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Citation V clone rebuild

The board sold by Erhard monitors each tube individually and maintains the bias level for each tube independently of the others that was preset by the user's adjustment during setup. Consequently it maintains quiescent current balance which is important in the output transformer for a push pull amp. It will also allow for easy substitution of other similar tube types or can be readjusted for use of other tube types such as KT88 or KT120 etc. assuming sufficient current capacity in the power supply.
 
hey-Hey!!!,
The Cree SiC schottky diodes can be paralleled to increase their current carrying capacity, but there is no increasing their PIV by connecting them in series like a PN-junction. For a doubler supply, maximum PIV is about B+. A full wave, feeding an LC filter is something else entirely...:)
cheers,
Douglas
 
Thanks Douglas. I have already rebuilt my power supply section and I did use the Cree SiC Schottky's.
 
Well, it's been quite a while since anything on this project. I got a top plate made last summer and that and the transformer bells painted by an acquaintance auto body painter. I finally made the first steps at assembly. The board in the center is an autobias board from @Erhard-Audio and the small board attached to it on its side is a DIY bias voltage regulator.
FA59DB16-926B-447A-A6ED-2AD9A7E926ED.jpeg
 
I have a question about wiring the input jacks. The original diagrams have them on opposite sides of the back panel but with a tie across the back panel directly between the commons and connected to chassis in the middle. I assume this was to mitigate ground loop problems with the separated RCA's and the preamp. I am not using chassis for any audio common. Do I need to tie the audio input grounds together like this or are there other ways to deal with ground loops at the input? I can mount them both on the same side, tying the commons together at the jacks, and run the audio around the front to the farther channel. Or should I just finish the assembly and see what happens? I have made some progress with the wiring lately and can begin to see the end. Just need to finish the woodworking on the base and then I can build the back panel.
John
 
Well, it's finally running. Have only run it on the bench with test signals so far. Here are a couple of pics. The scope pic is 10kHz. am using new Tung Sol 6L6GC-STR tubes from Jim McShane and I have the @Erhard-Audio autobias board set for 55mA. The autobias has a delay for B+ of about 30sec and then is ramps up the bias current over about 40-50 seconds to the set value. Works like a charm.

Square waves look pretty good. 100Hz has a little tilt and 10kHz has some ringing. There is a bit more ringing on the negative side. Not sure about the meaning of that. I will see what I can do with some feedback tuning. Suggestions welcome.

B+ is running a bit high. At 110vac line I am at 440v B+ which is the spec value. Filaments are about 6.15v. If I run the line voltage up to 120 which is my normal house voltage, the B+ goes to around 480vdc. Is this ok if I keep plate dissipations under control or should I try to reduce it? I don't really want to use a bucking transformer but will if I have to. Anyone have other suggestions?

63950958-DB1A-46E1-842E-D4A470BF0A06.jpeg A69D3343-EBED-4ACA-BDD7-6177028D8C86.jpeg 0ECCD939-8D2A-4A36-96A9-CFAC100686B3.jpeg
Thanks,
John
 
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Can you post your PS schematic?
Are you willing to modify it to achieve B+ you are looking for?
 
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Since it isn't a real Citation V, just the transformers, although I duplicated the schematic mostly, I can do whatever seems fit since it isn't a collector piece. The power supply schematic is right off the original schematic other than larger capacitor values and a couple of inrush limiters.
 
Schematic specifies 460v at 117v input. 480v is only 5% higher. What are doubler capacitors' values? The higher capacitance may bump voltage higher, besides loading PT harder on turn-on.
 
You are correct. I've been looking at the Sam's which says 440. I'm probably ok. At the bias of 55mA the tubes are under max plate dissipation. Guess I will not worry for now. I will watch the turn on voltage to see how high it goes just to be comfortable. I might add an inrush limiter on the primary.
Thanks,,
John
 
Do you have screen stability resistors installed? At 450v+ on screens and new (low ESR) PS capacitors output tubes are very likely to arc.
 
Nice build! This is an interesting amp. As in all pentode 6L6GC using 4k into 8 ohm output transformers. Rated for 40 watts, internet lore says makes 53 watts per channel. As a Citation II owner, it is rated 60 watts but I measured 63 watts per channel until I boosted the B+ with some KT90s and now get 83 watts per channel.

I hope you can test the output power.

I trust you used the 12by7 and 6cg7 small signal tubes as per the original?
 
rustnrot said:
Nice build! This is an interesting amp. As in all pentode 6L6GC using 4k into 8 ohm output transformers. Rated for 40 watts, internet lore says makes 53 watts per channel. As a Citation II owner, it is rated 60 watts but I measured 63 watts per channel until I boosted the B+ with some KT90s and now get 83 watts per channel.

I hope you can test the output power.

I trust you used the 12by7 and 6cg7 small signal tubes as per the original?
Click to expand...

I have the original small signal tube compliment. Thinking of trying a 6N6P Russian tube and maybe a ccs for the LTP tail.
 
nerdorama said:
I have the original small signal tube compliment. Thinking of trying a 6N6P Russian tube and maybe a ccs for the LTP tail.
Click to expand...

That is a very fine idea. The CCS for sure. The finals are pentode-rigged, so it is not like their input capacitance is of much significance. You will get a wee bit more output capability from 6N6's as their plate resistance is lower than the 6CG7's.
cheers,
Douglas
 
I have 4 or 5 hours on it now. It seemed a bit wooly and had an indistinct character initially but has really opened up now. With all new components other than the transformers, especially the new power tubes, it probably needed a little break-in. It is also dead quiet, and I mean like a tomb. I am having some good fun. It seems quite powerful and has a very clear and detailed presentation. And it doesn't hurt my ears at all with any harshness. Very happy so far. I have a couple things I still want to do and I need to tie up some of the wiring.
John
 
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