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classic valve ST-70 8b driver kit capacitor
classic valve ST-70 8b driver kit capacitor
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Old 10th February 2018, 12:07 PM   #1
indianajo is offline indianajo  United States
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Default classic valve ST-70 8b driver kit capacitor

I have a 1961 dynakit ST-70 that I've decided to try the Classic Valve "Marantz 8b" driver board kit. The triple 6SN7 dual triode board. Mainly because I can buy new tubes for it. The 7199 has gotten weak, and the design was good sounding for 1961 but is fuzzy compared to my solid state amps. Also my paper/phenolic dynakit boards have been soldered too many times with the new metal film resistors and polyester anode caps.
So Greg Van der Sluys refuses to guess what NFB capacitor C105-205 my kit might require, because the output transformer might be from any of 6 vendors. My transformers are 1961 dynaco. He won't even guess what decade capacitor to use, and suggests I buy a cap substitution box, a square wave generator, and a scope. My scopes are all dead with a hundred expired eletrolytic caps each, the square/sine wave generator kit is a failure awaiting action from about 10 years ago. My cap substitution is axial lead stock in a number of plastic bags marked with a sharpie.
Before this kit goes in the junk box, while I listen to vile Samsung TV sound or regret buying one of their patented $190 sound bars with the amazing 3" woofer:
Has anybody with a real dynaco ST-70 built one of these? the nearly equivalent triple 12AX7 driver circuit? What capacitor did you use? Even the decade would cut my trials down to 8 caps instead of 60. I can probably hear enthusiastic overshoot as too much IM distortion, slow slew rate as missing highs on difficult sound source. Does it have to be 630 V polyprophylene feeding the cathode circuit?
Schematic diagram is here: Classic Valve Design - Vacuum Tube Printed Circuit and Turret Boards the marantz 8b circuit.
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Dynakit ST70, ST120, PAS2,Hammond H182(2 ea),H112,A100,10-82TC,Peavey CS800S,1.3K, SP2-XT's, T-300 HF Proj's, Steinway console, Herald RA88a mixer, Wurlitzer 4500, 4300

Last edited by indianajo; 10th February 2018 at 12:14 PM.
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Old 10th February 2018, 12:33 PM   #2
DF96 is offline DF96  England
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Other things being equal, a 3.3k feedback resistor with a 2.4nF capacitor across it would give you an HF rolloff at 20kHz. You will need something a bit smaller than this, so I would look in the 100pF to 1nF region. In reality a 'good' square wave response is not always as important as some peoiple think.
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Old 11th February 2018, 12:10 AM   #3
audiowize is offline audiowize  United States
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The cap can be ceramic, polypropylene, polystyrene, silver mica, whatever really. It does not see much voltage. A 50V rated cap is fine.

The value will change if the feedback resistor values are altered from stock.

If you own a computer, a simple and cheap USB scope will get the job done for this task.
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Old 11th February 2018, 02:43 AM   #4
indianajo is offline indianajo  United States
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Okay, thanks for the decade (100 pf-1000pf) and voltage (50 okay). The original 7199 circuit has a 390 pf parallel 680 ohms for feedback, but the mica cap has a connection to the ultralinear primary winding this doesn't, so it is 500 v rated.
I have in stock 150 pf cog ceramic,100 pf 220 pf and 390 pf mica, some other values in the range in x7r ceramic. I'll start with 100 and listen to my reference top octave piano tracks for distortion. I'll also check the output with silent input with an analog AC VOM for oscillation. I don't think paralling the 100 pf with other caps with an alligator clip lead would mess this circuit up. As I listen to my piano track. Top octave piano is very difficult to reproduce, the 7199 driver board fuzzes this up some. (Original ST-70 amp tested at 1% hd a max power, and that was 1000 hz test, not high frequency I think).
Thinking of this as the analog of the feedback resistor on an op amp makes it a lot less scary. there a 33 pf ceramic I added around the feedback resistor on the 33078 op amp in my disco mixer, which stopped the oscillation caused by subbing for slow old 4558 with no feedback cap bypass. Tubes are RF devices, **** fast amps, if not op amps.
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Dynakit ST70, ST120, PAS2,Hammond H182(2 ea),H112,A100,10-82TC,Peavey CS800S,1.3K, SP2-XT's, T-300 HF Proj's, Steinway console, Herald RA88a mixer, Wurlitzer 4500, 4300

Last edited by indianajo; 11th February 2018 at 02:46 AM.
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