• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

6L6 PPP UL amp

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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,

PA schematic:


  • pa circuit.gif
    pa circuit.gif
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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):


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

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