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Old 20th February 2014, 11:34 PM   #11
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Thanks for the safety tips. I have added a 2.5A 120/120 xfmr to my bench with a female 3-prong AC socket, so I have no excuse for not protecting myself. Oh, and that was before I posted a thread in the Hi-Fi area, but not showing it on a schematic, so yes it got deleted (and I was directed to Note #2). All good, I want to learn from other's mistakes! Anyway, I'm trying to squeeze as much headroom out of this amp, and with so little plate voltage I am now learning how to use the tube data sheet plate characteristics graphs, draw load lines, and optimize voltage swing to input signal. Below is my current amp, with 1/2 a 12AU7 added to the original intercom. Yes, it has only 16v on the plate. Not even on the tube graph... The amp clips, not sure where (2nd or 3rd stage). Will boost B+ where I can, play with load resistors, and set bias between grid and plate cutoff points. Or, put a $25 Weber WRVBPT (260VAC sec) on it! Only has 6.3v for filaments, so will need to find other tubes....
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File Type: jpg AMP FINAL DRAFT w-xfmr2.jpg (89.4 KB, 35 views)
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Old 21st February 2014, 12:36 AM   #12
jjman is offline jjman  United States
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I would put the power switch on the hot leg of the primary side of the iso tranny. Otherwise there would be 120v on the tranny when it is off. Assuming the cap is on the switch to avoid a "pop" that cap won't be needed with the switch moved. I guess it was a noise filter but is no longer needed. A fuse should be used between the hot leg from the wall and the switch. 1amp slow blow should be fine.

My iso tranny has 133vac on it's secondary when running since the actual load is not the full rated amount. So I'm getting 8% more PS volts which is good. I reduced the heater current back to spec by increasing the heater-dropping-resistor in my setup. 3 prong main of course.

Have you measured the gain of each preamp stage? (ac volts out / ac volts in)
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Old 21st February 2014, 02:52 AM   #13
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"Have you measured the gain of each preamp stage? (ac volts out / ac volts in)"

jjman - I have just started doing just that, but using my scope, with a "guitar-level" 150mVp-p 1KHz from my SigGen. I just can't get used to preamp tubes working with only 16 VDC on their plates!! You notice I did add a 12AU7 to get more gain (and clipping to then control), so was able to eliminate the 300R heater dropper. Tube filaments are real close to desired. I did move the preamp (12AU7 & 12AT6) tubes B+ from pin 6 to 7 (on 50C5) to raise B+. Bad move.... Didn't move any voltages more than +/- 5VDC, but the amp was way squawky over 5 on vol pot. Reversed the change, and am back to (with 150mVp-p signal) 4.6Vp-p out of 12AU7, 32Vp-p from 12AT6, and 215Vp-p from the 50C5 (is this way too high, for 125VDC on 50C5's plate?).....Need to read up on AC signal voltage swings, gain and plate voltage.....and check my scope cal.
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Old 21st February 2014, 03:17 AM   #14
kuroguy is offline kuroguy  United States
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Quote:
Originally Posted by LATUBEGEEK View Post
Only has 6.3v for filaments, so will need to find other tubes....
12at7, 12au7, 12ax7 can be wired fot 6.3v filaments.

Edit: just noticed12at6. Never mind.
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Old 21st February 2014, 03:31 AM   #15
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Thanks, I know. How about a 50C5 and 35W4 replacement (7-pin of course)....
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Old 21st February 2014, 12:05 PM   #16
kuroguy is offline kuroguy  United States
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could always use 6aq5. it's a nice (cheap) tube. 6x4 is about your only choice for rectifier. i doubt one would be big enough.
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Old 21st February 2014, 04:09 PM   #17
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Quote:
Originally Posted by LATUBEGEEK View Post
All good, I want to learn from other's mistakes! Anyway, I'm trying to squeeze as much headroom out of this amp, and with so little plate voltage I am now learning how to use the tube data sheet plate characteristics graphs, draw load lines, and optimize voltage swing to input signal. Below is my current amp, with 1/2 a 12AU7 added to the original intercom. Yes, it has only 16v on the plate. Not even on the tube graph... The amp clips, not sure where (2nd or 3rd stage). Will boost B+ where I can, play with load resistors, and set bias between grid and plate cutoff points. Or, put a $25 Weber WRVBPT (260VAC sec) on it! Only has 6.3v for filaments, so will need to find other tubes....
Are you working from a known working schematic? In the first post you said it's based on the Gregory, but your schematic looks quite different and has some really strange connections. Before commenting on the design, I just want to make sure I am looking at the right thing...
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Old 21st February 2014, 04:48 PM   #18
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Yes, the original schematic (which I couldn't find, had to draw it) was what I posted earlier, minus the 1/2 12AU7 input stage. The "old intercom" had 3 tubes and heater dropping resistor. Replaced filter caps and it worked, but too clean, and wanted to push it to clipping. I used the Gregory schematic as a guide only (input tube is a 12AU6, not the AT6 I have). I'd like to add the Gregory tremelo later, how having a spare triode. Want to bias the 1st stage clean with headroom, and be able to push the 2nd stg to "dirty" with the vol pot (which was originally on the input jack before what was the 12AT6 1st stage!). The "strange connections" are original to the intercom (and probably the way I did the schematic...).
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Old 21st February 2014, 11:34 PM   #19
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So the 10M grid leak is connected to the heater?
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Old 22nd February 2014, 12:31 AM   #20
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Yeah, that's where the heater string ends....at ground. Starts at the 35W4 pin4. Looks convoluted on my schematic, but is cleaner in the actual chassis wiring.
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