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Stromberg ASR-880 Biasing
Stromberg ASR-880 Biasing
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Old 16th January 2017, 04:15 PM   #1
Lawrk is offline Lawrk
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Default Stromberg ASR-880 Biasing

I recently replaced a number of components including filter caps blocking caps and supply diodes and many resistor that had gone high. I used the same values for all parts including electrolytic caps. I see that B+ for plates is 500 V where Sams's notes 470 (maybe this is VTVM vs modern meter?)?

My issue is: in Setting bias Sams calls for 34 mA and the max I can obtain with the pot is 25 or so. Tried swapping my 7355 tubes for 7591xyz and get similar results. Is the lower bias caused by the higher plate voltage?

Is the lower bias value a problem? If so, should I install a resistor to knock down the max voltage? I assume the more efficient diodes and caps caused the increase. The amp is designed for 117 Volt input.

thanks
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Last edited by Lawrk; 16th January 2017 at 04:25 PM. Reason: clarification
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Old 16th January 2017, 04:28 PM   #2
Mooly is offline Mooly  United Kingdom
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Stromberg ASR-880 Biasing
Is the bias voltage to high (to negative) ? There is a -60 and -45 as indicated values. What are those actually reading.

You could perhaps try slightly reducing R91 to say 1meg to bring it all a little more positive. It sounds like you are only a fraction off where it all needs to be.

Those more in tune with valve circuitry might have some better ideas
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Old 16th January 2017, 04:58 PM   #3
6A3sUMMER is offline 6A3sUMMER  United States
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Were any of the diodes you replaced Selenium Rectifiers?
Especially the diode that is used for the negative bias supply.
That might be part of the problem, because selenium rectifier stacks had quite a bit of voltage drop (replacing with a silicon diode would give a higher bias voltage).

Then there is the 117v versus whatever power voltage you have.
A tube in pentode operating mode, like you have, will not have more plate current with more plate voltage.
But ... it will have less current if the bias voltage is higher (more negative).
The 7591 has a transconductance of about 10mA per volt bias.
Suppose you have 120V power, that is a 2.5% increase over 117V.
-45V * 1.05 = 46V, or about 10mA less current (that is about what you have).

My AC mains are about 121V. That is 3.5% higher than the 117V rating of some of my amplifiers.
It can be a problem. I thought about using a 10W power resistor in series with the primary to drop the 4V difference, i.e. 4 Ohms and 1A draw = 4V drop.
(More resistance and more watt rating for more current draw, etc.)
I will have to try that.
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Old 16th January 2017, 05:05 PM   #4
Lawrk is offline Lawrk
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I should be more clear. The bias voltage looks ok, its the plate current that is low. As background, I installed a one ohm resistor in series with the plate of each tube and measure the volage across it to derive the current. So, its the current that won't reach its reccommended value when I adjus the bias pot..
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Old 16th January 2017, 05:12 PM   #5
Lawrk is offline Lawrk
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Maybe selenium, the parts list says they were 1N1764.

So the bias supply is outputting -45 Volts (see attached schematic, if you save it you can magnify).

So maybe this is unavoidable. I looked at the 7591 data sheet to try and understand what happens if B+ goes up, but the bias voltage, current B+ curves all look linear at the high end and stop around 450 Volts.
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Old 16th January 2017, 05:18 PM   #6
Lawrk is offline Lawrk
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Sorry, I rechecked the bias voltage and the wiper of the pot reads -46.3 Volts. So I'm 1.3 Volts high.
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Old 16th January 2017, 05:19 PM   #7
Mooly is offline Mooly  United Kingdom
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Stromberg ASR-880 Biasing
The bias voltage determines the anode (plate) current. If the voltage is to far negative then the valve approaches cut off and the anode current falls. It sounds like the negative voltage derived from rectifier M14 is just a wee bit to high (in the negative direction).

There are a few easy ways to bring it down a small percentage.
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Old 16th January 2017, 05:20 PM   #8
Lawrk is offline Lawrk
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So, if I wanted the voltage back to spec, would an acceptable method be to add another diode in series with M14, doubling the drop?

Last edited by Lawrk; 16th January 2017 at 05:27 PM.
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Old 16th January 2017, 05:28 PM   #9
Mooly is offline Mooly  United Kingdom
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Stromberg ASR-880 Biasing
Quote:
Originally Posted by Lawrk View Post
Sorry, I rechecked the bias voltage and the wiper of the pot reads -46.3 Volts. So I'm 1.3 Volts high.
Yes, we are talking just percentage points here. You could also look at increasing slightly the 10k feeding the bias pot.

Another possibility that you could try in addition to the above would be to add a small resistance in series with the rectifier.

Whatever you try, do it by degrees because if you have insufficient bias then then the valves will conduct heavily.
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Old 16th January 2017, 05:30 PM   #10
Mooly is offline Mooly  United Kingdom
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Stromberg ASR-880 Biasing
Quote:
Originally Posted by Lawrk View Post
So, if I wanted the voltage back to spec, would an acceptable method be to add another diode in series with M14, doubling the drop?
You could certainly try that if you have a suitable diode. It might not be quite enough but you could see.
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