organ pull tranny?

[Buschman]

I have this huge power tranny from a Baldwin organ. It has a real nice 300ct output but when I measure the filament tap it reads 7.3vac. Is that high enough to cause problems on my tubes?

[lumox0013]

I use the organ donors all te time that # will come down once the circut is loading it down as long as it ends up running under 7 your probly ok most I encounter come down to about 6.7-6.9 which might shorten tube expectancy but not horibly.if you want it perfect there are ways to adjust.

[Cliff Schecht]

Most all of my personal amp builds use inexpensive (but high quality) older iron I either pulled from something, bought cheap on ebay or bought cheap here. The problem is most of this iron is wound for either 117V, 115V or for really old shit, 110V. So when you put modern 123-125V wall voltages onto these old transformers, everything on the secondary comes out higher than it should. For typical unloaded 117V iron on a 125V wall plug, you are looking at well over 7V on the heaters and a good 10% increase on the HV windings as well.

Another thing I've noticed with old PA or audio (non-guitar) based tube amps, they tend to cook the power tube plates pretty bad. I've got a couple of old PA heads sitting next to me that put out well over 500V after rectification onto 6L6 plates. Tubes were more abundant back then so it wasn't a big deal to ruin tubes more often to get more power out of them.


FWIW I use two methods for handling these two problems. For the 125V to 117V conversion, I use a simple bucking transformer configured in a box with a fuse, an input plug and an output plug. The transformer is a common filament transformer wired up so that the secondary (heater) side of the transformer reduces the wall voltage by the voltage rating of the heater tap. For example, I use a 6.3V transformer and it shaves a bit over 6.3V off of the incoming wall voltage. TBH I prefer my heaters to run a *tad* cold as opposed to hot so even if the wall voltage is only 120V, the input voltage will be around 114-115V which isn't as big of a deal as 125V onto a 117V winding.

To handle the high B+ of some transformers, I use the amplified zener trick as introduced by RG over at Geofex (or that's the first place I saw this trick). You essentially use a high power, high voltage BJT/MOSFET to drop the B+ by an amount proportional to whatever zener diodes you throw in the base/collector reference circuit. I have these installed in at least 7 amps now and none of the power transistors have failed.

[Kagliostro]

If the heater voltage didn't drop under when in use

you can drop it by a resistor

K

[Phil_S]

For the 125V to 117V conversion, I use a simple bucking transformer configured in a box with a fuse, an input plug and an output plug. The transformer is a common filament transformer wired up so that the secondary (heater) side of the transformer reduces the wall voltage by the voltage rating of the heater tap. For example, I use a 6.3V transformer and it shaves a bit over 6.3V off of the incoming wall voltage. TBH I prefer my heaters to run a *tad* cold as opposed to hot so even if the wall voltage is only 120V, the input voltage will be around 114-115V which isn't as big of a deal as 125V onto a 117V winding.

Cliff: This is a topic that interests me. I also buy transformers on eBay for cheap and old 4A or more filament transformers are not hard to find. I'm not sure I understand. Is this what you are describing? http://www.dms-audio.com/bucking-transformer-diy
(Google is our friend as someone else has probably done it and published it!)

[Cliff Schecht]

Yup that's it. Simple to build too. I like to fuse the transformer at it's rated secondary current to make sure nothing melts when used for hours on end.

[xtian]

Very useful, guys. Please go one step further and point to an example transformer for use as a bucker.

[Phil_S]

Look at the Hammond 166 series, maybe a 166N6 or 166Q6. These probably retail for under $20. If you troll eBay, you can buy a similar vintage one, maybe for less, but with shipping you won't save much. For $20, you can probably buy 2 or 3 vintage filament transformers at www.oldradioparts.com . If you get a voltage boost when you want a voltage buck, just reverse the leads.

[rp]

I have these installed in at least 7 amps now and none of the power transistors have failed.

Not applying for an Energy-Star rating I see

[Cliff Schecht]

Xtian I have a couple of Radioshack 12.6V at 3A transformers (i.e. 2 by 6.3V @ 3A windings) that I paid maybe a few bucks a pop for. They're pretty small and work fine for my needs. Probably would upgrade the secondary current handling capacity for a 100W amp..

[rdjones]

Check out the article about bucking transformers, in particular Fig 4, Proper Way To Wire A "Bucking" Transformer and the text that explains why this method is more efficient.

http://sound.westhost.com/articles/buck-xfmr.htm

[img:640:250]http://sound.westhost.com/articles/buckx-f4.gif[/img]

rd

[Cliff Schecht]

RD thanks for the link. That's the schematic I have implemented currently. Good catch.

[Matt D]

Thanks everyone who has contributed to this post. Great information!

[tubeswell]

You can also drop heater winding voltage by 0.6V with a couple of 6A nose-to-tail diodes in series with the heater winding

[Cliff Schecht]

I actually like resistors more than diodes for dropping heater voltage that way. The diodes dump 0.6V no matter what whereas the value of the resistors can be calculated to drop a known amount of voltage over a range of primary input voltages. Also resistors can be found with very tight tolerances to retain balance in the heaters, diodes can be matched for forward voltage drop but this still varies with the current through the diodes. I usually choose the resistors so that at 125V the heaters are around 6.5V so that if the wall voltage is actually closer to 117-120V, I'm not running the heaters too far below 6.3V (also not too good for ze tubes!).