Dynaco ST35 power transformer

[coolhandjjl]

I just picked up a restored ST35 with original transformers. The PA 774 power transformer sure gets hot, I guess this is normal. The two output transformers look good enough as is, but the 774 has lots of dried black lumps on the laminations and the top area of the covers. Is this most likely bubbled up or fried paint? I thought about removing the transformer and scrapping down the laminations with a carbide scraper, and respraying it and the covers with a flat black to closely match the output ones. Do I need to use engine paint to withstand the high heat? I thought crinkle/wrinkle paint would look cool, but I don't know if that type of paint will withstand the heat.

 

[ArtG]

The PA 774 does run warm, due to the fact that it’s closely sized without a lot of extra “designed in” capacity and the fact that it’s sandwiched in between the OTs. I’ve seen a couple of them fail, but in SCA-35s where ventilation is even worse. The “black lumps” are probably “tar” that’s used internally to seal it and prevent buzzing. Some oozes out in production, and little effort was taken to remove it. Dynaco didn’t worry too much about the cosmetics of the unit and put the money into more important places such as the quality of the OTs. Remember, the ST-35 sold for less than $100 in its “day”.

I remove the tar with 40-80 grit sandpaper, then sand the entire transformer with 220 grit paper to prepare it for painting. It's a good idea to wipe the unit off with lacquer thinner or "grease and wax remover" prior to spraying. Lacquer thinner will quickly evaporate but "grease and wax remover" must be completely removed with a clean rag before spraying. Soda straws work well to "mask" the wires. I’ve used standard “rattle can” enamel with good results, but if the heat worries you, automotive grade lacquer seems to tolerate heat even better. I was not happy with the quality of the last can of “crinkle” paint that I purchased, and tossed it out. I seems to me that the manufacturers are continually lowering the quality of spray can paint.

The ST-35 that I’m currently using has an outboard heater supply, which allows the PA-774 to run noticeably cooler, since the tube heaters consume 25% of the total power draw of the amplifier.

 

[Eli Duttman]

Art made good points. I would add don't scrape the lamination stack. You could remove the insulating surface that prevents eddy currents from flowing. If the rust is truly bad, consider Duro "Extend" or RustOleum fish oil primer.

 

[rcollege]

I love my ST-35. From what I hear, it has a good reputation for sound.
I've used various finishes, on various materials. RustOleum Hammered finish is one of my favorites. The results are good. Its made to give good reliable results and an attractive finish without scraping and priming. The finish is designed to hide imperfections, along with a subtle metallic sheen. I used the bronze on more than one item...
I would definitely look up the heat ratings...though 100F weather with a metal fence in the sun painted with RustOleum gets hotter than the power transformer of my ST-35 (still stock)...but thats not what I would call reliable data...you should find some heat ranges on their site.

 

[coolhandjjl]

There isn't any rust fortunately. I may leave it as is.

 

[rcollege]

I think leaving it as is may be the best idea.

I found some info on the Hammered Finish on the RustOleum site -
200 Degrees F Doesn't give me any comfort. Just too low. If your transformer did get that hot, you may have a problem...and you wouldn't want to deal with any paint failures on top of it.

Here's the info from the RustOleum site..

PRODUCT APPLICATION .
PAINTING CONDITIONS
Apply only when air and surface temperatures are
between 60-90°F (15-32°C) and humidity is below 85% to
ensure proper drying. Do not apply to surfaces that will
exceed 200°F (93°C). Do not use on galvanized steel.
SURFACE PREPARATION

ALL SURFACES: Remove all dirt, grease, oil, salt and
chemical contaminants by washing the surface with a
commercial detergent or other suitable cleaning method.
Rinse thoroughly with fresh water and allow to fully dry. All
surfaces must be dry at time of application.

 

[katabatic]

I use Krylon "High Heat & Radiator" paint (comes in spray cans) regularly on transformers. Supposed to be good to 1200 degrees. I've used the black (a matte-but-not-too-flat that goes on really well) on a PA774 and it has held up fine.

 

[rhing]

When I restored my Stereo 35, I first used a clean cloth soaked with lighter fluid to clean the surface of the PA-774 PT as best as I could. I followed up with using a tack cloth to remove any lint from the surface cleaning process. I also applied a small patch of masking tape over the PA-774 marking on the bell cover. After that, I applied a few coats of Rustoleum Automotive Flat Black High Heat Automotive Spray Paint. My PT also had that bubble-looking texture and there was some rust on it as well, but everything was looking great after I applied the paint and allowed it several days to properly cure. I did the same with the OPT's as well.

If I can also make another suggestion, please consider Dave Gillespie's Stereo 35 Enhanced Fixed Bias (EFB) modification. He sells blank PCB's that hold a replacement power supply circuit and EFB circuit. Many of my audiophile friends with expensive tube amps have been really impressed with the headroom, low distortion and tonality of my Stereo 35 with this modification. I didn't have to change anything on the chassis to accommodate this modification, and now I have the luxury of using new production tubes like the reissued Genelex Gold Lion EL84's without fear of frying them. The PT and OPT's will also run cooler with this arrangement.

 

[coolhandjjl]

If I can also make another suggestion, please consider Dave Gillespie's Stereo 35 Enhanced Fixed Bias (EFB) modification. He sells blank PCB's that hold a replacement power supply circuit and EFB circuit.

I had read about Dave's earlier mod that had four tiny boards underneath. Then I read about his new power supply/EFB board. That's the reason I bought this particular unit. I had Dave's new board already installed. I had been seeing loads of ST35's for sale on ebay, but this is the first one that had a proper restortation with new boards, and Dave's EFB already done to it. I just don't have the time to undertake kits or boards myself right now.

 

[coolhandjjl]

Are the Dynakitparts ST35 transformers made at the same place as Triode Store ST35 transformers are?

 

[rmyauck]

Somewhere I read they're not. I think dynakitparts owner said so in one of the forms. Magnequest is the 3rd maker of OPT's and they are made to exact original specs including secondary 8 & 16 ohm outputs.
They also offer nickel laminations.

 

[coolhandjjl]

Thanks!

 

[zibra]

Is tere any producent in EU wo is able to make power transformer for 240V as replacement of original one to fit cassis? I want to make some researc first before order from USA because of sipping costs.

 

[zibra]

Guys
I own original Dynaco wit PA774 power transformer. PB028 is expensive wit sipping (I live in EU) and maybe not optimal for amp. So I would like to order toroidal power transformer wit best parameters.
Original EI76 secondary windings are:
330-0-330V 0,18A
6.3V 2.5A
max output together with heaters is 90W

So lets say I would like to change secondary windings:

1x 330V 0.2A and adding 2 diodes

Is it better than symetric 330-0-330V? Is 0.2A enough or should I make it double like 0.35A? In second case it would be also iger eater voltage and I would need to increase value of resistors in power supply. I was told its bad idea because of higher impedance and resistance (?) of PSU? So is 0.2A ok if I changed 330-0-330V to 330V?

1x6.3V 3.2A
1x6.3V 1A

Separate 6.3V windings for EL84 and separate for 7247 tubes.

Toroidal transformer should be 100W or bigger? What do you think about it? Is 0.2A enough for 330V?

I would also make better power supply. I have PCB for that.