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 Post subject: Output tube biasing
PostPosted: October 27th, 2020, 7:56 am 
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Joined: January 15th, 2015, 7:19 am
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Location: Baltimore MD
Assuming adjustable cathode bias
Instead of measuring current why not measure plate voltage to make sure tube performance is matched?


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 Post subject: Re: Output tube biasing
PostPosted: October 27th, 2020, 8:58 am 
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Joined: February 28th, 2013, 1:19 pm
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Paul,

First, on a PP transformer, the windings may not be equal in resistance, so you need to know the resistance of each winding. The voltage may be different even though the currents may be the same.

If you KNOW the resistance of the windings feeding B+ to the output tubes, it IS possible to determine current through the tube by measuring the voltage drop across the winding and dividing by the resistance. It is sometimes difficult to get a good resistance reading off an inductive winding, so the best way is run a known, fixed current through the transformer and measure the voltage drop across (SE) or on each side of the winding (PP), and keeping a record of the measured winding resistance (measured Voltage/known current). However, it is so much easier to install a precision (i.e. 1%) 1 ohm or 10 ohm resistor (which can be unbypassed) at the cathode, and accessible test point(s), so to read current by measuring the drop across the resistor. With a 1 ohm resistor, the measure voltage drop is the same as the current, but may run into issues with multimeter digit display. Better to use a 10 ohm resistor, and divide the reading by 10 to get the current. This scheme works for both fixed bias (where this is the only resistor from cathode to ground) and cathode bias. You could put that resistor at the plate as well if you want, but your meter needs to be rated to the B+ voltage (most are, up to 1000V, but there are some cheap ones that may be rated at 600V), and you will need two test points. Much safer to measure at a point of low voltage in event you come into contact with lead, and with the resistor installed at the bottom of the cathode resistor with one end at ground, you only need a single test point since you can use the speaker ground connection as the other test point.

David


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 Post subject: Re: Output tube biasing
PostPosted: October 27th, 2020, 10:22 am 
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Location: Baltimore MD
I guess I did not state my question very well.
Does the plate voltage change when bias current changes? If so I would think a better way to balance the tubes in a PP amp is to adjust the bias current so that the plate voltages are the same.
I have a couple of amps with individual adjustments for each tube and usually adjust to a specific current measured as a voltage drop across a 1-ohm resistor in the cathode circuit. f the plate voltage changes with the bias current I just thought a better way to match tubes would be to match plate voltages.
If I am wrong then I am wrong


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 Post subject: Re: Output tube biasing
PostPosted: October 27th, 2020, 10:34 am 
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As can be deduced from David’s post, in a PP amp if the primary windings have different DCRs, which they always will, then if you adjust the bias to match the plate voltages, the current will be different in the two tubes. Adjusting based on plate voltage will only match currents if the OPT primary DCRs are identical...

Roscoe

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 Post subject: Re: Output tube biasing
PostPosted: October 27th, 2020, 10:38 am 
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Pelliott321 wrote:
I guess I did not state my question very well.
Does the plate voltage change when bias current changes? If so I would think a better way to balance the tubes in a PP amp is to adjust the bias current so that the plate voltages are the same.
I have a couple of amps with individual adjustments for each tube and usually adjust to a specific current measured as a voltage drop across a 1-ohm resistor in the cathode circuit. f the plate voltage changes with the bias current I just thought a better way to match tubes would be to match plate voltages.
If I am wrong then I am wrong

No. I have a bias meter that plugs into the output tube sockets and I can observe plate voltage and cathode current with a flick of a switch. I found that the plate voltages are stable at each tube position regardless of the cathode current.

However, the plate voltages are a little different at each position due to manufacturing tolerances of the associated circuit components including the output transformer.

It is a good idea to start with matched tubes, but do not expect the rest of circuit to be matched unless you upgrade with at least matched resistors (+/- 1/2% or better), then what is left is the balance of the output transformer primary.

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 Post subject: Re: Output tube biasing
PostPosted: October 27th, 2020, 10:47 am 
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The plate voltage is going to change as the current changes, but you’ll not be able to see it on your DVM, we’re talking about changes in the mV range, which is more resolution (and accuracy) than the DVM has when measuring hundreds of volts...

Roscoe

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 Post subject: Re: Output tube biasing
PostPosted: October 27th, 2020, 10:59 am 
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Joined: January 15th, 2015, 7:19 am
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Location: Baltimore MD
so it sounds like one cannot overcome circuit/tube variations completely by just matching the bias


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 Post subject: Re: Output tube biasing
PostPosted: October 27th, 2020, 11:08 am 
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Pelliott321 wrote:
so it sounds like one cannot overcome circuit/tube variations completely by just matching the bias

10-4

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 Post subject: Re: Output tube biasing
PostPosted: October 27th, 2020, 2:11 pm 
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Then getting matched pairs is not enough to get matched halves of the signal. You can never get rid of crossover distortion.
Sounds like a big limitation to PP design


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 Post subject: Re: Output tube biasing
PostPosted: October 27th, 2020, 3:07 pm 
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Pelliott321 wrote:
Then getting matched pairs is not enough to get matched halves of the signal. You can never get rid of crossover distortion.
Sounds like a big limitation to PP design

Yep. I found love with SET. Your Jazz speakers could sound even better than expected with SET amps.

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