EL36/6CM5 pp design critique

[aardvarkash10]

for all you horizontal deflection fans...

based on http://home.alphalink.com.au:80/~cam...ls_6CM5amp.htm but with obviously different pre and splitter.

As usual, based on stuff I have and with a serious buget limitation in mind. Semiconductors and passives total round $30, the valves owe me about NZ$4 each, and the OPTs were NZ$30. Transformers are ex-surplus affairs at a total of NZ$20.

THe PTX is a multi-tapped item providing up to 250vac @ 150ma. THe heater TX is able to supply around 28v @ 4A, so the heaters will be series connected - all outputs on one series, inputs and splitters on another. Total current draw will be around 1.6A - plenty of headroom, but am I opening up to crosstalk between channels?

Now, I reckon to get around 130v p-p out of the 6J7 with a 2Vrms input, but would appreciate guidance. I understand this converts to only 65v p-p at each of the splitter outputs, hence the MOSFET follies inspired followers to take the load to the EL36 grids.

I recall Tubelab discussing MOSFET followers and recomending +/- 100V supply, so thats that explained. THe various bias supplies are self-explanatory. THe start delay would appreciate a practised eye.

Question on feedback - the original circuit has a gfb circuit from the OPT to the input valve cathode. Since I am using LEDs to provide the bias on the input valve, can I still use the divider network to provide feedback?

[aardvarkash10]

circuit....

[Tubes4e4]

Hi aardvarkash10,

Question on feedback - the original circuit has a gfb circuit from the OPT to the input valve cathode. Since I am using LEDs to provide the bias on the input valve, can I still use the divider network to provide feedback?

No, you cannot.

Regarding the MOSFET drivers: There is no need for them when running the EL36 in BPT mode like you do. As a matter of fact they even wouldn´t be strictly needed when triode-strapping the EL36, since its CMiller will be comparatively lowish due to the lowish µ.

Regards,

Tom

[kegger]

I just built PP Triode EL86 with an E180CC direct coupled to a 6N6P in
a concertina style splitter, cap coupled to outputs run cathode biased
wth 285v on the plates (20v on cathodes) and 45ma per tube.

No problem here driving these things, there not difficult tubes to drive.

--------

Posted at the same time as Tom, he's right no need for such a drive.

[aardvarkash10]

yeah, I thought the mosfets were overkill, but its my first attempt at using them, so I figured they will work (or not) and I'll notice a difference (or not) and so leave them in (or not!)

[kegger]

Experimenting is always fine by me.

Have fun, by the way I like the sound of the EL86 quite a bit.

Recently got 20 NOS Seimens for $50.00, so I was pretty happy.

[kegger]

OH man, now I'm one of those idiots I talk about..

Darnit, EL36 not 86, DOPE!!

..........

Sorry, disregard anything I had to say!

Carry on.

[Yvesm]

Originally posted by aardvarkash10

. . .
Question on feedback - the original circuit has a gfb circuit from the OPT to the input valve cathode. Since I am using LEDs to provide the bias on the input valve, can I still use the divider network to provide feedback?

Sure, you can !

Just insert a small (10 to 100 Ohms) resistor between the cathode of the LED and the ground and return here some voltage from the OPT secondary from another resistor.
Then you have the divider network.

To void ground loop, ground the other end of this secondary at the ground point of the inserted resistor, not elsewere.

Yves.

[Yvesm]

On your schemo, swap 6J7 G2 and G3

[aardvarkash10]

I'm becoming famous for that Yves - thanks!