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My pal JIm received the PP 300B amps and after a month of fiddling (mostly encouraging him to get rid of all the tweaks he was using to make is other amps sound good) he's pretty hooked on them. He's in DC so if anyone is curious to hear them I could put you in touch with him. By all reports they sound pretty awesome on his ProAc 1S's. I went back to SE for a while, thinking I could live with that while I did a permanent rebuild of a pair for myself, but I missed the sound so much that I threw the PP circuit back in there for the time being.

Here's the issue: I'm using a shared cathode resistor in the 300Bs, with a 150 ohm balance pot, like the old Heath W5 circuit. Unfortunately I ran into a bad batch of 300Bs and had some runaway issues, with predictable results, mostly a fried bypass cap that shorts and then it's whoa nellie. I caught it before it took out the OPT. The solutions I see are:

1) Separate cathode resistors, but I like the sound of the shared cathode resistor and this would not necessarily help save the OPT if one tube took off.

2) Fuse in the plate circuit. Some say this affects the sound, but I can probably work around that in the interests of avoiding a meltdown.

3) Some sort of current or voltasge sensor that would shut down the plate supply?


If 2) then what size fuse? One for each plate? A single fuse before the OPT?

On 3) I have no idea what would work here. Any ideas?

Earth leakage circuit breaker can measure imbalance in plate current.....

That's an interesting idea. I'll look into that...

Me and my big mouth.... I just realized it won't work...! The ELBs that I am familiar with are Japanese Fuji models that we used to use in our semiconductor mfg equipment. It would trip if there was more than say 30mA imbalance in the AC current between phases. It didn't measure current leakage to ground, directly. Anyways, back to my bad idea.... in a P-P amp, every bit of AC (audio) signal would create an imbalance in the two plate circuits (it's a P-P amp, you know.... ) So, that idea is out.

Maybe the best thing is to fuse the B+ going to the center tap of the output primary winding.

Use a constant current source between B+ and the OPT... Of course, that'll only work for PP class A

Roscoe

_________________
I can explain it to you, but I can’t understand it for you.

Noooooo Put the CCS in the tail.
...and it's still PP Class A

Something that worked for Richard Sears in a PP toroid OPT application was a separate CCS in each tail of the output tubes. This allowed precise DC balance to avoid core saturation. Of course, this only works if you are running Class A.

I know you like the sound of a shared cathode resistor, but compared to what? It may be that the sonic improvement gained with true DC balance in the OPT core will swamp whatever other effects are causing the shared resistor to have better sound than separate cathode resistors.

We heard the 6AS7 PP amp at David McGown's and it was beautiful sounding. I remember that we were in the kitchen chatting when the amp came on line. It drew me into the room.

http://www.triodeguy.com/6as7_pp.htm

That's very interesting, Stuart, thank you. The voltage drop is perfect in my application. I'd have to figure out how to increase the current to 75mA. I might have to give that a go.

Does anyone see or hear from Richard anymore?

I don't know how this works, unless the cap connection between the cathodes allows the AC signal to vary between the two output tubes. With both tubes in a P-P arrangement each running constant current, how do you get any signal flowing in the O/P transformer?

That's exactly what happens. The CCSs keep the DC current in each tube fixed, and under AC signal conditions the cap allows the current in eac tube to vary with the signal, but the TOTAL current between the two tubes remains constant.

Roscoe

_________________
I can explain it to you, but I can’t understand it for you.