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HK Citation V rebuild - any advice?

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Jim McShane

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Joined 2006
DF96 said:
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.
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Your point is well taken. It should be said though that the amp has no problems with noises on startup and the 6CG7 life is completely normal. Also, I have not studied it in detail, but it looks like to me that the initial start up surge would blow 1N4148s up with regularity.
 
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DF96

R.I.P.
Joined 2007
myweb.tiscali.co.uk/g8hqp/
Jim McShane said:
it looks like to me that the initial start up surge would blow 1N4148s up with regularity.
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Less than 10mA, through the first stage anode resistor and the LTP tail resistor? The compensation network on the first stage anode won't produce a transient (bypassing the anode resistor) because the supply rail rise will be too slow even with a silicon rectifier. Works fine in my 5-20, although that has higher resistor values.

A small neon would work instead. Don't use a power diode, as that would have too much capacitance.
 
zigzagflux

zigzagflux

Member
Joined 2006
Jim McShane said:
It may be essentially a 5-20 layout, but the very wideband input pentode is a real good choice Stu Hegeman made.
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Given the C15-R8 network on the plate, which purposefully limits the HF extension of the amp, what good does a wideband input pentode do? I can understand if the argument is improved linearity or 'just the right gain' for the application or 'the sound is right', but I don't get the wideband characteristic.
 
E

Eli Duttman

Member
Joined 2004
There's plenty of NFB in the global loop. Problems with HF error correction signal induced slew limiting are mitigated by high gm. The 12BY7 is very much a high gm type. :)

Continuing with the high gm theme suggests replacing the 6CG7 in the LTP with an ECC99. The overall gain structure will be undisturbed and (IIRC) the Cit. 5 power trafo can supply the extra heater current needed.
 
J

Jim McShane

Member
Joined 2006
DF96 said:
Less than 10mA, through the first stage anode resistor and the LTP tail resistor? The compensation network on the first stage anode won't produce a transient (bypassing the anode resistor) because the supply rail rise will be too slow even with a silicon rectifier. Works fine in my 5-20, although that has higher resistor values.

A small neon would work instead. Don't use a power diode, as that would have too much capacitance.
Click to expand...

You're right on the 10 ma or less, I just took time to figure it out. Thanks for pointing that out. I spoke too soon.
 
L

llwhtt

Member
Joined 2008
The voltage on pin 7 of the 6CG7 measures low because the meter is loading down the voltage. The reading is thru the 1M resistor and will read low unless a very high imp. voltmeter is being used. Pin 2 should read normal, 145VDC or whatever the plate of the 12BY7 is.

Craig
 
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nuvistordave

Member
Joined 2009
My rebuild is moving forward. I stripped the vertical power supply plate and bolted in a piece of single sided copper clad. A Dremel tool was used to cut 'islands' to mount the low ESR Snap-in caps, HEXFREDs, and smaller decoupling caps. The doubler caps are 1200 uF and the screen grid decoupler is a 180 uF @ 500V. A couple of 33uf 450V caps round things out. I kept the OG 270 ohm cement block but used paralleled 2512 SMT resistors to the 33 uF caps. The bias supply is running 4 high speed SMT diodes in bridge feeding a 560 uF snap in and a 100 uf 100V cap completes this along with the original 5K 3W resistor. Then I stripped the back panel; in went the obligatory dual bananas and IEC connector. I appended a 3rd banana to provide 4-8 ohm selection.
 
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nuvistordave

Member
Joined 2009
6FQ7 is basically a 12BH7 with the filaments rewired. Although similar to the 6SN7 in terms of mu and rp the plate structure is much different 6SN7's, and they were produced only for a short time (6SN7's are STILL in production). In terms of the sound, no comparison to the older 'Chrome Dome' or vintage 'Tri Plate' 6SN7's. Then of course there are 5692's, Russian metal base, Brimars, etc etc etc. So there is a lot of opportunity to tube roll by punching out that driver socket.
 
J

Jim McShane

Member
Joined 2006
nuvistordave said:
6FQ7 is basically a 12BH7 with the filaments rewired. Although similar to the 6SN7 in terms of mu and rp the plate structure is much different 6SN7's, and they were produced only for a short time (6SN7's are STILL in production). In terms of the sound, no comparison to the older 'Chrome Dome' or vintage 'Tri Plate' 6SN7's. Then of course there are 5692's, Russian metal base, Brimars, etc etc etc. So there is a lot of opportunity to tube roll by punching out that driver socket.
Click to expand...

Excuse me, but I beg to differ on a number of points...

1. the 6CG7/6FQ7 is electrically identical to the 6SN7 (other than a few very small differences in things like interelectrode capacitance).

2. the 6CG7/6FQ7 is still in production now, and has been in production since the 50s (other than a few years). 50+ years of production is hardly a "short time"

3. have you heard the late 50s/early 60s Sylvania 6CG7 with the internal shield? Or the 1960s Tung-Sols? Or the black plate RCAs? These are every bit as exceptional performers as the 6SN7s. Which is better? Personal preference is the only difference in my opinion, both are great tubes.

I think you'd be very unwise to try to put 6SN7s in a Cit V. If you insist on them then get/make some adapters - don't punch the chassis of that classic piece! But it's your amp, so You'll do what you want I'm sure...
 
J

Jim McShane

Member
Joined 2006
nuvistordave said:
I also have a massive collection of EF86's. These are far less microphonic than the TV video output tubes that H=K is so fond of, and after all are the OG 5-20 input tubes.
Click to expand...

Wanna bet? I have serviced 100s of Citation amps, and I also sell tubes. I get far more complaints about EF86 microphonics than 12BY7As. The one exception seems to be the small plate Japanese tubes - they can be a pain re: microphonics.
 
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