6AQ8

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6AQ8

Postby dhuebert » Tue Oct 12, 2004 8:05 am

I bought a PA amplifier at a garage sale on the weekend for $10, it has two 12AX7s on the front end, a 6AQ8 as phase splitter and a quartet of 6BQ5s on the output (35 W). I have never heard of the brand but it says "Made in Japan" on it. I fired it up and it worked, but the tone controls are crap (more effect on the volume than the tone). I have been tracing out the schematic over the past couple of days, it has a biasing scheme for the 6BQ5s that I have never seen before, the AC 6v to the filament of the 6AQ8 is applied to the ends of a rheostat with the wiper connected to the cathode of the 6BQ5s, I'll have to scope the voltages to understand this! Other than that it all seems very conventional. I will scan and post the schematic when I get it, if anybody wants.
The question I have is: why the 6AQ8 (ECC85)? The data sheets I have found indicate this tube is mostly for RF work.

Don
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Postby erichayes » Sun Oct 24, 2004 5:00 pm

Hi All,

Don, that weird circuit you described was used by Eico, among others back in the '50s and '60s, and doesn't have anything to do with the bias. It's a hum balance control with the wiper going to the output cathodes, rather than ground, so that the heater to cathode voltage is reduced. This serves two purposes: first, it allows tubes with slight h/k leakage to be used for a longer time without noticeable hum. Second, it puts a little less strain on the H/K isolation, thus allowing the tube to last longer.

The 6AQ8 was used primarily in the front ends of FM tuners, although I've seen them in VHF TV tuners as well. There's nothing wrong with using RF tubes in AF circuits--the 12AT7, 12AU7, 12AV7, 12AZ7 and even the 12AX7 started out life as RF or multivibrator tubes. The 6DJ8, so highly touted by the Golden Ears, was designed for use as a VHF mixer, but don't let me go there.

The main problem with tubes that weren't widely adopted by the audio world was that they tended to be microphonic. Microphonics don't play a large part in RF, so the physical causes were never engineered out of them. The 6DJ8 still has this problem as well; a mikey DJ8 sounds kind of like a cross between a bell tree and a window breaking. But I'm not going there...
Eric in the Jefferson State
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Thanks Eric!

Postby dhuebert » Mon Oct 25, 2004 7:38 am

Ya know, I turned that pot from end to end and all I noticed was greater or lesser hum on the output, so it works, although I would like to know why... Do you mean the heater to cathode voltage on the 6AQ8 or the 6BQ5? I'll have to scope it with this new knowlege.

The other thing...The 12AX7 sockets are mounted on rubber grommets, but the 6AQ8 is not, I gave it a tap and didn't hear anything.

The tone controls are exactly the same circuit as my Fleetwood Stereo Hi-Fi, but they work on the Fleetwood (Baycrest). If I take the tone controls out of circuit on the AGS PA-35, the sound goes very "thin" and quiet. The power supply is one long chain of resistors and caps, some of the resistors are 50K, telling me that maybe differing loads can have a large effect on downstream DC bias. I will be playing with all this stuff this week.

On another note, a friend has a receiver of the same make (AGS)with a quartet of 6BM8s on the output, needless to say I'm dying to get my hands on it.

Don
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