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Old 23rd January 2008, 02:41 AM   #1
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Default 6L6 PPP UL amp

Greetings all,

First off, let me say this forum is a great source of information, and you guys put a lot of time into answering and helping out your fellow tube-heads. Anyways, I am relatively new to tubes, have built a few guitar amps and modded a few old Hi-Fi tube amplifiers, etc. I have recently built a 6L6 UL push-pull parallel ala Williamson amp (two channels mix into one) to be used as a vocal PA amp for my band; it sounds good (much more articulate and less distortion at higher volumes than the cheap ~200W solid state PA we previously used) but I wanted a few of the gurus to take a look at the schematic/voltages and let me know of any possible problems that might be lurking in my circuit. I know the concertina is somewhat lop-sided, but the AC voltages at the grids of the 6L6's are approximately equal (up to ~60VAC and higher depending on volume level, using my voice as a signal and using my voltmeter set to AC to measure--kind of a hack way, I know). If the cathode voltage of the concertina is at 21V (RMS) then it will pass a signal of 15VAC (peak to peak) without clipping right? My signal going into the concertina is admittedly weak because of the reverb bypass. I did hook up the traditional concertina (w/extra cap and bias resistor (bypassed) of 1.5K/22K and grid bias of 330K going to top of 22K and acceptable voltages) but switched back because I liked the sound slightly better (or is it distortion imparting a "warmer" sound?). Also, I was wondering if it is acceptable to use KT88 or 6550 without pushing the limits of the power TX (filament TX would be at ~9.9A for 10A rating—cutting it too close?).

Sorry for all the questions... Any help/advice would be greatly appreciated.

Thanks in advance,
Reagan

PA schematic:
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Old 23rd January 2008, 04:28 AM   #2
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Hi Reagan;

6L6s are pushed like in a guitar amp, I would not torture them such a way in a vocal amp. In my vocal amps I use similar schematics, with few differences: regulated lowered G2 supply for 6L6 tubes, global negative FB, less gain in driver stage that is long - tailed, and directly coupled tubes for better stability on low ends. However, it is a matter of taste, probably...

Here is the previous prototype (2004, power amp only):

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One more (unfinished yet; 2 microphones, guitar, and aux input):

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Old 23rd January 2008, 06:50 AM   #3
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Default 6L6 PPP UL amp

...here's the power supply...
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