Too much iron?

[ampgeek]

Woody,

I saw the lowish primary value of ~3.8k here:

http://tdsl.duncanamps.com/show.php?des=7027A

The first generic application condition cited. I also scanned through a couple of the linked data sheets and *thought* that I saw a similar value somewhere but I can't seem to go right to it at the moment.

Maybe that synopsized, generic data isn't as reliable as one would hope?

Your RC-30 data appears to stongly support the topology and measurements. Yes?

Thanks much,
Dave O.

[Alexo]

That would seem to make a 400:1 OT quite a mismatch.

It's funny what you find when you go digging in old amps... I was using some Fisher output transformers, having guessed at their ratios from what the tubes the donor was using would "want" at the voltages they were running, but when I got around to measuring them (after witnessing a screen arc in an EL34), they were actually mismatched by 100%. I've found others that were off too, but not by that much. I think we tend to be a little more choosey than the engineers of old were, or maybe they were choosey based on the sound of different primary Z's...

[wscrane]

It would certainly be cool to try with two EL34s.

Everybody probably knows this but... the way they got those numbers was just by figuring the max power loadline off the characteristics (the one that lands in the center of the knee.)

If you could find characteristics for the screen voltage you're using it would be easy to check. If not, and you have RDH4, there is a fancy procedure on p.40-41 for guesstimating characteristics that don't exist.

[Andy Le Blanc]

Big Iron gives you the chance to choose either big voltage or big current.
The intention of my current project was to find a load for 6550 that was very
close to the RCA data for a pair , a load of around 3.5k. which I found in a
100w marshal opt or a fender twin opt with an 8 ohm load on a 4 ohm secondary.
The pt is from a blown 200w head and is massive, but with a conservative
700v ct secondary. With this I want to push 6550a to max plate dissipation
and try to take advantage of the power handleing ability of the tube. I should
be able to get 75 to 80w. But of course the dynamic circuit will let me know
if I have looked hard enough and made good choices.

[wscrane]

Andy,

I have a few RCA amps (model SA1000) that get 100W from two 6550A. If you're curious, the specs they used are Vp=585, Vg2=300, and a 5K primary.

[Alexo]

Andy,

I have a few RCA amps (model SA1000) that get 100W from two 6550A. If you're curious, the specs they used are Vp=585, Vg2=300, and a 5K primary.

Straight out of the handbook. ...almost.

[Andy Le Blanc]

the limit with my tranny is the plate voltage. the average voltage D.C. for
the rectifier circuit is .45 x V A.C.. which is around 375 . it should come in
better than that but Im going with conservative guesses. should have the
supply up and running today and should have better figures. The power handling of the tube
is the real attraction. if you can supply the right conditions.
this tranny should give me a current reserve even if the plate volts are too low
to get the full potiential in watts, Ill see if its worth it, with tone v.s. a max power measurement.

[Andy Le Blanc]

surprise, I guessed way too low. It turned out to have a plate voltage of 585,
and a grid voltage of 328 when biased to .05 A (29W static plate dissipation).
I'm gonna recheck the opt tomorrow. Try to find a more appropriate load.
The ac measurement of the pt secondary was misleading. I should find
something around a 5k effective load with the opt I have.