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Old 7th October 2007, 07:13 PM   #11
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Quote:
Originally posted by Bandersnatch
hey-Hey!!!,
A quick look at the GE 6JE6C data sheet shows a pentode plate Z of 5k5 Ohms. Hardly more than a 211. Certainly less than the 211 running low current and high voltage... Still, for this particular design, running a high-slope diagonal across the plate characteristics at half the a-a load is the way to go. Keep it Class A bias; drawing an AB load line is a lot more difficult.

Also, see Pete Millett's write up on his SE E-Linear amp, the driver stage should be a high-Z element. Cascodes or pentodes; it doesn't do well with straight triodes.
cheers,
Douglas

Doug,

Using the Plitron "4070" trafo and "mundane" UL topology, with 400 V. on the plates, 150 V. on the screen grids, and -20 V. on the control grids of the 6JE6Cs seems to yield a reasonable Class "A" operating point. FWIW, that's well within the capability of the "El Cheapo" splitter/driver. No line stage needed, when a CDP is the source.

I know you are not fond of loop NFB around the O/P trafo. Frankly, I don't think there is anything to worry about at the 9% of the speaker drive voltage that the "El Cheapo" topology uses.
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Old 7th October 2007, 07:50 PM   #12
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I sorta recall an ultralinear amplifier design in Glass Audio magazine some years back that used these type of horizontal output tubes. It was called the "Snubber" I think, since it used snubber circuits on a Hammond 1650T transformer to clean up some UL transients. I will try to find it, but my magazine collection is in extreme disorder after moving.

Don
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Old 7th October 2007, 08:24 PM   #13
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Quote:
Originally posted by Eli Duttman



Doug,

Using the Plitron "4070" trafo and "mundane" UL topology, with 400 V. on the plates, 150 V. on the screen grids, and -20 V. on the control grids of the 6JE6Cs seems to yield a reasonable Class "A" operating point.

The 400V plate voltage and 2k load would need a lot of idle current to achieve Class A. Assume something like 325V of swing, and we're looking at needing 160 mA idle current per tube to keep it from cutting off.

We'd also need a lot less than 150V on g2. Likely something like 100V would yeild a peak g1=0V plate current of ~340 mA.
cheers,
Douglas
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Old 7th October 2007, 08:47 PM   #14
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hey-Hey,
For a class A operating point, it looks like a higher load a-a should be used. Say ~7k a-a, or a 3k5 load for each plate. 330V of B+ will leave about 300V of swing towards the top horizontal part of the curves. You set this g1=0V line with the g2 voltage. Since we've got a 30W plate, and there will be little g2 current, we'll run the plate to 30W. That's 90 mA. Then we need the g2 voltage so that g1=0V line comes to ~180 mA, or twice the idle current. For the pentode, plate current= C*g2_voltage^1.5. At 200V plate voltage, and 125V on g2, C is ~.0005. So for our 200 mA peak leaves g2 to be ~55V. Try 60 or so for the first try....

So, 7k a-a, B+ of 330, g2 of 60, and 90 mA of plate current you'll have a class A operation. Does it look like there are very many real Class A amps out there now?
cheers,
Douglas
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Old 7th October 2007, 08:48 PM   #15
cerrem is offline cerrem  United States
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Just a quick question,....What kind of power output do you need ....I assume you need to make 2 amps for Stereo operation?? If so, would you put both channels in the same chassis with a common power transformer....??
Do you really need Class A P-P ???
This should be straight forward design.....simply make screen regulator to keep the screens happy, under 200V....then choose a plate load depending on the applied B+ .....

Chris
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Old 7th October 2007, 09:07 PM   #16
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Quote:
Originally posted by Bandersnatch


The 400V plate voltage and 2k load would need a lot of idle current to achieve Class A. Assume something like 325V of swing, and we're looking at needing 160 mA idle current per tube to keep it from cutting off.

We'd also need a lot less than 150V on g2. Likely something like 100V would yeild a peak g1=0V plate current of ~340 mA.
cheers,
Douglas

Doug,

A VR105 would take good care of the screen grid supply.

The design max. plate dissipation limit is 30 W. With 400 V. on the plate, I make the Class "A" idle current to be no greater than 75 mA.

I wonder how much abuse the 6JE6C's control grid can take. Is a Class "_2" operating point on the horizon? L_RD knows the 6JE6C is not cathode current constrained.

FWIW, the full 2 VRMS I/P from a CDP and more will not drive the "El Cheapo" splitter/driver into clipping. Add Tubelab's "Power Drive" or circuitry along the lines suggested in MOSFET Follies and positive grid current is not problematic. IMO, the superior IRFBC20 MOSFET is appropriate for that kind of circuitry.
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Old 7th October 2007, 11:20 PM   #17
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Quote:
Originally posted by Eli Duttman



Doug,



The design max. plate dissipation limit is 30 W. With 400 V. on the plate, I make the Class "A" idle current to be no greater than 75 mA.

With what loading? The two are not separate or disconnect-able from each other.
cheers,
Douglas
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Old 8th October 2007, 12:14 AM   #18
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I'm thinking static DC conditions. P = (V)(I)
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Old 8th October 2007, 02:44 AM   #19
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Quote:
Originally posted by Eli Duttman
I'm thinking static DC conditions. P = (V)(I)
Hey Eli,
It works a bit differently. I'll assume the plate could swing to zero volts... Define Class A; equal current increase to the current decrease as the grid swings up and down.

1. DC conditions; 400V, 75 mA

2. So what's the load? V=i*R...5333 ohms. For PP double that for a-a or 10666 ohms. 400/.075=[a-a/2]

Higher load( lower numeric ) will have more current swing up than down...that's AB. Increasing idle current raises plate dissipation( usually past surviveable limits )

3. (a)Details to complicate things: Estimating lower limit of plate voltage swing to apply to (2.) in the voltage slot. We can get close with the curves, and a guess can be conservative.(b)The tubes aren't anything like linear; upward current swing isn't going to equal the lower.(c)U-L complicates the assessment of working g2 voltage in locating the g1=0volts line and thus the upper current limits of the anode swing.
cheers,
Douglas

seen many real Class A amps lately...heh-heh-heh!!!!
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Old 8th October 2007, 08:31 PM   #20
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Thanks Everyone!

Wow, there's a lot of great info to digest here. Could we agree on a schematic to start with, and to hone this info into a workable design?
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