Magna/Estey input stage

[Andy Le Blanc]

One thing I've tried out is a circuit configuration in old magnavox amps.
It's a regular one half of a 12ax7 with a 100k plate resistor, but it has a
large ww 470r resistor on the cathode and a 220k resistor going from the
supply side of the plate resistor to the cathode of the tube. I assume that
there is injection of supply ripple through the cathode to provide cancellation,
and it has a tone, the cathode resistor is un-bypassed. I was curious if any
had also fiddled with it.

[mojo2001]

Wow, that sounds freaky.

Consider also that it provides NFB and it is also a voltage divider laying some DC on the cathode (not a lot though with those values),

Which amp was this in. I'd like to see the schemo!

Joe

[Andy Le Blanc]

Here's one, it was used fairly extensively in they're lines. Ive tried it with
a few paraphrase inverters as a simple pre, it makes a neat tone.

[mojo2001]

Well, scratch the NFB suggestion since the 220k is on the filter cap side of the plate resistor.

Why do you think that 470R is WW? Very little current flows through that resistor.

All I can see is a voltage divider that puts 0.3V fixed bias on the cathode, in addition to the voltage developed across the cathode resistor.

Bizarre and interesting...

[Structo]

Just a guess but would they spec the WW because of the inductance they exhibit.

[mojo2001]

Can't see inductance playing much role... in a cathode circuit that would give a very slight peak way up in the VHF RF range...well beyond any frequency Estey's wiring techniques would support

Only reason to use WW would be dissipation (or low noise)...but its a dang 12AX7 and this is an Estey amp not a studio mic preamp!

[Andy Le Blanc]

I've wondered about the inductance, but would it be significant enough to
worry about. I think its because the ww has to handle DC as a PS bleeder
and handle AC as an un-bypassed cathode resistor, might wrong on both
accounts. It reminds me of "ultra-path" and other ways to introduce the right
amount of PS ripple so that it cancels in the output. Sounds good, has it's own
tone, those cement ww seem ok for the job. I've tried it with parallel stages,
kept the 470 and the 220k, and tweaked the plate resistor.

[mojo2001]

Dissipation is on the order of 0.25 W...don't need WW. Something like 1 mA bleeder current and what 2 mA cathode current?

I have measured the inductance of WW resistors before and it is quite low for something like a 470 ohm. Definitely talking high RF range effects in a cathode circuit like that.

Plus there are four sections of filtering before the input tube...that seems more than adequate, so why the hum concern?

I don't know, Andy, you stumped me, Bro!

Waiting for somebody SMART to come along...

Joe

[Wayne]

Waiting for somebody SMART to come along...

No, that's not me, but he may be along in a minute...

Maybe those resistors are WW for reliability, i. e. if the tube shorts it won't take out the cathode resistor with it. Just a thought...

Also, did anyone else notice the difference between the two channels? In channel 1, the feed for the plate resistor and the "feedback voltage" or whatever we're gonna call the voltage feeding the 220k come from the same node on the power supply - in channel 2, they don't. I wonder what the significance of that is?

W

[mojo2001]

did anyone else notice the difference between the two channels?

You may or may not be smart, but your eyesight is unquestionable!

the plot thickens!!

[Andy Le Blanc]

ya... in channel two the voltage on 220k is sourced from a higher voltage than in channel one.
It's not consistent in other models. Neat way to voice the stage , change the
voltage. Its very simple to build , have you tried it on the bench yet?