Fifty Fifty issue

[Jimmy Fournier]

Hi
I’m new in the forum, I’m a owner of several Mesa amps and I’m very proud to be part of this community.

However I have an issue with one of my pride and joy amp, the Fifty Fifty stereo rack that I use for Wet-Dry-Wet setup.

Last week the fuse blowed and then I change it (3 amp Slow Blow) with the same result. I tried to swap with other new power amp tubes 6L6 GC and new preamp 12ax7 tubes and I got the same result. I don’t know what to do next to troubleshoot it. I have experience with amps and I can do basic electronic reparations.

Is someone can give me an advise ?

 

[domct203]

Welcome to the forum.

Be advised, tube amps are different from their solid state counterparts. The power supply capacitors in a tube amp will store around 400-450V, even when unplugged and unpowered for days. That’s enough voltage to stop your heart. If you are unfamiliar with the process please educate yourself on how to properly discharge that voltage before doing any work inside the chassis.

Now with that out of the way…

Will the amp power up with all its tubes removed?

Have you opened the enclosure and visually inspected for signs of failed components?

Dom

 

[Jimmy Fournier]

Welcome to the forum.

Be advised, tube amps are different from their solid state counterparts. The power supply capacitors in a tube amp will store around 400-450V, even when unplugged and unpowered for days. That’s enough voltage to stop your heart. If you are unfamiliar with the process please educate yourself on how to properly discharge that voltage before doing any work inside the chassis.

Now with that out of the way…

Will the amp power up with all its tubes removed?

Have you opened the enclosure and visually inspected for signs of failed components?

Dom

Hi Dom
Thanks for the safety advise ! I will refresh my procedure, it’s been a couple years since I opened a tube amp.

Yes the amp power up (in standby) with all its tubes removed (power and preamp)
Visually everything seems to be fine inside. (See picture)

 

[domct203]

Ok, that’s a good start. Put just the 12AX7’s back in and try again, if the fuse blows there is most likely a failed component in the amp and it needs to be serviced.

If the fuse does not blow, working with just one channel at a time, put both power tubes back in and power up. If the fuse fails, something is wrong with either the those tubes themselves, the tube sockets, or the few components directly related to those sockets (screen grid resistors, etc).

If the fuse does not blow, repeat for the other channel. Same diagnosis applies.

How do the power tubes fit in their sockets? Are they tight, or do the seem too easy to remove? An intermittent connection from a loose fitting socket can blow a fuse. Is there any black carbon inside or between the holes in the sockets?

Dom

 

[Jimmy Fournier]

Ok, that’s a good start. Put just the 12AX7’s back in and try again, if the fuse blows there is most likely a failed component in the amp and it needs to be serviced.

If the fuse does not blow, working with just one channel at a time, put both power tubes back in and power up. If the fuse fails, something is wrong with either the those tubes themselves, the tube sockets, or the few components directly related to those sockets (screen grid resistors, etc).

If the fuse does not blow, repeat for the other channel. Same diagnosis applies.

How do the power tubes fit in their sockets? Are they tight, or do the seem too easy to remove? An intermittent connection from a loose fitting socket can blow a fuse. Is there any black carbon inside or between the holes in the sockets?

Dom

Do I have to do these tests in Standby mode or do I have to pass from standby to ON with a load box in each channel output ?

The test I have done previously with all the tubes removed was only in standby mode, never pass in all operation for this test, should I also redo it by running in full operation mode ?

 

[domct203]

Turn your input control all the way down, and don’t have anything plugged into the input, you are not passing a signal so a load box is really not necessary, but it’s good insurance and I would use one (or a speaker cab).

If the amp powers up in standby then yes, you can take it out of standby for each test as a final check.

Are you 100% certain the ‘new’ 6L6’s you are trying are indeed good?

If the tubes are 100% known to be good and the fuse still pops there is an internal issue with the amp.

