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A300 6L6/6V6 modified Harmon Kardan A300 (XAM)
A300 6L6/6V6 modified Harmon Kardan A300 (XAM)
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Old 14th January 2017, 05:28 AM   #1
KinklessTetrode59 is offline KinklessTetrode59  United States
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Join Date: Nov 2014
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Default A300 6L6/6V6 modified Harmon Kardan A300 (XAM)

i just got a Hi Fidelity XAM from EJ Kozette (HK A300) that i modified to use 6L6 by adjusting the bias on the cathode with a 50 Ohm 5 watt resistor and a switch. Hooked up headphones to the 8ohm output using 150 ohm resistors in series with AKG K142 3.5mm output. Slaved the filament to a new 6 amp filament supply to keep the 6L6's .9amp filament hunger from starving and man what a great sounding amp. Replaced all the electrolytics and getting ready to replace those paper and wax couplers. Sounds fantastic. Listening to Paul Hardcastle and it really sounds good.
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Old 15th January 2017, 01:45 AM   #2
6A3sUMMER is online now 6A3sUMMER  United States
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I owned a Harmon Kardon A300. If I remember correctly, The 4 output tubes cathode current was used to run the phono preamps 12AX7s filaments that were in series (12V + 12V = 24V bias).
That is 150mA, or 37.5mA per output tube.
Changing to 6L6s can change all that.
You still need 150mA to go through the 2 12AX7s that are wired series for 12V each.
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Old 15th January 2017, 04:30 AM   #3
KinklessTetrode59 is offline KinklessTetrode59  United States
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Default 12AX7 use 150mA on the filaments

As long as i dont overdrive the phono filament (i put in a 50ohm dropping resistor), the 12AX7's are happy since the cathodes of the 6L6' are only dependent on the filament voltage. When the 6L6's are driving they want almost 30 volts of cathode voltage due to the plate current they are seeing, so dropping the 30 volts after the series cathode bias down to 23 makes both the 6L6's happy and the 12AX7's. I'm listening to it now on the 6L6 setting and Linda Ronstadt on the garrard sounds just as fine today as she did in the 70's
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Old 15th January 2017, 07:55 PM   #4
6A3sUMMER is online now 6A3sUMMER  United States
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On my A300, there was a lot of filament to cathode leakage on the phase splitter tube.
It was a split load (concertina?) phase splitter.
That leakage from the 6.3V filament to the cathode of the splitter caused too much hum, and consequently hum at the amp output. It was primarily higher harmonics of the hum, like the interference caused by a drill motor, etc.
If you have hum or noise like that, you might try changing that tube.
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Old 15th January 2017, 08:26 PM   #5
KinklessTetrode59 is offline KinklessTetrode59  United States
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Default Hum was bad until i grounded the inputs or the chassis

Yeh I got bad hum on the output whenever i didnt have my phone on usb charge while inputting through the AUX input (phono too). Changed that phase splitter too. Got me thinking about ground loops and safety. Measured over 50 volts on the chassis while energized with the plug in one direction in wall socket and over 160 AC the other with respect to house ground. Actually scared me a bit since i am pretty cautious of these older tube amps. I always test the capacitor bodies i replace with respect to chassis but i never did with respect to house ground. The A300 directs part of the ac input signal after one of the rectifier diodes to ground for the return path which i have seen before but i dont think i have ever measured that kind of potential. I Measured again with the USB charger connected to the phone while inputting music to the AUX input and the potential was diminished to less than one volt. Not exactly sure what the pathway was (I think through the first diode to ground for the return path) but i grabbed a ground wire on a spare plug i had and plugged into the same plug in my amp room and touched the ground wire to chassis: no potential, no hum (night and day). Removed the old cord immediately and put in a heavy duty old computer cord with three prongs and made sure the chassis was grounded well. Power switch of amp now connected to HOT all the time instead of the 50/50 way it was before with the non polarized plug that the amp came with. Tested again and listened to it for about 2 hours on the new 6L6 setting. No hum, no potential and no ground loops that i could discern. Except for the dull white XAM face im really liking the sound of this amp. Im ready to put in new Vit-Q couplers later this weekend and get rid of those old (still nice sounding) American Radionic Caps it came with... thanks for the advice on the phase splitter.....
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Old 15th January 2017, 08:54 PM   #6
6A3sUMMER is online now 6A3sUMMER  United States
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All my old vacuum tube amplifiers had 2 wire power cords.
They all got converted to 3 Wire IEC sockets and cords with ground wires.
(before the modification with un-grounded amp chassis, the voltage on the chassis was elevated by nature of the capacitance and leakage of the primary of the power transformer to the transformer lams and end bells, then to the chassis).

Things like AM/FM tuners, CD players, Turntables, and small phono preamp all still have 2 wire power plugs. The Turntable has a ground wire that goes to the ground connector on the phono preamp. The chassis of these signal sources are grounded through the signal coax that connects to the power amps. So far, no significant ground loops.

As to those old vacuum tube amps, the Hot power voltage is wired to 2 fuses in series, a smaller value slow blow, and a larger value fast blow. Those values are adjusted to make the amp work both at turn-on inrush, and at warmed up operation. They are protected for both transient and long term current draws beyond those normal values.

The power path is: IEC connector, Hot voltage, 2 fuses in series, on/off switch, transformer primary, and return to Neutral IEC. That makes it the safest in the event like a shorted connection or part (fuses immediately after hot).
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Old 21st January 2017, 06:30 PM   #7
KinklessTetrode59 is offline KinklessTetrode59  United States
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Default A300 (XAM) before and After

OK guys her she is, kinda like Sammy Hagars Plain Jane but she performs well. New Vitamin Q couplers, resistors where necessary, 6V6/7408 to 6L6 mod, 6 amp filament transformer, HEXFRED rectifier diodes and safety caps and new electrolytics. Here are a few before and after pics. Burning in some new issue Mullard 6L6 while i contemplate if i really need an my arm and leg for a NOS or old school RCA black plate quad output. Kevin
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File Type: jpg After Top.jpg (416.3 KB, 47 views)
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File Type: jpg AfterUnder.jpg (538.2 KB, 41 views)
File Type: jpg After.jpg (344.0 KB, 41 views)
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