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184.56.78.220
I finally got these figured out .No disrespect but to you naysayer's there is a factory bias for the outputs .I have looked at a lot of schematics of these amps and they only show a level control .The pot's are definetly factory and not add on's .Was lucky enough to score four NOS RCA tall bottles and have the double halo of blue heaven going on .Have done some circuit mods on my own and paired through my La Scala's I have reached Audio Nirvana .Ran some new PIO vitamin q's,gold binding posts ,new gold plated RCA input per amp and redid some grounding ,etc .Can't wait to get these broke in .Thanks for the help I receive here .
Donnie
Donald
Follow Ups:
Since you have modded/upgraded your amps and you are not trying to keep it stock I have some suggestions to improve performance.
I would remove the power to the AC outlets and power supply tap if you will not use them. AC hum is "cancelled" in a push pull amp but the less of it to cancel, the cleaner the music signal will be. In other words the output from the speakers should have a darker background. You don't need to remove the outlets just the wiring. The next thing you might want to try is a film cap of about 20 to 30 uf x 600vdc for the first capacitor in your power supply. It will increase the dynamics to the sound and will last the life of the amp (no chemicals to dry out). Just remember that film caps like Solen or the newer box types from Panasonic will have a longer break in period.
Since this amp has fixed bias, then you should be using matched pairs of output tubes. If you are not, you should make each power tube bias adjustable. This will allow the power tubes to operate in balance and not have the stronger tube do most of the work and wear out sooner. Just some things to consider.
cheers, Dak
Hi Dak,
Man am I glad you weighed in . I've read a lot of the valuable knowledge you have posted . I am sort of new at this and as such I told film caps were non polarized and could not be used to replace electrolytics . Myth?? Also your are saying instead of replacing the multi can "which is the power supply ??? correct just to run film caps below the chassis there ? The value is 40-20-10 @500V .You recommend reducing the first to 20 or 30 or to stay at 40 @ 625 V for the film caps .Which film caps do you use or recommend .As we speak I have all ready done the power circuit bypass as you posted .Thanks in advance .
Donnie
Donald
film caps were non polarized and could not be used to replace electrolytics . Myth??
I don't like absolutes, so, for your amp replacing the lytic with a film cap is not a problem. Film caps have no positive and negative orientation but are sometimes marked to indicate which lead is connected to the "outer foil" of the windings. This lead should be attached to the lower potential such as ground.
I did not know the capacitance to the rectifier in your amp so I gave a generic response. But it is best to replace like with like, in this case 40uf. The problem maybe the size of this film cap. It may be quite large and I don't know how much room you have. Check out the dimensions of Solen and Panasonic EZPE series and also WIMA at Mouser and Digikey. I like to use Solen and SCR, and ASC depending on the situation. Your decision to use a film cap may depend on how much room you have. good luck, Dak
Thanks for the response . I have quite a bit of room to tuck caps in.Check these out,go or no go in your opinion .
http://www.thetubestore.com/Capacitors/Mojotone-Vitamin-T-Tin-Foil-Polyester-Film-in-Oil_2/Mojotone-Vitamin-T-Capacitor-0033uF-600V
Donald
.
Ok, these say for crossover network/ ok to use
http://www.parts-express.com/solen-047uf-700v-silver-mkp-metallized-polypropylene-crossover-capacitor--027-634
Donald
Did you replace coupling caps? What about power supply caps?
I have a pair to restore, and would like more information about these amps.
I have never seen a 4 tube version. I suppose later versions might have had solid state rectification.
I have seen a stereo version of the 6973 power amp. which seems to have solid state rectification.
RT Barr
Coupling caps were replaced w/ Tone factory PIO's because I prefer the sound but orange drops work fine.I have just ordered the CE multi cans which have a 40-20-20 tap at 500V .I will use the 40-20 tap and run a seperate 10uf F&T cap 10uf @ 500 V for that leg as the original is 40-20-10 @ 500V .There is a bias pot for the 6973's outputs and I ran a tail over to the two contacts on the unused ( secondary ) speaker taps so I can read w/ a DVM easily .My one bias was frozen but time and Deoxit allowed me to free it . I dry sat them @ 45 ohms on each leg while I had it apart which is dead center .You also have a 47 uf electrolytic at 50 V to replace on each amp along with ( optional ) binding posts,new RCA input .I resoldered all ground paths and tied them to a central point .My monos are 5 tubes and I have seen SS rectification on the stereo models .Buy RCA NOS tall bottles which you will pay out the yazoo for but Every other tube I have ever tried tends to red plate some .Have fun,they are pretty easy to work on .FYI The parts I listed can be bought at AES and the PIO's on ebay etc.
Donald
The schematic you supplied with a previous post showed an amplifier
with only four tubes. The amplifier you show with your second post
shows an amplifier with five tubes.
The information you received with your first post was correct. The
amplifier in that schematic is cathode-bias and not adjustable.
The common resistor connected from the output tube cathodes to ground
is fixed and determines the bias on the output tubes. My guess is the
adjustable resistor on the phase splitter would be set for lowest
distortion with an output a few watts.
Again, I believe you are showing/talking about two different
amplifiers?????.
MY apologies . I am running the 5 tubers that have a bias pot for the outputs .Can't seem to find out what to set it at and where to test .
Donald
Beautiful! Love the big honking Triads iron!
Jim
ARS VS-110
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