Just a quick question concerning my 59 Fender tweed Vibrolux.
I have never noticed before but when I turn my tone all the way down I have no volume even with the volume up? I usually run the tone on about 3 or 4.
Is this how these tone circuits work or should I still have volume with the tone all the way off?

joe

I think you would have some volume even with the tone all the way off. Do you get nothing at all even with volume all the way up?

So the volume and tone usually interact on the old amps. What you seem to be describing [zero volume] might be a little extreme though. John Eppstein prolly knows from working on those amps.

audioforce

The Tone control on the Tweed Vibrolux is simplicity itself. In the brightest position (12) it provides a high pass capacitor that allows all of the signal above a certain frequency to bypass the Volume control for maximum gain at those frequencies. In the "bassiest" (0) position it does something akin to the opposite using a different capacitor to leak off highs ONLY to ground and even those at a mere 6db/octave. If you are getting zero volume at the "zero-th" position of the Tone control that capacitor on the Zero side is bad, meaning either it has failed or someone thought they could get more bass by installing a too large value.

TLDR answer - No. Tone on Zero should not stop any and all signal. There should still be lows getting through. Get it fixed... an extremely simple single cap replacement..

I will fix it then. Is it the tone cap connected to the tone pot (which is the original Astron) or is it one of the tone caps on the the board ? (also all original Astrons)

I have the circuits diagram and the schem. I've done the power caps already as well as the Ot replacement so I'm pretty comfortable in there.


joe

The cap giving you obvious problems is the one on which one end is connected to the "bottom" of the Tone pot and the other goes to ground. If it was mine, given the all too common failure of Astron caps (especially the yellow ones) I'd replace them both and most likely with metal film polypropylene. The reason I would choose that material is for consistency and stability. I don't want any resonant peaks that I can't dial in and I want my amp to be as noiseless as possible. Metal film polypropylene does that job exceptionally well.

Since you have the schematic, note that the Wiper of the Tone pot, which is connected to the signal or "High" side of the Volume control, when in the "12" position provides a circuit identical to the circuit switched in, in later amps as the "Bright" switch. It is a means to give preference to highs by bypassing the resistance of the Volume control and get directly to it's Wiper/Output.

When the Tone control is in the "0" position it is exactly the same circuitry as any High Cut filter where a cap passes highs to ground to be dissipated and removed from the signal path. It is an extremely simple but very effective instrument tone filter that glides between cut and boost.

Thanks for the input and advice.
Although I did change that tone pot cap to a new Mallory .0047 600v and there was no change at all. In fact, I tried a few different values just to see if the was any change at all. Nothing from any of them. Is it possible it is the cap connected between the volume and tone pots? (blue circled in diagram)

The tone knob is acting like pretty much a volume only?

joe

What you're displaying isn't a schematic. That's a wiring diagram. They are obviously useful too but to visualize how a circuit works you really are much better informed by a real schematic. I'm including two. The only Vibrolux schematic I can find to link is very small and hard to read but I'm linking it so you can see any differences to the similar Tweed Tremolux schematic since the first stage circuitry is nearly identical and much easier to follow. There is a gain difference between a 12AX7 and a 12AY7 but other than that they are nearly identical and one can in fact be substituted for the other. The PI/Driver stages are entirely different but as long as we stick to the first stage up through Volume and Tone controls we are have safe, meaningful comparison.

Even if the .0005 was a dead short (rather unlikely... far more likely to be open or leaky) there should still be high cut occurring when the Tone is on "0", not zero volume. It is vaguely possible the 0.1 coupling cap is leaking B+ voltage which could negate the bias of the following stage putting it in a cutoff condition but that seems a stretch since the amp would always sound odd and weak at any Volume or Tone setting. The caps insure at least some frequency selection not all frequencies. It should not behave like a volume control which is essentially all band. The only simple way the Tone control could act as a Volume control is if either the .005 cap was a dead short or some other manner in which the bottom of the Tone pot is grounded. No condition of the .0005 could do that. Obviously check the low side of the Tone pot to see if it is grounded. It shouldn't be.

