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12B4 PP Full Differential Amp Notion
12B4 PP Full Differential Amp Notion
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Old 11th May 2008, 05:10 PM   #1
Sheldon is offline Sheldon  United States
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Default 12B4 PP Full Differential Amp Notion

I've got some transformers tubes and other bits left over from various projects and was thinking of a little low power amp. This one is direct coupled, shown here without the various extras, such as grid/gate stoppers, and trimmers. I've actually built the input stage and it performs well. Pretty standard.

Sheldon
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Old 11th May 2008, 05:25 PM   #2
zigzagflux is offline zigzagflux  United States
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Very nice. Do you find the DC balance between the first pair to be stable enough, such that the second stage shares the current equally? I know Gary Pimm has had some issues with this.

FWIW, the DN2540 is running at its minimum headroom at 12V negative rail. The output capacitance really drops when you reach about 20V, at least according to the data sheet. Might be a small improvement with a 25V rail in cascode.
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Old 11th May 2008, 06:05 PM   #3
Sheldon is offline Sheldon  United States
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Default And sailing off into incertian territory

One thing I wondered about is maintaining balance in the output tubes. I guess I should be able to put a trimmer in the driver tail to deal with that, but I wondered about other options. I toyed with the idea of three biasing schemes.

Left diagram: There was some discussion of this biasing scheme on the Baby Huey thread. For DC purposes, each tube is biased independently. If a low valued (or no) resistor is used for the tail to common between the bypass caps, it's esentially a standard cathode bypassed PP stage. If the resistor is a high value, it's essentially a differential stage. The AC current is carried between the cathodes with separate DC paths for biasing. OK so far.

Middle diagram: Now the shakey part (I'm guessing here). This shows separately biased and bypassed cathode sources - a regular PP stage - with a twist. I wondered if bypassing to the gate might be advantageous (if it works at all) in allowing a smaller cap. I'm guessing that the current set resistor would provide some degeneration.

Right diagram: Ok, if bypassing via the gate isn't complete lunacy, then can we get back to differential by making the AC path between cathodes via gates? Any thoughts?

Sheldon
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Old 11th May 2008, 06:12 PM   #4
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by zigzagflux
Very nice. Do you find the DC balance between the first pair to be stable enough, such that the second stage shares the current equally? I know Gary Pimm has had some issues with this.

FWIW, the DN2540 is running at its minimum headroom at 12V negative rail. The output capacitance really drops when you reach about 20V, at least according to the data sheet. Might be a small improvement with a 25V rail in cascode.

Good question. The balance is within a volt or so, but I wondered about the output stage too. So I wrote the above reply and didn't get it posted before you replied. Does any of this make sense as a way to deal with the balance?

As for the negative supply voltage, yes, it would be easy to increase and I probably will for the final.

Sheldon
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Old 13th May 2008, 03:18 PM   #5
Sheldon is offline Sheldon  United States
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Here are the 3A5 curves. Pretty linear at this end of the curves.

http://www.geocities.com/dmitrynizh/3a5-very-small.gif
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Old 15th May 2008, 06:17 PM   #6
Sheldon is offline Sheldon  United States
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OK, one more shot at feedback on my cathode biasing schemes (the text kind), especially on the third version. I can think of several possibilities: It's obvious and trivial, it's dumb and it's not gonna work, or, it might work with changes. Give me a hint, or I'm gonna have to lash it up to find out.

Sheldon
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Old 17th May 2008, 01:11 PM   #7
Jaime is offline Jaime  Uruguay
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Default Nice small amplifier

Nice small amplifier!!!
The power is 2w?

Could be a good option 6SN7-GTB paralel the 2 triodes in each branch?
12B4 is very good, but 6SN7 too.
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Old 17th May 2008, 03:24 PM   #8
Sheldon is offline Sheldon  United States
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Default Re: Nice small amplifier

Quote:
Originally posted by Jaime
Nice small amplifier!!!
The power is 2w?

Could be a good option 6SN7-GTB paralel the 2 triodes in each branch?
12B4 is very good, but 6SN7 too.
I haven't built it yet, but power should be in that range. I picked the 12B4 because I have some. The 6SN7 will put more demands on transformer design, as the plate resistance is pretty high, even with paralleled triodes. Could be nice, though, for 32 Ohm headphones or 16 Ohm speakers.

Sheldon
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Old 21st May 2008, 01:12 AM   #9
Sheldon is offline Sheldon  United States
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Default Re: And sailing off into incertian territory

Quote:
Originally posted by Sheldon
One thing I wondered about is maintaining balance in the output tubes. I guess I should be able to put a trimmer in the driver tail to deal with that, but I wondered about other options. I toyed with the idea of three biasing schemes.

Left diagram: There was some discussion of this biasing scheme on the Baby Huey thread. For DC purposes, each tube is biased independently. If a low valued (or no) resistor is used for the tail to common between the bypass caps, it's esentially a standard cathode bypassed PP stage. If the resistor is a high value, it's essentially a differential stage. The AC current is carried between the cathodes with separate DC paths for biasing. OK so far.

Middle diagram: Now the shakey part (I'm guessing here). This shows separately biased and bypassed cathode sources - a regular PP stage - with a twist. I wondered if bypassing to the gate might be advantageous (if it works at all) in allowing a smaller cap. I'm guessing that the current set resistor would provide some degeneration.

Right diagram: Ok, if bypassing via the gate isn't complete lunacy, then can we get back to differential by making the AC path between cathodes via gates? Any thoughts?

Sheldon
Upon further reflection, that right most schematic is, in fact, lunacy. The cathode sources will fight to a standstill. But maybe this one will work. The FET's should be closely matched and either R4 or R5 smaller than R11, and could be trimmed for perfect current match?

Sheldon
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