i have a set of single ended amp transformers. worth using?

[wsaraceni]

sorry i wasn't clear. the issue is the 269EX power transformer. the voltages seem low for anything worth building. so i was thinking of using the blues jr PT with the 125DSE output transformer

i guess i should really look at what single ended tube matches up well with the 125DSE and work backwards from there. how would i find that out though?

[Firestorm]

The 125DSE is a 10W transformer with a max. DC "bias" of 70mA so you just need to pick from output tubes whose power output is 10 watts or less and whose zero signal plate current is 70mA or less at the plate voltage you plan to use. The OT's impedance ranges from 2K5 to 10K so it will accomodate pretty much any tube that meets the above specs.

You can estimate a tube's SE output power from the plate disspation rating: the theoretical max output is half of that. Actual power out will be less unless you run everything right at the limits.

At 250VDC, an EL84 can put out 5.7W into a 4K5 load; zero signal plate current is 48mA; so that's a good choice.

At 265VDC, an EL34 puts out 11W into a 2K load; you can get away with the 11W and 2K5 instead of 2K, BUT zero signal plate current is shown as 100mA, so that's well above the transformers rating, so not a good choice.

6V6 would be good anywhere from 250-400VDC, load from 5K to 10K, output 4-6W.

6L6 or 5881: 300-400V, load about 5K, output between 7-11W.

There are other choices, but those are the common ones.

[wsaraceni]

wow. thanks for the great info.


according to the blues jr schematic
http://www.blueguitar.org/new/schem/fender/blues_jr.gif

i should get about 329V B+ after rectification

judging by that. I could build a single 6v6 amp like a fender champ or something that will work well with what i have. now to find out what to do.

thanks for your help.



http://www.hammondmfg.com/125SEsch.gif

[Firestorm]

As was mentioned earlier in this thread, SE power amps don't phase cancel the post-rectifier hum, so you really need to put a pi filter or even a choke between the rectifier output and the OT. That will drop your B+ a little below the 329VDC shown, but still perfectly good territory.

I should have been clearer in my earlier post re choosing tube types: the idle plate current given in the typical operation section of tube manuals is what is necessary to achieve the given output power, but the tubes do not have to be run that hot. Lower current simply results in lower power, so you can run an EL34 or 6L6, but you will not be able to drive them to 12W-15W output. Since the OT has 4 secondary taps, you can wire an impedance selector for 10K, 5K and 2K5 and swap in 6V6, 6L6 or EL34 at will. Just use cathode biasing and expect no more than 5-7W no matter what you plug in.

The Blues Jr. PT has an extra winding to power the solid-state stuff, so you have fun making various preamp functions switchable via relay.

[wsaraceni]

The Blues Jr. PT has an extra winding to power the solid-state stuff, so you have fun making various preamp functions switchable via relay.

don't start getting too complicated on me......



......what kinda relay switching stuff would you recommend? i like the idea of that

[Firestorm]

An SE 6V6 amp is essentially a Fender Champ. Tweed Champs have no tone controls, Blackface Champs do. So one option is to put in tone controls but make the ground connection at the bottom of the tone stack switchable so you go from the BF circuit with tone controls (which loads down the signal a bit) to the Tweed circuit with no tone controls (which is a little louder and rawer). If you do that with a relay, you can make it footswitchable. The 5F1 Champ schematic shows no bypass cap on the first preamp (but every one I've ever seen actually has one). Without the cap, gain is cut nearly in half so the amp would stay pretty clean. So you could make that cap's ground connection switchable and have a sort of rhythm/lead footswitch.

Setting up a relay supply and wiring the relay itself can be a bit of a chore if you haven't done it, so you could also implement either (or both of these) with conventional front panel switches.