• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

ST-70 Power Transformer

jonnykun

Member
2012-11-22 4:39 pm
I am building an ST70 clone and need to select a power transformer. I am ordering my output transformers from Edcor , CXPP60-MS-4.2K, and I would like the power transformer to also come from Edcor to save on shipping cost.

So far I found the XPWR030-120. Since there is no 5V winding I plan on buying another small transformer to supply this voltage to the rectifier tube. I also will need to drop the 130V bias to 55V, which I can do using a voltage divider. Will this work out?

Thanks!

Edit: From searching the forums the XPWR002-120 is also another option, but I am wondering if the maximum output current is too low at 220mA. Has anyone used this transformer?
 
Last edited:
If your circuit can tolerate a bit more B+ voltage take a look at the EDCOR XPWR233-100. It has 360-55-0-360 with both 6.3Vac and 5Vac filament windings.

As mentioned above you could specify exactly what you want, and sometimes is not much more money. Remember not much come off the shelf from EDCOR so any way you do it you have to wait a couple of weeks so custom might just be fine.

I like their products, very nice finish and good value and quality.

Sandy
 
Hi,
Well, the old ST-70 is up and running again, but there's still some transformer humm.
It's MUCH quieter, but the hum is still audible from the listening chair, which I don't find acceptable. It's only a mechanical humm, it's DEAD quiet through the speakers, you can only hear a slight tube rush if you have your ear right up to the tweets. I'll see if it gets quieter later at night when the AC is better.
It's still good to have it working again. It's sooo much sweeter than the Adcom GFA535II that I picked up as a temporary replacement, it's silly.



TV Repair Minneapolis Pros | 612-200-0064 | Your Electronic Experts!
 
Shoot, didn't catch the 100vac primary on that transformer. It would not work unless you plan on running a power cord to japan ;) I would just shoot EDCOR that you want the same transformer with 120vac primary windings, or spec the exact one you want. It may not be more then a couple of bucks more for the custom. Also I think I saw some specs for the ST70 xformer with a 360-0-360 winding so it might be fine even at that voltage.

Sandy
 

jonnykun

Member
2012-11-22 4:39 pm
I sent EDCOR a request to change the primary to 120V, hopefully I hear back from them on Monday. It's only my second amp build, and I'm really excited to order and put together all the parts. Maybe I'll even post some pictures along the way, since I probably will have some newbie questions for the forums.
 
This is probably blasphemy here, but you can dump the tube rectifier. Tubes are particularly ill-suited to that job. You get a much better power supply with solid state diodes there. That would solve your 5V issue.

I'm using the transformer that Sy referenced above. It's a good one. I don't use the 5V winding, so I have a bit more headroom in that thing as well.

Sheldon
 
You can look around for replacements based on specs, but there are power transformers available that are "drop-in replacements" for the PA060, meaning that they are designed specifically to fit the ST-70 chassis cutout and mounting holes. Unfortunately, the usual suppliers for these drop-ins are both out of stock, but you might contact the following vendors and see when their transformers are expected to be back in stock:

http://www.triodeelectronics.com/mq060.html

https://www.dynakitparts.com/shop/pa060-power-xfmr/

Mercury Magnetics is another source for a PA060 clone transformer. It's pricier than the others but does appear to be in stock:

https://www.mercurymagnetics.com/pr...el=pa60&filter_product-type=power-transformer

The A-470 output transformer specs are available here:

https://www.dynakitparts.com/shop/a-470-output-transformer/
 

Micror

Member
2020-06-25 2:48 am
You can look around for replacements based on specs, but there are power transformers available that are "drop-in replacements" for the PA060, meaning that they are designed specifically to fit the ST-70 chassis cutout and mounting holes. Unfortunately, the usual suppliers for these drop-ins are both out of stock, but you might contact the following vendors and see when their transformers are expected to be back in stock:

http://www.triodeelectronics.com/mq060.html

https://www.dynakitparts.com/shop/pa060-power-xfmr/

Mercury Magnetics is another source for a PA060 clone transformer. It's pricier than the others but does appear to be in stock:

https://www.mercurymagnetics.com/pr...el=pa60&filter_product-type=power-transformer

The A-470 output transformer specs are available here:

https://www.dynakitparts.com/shop/a-470-output-transformer/
Thank you, I checked all of those. looks like they are particularly doing the dynaco.... I am more interesting to find a transformer vendor that have fit transformer to reduce the price...
 
I am building an ST70 clone and need to select a power transformer. I am ordering my output transformers from Edcor , CXPP60-MS-4.2K, and I would like the power transformer to also come from Edcor to save on shipping cost.

So far I found the XPWR030-120. Since there is no 5V winding I plan on buying another small transformer to supply this voltage to the rectifier tube. I also will need to drop the 130V bias to 55V, which I can do using a voltage divider. Will this work out?

Thanks!

Edit: From searching the forums the XPWR002-120 is also another option, but I am wondering if the maximum output current is too low at 220mA. Has anyone used this transformer?
If you build a ST70 clone only A470 + PA060 will do. Other transformers will make your
build a "homebuild EL34 amp"
 
Yes, definitely I am planning st70. I believe must have some transformer manufacturers/venders have exact same products with a470 pa060 specs. to replace a470 pa060, cos I feel their price is too high
As mentioned, in order to preserve perfomance, it's always best to invest in accurate components.
Worrying about a "one time" cost and going for something cheaper is not the best way.
I invested in the Triode/Dynaco transformers, and I'm glad that I did.
 

Micror

Member
2020-06-25 2:48 am
As mentioned, in order to preserve perfomance, it's always best to invest in accurate components.
Worrying about a "one time" cost and going for something cheaper is not the best way.
I invested in the Triode/Dynaco transformers, and I'm glad that I did.
Agree. I will first try the economic one to see if their performance are really far away to the expensive one
 

Micror

Member
2020-06-25 2:48 am
I cut my diy teeth on the ST-70 about 15 years ago. I also extensively modified it and would suggest you get the kit from Bob Latino with the upgraded power transformer and front-end. Bang for buck it’s the best option.
I read a lot of topics regarding his kits and service but he retired recently so I guess the thing has been changed. I got helps here but unfortunately I couldn't found their dedicate threads here....