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Old 24th March 2012, 07:15 PM   #11
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I built a nice-sounding low-power amp using a 6SN7 output and a Hammond 125E output tranny, using the 17.5 KCT connections (pins 3 and 5). I used around 510 to 560 ohms for the cathode resistor, bypassed with a 250uF 35V electrolytic. For your setup, you might be better to replace the 12AU7 with a 12BH7-A or (if you have 6-volt heater supply) a 6CG7/6FQ7. Those should give you similar results. I'm not sure how well a 12AU7 would work as a power-output tube.

Take care,
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J. E. Knox "The Victor Freak"
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Old 24th March 2012, 07:47 PM   #12
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Quote:
Originally Posted by shonen View Post
how do you know how many Volts to deliver to the OT's B+? Is there a formula that I can use? Hammond's 125A has "universal" impedance matching chart, so I can match the 22k? impedance of the 12AU7 plate with an 8Ohm voice coil, but i'm not sure the amps i'm putting on the 12AU7's plates or the voltage to the B+ will be correct... and also don't know correct resistor for the cathode bias.
rojoknox has already answered your general question in depth, but I could add that the specific choice of voltage and current for your output stage comes from reading your tube manual. Get thee to an RCA tube manual shoppe and purchase one of about the RC25 to RC30 age range (sure hope I'm remembering this right - if not somebody near theirs please correct me). A tube manual is the beginning of tube wisdom, and has been for generations.

All good fortune,
Chris
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Old 24th March 2012, 09:03 PM   #13
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And if $$ is a problem for purchasing a tube manual, you can download LOTS of 'em (as well as many other tube-related reference works) from Pete Millett's Web site:

tubebooks.org - Vintage info from the age of vacuum tubes

(Word of warning -- RCA RC-30 manual in PDF form is 38 MB. Best to have broadband access when downloading.)

For reference, I ran my 6SN7 amp from a 400 V (!) supply. Yes, one can do that. Not every tube can (such as a 12AU7 which has a 330 V Vp max).

Take care,
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J. E. Knox "The Victor Freak"
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Old 25th March 2012, 02:27 AM   #14
shonen is offline shonen  United States
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Originally Posted by Chris Hornbeck View Post
Get thee to an RCA tube manual shoppe
... indeed, that's really the answer for the bias and plate info. i was looking at the pdf's on Hammond's website for the 125A OT though and there was no mention of what it takes for B+ voltage in. as you guys know, that B+ can vary greatly based on the power transformer.

Anyway, thanks for the help! I'm getting thee to a tube manual.

-Adam
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Old 26th March 2012, 12:32 AM   #15
shonen is offline shonen  United States
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Quote:
Originally Posted by rojoknox View Post
And if $$ is a problem for purchasing a tube manual, you can download LOTS of 'em (as well as many other tube-related reference works) from Pete Millett's Web site:

tubebooks.org - Vintage info from the age of vacuum tubes

(Word of warning -- RCA RC-30 manual in PDF form is 38 MB. Best to have broadband access when downloading.)

For reference, I ran my 6SN7 amp from a 400 V (!) supply. Yes, one can do that. Not every tube can (such as a 12AU7 which has a 330 V Vp max).

Take care,
--
J. E. Knox "The Victor Freak"

Just wanted to thank you for the link... these pdf's of the RCA original manuals are awesome. From RC25, "This manual, like it's preceding editions, has been prepared to assist those who work or experiment with home-entertainment-type electron tubes and circuts." Copyright marked 1966.

Yea, I guess that would be me

Actually, these are cool enough, I think I need to have them as hard copies too.

Thanks!

Last edited by shonen; 26th March 2012 at 12:35 AM.
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Old 26th March 2012, 04:55 AM   #16
shonen is offline shonen  United States
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so it looks like, based on the Firefly rev. 3 schematic, he's putting 265V DC on the B+ of a Hammond 125a OT and getting around 1.5 watts w/ a "self split" PI, after the output tube. the cathodyne PI on a 5E3 should be more effecient, albeit not as effecient as long-tailed pair. anyway, confidence level going higher on this idea now.

Last edited by shonen; 26th March 2012 at 04:59 AM.
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Old 27th March 2012, 11:06 PM   #17
shonen is offline shonen  United States
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I got a response from Hammond regarding how much B+ the 125a OT can take and they answered with this:

"The rated value of voltage at 3 watts 27,000 ohms is 285Vac or ( 402 pk ). We do a 1500 vac dielectric strength test for a short time. I would say that it would be OK at 360 Vdc continuous. It would be at its upper limit."


I'm not planning on pushing the OT & tubes this hard but I thought someone else doing a low power amp w/ the 125a might be interested in the response.

Last edited by shonen; 27th March 2012 at 11:09 PM.
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Old 27th March 2012, 11:26 PM   #18
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For someone needing more power, Hammond makes four other, larger, units in the 125 series. I'm using a 125E, rated for 15 W.

Take care,
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J. E. Knox "The Victor Freak"
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Old 28th March 2012, 07:02 AM   #19
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Also look at the 6n6p as a small output tube.
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Old 28th March 2012, 07:16 AM   #20
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I've done a dozen different flea power guitar amp circuits - most with 12AU7 power stages, simply because of the easy availability. 6SN7 works also great, as mentioned.

What I'm using these days is the EF80. This offers the benefits of extremely cheap price and true pentode sound.

12AU7 and 6SN7 do their triode thing great, but the pentode sound is a bit different, and to my tastes more suited for distortion guitar. More edge and bite and all that. I find triodes a bit muffled with distortion sounds. Still a great way to go, just a matter of flavour.

Put 300 volts on the plate of the EF80, with 6mA plate current and some 2 ... 3 mA of screen current. With cathode bias I'd start with 1k or maybe 820 R cathode resistor, for around 10 volts of bias. My supply of EF80's has been extremely rugged. Hammond OT configured for 27k primary impedance. Works great, good rocking.
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