SSS-style DCCF, separate bias ? LND150?

[sds1]

I have questions about a DCCF that drives the power tubes, specifically the one in the Dumble Streel String Singer:



1) How would you modify this circuit to use 2x separate bias trimmers (one for each side of PP)?
2) Thoughts/experiences swapping an LND150 in for the 12AX7 here?

[PRR]

That snippet makes no sense to me. Maybe I need to see more? Maybe I need the "DCCF" pointed-out? 130k and 820k??  220k seems large for 12AX7 cathode current. Unless these are seriously high voltages?

[2deaf]

That snippet makes no sense to me. Maybe I need to see more? Maybe I need the "DCCF" pointed-out? 130k and 820k??  220k seems large for 12AX7 cathode current. Unless these are seriously high voltages?

Maybe "E" is around +450V and "F" is around -370V? 

[sluckey]

Point "F" is coming from a simple bias supply but the voltage is -328V. The grids of V6 are connected to the phase inverter and the cathodes of V6 are connected to the grids of a quad of 6L6s. Here's the rest of the schematic...

     https://ampgarage.com/forum/viewtopic.php?f=5&t=17827

[sds1]

Power amp context:


Bias Power Supply:


I did not realize the bias supply was -328VDC. I guess this isn't a big deal just didn't realize it haha. It helps me understand the configuration better though.

RG's design is a little different:


I'm open to any other designs or ideas have from their experiences, this is going into a 100W high gain amp on a switch and I want to see if I like it or not.

[PRR]

> the voltage is -328V.

So not so much a "bias" as a distant negative rail, like an opamp.

OK. I did that back in 1973. It seemed like a good idea at the time. On the whole it maybe wasn't.

Merlin's plan is certainly not scaled for hundreds of Volts. You would have >9 Watts in the 0.5W part.

There is also virtue in a "weak pull-up" like 12AX7 instead of a brute like IRF820 yanking at delicate grids.

[2deaf]

What is Merlin's design/plan?

[silverfox]

Point "F" is coming from a simple bias supply but the voltage is -328V. The grids of V6 are connected to the phase inverter and the cathodes of V6 are connected to the grids of a quad of 6L6s. Here's the rest of the schematic...

     https://ampgarage.com/forum/viewtopic.php?f=5&t=17827

I'm curious but unable to determine the actual bias on the grids. I know it can't be -328V. Any ideas anyone? My math fails me at the moment.
que shooter. . . .

[2deaf]

I'm curious but unable to determine the actual bias on the grids. I know it can't be -328V.

The 12AX7 (V7) is configured as a cathode follower and it has -43.5V on its grid, so it will be within a volt or two of that at the cathode which feeds the 6L6 grids.

[PRR]

....actual bias on the grids. I know it can't be -328V.

A 820k:130k divider runs from -328V to ground. The 130k is grounded. You should be able to work it out.

The noted voltages are a few % off from calculation because of meter loading.

There WAS an image from Merlin's site but it has been vanished?

[silverfox]

A 820k:130k divider runs from -328V to ground. The 130k is grounded. You should be able to work it out.

Yes, thank you all: An approximate 8:1 voltage divider agrees closely with 2deaf's calculations. About -40 volts. That resistor network has always confused me.


silverfox.

[2deaf]

Yes, thank you all: An approximate 8:1 voltage divider agrees closely with 2deaf's calculations. About -40 volts.

I didn't actually do any calculations until after I posted that.  It got to where it was bugging me earlier today, so I taped a piece of paper to my 12AX7 graph and pulled out my calculator.  I used -44.4V for the 12AX7 grid and 707V for the B+.  I decided that the thing idles at about 420V plate-to-cathode, -3.7V grid-to-cathode, and 1.3mA.  This would put the cathode at -40.7V, which is closer to the observed value than I usually get with this kind of exercise.   

[sds1]

Merlin's plan is certainly not scaled for hundreds of Volts. You would have >9 Watts in the 0.5W part.

That is RG Keen's drawing, my bad for saying otherwise.

Also from same page on his site:

The follower is set up so that its cathode sits at the normal negative bias voltage of the output tube, maybe -10V to -40V depending on the tube type. This means that the cathode resistor is tied to a more negative voltage of something more than twice the bias voltage...

(source: http://www.geofex.com/Article_Folders/mosfet_folly/mosfetfolly.htm)

So what does a +/-70VDC supply do in terms of voltage drop across the 10K resistor, is that 35 volts across it for 1/8W or am I looking at that wrong?

[2deaf]

1) How would you modify this circuit to use 2x separate bias trimmers (one for each side of PP)?

Like this?