Marshall 2020 transformer reliability

[Mick Bailey]

If I hadn't already spent time and money on it I would. Just trying to save it at the moment and at least get a better balance between performance and reliability, but it's difficult. I'm getting 9.3W per channel at the point where the OPTs don't get hot. Perhaps I'll be able to squeeze a little more, but not much.

[pdf64]

Can another fan or 2 be squeezed in, maybe blowing out of the top / bottom, at the 'other' side of the OTs with regard to the existing fan, to get some airflow across them?
eg 40mm 12V pc fans, can be pretty quiet when driven from rectified heater supply.
Pete

[Mick Bailey]

There is a little more room in there to improve airflow. The OTs don't get much air at the moment and the thermal contact with the chassis is poor. I'm going to machine up a heatsink mounting that's in contact with both the chassis and lid (bolted through both) and get rid of the present arrangement, and at the same time move the transformers further apart. Bit like how the older Edens mount their power amps. Then stick an additional fan in the case at the side of the OTs.

Even so, the thermal gradient between the bobbin and core is the limitation, but it should go some way towards squeezing a bit more out of it.

[J M Fahey]

Try to stick a new small fan blowing *straight* into the OTs and make some kind of baffle (even cardboard will work) to guarantee they are in some kind of "wind tunnel".

What you really want to cool is copper, but keeping the core fresh will help suck heat from the inner windings.

FWIW I'm sort of obsessive (who isn't around here? ) and measure *everything*
I found that my bigger amps started losing an appreciable chunk of power after 1 hour or less.
The cause?: dropping rail voltage.
The cause?: hot copper= higher winding resistance.
In capacitive input PSU, they get charged in short narrow pulses, so even *a little* resistance increase causes big damage.

So for the last 20 years or so, I have been *side* mounting my fans, with the transformer almost touching them, and mounted sideways, so strong airflow blows all around the coil winding, including between coil and iron (there's 1 or 2 mm separation).

Don't have one of them now for a gut picture, but here's an external one.
The transformer is bolted to the cabinet itself , to reduce stress on the chassis; it's also thick contact cement glued to the cabinet.

[Gtr_tech]

Some of the larger Traynor amps were setup like that with the fan in the side of the cab.