tube substitution question

Hi all,

Is 5751 a direct replacement for a 12AX7? I've looked around but I can't seem to find a definitive answer. The intended potential use is as the tremolo tube in my Marshall 18 watt clone.

Thanks

Pin for pin, yes, but gain is less. Mu of 100 (nominal) for 12AX7, 70 for 5751. It might not have enough gain for use as the oscillator tube.

Thats kind of an audiofile tube, so it might be a waste in the trem circuit.
I would swap it for a preamp tube, and put the ax in the trem circuit.
Also, if you want a killer trem circuit, check out the Magnetone version.
Probably the best ever, i think you need stereo speakers though. cabn't remember. :sam:

Thanks Scott. That was quick!

The odd thing is that I sat down with a bunch of old 12AX7s to try to find a quieter tremolo tube. I've been having a ticking sound when the tremolo is on. I discovered that I only have the ticking sound if the volume on the normal channel is up. If the normal channel is down, the tremolo channel is quiet. :shock: :?: :roll:

Anybody know why that might be?

Thanks again.

And thanks CJ!!

I have a pair of used GE 5751s out of an old conrad-johnson preamp. Might be worth a try in the preamp section, it sounds like. If the gain is less that might actually be a good thing. I wouldn't mind getting the tone at less volume in the room. The amp is bloody loud for 18 watts!

Yes, the 12AY7 is my favorite first stage.
I think gain is 70 also. Warmer sound.

A lot of the old Fenders use a straight up 12Ax7 for the tremolo...

The 5751 performs differently "sound wise"..I've never tried it in a tremolo.. you might try a 12AT7.... check your caps for the ticking...weird..

12AY7 is a gain around 40,isn't it? 12AT7 is 70..

http://tdsl.duncanamps.com/show.php?des=12AY7

Seth,

I assume you've checked the archives at the 18watt community. I know it's a lot to wade through, but many people have reported that problem with their 18 watters.

Kent built one of these, maybe he could chime in.

Yes, I did check the archives. The most common solutions are changing tubes and re-routing wires. Kent said he put a cap across the tremolo switch and it fixed the problem. I guess I'm thinking the problem is temporarily fixed because the ticking in the tremolo channel only happens when the normal volume is up. I'd like to know why that is, though, and fix it. I should go look there some more.

The phase-shift oscillators usually used in trem circuits have fairly sharp transients since they're not amplitude-stabilized like a sine oscillator would be. Try a small-valued cap--say, less than .1uF--across the trem oscillator plate resistor. Make sure it's of appropriate voltage rating, of course. Keep all signal leads well away from the oscillator circuit as well.

Thanks, Dave. I'm trying to understand this, so be patient with me - that small cap would filter out the high frequencies of the transients? If the cap is too big it would filter too low and lessen the effect. Is that right?

Man I just thought of a great idea!
A T4 in the trem circuit instead of that sluggish neon bulb and photo resistor!
what say?
:guinness:

A T4 in the trem circuit instead of that sluggish neon bulb and photo resistor!

Click to expand...

That's in a Fender circuit, right? There's no optical stuff in the Marshall. How about in a stomp box?

Randall Aiken has some good info on his site about phase shift oscillators.

http://www.aikenamps.com/PhaseShiftOscillators.html

Looks like you could try raising C7 on the reference schematic at the 18watt site from 50n to 100n or even 200n. According to the info in the link above C19 wants to be much larger as well, maybe 220u or even 470u.

Keep us informed...

[quote author="jrmintz"]Thanks, Dave. I'm trying to understand this, so be patient with me - that small cap would filter out the high frequencies of the transients? If the cap is too big it would filter too low and lessen the effect. Is that right?[/quote]

Correct on both counts.

> A T4 in the trem circuit instead of that sluggish neon bulb and photo resistor!

Been done many times. I have a Fender Stereo Rhodes carcass here with two incandescent/LDR for stereo tremulo.

The neon bulb is very fast, though grossly nonlinear.

If I understand the electroluminescent panel in a T4, it is frequency sensitive and may be exceptionally hard to drive at few-Hz rates. An LED/LDR makes a lot more practical sense.

Also an LDR tends to have slow release, and the T4 is selected for slow release, while in a tremulo we probably want attack and release times similar and faster than we'd use in a limiter.

The classic guitar amp tremulo is not photo-cell, but tube bias, because working through the tube nonlinearities gives a more interesting sound. Opto is a fall-back for transistor boxes where bias shift is harder to do and less interesting to hear.

[quote author="PRR"]>The classic guitar amp tremulo is not photo-cell, but tube bias, because working through the tube nonlinearities gives a more interesting sound. [/quote]

Right you are... Many people consider it a big step backward when Fender went from bias-modulating to neon-and-LDR coupled tremolo. I used to modify my Fenders to a version of the older circuit, that modulated the bias of the output tubes. Sounded great...

Again, check out Magnetone amps.

The best type tremolo Fender came out with (IMHO, better than the "fixed bias" type) was on the brownface "Professional" series of the early 60s, like the Bandmaster and the Showman. It's called "harmonic tremolo" and it's pretty cool...

The guys at the Fender Forum talk about it here:

http://www.fenderforum.com/forum.html?db=&topic_number=327608

CJ,

The Magnatone trem is cool, but requires a redesign of most of the preamp circuitry--and uses impossible-to-find copper oxide varistors.