Filament center-tap

I saw a schematic for a Marantz Model 5 tube amp from the 60's that took the -40 bias supply and connected it to the filament winding center-tap for hum cancellation.

Why does this work?

[quote author="Larrchild"]I saw a schematic for a Marantz Model 5 tube amp from the 60's that took the -40 bias supply and connected it to the filament winding center-tap for hum cancellation.

Why does this work?[/quote]

Typically the only noise-reducing d.c. bias strategy is to bias the filament positive w.r.t. the cathode, to reduce the tendency of the filament to act like a little electron emitter.

But in a amp the hum is more likely to get in in earlier stages, depending on power supply bypassing. Is it a separate filament supply for these earlier stages, or are their cathodes biased at a still lower d.c. potential?

It sounds a little odd in any case, but maybe the schematic would reveal the purpose.

Right-on, B ..here's a scan. It goes to all fils.
It is positive. So sorry.

Cool---my faith in Saul et al. is restored :grin:

As a side note, those old Marantz tube amps are killer. I repaired an 8B for a customer several years ago and it just pained me to give it back to him. I wanted to keep it!

Here are some pics of an 8B.

Pic 1
Pic 2
Pic 3
Pic 4

Manual:

Page 1
Page 2
Page 3
Page 4

I met Saul Marantz at a Hi-Fi show in detroit in the late 70's. He was cool.
His model 7 preamp aint bad either. passes 100khz easy. Just slightly load-dependent.

and that bias meter on the 8b. tres chic!

or the CRT on the 10B, from the LBZ, with the Snoop Double G.

Not as cheeky as the Panadapter(Spectrum Analyzer) in the Sequerra Tuner, but the 10b's scope (multiipath i think) was the shanizzle. :thumb:

Panadapter? What was that?
Sequerra had his transformers wound right down the street at Zambre. Starting asking for cheaper prices so Mr. Zambre and him parted ways.

Point to Point - Definition:



I think the four gang tuning cap was 300 bucks back in the seventies.

http://vacuumbrain.com/The_Lab/TA/Marantz/saul.jpg

The panadapter is a spectrum analyzer with less precision used to see multiple signals for tuning etc.. I would swing the Alliance U-100 roto-yagi around on the hi-fi stores fm antenna and watch the blips go up and down on the Sequerra.
I was hooked.

Wow.

They used torroid inductors in the 10B but were having problems aligning the recievers. Found out that the torroids were getting magnetized by the Weller soldering guns which changed the bandpass. Sheesh. Those monsters are hard enough to align without residuals.

Permeabilicly challenged toroids. Why is it when CJ posts, now we are talking about windings on a core..eh? LOL
Tuning those IF's with a sweep generator..pre ceramic filters was no swiss-picnic, thats for sure.


Since the esteemed Bcarso answered my question on the first round=), this frivilous offtopicness seems an appropriate indulgence. Here's some marantz stuff from their site.

Freelance graphic artist Saul Bernard Marantz, a native of New York born in 1911, founded the Marantz Company in 1953. As a record collector and amateur musician, he felt that commercially available amplifiers were simply not good enough for the recently introduced (1948) long playing (LP) records, so he built his own. The resulting ?Audio Consolette? became the prototype for his young company?s legendary ?Model 1? Mono Preamplifier. It included features like a rotary volume potentiometer, seven inputs, including one for TV Audio thus pre-empting home cinema by about 40 years! It can be considered revolutionary by far

The Model 2 Power Amplifier followed in1956 and in a year later, the Model 3 Mono Two-way Channel Divider and Model 4 Power Supply were introduced. In 1958, he announced the Model 5, a less powerful version of the expensive Model 2. This year also saw the introduction of stereo, so Marantz launched the Model 6 stereo adapter for use with two Model 1 preamplifiers plus what many consider his finest creation, the Model 7c Stereo Preamplifier. This became one of the biggest selling high-end audio units of all time, totaling 130,000 pieces. At the start of the sixties, Marantz launched its first and only stereo power amplifier, the 30 watt per channel Model 8. The 1963 Model 9 was a 70-watt mono power amplifier while the Model 10 FM tuner featured a built-in oscilloscope to verify the quality of the received signal.

A change of ownership

The high development costs of the Model 10 forced Saul Marantz to sell his company in 1964 to Superscope Inc. resigned a few years later.

You know the rest.

I was readin Saul's sheet on the 10b. Double-balanced mixer and balanced tuning thruout. Pretty bold. Had a perky front end for the day 25uv=20db quieting. I guess if you can design an FM tuner in New York City, it will work anywhere. But this must have been like trying to align a 1955 color set.

Here is some more:

http://vacuumbrain.com/The_Lab/TA/Marantz/10b_2.jpg

http://vacuumbrain.com/The_Lab/TA/Marantz/10b_3.jpg

I love how both of the guys who designed this 10B didn't have BSEE's.

Stuff like a double balanced ring mixer was state of the art. Although they requre massive local oscillator injection level, they are almost imageless.

And Dr. Dishell, who's IEEE paper they used as the basis for their phase linear IF filter, came up with an interesting technique of stagger-tuning the frequency centers of several tuned bandpass circuits to gain linear phase response across the wider passband. Nice trick.

The totally balanced rf sections, while a tracking nightmare, provide some amazingly tight filtering. It reminds me of the 10 station FM antenna combiner at my ex station.

They take 10 20kw FM transmitters and feed them into a single broadband antenna. And it uses the exact same topology to achive the tight filtering required. A balanced symetrical set of filters. They are just 4 feet in diameter.=)

Audio seems simple after reading rf stuff, eh?

bitmap this time:

http://vacuumbrain.com/The_Lab/TA/Marantz/10b_4.bmp

sorry, new scanner.
still messin with the settings.
sometimes I think the digi camera is better than this HP crap.

http://vacuumbrain.com/The_Lab/TA/Marantz/10b_5.bmp

good stuff CJ! I don't wanna take anyone down the RF road on an audio forum, but other than the occasional dead-short that acts like a tuned circuit, and too-big holes let your signal out, same immutable physics in action.

Using Dishell's function, one could build a phase-coherent audio filter, i imagine.

The 10B tuner is one marantz they won't be cloning soon. ha
I mite clone me a Model 7c Preamp though.

It's tube audio, so it's all good.
A shame that Sequerra is doing spark plugs. Oh well.

In case anyone missed it, In the first Saul Marantz 10B press release CJ posted, some wiseacre on vacuumbrain.com doctored Saul's picture holding a cigarette to have marahuana-overtones.

I almost busted a kidney laughing. Damn kids.