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Bias supply for Dynaco ST70 replacement power transformer?

The problem with lowering mains voltage is that it also lowers filament voltage. So, unless the PT is intended for 220 (or whatever) instead of 240, the filaments may be too low operating down at that voltage.

So, if connected to 240 (or whatever your normal line) and the filaments are about right, then you don't really want to buck or otherwise lower the incoming voltage. You'd either run higher plate and lower bias or add more drop on the B+.

The VTA board is designed for 380-400 supply voltage (at least the newer one is, like I have). If you put that to the board's B+ terminal the rest of the stuff on the board should be fine.
 
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gadget73 said:
The backwards transformer in my Bogen gets me 90 vac out of a 120v primary, so ~75%. probably depends somewhat on the transformer, the one in the Bogen is a wee little thing.
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Possibly true. The transformers I've used when wired backwards this way are filament transformers that can source something like 2 to 3 amps on their 12.6V winding, when wired normally, so that may be part of the difference. It probably also depends on the input voltage driving it. In my case I had a spare filament winding not doing anything, which put out about 6.6V unloaded.
 
Edit: Sorry, I hadn't updated my browser to see the two posts before me.

Check your filament voltage. You may be low with the variac turned down. 222V is 92% of 240 so you are probably ok, at least you should be within 10% on the filaments. Check them anyway since you have a somewhat unknown transformer. Your filament load may be more or less than the original application and the voltage will vary with the load.

The Mullard data sheet says EL-34 is good for 800V on the plate, but only 500V for the screen grid. This would be for pentode operation. In UL the screen can see higher voltages but not steady state. I don't how far they can be pushed but you should consider adding screen stopper resistors if you don't already have them. Dave Gillespie, who is well known on AK, has written a very nice article on power tube life which discusses the use of screen stoppers among a number of other things. Good reading: http://www.tronola.com/html/maximize_tube_life.html

If you want to reduce B+ but not affect the filament voltages, this page has a neat idea for a mosfet B+ reducer. Scroll to the last item. If will need some heatsinking and a carefully chosen device depending on how much dissipation will be required.
http://www.geofex.com/Article_Folders/mosfet_folly/mosfetfolly.htm

Hope you're having fun,
John
 
Thanks who and nerd! I had already printed out Dave G's article on the ST70 for careful re-reading, so thanks for the other one.

I am having fun--half the fun is learning more about our hobby. The filament voltages were high at 240V and about right at 220V, suggesting that the transformer was intended for a lower mains voltage as is often the case. I'll check again though--I've been messing about with voltage changes on the variac today getting to know the PT under load. I can get between 380 and 400V going into the VTA board, so that's okay.

Will also check out the screen stopper resistors. Apart from the 'hours of fun', it is an audible improvement to the amp.
 
If the 12V winding is still available probably could use that for bucking...if that would be close enough. ~227V.
 
As you found it may be better sonically with the high voltages which won't hurt if you can reduce the bias for the correct dissipation. Dave Gillespie's screen grid resistors idea should help greatly also as mentioned. There is a present day PP EL84 amp that I can't recall the name of that runs the tubes at those high voltages, but runs the screens much lower and the tubes last normally with a nice powerful sound according to owners. As long as the tubes don't redplate you should be good I would bet.
 
kward said:
The transformers I've used when wired backwards this way are filament transformers that can source something like 2 to 3 amps on their 12.6V winding, when wired normally, so that may be part of the difference.
Click to expand...

I want to say the dinky little thing I used is rated for 500ma or something. Probably has fairly high DC resistance, so less voltage out the other end. I needed something compact, and compact transformers tend to be fairly wimpy transformers.
 
Thanks for more great ideas. I am using the 12V winding for bias at the moment, but I like the idea of not adding another transformer externally. It's a very robust wiring, and I guess it was used to supply filament voltage to a large bank of 12A*7s or similar. I'll just check if 227 is low enough.

Dave G's AudioXpress article is really helpful.
 
Yeah, just depends on what's most best in the big picture. The reverse transformer for bias works great as you see, and is about simple as it gets.

Not sure what's reasonably available in UK, but a 20V, 50VA Antek toroid is less than $20 here and would provide more than enough bucking capacity.
 
227V would work fine (5.3V and 6.3V for filaments), and B+ of 455VDC with 425V on plates and 435V on the grids. 423V to the board, but that can be dropped. So I'd like to use the 12V winding for bucking.

