Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: sds1 on December 09, 2020, 11:02:16 am
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I have questions about a DCCF that drives the power tubes, specifically the one in the Dumble Streel String Singer:
(https://i.imgur.com/XqOtmy9.png)
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?
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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?
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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?
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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
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Power amp context:
(https://i.imgur.com/MhHCL6G.png)
Bias Power Supply:
(https://i.imgur.com/2OR5H55.png)
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:
(https://i.imgur.com/7vqmkvP.png)
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.
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> 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.
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What is Merlin's design/plan?
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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 (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. . . .
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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.
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....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?
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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.
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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.
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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?
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1) How would you modify this circuit to use 2x separate bias trimmers (one for each side of PP)?
Like this?