Choosing output transformers. . .

[Plimpington2]

I’ve been getting lucky lately with a handful of nice, complete vintage amps in pretty good condition. But my luck ran out last night when I opened up a Sherwood S5500 IV and discovered someone had swiped the output transformers.

I made the decision, then, to use this as a donor for other projects. . . In particular the pots look very fresh as does the power transformer.

But out of curiosity, if I don’t do that, what does one look at to determine what an appropriate output transformer is for a given output tube? This uses the 7868, a tube with which I am unfamiliar.

 

[gadget73]

Off the top of my head I want to say Sherwood ran a 5.5k transformer. I know I tested them in my S-5500II, and it was 5.something K, lower than you usually see for this tube.

Picking one depends a lot on operating voltages, and its honestly not a subject I'm well versed in. I usually get lazy and just rely on datasheet conditions. The 7868, 7591, and 6GM5 are effectively identical other than their sockets so you can use those datasheets for info if it helps. Performance-wise these give output like a 6L6GB but require the input of a 6BQ5.

 

[zackthedog]

Every tube has a corresponding data sheet that details the tube's operating parameters. The data sheet will give a set of typical operating conditions and the appropriate load for that tube to achieve optimal performance. In the case of push-pull power tubes, there's a plate-to-plate impedance specified for an output transformer. Most manufacturers followed these fairly closely. The final arbiter is the schematic for a given device, which should tell you what the manufacturer chose when they designed the piece.

 

[maxhifi]

Pick one which has approximately the same primary impedance, secondary impedance taps, and physical size as the old ones. You do have to make sure they actually fit inside the amplifier's case, after all!

Once you get them installed, you'll need to play with the compensation components to make sure the negative feedback is stable still. Only way around this is to get exact replacements from a junk amplifier.

This sort of mod can result in a very good sounding and usable amplifier, but will have the resale value of a DIY project (i.e. parts value only) That said, you haven't got a lot to lose - why not order up a pair of Hammonds or Edcors and go nuts making it work?

 

[gadget73]

Not impossible you can turn up a set of genuine Sherwood iron, but it will take some patience and shopping for that. Pretty sure I recall a set for sale here on BT, but thats been a while. Someone might have a parts hulk if you post a want-ad.

Just be aware that any non-original iron is probably going to need some fussing with to get the performance right. Differences in HF characteristics and such will need tweaking, and if it doesn't have the same secondaries as the original you may need to alter some of the feedback stuff. I believe these are 4/8/16 but I don't remember which tap the feedback comes from. If the new transformer doesn't have the same taps you can make it work but the resistors and caps need to be altered in order to maintain the same feedback levels.