300B SS rectifier with CL-90

[badassbob]

I recently built another GSG board, but want to go 300B with a SS rectifier to get a little more B+ to feed those hungry power tubes. I know I need to run the diodes from pins 3 and 5 to pin 8, but where does the CL-90 get wired in? I dont wanna hit the switch and hear a loud pop through my beloved 12TRXBs.

Thanks,

Bob

[TomMcNally]

The CL-90 would go in series with the hot side of the power line,
after the fuse, doesn't really matter if it's after the power switch or
not.

You might also want to put resistors, maybe 150 to 250 ohms in
series with each diode to limit inrush, maybe instead of a CL-90.

... tom

[20to20]

I recently built another GSG board, but want to go 300B with a SS rectifier to get a little more B+ to feed those hungry power tubes. I know I need to run the diodes from pins 3 and 5 to pin 8, but where does the CL-90 get wired in? I dont wanna hit the switch and hear a loud pop through my beloved 12TRXBs.

Thanks,

Bob

Would there be a pop before the cold tubes conduct? Or are you refering to hot switching?

20

[badassbob]

If I wire resistors in series with the diodes, will I still get the higher B+? If I use resistors, will they come before or after the diodes?

[TomMcNally]

Before or after, doesn't matter ... yes, you'll still get higher voltage.
There are some threads about this in the get*set*go section, do
a search for DIODE.

... tom

[Shannon Parks]

The CL90 is what you want, as it prevents a high floating voltage before the tubes start conducting current. Namely we want the turn on voltage to stay below 450V due to the cap voltage rating. But once things warm up we also want the B+ as high as possible to make those 300Bs sing. The CL90 is the preferred route in this application as its resistance drops to less than 10 ohms after warm up. This similar setup has worked well in my ST35 for almost ten years.

A popping sound comes from the on/off switch and the inductive kick from the power transformer primary. A 4.7nF, 1kV ceramic cap (off the top of my head) across pins 1 and 2 of the power switch should do the trick.

Shannon

[badassbob]

Is this the correct ceramic disc capacitor? http://www.mouser.com/ProductDetail/TDK ... isCcILo%3d

Thanks for the help guys!

Bob

[Shannon Parks]

That looks fine, Bob.

Shannon

[badassbob]

How much output power am I looking at running 300Bs and a SS rectifier? I see Toms run ~5W with a 6AX5...would the higher B+ bump me closer to 6 or 7W? Not that it really matters, just wondering. I run my current GSG off of the 3.5k connections on my James 6112HS OTs. I'll probably pick up the larger Edcor irons for my next one

[Shannon Parks]

No prob, Bob. I will add this to the main GSG thread, too. I did this data early this summer before I changed my max output triode testing methodology, so max output power in these tests is relative to 5%THD. I used my 3.5K primary connections, but I think you can use the 2.5K for a little more output power. A percentage of the 2nd harmonics is probably from the single stage 6SL7 which is swinging a lot of voltage for the 300B.

get*set*go 300B with 6AX5GT rectifier at 1W
get*set*go 300B with 6AX5GT rectifier at 4W, 5%THD+N
get*set*go 300B with 1N4007 rectifiers at 1W
get*set*go 300B with 1N4007 rectifiers at 7W, 5%THD+N

Shannon

[badassbob]

The THD really isnt that bad at 7W with a SS rectifier. Looks like this game is on for me :D.

[badassbob]

Had a bit of an issue desoldering R19 and R20 on my PCB, one of the solder pads came off . Does this mean the board is smoked, or should I just solder in the 1.5 ohm resistors from the other side of the board? Sorry about all the newbiness, but Im having a good time learning and messing things up LOL.

[Shannon Parks]

Had a bit of an issue desoldering R19 and R20 on my PCB, one of the solder pads came off .

Fortunately that won't be a problem. You can mount the resistors on either side, just be sure that the resistor leads are well soldered to the pads that connect to traces (i.e. the cap side).

Shannon

[badassbob]

Could I run this same SS rectifier setup with the new production Sovie 6B4G monoplates? I know they can take more abuse than the twin plates. The old gears are a crankin'...thinking SS rect and 5k impedance?

[Shannon Parks]

Sure! I think 3.5K would be fine, too.

Shannon