hum problem in tube amp

i have a small musicmaster bass amp from the 70s thats starting to hum. Its volume control dependant so im thinking its in the preamp stage.

obvously resistors are easy to check, how do i know if one of the caps are bad? how do you 'check' these ???

did you change the tubes? Do that first. then check the fil hum bal pot or two resistors used for hum balance. You need a good cap tester to check a cap other wise you sub them in.

The change with vol could mean all that is bad is a preamp tube(s)

[quote author="buschfsu"][...] how do i know if one of the caps are bad? how do you 'check' these ???[/quote]

Just put a 47uF or similar size cap (with correct voltage rating and polarity) temporarily in parallel to the suspect cap. If the hum goes away, that cap has lost most of its capacity and needs replacing. It won't hurt to replace all electrolytics anyway...

;Matthias

will do,

i replaced the preamp tube (1 12ax7) and both power amp tubes (6v6) no help...

when i remove the preamp tube the hum goes away. im thinking preamp stage...

I will report back after changing out the caps.

thanks

I thought I was the only person who played a Musicmaster Bass!
Mine had 6AQ5's .
Weird amp. Notice that extra transformer? That's an innerstage. Only music instrument amp I know of with one of those.
That was a werid time for Fender, they hired Rivera to do some circuits, were building overdrive specials, new box out every week, etc.

That amp was probably one of the best examples of cheapmanship gone bad. Single strand wire, cheap tube sockets, etc.


That's an easy circuit so it should not take long to fix.

Call me lame but start with the guitar chord. A bad sheild connection can cause hum.

Then go for the power supply. Bridge the main filter cap with a beefier one and see what that does.


I used that amp for guitar, so I ripped out the innerstage and put in an extra gain stage to make it crank.

Nice light amp, probably the lightest 12 inch amp around.

hey thanks for the reply!!

this was built in 80 so it has the 6V6s.

it does have an audio transformer before the power section, (then the usual output and power transformers)


I did the Torres mod. helped it at bit.

http://www.torresengineering.com/musbasmag.html

no luck on the cable, i turned it on with no cable plugged in and still the volume control dependant hum (sounds like 60Hz)

I'll check all the caps based on what you said.

Here is the schematic

http://www.ampwares.com/ffg/schem/musicmaster_bass_schem.jpg[/[/url]

I assume the filter caps are the ones marked 20 300v. (20 F??? i doubt it, what is the default when looking at cap values, uF?)

yes, thats right. just use a couple of alligators.
do it hot, live a little!
(oh boy, here come the saftey freaks! never paddle down the Amazon with a safety freak. it won't work out)

wow that looks cheap. CJ did you change the fil wiring?

why is there an input cap? .0047uf

470k not a 1meg???????

300V caps with 295V??????????? at the first power supply cap Cheap. It might be overvoltage with todays higher wall outlet voltages in the USA mine often is 120 to 122.

If it is the pre, the weird thing is that is the last cap in the RC power supply and most of the hum should be reduced by then.

Check/ change the 2uf at the cathode of the first triode. use a 2.2uf to 10 uf or even bigger if you want.

Whats is the torres mod?

6V6 amps can be cool I would leave the interstage and increase gain some where else before the interstage driver or just stick a clean boost in front.

thanks.

Torres mod link given above. changes out a few parts. pot values, adds some caps. 18bucks for 'musicmaster bass magic'

It sounded great until this hum thing. i may have fried something cause i just replaced the 1980 fender power tubes with brand new JJ/tesla 6v6s tubes (matched but this thing is cathode biased).

a les paul at 7 with this amp at 9 is really nice!!! raw but nice

i reread your post.

if i want to beef this up i should increase the voltage rating on the 20uf power caps?

replace single strand wires

anything else???????

wow, didn't think this post would get anyones interest.

:guinness: :guinness: :guinness: :green:

Is the input jack grounded to the chassis? Make sure this ground is secure; if it's via a lock-washer, maybe replace the lock-washer.

Peace,
Paul

the 0.047, might be for bass limiting, since this is a bass amp. maybe the speaker was complaining about "Into the Void" basslines.
(I think it's in D flat)
that innerstage might not have the top end if using this amp for guitar.
since they knew it was a bass amp....

yes I would change the heater circuit, unground the green wire and run two 100 ohms to ground thing.
have to run an extra wire to the now ungrounded heater tube socket terminal. ( :evil: )

HOLY SHIT. i just found the fender tube label inside the amp and it asks for a 12ax7 a 6V6 and a 5y3gt. Everything ive seen about this amp says it runs on 2 6v6s. could this have caused my hum and reduced power? i guess ill be buying the 5y3gt

The Sovtek 5Y3GT is an indirectly heated diode intended for rectification in general purpose audio equipment.


does that mean its not even a real tube????

baffled.

does that mean its not even a real tube????

Click to expand...

No, it means it's a rectifier tube; it's definitely real! Comes right after the XFMR, rectifies your input voltage. (120)

oh yeah, cuts the negative waveform off the ac cycles. thats right.

that sounds like the culprit. No recifier would equal 60Mhz hum correct? hope i didn't fry anything!!

Well, the question is, what do you have in place of the rectifier tube? If the tag in the amp is the original, then you definitely would want to get that tube (or it's closest substitute). I only saw mention of 6V6 (output tube), and 12AX7 (preamp tube), so what is currently residing where the rect. tube (5Y3GT) should be? :grin:

another 6v6.

Everything ive seen says that this amp runs on 2 6v6s. it shows a center taped power transformer with diodes.

No diodes in my amp so i guess thats the answer.

it seems strange that this amp runs on only 1 power tube.

it seems strange that this amp runs on only 1 power tube.

Click to expand...

If you mean power tube as in power supply (rectifier), then not too strange, because I believe the 5Y3GT is a dual rectifier (2 in one tube). You might want to confirm that, though. (My '65 Vibro has a GZ34 for the rectifier, so that wouldn't help ya!) lol

thanks but i meant its strange that it would run on only 1 6v6

it seems strange that this amp runs on only 1 power tube.

Click to expand...

Yeah, ya didn't specify OUTPUT power tube, so I assumed you were talking about the rect! The amp doesn't "run" on the 6V6, meaning the rect. tube is the one that runs it; without the DC, nothing would run at all! (6V6 just drives the output, spkr, dummy load, etc.,)
Anyway, sounds like a good place to start! :thumb: :sam: