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 Post subject: Sherwood S-8000: Increasing the negative bias supply voltage
PostPosted: May Sat 25, 2019 1:17 am 
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Location: Windsor, CT USA
I am restoring a Sherwood S-8000. My variation uses 12ax7s and 7591 outputs. It's a fixed bias amp. I can't seem to find the exact schematic for my variation (I guess there were many!). However, I did find one for 12ax7s and 7868 outputs which seems to follow what I have except for the use of 7591s. Anyway, this is a fixed bias amp with a -18.5 bias supply. The bias supply is working and the voltage is up to spec. Plate and screen voltages on the 7591s are also up to spec. Everything has been recapped, and 150k and 1k screen resistors were replaced. All voltages are close to normal. The amp works fine. However, the cathode current on the 7591s is high - anywhere from 33 to 46 ma. I measured it across 10 ohm 1% resistors from the cathode to ground. I think I need more bias voltage if I am to get the cathode current down to 26-30 ma. Here is the schematic for the bias circuit:

Image

The circuit seems to use a voltage divider to get the -24 volts down to -18.5 volts. I'm guessing that to increase the negative bias voltage, I need to reduce the value of R201 (currently 18k) and increase the value of R212 (currently 27k). Problem is, I have no idea as to where to start. I've looked at some online voltage divider calculators, and they don't even explain how the current circuit yields -18.5 volts. So, rather than inserting random values, I thought I would ask for help!


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 Post subject: Re: Sherwood S-8000: Increasing the negative bias supply vol
PostPosted: May Sat 25, 2019 1:36 am 
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I am tentatively considering 15k for R201 and 33K for R212. Thoughts?


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 Post subject: Re: Sherwood S-8000: Increasing the negative bias supply vol
PostPosted: May Sat 25, 2019 2:04 am 
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Location: Rochester NY USA
The input voltage isn't known exactly - but we can estimate it. The bias circuitry doesn't appear to draw any current, so with 18.5V across 27K, there will be about 12.3V across the 18K resistor. Which adds up to -30.8V at C209. And if the preamp tubes draw 300 mA, there will be 6.6V dropped across 22 Ohms, so they'll get 24.2V, which seems about right. So you can calculate a voltage divider with an input voltage of -30.8V - but do you know what voltage you want?

One way (if there's space - it's a Sherwood, after all) would be to replace R201 with a 10K resistor and 10K pot in series - then adjust for the idle current you want. Some versions of the 8000 had bias pots, one had cathode bias.

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 Post subject: Re: Sherwood S-8000: Increasing the negative bias supply vol
PostPosted: May Sat 25, 2019 12:33 pm 
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Location: Windsor, CT USA
Tom, you are right! Using an online voltage divider calculator, if I assume 30.8 volts input voltage, and enter the 18k and 27k resistor values, I get -18.48 volts - pretty much exactly as I measured. This morning, I replaced the resistors with 18000 and 33000. Resulting voltage was -21.2 volts. This reduced the idle current draw on one pair of 7591s to ~27 ma (right where I wanted it). The other pair now draws about 17 ma - too low. So, I think I'll insert a second voltage divider for the second pair of 7591s. Yes, I know most people would use potentiometers. However, pots oxidize, get dirty, and go intermittent. I think I would prefer the safety (and reduced noise) of fixed resistors. I don't want any interruptions to the bias voltage. Thanks for your advice!


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 Post subject: Re: Sherwood S-8000: Increasing the negative bias supply vol
PostPosted: May Sat 25, 2019 12:36 pm 
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Before you build another bias supply, you might want to swap the output pair from side to side to see if the increased current is related to the tube(s) or the circuit?

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 Post subject: Re: Sherwood S-8000: Increasing the negative bias supply vol
PostPosted: May Sat 25, 2019 12:52 pm 
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Don Cavey wrote:
Before you build another bias supply, you might want to swap the output pair from side to side to see if the increased current is related to the tube(s) or the circuit?

Indeed!!
e.g. some tubes with grid leakage....

Also, a cathode resistor is a lot quicker and easier way to reduce tube current

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"Measure voltage, but THINK current." --anon.


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 Post subject: Re: Sherwood S-8000: Increasing the negative bias supply vol
PostPosted: May Sat 25, 2019 1:01 pm 
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[quote="pixellany"...
Also, a cathode resistor is a lot quicker and easier way to reduce tube current[/quote]

But won't that affect the frequency response of the amplifier if it is increased too much (= more shallow bass?)

