6au6 pentodes in push pull clean low watt practice amp. uses 1363 smps and 9v

Started by zambo, September 19, 2011, 12:40:33 AM

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QuoteIts 44k for 2.
Half it.  It's all Ohm's Law.  Voltage is essentially the same for either 2 or 4 tubes.  So doubling the number of tubes will double the current draw, so to allow the same current for each tube with 4 tubes as you would with 2, impedance has to be halved. 

QuoteI was sondering about the feedback on only one side of the PI, would this not come to the imput to the PI?
It's actually not going into the grounded input, it's going to the cathode biasing resistor cluster, the grounded input just has to be connected to that string for it to work.  LTPIs are kind annoying because at first glance, it's not intuitively obvious what's going on.  Basically, one triode is amplifying the signal in one phase, and the other triode is amplifying the same signal, but 180 deg out of phase.  To do that, the first triode gets connected to the "input", the there triode gets connected to ground.  Because of the biasing arrangement, the cathodes sit above ground potential, so you've sorta kinda made those two triodes work like they had a bi-polar power supply.

More info: http://www.aikenamps.com/GlobalNegativeFeedback.htm

If you just lifted that NFB scheme from another amp, expect to have to adjust it to make it work.  You'll need Aiken's page above to do that.  The reason why is because the OT and power tubes are part of the circuit, and need to be accounted for.  Do yourself a favor and mess with the NFB last once you know your build is stable.  I say to do this because NFB, if wired incorrectly, will be positive feedback, and will cause you amp to go unstable and squeel.  If you don't have a confirmed working build when this happens, you don't know if it's the amp or the NFB.  It's one of those things that experience teaches you. 
-Mike

zambo

Thanks for the explanation! So its kind of like when you run a 12au7 self split with the second grid grounded then. Makes much more sense. Yeah that nfb circuite is going to get tweaked alot. I dont want a lot of it so I think the 100k resistor will be ok. I may up it. A bigger resistor should provide less of it I am assuming. The PI values I will need to tweak with too. Thanks for the help  :icon_smile:
I wonder what happens if I .......

zambo

sounds pretty good but still needs some tweaking. Will post a vid and updated schematic asap. had some questions about how to turn off the two outside power tubes with a switch and how will that affect the ot impedance and cathode r. do i need to worry about it? using the 269ex pt and 125c ot. pretty loud. about 5 watts if i had to guess.
I wonder what happens if I .......

zambo

here is the newest one with 4 power tubes and a 269ex power trans  :D
I wonder what happens if I .......

iccaros

Quote from: zambo on October 21, 2011, 01:37:36 AM
sounds pretty good but still needs some tweaking. Will post a vid and updated schematic asap. had some questions about how to turn off the two outside power tubes with a switch and how will that affect the ot impedance and cathode r. do i need to worry about it? using the 269ex pt and 125c ot. pretty loud. about 5 watts if i had to guess.

I believe that since the 6au6 the OT is not as big a deal, It will change somethings in tone, you can play with that.. I would split the cathode resistor in half and have one on each set of tubes.
In the words of the immortal R.G. "I reserve the right to be wrong.."