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807 Push-Pull Oddwatt amplifier
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Author:  rparsh [ 07 May 2012, 08:23 ]
Post subject:  807 Push-Pull Oddwatt amplifier

Hello all, I am well on the way to completing my first 807 PP Oddwatt. Using the same transformers as on my KT88 ones and some Rogers 807s and a National Union 12SL7GT for the main parts. I am using a 12vdv @3.5amp SMPS for the heaters as in the KT88s. I used one of the delay relay tubes a 115C120 and a Omron DPDT "ice cube" 120volt coil relay to switch the HV before the bridge rectifier. Oddly enough on the first test of this HV delay system the delay tube switched at 80 seconds instead of 2 minutes. Possibly because we now have 120-122 volts at the wall plug nowadays. Should be enough delay anyway. I have a question though. I know what screen resistors are and where to put them. What about grid stoppers? Do they just go from the grid to the signal ground? I was advised to use them as the 807 tubes are also used as transmitting tubes and could do some high oscillation on occasion. Thanks, Robert Parsh

Author:  Geek [ 07 May 2012, 16:04 ]
Post subject:  Re: 807 Push-Pull Oddwatt amplifier

Hi,

Grid stoppers go in series with the grid and as close as possible to the socket. 10K to 47K would be OK for a pentode (divide by 10 if triode mode).

The resistor you speak of is the grid leak, from grid to ground (place before the grid stopper, not after).

Other than that, treat the 807 as you would a 6L6 :D

Cheers!

Author:  gofar99 [ 08 May 2012, 20:40 ]
Post subject:  Re: 807 Push-Pull Oddwatt amplifier

Hi, If you find the high frequency reduced, you can use smaller grid stoppers. I find that with this design and if you use similar values in a NFB circuit as in the original project (only adds about 3-4 db NFB) then you can use grid stoppers as small as 220R.

Good listening
Bruce

Author:  EmptyPockets [ 14 Feb 2020, 22:17 ]
Post subject:  Re: 807 Push-Pull Oddwatt amplifier

:thumbsup: I’ve been searching for a PP 807 for some time and was wondering if I would be able to use 807’s in the oddwatt. Do you have any schematic changes or was it just using the 807 without any changes. Which PT did you use? The Edcor XPWR101 isn’t available. Thank you

Author:  gofar99 [ 16 Feb 2020, 16:12 ]
Post subject:  Re: 807 Push-Pull Oddwatt amplifier

Hi, The 807s are similar to 6L6 tubes and no circuit changes are needed to run them at about 60ma and 425V. The dissipation will be fairly high, but with about 25 volts on the cathodes it should be OK. Use an Edcor XPWR105 for each channel (it can not do both) An Edcor EMO813 would be suitable for a stereo build (it is OEM to Oddwatt, but I allowed them to sell them to anyone). It has another XPWR number but I don't know what it is. You need about 360VAC to the bridge rectifiers and a current of over 350ma (about). The 813 has the 11 volt heater windings for use to rectify and feed the tubes DC heater power. For AC heaters you will need to find a different transformer. BTW the XPWR105 has 12 volt heaters and if you rectify it you will need to bleed off about 2 volts DC to power the heaters. A large power resistor (10 watt) in the 1-2 ohm range will be suitable. A few 0.5 ohm ones in series would be the easiest way to set it. The exact value will depend on the efficiency of the regulators and filter caps. To set the idle current at about 60 ma per tube you will need to use sense resistors in the LM317 of about 11 ohms. I would probably try 12 ohms to start.

Good listening
Bruce

Author:  EmptyPockets [ 17 Feb 2020, 08:04 ]
Post subject:  Re: 807 Push-Pull Oddwatt amplifier

Thanks for the info Bruce and posting question

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