Silvertone 1482 tremolo problem

[davohilts]

Yes this was a big learning moment! I now know that if I'm having crazy sound problems that it is the grid voltages that tell me more than plate voltages. Also I've shit canned my pile of old caps!

[davohilts]

Well, I thought my problems were over but they are not.
What I have done to this amp is (to this point), changed out all the capacitors, most of the resistors including the cathode resistors, put in new tremolo intensity control, new 6v6 power tubes and changed out 12AX7 and 6AU6 tubes with known working tubes. Using Regis Coyne's suggestions ( http://www.regiscoyne.com/silvertone1482/ ) I also modified the microphone input by installing 33K/1Meg resistors and a 1.5K cathode resistor with a 25mf bypass cap. On the instrument inputs I rewired them like a Fender Hi/Lo setup with 68K/1 meg resistors, and added a .68 mf bypass cap to the 2.2K cathode resistor. I took the tone controls out of the circuit and added 150pf caps to both volume controls
The amp is sounding good, I have 3 levels of gain by choosing the microphone input, or the hi/low on the instrument inputs.
Now for my problem.
When I have one or the other of the volume controls on with the other shut off everything sounds fine but when I turn up the volume control for the other channel the amp starts to pulse. The B+ voltages fluctuate between 350 and 175 volts and the sound of the amp is like someone is turning the volume up and down. The voltages all fluctuate and sound like a very slow and intense tremolo and continue to do so even if I have the trem footswitch shorted and the tremolo control turned down all the way. The speed control doesn't effect the speed of this pulsing. When only one volume control is on the amp sounds good, however the tremolo is a little weak but I don't know really what it is supposed to sound like.
I've checked voltages and have the following to report:
V1 12AX7: P1 232V, P2 0V, P3 3V, P6 106V, P7 0V, P8 .8V
V2 12AX7: P1 190V, P2 23V, P3 82V, P6 164V, P7 .001V, P8 1.2V
V3 6v6: P3 249V, P4 338V (fluctuates) P5 - to .02V, P8 19.5V
V4 6v6: P3 341V, P4 338V (fluctuates) P5 - to .02V, P8 19.5V
V5 6AU6: P1 -.1 to 1.8V (fluctuates), P5 170-260V (fluctuates), P6 86 to 120V (fluctuates), P7 0V
I checked the B+ voltages and they are 350V, 335V and 273V.
I have 19V across the cathode resistor and bypass cap
When I check the bias I get 35mA on V3 and 32.5mA on V4
Once again I am mystified as to what the heck is going on. It seems to be related to the gain stages from each channel causing the power tube control grids to turn up and down.
Any help would be greatly appreciated!

[RWood]

When I have one or the other of the volume controls on with the other shut off everything sounds fine but when I turn up the volume control for the other channel the amp starts to pulse.

Are you using the stock/non-shorting input jacks? If so turning the volume on with nothing plugged in would make for a nasty ground hum. I recommend switching to the Switchcraft shorting jacks so that your input in sent to ground when nothing is plugged in. That way raising the volume will have no audible effect.

If you have already made this change, then something is oscillating and wreaking havoc with your power tubes' current draw. You can pull preamp tubes individually to see which one's removal makes the voltages stabilize, then look at your lead dress around that part of the circuit. You can also chopstick it with the tube in to see what, if anything, makes it change.

I've checked voltages and have the following to report:
V3 6v6: P3 249V, P4 338V (fluctuates) P5 - to .02V, P8 19.5V
V4 6v6: P3 341V, P4 338V (fluctuates) P5 - to .02V, P8 19.5V

Are you sure this is correct? One plate is 90 volts lower then the other in a push/pull circuit? If so, this is a problem.
Your grid-to-cathode voltages are identical from tube to tube.
I would swap power tubes and see if the lower voltage follows the tube. If it does, replace that tube and recheck voltages. If it does not, something else is wrong and I would start by measuring, with the amp off and drained, the resistance from the OT center tap to each plate connection (pin 3 on your power tube sockets.) Original schem says you should get approx 175Ω to each leg. This will not prove that the OT is good, but it might indicate if it is bad.

RWood

ps 1482 was my first amp, purchased third-hand in 1968! Had a lot of fun with that amp; even had the footswitch and the plaid cover.

[davohilts]

I did put in grounding jacks and added a ground near the jacks which I have grounded all the jacks plus the footswitch jack to.
I am sorry to report that I did make a mistake typing in the voltages on the power tubes, V3 should have read 349V not 249V! Sorry!!
I am not getting any tremolo at all. I can measure voltage fluctuation on P5 of the power tubes but get no tremolo. When I turn up both volume controls all the way I get the pulsing which is out of phase with the guitar and sounds like a delay pedal, the speed control doesn't effect the pulsing. Turning one or the other volume control half way down stops this.
Checking my output transformer I get 176 ohms from center tap to p3 on V3 and 157 ohms on p4. My Bias mA measurements were 35mA on P3 and 32.5mA on P4.
The amp is really sounding good with 3 gain steps between the Microphone input jack and the hi/low instrument jacks. I'm missing something!! I've gone over the schematic and carefully followed the wiring to verify I've got it connected properly but I must be missing something!
Thanks for your help and suggestions.

