Configuring Out-put Transformers ?

[Thor61]

Im currently constructing a ST35 based amp, after reading many posts ect.. I have noticed many are using different OP transformers. Could someone please explain how to configure there use for obtaining the best results. First I've seen primaries between 6.8K to 10K used, Second varied winding ratio's, And Third different core types ! Could someone please explain the above in simple terms, so myself and others may be able to have more purchase options when constructing various amps ect...

[Thor61]

With the wealth of knowledge in this forum can't anyone contribute some input as to how and why various output transformers can be used on a given amplifier. Is there not some kind of thumb rule, or simple formula for there applications when used in other than intended designs ? Example the ST35 calls for a 8K, one guy uses a 6.6K another uses a 9.6K ! What kind of results can be expected from from using varied primaries of different resistances or winding ratio's ?

[Ty_Bower]

Your question is innocent, but the answer is complex. Entire textbooks have been written to explain the topic. There are no "best" results, only various compromises.

Primary loading is based on the tube type selected, available voltage, and the desired output power / harmonic distortion. Generally speaking, lower loading gives more power at the expense of higher distortion. Keep in mind this is a grotesque oversimplification.

The winding ratio determines what impedance load is suitable for connection to the secondary. Once you've decided your "ideal" primary load, identify the typical impedance of your speakers. Get a transformer that fits. Of course, this is taken with a very large grain of salt, since no speaker has a constant impedance across the entire frequency spectrum. They tend to vary (wildly!), which makes a perfect match impossible.

I'll leave your question on core types for someone else...

edit... Here's a nice little article that coincidentally addresses your three topics: http://electra-print.com/techblog.php?blogid=4

[Thor61]

Thank-you for your reply ! The Electra-print site has answered some of my questions. The original basis for my questions stem from using Heathkit SA-2 transformers in my build ect... Wanting to obtain the best possible results, I'm hoping they will perform as well as the intended OP transformers described in the manual ect... Again Thank-you !

[Ty_Bower]

The SA-2 has the same output transformers as the AA-151. These have been used with very good success. If you've still got the original power transformer, you can use that too. It has a 5 volt winding for a tube rectifier, which you could you if you like. Here's a short thread with a wiring diagram for them:

http://www.diytube.com/phpBB2/viewtopic.php?t=187

Search for SA2 or AA151 and you'll probably find others.

[Thor61]

Yes I have the org power transformer which I plan on using. If I were to omit the tube rectifier and use the on-board SS rectifiers, would this free up some current and help the transformer operate a little easier and/or cooler ? Somewhere I had read vintage transformers were designed for say 110vac, with today's 117-125 could this heat things up ? Its really great to have someone to help answer my questions. Ty_bower > THANK-YOU !

[Ty_Bower]

Yes I have the org power transformer which I plan on using. If I were to omit the tube rectifier and use the on-board SS rectifiers, would this free up some current and help the transformer operate a little easier and/or cooler?

I think Ed (EWBrown) has an AA-151 project in progress. Maybe he can chime in and tell us more. It sounded like the voltage from that PT was going to end up a little on the high side. I'd be tempted to keep the vacuum tube rectifier.

If you want a power transformer to operate a little cooler, you might try downsizing the first capacitor in the power supply. A smaller cap here makes for (slightly) cooler power transformers - something to do with conduction angles and stuff I think. If you go the vacuum tube rectifier route, you're going to have to reduce that first cap anyway, as the stock design has a 120 uF (?) part there, which is way too big for a vacuum tube rectifier. Maybe try going with 20~30 uF, or whatever is recommended in the "Tube Rectifier Mod" links found here:
http://www.diytube.com/phpBB2/viewtopic.php?t=837

[Thor61]

This changes a few things for me ! First the chassis I'm currently using is a Hammond 12 X 10 x 2 luckily I have not made the cut-outs for the power trans yet. There is not enough room for the additional tube regulator and a choke. What are your thoughts on installing the regulator and choke on the above mentioned chassis, than remote mounting the power transformer on another smaller chassis ? If possible what are the "pro's and/or con's" for this type application ect...

[Vince]

Thanks Ty,
Very helpful and informative. Vince

[Thor61]

Ty, I also would like to thank you ! Had it not been for this additional info I may have made the last set of cut-outs leaving no room for a rectifier tube ect... I found this link > http://www.tubecad.com/2006/11/blog0087.htm About "shunt regulators" would something like this work in conjunction with SS regulation ? Just trying to explore more option's sense I hadn't planed on using a tube reg and really don't want to mount the power transformer in a separate chassis.

[Ty_Bower]

Glad to be of some help. :)

I've never put the PT on a separate chassis, though I know that many do. I guess I'm not keen on having an extra piece hanging around, and I'm really not happy about the idea of an umbilical cord with 360 volts running through it.

If I had no space for the tube rectifier, I'd probably just build it stock and see how it works out. I made my ST35 with an old Knight PT, and the resulting B+ is just under 410VDC. I was worried about it at first, but the amp seems to hold up just fine. I would recommend checking the 6.3V AC heater voltage when it is done. You want to make sure it isn't too high, or you'll shorten the life of the tubes unnecessarily. If you need to drop the line voltage a bit, trying reading up on bucking autoformers. They're easy and cheap to build out of a spare 12 volt transformer.

Can't comment on the shunt regulator... I haven't seen that one before.

[Thor61]

Ty, I read several post's including some of yours in other forum's, after continued searching I found this > http://www.siteswithstyle.com/Voltsecon ... djust.html Please take a look and let me know what you think ! Again please let me express my gratitude and thanks for all your help !

[TomMcNally]

That's a nice schematic for a boost/buck box.

When I was a kid they used to sell one that boosted only
in a black minibox for CB radio guys. The thing boosted
the line voltage by about 10 volts to supposedly increase
the transmitter power above the legal 5 watts input.

I think all it did was burn out the lights in the meters
and shorten the life of the tubes.

[Thor61]

Tom, What are your thoughts of using this or something similar for dropping the line voltage when using vintage power transformers with lower rated primaries ect . . .

[Ty_Bower]

...I found this > http://www.siteswithstyle.com/Voltsecon ... djust.html

...let me know what you think!

Yep, that's the right idea. I don't think I'd build it with all the fancy switches, but do whatever pleases you. If you're looking for the transformers, Radio Shack sells an inexpensive one that should work just fine:
http://www.radioshack.com/product/index ... Id=2102702

I've also discovered that I have an old junk computer UPS. It's a Belkin, and it's roughly shaped like an oversized power strip. Inside is a nice hefty transformer. One of the secondaries is a heavy gauge 16VCT winding, which I'd imagine would probably work pretty well for this application. The bonus is that this part is basically free if you can find a discarded UPS. Maybe something to think about...