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Anyone ever build a clone of Citation V? (schematic pics)

184.96.174.19

Posted on May 12, 2015 at 11:45:21
PeterI
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Has anyone tried to build a clone of a Citation V tube amp? It seems you could use 5881s (or 6L6WGB) instead of the original 7581s. The OP trannies wouldn't be the same but you could maybe find something close? What is the ideal OPT impedence for 5881s? (I have lots). 6.6K or 7Kohm?

Is this a poor choice for a project?
Thanks for any info.

Pete

 

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RE: Anyone ever build a clone of Citation V? (schematic pics), posted on May 12, 2015 at 12:10:23
Eli Duttman
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You should use 6.6 Kohm primary "iron" that has PLENTY of magnetic headroom. Core saturation sounds terrible. :>((

Make darned sure you don't exceed 5881 dissipation limits.

The Cit. 5 is a superior implementation of Mullard style circuitry. Save 12BY7s for already existing builds. I'll post a 6922 cascode for the voltage amplifier later, from home. In keeping with the high gm theme, use an ECC99 as the LTP phase splitter. A 10M45S CCS as the LTP tail load is another refinement. Yet another refinement is regulated O/P tube g2 B+. FWIW, Jim McShane shoehorns a choke into Cit. 5s, to improve O/P tube g2 B+ regulation. Space is too tight for active regulation circuitry to fit. Be sure to allow yourself maneuver room.


Eli D.

 

RE: Anyone ever build a clone of Citation V? (schematic pics), posted on May 12, 2015 at 14:22:01
Triode_Kingdom
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I think this would be an excellent project. The Citation V is much less complicated than the Citation II, more in line with HK's smaller 25W and 30W amplifiers. The basic circuit configuration should be easily adaptable to a wide range of tubes, transformers and power levels.

 

RE: Anyone ever build a clone of Citation V? (schematic pics), posted on May 12, 2015 at 14:25:05
Caucasian Blackplate
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This isn't a bad place to start. I agree with Eli - get that 27K cathode resistor out and put a CCS in for significantly better performance.

For today's applications, an output transformer with an 8 Ohm secondary having a 4 Ohm tap is a fine choice, and the feedback network can be tweaked accordingly.

The power supply could be changed for potentially better performance if you slip out of class A, but if you're going for a budget build, you may end up using what you have that gets you close.

I don't see any numbers on what the actual gain of the amplifier is, but you might be able to mess around with the pentode operating point a little bit to tone it down if it's unreasonable.

 

6922 Cascode Voltage Amplifier, posted on May 12, 2015 at 18:03:37
Eli Duttman
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Here's some "hen scratches". I don't do pretty pictures.

If somebody sees an error on my part, please speak up.

Power supply rejection ratio (PSRR) in cascodes is essentially non-existent. That's why cascode B+ must be regulated. The little TO92 case LR8 could easily be up to this job.


Eli D.

 

RE: Anyone ever build a clone of Citation V? , posted on May 12, 2015 at 19:18:21
PeterI
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Thanks for suggestions. Of course, implementing them would result in an amplifier that had nothing in common with a Citation V. I have 12BY7s, 6CG7, 5881s, lots of 12BH7 (ECC99) etc. The point of following a proven plan is that you know it will at least work when complete. Too much experimentation and solving the problems is something the engineers love (not me). I have the parts for CCS, can be added later. What is the supply for the CCS though? I can't invent it.

Edcor makes a 25W 6.6Kohm/8ohm OPT w/ screen tap for 50 bucks, not bad. Gotta wait the 6 weeks.

I can definitely build a beefier PS that what the schematic shows. But do you really need 460V to the plates of the OP tube? Seem like one should go with less, especially on 5881s (I also have T-S 6L6GWBs).

Thanks

Pete

 

RE: Anyone ever build a clone of Citation V? , posted on May 12, 2015 at 20:25:43
Eli Duttman
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"Edcor makes a 25W 6.6Kohm/8ohm OPT w/ screen tap for 50 bucks, not bad. Gotta wait the 6 weeks."

30 W. should very easy out of a PP 5881 pair. 40 W. would not be at all surprising. Remember what I said about PLENTY of magnetic headroom, lest core saturation occur. Edcor's CXPP60-MS-6.6K is what you need. TANSTAAFL!


Eli D.

 

Thanks. Will keep a note of this. nt, posted on May 12, 2015 at 21:02:08
PeterI
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nt

 

+1 :^), posted on May 13, 2015 at 06:27:15
Paul Joppa
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With respect to "...Too much experimentation and solving the problems is something the engineers love (not me). " - speaking as a retired professional engineer, I always want to start with something that works. You never actually know what you don't know, and learning happens best when you understand each change - which is best done by trying it, one change at a time. Theory is useful for deciding what to try, but only trying it can tell you whether it works they way you think it does.

