Fender Studio Bass Amp

[Tone Snob Geezer]

My '78 200 watt Studio Bass has been my sidekick for 30 years but over the last few years it rattles pretty good for a few seconds when I take it off Standby, and then after about five minutes of idle warm up is sounds exactly like dropping the needle between the grooves on an LP with the volume way up. Pop, hiss, pop for about three seconds and then abrupt silence and the amp works normally. It doesn't seem to matter if the signal is going through the EQ or not. It does it with all the volumes at zero as discovered during Sunday Service.

The tubes have all been checked and the filter caps are all within spec, however, the peaks on the scope have a little wiggle at 1K with a dummy resistor in place of the speaker, so I know they are not perfect and due for replacement but there is no hum to the speaker... yet. I've done the chopstick test and sprayed the pots and it all seems well connected.

It has a multi channel EQ and so the chassis is loaded with components which is a departure from the AB763 design I am used to.

Do these symptoms point to anything that you would say is typical in the failure of a certain component? I think the sound coming off Standby is key in the diagnosis.

Thanks!

My daughter says I need to use these things because she can't tell my mood.

[pdf64]

http://bmamps.com/Schematics/fender/Stu ... ematic.pdf

How have the tubes and ecaps been checked? Only a tiny % of testers would put as much voltage stress on these as your amp does.

The best test for tubes is to swap in known good ones of the same type (and if power tubes, similar idle current).
Hence keep known good spares of each type available.

As none of the volume controls affect the noise, the problem almost certainly is in the V4 LTP, V5 CF buffer or power tube circuits.

If the tubes really are good, then 220k LTP plate resistors are the next most likely suspects; suggest that you just change them out, suggest 1 or 2 watt carbon film type.

[Stevem]

Replace the standby switch with a DPST type and use both sides of it, next replace all the plate load resistors with 1 watters , and I don't care how the power supply looks at 1K , test it at 80 hz, in fact don't bother just stuff in all new filters including the bias circuit filter which should be increased to 75 to 100 volts!
Once all the new filters are in wind up the AC into the slowly over the course of 4 hours and you'll have your buddy back better than ever!

[Phil_S]

+1 on what Steve says. My first reaction was plate load resistors for the rice krispies. Due to age, changing all the electrolytic caps would be routine maintenance and probably way over due if not previously done.

[Tone Snob Geezer]

Thanks for the suggestions.

The tubes were tested on a TV-7/U tester which may not push them even close to their operating load. The problem with swapping them out is there are six 6L6's that don't come cheap. If I had that kink of change in the ashtray I'd just replace them. I could do a couple at a time I suppose. The caps were tested with a Klein multimeter with a capacitor tester, but again, no load conditions.

The DPDT is a a great idea. I replaced it not long ago with a Carling because the old one was getting a little sloppy. The replacement didn't help the noise.

The schematic specifies a 1k signal which I thought was a little high for a bass amp but I guess it shares much of the same construction as the Super Twin.

A problem with a plate resistor makes sense with the amount of time it takes for the problem present itself.

Would it be ill advised to pull one tube off of each side of the output and go through them my elimination instead of substitution? It is a bias balanced between 3 of the 6 outputs. Just a thought.

Thanks!

[pdf64]

...Would it be ill advised to pull one tube off of each side of the output and go through them my elimination instead of substitution? It is a bias balanced between 3 of the 6 outputs. Just a thought...

I don't see a problem with that - good idea!
But if it doesn't make the noise without tubes in the V4 and V5 slots, that would also help eliminate the power tubes.

[Stevem]

If the Cap tester does not test for ESR then pretty much concider the results useless!

For killer none brake the Bank output tubes , check out SND tubes and the 6BG6GA tubes and adapters they have for them, I think you have the needed 1 5/8" hang down height to make them work!

[pdf64]

Perhaps literally killer - the combination of plate cap tubes, open back amp cabs and high capacity 500Vdc supplies seems way too hazardous to me, a (fatal) incident waiting to happen.
The open sections of the rear panel should be meshed over, like a Marshall, if such a sub was made.

[Stevem]

There are many fixes for that issue , and many of us who are over 45 and have been full time players have got the shock of a amp with a ground switch in the wrong position more times then we have fingers to count on and lived!
I think it's pretty dam easy to just keep you hands out of the back of the amp while it's on!!

[JazzGuitarGimp]

Which works fine for the owner, not so much for inquizitive toddlers....

[TUBEDUDE]

Darwinian law.

[martin manning]

... got the shock of a amp with a ground switch in the wrong position more times then we have fingers to count on and lived!

No surprise there... The 0.047u ground cap puts 56k ohms between the 120V AC line and the chassis so the max current is only a couple of mA.

[Tone Snob Geezer]

The voltages within this thing are no game. One hand in the pocket whenever it is open and powered up, and the filter caps get drained every time! I got hit by a Super Champ X2 after it had been off and unplugged over 24 hours. Never take a chance.

The parts are on order.

[Stevem]

All you need to do to drain it out in 30 seconds is to turn on the standby switch with the power switch off!
You should never drain out a cap by just jumping it right to ground, you can do big time harm to them that way!

I've been hit by a SVT a few times to the point where I had little black hole in my finger tip, that wakes you up darn fast!!