Trying to repair the tremolo in a Silvertone 1472

[rollo greb]

I picked up a Silvertone 1472 a while ago, and the tremolo is very weak. It pulses in the background, but when I play the guitar it's virtually unnoticeable. I've gone through it and replaced all the resistors in the tremolo circuit, and put all new tubes in it. I did notice something strange - in the schematic here:

http://www.freeinfosociety.com/elect...ertone1472.pdf

I noticed R23 and R25 (by the power tubes) are missing in my amp. Looking at the model 1482 schematic, I see these two 330K resistors are not shown, so they were obviously removed from the design. Could this be the source of my problem? I'm a little leery about going ahead and adding the two resistors, because this is my first amp project and I don't want to damage anything. So far I've just been replacing old parts.

It does look to me like these two resistors could have a large effect, because the signal from the preamp would pass right through them. But what do I know?

Thanks to anyone who can help me out! These amps sound wonderful and are a great bargain, in my opinion.

Also, if anyone else owns this amp - how does the tremolo work for you? I had a 1482 in the past and remember it being strong and gorgeous.

[52 Bill]

I noticed R23 and R25 (by the power tubes) are missing in my amp. Looking at the model 1482 schematic, I see these two 330K resistors are not shown, so they were obviously removed from the design. Could this be the source of my problem?

No. Those resistors are generally refered to as "grid stoppers". They are used in some designs and as in your amp, omitted from others. They will not cause your tremolo problem.

Do you have a voltmeter? If you do read the voltage at the wiper of the strength control. Be sure that it is turned up fully on. You should find a pulsating voltage, that varies in time with the speed control.

Generally, I find more problems with the caps in the trem circuit than the resistors. As a rule those original resistors were 20% tolerance and often varied far beyond that.

[rollo greb]

Thanks for the reply. Yep, I've got a voltmeter, so I'll check that out.

The resistors I removed all measured about 20-50K above the proper value, so they either drifted or were off to start with. But that obviously wasn't causing the problem. I'll have to try replacing the caps next.

[rollo greb]

I'm getting a pulsating voltage of about 5VDC on the wiper. I did notice that the voltage off the power tubes was around 300V instead of the 260V on the schematic - is this something to worry about, or is this kind of variation normal? Maybe a sign I need new filter caps?

[52 Bill]

I'm getting a pulsating voltage of about 5VDC on the wiper. I did notice that the voltage off the power tubes was around 300V instead of the 260V on the schematic - is this something to worry about, or is this kind of variation normal? Maybe a sign I need new filter caps?

The tremolo voltage sounds good, next check R21 and R22. Also read the voltage to ground at pin 5 of both output tubes. If C7 or C8 become leaky they will throw off the tremolo effect.

The increased plate voltage could be a design difference, as well as a line voltage increase, since the time the schematic was drawn. Nothing to worry too much about for now.

[rollo greb]

The tremolo voltage sounds good, next check R21 and R22. Also read the voltage to ground at pin 5 of both output tubes. If C7 or C8 become leaky they will throw off the tremolo effect.

The increased plate voltage could be a design difference, as well as a line voltage increase, since the time the schematic was drawn. Nothing to worry too much about for now.

I get similar readings at all those points - pulsing of about 5 volts. I did notice that turning the speed pot down reduces the pulsing to a point where the voltage doesn't change, instead of just slowing down the rate of the pulses. It seems to me that this would mean it's affecting the intensity as well as the speed.

[Enzo]

In other words the oscillation doesn't hold up through the speed range.

I would just replace caps C10, 11, 12, 14 in the trem circuit. Not surprised at this amp's age.

Might as well replace C15, 16 as well, C15 could load down the screen of the trem tube, and a leaky C16 could shunt trem signal past the intensity control.

ANother test, turn the intensity control to zero so no trem effect should be present, then measure the DC voltage at those two missing resistors. In other words at pin 5 of each power tube socket. if there is any positive DC at all, then caps C7,C8 are leaky.

And no offense, but is there something plugged into the footswitch jack to turn it on?

[rollo greb]

In other words the oscillation doesn't hold up through the speed range.

I would just replace caps C10, 11, 12, 14 in the trem circuit. Not surprised at this amp's age.

Might as well replace C15, 16 as well, C15 could load down the screen of the trem tube, and a leaky C16 could shunt trem signal past the intensity control.

ANother test, turn the intensity control to zero so no trem effect should be present, then measure the DC voltage at those two missing resistors. In other words at pin 5 of each power tube socket. if there is any positive DC at all, then caps C7,C8 are leaky.

And no offense, but is there something plugged into the footswitch jack to turn it on?

Thanks - yeah, it sounds like I'll be replacing the caps (not much else there to replace!)

The tremolo is active when nothing is plugged into the footswitch jack. A footswitch is only needed to turn off the tremolo.

[Enzo]

Sorry, it looked initially like the FS jack was a cutout.
COnsidering the age, if a couple of the film caps - or paper caps - are leaky, then they all most likely need replacing.

[rollo greb]

Wow, I never realized there were so many varieties of capacitors, with supposed differences in sound quality. Orange drops seem pretty popular, maybe I'll go with those.

[Enzo]

The differences will be subtle, like the differences between brands of guitar strings of the same size.

[pila]

There is quite a thread about the trem in those amps on Doug's forum, like wiring in a Fender circuit etc.

[rollo greb]

There is quite a thread about the trem in those amps on Doug's forum, like wiring in a Fender circuit etc.

Doug's forum? Sorry, I don't know what that is.

[pila]

I should have said Doug Hoffman's forum. Don't know if it's polite to reference another forum here, but try el34world.com for that thread.

[Enzo]

Of course it is OK to mention other forums.