Vibro Champ issues...

[jonrpick]

Hi all... first post here. I used to hang out on AGA (alt.guitar.amps) years ago, but was more of a Lurker than anything else.

I have a SF Vibro Champ that I'm trying to get into shape. Right now I have 2 issues of concern. The tremolo is *very* weak, and the bass is very flabby when it gets pushed into overdrive. I installed all new tubes a few years back and it's seen very little use.

Now, I think I've determined the culprit of the weak tremolo to be a bad intensity pot. It peaks at around 4k in either direction instead of the 25k it should read. Gonna try to pick one up today.

Concerning the flabby bass... it's not a speaker issue. The chassis is in a SF Musicmaster Bass cab (cut for 12" speaker), though there's no speaker so it's actually connected to a '67 Guild Thunderbass 2x15 cab with a pair of JBL E130's (8ohms each, in parallel for a 4ohm load).

It's NOT the speakers. 

I'm playing a nice Les Paul copy with with Duncan Antiquity II humbuckers (hand wound PAF clones, unpotted & scatterwound and the Peter Green magnet mod) with 11 gauge strings. The low E string snaps big and hard through other amps. The only way I can keep the low end tight on this amp is to run the bass knob at zero.

Are there some small cap changes that can be made to help bring out more lows while keeping it tight? I want lots of lows, just not flabby lows...

Thanks...

~Jon

[Leevi]

The tremolo is *very* weak,

Try to change the resistor values around the Intensity pot.

I have built recently a similar tremolo and it works fine with the following settings (see the schematic below).
Of course it's good to check the voltage values of the tremolo tube.

/Leevi

[eleventeen]

The intensity pot measures 4K *in circuit*? High end to ground (low end)? Assuming the ckt is an AA764, the high end of same is in parallel with some pretty low-value resistors....the cathode stack under the non-tremolo 12AX7 = 1500 + 47 = ~~ 1550 ohms to ground, plus the neg feedback resistor through the output tranny = 2100 ohms + 1500 ohms + "zero" [4 or 8 ohms output winding] if you are measuring 4K *from the wiper of that pot* that would not be wrong as I guesstimate it. The wiper should read in the area of 1K to ground under *all* positions of that pot. Not to mention that bad pots ("bad" meaning serious resistance drift of the whole "horseshoe" inside the pot) are at least a little bit rare.....Now if that pot is really 4K end-to-end, that's NG. Pots, by the way, often measure +/- 20% brand new. I would measure the end-to-end resistance of that pot with BOTH non-grounded terminals unsoldered before rushing out to replace it.

Others may comment more intelligently than I might over the cap changes...but in your assessment of the amp, be sure to check the values of the resistors that separate the power supply nodes. The 1K and 10K 1 watts. I also know these resistors changed over several versions of VC's and were not accurately shown on the schematic. (Back then, a Champ/VChamp was literally one step above a record player, nobody paid much attention to them) I had a SF VChamp where those resistors, for some unknown reason, had drifted massively away from their factory values. I think the 10K had gone to 3K (!) but I can't recall exactly.

[sluckey]

Now, I think I've determined the culprit of the weak tremolo to be a bad intensity pot. It peaks at around 4k in either direction instead of the 25k it should read.

You will have to disconnect the ungrounded end of the intensity pot to get an accurate resistance reading. Did you do that?

Concerning the flabby bass... it's not a speaker issue.

But the OT may not be up to reproducing bass notes very well. This could just be the way it is. You can try using smaller value bypass caps (1 to 5µF) on the tubes. That will reduce flabby bass.

[sluckey]

The wiper should read in the area of 1K to ground under *all* positions of that pot.

Sure 'bout that?   

[eleventeen]

Sorry..the top end will read in the 1K zone, not the wiper.

[jonrpick]

I was measuring the pot from the center lug to each of the outers while sweeping back and forth. It would go from a very low reading to about a 4k peak around 1/3 of the way up then drop again. Measuring from the opposite outer lug to center, the pattern reversed... the peak occurred around 2/3 of the way.

The pot is also quite scratchy "feeling" compared to the others in the circuit. Pot cleaner had no effect.

All measurements were taken with the amp powered down and unplugged.

Re: the stock OT... I figured that the OT could simply be at its limits, but was hoping for a quick fix. The good news is that there is an extra (what appears to be factory) hole further out from one of the mounting tabs of the stock OT. I measured and located a suitable replacement that'll fit that hole spacing. The only issue with that is that the new tranny is bigger in all dimensions and the cap can would interfere with its placement. I figure the easiest work around would be to remove the cap can and replace it with 3 electrolytics mounted inside the chassis.

