silvertone 1484 tremelo issue

[Steve S.]

Thanks for the responses but I'm still stumped so I'm re-posting this.
Hello
I am working on a Silvertone1484 which belongs to a friend of mine. The amp has been sitting unused for quite a few years.  I replaced the filter and bypass caps, put in new tubes, jacks and a few other odds and ends.  The amp works well now except for the reverb, the tank appears to have a bad transducer.
The problem is with the tremelo unit.  It works well but when it is turned all the way off you can still hear a faint tremelo effect which is as though the speed is up all the way.  Turning the speed control while this is happening has no effect one way or the other.
Grounding the footswitch has no effect. The control pots test good as far as resistance goes.  If you remove the tremelo tube the effect stops and there is a noticeable rise in volume.
One by one I tried disconnecting connections on the tremelo tube to try and isolate where the effect was coming from.  The only thing that stops it is to disconnect the plate on the second half of the tube, the one that connects to the oscillator. Also , disconnecting the cathode stops it.  I currently have a switch wired to the cathode which works but I'd really like to figure out the problem and not just put a "bandaid" on it.
I own a 1484 in really good shape, so I've been able to compare voltages and other readings between the two.  Everything seems to be within normal range on the problem amp.
Thanks for your help.
Steve

[thermion]

if you disconnect the intensity pot, the noise goes away? if so can you open up the pot and using a razor scrape away a tiny bit of the trace at the "off" end of the pot, thus when the wiper goes all the way to the end it falls off the track and kills the oscillation?
maybe not the best but could get you that little bit of volume boost when the pot is out of the circuit.?.

[Steve S.]

Disconnecting the intensity pot does not stop the problem. I also tried rerouting the ground from the pot to a different location, no effect
on the problem. 
Any other thoughts?
Thanks,
Steve

[thermion]

looking at the schematic...check resistor values around the oscillator (R49-56) and (R60-61). maybe a leaky cap (C23,24)?
does the ldr module have it's own case grounded?

[Steve S.]

The LDR is grounded right at the unit.  I'll go back and check those resistors and caps.
Thanks, Steve

[Steve S.]

I checked the resistors and caps that you suggested.  The resistors are in the range that they should be and I temporarily wired in new caps.  Still no effect on the problem.  The problem has to be in, or related to, the circuit for the second half of that tube since disconnecting either the cathode or plate stops the problem.  Disconnecting the grid on that half of the tube does not stop the problem. 

[thermion]

i dug a some more on this... found some references to a fender "service bulletin #9" that suggests in the fender deluxe reverb ticking was caused by poor lead dress at assembly. their solution was to use a shunting cap as a band-aid or redress (shorten leads). maybe the same approach will work here?

[Steve S.]

Thats interesting because when I checked voltage on the lead off the LDR with a VOM, the effect seemed to go away when I touched the lead on the side away from the plate.  I wonder if this had a shunting effect.
Where would I connect the cap and what value should it be?
Part of my problem with thinking this out is that I don't fully understand how the tremelo circuit works, the LDR in particular.
Thanks again,
Steve







 

[sluckey]

The LDR (Light Dependant Resistor) is a two terminal resistor whose resistance changes depending upon light shining on it. When dark, it's resistance is verrry high, typically many megohms. When you shine a bright light on it, the resistance becomes verrry low, typically 1K or less. When you shine a dim light on the LDR, it's resistance will be somewhere between verrry high and verrry low. If you put a light source close to a LDR and turn the light on and off repeatedly, the LDR resistance will change between verrry high and verrry low at the same rate that you turn the light on/off.

More specific. The 1484 uses an optocoupler (roach) that contains an LDR and a neon lamp bulb placed next to each other in a light tight enclosure. The LDR is connected between ground and the guitar signal path at the junction of C9 and R9. When the LDR is dark (high ohms) it has very little or no effect on the guitar signal. When the LDR is lit up (low ohms) by the neon lamp it shunts the guitar signal to ground, decreasing the signal amplitude. Now if you can just flash that neon lamp on and off at some slow speed, say 1Hz to 10Hz, you will effectively vary the amplitude of the guitar signal at that rate. This is called amplitude modulation, or in the guitar world, tremolo.

