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In Reply to: RE: What !!!! ........... posted by Tre' on August 19, 2007 at 11:07:14
No.
JM
Follow Ups:
Jeff, will you take time to explain to me what this cathode negative supply is for?BTW, the negative supply and schematic FlaCharlie is talking about is for a Scott 299B. The negative supply is for the grid bias of the output tubes. It also runs the heaters in the preamp tubes. But no cathodes!
Thanks, Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Hi Tre' - I'm the cause of your confusion. I just posted the Scott schematic as a general example of a negative supply. The negative supply I want to try will supply -125v to the cathode of the input tubes.
In the post linked below, I posted a couple of input schemes and asked if they could be used for this PP 6B4G amp I'm building. One was a paraphase which someone had once suggested I could use as the input for a modded Dynaco ST-70. I also posted this schematic of an Eico HF-87, which uses one 6SN7 and one section of a 12AX7 per channel. I wondered if the 6SN7 section (only, no 12AX7) could be used. As stated previously, I want to use some 2C22s in this thing since I've read some posts which praise them highly. Mikeyb suggested that I could adjust the voltages so that the grid would be at 0v, the plate would be 180v and the cathode would be 5v and this could be accomplished by hooking the cathode resistor up to a negative supply. I'm not sure if this is really a LTP or a quasi-LTP or what since my technical knowledge is limited. Anyway, that's what the negative supply will be used for. I'll be using cathode bias on the outputs.
I'm not sure if either of these inputs (paraphase or the modded Eico) will work correctly but I may try them out. If I could get one of these to work I can use four 2C22s - two per channel. It was later suggested to me that I would have better luck using one 2C22 (half a 6SN7) and one 6SN7 per channel. I'm still open to suggestions . . .
. . . Charlie
Sorry, I missed all that.
Ralph does something similar with his OLT driver cathode follower so the output tubes can be both direct coupled to and biased by the cathode follower.
And yes, I agree with Jeff. That negative supply (or at least the last cap) is in the signal path.
Have fun, Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
"That negative supply (or at least the last cap) is in the signal path."
Suppose there's a CCS in the tail. How much signal gets through?
I know from experience that power supplies are more important than it seems they should be, and in ways that it seems they shouldn't. But, if we rank things in order of importance, I think the negative supply under a LTP is way down the list. The longer the tail, the less important it becomes.
-- Dave
As I mentioned, I'm not that familiar with negative supplies since only a few of my vintage amps use them. Probably a dumb question, but: All the schematics I've seen show an electrolytic cap with polarity reversed, as in the Scott I posted. Is it possible to use a non-polar motor run in a negative supply?
. . . Charlie
The polarity of the electrolytics in a negative supply is not reversed. Negative is connected to negative and positive is connected to positive, as in any PS. What is 'reversed' is the 'common' or 'ground', which is the negative line of a positive supply and the positive line of a negative supply.
So, can I use a motor run (non-polar) cap in a negative supply or do I have to use a polarized electrolytic?
. . . Charlie
Motor run caps would be fine.The reason the caps in a negative supply look upside down is the voltage is upside down. If you use a polarized cap, the + needs to go to the positive voltage point in the circuit and the - needs to go to the negative voltage point in the circuit. No matter where those voltages go.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
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