I can't find the link right now, but the replacement OT has taps for 4, 8 & 16ohms and is supposed to be good to 15W.

Unrelated: I'm also planning to put a screwdriver pot (like a bias pot) in the jack for input #2 to use as a stealth mid control.

[Willabe]

Unrelated: I'm also planning to put a screwdriver pot (like a bias pot) in the jack for input #2 to use as a stealth mid control.

That might cause problems putting a mid control right next to the input jack. The guitar PUPs signal is so small that any signal larger than the input signal could get on the input signal and get fed to the input tube grid. That could cause parasitic oscillation.

                Brad      

[eleventeen]

"That might cause problems....."

Was gonna say the same. *Very* close (could you actually get closer?) to the guitar input signal!

[jonrpick]

BTW, this is the OT I'm looking at getting:

http://www.classictone.net/40-18031.html


And the spec sheet: http://www.classictone.net/40-18031.pdf

Classic Tone 40-18031.


I attached a pic of the extra hole in the chassis next to the OT.

[jonrpick]

That might cause problems putting a mid control right next to the input jack. The guitar PUPs signal is so small that any signal larger than the input signal could get on the input signal and get fed to the input tube grid. That could cause parasitic oscillation.

                Brad      

Well...  Durn. 

[jonrpick]

Well...  You can tell it's been a while since I studied anything electronic.  I'm having to regain my bearings.

I completed unsoldered the Intensity pot, and measured outside of the circuit it goes to 21.3K on either end.

So, while it's good that the pot is functional, I'm back to square one on why my tremolo isn't working.   

[jjasilli]

I'm back to square one on why my tremolo isn't working 
The good news: there's only 1 square (and a tube)!    1st check the tube - swap it out for a known good one.  If not fixed. . .

Then it's easier to rebuild an entire new trem circuit than to diagnose the old one. It's OK to use ceramic or mylar caps, as they are not in the signal path.  Post a schematic or layout above your workbench and mark-off each component with a yellow highlighter as you go, to keep track of your work.

[eleventeen]

If you are religiously committed to not drilling another hole in the chassis, perhaps you could mount the new OT on a piece of flat aluminum stock, say 1" x 3.5" x .25 or .375". An adapter plate, as it were. You'd have to figure out some way to get to the old holes which by definition will be covered up by the footprint of the new OT. Drill & tap....drill & countersink/counterbore.

[jonrpick]

The good news: there's only 1 square (and a tube)!    1st check the tube - swap it out for a known good one.  If not fixed. . .

I already tried 2 different 12AX7's, (one brand new) so I know it's not that.

Then it's easier to rebuild an entire new trem circuit than to diagnose the old one. It's OK to use ceramic or mylar caps, as they are not in the signal path.  Post a schematic or layout above your workbench and mark-off each component with a yellow highlighter as you go, to keep track of your work.

True...  I just don't like doing that.  Not that I'm not sold on the idea of replacing all caps and resistors with new (I like that idea).  I just really want to be able to 100%, conclusively diagnose it.  Mainly just from a learning perspective.  I'm more in this for the knowledge gained than trying to save money by DIY'ing it. (though that's nice too...)

Also, so far, every cap and resistor I've tested has been very close to spec.

If you are religiously committed to not drilling another hole in the chassis, perhaps you could mount the new OT on a piece of flat aluminum stock...

Yeah...  I actually thought of using some sort of adapter plate, but I'd rather just inboard the electrolytics and do away with the cap can to free up the space.  The existing holes plus the additional unused factory hole will suit my needs just fine.  I want to put all new caps in here anyway, so it's no biggie.  I'd just get some plastic snap-in hole cover/filler plate to cover up the hole left by the cap can.


On a side note, while I was resoldering the Intensity pot back into place, I decided to go ahead and swap out the RCA output jack for a 1/4".  No more 2" long adapters hanging out of the bottom of the chassis.   

[jonrpick]

Thanks for the tip on desoldering the components before measuring... (slaps forehead)

I measured everything in the tremolo circuit except the 25uF cap.  My meter is a cheapo model from Harbor Freight and won't read down that low on capacitance.

I've taken the schematic image and edited it.  You'll see the factory specs for the resistors and caps, and in the newer font, you'll see my measurements.  Unfortunately, the values seem *pretty* darn close to where they should be.      

[jjasilli]

I'm more in this for the knowledge gained than trying to save money by DIY'ing it. 

General rule:  1st get the circuit working to manufacturer's spec.  Then try various mods to expand the learning experience.  The best way to deal with or repair a malfunctioning trem circuit is to entirely rebuild it, rather than diagnose it, according to published authors on the subject.  Then try various mods to speed and intensity later.

