Fender MV Twin Help.

[beckspeed]

I recently acquired a Master Volume Silverface fender twin with the push/pull switch. I'm chasing a few gremlins which I was hoping you could help with.

1) it's got a super noisy bass sound on the reverb channel which goes away if I turn the bass pot all the way down, but then it has no bass at all. any ideas? I'm going to do the tube check as outlined here. http://el34world.com/Forum/index.php?topic=14841.0 But I don't know where else to look.

2) the Master volume pot is busted. the knob falls right out with the shaft for the switch on it. I would like to repair / replace this, but I'm having a hard time finding the correct "1m audio taper potentiometer" pot. I would gladly eliminate the push/pull feature as the "1M audio taper" pots are available all over the place, but I'm not sure what to do with the other leads going to the switch.

3) the vibrato speed potentiometer is busted off. it looks like I need a 3.0M-RA pot

Thanks in advance!

[pompeiisneaks]

Doug sells a dual pot here: http://hoffmanamps.com/MyStore/perlshop.cgi?ACTION=enter&template&thispage=PotsKnobs&ORDER_ID=!ORDERID! look for "Alpha Dual amp pots" That would work for your MV, if it is PPIMV type. 

CTS Amp pots have a 3m as well.
As for the bass issue, I'd have to look at the schematic to have good ideas, but maybe a cap somewhere in the line that's supposed to pull down the bass isn't working right?  Also the tube related to the driver or recovery part of it may be having issues.  I'll look at the schematic and see if anything screams out to me.

~Phil

[pompeiisneaks]

If this is the schematic http://acruhl.freeshell.org/mga/schematics/amps/mid70s100w.jpg it seems like there's a different kind of MV, push pull of some kind as well.  As for the reverb circuit there are quite a few caps in that chain, worth checking out.  Again, though, it may just be a tube going out.  I'd guess for the push/pull pot, you may have to hunt around.  This may work: https://www.tubesandmore.com/node/3818

Otherwise some of the other guys here are pretty knowledgeable and may know what else to look at.

~Phil

[beckspeed]

Thanks pompeiisneaks - wouldn't it be easier to eliminate the switch and just get a plain ole pot? this is the diagram i'm working from - not sure if its correct - I need to double check it to my amp. should be close though. http://ampwares.com/schematics/100W_Mast_Vol_SF_Twin_Rev.pdf

[pompeiisneaks]

If this is for you, and you don't think you'd ever use the switch, then yes, I'm sure you can just replace it with a standard pot.  The MV is pre PI, so you don't need a dual pot.  I am a bit confused at the schematic, though, as it seems to show two variable resistor arrows, so it may still need a dual pot just not the push./pull part.  It seems like one half goes to one side of the PI the other to the other half.  I'd take a look a the current one.  If it looks like a double stacked one, then it still needs to be a dual pot. 

~Phil

[beckspeed]

thanks again! I'll pull it apart and take some pics this evening.

[Willabe]

No it's a 'tapped' pot. May prove hard to find, even without the switch?

Question is.... is that particular MV with a tapped pot sound that much better or even sound good to begin with to make it worth chasing down a new 1M tapped pot?

Wait for more suggestions.

[PRR]

Heavy bass rumble/hiss can be decaying composition resistors, dirty sockets, bad tube, or bad cap.

Doesn't Doug have a page on such troubles?

[beckspeed]

ok I opened her up last night and got a ton of pics. I also noticed at least one chassis ground which is busted loose - that will be my first fix. This amp lived a hard life before I got it, as you can see from the "wang" the faceplate has. This seems like an opportunity for something to ground out. I can say that the amp played very nicely when I tested it out before I bought it. I build a new cabinet for it (treating it very delicately) and worked on correcting some of the damage to the chassis at the same time (also delicately) - and now it has the gnarly bass sound / hum. Thanks!

Photos:

[beckspeed]

while poking around in there I noticed a bundle of wires electrical taped together? (pic #5) I don't see a switch on the MV pot either? Did someone already disable the pull-gain?

[sluckey]

Looks that way.

[beckspeed]

Still hunting for my bass-noise issue. Is there a good process to determine if I have a bad tube? Is it safe to pull preamp tubes and fire it up? Power tubes?

Thanks!

[sluckey]

Substituting a known good tube is the best/fastest way to find a bad tube. You can pull tubes but I don't think that is necessary. Eliminate the bass pot first. Have you cleaned the bass pot? Swapped the pot with the bass pot from the other channel?

[beckspeed]

no I have not. I will do that!

thanks

[beckspeed]

Before I started swapping pots I decided to look at some easy and obvious stuff first. I unwrapped the electrical tape bundle and saw that they were attempting to isolate all of the wires but they all looked to be touching/shorting to each other. So I was incidentally getting some of the unwanted gain boost. I heat-shrink insulated all of the wires.

Then i started inspecting the tubes and found this. Must have been banged in transport (reverb tank was loose in the original cabinet.)

I luckily had an ECC83 sitting in a box from a previous project, snapped it in and we're back in business!

Thanks again for all the help.

[beckspeed]

well I have discovered a follow up issue to my bass noise problem. The bent tube was in V5 - the amp now has no vibrato.

I have checked with footpedal and with rca grounded out - no vib but I do hear an audible click qand there is a slight volume change.
I have good ground at the RCA for the pedal and at the INT pot

Any other checks I can do to determine the culprit?

Thanks again!

[beckspeed]

attached are the V5 voltages as seen from inside of the chassis. Everything was powered up, connected to guitar, connected to cabinet and Vib engaged. Reverb tank was not connected though.
 Thanks