[Regis]

Hey Dave, thanks for looking at my 1482 page.

Did you clean the tube sockets? Doing it on my 1482 pretty much cleaned up the motorboating.

The trem on mine is very weak tho, no idea why.

With those .68 caps in it the amp is very midrangy, I think I might fool with those some, maybe change the one on the phase inverter.

Regis

[davohilts]

Hey Regis!
Your page is really great! I have installed 1 mfd caps in place of the .68 on the phase inverter and really like the sound! I've cleaned and tightened the tube sockets and everything is pretty stable as far as the tubes go.
There were so many out of wack resistors so I just changed all of them out and have installed .022 mfd caps in the trem circuit along with a .047,and .47 in place of the .05 and .5. I have a couple of schematics and one calls for a .0068 mfd cap and on another schematic is calls for a .0068 which I think must be a typo.
This amp is pretty much driving me crazy because of the out of phase pulsing when I turn up both volume controls. The trem is gone completely now. It was working when I got the amp but everything else was so screwed up that it hardly mattered. I've got two 6AU6 tubes that test good on my Precision Tester and I'm getting some voltage change at the 330 K voltage dividers and voltage changing on P5 of the power tubes so I don't know why i'm not getting tremolo out of the speakers!
Anyway, its great to hear from you and be able to say thanks for such a great web page on the 1482. I also enjoyed looking at another page from a guy who somehow managed to get a new old stock 1482!! My mint Silvertone 1482 - poor man's Tweed Deluxe - Telecaster Guitar Forum

[davohilts]

here are a couple of gut shots in case anyone might notice something i'm overlooking!

[davohilts]

and the other side of the chassis

Note: i used some stereo input jacks but didn't connect to the other tip lug so they are mono!

[RWood]

Glad to read that your plate voltages are in line; that's a relief I'm sure.

In your first picture I see that the .47 trem cap is parallel to one of the heater wires. Have you chopsticked or carefully lifted those up to see if it has an impact? What you want to do is try to get an idea where the oscillation is coming from.

In general your heater filament wires could stand to be re-done. They should be tightly twisted pairs and either all hugging the chassis or all flying in the air - in both cases away from the signal carrying wires. I realize your amp came like this and it probably worked, too, as many of them do. Check out Merlin's site, Valve Wizard (or better, his books) for a best-practice example of how to wire a 12AX7 socket. No looping involved.

I's also be inclined to get the trem working with original values before you start to tweak. I personally like trems on the slower side, so I'm a tweaker too, but it is fairly easy to parallel a cap "by hand" to increase the capacitance of the trem caps.

You are probably closer than you realize.

[davohilts]

I've poked and prodded this baby until I am blue in the face without any effect. My trem circuit is to spec. I used .022 caps and .047 and .47 caps where they were supposed to go. I hope I'm close but don't know what to do. I've put an 8 ohm resistor in place of the speaker and run a signal through the amp and when I turn the volume controls up all the way the pulsing is present throughout the circuit. It is out of phase with the input like a delay pedal. I've got a bald spot on the side of my head from scratching it so much! I know the lead dress is horrible but it is a silvertone and I think by putting in the correct value resistors in place of the out of tolerance ones I've managed to insult my amp and it is just not going to work for me.
So if I want to slow the trem down I add a .022 cap in parallel and get a .044 cap value, right? I might try that and see what happens.
One other item of interest:
The original OT connections on this amp are supposedly as follows: looking at the transformer side with the connection strip there are three connections. The middle is grounded to the chassis with a screw, the left is 4 ohm connection the right is 8 ohm connection. The amp came with the 2 speaker wires connected to the 4 and 8 ohm connections but not the ground. I've tried moving the neg. lead to ground and keeping the 8 ohm connection on the positive side going to the positive post of the speaker but there doesn't seem to be any difference in sound or any effect on the pulsing delay issue. Doesn't it seem strange to have a connection like this (4 and 8 ohm posts going to the speaker) and have it work normally? Sheesh!

[davohilts]

Solved my problem! I bypassed the 2.2M resistor coming off he 6AU6 tube for more gain. I went back and removed all my supposed modifications one by one and finally the trem came back and is sounding great and the amp is stable. Sometimes (not always) i'm my own worst enemy!
Thanks for all you ideas and help!
Dave

[RWood]

Nice work! Bet you learned a lot along the way, too.

[davohilts]

yes, that is what is the most rewarding part about fixing a broken amp, not only does it sound great and it will live another 50 years but I feel like i've learned so much, yet there is so much more to learn!