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Old 7th August 2007, 04:09 AM   #1
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Default 6L6WXT in a Dynaco ST-70

I have a question I know someone here has a answer for me. I just finished rebuilding my 2nd Dynaco ST-70 and although I have a complete set of EL-34's and 5AR4 tubes for it. I decided to put a quad of Sovtek 6L6WXT to try them out. I also plugged in a NOS RCA 5R4GYB to drop the plate and bias voltage a bit.

I measure 367 volts on the plates of the 6L6WXT tubes and I can lower my bias to 1.1 vdc which comes out to 70.5 milliamps per pair or 35.35ma per tube.

Is this ok, or should I raise the bias a little bit to maybe 40ma or 45ma per tube?

I know the plate loading is different for the 6L6 tube vs. the EL34 tubes and I read somewhere the output will be a little lower which is OK by me.

Opinions anyone?

Thanks,
Sal Brisindi
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Old 7th August 2007, 04:11 AM   #2
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And a photo of the 6L6WXT ans 5R4 up close.
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Old 7th August 2007, 06:39 AM   #3
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6L6WXT+'s allegedly can handle 30W on their plates just like any other 6L6GC. I don't know how much B+ you're burning off with cathode bias because I'm too lazy to get out my datasheet, but assuming zero losses to biasing, you can probably get away with feeding around 50-60mA to each tube. Do you have the upgraded PT that puts out 300mA? If so, this might be an interesting experiment.

edit: saw you just have non-extra-special WXT's. I'm not sure about the wattage but 80% of 20 is around 45mA, again not accounting for cathode bias losses.
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Old 7th August 2007, 01:23 PM   #4
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That B+ seems awfully low. What did you do to the power supply? You should have at least 400V there.
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Old 7th August 2007, 02:16 PM   #5
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Quote:
Originally posted by SY
That B+ seems awfully low. What did you do to the power supply? You should have at least 400V there.
I replaced the 5AR4 with a 5R4. With the 5AR4 the plate voltage was 401V but the lowest I could bias was 1.7V on the test point on the front of the amp which is about 54ma per tube.

I did upgrade the quad can from 30/20/20/20uf to 40/80/30/20uf and on the bias circuit I replaced the selenium rectifier with a diode and replaced the 50uf caps with 100uf.

I can replace a 10K resistor with a 15K in the bias circuit to give me more of a bias adjustment.

Soren, my ST-70 has the original power transformer in it.

Thanks,
Sal Brisindi
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Old 7th August 2007, 03:10 PM   #6
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I didn't think the drop could be that much, but your result and running a PSUD2 sim convinced me. Why use that tube in a hifi amp? (I could see it in a guitar amp, maybe)
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Old 7th August 2007, 03:15 PM   #7
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Quote:
Originally posted by SY
I didn't think the drop could be that much, but your result and running a PSUD2 sim convinced me. Why use that tube in a hifi amp? (I could see it in a guitar amp, maybe)

Sy,
I am just experimenting with different tubes just to have fun. If the 6L6WXT can handle 400v then I will leave the 5AR4 in.

Sal
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Old 7th August 2007, 03:39 PM   #8
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Hmm, 401V with a 5AR4 and a stock Dyna transformer seems very low.
I've worked on more than a few (dozens actually) and typically at 200mA load current the B+ is around 480V with a totally stock supply. There are a few variations in that power transformer but 720VCT on a 120V supply is not unusual. What are you doing differently from the typical dyna supply? Meter ok?
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Old 7th August 2007, 08:04 PM   #9
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Looking at the Dynaco ST-70 building instructions, it says 410vdc per plate of the EL34, I am off by 9 volts, not so bad. I did measure my outlet and it was 114 vac. I connected a variac and brought it up to 117vac and now my plate voltage was 405vdc, close enough by government standards.. :-)

I used a Fluke 8060 DMM which is accurate and I measured the plate voltages with other meters with the same results (give or take a volt here or there).

Regards,
Sal Brisindi
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Old 11th August 2007, 07:39 PM   #10
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Sal:

Your 6L6's are biased way too cool. The power supply will sag when the amp is pushed.

ST70's are supposed to be biased in Class A territory because current demands are fairly constant at that setting. I understand Hafler designed the ST70 this way to avoid the cost of a more expensive power supply, (i.e. one that could supply higher current peaks.)

Regardless, if you like how it sounds try connecting your speakers up to the next higher impedance setting.

Bubba.
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