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RE: These 300B came in today and they Tube DIY asembled but didn't work

Posted by Thorsten (M) on April 13, 2014 at 14:02:05

In Reply to: RE: These 300B came in today and they Tube DIY asembled but didn't work posted by Michael Samra on April 13, 2014 at 13:52:15:

Hi,

> I thought that they had an unusual amount of power.
> Can you really get 16 watts with a standard 300B?

Yes, the output tube runs at > 400V / 80mA (pretty much the legal limits) plus 3K load. And the grid circuit with two parallel 5687WB can put a lot of power into the grid when called upon (Class A2 on peaks).

> I just crossed the number on Q2 and I did have an 82v zener that
> opened which is rare but that could have been due to mechanical
> stress from the crack in the board because it was snapping really
> loud..

Could be. These amp's are a bit cramped, there are cables under the board that need routing carefully to avoid stressing the board.

> I love working with solid state circuity but many times it's over
> engineered from what it has to be.

These units may (or may not) be guilty as charged. I did try to max out certain things.

> I was thinking about implementing your circuit in these.

They implement my circuits... 100%.

;-)

Keep them as they are, they are quite a bit special.

> The 5687 runs at a blistering temp

That is the heater alone, the current is low...

> but that's because it's almost like a small power triode lol.

Yup and it is used as a cathode follower with a choke in it's tail. It has control over the 300B Grid well past "Iron Fist" levels. And no Velvet Glove either...

I'd avoid overly expensive 300B's, because the Amp's does run them hard, the price to pay for power.

Thor

At 20 bits, you are on the verge of dynamic range covering fly-farts-at-20-feet to intolerable pain. Really, what more could we need?

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