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Old 13th April 2018, 03:35 PM   #1
ziffel is offline ziffel
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Default Schematic Critique – Micahel J Koster adapted for 6W6GT in PSE

Hello,

I am attempting to adapt Michael J. Koster’s Schadeode 6L6GC/5881 amp to work with 6W6GT in PSE. I am a paint-by-number builder so have limited knowledge, but am trying to learn. I would be very grateful for any criticisms and suggestions for the circuit I have come up with.

The output transformer I have is a Transcendar 10 watt 5k:8. My speakers are 4 ohm so the schematic shows it as a 2.5k:4. I hope this is OK. Would it be OK to use IXYS10M45 instead of the IXYS01N100?

Here is the link to Mr. Koster's amp that I am trying to emulate. I've attached the schematic for that as well.
Tube DIY Asylum

Thank you all for your help.
Attached Images
File Type: jpg Schadeode - 6W6GT PSE.jpg (91.9 KB, 446 views)
File Type: png schadeode-6L6-benchtest1.png (6.9 KB, 435 views)
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Old 13th April 2018, 03:45 PM   #2
GoatGuy is offline GoatGuy  United States
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Well…

First and foremost, your MOSFET. ⁰¹n₁₀₀ is an enhancement mode device, meaning that it is OFF at 0 V gate bias. Having the 47 kΩ input impedance resistor hooked from gate to ground is a zero bias nominal position. Worse, with whatever current that flows thru MOSFET, the source-resistor (akin to a cathode resistor in a tube circuit) ends up negative relative biasing the gate. Even more into device's OFF condition.

“Apart from that Mrs. Lincoln, how did you enjoy the show?” … mmm is apt.

There are other oddities: why the powering of the 6W6 pair's grid–2 thru a Shotkey diode? Or doing so without a reservoir capacitor in parallel to ground? Or the function of the 1Nxxx diode on the source of the input MOSFET? Or the criticality of the input MOSFET's transconductance on actually establishing a 9 volt operating point for its drain?

I'll stop there. The consequences just seem to cascade from there.
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Old 13th April 2018, 03:49 PM   #3
GoatGuy is offline GoatGuy  United States
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HOWEVER… since you're asking for HELP, try using a depletion mode MOSFET as the input. Depletion mode MOSFETS work much more akin to JFETs, which in turn work more like triodes, which in turn like "cathode self-bias" (source resistors).

Here's a good device: https://www.mouser.com/datasheet/2/2...50-1181133.pdf

LND150D is the device. Has very JFET charateristics. Easy to use.
_______

However, establishing the right operating point seems dicey. I've got to leave now, so I can't offer a concrete circuit diagram alternative. Good luck!

GoatGuy
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Old 13th April 2018, 04:23 PM   #4
ziffel is offline ziffel
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Thank you for your suggestions. I think it's supposed to be a zener diode string going to grid 2.
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Old 13th April 2018, 05:25 PM   #5
rongon is offline rongon  United States
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I think the MOSFET is supposed to be an 01N100D, which is the depletion mode version of that MOSFET.

https://www.mouser.com/ds/2/205/98809-1109866.pdf
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Old 13th April 2018, 05:33 PM   #6
SpreadSpectrum is offline SpreadSpectrum  United States
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You have to keep in mind that Michael Koster was one of those brilliant idea-generator type of guys and didn't always spell things out well for beginners and frequently took shortcuts in his drawings that would cause confusion.

The symbol is clearly for an enhancement device but I am confident that Koster used the 01N100D depletion device.
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Old 13th April 2018, 05:44 PM   #7
rongon is offline rongon  United States
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Yes, I think the symbol in the schematic is for the enhancement mode version, but the depletion mode version is what is actually used in the real world.

I hadn't given much thought to MOSFET symbols and what they mean, so I did a quick search and found this image. Is it accurate?
--
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File Type: png MOSFET_symbols.png (17.2 KB, 401 views)

Last edited by rongon; 13th April 2018 at 06:06 PM.
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Old 13th April 2018, 08:31 PM   #8
ziffel is offline ziffel
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I corrected the schematic with the info from the previous posts. Thanks for the input.
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File Type: jpg Schadeode - 6W6GT PSE.jpg (92.2 KB, 367 views)
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Old 15th April 2018, 11:49 PM   #9
6A3sUMMER is offline 6A3sUMMER  United States
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Your amplifier:
1. You asked about using a 5k:8 Ohm transformer in place of a 4k:4 Ohm transformer.
Lets suppose the 5k:8 Ohm transformer has 250 Ohm primary DCR, and 0.4 Ohm secondary DCR.
The transformer loss due to DC resistance will be 1dB.
Now, use the same 5k:8 Ohm transformer in a 4k:4 Ohm circuit, the loss due to DC resistance will be 2dB.
Conclusion: Whatever loss there is due to DC resistance in the 5k:8 Ohm setup, it will be twice as much in the 4k:4 Ohm setup if you use a 5k:4 Ohm transformer.

2. The parallel 6W6GTs bypass cap of 20uF and effective 375 Ohm cathode resistor has a -3dB response at 21.2 Hz. Actually -3dB occurs at a higher frequency than that, because the 375 Ohms is in parallel with the two cathode’s parallel impedance. Plus there is the additional low frequency rolloff due to the output transformer’s low frequency rolloff.
Fix that:
It would be better to use a pair of 47uF or 100uF caps, one cap to one 700 Ohm resistor, and the other cap to the other 700 Ohm resistor, with the other ends of the caps to the bottom of the output transformers primary. Then, disconnect the connection from one 6W6GT cathode to the other 6W6GT cathode.
And get the following benefit:
That way, you can measure the individual voltage across each 700 Ohm resistor, to see if one 6W6GT is hogging more than its share of the current, Then you can find better matched 6W6GT tubes (matched in the circuit voltages and currents you are using, instead of at a different voltage and current rating on some tube tester).

3. Your amp is in pentode mode, even though it has Shade negative feedback, I would advise that you never operate it without a proper load resistor, or a loudspeaker.

4. The original Koster amplifier has an output tube power dissipation problem:
The 5881 plate has 470V and the cathode has 70V. The plate current plus screen current is 70mA, most of that is the plate current. 470V - 70V = 400V plate to cathode.
470V x .07A = 28 Watts
The plate is rated for 23 Watts, and the screen is rated for 3 Watts. 23 + 3 = 26 Watts.
The tube is being used at 28 Watts, 2 watts beyond its maximum specification.
And most likely, most of that excess dissipation is likely going to the plate, not the screen.

Last edited by 6A3sUMMER; 15th April 2018 at 11:53 PM.
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Old 16th April 2018, 03:22 PM   #10
ziffel is offline ziffel
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Thank you 6A3sUMMER,

I've redrawn the schematic.

Do you think it would be better to use other output transformers?
Edcor has a GXSE 15 -2.5k, available with 4 ohm, 6 ohm and 8 ohm secondary. I am considering the 6 ohm. Would you advise using these instead of the Transcendars I have?

Thank you again for your help.
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File Type: jpg Schadeode - 6W6GT PSE.jpg (96.7 KB, 281 views)
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