on top, and the front one is a line out... still not an optimal placement for an output next to an input, for sure.

~Phil

[thetragichero]

nothing stamped on the tops of the choke or ot?

[Phil_S]

if I'm looking at the wiring right, the speaker output is really on top, and the front one is a line out... still not an optimal placement for an output next to an input, for sure.

Yeah, LOL, the "designer" was a real thinker. He managed to not leave a good spot on the back wall for the output jack and wasted a significant amount of that real estate by mounting the OT on the inside. I suppose the OT can go topside and then there would be a good spot for the jack.

[dartanion]

Bit of a rats nest for sure. That ground bus looks pretty shaky as well as the other items that have been pointed out.

Phil_S, you mentioned that the power supply wouldn’t have much sag. Let’s remember the S.E. amps (class A) don’t sag. To be clear, biased to true class A would not sag at all, biased outside of true class A would have a small offset current and some asymmetric clipping. That offset current would be quite small when compared to a saggy class AB amp, and likely imperceptible to the ear. Class A is biased to where max current draw is at idle.