Caps are one of the first components to fail so I'd test caps thoroughly but I strongly suspect an improper wiring connection somewhere and any decent tech would do the basics and test for proper voltages everywhere and signal trace, preferably with Pink Noise and a Spectrum Analyzer but only because it is more comprehensive and a bit faster than sine wave and Oscope. Don't get me wrong. It is quite possible to discover the problem with nothing but a voltmeter but it is slower.

If you have a voltmeter and know how to check for leaky coupling caps and have the inclination, do that ASAP but even a leaky coupling cap should also show it's effect on the following stage's bias voltage if it is measured between the Cathode and the Grid. It should be roughly 1.5v with the grid negative relative to the positive cathode. It is an implied voltage created by raising the Cathode above ground slightly. If this is at all confusing or daunting take it to a proper tech and tell him the Tone is acting like a Volume and outline what work you've done so far.

Here's the two schematics

It looks to me like .005uf capacitor on the tone control is shorted.

If it's a dead short it will short the signal to ground and cause zero output.

Here's a better schematic. NOTE =- it's a PNG image. It will display online in Firefox but won't display in Windows Picture Viewer, although it will in Windows Paint (Win7).

That is the only simple answer but since he just installed a new one the fault is not likely the cap itself but the installation. It is also possible the leg of the tone pot has been shorted against the body or the chassis. Astron caps are so notorious for failure that I wouldn't stop at just fixing the short whichever is the cause. I'd check every one of those yella bastids.

I agree.

Unless he overheated the new cap while installing and burned it.

well thank you guys for the insight and help. I'll get it to my tech if it continues to elude me.

I'm pretty certain the cap installation wasn't botched and the pot isn't shorted to the chassis as far as I can see.

I am no tech that's is for sure but I was able to do a power cap and output transformer repair when I got the amp quite nicely so this cap replacement seemed simple enough?

I'll keep at it.

joe

found it... duh...cold solder joint on the middle tone pot terminal. original cap is fine


joe

???? How can that be? If the connection at the Wiper ("middle terminal") of the Tone pot was good at any time then it should have worked as a Tone control, not a Volume control. For any time that there was no connection due to the cold joint, it would effectively been out of the circuit and still NOT a Volume control. Those are the 2 most likely conditions with a cold solder joint neither of which turns a Tone into a Volume so how and why do you suppose this is the problem?

Just FTR This has no reflection on you personally at all. You are apparently doing the best you can with the tools you have at your disposal. I am just genuinely curious as to what is going on in your amp and if fixing the joint will actually solve the problem as it currently makes no sense as described..

...Just a guess I suppose. I started from resoldering the original cap back in place. I then freshly soldered the center tone post and it worked from then on. Perhaps the bad solder connection was actually the at the cap to pot ground.


Nontheless, it is working wonderfully now. I'll keep an eye on it.

joe

Congratulations. While I personally hate it when things start working and I'm not sure why (little worse than an intermittent problem) , I am also thankful it works and I'm sure you're relieved, too. BTW these days it is possible to get a simple temperature controlled soldering station that helps greatly in preventing cold solder joints without requiring as much time as a dumb soldering iron often does on your components, for under 100 bucks. Here's an example at ~$70.00 USD, a wise investment when maintaining old electronics. For a similar cost a basic multimeter that measures ac voltage, dc voltage, resistance and capacitance can also be had these days. These are not pro shop tools but they are light years better than guns and dumb irons from Home Depot.

https://www.amazon.com/21-19800-Temp...837123&sr=8-47

With the one exception that if you have to solder grounds directly to the chassis you're going to need one of those big 100-150 watt Weller guns. They also sometimes help with soldering grounds directly to the cases of pots or larger switches.

You won't need the big gun very often but when you need it you really need it.

Good point, John. Big irons work but they are used so rarely they are impractical and downright wasteful to keep hot. The Weller Guns are basically on-demand, fast and great for chassis.ground work