V6B is a simple low frequency phase shift oscillator that produces a sine wave output. This sine wave is applied thru the STRENGTH control to V6A which controls/switches the neon lamp. The neon lamp will flash at the frequency of the oscillator and the STRENGTH control determines how bright the neon will flash.

The typical Fender tremolo circuit with a roach operates just like the 1484.

[Steve S.]

So, is the plate side of the LDR the neon bulb and the resistor the side that goes to the bass pot?
Also, is the V6A circuit just to control the speed and boost the signal?  I'm also a little fuzzy on that part of this, any further explanation would be appreciated.
As previously stated, the oscillation that I hear is as though the speed control is on high and the only thing that stops it is switching off the cathodes of V6, or the plate of V6B.  Another possible clue is that when I switch off the cathodes, the oscillation stops and the bass response goes way up.
I guess when you take away the signal to the bass pot it swings the signal path more through the low pass part of the bass pot circuit?
Thanks so much for any info on this,  I'm always into expanding my knowledge of all this stuff.
Steve

[thermion]

Thanks for the great explanation, sluckey.
I see reference to two different ways to shunt:
The official Fender solution (Service Bulletin number 9):
******************************************************** *
"The ticking caused by the Vibrato is caused by improper lead dress.

It can almost be "cured" by connecting a .01 mfd 600 volt mylar capacitor
on the 10 meg ohm resistor in the vibrato circuit. this resistor is located
on the parts panel. Remove capacitor across the 10 meg ohm resistor (old
modification) if in place."

(IOW, Solder the cap from the junction of the 10 meg resistor and the
optoisolator to ground.)

"If this does not produce the desired results, then the leads should be
dressed as follows and excessive lengths shortened.

1. Dress the leads to the vibrato speed and intensity controls away from
the tone controls and filter leads.

2. "Bunch" the leads to the components on the parts panel which connect to
the tube socket of the 7025 (12AX7) vibrato tube."

Another  reference says to put the cap across (in parallel) with the same resistor. Maybe try the first one and see how it goes...

[Steve S.]

Thanks for the info Thermion.
I tried various shunting ideas.  On the 1484 there isn't a 10meg resistor as in the fender circuit so I tried adding one.
I also tried more lead dress work, rerouting and shortening here and there.  Nothing had an effect on the problem.
I think that I will rewire the footswitch jack so that it switches the cathode of V6 instead of running the signal to ground.
Its a bandaid but I think it will work.

[thermion]

As sluckey said,
"The typical Fender tremolo circuit with a roach operates just like the 1484."
Compare the 2 schematics, find the analogous part in the 1484 to the 10M fender resistor, and shunt around that. It may not be exactly 10M but should be in that order of magnitude (millions of ohms).
If that doesn't do it, rewire the footswitch as you describe and see how that goes.

[sluckey]

Compare the 2 schematics, find the analogous part in the 1484 to the 10M fender resistor, and shunt around that. It may not be exactly 10M but should be in that order of magnitude (millions of ohms).

There is no such part in the 1484. Sears was known for building cheap, stripped down electronics.

[Steve S.]

I think I figured out the problem.  In the actual layout of the amp, C23 is soldered directly to the ground side of R50 [560K].  I re-wired it so it connects to the cathode, as in the schematic, with the grounds connecting to the chassis at different points.  I've played the amp several times and so far no oscillation problem!
As I mentioned before, I own a 1484 also so I have it here to compare my friends amp to.  Its interesting that my amp wired all stock has no oscillation problems.  One thing that really stands out in comparing the two amps is that the lead dress and overall work on my amp is very neat and tidy and on my friends amp the original work is very messy.  The person who did the work must have either been new on the job or just didn't care much about his work.  In side by side comparisons his amp sounds pretty good but mine sounds noticeably better to my ears.
It seems to me to really point up the importance of proper lead dress and just overall attention to detail and neatness.
By the way, I did re-work the footswitch so that it switches the cathode instead of running the signal to ground.  It actually works quite well and gives a boost to the volume when you open the cathode connection.  I think I'll leave it wired that way.
Thanks for your help Sluckey and Thermion!
Steve

[Willabe]

Steve S, glade you got her fixed!

