Weird heater arrangement 12ax7/6bm8

[rotaspec]

Guys.
I am working on restoring (and some small mods to the input switching for todays impedances) a 1965 New Zealand made integrated 10wpc amp.
It uses PP 6BM8 for outputs, splitters and drivers, and a 12ax7 for a voltage amp to drive the tone controls. I have no schematic other than what I have drawn so far and attached.
While tracing around to find the suitable electros to replace, I saw a heater arrangement I hadn't seen before. The 12AX7 appears to get its heater supply from the cathodes of the 6BM8 pentode sections (all 4 paralleled).
This makes for a very long warm up time and elevates all the heaters by the typically 23v once the tubes conduct, but does it have any other advantages (or disadvantages). It certainly doesn't help fault finding when you pull the 12ax7 and everything stops.

Gary

[payitforwardeddie]

Looks like a pretty good way of getting DC into the 12ax7 heater. No hum there.

[trobbins]

The AWA PA-1005 12W quad 6AQ5 uses the 12AX7 preamp heater as part of the cathode load (which is also capacitor bypassed). Given the application was typically voice PA, then the short duration rise of cathode bias would not be a significant reliability concern, especially if the nominal heater power was at the -5% level for idle. For heavier duty music or guiter use, then I agree that it is not a good idea to retain this configuration.

The other neat trick I've seen is to use the cathode bias as the elevated DC for a tuned humdinger pot wiper (RC filtered). This is probably a good modification for your amp, as it should achieve suitably low hum levels (if you don't have other hum causing issues).

Ciao, Tim

[rotaspec]

Well, this is (according to the front panel) a "High Fidelity Stereo Amplifier".
It has other hum issues, but I haven't finished replacing the PS caps yet. It also appears to become a little distorted with operation time. Could that be related to the heater arrangement? Maybe the output tubes cathode current drops when run for a period of time (I haven't tested them) and there is not sufficient to properly run the 12ax7?

One thing I did figure is that pulling the 12ax7 while the output tubes are conducting would be very detrimental to the 100uF cathode bypass, as it is only 50v rated.

Gary

[trobbins]

You probably need to do any electrolytic replacements before trying to assess the level of hum. Modifying the 12AX7 heater to come from a transformer winding of course presumes the PT heater rating can handle the extra load.

Heater lead dress, and using a tuned hum dinger with elevated DC wiper should nearly always achieve hi-fi level of hum - but that also presumes a good star-point grounding scheme, and suitable isolation of PT and OT and choke and high level signal fields from high impedance nodes. Many older amps need a fair bit of subtle wiring modification - the toughest usually being to change an old chassis earthed scheme.

Ciao, Tim

[rotaspec]

Yes, this thing has chassis earths in about a dozen places.
Once I have looked at the PS caps, I'll put an extra load on the PT heater winding and see how hot the PT gets, but the fact that there are a couple of extra cutouts in the chassis for more valves gives hope that the PT can take some more heater load.

Thanks for your help Tim,

Cheers, Gary