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Old 15th February 2015, 04:12 AM   #1
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Default SE Power amp SG2 voltage

question:
in a SE power amp, is it kosher to have the screen grid (SG2) at a slightly higher voltage (6volts) than the plate?

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Old 15th February 2015, 04:33 AM   #2
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Very bad idea. Max. open loop linearity is achieved by regulating g2 B+ at a fraction of anode B+.
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Old 15th February 2015, 05:55 AM   #3
Keit is offline Keit  Australia
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Eli is incorrect.

If you check the datasheets or application notes for audio output tubes, they are generally designed to have the screen at the same voltage as the anode. Tubes for audio are designed that way so that power is not wasted in a resistive divider.

Since screen current is almost linear with screen voltage, it will not matter a jot if the screen is a few volts higher than the anode.

The resistance of the output transformer will drop a few volts, so the anode DC voltage will be a few volts below HT while the screen is getting full HT. This is of no consequence. Neither the tube life nor linearity will be affected.

However, what is the reason for asking? If the reason is a separate power rail for the screen (the usual reason for asking), you may have another problem. If, during power up or power down, the screen supply come up faster than teh plate supply, or (more likely) teh screren supply collapses weel after the anode supply when you turn the amplifier off, the tube's screen will each power up/down be seriously overloaded and will quickly fail. Not because the screen supply is a bit high, but because at power up or down all of the cathode current will go to the screen.

Some folk build amplifiers with tubes not designed for audio, such as TV deflection tubes and some types of transmitting tubes. These tubes MUST be operated at the maximum permitted screen voltage, which will generally be well below the anode voltage for efficient operation.

When using TV deflection tubes for audio power amplification, the sound quality results should be very good. But a separate screen supply is mandated by the tube ratings, and you must design the power supply so that the screen supply comes up last and falls first. If fuses are used on the secondary side of the power transformer, it MUST be the case that a blown fuse collapses the screeen supply as well as the anode supply. Otherwise a blown fuse will take out the tube as well.

Last edited by Keit; 15th February 2015 at 06:09 AM.
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Old 15th February 2015, 06:38 AM   #4
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Quote:
Originally Posted by Eli Duttman View Post
Very bad idea. Max. open loop linearity is achieved by regulating g2 B+ at a fraction of anode B+.
This applies to small signal voltage amps. In that case, get that screen voltage as low as possible, and supply it from a voltage divider. A single series dropping resistor can work if the plate current doesn't vary by much.

As for power finals, that depends. Like Keit says, audio power finals (6V6, 6AQ5, 50C5, 50L5) all operate the screens at or close to the main rail voltage as a means of allowing easy application of UL, pseudotriode operation, or to eliminate a screen supply.

Other types, like the TV vertical and horizontal deflection finals, and a good many RF pents, have low screen voltage specs that should be adhered to unless you really know what you're doing. In that case, your best distortion performance is had with active screen voltage regulation.
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Old 15th February 2015, 08:07 AM   #5
Keit is offline Keit  Australia
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I made a typo in my earlier post.

The last sentence in my 6th para should read:-
These tubes MUST be operated WITHIN the maximum permitted screen voltage, which will generally be well below the anode voltage needs to be for efficient operation.

It does not of course matter much what the exact screen voltage is, providing it does not vary with signal voltage, or varies specifically in the case or ultra-linear operation.
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Old 15th February 2015, 04:58 PM   #6
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Thanks for your responses. My amp uses a 6V6 o/p stage driven by a common cathode 12AT7. B+ is 294 volts, o/p xfr has pri Z of 5K and DCR of 500 ohm,the idle current of 6v6 is 43 mA (RK=330R) and SG2 current of 3.7 mA.

The measured plate voltage=272V, SG2 voltage=283V.

The amp sounds very good. here are some measurement data:
Open loop gain= 25 dB
Closed loop gain= 16.7 dB
S/N= 90 dB ref to 1w/ 8R

Here are the AP graphs:

I noticed that in the Mullard EL-84 se amp plate and SG2 voltages are equal.

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Old 15th February 2015, 11:47 PM   #7
Keit is offline Keit  Australia
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Quote:
Originally Posted by audiohead View Post
I noticed that in the Mullard EL-84 se amp plate and SG2 voltages are equal.
That is because the screen of the EL84 is fed via a 3.9k resistor (R12) which also feeds the driver tube (EF86) and is bypassed by a 50 uF electrolytic. This is a hum reducing arrangement. The small amount of hum on the anode supply remaining after filtering would otherwise be amplified a little by the EL84 screen, so the screen is fed from a third stage of filtering.

The DC currents drawn by the anode and screens is only marginally affected (about 5%) by the slight drop in screen voltage that this entails, (20V, evidently the same as the voltage drop in the output transformer due to the anode current) and is of no consequence.
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Old 16th February 2015, 01:28 AM   #8
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Quote:
This applies to small signal voltage amps. In that case, get that screen voltage as low as possible, and supply it from a voltage divider. A single series dropping resistor can work if the plate current doesn't vary by much.

As for power finals, that depends. Like Keit says, audio power finals (6V6, 6AQ5, 50C5, 50L5) all operate the screens at or close to the main rail voltage as a means of allowing easy application of UL, pseudotriode operation, or to eliminate a screen supply.
When I said fraction, I should have been more specific. Large fraction is appropriate. Full pentode mode is somewhat notorious for having high levels of IM distortion. A well regulated g2 B+ supply held somewhat below anode B+ has a beneficial effect on open loop IMD levels.

The 6AQ5 is a type I'm familiar with. Screen grid fragility is what keeps the type from being a "perfect clone" of the 6V6 in a 7 pin mini package. IF g2 B+ is regulated at 250 V., a fair amount of liberty can be taken with the anode B+ limit, PROVIDED the plate disipation limit is scrupulously adhered to.
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Last edited by Eli Duttman; 16th February 2015 at 01:29 AM. Reason: spelling
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