is this correct?

[dmartn149]

Here is a schematic I want to use for a tape recorder to guitar amp conversion. The two resistors coming off the heater supply look wrong to me. (R1 & R2) Shouldn't they be connected directly to ground? or to put it another way, shouldn't that lead that goes to C10 and R15 be on the other side of C10 and R15? I can't find this arrangement on any other schematic. Is it supposed to reduce hum?Micro-Champ-R2B.pdf

[g1]

Floating the heater CT at some DC voltage is not that uncommon. And yes, this arrangement is supposed to reduce hum.
Sorry, I can't think of any commercial schematics showing this method right now, I believe some of the early ampeg guitar amps used, it.
I'm sure other members can provide some examples of mainstream production amps using this method.

[dmartn149]

Thanks g1. I've looked at some Ampeg schems and see a hum balance pot on the heater leads, but these are between the 6.3AC and ground. On my schematic those resisters are connected to the cathode of the output tube.

[Leo_Gnardo]

Thanks g1. I've looked at some Ampeg schems and see a hum balance pot on the heater leads, but these are between the 6.3AC and ground. On my schematic those resisters are connected to the cathode of the output tube.

Some of those old Ampegs stand the filament CT on a 0.1 uF 200V or 400V film cap to ground, no DC bias. One more arrangement to try. The "right solution" is whatever yields the minimum hum, hmmmmm?

[g1]

Here's one of those ampeg examples:

[MarkusBass]

No, it's not a mistake. In the 60'ies many amps had heaters connected in exactly the same way. For example Harmony amps. I don't remember the exact model but it could be 300-something.
It is convenient to connect the winding directly to the ground but it can be connected to any DC reference voltage, assuming it is not very high (it fits in the range allowed by the manufacturer of the tube).

Mark

[Tom Phillips]

...I can't find this arrangement on any other schematic. Is it supposed to reduce hum?

Dan,
There is a good article at The Valve Wizard that includes several examples of heater hum reduction circuits including the configuration that you asked about. The article is a nice read and includes good illustrations.
Cheers,
Tom

[dmartn149]

Thanks everyone. I found the Harmony schem. like that (H310) I'll read the Valve wizard article now.

[dmartn149]

Yup, that Valve Wizard article describes it very well.
Thanks again, everyone.

[Enzo]

Even today they do that. The Peavey 5150/6505 has the AC heaters elevated by +24vDC, for example.