DIVIDED BY 13 RSA23

[RCGPNY1]

Does anyone know the story with this amp...used to own one and it was great...thr 5879 pentode channel was outrageous, the 12ax7 channel creamy....how was he getting those 23 watts out of a pair of KT88's ? Was he biasing very cold? lol...

[Stevem]

Likely low plate voltage, cathode biased and OT primary impedance all set for cream!

[TUBEDUDE]

Class A push pull, or triode mode maybe.

[matt h]

(deleted)

[RCGPNY1]

from the tone it gets..I cant imagine that its triode....it's bright lively amp...most triode amps are dark and kinda flat sounding. On the /13 page Fred says the amp is AB1/A..what the hell does that mean? Also I wish I could build the click channel only (5789 pentode) with 2 el84's or if I figure out how to do those KT88's. The only amp I ever played that overdrives as nice as the RSA23 is my RJ Lerpoole 15.

[matt h]

(deleted)

[gingertube]

KT88 Push Pull for 23 Watts
That would be Class AB1 Triode Mode with a B+ of 400V to 425V and an Output Tranny with an Raa of 4K, probably cathode biased with a 525 Ohm cathode bias resistor on each output tube - see here (and the linked datasheets) - well that is what I would try first anyway.
http://tdsl.duncanamps.com/show.php?des=KT88

Triode Mode Amps can sound dark if badly designed - with the output tubes in Triode Mode there is VERY significant Miller Capacitance at the output tube grids, you have to make sure the Phase Splitter is up to driving that capacitance to the highest frequency you want to reproduce. That is not a difficult thing.

Cheers,
Ian

[printer2]

Triode output tube amps have a lower output impedance than pentodes or beam power tubes. The higher impedance of those tubes in the circuit makes the amp behave more as a current source as compared to a triode amp that acts more as a voltage source (relatively speaking).

What this does is makes the amp try to keep the power flowing to the speakers constant even though the speaker's impedance is rising. This gives you more treble. You can get the same thing happen with triodes if you put NFB on an amp that has pentodes and no NFB.

[drew]

from the tone it gets..I cant imagine that its triode....it's bright lively amp...most triode amps are dark and kinda flat sounding. On the /13 page Fred says the amp is AB1/A..what the hell does that mean? Also I wish I could build the click channel only (5789 pentode) with 2 el84's or if I figure out how to do those KT88's. The only amp I ever played that overdrives as nice as the RSA23 is my RJ Lerpoole 15.

Check the Hoffman forum for 5789 builds.

[gingertube]

"5789" pentode?
Is that really the number or did you mean 5879.

Either way this is not a tube we find in Oz. How close is it to an EF86 or 6BR7?

Thanks,
Ian

[RCGPNY1]

Sorry about my intermittant dyslexia...yes I meant 5879 pentode.Check my OP. As I undertand it has slightly less gain than the EF86, but it still has plenty, and I think a slightly better sound-also less problems with microphonics.

[10thTx]

Check the Hoffman forum for 5879 builds

Do a search there on Geezer's "HoSo56". You might find some useful information there.

The schematic is here (but there is a long thread on the forum also):
http://el34world.com/Forum/index.php?topic=6176.0

5654 or 6AU6 could be tried as a substitute for the 5879. I built an amp with both a 5879 and another socket that was in the V1 position that was a 7 pin socket where 5654 or 6Au6 could be used.

In order of preference: 5879, then 5654 and lastly 6AU6. However, I liked all of them and there wasn't a real big difference between the 5879 and 5654, IMO.

With respect, 10thtx

[gingertube]

Ah!
I must build another amp with a pentode front end.

The last one I played thru was a 1942 job. 6J7G Pentode to triode strapped 6J7 to 6V6 (single ended) with an 80 rectifier and a field coil speaker.

Mind you that was glorious and I have 3 standing orders from guys who played thru' it, should I decide to build a few.

Cheers,
Ian

[TUBEDUDE]

Good luck acquiring the field coil speakers.