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In Reply to: RE: Scott 233 tube recomendations posted by Mark W. on June 26, 2008 at 14:36:32
Talk to AA sponsor Jim McShane about your tube needs.
It is not possible to do better than a Mullard made GZ34/5AR4. You are set there for many years.
ElectroHarmonix (EH) 7591As are your best bet in O/P tubes. Yes, JJ is also producing 7591As, but JJ's Octal based stuff has been plagued by QC problems.
The Sovtek 12AX7LPS gets (IMO) the nod for those "holes". The Russian tube is clean and quiet. Of particular interest to you is the fact that tube shields fit over the 'LPS well. The other good current production 'X7s could give you trouble with the shields.
RCA made GOOD tubes. More of the same, if you can get 'em, for the 6U8 spots. Siemens made 6BL8s would be fine too.
Look at the schematic for the Scott 233. Notice the 470 KOhm grid leak resistors on the 7591A O/P tubes. That must change. Scott and Fisher "cheated". 300 KOhms is limit for "fixed" bias 7591A grid leak resistors. Tubes from circa 1960 tolerated the liberty taken. Current production will not. Drop the grid leak resistor value to 270 KOhms or less.
BTW, in a unit of that age, replacement of all OEM electrolytic capacitors is absolutely routine.
Eli D.
Follow Ups:
Thank you very much for the insight Eli. I'm very interested in bring this Little Scott up to snuff it sounds so nice now I can only imagine how it will sound with the caps replaced and a fresh set of tubes.
My main system is based on a Pioneer SA-9100
but I have room for a complete tube system in the little alcove under our living room stairs and I intend on doing it up right. thanks
Mark W.
Mark,
Refer to the schematic again. Please notice that the OEM values for the caps. (C16/C17) coupling the phase splitter to the "finals" is 0.1 μF. That's fine, when the 7591A grid leak resistors are 470 KOhms. Unfortunately, that value is too small in combination with the essential 270 KOhm grid leak resistor value. Deep bass performance is adversely affected.
Mere age made the coupling caps. candidates for replacement. Electrical reality demands the change. Use 600 WVDC/0.22 μF. 716P series Orange Drop parts in those coupling cap. positions.
C14 combines with R26 to form a high pass filter that's outside the NFB loop. Decreasing the value of C14 to 0.01 μF. will keep infrasonic noise out of the power amp section, which protects the O/P trafo against nasty sounding core saturation. Soviet surplus paper in oil (PIO) parts are a good choice here, as NFB will not cause sonic homogenization.
Eli D.
Eli I'm printing this out as you write it. So when I get ready to attack the amp this coming fall I will contact you and see about a translation into what ever simplified version of this language I can understand.
Might have to resort to pictures and simple drawings. Good thing I do photography as my other hobby!
I have a 299D with the same tube setup. I restored it using JJ power caps and Dayton bypass caps. They were the budget end of it. The RCA 6u8's are what I have and you should not ever have to replace them. My 7591's are JJ's and they are not bad if you dont over drive them. I also did the 270k resistor. The 12ax7's I use Telefunkins on the pre amp part and I roll the phono ones between JJ and old RCA's. The JJ's in the pre stage were a bit tinny sounding. After 3 years there seems to be a hum developing. I will have to recheck my work and see where that is coming from. Maybe the JJ7591's are near the end of life. The 5ar4 is a sovtek. This is a strong tube and has held up for 2+ years. I had a JJ in there and it didnt last but 3 months and shorted. Otherwise, this is a good heavy amp for the wattage and easy to work on.
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