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 Post subject: Bias adjustment for my Sherwood S-5000 ii 7868 amp
PostPosted: Jul Sat 28, 2018 7:06 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 302
Location: Windsor, CT USA
I'm having trouble determining optimal cathode bias current(in MA). Plate voltage is 435 volts (actual measurement). The S-5000 ii is a fixed bias amp.
I'm using the "10 ohm cathode resistor" method. At maximum bias voltage adjustment ( a little over -21 volts), I measure about .275 volts (or 27.5 ma). Trouble is, I don't know what an appropriate (reasonably conservative) bias MA value is. 28ma? 30ma? The 7868 is supposed to be electrically equivalent to a 7591. An online bias calculator seems to suggest 24.4 ma (70% maximum safe dissipation). However, I can't achieve that with only -21 maximum bias volts.

Suggestions? Thanks!


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 Post subject: Re: Bias adjustment for my Sherwood S-5000 ii 7868 amp
PostPosted: Jul Sat 28, 2018 10:32 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 18544
Location: Albuquerque, NM 87123
I'm having a little trouble understanding your question. Doesn't the S-5000II have no bias adjustment as the cathodes are tied to ground? The 435 volts you measure is probably correct; schematic says it starts with 445 at the power supply but is dropped to 435 at the plates.
If distortion and power out are correct, then isn't the amplifier working properly?


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 Post subject: Re: Bias adjustment for my Sherwood S-5000 ii 7868 amp
PostPosted: Jul Sun 29, 2018 1:07 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 302
Location: Windsor, CT USA
The cathodes are tied to ground. The S-5000 ii has a separate adjustable bias supply with a range of 17-21 volts.


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 Post subject: Re: Bias adjustment for my Sherwood S-5000 ii 7868 amp
PostPosted: Jul Sun 29, 2018 1:30 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 18544
Location: Albuquerque, NM 87123
The specs say -21 volts for a plate supply of 450 volts, 40ma cathode current with 0 plate current. Is that what you need?


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 Post subject: Re: Bias adjustment for my Sherwood S-5000 ii 7868 amp
PostPosted: Jul Sun 29, 2018 2:39 am 
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Posts: 20479
Location: Utica, NY 13502 (USA)
Johnnysan wrote:
The specs say -21 volts for a plate supply of 450 volts, 40ma cathode current with 0 plate current.
I'm curious how there can be 40 ma of cathode current when the plate current is 0 ?

Dave


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 Post subject: Re: Bias adjustment for my Sherwood S-5000 ii 7868 amp
PostPosted: Jul Sun 29, 2018 4:33 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 18544
Location: Albuquerque, NM 87123
Sorry; meant to say 0 signal plate current.


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 Post subject: Re: Bias adjustment for my Sherwood S-5000 ii 7868 amp
PostPosted: Jul Sun 29, 2018 12:03 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 20479
Location: Utica, NY 13502 (USA)
Lots of variables here. I tend to agree with Johnnysan's earlier post that, at 27.5 ma (each tube), you are well within the range of proper operation. Don't forget about the role that screen voltage plays in determining the operating parameters.

https://frank.pocnet.net/sheets/168/7/7868.pdf

Dave


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 Post subject: Re: Bias adjustment for my Sherwood S-5000 ii 7868 amp
PostPosted: Jul Sun 29, 2018 2:55 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 18544
Location: Albuquerque, NM 87123
I question using a 10 ohm resistor on the cathodes as this is not the original design of the circuit. If you want to measure cathode current just use a milliammeter. Also, if more bias voltage (more negative) is needed for testing then changing the two resistors on either side of the bias pot can be done, but I really don't see any point in doing this as the plate and screen voltages are going to stay the same.


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 Post subject: Re: Bias adjustment for my Sherwood S-5000 ii 7868 amp
PostPosted: Jul Mon 30, 2018 4:07 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 5279
Location: Rochester NY USA
There will be a bias level that gives minimum distortion, which will depend on load impedance and other circuit conditions, but if you don't have a way to measure distortion, use the lowest level that still sounds good (at too low a point, crossover distortion will increase). Tube life will increase as the idle current is dropped, and of course the temperature of the whole unit drops somewhat too. At the factory, tube amps were biased at the dissipation limit if that's what gave the lowest distortion, since they had specs to meet.

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 Post subject: Re: Bias adjustment for my Sherwood S-5000 ii 7868 amp
PostPosted: Aug Thu 02, 2018 4:57 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 302
Location: Windsor, CT USA
" If you want to measure cathode current just use a millimeter." Inserting a 1 or 10 ohm resistor in series between each cathode and ground seems to be a commonly-used technique for measuring cathode current in fixed-bias amps.


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 Post subject: Re: Bias adjustment for my Sherwood S-5000 ii 7868 amp
PostPosted: Aug Wed 08, 2018 3:22 am 
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Joined: Aug Thu 27, 2009 7:47 am
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Location: Seattle 98125
10 ohm 1/4W resistor gives a measure of fusable protection as well in the event of a catastrophic failure.


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 Post subject: Re: Bias adjustment for my Sherwood S-5000 ii 7868 amp
PostPosted: Aug Wed 08, 2018 11:22 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 302
Location: Windsor, CT USA
Took the advice here and biased the 7868s at ~28ma. My restoration is now complete and I must say that I love listening to this amp! I replaced all the electrolytic caps (yes, I checked the voltages), all of the film caps with metalized polypropolene, the four diodes, and a few resistors. I also installed a CL-80 thermistor. The 12ax7s that came with it were all vintage Telefunken smooth plates. They all tested good and reasonably balanced. I bought a bunch of used 7868s and swapped them out until I found the closest matched pairs. Anyway, I couldn't be more pleased. The S-5000 ii replaces a pair of Grommes 61pgs, which also sound great but can't match the S-5000 ii. Also, the phono preamp on the S-5000 ii is awesome.


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