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Old 16th April 2018, 05:06 PM   #11
rongon is offline rongon  United States
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Location: Across the river from Rip's big old tree...
I have a couple of questions about this proposed circuit.

First, regarding the OPT pri:sec impedances. Since the plate-grid local negative feedback reduces the output tubes' plate impedance to down in the triode range, does the OPT primary:secondary impedance make that huge of a difference? I'm thinking that with triode operation of the output tubes, the exact primary impedance becomes less of an issue than with straight pentode operation. Generally speaking, a lower primary impedance allows the output stage to deliver a bit higher power output, but with higher harmonic distortion. A higher primary impedance (relative to the load) reduces both the possible output power and the harmonic distortion present in the output signal. Does something similar to that exist with this 'Schade' plate-grid negative feedback arrangement?

Second, I think the Sovtek 5881WXT (which I think is a repackaged 6P3C-E) is a very different beast from the original USA-made 5881 (6L6WGB). The Sovtek 5881 is very robust, probably good for 30W Pdiss, similar to the later USA-made 6L6GC. However, you'll notice that the Sovtek 5881 (6P3C-E) biases up very differently from any US-made 6L6 type. I've actually found that you can drop a Sovtek 5881 in place of an EL34 in a cathode-biased output circuit and the two will bias up nearly identically. However if I drop in a US-made 6L6GC into that same circuit, the 6L6GC will draw significantly more plate current. That tells me the Sovtek 5881 is very different from any US-made 6L6 type. MJ Koster's choice of 5881WXT may have been deliberate, and not meant to be any other 6L6 type in that particular circuit. I'm sure you could increase the value of the 1k cathode bias resistor and get the circuit to work with a 6L6GC or even an old American 5881 (6L6WGB).

This might explain the situation better.
Sovtek 5881 WXT | 6l6 WGC | 6Π3C-E Tube Guide by Jay Skyler
I'm not sure if that's all 100% accurate, but it agrees with my experience with these tube types.
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Old 17th April 2018, 11:12 AM   #12
6A3sUMMER is offline 6A3sUMMER  United States
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ziffel,

You should measure the DCR of the primary and secondary of the transformers that you have. That way you can calculate the approximate insertion loss of the transformer at mid frequencies.

You ought to try using the transformers you have first. See how they sound, and how much power you get.

If you are going to have to cut holes or drill holes in the chassis to use the transformer, and a different transformer will not be the same size, holes, spacing, etc., then you might find a way to hook them up before you cut the chassis holes (but be sure to ground the transformer frame for safety reasons).
I once had a new transformer that had its primary shorted to the frame; that could have been a 'shocking' event, if I did not have the frame grounded (you might not see those shorts with an Ohmmeter, but could happen when you apply B+).

Some speaker impedances are lower than you think. i.e. I have some 8 Ohm speakers that are 6 Ohms at some frequencies; and some 4 Ohm speakers have 21/2 Ohms at some frequencies. Consider your speakers actual impedances at different frequencies, especially if you are going to purchase an output transformer that only has a single impedance output tap.

Last edited by 6A3sUMMER; 17th April 2018 at 11:19 AM.
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Old 17th April 2018, 05:23 PM   #13
ziffel is offline ziffel
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The primary resistance is 370 ohms.

Do you have an estimate for a good load resistance for the 6W6GT in triode?
My GE data sheet has the plate resistance at 1600 ohms. This is with 225v and 22 ma at the plate.
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Old 17th April 2018, 09:52 PM   #14
SemperFi is offline SemperFi  Wake Island
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Too bad 6W6 has such low voltage rating for the screen.
Just wondering about the +9V on the grids... 26V on the cathodes... shouldnt you have at least a clean +/-26V drive?
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Old 18th April 2018, 05:07 AM   #15
6A3sUMMER is offline 6A3sUMMER  United States
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In Triode Mode, the 6W6 plate/screen is rated up to 330V.
But you will not be able to run as much current at 330V as you can at 225V.
As a result of the lower current, the plate resistance in Triode mode will be higher at 330V plate/screen to cathode versus the plate resistance in Triode mode at 225V plate/screen to cathode.

Two Parallel 6W6GTs will be about 800 Ohms. You might try a transformer with 3 times that - a 3,200 Ohm primary. The 22 x 2mA = 44mA, and 30V self bias, with the u = 6.2 should work together there quite well.
The B+ should be:
225V + 30V self bias + (44mA x the DCR of the 3,200 Ohm primary = X volts drop).
Measure the primary DCR, and calculate the total B+ voltage you need.

