Quiet champ, blown OT?

[axpro]

Hey guys,

I been away so long i probably don't know anyone on the boards anymore!

I inherited a project champ a while back, a 5f1 clone. never liked the sound of it, always seemed overly quiet (output wise) and fizzy (tone wise)

Took it apart today to try another preamp, realized that the previous owner must not have been able to read schematics, and there were some mighty large resistor values in the pwoer supply, thus starving the tubes (i think down to 60-ish volts on the ax7) I didn't use it more than once or twice, it just sat in my office at home.

Just quit my last band, and don't need the dual rectum friar anymore, sold it and found myself ampless, thus the reason why i opened it up.

So short story long, it's been starved since it was built, it was made of leftover kit parts, and now that i put a blackface champ circuit into it, it works nice and is clean, but seems REAL quiet. dimed, it doesn't come anywhere near the vox ac4tv that i am also playing with.

I tried it with an external cab, and it got REALLY noisy and farty sounding.

Think its a blown ot?

[acorkos]

there are a lot of other things i would suspect before trying a new OT. measure all the voltages on all the tube sockets and see where you stand vs. the schematic

[R.G.]

Think its a blown ot?

This comes up over and over.

Transformers are probably the most intrinsically reliable part in the amp. Certainly among the most reliable if not the most.

Why is it that whenever an amp malfunctions, the post here frequently reads "My amp (died/sounds bad/is quiet/farts/etc.) do you think it's the output transformer?"

Maybe. Conceivably it could be. But there are a bunch of other reasons, including failure of the filter caps or tubes, corrosion on the sockets, and so on, that are much more likely.

[Enzo]

Amen, transformers are THE single most suggested diagnoses I get from customers.


My mom used to think ANYTHING that went wrong with the TV set was the picture tube failing. No sound? Picture tube. Can't pick up UHF? Picture tube. "Well, they haven't perfected them yet," she liked to say.

[axpro]

lol, didn't think i'd get the "blown ot rarely happens" treatment!

New tubes,
voltages are within 10% of spec,
sockets aren't old, (was a kitbuild a year or 2 ago)

GOod (new) mallory caps and new resistors. rebuilt the new blackface circuit this morning, 99% to spec (no mods or changes) Old tweed circuit as stated always seemed kinda quiet.

I asked about the possibility of a blown OT, since i have never had one blow on me before!!!!

When i tried dropping in a 6l6 as an experiment, amp became noisy and farty.

[axpro]

Oh yeah,

the output levels seem about on par with a voc ac4tv i have in the shop, with the output set to 1 watt,

[axpro]

Ok, been thinking about the build more.

I did re-use the Power supply section from the tweed, so it could be a cap in there, but wouldn't that affect the PS voltages if there was a dead or faulty cap? I'll go over the amp again in the morning (it is at my workshop) but it is just strange, it actually "sounds" pretty much fine, just too quiet. I am not expecting roaring amps, and my hearing isn't "THAT" bad, but this is a problem i have yet to encounter, sounds fine, but quiet, and voltages are reading fine. Which is why i was thinking OT.

i may have the old OT out a pro junior project laying around, but i dont think the specs match up.

THough i guess with the issues in the orginal PS build (some quite large resistor values dropping voltage a LOT) may have stressed the filter caps..

Other champs i have built were much happier! and until my princeton high power kit comes, i need something for an amp that i can play at home!

[Steve Conner]

My mom used to think ANYTHING that went wrong with the TV set was the picture tube failing.

Well it's obvious innit. If your amp lacks output, it must be the output transformer, because it has the word "output" in its name.

And if the TV has a bad picture, then by the same reasoning, the picture tube must have got clogged up.

Maybe the power company sent a dirty batch of electrons, and the filter caps need backflushing too.

On a more serious note, I thought the Pro Jr. was a push-pull amp. The OT won't work in a single-ended one. Champ OTs are cheap enough that you could just buy another one if you suspect it.