Are you absolutely sure that’s not supposed to be a 4A fuse? Most 100W amps will use a 4A fuse. Is 3A what is printed on the amp next to the fuse holder? Your 50/50 doesn’t have a full preamp so it may indeed be 3A here, just covering all the bases.

Dom

 

[domct203]

Forgot to add that you don’t have to redo the preamp tube test, the standby switch is for the power tubes.

 

[Jimmy Fournier]

Forgot to add that you don’t have to redo the preamp tube test, the standby switch is for the power tubes.

3 amp Slow Blow is according to the operation manual p.4 :
«
LINE FUSE
The Fifty/Fifty is protected by a 3 amp fuse when operating on 100 - 120 V. AC. Should the fuse blow, replace it with a Slo-Blo fuse of the same amperage. »


Here’s tests I have done so far, no input, volume ctrl at 0 :


Test A
1 - Remove all the tubes (12AX7’s and 6L6’s)
2 - Power up, Standby mode, Low Power switch : for 2 minutes everyting ok
3 - Take out Standby mode : for 30 secondes everyting ok
4 - Go back to Standby mode
5 - Switch to High Power mode
6 - Take out Standy mode : FUSE BLOW

Test B
1 - Put a new fuse, put back 12AX7’s tubes, NO 6L6’s
2 - Power up, Standby mode, Low Power switch : for 2 minutes everyting ok
3 - Take out Standby mode : for 30 secondes everyting ok
4 - Go back to Standby mode
5 - Switch to High Power mode
6 - Take out Standy mode : FUSE BLOW

Do you think I have an internal problem or should I perform tests with 6L6 (brand new) ?

 

[domct203]

I’m pretty sure you have a problem other than tubes, the fuse is blowing without them even in the amp, and you already tried with a different set of power tubes.

Maybe someone else reading this can offer more specific help.

In the meantime I’m going to see if I can find the schematic to see what exactly is going on with that low power/high power switch.

You might want to start looking for Amp Tech’s in you area…

 

[user 41014]

You might want to start looking for Amp Tech’s in you area…

Agreed.
I have a 50/50 here and am happy to help with any reference voltages, etc but it sounds like there is a problem.

If it's blowing fuses and the wall volts are the same that means the current draw is higher than is good therefore something blocking (capacitors block DC) or stepping down volts is missing.

I would 1st get "honey I shrunk the kids" goggles on and look over everything for traces of heat damage, filter capacitor positive end bulge/leak and wouldn't proceed with anything until draining the big blue caps (they may have bleeders but you don't know if they are working) and then doing a dental probe with a continuity tester on every filter cap, orange drop, varistor(s) and smaller e-caps. That was a mouthful.

Any wires loose? I'd be looking for a short and if none of the low hanging fruit leads you to an answer I would want to know what @domct203 finds out about the power switch and trace that visually for a bare wire touching something. Next up for me would be to test the voltage drop resistors on the PS. PS diodes maybe too.

For transformers I would like to think you can rule out the OT if there is no signal and no tubes to amplify but B+ does live on the primary of the OT.

 

[domct203]

Looking at the schematic in this thread it looks like the Hi-Low switch simply by-passes a resistor when closed.

It’s hard to read the part ID’s but I would start with checking that resistor and the switch itself. There’s not much to the circuit, it might just be a bad switch. The resistor will need to be checked out of the circuit, unless it’s obviously damaged on the exterior.

https://boogieforum.com/threads/50-50-first-version-schematic.85156/
Dom

 

[John Micka]

2 cents to check.
When I got my 50/50 after a while it was blowing fuses and I checked most everything.
The fuse holder didn't seem right after replacing fuses a few times. I took it out and replaced it. Never had that problem again. It's been many years.

 

[user 41014]

2 cents to check.
When I got my 50/50 after a while it was blowing fuses and I checked most everything.
The fuse holder didn't seem right after replacing fuses a few times. I took it out and replaced it. Never had that problem again. It's been many years.

I had not heard this one- thanks for sharing.