Interesting Magnatone low-noise wire-wound resistors in preamp

[nuke]

You know, you learn things when working on interesting stuff. My Magnatone M260A from 1959 is a great example. Magnatone went to interesting lengths to reduce noise; one was floating the filament supply center tap to the 6L6GB cathode bias raising it to about 25v above ground. The other interesting thing was the pre-amp and amp circuitry on a sub-chassis, mounted on grommets was actually a single-point grounding scheme and not a shock-mount as is often said on the 'net.

Another interesting thing I discovered is the use of wire-wound resistors in the preamp stages to reduce current noise. Current noise in resistors is correlated with their composition with carbon-comp being among the worst, and the best being bulk metal-foil ($$$). Wire-wound are pretty close to the metal foil resistor, but much less expensive.

magnatone 260A schematic.jpeg

Also interesting is the fixed-bias operation of the 12AX7 triodes, where the cathode is biased by a voltage divider, not self-biased as we normally see.

[Olomert Brever]

I love the sound of those current boosted cathode preamp stages.

[lonote]

Those are pretty interesting little details, thanks for sharing & pointing them out.

Do you have a lot of playing time with this particular amp? I am mostly wondering about the tone circuit, pretty close kin to the Ampeg stuff, wondering how you like it?

[trobbins]

The input stages don't use cathode bypassing (perhaps to constrain low frequency gain), but that then allows more hum to ingress from the heater, which they then try to mitigate with dc elevation of the heater. That also means they need to keep cathode resistance as low as practical, which they then try to mitigate by raising the triode bias with a bleed from B+. They may then have realised that noisy resistors in the first stage were a concern and gone to ww, although that seems a little strange as the 100k anode resistor would also benefit from being ww. You may also get some noise benefit from dropping the input 47k stoppers to say 10-15k, and using a humdinger trimpot instead of just a fixed humdinger.

[nuke]

Yeah, just interesting tidbits. My particular amp has 270k plate resistors, but same small wattage wire wounds to set up the cathode.

No lack of bass response in the amp overall.

[Helmholtz]

The input stages don't use cathode bypassing (perhaps to constrain low frequency gain)

Why only low frequency?
A bypass cap won't boost only the lows.

[mhuss]

Agree, it seems the 4M7 on the input and the plate resistors would be a better noise-reducing target then, say, the 470R cathode resistor.

Maybe they were ahead of their time anticipating the "carbon comp distortion is good" fad.

[pdf64]

I wonder what purpose is served by the V2 pin8 10nF cathode bypass on the modulation oscillator triode?

[Helmholtz]

Agree, it seems the 4M7 on the input and the plate resistors would be a better noise-reducing target then, say, the 470R cathode resistor.

The 4.7M grid leak will be shunted by the much lower resistance of the guitar.
Shouldn't be much difference in noise between a 1M or a 4.7M.

[nuke]

This attached schematic is closer to what I actually have in my 1959 260A.

The one-point ground for the entire amplifier sub chassis, and the wire-wound resistors were known, early, "hi-fi" tricks even as far back as the 1950's. Given the other cleverness of the amp's electronic design, it isn't a surprise to find such.

Maybe they couldn't source 270k wire-wounds for the plates? I don't know.

It is actually very quiet, no hum to speak of, very little hiss. (I did put the original Amperex Bugle-Boy ECC83 back in it).

I took the amp out to a jam last night played it and another guitarist played it so I got a chance to hear it from the audience perspective. It's quite a good amp. The vibrato effect is really quite useful. It's not a very powerful amp, but it voices out really nice and the vibrato sounds really big.

Really glad to get this restoration done after all these years it has been in a box in my garage. It was a ton of work duplicating the ruined cabinet and cleaning all the dirt and caked on dust from the electronics.

[trobbins]

Why only low frequency? A bypass cap won't boost only the lows.

Yeh, arse-about comment - doh.

Wrt the 10nF cathode coupling in the LFO stage, most Maggies just used 3M3 grid leak biasing, but a few models added 1k of cathode bias, and only the 460 (I think) also has a 10nF bypass. My only thought is that it may reduce the likelihood that the oscillator doesn't start or is slow to start (for whatever reason).

[nuke]

Without any deep thinking, perhaps they were trying to shape/cleanup the waveform of the LFO before passing it on to the varistor stages?

I think the waveform of the LFO is important in this circuit, more so than with an optical tremolo with a neon bulb. The neon would mostly just fire on off, and it is the LDR ramp time that determines how the audio is modulated.

Unlike the typical Fender circuit this LFO doesn’t do weird stuff while you turn the speed knob. It is a good trick to greatly reduce the bypass cap on the Fender tremolo oscillator to stop that.

Have to think about a bit and maybe look at the waveform sometime. After so many hours spent on that restoration I’m not eager to open it back up anytime soon.

[trobbins]

I had a quick look at how an LFO output changed with minor part alterations. There could well be an audible difference to using an LFO that has lower harmonic levels, as low frequency content getting through to the output stage and speaker could easily cause noticeable problems.
https://www.dalmura.com.au/static/Magna ... design.pdf

[sluckey]

Nice document. Thanks