Silvertone 1484 help needed

[waltk]

Hi All,

I just finished rebuilding my Silvertone 1484. This is my first tube amp project, so I guess it's not a huge surprise that it doesn't work (although I was pretty careful with my work). If anyone familiar with this amp can point me in the right direction, I sure would appreciate it.

So by "rebuilding" I mean that I replaced almost every cap and resistor in it. After putting it all back together, here's the state it's in...

• It has power, and the red jewel lights up.
• When I switch from standby to operate, I can hear a faint hum from the speakers, but nothing from the guitar makes it through the preamp section.
• The voltages around the 6L6 tubes appear to be normal (~475 at pin 3, and -34 at the junction of 330K resistors from pin5).
• The voltage at the center input of the output transformer is ~480.
• The voltages specified on the schematic for the 6CG7 tubes are all very close.
• The voltages on the 12AX7 tubes (V1 and V2) are way off - with pins 1 and 6 reading about -.7 for both tubes (instead of the 93v or 100v they are supposed to be.

Any suggestions about where to look for my mistake would be very helpful (as I am quite stumped). I'll gladly answer questions and take measurements where needed.

-Walt

[olddawg]

I have one waiting for me to restore. It is bone stock, even down to the original tubes. There was a recent on this site post where someone very knowlegebly stated that there are misprints in the schematic. If you are following that schematic you may be grounding something you shouldn't.

[waltk]

Thanks for the reply. I saw that other post, and have correctly wired the socket in the trem section (which has pins swapped on the schematic), and also have not grounded the two 330K resistors that go to pins 5 on the 6L6 tubes.

So I guess I'm looking for something else... anybody? Please?

[olddawg]

Do you have a scope or an injection probe to signal trace?

[JerryP]

Do you hear noise thru the speaker when you test the voltages at the power tubes?
Start back tracking the voltages to the power supply from the 12AX7. What's the voltage on the other side of plate resistors R7 & R8? Since the voltage at the 6CG7's is about right I would expect you have the correct voltage before the plate resistors. If not I would check wiring from the plate resistors to the power supply.
Jerry

[soundmasterg]

Walt, if you have such a low voltage on the plates of the 12AX7's, then they are not drawing any current. Something is miswired on those tubes. Check the dropping resistors (R58 and R59) over at the can cap and you should have voltages above 300v there....at least at the first one. If you have voltages there that are in that range, then you've got a problem at the tube sockets. Plate resistors faulty or incorrect values or grounded out. Cathode resistors the same. Pins shorting that shouldn't be, etc. Check over your work very carefully. If you had a signal genrator and a scope you could track the signal from the input through the amp and find where your problem is.

Greg

[waltk]

Thank you all so much for the suggestions! I'm new to tube amps (other than playing through them), and new here - so your experience and willingness to share it is invaluable to me.

I do have a scope, a homemade test oscillator for signal injection, and a homemade audio probe for tracing a signal - however I didn't need any of these to find the problem - just your good advice, a little extra thought, and a fresh look at my work.

My approach in this rebuild was purely mechanical - without taking the time to understand the circuit. The can cap values were way out of spec and leaky, so I replaced it with three separate caps. My mistake was that I simply forgot to put the 68K resistors (R58 and R59) between them, so most of the plates had no power. I put these in and everything worked - tremolo, reverb (such as it is), and everything.

Now I only have to wait 'til the wife and kids are out of the house and crank it.

-Walt

[Enzo]

You are missing your B+ voltage on the tube with no voltage on the plate.

Look for an open R58,R59 over by the main filter caps. And if they are OK< look for the wires trailing from the filters over to the points in the circuit. You may have neglected to connect one. SInce V2 is also dead, R58 becomes the main suspect.

And if you replaced a lot of resistors, make sure you installed a 68k there and not a 680k.

[waltk]

Hi Enzo,

The problem has already been fixed (see my last post). You were exactly correct - R58 and R59 were completely missing after my rebuild. I just forgot to put them back in after replacing the can cap with individual ones.

Thanks for your reply! It's amazing to me that some of you are so good at this that you can diagnose the problem accurately from a sketchy description of the symptoms.

Side note: I notice that you are from Lansing. Coincidentally, I bought this amp from a guy in the mid-70's in Lansing/East Lansing while at MSU.

-Walt

[Enzo]

Great! Not sure how I overlooked your last post, I only recall the one above yours.