Could you please post gut pics of Both amps so all of us can see the lead dress?

I'm sure all of us would like to see the differences.


            Brad          

[thermion]

good outcome!
just so I understand, now both amps have c23 connected to the cathode side?
+1 on pictures.

[Steve S.]

My amp is wired stock and it works fine, sounds great.  My friends amp, the one that had the problem, is the one that I rewired C23 to the cathode ground. I'll take some pics and post them, I'll be away for a few days so it will probably be when I get back.  Thanks for your interest.
Steve

[Steve S.]

This is my friends 1482, the one that had the oscillation problem.  I left C 23 in place so you could see it in its original position , its the one with the bit of blue tape on it. The mallory cap [yellow] is the one I replaced it with and relocated the ground point. 
I need to go back and mount it permanently, I just did a quick hookup for test purposes.  The lead dress looks better now than when I started since I moved things around and shortened various leads in my attempts to fix the problem.  I also added a selector switch for 4 0r 8 ohms since he doesn't own the speaker cabinet for the amp.  Hopefully you can see some points of interest in these photos.
Steve

[Steve S.]

These are the photos of my amp, for comparison. You may not be able to see much in the photos but when you look at the two amps side by side on the bench it is much more plain to see.
These really are great amps, I've owned mine since 1970.

[SoundmasterG]

The stock Silvertone 1484 and 1485 use the same output transformer, and the stock taps on them are 4 ohm and 2.6 ohm, NOT 4 ohm and 8 ohm. The 1484 used the 4 ohm tap for two 8 ohm speakers in parallel, and the 1485 used the 2.6 ohm tap for six 10's.

Yuo can mismatch the 4 ohm tap into an 8 ohm speaker with the 1484 and get an interesting sound...very compressed and distorted, but the amp puts out quite a bit less power. These output transformers are extremely small for the tube complement and as a result the amp is only 25 watts RMS stock, and the output transformers often blow.

Greg

[Steve S.]

Thanks for the reply,
This is actually a Weber replacement OT and they differ from the original in that they are 4 and 8 ohm taps.  You're right , they are really undersized but I have one that I've had since 1969 and it still has the original OT.  I can remember when I was in high school in the early 70's running three guitars and a bass through it all at once at full volume at wild parties.  I blew a few speakers but not the OT, just dumb luck I guess.
Steve

[Platefire]

Har! I remember going to a dance in the late 60's where the whole band was playing through one of those 6-10 cab ST's. I think a lot of that went on in those days.

[sluckey]

I remember those days. Then we got lucky and each got our own amps. 1484 cost $149 back then. Everybody dreamed of a Super Reverb but those were nearly $400!

[dude]

Steve,

Which kid is you?

Yeah, those were the good old days, a Super Reverb for $400, I take five.

al

[Willabe]

I bet the one on the left?

But wait a second, a left handed drummer? What kind a radical rock band were you in?


                         Brad       

[sluckey]

Yeah, I'm the tall geek on the left. Wish I still had that old Strat!   

We were really radical. Wipeout, Steppin' Stone, Kicks, My Girl, Midnight Hour, etc., etc. The good stuff was not yet popular at teen clubs and high school dances.

[bigsbybender]

Great pic!  I notice you had a southpaw drummer....

[Rev D]

 I remember well mom and pops would leave me in the TV section (electronics as it was at the time) and I'd drool over the amps and Teisco (Sears) guitars... I knew the good stuff as my brother had Fenders and other stuff but I just knew I'd rock if I had one of those Silvertones... I never found out heh... I rocked a small K-mart solid state instead and was darn happy to have it, for distortion I'd turn it up and let the plastic handle rattle like crazy..  

Regards,

Don

[Steve S.]

I guess thats one of the reasons that we all love these old amps.  It reminds us of a time when everything was a whole lot simpler!

[punkykatt]

Back in the early 70`s I use to have two of those 1484`s and double stacked them.  I made a 4 way Y chord so I could play through both channels on each amp. Cranked the volumes to 10, treble all the way up, LesPaul in hand, Vox Tone Bender, Big Muff, and King Vox wa on the floor.  I was in Rock Heaven. Not a bad sound. I can remember getting controlled feed back that could last for days.
Punky