Tricky 800c problem, need help

[sloober]

I have completely rebuilt my 800c: new caps, SDS power supply, new EH outputs and adjustable bias pot. I have it working great. But one thing I wanted to do, and may do to others, was to ad a high voltage delay to prevent cathode stripping when turned on. I found the Rubli HV delay board online and it has been installed on the 800c. He sent a diagram that could not be more simple, but since I have the SDS board, I am a little unsure how to proceed.
Here is a photo of his instructions and the board layout for the SDS board. Can anyone help figure it out for me?
Thanks-John
Here is a link to the rubli board.

http://nervenet.info/HT_delay/

 

[Rocjr]

I'm working from memory here: Each output transformer's red wire (CT) is tied to a common point on a tie strip? You should have a wire feeding that point from the SDS board coming from the 430v feed on the board. As he drew this, simply insert the delay between those two points. In other words, feed the "B+ in" of the delay board from the 430V out of the SDS. Connect the "B+out" of the delay board to the point where both output transformer red wires are connected. If they happen to be tied to seperate strips, you'll have two wires exiting "B+out" to feed each output trans red wire (Center tap).

 

[sloober]

Does this look to be right?

 

[sloober]

Thanks Rocjr. I just wired it up that way. Will be testing in about 15 minutes...Crossed fingers.

 

[shelly_d]

 

[sloober]

It works!

I got it in after getting this diagram from Alex. I also had to jump a diode to get it to kick in. The LED says on for about 25 seconds while HV is off. Then the light goes out and the sound kicks in. I will post photos in a few.
John

 

[sloober]

 

[dshoaf]

That's quite the sophisticated way to deal with HV delay - I'll bet its getting crowded in that Fisher. The good news is that there's no additional heat being generated.

FWIW, Jim McShane uses a slightly different approach where 2 CL-150 thermistors are installed in series with both sides of the high voltage winding. They will exhibit a high resistance on the high voltage B+ while the filaments and bias circuits power up and stabilize. Then, as the resistance of the thermistors begins to drop as current flows through them, the high voltage comes up in a delayed fashion. They'll end up dropping about as much voltage as the HV board when engaged.

This is done to both the Citation I and II gear from his kits and I've applied the technique to other amps, too. Funny, the McIntosh tube amps of the 60s also employed thermistors but, in their case, only one was used on the input side of the power transformer. This effectively caused the entire amp to come up slowly and eliminate inrush to the power supply caps, bias circuit and filaments. In orther words, Mac had a soft-start feature while many other tube amps of their day didn't really have much in the way of controlling high voltage ramp up. I'm sure it was a parts-count/financial design decision.

Still, quite the elegant approach you have there.

Cheers,

David

 

[sloober]

It is a very nicely designed unit. For $25 assembled I am very impressed. I ended up changing the 47uF cap to a 68uF and jumped 2 diodes per Alex's instructions. It now kicks in after 31 seconds. Since the sound comes on as soon as it kicks in, I am sure that everything has warmed enough for safe HV engagement. I may add one of these to all my amps. John