ST35 with EL86 / 6CW5?

[EWBrown]

Just another one of my wild 'n' crazy ideas... And no etch cutting, slicing or dicing needed, just change some resistors! And besides, I have a modest quantity of 6CW5s, and a nice matched quad of Brimar PL84 / 15CW5s.

Here is the W&C idea:

Take one ST35 rev C or D board, and configure it to use EL86 / 6CW5s, instead of EL84s. Why? Lower B+ voltage, 250VDC, though at higher operating current. OPTs could be 6600 ohm CT, but load the 16 ohm secondary to 8 ohms, to make it behave like a 3.3K OPT for the 6CW5s.

Power trannnie could be the MPS 190-0-190 V / 220 mA unit (which may currently be discontinued, but then I have a couple of them looking for gainful employment)

Of course the VA and PI stages will need appropriate plate and cathode resistor value changes in order to operate on the lower B+ voltage. The 6CW5s will also require a lower drive signal, so reduced plate voltages (and gain) should not be an issue here.

Izzat Worthwhile, or is this just another "flight of fancy" which should never get off the ground......

/ed B

[Geek]

Such a beautiful tube

I still think they sould like little EL34's ;-)

Cheers!

[20to20]

Izzat Worthwhile, or is this just another "flight of fancy" which should never get off the ground......

DO IT!

....(peeks out from behind a wall)

[EWBrown]

I've been running some sims for the VA/PI stages on reduced voltages, with TUbeCAD, and it looks entirely feasable.
Probably better to start out with a "clean" blank board than desoldering and replaceing lots of resistirs.

I'd SWAG that EL86s would work well in UL mode. If they don't, no biggie to change it to pentode mode, with no PC board surgery involved.

/ed

[Geek]

Hi Ed,

I'd SWAG that EL86s would work well in UL mode.

They do!

But don't use more than ~240V B+ then, as they will "runaway" like an overheating bipolar transistor.

Cheers!

[EWBrown]

After some further calcs, I figured that 230V B+ would be about perfect, and I can use cheap isolation trannies and voltage doublers to generate the necessary B+ and higher current.

If I decide to use the Brimar 15CW5s, then I'll have to so some etch surgery, but I'll go with the 6 volters, first, to get it working.

/ed B

[Geek]

After some further calcs, I figured that 230V B+ would be about perfect...

That's what the specs say.... 230V. When I did the guitar amp, I ramped it up..up...up and found it can *hold* at 250V and at 260V on the button, they go ZOOM! and will conduct over 2A current before going nuclear :-o

Doing a 6/15V switch is pretty easy :-)
http://www.ckamps.com/ToneGeeks/ToneGee ... 180#msg180

Cheers!

[EWBrown]

Some of the later poduction 6CW5s / EL86s have smaller plate structures, and they will go ZOOM!!!!! above 200VDC...