But: Check the voltage across your power tube cathode resistor, what is it? Check the value of the cathode resistor, is it correct? How much AC voltage do you get at the power tube grid when you strum your guitar? Is the OT wired up correctly? And so on.

[axpro]

On a more serious note, I thought the Pro Jr. was a push-pull amp. The OT won't work in a single-ended one. Champ OTs are cheap enough that you could just buy another one if you suspect it.

But: Check the voltage across your power tube cathode resistor, what is it? Check the value of the cathode resistor, is it correct? How much AC voltage do you get at the power tube grid when you strum your guitar? Is the OT wired up correctly? And so on.

I'll dig a little deeper in an hour when i am at the shop.

And I meant valve junior, instead of pro junior... My brain was a little fuzzy

[Enzo]

Well it's obvious innit. If your amp lacks output, it must be the output transformer, because it has the word "output" in its name.

This may be some sort of universal phenomenon. I was once with a young lady, and I mentioned "output" and she immediately was transformed, extremely cold on her bias. No, wait, come to think of it, I mentioned "put out," not "output," perhaps that is different.

[Amp Kat]

Enzo , you always have the best analogies for comparing amps to real life.
Since sending posters to RG's site that has the OT troubleshooting guide that seems to be a waste of time because they never read it. There are two easy checks for determining bad output transformers and both are solid and can't be any more accurate to use. You can ohm it out or you can do a reverse voltage test from secondary to primary yet nobody seems to want to do it.

[MWJB]

Axpro wrote: "sounds fine, but quiet, and voltages are reading fine." Please specify dc voltages (no harm in confirming heater voltages too). you say they are within 10% of spec, whose spec?

How much W are you getting at the speaker (clean and at regular playing levels)?

[axpro]

A-HA!!!!!!!!!!!!!!!!!!


Looking back over the voltages (the 10% variance was from the original schematic) and......

the second stage of the 12ax7 was biased at 3v, instead of the 1.7-1.8 i expected, turned out to be a bad solder joint... once i hit it with the probe, i got a big volume jump. turned it off and redid the joint.

YAY! champ-ey goodness!

did a few circuit tweaks (I found it a little too bright) changed the slope resistor and upped cap values here and there. MUCH better, and not so much bass that it gets too flabby.

next is tube swaps... i think i'll try a 12ay7 or at7 to cut down the gain a touch, now that its working properly.

Thanks for the tips, I wasn't sure about the ot (thus the question mark in the post title) since i haven't had trouble with them in the past.

NOw to tweak a lil more and do a burn in test.

Thanks again!

[R.G.]

Enzo , you always have the best analogies for comparing amps to real life.
Since sending posters to RG's site that has the OT troubleshooting guide that seems to be a waste of time because they never read it. There are two easy checks for determining bad output transformers and both are solid and can't be any more accurate to use. You can ohm it out or you can do a reverse voltage test from secondary to primary yet nobody seems to want to do it.

In the computer/programming biz, we postulate a "DWIM" instruction or key on the keyboard, depending on whether you're writing kernel code or running applications. "DWIM" is an acronym, of course. It stands for "Do What I Meant".

There seems to be a universal human longing for a magic wand to do whatever is intended, and for the *wand* already knowing what to do to get whatever was intended accomplished. Actually learning what can be done and stringing together bunches of what can be done to get what you want done is a dying art I sometimes think.

[Steve Conner]

The DWIM button thing would be funny if it weren't true.

I often get hired by people who genuinely have no idea what they want on a nuts and bolts level. I can ask "What signal-to-noise ratio do you need?" and the answer is "We have no idea, just make it work".

So part of my job is to be a kind of human DWIM button. Mostly it works, but on Monday mornings I'm more HCF (Halt & Catch Fire)

I should start answering posts in the troubleshooting forum with FTIIWHGIIWT: "Follow the instructions I would have given if I were telepathic."