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Old 14th April 2013, 11:26 PM   #1
rkoonce is offline rkoonce  United States
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Default Diodes for Marantz 8B

I have a Marantz 8B in for major repair. It's been hot and I'm in the process of recapping it and replacing out-of-spec resistors. I want to replace the rectifier diodes as well. Suggestions for the best component appreciated. I'll replace the selenium bias rectifier with a 1N4007.
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Old 14th April 2013, 11:37 PM   #2
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If you use a 1N400* then as the voltage drop is considerably less, you must select a new value for the bias voltage resistor R20.
1N4007 is ideal for the half wave bridge rec. 1N4003 is more than high enough voltage rating for the negative bias supply.
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Old 14th April 2013, 11:42 PM   #3
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Hello,

A couple years back I built an 8B clone using ST70 transformers. The amp sounded wonderful. In that thread Eli gave me the needed information to build the power supply. Check post #11. The title of the thread is: Marantz 8b power transformer
It was dead quiet.

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Old 15th April 2013, 12:12 AM   #4
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Use 1N400n series diodes? No, no, a thousand times no! The 1N400n series parts are nasty, noisy, trash. Use UF400n diodes in place of the corresponding 1N400n parts. The difference in cost is pennies, but the difference in switching noise levels is HUGE.

I've long since forgotten what I said in the thread mentioned by m2racer.

Take a close look at the 8B schematic. It employs stacked diode pairs in a Greinacher doubler B+ PSU, just like the H/K Cit. 2 does. Contact Jim McShane and buy the same diodes he puts into a "Deuce". Buy the film snubber too. If replacement PSU caps. have not already been purchased, talk to Jim about increasing the value of the parts in the doubler stack. If you do increase the cap. value in the doubler stack, buy Jim's NTC inrush current limiting thermistor kit. What's going on in an 8B is quite similar to what's going on in a "Deuce". We are talking about 2 of the very finest "vintage" amps ever offered to the public.
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Old 15th April 2013, 09:33 PM   #5
rkoonce is offline rkoonce  United States
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Thanks Guys, that's exactly the information I was looking for.
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Old 16th April 2013, 08:31 PM   #6
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rkoonce,

Somehow a couple years back I said or did something to tick Eli off. But when he was speaking to me he was very helpful and maybe a little too patient. I've learned a lot since then and don't need to ask so many stupid questions but here is what Eli recommended back then:

Parallel 2X UF5408s with high WVDC 10 nF. caps. and use the assemblies on the ground side of the bridge. Use 2X 600 PIV Schottky diodes on the "hot" side of the bridge. The rail will be dead quiet, while costs are contained. Insert a CL150 negative temperature coefficient (NTC) inrush current limiting thermistor between the Schottkys and the PSU filter, to slightly "soften" B+ start.

SS or vacuum for the rectification, CLC filtration is the way to go. If you use a large valued 1st filter cap., install a "hash" filter (as I have recently described more than once) between said cap. and the choke.

Mount ferrite beads on the heater winding leads, to protect against noise sneaking into the signal path, via the heater supply.

Panasonic (Matsushita) makes excellent 'lytics. Buy 'em from DigiKey. Nichicon and United Chemicon also produce suitable stuff.

The one thing I can say for sure is he was right. The power supply was black quiet and the amp sounded wonderful!
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Old 6th July 2013, 09:40 PM   #7
rkoonce is offline rkoonce  United States
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Default Marantz 8B again

I got the amp recapped and I replaced any resistors which were more than 5% out of spec and it seems to behave as far as the power supply, but the amp doesn't pass a signal on either channel. I'm running it at reduced input voltage for testing which produces about 275V B+. I inject a 1V signal and I can follow it through to pin 2 on the 6CG7 where I measure about 35VAC, so good gain from the Input tube. I get no signal in pin 7. B+ appears where it should. The meter was replaced with one from VAC and I was told it has an internal shunt. I get no meter deflection with bias adjustment. I do measure B+ on both meter terminals. Suggestions appreciated.
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Old 6th July 2013, 10:21 PM   #8
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You say there is no signal on pin 7. Pin 7 of what? which valve?
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Old 6th July 2013, 10:29 PM   #9
rkoonce is offline rkoonce  United States
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Sorry for not being clear. I'm referring to in 7 of the 6CG7 phase splitter.
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Old 6th July 2013, 10:34 PM   #10
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There should be no signal on pin 7 as it is dedcoupled with a 330nF capacitor, C6. Pin 2 should have signal on it.
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