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 Post subject: Re: Sherwood S-8000: Increasing the negative bias supply vol
PostPosted: May Sat 25, 2019 1:09 pm 
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An unbypassed cathode resistor reduces the stage gain at all frequencies--and reduces distortion.

If the objective is just a "tweak" in addition to the existing fixed bias, then the added resistor is pretty small.

As an aside, when I was restoring a Fisher 400, I tripped across an article recommending small cathode resistors to protect against meltdown caused by output tubes with grid leakage.

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"Measure voltage, but THINK current." --anon.


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 Post subject: Re: Sherwood S-8000: Increasing the negative bias supply vol
PostPosted: May Sat 25, 2019 1:19 pm 
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pixellany wrote:
An unbypassed cathode resistor reduces the stage gain at all frequencies--and reduces distortion.

Thanks, I am not good at "engineering" changes, rather I try to follow what was engineered. I have often seen cathode resistors
bypassed by capacitors in smaller amplifiers, and cathodes tied directly to ground (or chassis if you prefer) in HiFi amplifiers.
pixellany wrote:
As an aside, when I was restoring a Fisher 400, I tripped across an article recommending small cathode resistors to protect
against meltdown caused by output tubes with grid leakage.

Mark, I have seen that numerous times too; they often advocate a ¼-watt or even ⅛-watt resistor in the cathode path to ground. Small price to pay to protect those output transformers.
And speaking of a Fisher 400, I have on which shows evidence of overheat coming out of one of the output transformers. (smoking gun, pun intended).

I hope that my discussion is not hijacking or derailing this thread, guys.

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 Post subject: Re: Sherwood S-8000: Increasing the negative bias supply vol
PostPosted: May Sat 25, 2019 1:34 pm 
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When the Fisher came to me, it showed the benefit of a previous repair attempt (including the replacement of some, but not all, of the electrolytics!!!)
But they did replace several output tubes (7868s). Once it was working, I tried the old tubes---all were suffering from major grid leakage.

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"Measure voltage, but THINK current." --anon.


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 Post subject: Re: Sherwood S-8000: Increasing the negative bias supply vol
PostPosted: May Sat 25, 2019 1:39 pm 
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pixellany wrote:
When the Fisher came to me...I tried the old tubes---all were suffering from major grid leakage.

No doubt. I have some NOS 7868 tubes which show major grid leakage. And several of my 7591 tubes, I tossed into the box labeled, "Weak, Grid Leakage etc."
Not thrown out yet...

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 Post subject: Re: Sherwood S-8000: Increasing the negative bias supply vol
PostPosted: May Sat 25, 2019 3:53 pm 
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Location: Windsor, CT USA
I have an assortment of (real) 7591s. All were tested for emission and leakage in my Eico 667. Also, I tested again for leakage in my Sencore Mighty Mite. Once I determined the "good" tubes, I popped them into the Sherwood and then (after 15 minutes) checked the cathode current. I resorted the tubes into pairs with the closest cathode current match. I then inserted a second bias voltage divider, and varied the bias voltage so that both pairs registered 26-30 ma idle current. One pair used the stock 18k/27k resistor pair to yield a bias voltage of -18.5. The other pair used a 15k/33k pair to yield a bias voltage of -21.1. I now have two closely matched pairs. I left it on for 30 minutes and rechecked the results. Satisfied, I put the receiver together and listened for a while. I'm very happy with the result. I now have two Sherwoods: an S-5000 ii (using 7868s) upstairs, and the S-8000 (using 7591s) downstairs. I feel confident that my sweep alignment (using my RCA WR-50b) of the FM worked pretty well. I wasn't able to do the multiplex because the schematic that most closely resembles my version does not correlate with the alignment instructions I have been able to find. However, I think I am going to live with FM/mono for a while until I am confident that the rest of the receiver is stable. Thanks to all who replied!


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 Post subject: Re: Sherwood S-8000: Increasing the negative bias supply vol
PostPosted: May Sat 25, 2019 8:57 pm 
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Excellent...

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 Post subject: Re: Sherwood S-8000: Increasing the negative bias supply vol
PostPosted: May Sun 26, 2019 5:21 pm 
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Location: Windsor, CT USA
A little research revealed that the multiplex circuit for the Sherwood S3MX multiplex adapter is identical to that used in my S-8000. Same tubes, same component values, everything. And the schematic and alignment instructions for the S3MX are available for free. And the alignment steps appear to be very easy and straightforward, and I have suitable test equipment. So, back in I go...


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