66 Vibro Champ needs some help

[eurekaiv]

I just picked up a '66 Vibro Champ that's obviously had a lot of "maintenance" done over the years and isn't sounding its best and so I'm looking for some suggestions on where to look for potential problems.

The main issue is that the amp seems very low on gain. It barely distorts at all until turned up until about 7-8 and the tone controls, bass in particular, aren't having much effect of the overall tone of the amp so I feel like somehow they're not being driven properly or being recovered enough. My other silverface Vibro Champ sounds much better and will overdrive in the 4-5 neighborhood.

I have already gone in and done a cap job (there was some obviously old cheap replacements used) with the exception of the filter section. I don't have a can on hand to replace that just yet. The vib was barely working at all and I managed to fix that after I identified the 68k resistor off the intensity pot had drifted up to over 150k.

The power transformer is a replacement and is delivering a bit more voltage then "spec" for the VC. For supply I'm getting 390v, 380v and 356v—higher but not too high I think and that goes for the other measurement points as well compared the the AA764 schem. They're all up about 10-20% but nothing seems particularly awry.

All other resistors in the amp are measuring within tolerance though now that I think of it, I forgot to check the 3 on the input jacks. And of course the first thing I did was put the known good set of tubes from my other champ in. I can always crack that one open and start comparing since as it stands right now, that one is definitely sounding better.

Any thoughts? Thanks!!

[MWJB]

If anything 390v B+ sounds low/low end of normal. What does the other champ measure?

You can replace the filter caps with discrete caps (500v rating), mounted in the chassis, main & screen grounded to PT bolt, preamp filter grounded to input jack.

DC voltages at the tube pins would be useful.

Have you tried the weak amp with the good amp's speaker? Compare resistance readings accross the OT primary.

What kind of power do you get with the pots wide open, maybe compare pot tapers between the amps.

[eurekaiv]

I'd prefer to just replace the can I think so I don't have to worry about how to secure the discrete caps in the chassis and I'd like to eliminate other potential problems before I spend $30 on caps for it one way or the other.

I haven't measured the other champ in ages but I seem to recall it being in the 370-380v range off the original power transformer. I'll have to double check though of course.

From memory (I wrote it down at home but I'm at work now) DC voltages were approximately:
V1.1 240v
V1.3 1.9v
V1.6 240v
V1.8 1.9v

V3.3 356v (don't recall what I measured here but it wasn't too far off)
V3.4 380v
V3.8 22.5v

V2 was giving weird measurement due to the oscillator and I didn't go back and re-check those after I swapped out the 68k off pin 8 since that seemed to get the vib working beautifully.

The weber sig speaker in this amp is blown (maybe this is a clue?) so I've actually tried it with a few other speakers. I'll defenitely check the resistance across the OT primary when I get a chance.

As for taper, my old champ distorts sooner and a fair amount more when full up. In general it sounds like the whole amp is being driven harder and the tone controls sound much more Fender-like and interactive to me. I get a similar amount of bottom end with the bass on 2 or 3 of that amp compared to the new one pretty much dimed.

[MWJB]

Check that all cathode bypass caps are in good order & fitted (if any have been taken off for any reason gain would be low).

Then I'd sub the tone caps (treb/mid/bass), after eliminating the OT.

[eurekaiv]

Did all the cathode caps last night with spragues and replaced the tone caps with some very nice Jupiter condenser caps I already had in my parts in. It sounded pretty much the same after as it did before.

[booj]

Hate to suggest this but it sounds like a bad O.T. to me. Try swapping that out. It's not hard to do.

[eurekaiv]

I may have simply put V2 tube from the old champ in the V1 spot or something when I moved them all over but when I swapped them back to the other amp I noticed the trem wasn't working anymore. Pulled that tube, replaced it with a good one and the trem worked. Put them all back in the new champ and everything seemed fine. I guess somewhere in all this moving things around half of one of the tubes went bad or weak? That might explain the loss of gain I think. Either that or the tube simply wasn't well seated. I'm going to tension and clean the tube sockets but I'm done for the night. Thanks for all the help/suggestions everyone!