If you want to use triode mode:
You can do without the Shade feedback resistor. And you can use a triode input tube instead of the MOSFET, with a 450V or 900V IXYS current source in the input tube plate.
Then you can use RC (coupling cap from the driver plate, resistor to ground) connecting to the two 6W6GT grid stopper resistors.

If you want to use triode mode and the Shade feedback with the MOSFET, I think you may have to change the feedback resistor.
There is so much less gain in triode mode, and the open loop to closed loop ratio will not be the same as in Pentode mode.

Last edited by 6A3sUMMER; 18th April 2018 at 05:28 AM.
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Old 18th April 2018, 05:22 AM   #16
6A3sUMMER is offline 6A3sUMMER  United States
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I do not know how I did it, but on post #9, I talked about what happens when you use a 5k:8Ohm transformer to act as as a 4K:4 Ohm transformer.
I meant 2.5k:4 Ohm, not 4K:4 Ohm.

Using a 5K:8 transformer as a 2.5k:4 transformer will have 2 times the loss caused by DCR of the primary and secondary.

At this hour, not a later hour, I seem to be able to type 2.5k:4. Sorry.

Last edited by 6A3sUMMER; 18th April 2018 at 05:26 AM.
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Old 18th April 2018, 07:53 AM   #17
SemperFi is offline SemperFi  Wake Island
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How's the tempco of the input stage? Just a minute shift in Iq will give large shift in grid voltage...?
Also, it doesnt seem to me the input stage will ever drive the output tubes even close to max, the available symmertic voltage swing is so much less than the tube's cathode bias... ? Is that intentional so the amplifier never clips on the tube output but rather on the FET input? Someone prefers SS clip to vacuum clip?
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Old 18th April 2018, 09:12 AM   #18
6A3sUMMER is offline 6A3sUMMER  United States
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SemperFi,

I did not even question the MOSFET input stage. I was only looking at the output stage.
That takes me back to my idea of using a triode tube for the input stage, and RC coupling.
Can't get much simpler than that. And with a current source in the plate of the input tube, it is very linear (maybe about as linear as the output stage, and so some of the 2nd harmonic distortion is partially cancelled).
For a 30V self biased output tube, there needs to be +/- 30V from the driver for full output.

It is true that clipping (in the cut-off direction) of a single ended output stage causes the amp to loose control of the woofer cone, etc.
But for anybody that does not like the sound of hard clipping or loss of woofer control, just do what I do. Either turn the volume down, or hook up a more powerful amplifier.

Last edited by 6A3sUMMER; 18th April 2018 at 09:20 AM.
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Old 18th April 2018, 10:06 AM   #19
SemperFi is offline SemperFi  Wake Island
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Turn down the volume?!?! Never even concidered :-O

Other than that I agree with you on the triode and RC coupling.

Back to the first post it appears as if the original ckt (Koster's with 6L6) works...So has the circuit inventor created a version with some feedback to stabilize the bias? Or has this circuit never really worked? Or is the tempco in real life not actually a problem and the amplifier works well enough as a very simple circuit, accepting limited drive on the grids?
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Old 18th April 2018, 04:37 PM   #20
ziffel is offline ziffel
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Hi 6A3sUMMER and SemperFi,

I keep reading that a pentode run with Schade feedback show plate resistance similar to that of a triode, that why I referred to the GE triode specs for the 6W6GT.

Your wrote:
"Two Parallel 6W6GTs will be about 800 Ohms. You might try a transformer with 3 times that - a 3,200 Ohm primary."

Would that be loading at 2400 ohms? Or do I want to load them at 4 times plate resistance?

I could have simply used a circuit with a fairly standard tube input RC to triode output, or even adapted the RH-EL84 circuits. Instead, I want to stick with Koster's topology because it seem so novel. I like the 6W6GT so much that I really don't want to hear the influence of any other tubes.

Regarding having +9v on the grids? What would be a better value? +30v? Koster's 6L6GC is running +55 at the grid and +70 at the cathode. I have included a picture of his circuit.

Thank you for your help.
Attached Images
File Type: png Koster 6L6GC.png (11.2 KB, 129 views)

Last edited by ziffel; 18th April 2018 at 04:39 PM.
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