Dynaco Mark Iii Power Transformer Hum-help Please!!!

[pontiacpete]

I own a pair of Dynaco Mark III Amplifiers.I just had the amps gone through with all new parts,such as the sds capacitor boards and the sds preamp boards.They also replaced all the filters and resistors.Now the problem is that these start off with a very slight hum from the power transformer in both amps and after a couple of hours the hum gets worse in both amps.The amps sound great and the hum does not come through the speakers.When i spoke to the tech he told me that he checked all line voltages and made sure they were what they are suppose to be.Can anyone give me some advise on these before somthing happens to them? Thanks
pete!!

[currituckco]

Make sure the bolts holding them to the chassis are very tight. Beyond that, I'm not sure. I'm having a PT rattle problem with a Fender Princeton Reverb guitar amp that I'm also not sure what to do about...

[PakProtector]

hey-Hey!!!,
Your tech isn't telling you the whole story. Those power TX's get quite warm with any more than 115V on the primary( and it only takes about 112 on mine to deliver exactly 6.3 to the heaters). They're just too marginal. Drop the voltage with a variac or get a proper TX.
cheers,
Douglas

[pontiacpete]

Hey douglas,thanks for the reply.I am no expert at this,but i thought that it was a voltage problem.are you telling me that these have bad trannies or can the voltage be reduced to stop this? These have the original trannies on them and i have not had any problems with blowing fuses!!! Thanks
pete.

[PakProtector]

It is not so much 'bad' trannies, just underbuilt ones. Lower input voltage was the answer I found most suitable until I put the amps into retirement.
cheers,
Douglas

[mqracing]

pontiacpete:

I used to have a pair of Mk III's--- and they run hot. I used to use a good sized muffin fan on top of the cage centered over the power trans. This cooled the units down considerably.

the powers weren't the best and they do run hot--- but seemingly they survive--- there are still tens of thousands of MK III's with original powers still in service.

give them a little help in cooling off---- and service life as well as performance will probably stabilize a bit more.

msl

[currituckco]

I thought you were saying you baked muffins on them, which would have REALLY impressed me.

[NOSValves]

I've most likely rebuilt/redesigned near 100 pairs Dynaco Mark III's and have replaced maybe two power transformers out of this sample set.

#1 If they have not been previously abused

#2 The rebuild/redesign was done properly

You should not have audible hum from the PT's unless you have your ear right up to the amps unless you have extreme high wall voltage above 120VAC. Since it sounds like both amp exhibit the same noise I'd suspect a error in the rebuild/redesign (but check your wall voltage). Also make sure it truly is the power transformer humming I've seen KT88/6550 or 5AR4's exhibit notable physical noise and folks mistakenly think its a transformer (this usually happens from a high wall voltage situation). I've cured it a few times by installing a pair of CL80 thermistors in series per amp on the AC hot line to the fuse this will effectively knock about 3 or 4 VAC off.

Craig

[PakProtector]

hey-Hey!!!,
The inrush limites are also useful for reducing the current spike from cold heaters. At ~50R cold they do a good job of limiting primary voltage when the demand is high. One can wrap them to allow them to get hotter and reduce their working resistance to the ~1/2 Ohm when the load isn't as big as the CLxx's rating.

The inrush limiter is also quite nice for dealing with the turn-on spike one gets with Mercury vapour tubes like 866's upon flipping the switch to the plate Iron primary.
cheers,
Douglas

[NOSValves]

hey-Hey!!!,
The inrush limites are also useful for reducing the current spike from cold heaters. At ~50R cold they do a good job of limiting primary voltage when the demand is high. One can wrap them to allow them to get hotter and reduce their working resistance to the ~1/2 Ohm when the load isn't as big as the CLxx's rating.

The inrush limiter is also quite nice for dealing with the turn-on spike one gets with Mercury vapour tubes like 866's upon flipping the switch to the plate Iron primary.
cheers,
Douglas

Pak,

I'd rather not limit the resistance and why I suggest the CL-80 I'm shooting for the added benefit of knocking the wall voltage down after warm up not just during it. If the user is driving the amp so hard that this device ends up being a limiting factor he needs a higher powered amp anyway. I've played with many different Thermistors and found this to be the best over all compromise. The spec sheets on these things really do not translate all that well to a tube amplifier electrical environment since they were really designed for instant on, instant full current modern electronics. I've found if you use thermistors rated closer to the 1 amp idle current for instance that Mark III has they end up blowing in about 6 months of use. Seems they do not like the 2 to 3 amp repeated turn on spike.

Craig