But, here's the crazy question: can I still access 12V from the same winding for the bias transformer? I think not, but maybe I'm wrong. I don't really want to use the B+ for bias unless I have to.

I'm working on this wiring method from Rod Elliott's ESP site: http://sound.westhost.com/articles/buck-xfmr.htm buckx-f4.gif
 
I've decided to go the bucking transformer route. How do I calculate size? Whoareyou, is 50VA really enough for the job? The one locally available has an output current of 2A. I can get this for about £15, but I thought I might build something bigger, say 160VA, with ability to switch in one or two secondaries, for smaller or bigger voltage drop.
 
In a bucking situation the VA rating of the transformer is relative to the amount of load bucked, not the full load applied.

In the example I mentioned of 20V, 50VA transformer, the secondary can handle about 2.5A (50VA / 20V = 2.5A) So, if the line is 240V and you're bucking 20V of that, that 2.5A at the resulting 220V means the 50VA transformer could handle as much as 550VA load in bucking mode. Despite that fairly big load, the bucking transformer itself would still be at its rating of 50VA (20V bucked and 2.5A current).

If you were thinking of a 12.6V transformer with 2A rating (~25VA) that would support roughly 455VA (240-12.6=227.4; 227.4×2=454.8).
 
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Dandy said:
But, here's the crazy question: can I still access 12V from the same winding for the bias transformer?
Click to expand...

It will work, but there are several things to be aware of:
  1. When wired in autoformer bucking mode with the primary, the 12V winding is no longer isolated from the mains, even though there is still 12VAC across that winding. (It's easy to forget this--see below). But if you connect the leads of the 12V winding to an external 12V:120V transformer run backwards for bias, that will provide isolation and at the same time power the external transformer to give you your bias voltage.
  2. The buck load will pull down the voltage available on the 12V winding a bit.
  3. The 12V winding must have a current rating equal to or larger than the current seen on the primary under fully loaded conditions.
  4. I believe the 12V winding will now be raised up to the mains voltage, so it seems wise to only use transformers with hipot ratings of 1000V or larger, to avoid any potential voltage breakdown between windings.
To validate, I wired up one of my spare multi-wind power transformers in auto former buck mode as well as at the same time connecting that same winding to an external filament transformer run backwards. Everything worked as expected--the 12V winding provided buck on the primary as well as powered the external transformer. However, before I could validate #4 with my oscilloscope, I blew the fuse on my variac because I "forgot" that the 12V winding is not isolated!
 
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Kward, thank you for investigating my crazy question and providing a few good reasons for why it was a bad idea!

I remembered I had a 16V transformer bought from a railway enthusiast to experiment with 16V heater PCL82 tubes. Wired in bucking fashion, I get 226V which gives almost perfect results 5.3VAC, 6.25VAC on the filaments, 440VDC B+, 405VDC to the board. I added 120ohm screen stability resistors and I now get about 428V on plates and screens. Any comment on whether it's okay to run the EL34s on those voltages, with the tubes biased at 0.4VDC as the VTA manual specifies?

I'm pleased with this outcome, especially as I haven't had to spend anything on the three transformers, and I've learned a lot from all the kind input here.
 
Thanks John. I'll get the bucking transformer safely into something, and then post a pic of the completed amp. It sounded even better with higher EL34 voltages, but those tubes aren't cheap anymore.
 
I've gone a bit quiet on this because I had to rebuild the VTA board and rewire the amp. After I replaced the transformer, I was getting intermittent drop-out in one channel, and intermittent interruption of the NFB connection in the other. Turns out that some of the solid core wiring had got metal fatigue and another redundant ground wire had hooked itself to an empty hole in the board. It's crazy that I didn't spot this, I know.

While the board was disconnected, I decided to put heavier resistors to carry the power voltages on the board, and to refresh all the solder connections which were done with an inferior soldering iron ten years ago. Getting there now.
 
IMG_0442.JPG

I thought I'd add this pic to complete my rebuild story. The VTA board responded well to a clean up and the use of some larger resistors in most places. With the 16V bucking transformer, the voltages are good.

Another point of interest is that I found some very cheap new Chinese EL34s on that auction site, so I decided to give them a try. The vendor is a clearance house, so the tubes are not matched. However, they bias up fine with the individual pots on the VTA board and the results are very acceptable with the voltages on grids and plates of around 430V.
 
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