Choosing an output xformer

[CitizenCain]

I'm trying to line up a few things for my next project and have been investigating some ideas based around using some power tubes I have on hand. Namely a couple of Svetlana 6550Cs and a couple JJ 7591Ss.

While looking at tube data sheets for plate load readings, I noticed some odd things. Odd to me at least. Time for my next lesson in the wonderful world of tubes I guess

It seems that the plate-to-plate load ratings change based on a number of things like the plate voltage, fixed vs cathode bias, ultralinear vs not.

It's making it tricky to try and match an OT to possible intended uses for these tubes.

For example, the 7591s seem to list 6.6k load for two tubes in a PP circuit...except when the plate voltage is at 450v and cathode bias. Then it shoots up to 9k. But it does list a 6.6k load for a cathode bias config at 425v plate volts running ultralinear at a 40% tap. I'm looking at Tung-Sol data sheets, BTW. The JJ sheets are nowhere near this detailed. The 6550s are even more all over the place.

What all affects the plate load readings and how do you figure what they would be for a given set of conditions? Or is it something you just have to put together and measure?

As you can tell, I'm a little confused on this subject

[Diablo]

Here's some good links:

The Valve Wizard -Single Ended
The Valve Wizard -Push-Pull

[defaced]

As you can tell, I'm a little confused on this subject

Simple, if there are too many variables, eliminate some. First, what sorta configuration do you want to run these in? Do you want to do cathode bias at 450v or fixed bias at 400v or UL at 350v or whatever. As you have noted, this will rule out a host of possibilities that you're not interested in. Start there, then work your way to the graphs and then to impedance.

[Chuck H]

Also... You by no means have to replicate what the tube data sheets have done. If your Fender or Marshall it seems you don't even have to get close. There's another consideration with guitar amps. Most of the watts will be produced in a much narrower frequency band than these tube specs are based on. In general, AFAIK, it's better to use a higher primary at a higher plate voltage. That's not very specific, I know. But really it's all you need to consider other than the tone. If you choose an off the shelf "hi fi" OT tranny you can even fudge the watt rating and use a slightly lower watt rated transformer since as a guitar amp transformer it won't have nearly the low end current capacity requirement. If you know of an amp that you like the sound of and uses the tubes you plan to build with, just duplicate those trannies. Or listen to amps that use the tubes you plan to build with and see if you like anything. Again, I know this isn't real concise but guitar amp OT specifications never have been. And don't have to be.

My advice is to Email Alden at Heyboer. Tell him what your building, voltages, tubes, etc. and tell him what kind of tone your after and he'll make it for you. Having a one off build is more pricey than "off the shelf" but not by as much as you'd think. Alden has been very good to me. And my amps sound and perform like I want.

JM2C

Chuck

[CitizenCain]

Thanks for the replies, guys!

defaced, that's what I started out trying to do, but everytime I'd come up with a possible setup I'd like, I'd get stymied by the settings in the data sheets. I either couldn't get the plate-plate load right or run in the bias mode I thought, etc.

Chuck, I often wondered if those use cases in the data sheets were just some general ones the engineers thought up. Seems like I'd compare known setups in popular amps and couldn't make any definitive matches according to the data sheet. I might be in a tad deep for my skill level right now. Maybe better to stick with a known circuit or kit.

You learn some more everytime you do one of these though, and that's the fun part!

[defaced]

We can probably help you out more, what operation mode do you want to operation in? Or if that's still to much tech crap, what sorta sound are you after?

Me, I like powerful, stiff, clean, power amps. Hence my amp has ~540v B+, 4 x kt-88, 1.9k primary, fixed bias, and so on.

[CitizenCain]

Here's where all this stems from, in the Tweed Builders forum. http://music-electronics-forum.com/t17251/

I was just thinking of ways I could build a non-traditional tweed type circuit. I like the idea of a 5E3 using Ampeg type tubes, 6EU7s and 7591s. That's where I ran into the info re: cathode-biasing 7591s that seemed to stop me whichever way I turned.

A "Super Champ (5F1)" would fun too using a SE 6550