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HK Citation V rebuild - any advice?

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taj

diyAudio Member
Joined 2005
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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taj

diyAudio Member
Joined 2005
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
 
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.
 

taj

diyAudio Member
Joined 2005
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
 
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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.
 
Should I add an NTC thermistor to the B+ supply maybe?

Is the Pope Catholic? Damned right to add NTC inrush current limiting, to protect against cathode stripping. Another good reason for the thermistor(s) is to stop power trafo clang at turn on. Clang will only get worse if you increase PSU capacitance, which (of course) you should. The only reason Hegeman used the OEM values is the fact that larger values were not available to him. Capacitance per unit volume has gone way up over a 1/2 century.
 

taj

diyAudio Member
Joined 2005
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.

I had one of the doubler cap polarities backwards (fixed now), but otherwise it seems to match up with HK's drawing. I don't think the ground symbol implies chassis earth, just a circuit common.

I haven't received the amp yet so I don't really know how its ground scheme is set up.

[edit] oops, just noticed the diodes are indeed messed up.

..Todd
 

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as mentioned by Jim, the Cit V is a tube eater in stock form. Being able to adjust the bias is a must-have and a worthwhile mod to pursue.

Waaaay back in the mists of time before the popularity of the internet and when I was a tube-noob, I had a Citation V. With RCA Blackplate 6L6GCs, I would get a red glow spots on the plates. The only tube that could handle it was the (then called) Sovtek 5881WXT.

I pretty much just replaced the caps in the voltage doubler and all the signal caps. Partnered with the Snells and Adcom preamp I had at the time, I wasn't a happy camper. The V isn't exactly a "tubey" amplifier and at the time I wasn't pleased with the results I was getting. I blamed my system woes on the amplifier which was pretty silly at the time. Now I wish I had kept the amp!
 
What with the size of the transformers and the amp itself it seemed to me that my cit V was just begging me to run 6550s. They look 'right' in the amp sitting near that massive iron. I tried advancing the output tap and pushed well over 60 watts to the load. That was with zero mods to the power supply.

I snagged mine recently at the auction site. The price was pretty much what one would pay for the raw parts. The iron can do the full 60 watts into the load at 20 Hz; top end power BW is well over 20kHz.
 
I would put a signal diode (e.g. 1N4148) between grid and cathode of the first 6CG7 triode. This protects the grid from 400V+ on switch-on, before the valves heat up. It also helps establish something like the correct bias for the second triode so might reduce thumps and bumps as cathodes begin to operate. In normal operation the diode ends up reverse biased and its small capacitance is half-bootstrapped so does no harm.

The original Mullard circuit did not need this diode as it used a thermionic rectifier.
 
Get some 7027A's. Not out of this world (yet) on ebay. They will take the citation's operating point. Pin out is the same as 6l6....except there is an extra connection to g1 on pin 6 and g2 on pin 1. Check to see if those pins are wired in your amp. If not it will plug in. If they are wired you can just snip the offending pin off the 7027A if you don't want to rewire your amp. They are a very under appreciated tube IMHO most likely because of the weird pin out. Check them out here:
http://www.drtube.com/datasheets/7027a-rca1959.pdf
 
Jim the man on this . I would use kt-120 if you can get the bias value more neg. redoing the bias for full wave . The kt-120 acts like 2 6l6 in one bottle it bias up pulling 2 times the current of the 6l6 and 2 times the output . So up the bias and go . This was an amp designed to be mostly pentode and it sound great that way .Stu was clear about what he did and why he did when I meet him in about 1983 . The lafayette 550 that Stu work on had a delay in the power supply on the high voltage . A fully mechanical bi metal thing with resistors . The relay was open when cold then it heated up and closed delay was about 30 seconds.
 
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