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AC-30 PT? Now I'm Confused.....

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9/1/2000 11:57 AM
BWilliamson	AC-30 PT? Now I'm Confused..... Looking at the schems on Steve's site the PT listed at 180-0-180 on the primary. The factory schem with the voltages added, after the power tubes and OT lists 320v--according to my math. 180 X 1.3(GZ34)= 234vdc Not enough voltage there before load. If you take 280 and drop this in you get 364vdc, which after load may work real close to the 320vdc mark. bw

9/2/2000 7:11 AM

BW:

Thanks for the math fix! I guess I better edit the PDF file with the erroneous voltages listed (since it was from Germany maybe they use metric volts...

)

Steve Ahola

P.S. Hmmm... that drawing shows ss rectifiers followed by a 47 ohm resistor! I like the artwork but didn't all of these amps use tube rectifiers?

I just edited and uploaded the drawings:

http://www.ampage.org/blueguitar/ac30elek.pdf

-- or --

http://www.ampage.org/blueguitar/ac30elek.gif

9/11/2000 9:44 AM

quote:
"didn't all of these amps use tube rectifiers?"

No. They went SS in about '71 or '72. Should be listed under "amps to be avoided"

9/2/2000 7:16 PM
Carlo	BW - I've built a few AC30 style amps using the Hammond 270HX. Usually end up with 360vdc on the plates. The higher voltage is due to the hammond being designed for 115VAC while my outltets are closer to 124VAC. BTW, i'm using a common 50ohm cathode resistor and a Chinese GZ34. Carlo

9/9/2000 2:37 AM

Carlo:

I had heard that the AC30's use/need a low voltage/high current PT to work right with the quasi-Class A output section. I guess the 200mA rating should be okay- right?

Do you see any problems with adding a few 10V/5W zeners to the PT CT circuit to drop the B+ voltage maybe 20 or 30 volts? Mouser sells the #610-1N5346B zener (9.1v/5w) for 52 cents apiece and they are much easier to use than the heavy-duty 50 watt stud mount ones, especially if you are not exactly sure how much you want to drop the B+ voltage!

--Thanks!

Steve Ahola

9/9/2000 6:04 AM

quote:
"I had heard that the AC30's use/need a low voltage/high current PT to work right with the quasi-Class A output section. I guess the 200mA rating should be okay- right?"

i believe 200mA is plenty. It works for me...YMMV. FWIW, I know at least 3 of my amps built with said tranny that have been in relatively heavy usage (3-5 nights/week) for over a year with no problems. Bear in mind, these amps are all running the plates around 350-360VDC, which is rather high. Then again, a year isn't very long in the life of an amp...time will tell. As for voltage, the number i've heard most often as the correct value is a plate voltage of 320. You may want to check out the following schematic. it's for a 1964 AC30. It has all the tranny numbers...OT @ 4k and the PT 280-0-280 @ 160mA. Be warned the schem is rather large.

http://members.aol.com:/funkyloon/ac30.html

quote:
"Do you see any problems with adding a few 10V/5W zeners to the PT CT circuit to drop the B+ voltage maybe 20 or 30 volts? Mouser sells the #610-1N5346B zener (9.1v/5w) for 52 cents apiece and they are much easier to use than the heavy-duty 50 watt stud mount ones, especially if you are not exactly sure how much you want to drop the B+ voltage!"

Sounds reasonable to me...i seem to recall Bruce mentioning this before, (stringing smaller/cheaper zeners together to get a safe wattage) but i have no experience with using zeners to drop voltage. IOW, you better ask someone who knows. ;o)

If it were me, since the Hammond PT is designed for a 115VAC, i would use the transformer boost/buck method outlined on RG's page.

Carlo

9/13/2000 10:50 PM
Pete	I'v always been a bit mystified about the '160mA' label on the 1964 circuit diagram. Should this be read as 160mA per side and hence 320mA really? 320mA sounds much more in line for use in a class A 4xEL84 amp running at 50mA per valve on tickover. Anybody any ideas on this?

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