What to build with 580VDC B+?

[tedmich]

I have a spare tranny which makes ~440VAC (no CT) and about 580VDC under load with a FWB and 110uf filtering, what small PP amp can use this? (I am NOT looking for a 100w Marshall!)

of course I can dump the extra volts over a resistor and run 2 el84s (and heat my room) but.....

Thanks in advance!

[octal]

Well, I'm not sure this is what you want to hear, but probably the best thing to do is to put the tranny on e-bay and use the $ from the sale to buy a transformer better suited to building "a small PP amp." Perhaps you can find someone to trade with?

Dropping resistors aren't going to do you much good, unless you REALLY like power supply sag.

You could regulate it down (perhaps using one of those power scaling boards?) to a useful voltage, but that's a lot of trouble to go to for the price of one transformer.

my .02

[Enzo]

You could wire up an EL34 stage and just not push it hard.

[tubeswell]

Ummm... single-ended 2 x KT88s in parallel - cathode biased (with big cathode resistors to keep the tube current down). You'll need to drop the pre-amp supply quite a bit tho'

And you'll need a grunty SE OT to handle all that power. But this should give you something like 25W-30W (at a wild guess)

[Grun]

How about using a half wave SS rectifier? Or would that lower the B+ too low for el84's?

[black_labb]

half wave gives the same voltage on the peaks, just half as often. this prettymuch makes the same voltage (more sag) but the sparse peaks make the filtering for the DC less effective as well meaning more hum most likely.

i think a single ended 6550/kt88 would be a good project.

you could also run a choke input to get around 380Vdc. would be good for some 6v6's.

[tedmich]

thanks all! I'll look into all of them.

As to regulating, I have the Swenson/Gary Pimm regulator BOM down to ~$13 from Mouser, which would let me run parallel taps for power/pre rails, or get me to the 400VDC I like for PP el84s

[Steve Conner]

What VA rating is this transformer? (If you don't know, you can reckon about 50VA per pound weight.)

[Marc]

super scaler?

I have a spare tranny which makes ~440VAC (no CT) and about 580VDC under load with a FWB and 110uf filtering, what small PP amp can use this? (I am NOT looking for a 100w Marshall!)

of course I can dump the extra volts over a resistor and run 2 el84s (and heat my room) but.....

Thanks in advance!

You could build a super scaler..... if you have a pair of spare Ot's, one for the output one for the input to drive the screens. The input grids are grounded. Kevin O'Conner has an example in one of his books.....

I built one recently with a PT from a Webster electric tube paging amp. The B+ is about 900V. The Slyvania tubes run about 8 ma's at idle. I ran my vibro champ into it and it was a monster..... I'm thinking about changing to power tubes to 6BG6's just be cause I have a pair and I'd hate to toast my Slyvanias....

Anybody else built one of these?

Marc

[zhyla]

Have you thought of using a big Zener diode to drop the B+ a bit? You can drop 100's of volts if need be though I think the ones I have are only 25 watt, so there is a limit to what you can do with them.

I used one on a organ amp conversion to drop my ~500 VDC line to 400 VDC.

[Diablo]

Well, it sounds like you're in the range of Ampeg VT-40 voltage. So, you could use NOS 7027A tubes or perhaps these:

6BG6 To 6L6 Vacuum Tube List

[groupof12]

Ummm... single-ended 2 x KT88s in parallel - cathode biased (with big cathode resistors to keep the tube current down). You'll need to drop the pre-amp supply quite a bit tho'

Yep. That sounds sweet. That's what I would do too.

Cathode-biased 6550's sound really nice, there's quite a few low-powered amps (maybe some boutique amps, like Fargen and so on) that do really nice recording in that configuration, and.. um.. with those big bad fat tubes in place, um... the hotter they run, the better they sound?

Check out some of the old Grommes amps too, you can find plenty of schematics on the vintage audio pages. I cloned a Grommes design and used it on the latest Jason Chance record which is about to be released, it sounds great through a Vintage-30, it's got plenty of presence and beef. Some of those old Grommes amps used high plate voltages, those designs might be just what you're looking for. They'll give you a real nice sound at right around 25-30 watts, and so if you can find the right OT you'll be smiling.

And you'll need a grunty SE OT to handle all that power. But this should give you something like 25W-30W (at a wild guess)

Yep, you sound like you have some experience with these things.

This is my first post here, by the way, I actually have a site with lots of audio schematics that I'll make available to you guys if you wish, don't want to violate the rules of this forum (bein' a newb and all), so maybe I oughta go read 'em - but if I can pass you a few schematics or something, you're welcome to 'em, if there's a way to post 'em here you can please let me know, or otherwise if I can post links to 'em we can do it that way too.

Gotta love those 6550's, they just have "the stuff" for guitar amps (at least to my ear - YMMV).

[tedmich]

Got a Heathkit SA-2 carcass for $10 at the local tube shop; it has a newer PT with 630VAC CT and 2 x 6.4VAC and a 5VAC rail.

The two OT are for 2 x EL84 PP with UL taps and 4/8/16 ohm output,

I plan to build into a Bipolar stereo guitar head with a clean (Matchless?) side and a PV Bravo side both with 2 new Belton Digilogic units that are cheap and well loved.

I'll have true stereo with 2 speakers and a balance control!

need to figure how to bias as there is no bias tap...maybe a CCS oddwatt....

[defaced]

I had the same problem with no bias tap on the build I am working, so I took a switching power supply I trash picked, and removed the front end for a bias supply: mains input, isolation transformer, FWBR, filter cap. I fed that into a RC network to get the voltages I wanted. I have not built the amp yet, so I don't know if I'll be exceeding the isolation transformer's VA rating, but for a bias supply which only needs 5-10 ma, I think it'll be ok.

[Ben N]

Old Sparky?