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In Reply to: RE: Which tube integrated to keep forever? Pilot or Sherwood? posted by Michael Samra on December 02, 2014 at 19:27:21
Hi Mike,
I take it you are a fan of the Mullard drive stage. I have never heard it but I like how it looks on paper. With the standard 5/20 style EF86 driving a EC83/ 12AX7 cathode-coupled phase splitter, do you think the is enough juice to drive a pair of 6550/ KT-88's in PP, class AB ultra-linear mode?
TIA,
Dave
Follow Ups:
I like all types of topology depending on the amp..I like the Williamson in that is has very wide bandwidth and low distortion and I like the Mullard as well for certain circuits like the Sherwood and small UL amps.The EF86 and the 12AX7 can drive a pair of KT88s in UL but its not the best choice.The EL37 requires similar drive to a KT88 and the Knight KB85 does ok with it but you really should use a 6CG7 or ECC99 as ELi said because a 12AX7 can't fight it's way out of a paper bag.It's a fantastic voltage amp but the Citation 5 circuit is perfect. I like a pentode in the initial stage but the 6922 in cascade is perfect and they are currently manufactured.
"
Thanks! Is there a schematic for such an amp or maybe and EF86/6992 driver stage?
Dave
Its an EF86 into a pair of 6SN7s..It also has a 12AY7 but don't use that as its a preamp section for the organ..The circuit Eli suggested is perfect because even tho it's a triode in the initial stage,the way it is used it doesn't really have to fight miller capacitance.
"
Hi Mike,
Thanks for the suggestion! My fist tub amp was a Craftsman KT66 Williamson, so I like 6SN7's. I also have a lot of NOS & used 6SN7's because an amateur TV repairman neighbor gave me his stash of tubes. I will look at Eli's suggestion. I trust both of your judgments.
Dave
"With the standard 5/20 style EF86 driving a EC83/ 12AX7 cathode-coupled phase splitter, do you think the is enough juice to drive a pair of 6550/ KT-88's in PP, class AB ultra-linear mode?"The Mullard topology is very good. Mullard style circuitry is less dependent on O/P "iron" quality than Williamson style circuitry. OTOH, the small signal complement in 5-20 leaves (IMO) MUCH to be desired. Never forget Mullard published their designs in order to sell the tubes they manufactured. Both the EF86 and 12AX7 are low gm types. High gm is your "friend" in designs with long loop NFB, as it provides resistance against HF error correction signal induced slew limiting. IMO, the best vintage example of Mullard style topology is the Harman/Kardon Citation V. The Cit. 5 contains a 12BY7 as the voltage amplifier and a 6CG7 as the LTP splitter. Compare the gm of those 2 to the pair in the 5-20.
If you want PP UL mode KT88 "finals", Mullard style circuitry generally following the Cit, 5 example is fine. A 6922 in cascode would provide a high gm voltage amplifier in a current production tube. A current production ECC99 twin triode, with a mu of 22 and HIGH gm, is my choice for the LTP phase splitter. As cascodes exhibit poor PSRR, regulate 6922 B+ to put 180 V. on the upper triode's plate. Force symmetry in the differential gain block by using a 10M45S CCS in the tail, instead of a resistor. A modest negative rail under the CCS rates to be beneficial.
Eli D.
Edits: 12/03/14
I just want to turn the pretty knob and listen!
Hi Eli,
Thank you! I had to look up transconductance to get what you are talking about. I am also glad you translated the Citation circuit to readily available tubes, which is very important for me. It there a schematic available for such an amp? I don't think my current skills are up to designing an amp from scratch.
What attracted me to the Mullard is that not only do they give you a schematic, but they tell you how to build it from the ground up, including the chassis, wiring and layout (see link). It is almost "monkey see, monkey do" easy. I was planning to do an EL-34 amp first, and then use a Dynaco style output circuitry for the KT-88's (iron from Uncle Ned). I would use a similar sized Hammond aluminum box.
Dave
Well, let's start with the Cit. 5. Then we'll look at changes.
Eli D.
Hi Eli,
Now that looks very interesting. I am pretty sure I have some 12BY7A's in my TV tube collection. They also seem to be going for a reasonable price. The 6CG7 is available new and is (if I recall correctly) is a 9 pin version of a 6SN7, which is a tube I like and have a few of. I feel pretty foolish, never having taken a close look at the Citation V circuitry, give all the praise it has received. I assumed I never would have found one for a reasonable price. Has anyone successfully made a DIY copy?
Dave
Dave,
Rebuildable Cit V cores do come up for sale from time to time, and that would be a great start for a killer amp.
12BY7As are not as plentiful as they used to be, so if it was me I'd wire the 12BY7A sockets to use the 6CL6 or 6197 and stock up on them. They're cheap and common right now.
Yes, the 6CG7/6FQ7 is a "9 pin 6SN7". The EH 6CG7s work quite well BTW.
As well, the stock power supply can be dramatically improved, especially as relates to improved screen regulation. There's also a few minor alterations to the audio path that you can apply and when you are finished you'll have an AMAZING amp - and an amp that can image like nothing I've ever heard.
If you need an integrated you can easily use a passive unit ahead of a Cit V. The V has an input impedance of about 1 meg and it only takes 1.2 volts RMS to drive it to full output. Most any line-level source can drive that load - just keep the IC cables from the passive to the amp as short as possible and use low capacitance cable. A 50K volume pot is a good choice.
You could clone a Cit V - but just like with the Cit II the transformers are a problem to reproduce.
Thanks for the great advise Jim, including the rewiring. If I can get my hands on a Citation V That sounds like the way to go. It sounds like it would be just the thing to drive my Spendor BC-1's.
Dave
I will attempt to augment Jim's excellent remarks.
A great thing about Mullard style circuitry is the fact that so/so O/P "iron" yields a decent result. Never forget that the better the "iron", the better the amp. Having issued the warning, I can say it is possible to "clone" the Cit. 5 using Edcor's model CXPP100-MS-6.6K O/P transformer. Will the result be as good as the superb H/K original? Almost certainly not, but the result will be GOOD.
Eli D.
I will add the Edcor to the list. I wasn't really aware of them until you mentioned their single ended transformers in another post. I was wondering if they made good PP transformers too. AT $98, the price it right.
Dave
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