I took over a Mesa Boogie Fifty / Fity tube amplifier.
This is a stereo tube power amplifier.
However, channel B has a problem (which I knew when purchasing). I will quote a piece from the seller:
"2 years ago a transformer and a tube broke. I then had this checked at NEL (a local fix) and had a new tube inserted. Since I still play mono, I decided not to use the broken transformer. According to the manual, the poweramp can remain perfectly 'on one leg', as long as you turn the volume knob of the unused channel closed. "
I then asked the seller what caused the output transformer to be killed and whether all the tubes were replaced. To this I got the following answer:
"Indeed, an output transformer. There is still sound coming out, only very weak. Earlier whisper level. No idea, at NEL they said it could have multiple causes. One of the four lamps was cracked so that pair was taken out and replaced by a new pair. Channel B now has the old lamps. Channel A new. Furthermore, they have (if I remember correctly) also replaced some diodes and capacitors. There is nothing wrong with that. "
I do not have the amplifier yet, but it should be sent today.
I can order a new OT from TAD but I first want to know whether it is / cannot be something else.
Before I am already gaining information.
Here I have some questions:
- If an OT is broken, does any sound come out at all? Although the volume is very low.
- Is it plausible that an OT is broken. Or is it better to look into surrounding components? Or perhaps solder joints.
- Is there an easy way to measure or check an OT?
This is a stereo tube power amplifier.
However, channel B has a problem (which I knew when purchasing). I will quote a piece from the seller:
"2 years ago a transformer and a tube broke. I then had this checked at NEL (a local fix) and had a new tube inserted. Since I still play mono, I decided not to use the broken transformer. According to the manual, the poweramp can remain perfectly 'on one leg', as long as you turn the volume knob of the unused channel closed. "
I then asked the seller what caused the output transformer to be killed and whether all the tubes were replaced. To this I got the following answer:
"Indeed, an output transformer. There is still sound coming out, only very weak. Earlier whisper level. No idea, at NEL they said it could have multiple causes. One of the four lamps was cracked so that pair was taken out and replaced by a new pair. Channel B now has the old lamps. Channel A new. Furthermore, they have (if I remember correctly) also replaced some diodes and capacitors. There is nothing wrong with that. "
I do not have the amplifier yet, but it should be sent today.
I can order a new OT from TAD but I first want to know whether it is / cannot be something else.
Before I am already gaining information.
Here I have some questions:
- If an OT is broken, does any sound come out at all? Although the volume is very low.
- Is it plausible that an OT is broken. Or is it better to look into surrounding components? Or perhaps solder joints.
- Is there an easy way to measure or check an OT?
The transformer is the most reliable part in the amp. It COULD be bad, but so could a lot of other parts.
A OT can fail in various ways. And that goes from blowing fuses down to making noise or weak sound.
A OT can fail in various ways. And that goes from blowing fuses down to making noise or weak sound.
An OT can be severely damaged by running the amplifier with no load connected. Even some short moments will do, if the output level is high enough. It will arc internally, damaging the wire and layer insulations, and finally cause shorted turns.
You've got a stereo device. Simply swap both OT's and observe if the issue swaps with them.
Best regards!
You've got a stereo device. Simply swap both OT's and observe if the issue swaps with them.
Best regards!
You got that absolutely right. Only I would like to be careful that I do not immediately have two broken OT. Because there may be a problem elsewhere in the amplifier.
Just spoke to the seller and he says the following.
"A 212 cab was linked to that.
And the sound subsided, crackles came out and the amp smoked. There was literally a crack in one of the tubes. The smoke came from the capacitors so no idea what exactly was the cause. "
So there would have been a load attached to it. The 220uF power capacitors have been replaced + the diodes.
Here is the schematic for clarification.
I just added channel A. Channel B is of course the same.
I just added channel A. Channel B is of course the same.
It looks like all you need to do is disconnect the feedback wire from the transformer's output, maybe pull the output tubes from their sockets and then you can test it with a battery and your fingers. Although a neon bulb might be better.