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Old 17th December 2010, 12:00 AM   #41
wabun is offline wabun  United States
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Quote:
Originally Posted by Ty_Bower View Post
I don't think that part about the positive feedback is true. Take a look at the PC-13 circuit. R4 (150k) provides a positive feedback loop from the phase inverter to the cathode of the voltage amplifier.

The driver circuit of the SCA-35 has more gain than the ST-35 because it uses the pentode section of a 7199 tube (mu > 1500) as the voltage amplifier. The ST-35 uses the high mu triode section of a 7247 (mu = 100) for this duty.
500K resisitor in parallel with 1nf polypropelene cap - 1K resistor at the grid input of 7199 solved the problem..quite a good sound, but bit slow sounding and blur bass..wonderful vocal & high through
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Old 17th December 2010, 02:40 PM   #42
Jim W is offline Jim W  United States
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Originally Posted by ArtG View Post
Dave Gillespie has devised an innovative EFB fixed bias supply for the output stage of both the SCA and ST-35 amplifier. Whatever route you take, I'd strongly suggest that you look into this low cost, easy modification, as it should make a marked improvement in the amp's output. Follow the link below.

http://www.tronola.com/A_New_Look_At...riend_Rev0.pdf
This approach using a LM339 to provide bias is appealing in it's simplicity. But I wonder about putting a voltage regulator directly in the signal path. How does it sound?

Jim W.
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Old 17th December 2010, 03:21 PM   #43
ArtG is offline ArtG  United States
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I don't know how it sounds at this point. I'm in the process of doing a total rebuild on an ST-35, which is now 75% complete, so I should be able to present my opinion in a few weeks. Dave Gillespie states that the measured impedance of the regulator is very low, so I'm guessing that the effect of having a SSD in a cathode circuit should not have any detrimental effect on sound quality. Others have been routinely using diodes, LEDs, BJTs and MOSFETs in cathode circuits without complaint, so I'm confident of good things to come.

Last edited by ArtG; 17th December 2010 at 03:23 PM.
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Old 17th December 2010, 03:24 PM   #44
DF96 is offline DF96  England
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The voltage regulator takes care of DC and very low frequencies. The bypass capacitor handles higher frequencies i.e. most audio. The net result should be better than an unmodified circuit, even if the regulator output impedance is a bit non-linear. A rather clever upgrade! Like all the best ideas it is simple.
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Old 17th December 2010, 04:03 PM   #45
Jim W is offline Jim W  United States
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Originally Posted by DF96 View Post
The voltage regulator takes care of DC and very low frequencies. The bypass capacitor handles higher frequencies i.e. most audio. The net result should be better than an unmodified circuit, even if the regulator output impedance is a bit non-linear. A rather clever upgrade! Like all the best ideas it is simple.
My concern was with the regulator's dynamic response, especially at high frequencies. But now that you point it out (and I open my eyes while looking at the schematic), C24 provides a signal path to ground so that's not so much of a concern. Clever mod.
Jim W.
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Old 30th May 2011, 01:31 AM   #46
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I just got one sca35. It seems to have a problem wity the preamp section. According to the schematic 1st half of 12ax7 is for phono, second half is for high level input eg radio, tape etc.

The schematic state that b+ for the 2nd half of 12ax7 should be 180v. In my amp it's only about 130v on both channel. Gain seem lacking when i listen to it i have to turn up volume pass 12 oclock.

Anyone has suggest where the problem is? B+ on the firzt half of 12ax7 seems correct according to schematic 105 v. I under stand that phono amp usually run at lower b+
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Old 30th May 2011, 02:14 AM   #47
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Quan -- On an SCA35, the 12AX7 tubes only serve as a preamplifier for phono or tape head service. When hi level inputs are selected, these tubes are not used at all; the input is sent straight to the passive control section, and straight into the power amp boards. While the gain for high level inputs is usually adequate, it is quite normal for the volume control to operate at a straight up position or higher based on the source's signal level. The input sensitivity on the hi level inputs for the SCA35 is 1.0 vac for full power output.

Dave
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Old 30th May 2011, 12:25 PM   #48
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Thanks Dave, i looked at the schematic again and it seems that the spare input goes directly into the passive control section. But the radio input and "from tape amp" input seems to go through the 12ax7. I might be confused by the drawing of the selector switch.

I have a turntable and also need to use the phono input. Still wondering why b+ on 2nd half of 12ax7 is much lower than spec in the manual and it's the same on both channel.

Cheers
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Old 30th May 2011, 03:56 PM   #49
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Quan -- I think you are being confused by the drawing of the selector switch. As drawn, it is shown in the "Tape Head" position, the most CCW position. If you rotate the connector ring clockwise, you will see that the last three positions connect the three high level inputs directly into the passive control circuits.

Besides off value plate load or cathode resistors, the only thing that could really cause the plate voltage to drop on the second section of the 12AX7s (assuming "normal" 12AX7s are installed), is leaking coupling caps between the first and second sections of these tubes. Check to make sure that the voltage on pin #2 is no more than 0.00 vdc on these tubes. It should in fact even be negative, which is normal for the way these stages are operated.

Dave
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Old 21st December 2011, 02:08 AM   #50
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Hey All,
I'm rebuilding an SCA-35 right now. I was planning to get rid of the 7199 and replace it with a 6AN8A by rewiring the socket. But after some research this may not be a good idea? Then I looked at the ST-35 schematic. It seems very similar. Any reason why I couldn't use the ST-35 values on the SCA board? Then I could use the 12DW7/7247 twin triode instead.

Kevin

Last edited by mr2racer; 21st December 2011 at 02:13 AM.
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