 

"Really need 460V to the plates? ", posted on May 13, 2015 at 06:51:34
I'd make the amp (and PSU) capable of running 5881, 6L6GC, and 7581 tubes. This will give you more options for replacements, in the future.

 

What I have so far (picture), posted on May 13, 2015 at 08:46:49
PeterI
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Instead of putting tubes on top like I usually do, I could gut this old Hewlett Packard test instrument case, and put everything inside, w/ ventilated top. (tried to sell the HP nixie electronic counter - no one wanted it). Wouldn't use voltage doubler like OEM Citation. Just need some 6.6K OPTs in case anyone wants to sell a pair.
Nearly all these parts are vintage including octal sockets from sealed military packing.

 

RE: What I have so far (picture), posted on May 13, 2015 at 11:20:28
Eli Duttman
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There's NOTHING wrong with well executed doubler PSUs. For starters, copper losses are low.

You need 2 small signal tube sockets in each channel. If you are going for a "copy" of the Cit. 5, you can use Noval 6CG7s, Loctal 7N7s, or Octal 6SN7s as phase splitters.

As long as air flow is good, I don't think mounting the tubes inside an enclosure will be problem.


Eli D.

 

RE: What I have so far , posted on May 13, 2015 at 11:48:59
PeterI
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I certainly agree about voltage doubler PS - so many classic tube amps used them, Harman Kardon, Marantz, etc. But I'm trying to work with parts I have. I don't have a 220VAC output power tranny. And voltage doublers require big current trannies, cause you end up with half, right? Mine is 318-0-318VAC, should give me approx 440VDC B+ after diodes.

What I don't want to end up with is an amp like the HK A300 or A500 - I had both of those years ago and didn't care much for the sound, even after upgrades. Had a hard time selling them too. Anyway, 12BY7 pentode is an odd tube for input, but you can't argue with the Citation II.

Thanks for info

Pete

 

RE: What I have so far , posted on May 13, 2015 at 12:08:33
Eli Duttman
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Use Mouser stock # 941-C4D02120A to rectify the B+. That's a 1200 PIV/2 A. Schottky. You get very low forward drop and "ZERO" switching noise, so diode snubbing is unnecessary.

I hope the rectifier winding's RMS current rating of the power trafo you have on hand is substantial. Remember, only approx. 50% of the RMS current is available as B+, when cap. I/P filtration is employed.

With the B+ rail somewhat "short", slightly increase O/P tube "idle" current. "Hot and low" operating conditions enjoy a solid reputation.


Eli D.

 

A500, posted on May 13, 2015 at 22:38:21
Triode_Kingdom
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"What I don't want to end up with is an amp like the HK A300 or A500"

That's exactly what you're proposing to build, at least in terms of the output section. The Cit V is essentially a more powerful version of the A500 power amp topology. FWIW, I just finished testing a prototype using the A500 as a foundation, and the results were very, very good. I changed the 7355 output tubes to a 6L6 variant, grounded the cathodes and applied fixed bias. THD/IM and frequency response were excellent, and I believe performance will be even better in the final build ('SL7s and 'SN7s in place of the minis). Just to be clear, this is the A500 power amp/power supply only, no preamp or tone controls. I was never satisfied with the control sections of the amp, so it's being modernized on a new chassis without them.


 

RE: A500, posted on May 14, 2015 at 07:52:16
Eli Duttman
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The Cit. 5 uses "routine" Mullard style topology. What's special is the high gm exhibited by the small signal tubes, particularly the 12BY7. High gm provides resistance to slew limiting induced by the GNFB HF error correction signal.

Higher gm is precisely why I suggested an ECC99, instead of a 6CG7, as the LTP phase splitter.


Eli D.

 

CCS, posted on May 14, 2015 at 08:01:23
Eli Duttman
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"I have the parts for CCS, can be added later. What is the supply for the CCS though? I can't invent it."

The CCS in the tail of a LTP need not be "super/duper", when the non-inverting triode's grid is at AC ground potential.

AFAIK, no additional PSU is needed if the tail resistor in a Cit. 5's LTP is replaced by a 10M45S integrated circuit.


Eli D.

 

RE: A500 - changing phase splitter tube, posted on May 14, 2015 at 08:03:43
PeterI
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If you change phase splitter from 6CG7 to ECC99 (12BH7) what else do you have to change (besides heater pin wires)?

