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 Post subject: Harman Kardon A300 - change BIAS method
PostPosted: Apr Mon 13, 2020 11:18 pm 
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Joined: Apr Mon 13, 2020 11:14 pm
Posts: 1
Hi all, I need some advice and guidance.

I have an HK A300 stereo tube amp.
Presently the output tubes are biased throught the 12AX7 tubes
I want to split the 4 output tubes from the 12AX7 preamp tubes to allow me to individually bias each output tube (orignal Schematic of section attached)

1. Is my proposed wiring diagram correct and functional? Attached
2. I could use some help in determining the R values and wattage rating, and the C values.
I see some tube specs provide a rating for the resistor, there is none provided on this tube spec.
I assume I need to calculate it. I estimate is at 143 ohms. 21.5 vdc / .15 = 143 ohm for each R, correct?
I've researched and researched, but only confused myself by finding lots of contradictory information.
I would really appreciate some help determining the correct values and an explanation of the calcs, or a few links to some explanations you know to be accurate so I can learn.

Misc Notes:
I will have a separate transformer wired to provide regulated 24Vdc to 12AX7 heaters
I estimate that the 12AX7 tubes are providing 143.33 ohms total resistance
Resistance and voltage drop was measured from OT center tap to pin 3 of each 6v6GT
The amp is powered via a 117Vac bucking transformer at all times
This is a Harman Kardon A300 stereo tube amplifier from the 60's
The output tubes are 6V6GT EH. Spec sheet attached.

My specific BIAS readings are attached.
The output section of the schematic is attached.
My proposed design schematic is attached.
The datasheet for my output tubes is attached.

Thanks in advance

Bias Readings.jpg
Bias Readings.jpg [ 121.71 KiB | Viewed 441 times ]
Orig A300 Schem.jpg
Orig A300 Schem.jpg [ 285.81 KiB | Viewed 441 times ]
proposed A300 Schem.jpg
proposed A300 Schem.jpg [ 290.44 KiB | Viewed 441 times ]
6v6eh tube spec datasheet.pdf [442.41 KiB]
Downloaded 16 times
 Post subject: Re: Harman Kardon A300 - change BIAS method
PostPosted: Apr Tue 14, 2020 1:59 am 

Joined: Jan Thu 01, 1970 1:00 am
Posts: 26159
Location: Detroit, MI USA
First of all, what is your reasoning behind wanting to change a proven design that was used in many different amplifiers back then, and works just fine as designed?

Second, DO NOT apply power to the unit if you wire it as you propose as the bias will be incorrect and likely all 4 of the output tubes will immediately red plate and self destruct, possibly taking the output transformers along for the ride.

You can't use four 143Ω resistors with all 4 cathodes tied together, that puts the resistors in parallel and you end up with an effective cathode resistance somewhere around 35Ω! As mentioned, that will immediately destroy the tubes. You would use ONE 143Ω resistor with that connection. The resistance value you calculated is for all 4 output tubes cathodes connected together.

For a reasonable alternative, you want the two tubes in each channel biased as a pair, not individually. A pair of 6V6's typically needs around a 270Ω cathode resistor, however that probably won't give you the -21 volts bias that you are seeking.

Also, to properly break up the cathode bias network you have to eliminate that connection between the 4 cathodes, and the original electrolytic cap which was across the 12AX7 filaments. If you are trying to bias the two channels separately, then only the two output tube cathodes in each channel should remain connected together and each channel should have it's own bypass cap.


Experience is what you gain when the results aren't what you were expecting.

 Post subject: Re: Harman Kardon A300 - change BIAS method
PostPosted: Apr Thu 23, 2020 12:39 pm 

Joined: Nov Wed 30, 2016 7:35 pm
Posts: 7312
Location: Sunbury, Ohio 43074
I am agreeing with the above... what is it you are trying to accomplish? If it's just .001% decrease in THD.... it's not going to do that.

And yes, if you want a second opinion, what you show is basically a "flashbulb tester" circuit. It will pop the output tubes in short order. Hopefully, that's all. Be a real shame to see those output transformers go open .......

Unless you are going to be running this amp at 100% power continuously, I doubt you are going to see much benefit from individually biasing the tubes either, although possibly some benefit could be realized that way. But ... not much.

Preserving the hist. of electronics, one boat anchor at a time! :)

 Post subject: Re: Harman Kardon A300 - change BIAS method
PostPosted: Apr Thu 23, 2020 1:34 pm 
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 11665
Location: Fernandina Beach, FL
The OP has not been back since the first day, 13 April. Maybe he/she just gave up?


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