What's going on in this circuit?

[AlanInSitges]

Note: this probably belongs in some other forum but I feel more comfortable here so hopefully it's not too-too out of place. Besides, I inherited a tuner along with it so together they make a radio and that counts.

I've been looking at a schematic for an old integrated amp I inherited when I was a kid, and it seems like a pretty straightforward design, but there's something that seems out of place and that I don't understand: what are those two massive resistors doing around the cathodes of the output tubes (R48, R49)?

There is an earlier version of the amp that has the same big one from the screen to the cathode, but the second one from cathode back to the B+ isn't there.

If there are two resistors in that circuit dissipating nearly 30W I can't figure out where that much current is going.

Can anyone explain?

Thanks.

[jr_tech]

Seems like a pretty wasteful way of providing DC power for 2 12AX7s heaters.

jr

[zeno]

Cant blow up the schematic so I could only guess. Whatever the case
the cathodes can have high current at volume. Also note I can see no bias
adjustment. The other thing is HH Scott loved to make production changes
& without a visit to the factory it could be a challenge. The cats there
were always helpful & sometimes would run off an actual blue print !

BTW I lived & worked a few miles from the HH Scott factory. There were
piles of it in the area & had I known I could have had a barn full of it
for free or near so from the dump, yard sales etc. I would guess at the shop 75% of the mid to high end audio repairs were Scotties.

73 Zeno
LFOD !

[jr_tech]

This copy of the schematic shows more detail when blown up and indicates (in two notes) the connection of the output tubes cathodes to the 12AX7 heaters. Would this constitute a fixed bias supply for the 6L6s as well as providing filtered DC to the 12AX7 heaters?

https://www.vintageshifi.com/reperto...-Schematic.pdf

jr

[old_tv_nut]

The heaters are not run on DC, but on AC from winding XY.

The Z tap raises the heaters to the same DC voltage as the cathodes.
This high power resistor network (R47, R48, R49) provides the cathode bias without requiring a separate low voltage rectifier and transformer winding, but I haven't figured out why they draw current from both the 285 volt and 375 volt points to do that.

Edit: probably because a simpler arrangement would require an even higher wattage resistor somewhere?

[AlanInSitges]

Yeah it seems like R47 should provide cathode bias on its own. I'm perfectly happy if the correct answer here is "it's a mystery", I mostly wanted to be sure it wasn't something that should have been obvious to me.

[old_tv_nut]

R47 could provide the needed bias for class A stages (both sides conducting all the time), but this is a class B push-pull circuit, so both sides conduct only near the middle crossover voltage of the waveform. The cathodes are biased to a higher voltage so that only one side conducts for positive or negative half cycles.

[jr_tech]

Originally Posted by AlanInSitges

Yeah it seems like R47 should provide cathode bias on its own. I'm perfectly happy if the correct answer here is "it's a mystery", I mostly wanted to be sure it wasn't something that should have been obvious to me.

R47 appears to be only a minor contributor to the cathode bias of the 6L6 output tubes, the lowest resistance path to ground for the 6L6 cathode current appears to be through the series connected heaters of V1 and V2 (12AX7s) through connection “Z”.

jr

[old_tv_nut]

V1 and V2 heaters are DC powered from point Z.
Some peculiarites, though:
Noted voltages are 10 V DC for V2 heater and 8.5 v DC for V1 heater. Heater resistance must have been different for the two tubes actually installed.
DC at V2 pin 4 (point Z is noted as 18.5 volts, DC at V4 and V5 cathodes (also point Z) is noted as 20 volts, should be the same.

V1 and V2 nominal heater voltage should be 12.6 volts according to 12AX7 specs. (This same value should be read either by DC meter for DC heater supply or by AC rms meter for AC supply.) The lower values on the schematic caught my eye. I found some discussion here:
https://audiokarma.org/forums/index....antage.845172/

[old_tv_nut]

Originally Posted by jr_tech

R47 appears to be only a minor contributor to the cathode bias of the 6L6 output tubes, the lowest resistance path to ground for the 6L6 cathode current appears to be through the series connected heaters of V1 and V2 (12AX7s) through connection “Z”.

jr

Yes. I think R47's main purpose is to prevent point Z from rising too high when V1 or V2 is pulled or has an open filament.

[Electronic M]

Making the output cathodes supply 12V to the preamp heaters was a favorite trick of Bell/TRW amplifiers in the 50s and 60s....I own one with that configuration.

[AlanInSitges]

Interesting! Thanks everyone.

[Opcom]

Most all the good points have been made. A closer look is worthwhile, for me anyway, to try to get into why and how. So here goes.

Comments:
1.) C3 filters noise and hopefully the V4 and V5 cathode current fluctuations out of the heater current.

2.) I do not presume to state the designers' reasons for V1-V2 lower heater voltage, but a low voltage filament operation is found also in high reliability and low noise communication radio and testing circuits. H.H. Scott was not looking for power here. It is not for just any tube but apparently the 12AX7 will work this way.

3.) Normally the two 15K resistors R48 and R49 make up a small part of the heater current for V1 and V2, about 31mA. This may serve to somewhat stabilize the heater power for V1 and V2 as the output tubes V4 and V5 age or other conditions change. The 3K resistor R47 conumes about 6.4mA with the V1 and V2 filaments operating. R48 and R49 dissipate about 5 and 9 W respectively. Was it more economical to buy double the 15K 15W resistors than a set of 15W and 7W ones each?

4.) If V1 or V2 should have filament failure, then R47 the 3K resistor prevents V4 and V5 cathodes from going dangerously positive as mentioned, and also it cuts off V4 and V5 by the resulting positive cathode bias of around 94V. It also passes about 30mA when that fault occurs. At the same time R48 and R49 help hold down the unloaded voltage of the power supply.

5.) The above arrangement, besides supplying heater current to V1 and V2 from V4 and V5 cathodes and from R48 and R49, also puts all the other cathodes 19.5V above ground which is enough to reduce heater-to cathode leakage current in them.

I feel that the discrepancy in the shown heater voltages between V1 and V2 are a typo.

There are inaccuracies due to averaging but I'm satisfied. If there are errors, please point them out so I can understand better.