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Hallo ! I would like to try a Consertina to drive a pair of Gold Lion
KT88 in penthode , fixed bias PP. I have earlier used 6SN7 but in a Williamson type of set up .
But I guess there are better tubes ??? Would a E88CC be better as a directcoupled gainstage and Consertina ? Or perhaps some russian tube ?
I am also open to use two different tubes , a penthode + a triode ??
What would You suggest for low distorsion ? I have about 415 VDC to use here.
Thanks for some advice Stewen
Follow Ups:
Hi,
First you likely should drop the screen voltages low to make sure you can produce enough swing (lower screen voltage = less bias). Crank up the current (more current = less bias).
Secondly, use a high perveance tube as splitter, run with loads of current. Make sure you can drive the tube close to 0V across it (read low Vak(min)) remaining linear.
The cathode voltage should be (Va - Vak(min))/4. This will maximise the swing of the splitter. Unless your input tube can swing close to 0V while remaining linear, or at least far enough to drive the output tube driven from concertina cathode into cut-off, you need to AC couple concertina and gain stage.
Sometimes you have to raise the gain-stage anode voltage and thus the concertina cathode voltage, reducing available swing in the concertina, if you want to direct couple the two. Going SRPP/Mu-Follower for the gain-stage may help to allow more swing with limited anode voltage than a plain common cathode gain stage.
If you can work out the conflicting requirements to something that works, I find Gainstage -> DC -> Concertina -> Coupling Caps -> Output Tubes one of the best PP Amp's I can make.
A way out would be to have fixed bias available and make the concertina tube also a cathode follower driving the output tube grid. Anode signal drives a second cathode follower via cap, make sure it is in effect an "output-less concertina", manipulating the resistors values somewhat can adjust the harmonic profile...
Yes, I know it is a crazy proposal, but it actually makes a surprisingly capable self-phase-splitting Amp with the potential to get much more swing and drive the output tube grids positive. Use a single 5687WB as driver and a single 6072A Mu-Follower as gain stage, or maybe a 6SL7 Mu-Follower etc.
Output stage - my recommendation would be ultra-linear with cathode feedback from the secondary (needs 0-4-8-16R secondary) near max. allowed plate dissipation (70-80%), this can run open loop without needing loop feedback.
Not the most powerful amp under the sun, but definitly something else.
Ciao T
Sometimes I'd like to be the water
sometimes shallow, sometimes wild.
Born high in the mountains,
even the seas would be mine.
(Translated from the song "Aus der ferne" by City)
Being a tricked out voltage follower, a "concertina" phase splitter has a small insertion loss. In Williamson style circuitry, the phase splitter is followed by a differential gain block. That gain block ensures that sufficient drive voltage for the "finals" is present.
IMO, you are LIKELY to have voltage swing problems with a triode "concertina" phase splitter directly feeding KT88s. Dyna barely got away with such a scheme in the MK3. Fact: FETs can swing closer to the B+ rail than triodes can. An IRFBC20 enhancement mode MOSFET DC coupled to a high mu/high gm triode like the 12AT7/ECC81 or the 6GK5 might be satisfactory in the "concertina" phase splitter role, when directly driving KT88s.
Eli D.
Stew
The one that comes to mind is from Allen Wright,Vacuum State Audio. I can't find that one right now but tubes like the ECC99 or the 6CG7 are wonderful drivers..The 6922 you can run an SRPP and that's quite a muscular circuit with low distortion but I found something here that may interest you.You can use a 6922 or a 6NIP for the first stage with a little tweaking of course but the other tube I've grown very fond of in recent weeks is the 5687.Talk about having a set of cojones.
Honest amplification is better than excessive 2nd order distortion anytime.
Edits: 04/20/14
Hey Staffan, how goes it?
Since most concertina splitters use 1/2 of the duo-triode, maybe you need to use a high mu tube? Try a 6SL7GT or the trusty 12AX7?
Thanks for Your answers ! Well , I have also heard some good talk about the ECC99 . I have some 6N6P which are said to be equal to ECC99 . A E88CC
as first , gaintube might be good as I think it can work well with +100VDC .... if I directcoupling . Dynaco used 7199 and Sunn , 6AN8 . Bo Winberg , here in Sweden , 75 years old and still performing with The Spotnics !!!
used 6AN8 , 55 years ago , and a quad of GEC KT88 . Very powerful amps at that time . ( When I started to play Swedish dancemusic in 1963 , we only had
amps ( Hagstrom ) with 10W and up to 20W ! And tapeecho of course !!
As You point out , Steven , most of these good tubes are double-triodes and
one half might be unused if I would like another tube , perhaps a small
penthode as directcoupled gainstage . Letīs say Iīd choose a penthode .
Would EF86 be a good choice or are there better small penthodes ? (Bo W.
uses one EF86 as inputstage in his signature amp and when I get one of them in for service I always put one or two siliconrings over this tube ).
Steve
Those would be my choice or a 12AT7 especially for voltage gain.
Honest amplification is better than excessive 2nd order distortion anytime.
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