[eurekaiv]

So this thing is sounding really amazing right now but in getting the cathode resistor value right to set the bias range where it needed to be given the higher B+ in the amp, I've noticed that my voltages are oscillating slightly. I don't think I've ever seen anything like this before but the voltage on the cathode goes up and down within a range of about .5 - 1 volt and the screen grid is oscillating within a 2-3 volt range. I tried searching and couldn't find much of anything regarding this situation but it doesn't seem normal.

[MWJB]

Have you turned off the vibrato? (not just turned it down).

[eurekaiv]

Turning off the vibrato doesn't make it stop. It does seem to lessen the effect slightly though but only slightly if at all.

Unfortunately, the old 6V6 in this thing shit the bed this AM and exploded the cathode cap inside the chassis—loud and messy. I can't really find anything else that could be wrong (other then the voltage oscillating slightly) as all my measurements seemed to check out after replacing the tube and the cap. I've got 398 on the plate, 396 on the grid and 30 on the cathode. With 830Ωs on the cathode that gives me around 34ma current if I'm using the weber calculator correctly.

[MWJB]

Well, if the amp sounds amazing, do you really have a problem?

The only other thing you could try is a new coupling cap feeding the 6V6.

Still concerned about the low B+, you shouldn't need a larger than stock cathode resistor with such low voltages, try 470ohms 5W. I'd replace the filter caps just to be sure that they are not pulling down the B+...then look again at the bias issue.

I hope you fitted a cathode bypass cap of at least 50v rating ?

[eurekaiv]

Don't all amps sound amazing right before they explode?

I am using a 50V cap cathode bypass cap. It was interesting seeing it blow up like a balloon right as it was going. Should have removed the power tube before turning the amp back on to check the voltages. Haha... lesson learned there.

So it hit me last night that this thing is making an awful lot of power at these voltages and cathode biased... it works out to around 13-14 watts if I'm doing the math correctly. Going down on the bypass resistor decreases the plate voltages as opposed to increasing them and also shoots the bias up into the 50ma range which is silly hot and pretty crappy sounding. It appears it's also trying to put out more power that way... like 17 watts or so. A stock blackface champ should have around 350 (give or take) at the plate but someone replaced the power transformer in my amp with a bigger one apparantly. I do plan on changing the cap can out soon along with the cheap previously replaced tube socket for the 6V6. I guess I should get a power transformer that's right sized while I'm at it?

[MWJB]

Keep some air between the cathode resistor & bypass cap, the resistor gets hot & can cook the cap.

You are confusing W dc dissipation (basically heat, like with a speaker or resistor W rating) with W AC power (sound out of the speaker). They are not directly related. Stock tolex champs might dissipate 18-21W (& will do for decades), they still only make 5 or 6W at the speaker, and that's not 5-6W clean. Even a conservatively biased champ might run 14-18W plate dissipation.

No tolex champ runs 350vdc at the plate (not with the original PT, rectifier and cathode resistor anyway).

Yes, a smaller cathode resistor pulls down voltage & raises current (conversely, less current = more volts)- it is normal for guitar amp power supplies to see saw like this - bias to the current you like the sound of and live with the voltage you get (as long as cap & tube reliability limits are not exceeded).

Your power transformer is fine (or at least it wiil be if the filter caps are doing their job). It has to cope with 2x6V6s and 4x12AX7 in a Princeton, plus 60mA+ current draw (it's typically rated for a sturdy 70mA).

This is one of those instances where you didn't know you had a voltage problem until you measured it (you're not the first, you won't be the last), but if you measured a bunch of other champs, you would see that you are actually on the low side of normal.

[eurekaiv]

Ok, that certainly makes a lot more sense. I'm going to order a new cap can today to see where that gets me and I'll go from there.