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Old 10th October 2010, 09:02 PM   #11
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Quote:
Originally Posted by grungeman91 View Post
I'm trying to design a 25 W tube amplifier with push pull output for my senior project in electrical engineering. I'm using 6l6 for the output tubes and I know I want my output power to be 25 watts, but I don't know how to start designing the amp from there. Any help would be greatly appreciated

Dude,

Start with the 6L6GC data sheet. Notice the condition set for PP AB1 using 360 V. on the plate, 270 V. on g2, and a 6.6 KOhm (end to end) O/P trafo. That yields 26.5 W.

The open loop 2% THD appears discouraging, but it's not really an issue. Use ultralinear mode "finals", instead of pure pentode, and that open loop number will come down. Add some GNFB and you'll get down to an inaudible 0.3% THD easily enough. Be certain to place current limiting resistors in the lines leading from the screen grids to the O/P trafo's UL taps.

Edcor's model CXPP60-8-6.6K O/P trafo will do very nicely. Yes, you do need the magnetic headroom that comes with the power handling shown. The low freq. error correction signal GNFB generates requires that magnetic headroom be present.

Soviet surplus 6Π3C-E (6p3s-e) tubes have decent sonics and an attractive price.

You also asked about gain structure. Sufficient open loop gain is needed to raise the -22.5 V. to zero and drive the GNFB loop. "Classic" Mullard style circuitry will do the job for you.
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Old 13th October 2010, 05:19 PM   #12
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Start with the 6L6GC data sheet. Notice the condition set for PP AB1 using 360 V. on the plate, 270 V. on g2, and a 6.6 KOhm (end to end) O/P trafo. That yields 26.5 W.

How do I design for class AB1 operation? Do I need any extra transformers?
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Old 13th October 2010, 07:24 PM   #13
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How do I design for class AB1 operation? Do I need any extra transformers?
What are the schools teaching new EEs these days? No, you don't need any extra trafos. Class "A" operation means the devices have a 100% duty cycle. Class "B" means the devices have an exactly 50% duty cycle. Naturally, Class "AB" means that the duty cycle is > 50% and < 100%.

The 1 or 2 seen in tube operating classes refers to presence of absence of positive control grid current, Class "x2" means positive grid current is present all or part of the time, while Class "x1" means no positive grid current is present. It's more or less a given that SS operates in Class "x1", which is why you may not have seen the 1 or 2 before. If it says Class "A" ... the 1 is implicit.

The data sheet has your "recipe". All you have to do is follow the directions. A fair amount of drive voltage is needed before the tubes alternately enter a cut off state.
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Old 13th October 2010, 09:39 PM   #14
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Originally Posted by Eli Duttman View Post
The open loop 2% THD appears discouraging
No it isn't. For audio finals, it's quite impressive. What counts is the harmonic content, and, unforch, this type likes to make lots of h3 and higher order harmonics. It definitely needs the help of local NFB. Open loop, they sound nasty with some program material.

Quote:
Use ultralinear mode "finals", instead of pure pentode, and that open loop number will come down. Add some GNFB and you'll get down to an inaudible 0.3% THD easily enough. Be certain to place current limiting resistors in the lines leading from the screen grids to the O/P trafo's UL taps.
I favour parallel local NFB (what a lot of folks around here have taken to call "Schade feedback"). You don't need the special UL OPTs, and you can regulate the screen voltages.

This type also tends to like to Barkhausen oscillate, so you will need screen stoppers as well as plate stoppers. 1K5 on the screens prevents that.
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Old 13th October 2010, 11:11 PM   #15
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Miles,

It's hard to get a better deal than that offered by Edcor. Their off the shelf stuff comes with the UL taps, standard.

You say 1500 Ω for the screen stopper. A 1 W. rated Carbon composition part right at the socket will do double duty in suppressing both parasitic oscillation and excessive g2 current.
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Old 14th October 2010, 01:48 AM   #16
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Next question. What tube and configuration is good for an input stage? Would a simple common cathode triode work?
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Old 14th October 2010, 02:55 PM   #17
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I previously recommended Mullard style circuitry. I've uploaded Mullard's seminal 5-20 schematic. Please observe the key features, which are a common cathode voltage amplifier DC coupled to a differential, AKA LTP, phase splitter and a GNFB loop.

Use a different small signal complement, as the EF86/12AX7 combination shown yields too much gain for mating to today's digital signal sources and both types are low transconductance (gm). High gm is protection against slew limiting, in a circuit with loop NFB.

Another improvement is to replace R8 (the LTP tail resistor) with a 10M45S constant current sink (CCS). A CCS in the LTP's tail, instead of a resistor, forces symmetry between the 2 sides.
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File Type: jpg Mullard 5-20 Schematic.jpg (380.3 KB, 55 views)
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Old 14th October 2010, 04:36 PM   #18
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Originally Posted by grungeman91 View Post
Next question. What tube and configuration is good for an input stage? Would a simple common cathode triode work?
Highly unlikely, especially if you include NFB.

As far as the front end goes, there are lots of possibilities.

Ex 1
Ex 2

Other possibilities include the "Williamson" topology (uses cathodyne splitter behind a second stage of differential gain). There are various other kinds of splitters, but none as good as either the cathodyne or LTP.
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