 

RE: CCS, posted on May 14, 2015 at 08:13:21
PeterI
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Thanks. I used 10M45S in my first amplifier build 3 years ago, the "Music Machine" triode EL34 version. Nice sound but not nearly enough gain. I had to use a preamp, which defeated the purpose, so it ended up sitting on the shelf.

In that application the CCS required a supply of -66V DC and a 100ohm adjusting pot. It went to the cathodes of the 6GK5 input tubes.

Incidentally, I used those Cree Schottky diodes in another build, they're great.

 

RE: A500 - changing phase splitter tube, posted on May 14, 2015 at 09:48:04
Eli Duttman
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The triodes in the 6CG7 and 12BH7 are so close electrically that, most of the time, rewiring the heater connections is all you have to do.

The ECC99 is a different story. That type exhibits very high gm, very low RP, and a big heater draw. Given those facts, the ECC99 falls into a so called "super triode" group, along with the 5687 and the Russian 6n30p. What I find attractive about the ECC99, for service as the LTP in a Mullard circuit, is: its mu of 22, the highest B+ limit among "super triodes", and the smallest heater draw of the threesome.


Eli D.

 

RE: CCS, posted on May 14, 2015 at 11:16:50
Eli Duttman
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It takes roughly 30 V. to operate a CCS. Additional volts are needed to "track" the I/P signal. Now, consider the potential at which the LTP cathodes "sit", in both Poinz's "Musical Machine" and a Mullard circuit.

In the "Musical Machine", the cathodes "sit" a few volts above ground. The negative rail is needed in order for things to work. The grids "sit" at 0 VDC.

OTOH, in a Mullard circuit, the cathodes "sit" a few volts above where the voltage amplifier's plate "sits". Remember, the grids of the LTP's triodes "sit" at the same potential as the voltage amplifier plate they are DC coupled to.

BTW, before Poinz went to the CCS tail load, he too used a resistor. The negative rail was also necessary in that situation.


Eli D.

 

Don't forget the 7044 :) nt, posted on May 14, 2015 at 11:33:56
Triode_Kingdom
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nada aqui

 

30V, posted on May 15, 2015 at 15:06:48
Caucasian Blackplate
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This number is very dependent on the actual CCS in question. I could think of some that would operate with much less (LM334) or potentially much more (pentode CCS).

 

Filament Windings, posted on May 16, 2015 at 05:33:40
Eli Duttman
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What sort of filament windings do you have on that 318-0-318 power trafo? If, by circumstance, a 5 VAC winding for a vacuum rectifier is present, you can voltage multiply it to obtain the negative bias (C-) rail.


Eli D.

 

RE: Filament Windings, posted on May 16, 2015 at 10:15:39
PeterI
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Yes, the power tranny, being older, has 5V supply. Was going to use a (physically) small, low current tranny for the bias. I can also supplement the 6.3 VAC filament supply with an extra tranny, but probably won't need to.
But does overdrawing a 6.53VAC winding make the whole tranny work harder, hurting or limiting its ability to supply HV current? I always wondered that.

Pete

 

RE: Filament Windings, posted on May 16, 2015 at 17:04:27
Eli Duttman
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It's NEVER a good idea to draw more current from a winding than it's rated for. Buy a filament trafo with plenty of "stones" for the tubes. You can use the 5 VAC winding in series with the primary to boost the 318-0-318 winding up. :>D The HIPOT rating of rectifier filament windings is necessarily good., Voltage multiply the other filament winding to get the bias rail.

I'm uploading a graphic that shows what a multiplier bias supply looks like. You will need more stages, but for the few mA. of current needed, that's a non-issue.


Eli D.

 

Thanks. NT, posted on May 16, 2015 at 21:06:25
PeterI
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nt

 

Ditto on ECC99, posted on May 17, 2015 at 11:27:26
Lew
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What I like about it is that it's a drop-in replacement for a 12AU7. Thus you can quickly supercharge any vintage piece that uses 12AU7 either as a CF or in a gain stage, without re-wiring the socket. Of course, the PS has to be able to make the extra filament current and the B+ current needed to make the ECC99 sound its best. I also use it in cascode as a CCS in my preamplifier, which has a -250V supply.

 

RE: 30V, posted on May 18, 2015 at 05:18:46
PakProtector
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The battery biased, depletion-mode MOSFET CCS operating envelope is dependent on how many batteries you use. With 3x 3.3v LI button cells, 15V is about the minimum it should ever see under maximum signal condition.
cheers,
Douglas

Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

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