[jonrpick]

General rule:  1st get the circuit working to manufacturer's spec.  Then try various mods to expand the learning experience.  The best way to deal with or repair a malfunctioning trem circuit is to entirely rebuild it, rather than diagnose it, according to published authors on the subject.  Then try various mods to speed and intensity later.

Got it...  But still, where to start?  Should I just replace all components related to the tremolo circuit?

[sluckey]

Should I just replace all components related to the tremolo circuit?

I wouldn't. I'd replace that 470K that measured 540K and I'd replace the 25µF cap on pin 3 of the oscillator. Then measure voltages on pins 1,2,3,6,7,8 of the trem tube and compare to schematic and also post the numbers here. Then change the other 12AX7 and measure voltages on pins 6,7,8 and post here.

Now troubleshoot. You say my trem is weak, but what does that mean. Do you hear the trem effect at all? Is it just too weak for your taste or is it barely noticeable? Will the speed pot adjust the speed?

Divide and conquer. A trem circuit consists of a low freq oscillator and a modulator. You can tell if the oscillator is working by checking the voltage on pin 1 of the trem tube. If it's working, the voltage will not be a steady 170vdc  as indicated on the schematic. It will be bouncing around. On an analog meter the needle would be swinging up and down or fluttering. On a dvm, the numbers will just be changing constantly. If you have a dvm that can measure low freq AC, you can actually measure the amplitude. If the oscillator is not working properly, the voltage on pin 1 will be pretty steady.

I'm gonna stop right here and let you do the stuff I've suggested and report back. BTW, re flabby bass, have you changed the cathode bypass caps as I suggested earlier?

[jonrpick]

I wouldn't. I'd replace that 470K that measured 540K and I'd replace the 25µF cap on pin 3 of the oscillator. Then measure voltages on pins 1,2,3,6,7,8 of the trem tube and compare to schematic and also post the numbers here. Then change the other 12AX7 and measure voltages on pins 6,7,8 and post here.

Now troubleshoot. You say my trem is weak, but what does that mean. Do you hear the trem effect at all? Is it just too weak for your taste or is it barely noticeable? Will the speed pot adjust the speed?

Barely noticeable.  If there's any other noise in the room you can't tell it's on.  In a dead quiet room you can hear it slightly.  The speed pot does affect the speed as it should.  Turning the intensity pot down to 0 does make it go away entirely.  So I'm assuming that the LFO is functioning and doing its job.

Divide and conquer. A trem circuit consists of a low freq oscillator and a modulator. You can tell if the oscillator is working by checking the voltage on pin 1 of the trem tube. If it's working, the voltage will not be a steady 170vdc  as indicated on the schematic. It will be bouncing around. On an analog meter the needle would be swinging up and down or fluttering. On a dvm, the numbers will just be changing constantly. If you have a dvm that can measure low freq AC, you can actually measure the amplitude. If the oscillator is not working properly, the voltage on pin 1 will be pretty steady.

I'm gonna stop right here and let you do the stuff I've suggested and report back. BTW, re flabby bass, have you changed the cathode bypass caps as I suggested earlier?

Got it...  The "good" electronics parts house in town is too far away to get to at this point in the day (Atlanta traffic and all).  I'm going to head to Radio Shack and see if they have the 470k resistor in a 1/2W / 10%.  If not, I'll have to postpone my tweaking until I can get down to the good shop.  Then I'll check out all of those things and post back when possible.  This will be my first time checking anything in a live, running amp, so bear with me.  My progress may be slow as I'll be super careful while doing it.

Regarding the cathode bypass caps...  No, I don't have spare parts on hand.  I was hoping to make better progress today so that I knew which specific parts to pick up.

[jonrpick]

I wanted to conclude this thread... it's been a while.

Life, in general, got in the way. Let's just say it's been a crazy few months.

Either way, after all was said and done, it ended up being the 10uF cathode cap on V1b.  I wasn't even paying attention to that because I had tunnel vision from staring at the schematic and the tremolo "section" is separated and down by itself on the schematic.

I just wanted to properly end this thread just in case someone else has the same issue.  Maybe it'll help.

Also, in reference to swapping out the original OT:  I ordered the Classictone OT mentioned earlier in the thread, it's on the way.  The cap can would have interfered with its placement since it's larger, even though it'll mount using existing holes.  I removed the cap can last night and replaced it with three 20uF electrolytics mounted inside the chassis.  Pic included...

Thanks very much to all that helped.