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Old 21st February 2011, 06:11 AM   #1
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Default HK Citation V rebuild - any advice?

I just bought a Citation V on impulse since the price was quite reasonable. I didn't need yet another project, but oh well...

It looks like a typical Mullard 5-20 circuit.

Aside from the obligatory replacement of capacitors, what performance enhancers would suit this amp?

I'm thinking... CCS on the LTP tail? screen regulator? Choke on the power supply? (probably no room for that in the chassis)

Geeze, no grid stoppers on the LTP. That's unusual? Would it make sense to throw some on there?

..Todd
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Old 21st February 2011, 06:45 AM   #2
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Nice score Todd.

Quote:
Originally Posted by taj View Post
I'm thinking... CCS on the LTP tail?
Sure

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screen regulator?
Run it triode strapped.

jeff
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Old 21st February 2011, 06:46 AM   #3
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Since the amp comes tubeless, I will need to provide some. I have plenty of 12BY7A's and 6CG7's in my box, but only 1 NOS 7581, which isn't very helpful. So I'll have to buy output tubes.

I don't want to spend the cash for NOS, (except Russian maybe), so which current production 7581 or 6L6GC tubes are worth considering?

What about Russian tubes? Would a set of 6P3S-E's work survive and get the job done?

..Todd
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Old 21st February 2011, 06:50 AM   #4
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Quote:
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Run it triode strapped.
That's a good idea, Jeff.

..Todd
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Old 21st February 2011, 06:57 AM   #5
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What about Russian tubes? Would a set of 6P3S-E's work survive and get the job done?
Yup, they will work fine. I have a quad in my Jolida, which has more B+ than you amp. Biased around 40ma each.

Reissue GL KT66 when funds permit.

jeff
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Old 21st February 2011, 10:47 AM   #6
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Todd,

Stop!! Jim McShane, Mr. Citation, IS your man.

You've noticed how tight things are. Jim has a choke tweak for g2 B+ that slides right in. Run the beast full pentode, as Stu Hegeman intended.

As for tubes, SED (=C=) 6L6GCs are fine. The Saratov made and TungSol labeled ST bottle 6L6 rates to be very good too. Jim will tell you if the 6П3С-Е (6p3s-e) is suitable.
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Old 21st February 2011, 01:45 PM   #7
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Thanks Eli. I always appreciate your input.

I'm familiar with Jim's expertise, and I'll see what affordable (I'm really frugal) tweaks he's got, and which tubes he recommends. I just hate to bother him when I don't plan to buy anything from him. He's got a business to run; I respect that. He is my only new-issue tube supplier.

..Todd

Last edited by taj; 21st February 2011 at 02:02 PM.
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Old 21st February 2011, 02:12 PM   #8
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One thing puzzling me about this amp is the lack of any B+ delay. It uses only silicon rectifiers. How does it prevent cathode stripping?

Should I add an NTC thermistor to the B+ supply maybe?

..Todd
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Old 21st February 2011, 02:22 PM   #9
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I specialize in the Citation series tube amps. The V is a killer amp and can be made even better. It may be essentially a 5-20 layout, but the very wideband input pentode is a real good choice Stu Hegeman made.

There are a number of improvements you can make. As Eli mentioned I've got an L-C section that gets inserted into the screen supply which makes a big difference. Don't bother with triode strapping the outputs - the amp sounds much better in pure pentode. BTW, your schematic drawing is very nice and neat - but you have the voltage doubler caps & diodes drawn wrong. Neither of them goes to chassis ground, the caps are connected wrong - well it kind of needs to be redone in that area.

BTW, the B+ supply itself benefits from some component value changes.

As far as power tubes go, I usually recommend either the SED 6L6GC or the Tung-Sol reissue 6L6G. Yes, the 6P3S-E will work fine in the Cit V. But regardless of what tube you use they are biased too hot with the stock setup. There are some changes in the bias supply that are must-do to keep from frying your power tubes.

If the octal sockets aren't in excellent shape they must be replaced.

There's more - if you want drop me a note.
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Old 21st February 2011, 02:29 PM   #10
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Thanks for the heads-up about my schematic mistakes, Jim. I'll update that.

And I'll send you an email shortly.

..Todd
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