 
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
|
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
4.244.195.127
In Reply to: RE: 2 Comments for Fla Charlie ........... posted by FlaCharlie on August 19, 2007 at 07:19:24
Fla Charlie,
Optimally speaking, the Cathode negative supply should be designed just as good as the Plate's positive supply.
Forget 1K resistances, in R's ..... or in Ls.
This past week, someone posted a schematic on an Eico vintage amp, and they were using ( back then !! ) 13 ohm Rs between caps on the negative supply, and, as I reall, 100 uF for the C's. Ray Moth sub-posted to it I recall. Everyone seemed to miss the point of the 13 ohm Rs, never openly commenting on that value, which, in MY MIND, is to maintain low series-resistance in the Cathode's negative power supply.
On thick wire, you tin the ends first, and then solder it where needed, using WONDER solder...no problem at all. How do you expect to keep intact the peak dynamic crests of the music, and "copy " the " low everything " supply implementation, if you change the parameters - and wire it with chintzy JUNK !!!!!!, especially after I almost ALWAYS post up here, " use heavy AWG, highest-quality wire ??.
Do it right, the first time, its NOT a budget breaker as there was just suggested for you a Radio Shack wire option .
Jeff Medwin
Follow Ups:
Jeff - Actually, I was the one who posted the schematic of the negative supply. It's not an Eico, by the way, it's a Scott 299B. I also posted an Eico HF-87 schematic and asked for comments on the viability of the 6SN7 portion of its input section as a possible input for the 6B4G amp since I want to use 2C22s (half a 6SN7) or 6SN7s or some combination of the two.
I have a rather large collection of vintage amps. The DIY interest came along with my doing restoration/repair on the old stuff. Out of all the vintage amps I have there are only a few that use fixed bias or some kind of negative supply - the Pilot 260, Dynaco ST-70 and the Scott 299B. I just posted this as an example of a negative supply, not with the intent of copying it.
The resistors are actually 18 ohm each and preceded by a 10 ohm, so don't these result in 64 ohms? That's a far cry from 1k, but as I said, the 1k was just an example. Hopefully I'll be able to come up with a combination of primary and secondary connections with the Signal transformer that will get me close enough for both the two B+ sections and the negative section that I won't have to use high value resistances. Mikeyb suggested previously that the Eico HF-87 input voltages could be adjusted and doing so I would need -125v to the cathode resistor. That's what the negative supply will be for.
I don't know what the Scott engineers were up to in this design somehow I don't think it had anything to do with "low DCR" since you'll notice in the Scott circuit there's also another leg with an 8.2k in it. So neither of these values are truly "low DCR" in your book. Correct?
I'll try to experiment with some heavier wire but, honestly, I wonder if 12 AWG wire will even fit through a standard tab on a terminal strip even by itself, not to mention on a tab that has other parts on it. I'm not sure if I even have any 12 guage around but I'll maybe try a pair of 16s or some 14 to see if it might fit.
I know there have been a couple of other guys who have built the "low everything" power supply. One, I believe you mentioned yourself, and you noted that he was pleased with the results even though he hadn't used heavy guage wire. Hasn't John Swenson (who I believe is a very experienced builder) built this PS too? Did he also employ your heavy guage wire strategy?
I'm going to be trying out various people's ideas in the process of building this amp, not building it according to, or trying to prove or disprove, any one person's theories. Sorry if that disappoints you. My purpose is to learn from it all and, of course, to hopefully end up with a great sounding amp. In the end, ALL of you guys will probably be able to call me an idiot and say, "There's NO WAY that could sound good because of . . . X, Y or Z." ;^)
. . . Charlie
"Optimally speaking, the Cathode negative supply should be designed just as good as the Plate's positive supply. "
Don't you mean the grid's negative supply?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
No.
JM
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"
| FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
||||||||||||||
This post is made possible by the generous support of people like you and our sponsors:
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2025, Audio Asylum, all rights reserved ]
