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Old 22nd March 2004, 07:38 AM   #1
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Default transformer coupling, magnetic amplifiers, grid leak current

Hi,

If driver stage is transformer coupled to a power stage, do we still need grid stopper, grid-leak resistor for power tube ? Does transformer secondary impedance work for us ?

How much grid leak current flows approximately for preamp and power tubes ?

Since we are concerning voltage gain in the driver stage, is it possible to design a passive magnetic voltage amplifier stage with transformers ?

When the costs are not a problem, do you prefer transformer coupling to RC or LC coupling ? If you do not, what are your reasons ?

Thanks in advance...

MB
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Old 22nd March 2004, 10:36 AM   #2
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Hi MB

Quote:
If driver stage is transformer coupled to a power stage, do we still need grid stopper, grid-leak resistor for power tube ? Does transformer secondary impedance work for us ?
No need for either but you may see a resistor loading the secondary of the transformer to reduce resonances.


Quote:
How much grid leak current flows approximately for preamp and power tubes ?
This may vary a lot with power tubes, especially if the grid is driven positive.

Quote:
Since we are concerning voltage gain in the driver stage, is it possible to design a passive magnetic voltage amplifier stage with transformers ?

Very possible. I think Sakuma does it. IME interstage transformers sound much better as step-down. 1:1 is already pushing it and asking the transformer for voltage gain is generally not a good idea.

Quote:
When the costs are not a problem, do you prefer transformer coupling to RC or LC coupling ? If you do not, what are your reasons ?
I would prefer transformer coupling only if 2:1 or 4:1 ratio is possible. It's much easier to get good sound with LC coupling. As i mostly use cheap Lundahls my findings may be hardware-dependent.

regards
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Old 22nd March 2004, 11:57 AM   #3
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Hi,

I think my question is a bit misformed. In a correct way:

How much grid leak current flows approximately for preamp and power tubes assuming the grid is always negative (no grid current flows) ? Is it in the range of uA or in mA ? Is 5mA grid leak current even possible ?

Thanks...

MB
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Old 22nd March 2004, 12:33 PM   #4
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Quote:
How much grid leak current flows approximately for preamp and power tubes assuming the grid is always negative
If you really mean grid leak current it is the same as the grid current, (the grid current however small need to flow somewhere and it flows trough the grid leak).

As long as the grid is kept at ~ -1-2V or more negative the grid current is almost neglible, typical could be nano amps, there is a curve of 6SL7 in Valley and Wallman's book "Vacuum tube amplifiers" on page 418 in the original edition. This curve show grid current to approx -10nA at negative voltages more negative then ~-1.7V, at voltages less negative then this the grid current is positive.

If a stage is operated in class A2, AB2 or B2 positive grid current is flowing as the grid is deliberatly driven positive, grid current in this case is very tube dependent.

There has been special "Electrometer" tubes built with grid current in the order of 10^-16A! which is quite low but these where made for measuring equipment.

In normal tubes the input impedance therefore is extremely large for DC but for AC you need to consider the tube capacitance and eventual Miller effect.

Regards Hans
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Old 23rd March 2004, 01:12 PM   #5
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> As long as the grid is kept at ~ -1-2V or more negative the grid > current is almost neglible, typical could be nano amps.

OK. This is what I want to know.

Thanks so much...

MB
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Old 23rd March 2004, 01:53 PM   #6
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Power stages can draw far more grid current, even when the grid is biased negative. The maximum recommended value of grid leak resistor in fixed bias mode gives a hint as to the expected grid current, the smaller the resistor, the higher the likely current. Grid current is extremely variable, which is why you very rarely see any numbers.
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Old 23rd March 2004, 06:09 PM   #7
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If grid is biased using a voltage source (fixed grid bias), it is important to know the grid current (even grid is always negative). This is why I want to know the approximate value.

MB
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Old 23rd March 2004, 07:18 PM   #8
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Approximate for what? Do you have a specific tube and op-point in mind? You may find it sonically beneficial to proide for some grid current anyway.
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Old 23rd March 2004, 07:48 PM   #9
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Quote:
Originally posted by metebalci
If grid is biased using a voltage source (fixed grid bias), it is important to know the grid current (even grid is always negative). This is why I want to know the approximate value.
I understand your concern, but even if we knew the type of valve you were using, we would be lucky to predict grid current within an order of magnitude. Assume the worst, cater for it, and you won't be disappointed.

Which power valves do you intend to use, and under what conditions?
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Old 24th March 2004, 11:22 AM   #10
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>Which power valves do you intend to use, and under what >conditions?

Possibly 300B. However, I think I should forget about fixed-bias for the moment and return to cathode resistor bias.

I have one another question. Suppose:
We have 300B. We are biasing by cathode resistor. We have DC heater supply. What is the proper connection of cathode resistor to the filament ?

Should cathode resistor be connected to the center of 5V DC supply ? Or simply connecting the DC supply across filament is enough ? Should I have to elevate DC supply to somewhere around 65V ?

PS: I had asked this question before, but I did not think this way that time.

Thanks...

MB
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