Silvertone 1474 tremelo

[lowell]

I cannot figure out why the tremelo isnt working in this amp. The feedback caps were leaky and have all been replaced. The screen and plate resistors were either way off in value or open. I've replaced them too. Continuity in the circuit is good. I've tried two NOS 6AU6 tubes.

Main issue may be the screen voltage but I'm not sure.

Va 166vdc
Vs 74vdc

with 6AU6 removed;
Va 337v
Vs 231v

Any tips? There are no voltages in the schematic.

[Enzo]

Schematic?

[Jazz P Bass]

http://www.freeinfosociety.com/elect...ertone1474.pdf

[lowell]

Oops sorry guys haha. Welp... ofcourse right after posting the trem came alive. My guess is I pulled out and put in the 6au6 enough times to clean the socket.

[Leo_Gnardo]

Oops sorry guys haha. Welp... ofcourse right after posting the trem came alive. My guess is I pulled out and put in the 6au6 enough times to clean the socket.

The friction may have helped clean decades of corrosion and crud off the tube pins as well, just in those critical spots where it could make good metal to metal contact. There are better ways, but the lesson is, you can't expect a tube to work unless the pins are ALL making good contact with the socket electrodes.

[lowell]

Agreed! Was tricky though cause everything seemed mostly in line... except for the low screen voltage. And it's hard to know if it was truly low because the schematic doesnt have voltages. But my theory is that the grid pin wasn't making contact, therefore had no reference to ground, and the tube was fully saturated on. Would explain low screen or plate voltage.

[Enzo]

The schematic showed 335 as the B+ at that node, and your tube-pulled plate voltae at 337 agrees. The screen voltage is a voltage divider of two 330k resistors. So the voltage I expect at the screen pin would be half of 335, or about 170v. So your 231v seems high. With the tube in place then tube currents affect it all.

[lowell]

Gotcha.

So anyone know what the transducers tend to measure DC resistance wise for the reverb tank? Reverb squeals when I turn up the Reverb control or Threshold. I get 12M on one coil and 6M on the other. Do we have ourselves a dead tank here?

[Leo_Gnardo]

So anyone know what the transducers tend to measure DC resistance wise for the reverb tank? Reverb squeals when I turn up the Reverb control or Threshold. I get 12M on one coil and 6M on the other. Do we have ourselves a dead tank here?

No coils in those Dan/Silvertone tanks. The transducers are piezoelectric crystals, similar to what you find in a cheap piezo tweeter or inside a "crystal" microphone that some harmonica players favor. Ideally they should measure infinite resistance. They act more like a capacitor electrically. In all except 2 or 3 cases, in Dans and Silvertones I find them kaput, simply not working. One possible dodge, and I've never personally done this, if you can mount the spring assembly from a short modern reverb in place of the defunct piezo one, you can drive the tank - thru a capacitor say 0.1 to 0.47 uF, 400V or more to block the DC on the plate of the tube driving it. Take a look at a schematic for an old Ampeg say Gemini I or II and you'll see how they did it. You'll need a tank with what's called high impedance transducers at both drive and pickup ends. When they say "high" it's a relative term: the drive coil measures 200 ohms more or less with an ohmmeter, and the impedance is rated 800 ohms. Here's an example from tubesandmore.com: P-RMOD-8EB3C1B .

You'll be entering new territory here, as I've proposed this kind of replacement dozens of times but no customer has ever wanted me to follow through. Maybe another tech here has managed to do it. I know I'd be happy to hear how they made out.

[Enzo]

I have actually repaired those, taking a piezo from a cheap tweeter. But I had to reengineer the clamp mounting the piezo chip, as one was two sided and the other not. But I made it work, and would never recommend doing that again as the result is that same god awful reverb that the stock item makes.

[lowell]

Thanks guys... yeah the piezo idea sounds very cool... but not cost effective from a labor standpoint and we're already over budget as this amp has had so many drifted resistors... and every single coupling cap was leaking DC to the following stage. Gonna make sure the circuit is functioning first... then propose a standard high impedance tank. We shall see... thanks!