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 Post subject: Re: Silvertone Model 1484 Guitar Amp
PostPosted: Nov Mon 19, 2018 11:42 pm 
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Joined: Dec Wed 24, 2014 7:34 pm
Posts: 2017
Location: Weimar, Texas
Well I ran into my first snag. I left the wiring to the 3 terminal strip on it when I took the terminal strip off to clean the chassis. I cut the two (green) reverb tank wires off and left a little green on them. In other words, I know it was wired this way. Besides that I have photos. Now that I have explained all of that, heres some photos followed by a question-

The 2.2MΩ (actually it's about 15% high which is not a good sign for the rest of them) connects from one wire from the "out side" of the reverb tank and S1 Reverb Depth switch to ground. The other wire on the "out side" of the reverb tank goes to ground. If the switch is closed then the reverb tank is in the circuit. I think...

Image

In the following snippet of the schematic I've added the 2.2MΩ resistor where I think it is in the circuit. Actually I know that's where it is :) But it's not on the schematic so I guess I have the wrong, only one I could find, schematic... OOICF schematic...

Why did they add a resistor?

Image

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 Post subject: Re: Silvertone Model 1484 Guitar Amp
PostPosted: Nov Tue 20, 2018 4:37 am 
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Joined: May Sat 06, 2006 4:03 am
Posts: 3476
Location: ZIP 23831 South of Richmond, VA 25 miles down the pike.
If I understand you need to know where the 2.2 meg is located. It is R49. Here's a schematic and parts list. My SAMS copy is so marked up I can no longer figure what I did on one that I fixed for a fellow. So it would be no good. I had parts with different values from what was shown on the schematic. And it came from the factory that way. Link below to SEARS data.

Bill J.

https://app.box.com/s/cqt834sv4gqsfl1yb6o5r0ecbdm5e3mc


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 Post subject: Re: Silvertone Model 1484 Guitar Amp
PostPosted: Nov Tue 20, 2018 6:05 am 
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Joined: Dec Wed 24, 2014 7:34 pm
Posts: 2017
Location: Weimar, Texas
NumberMaj 1 wrote:
If I understand you need to know where the 2.2 meg is located. It is R49. Here's a schematic and parts list. My SAMS copy is so marked up I can no longer figure what I did on one that I fixed for a fellow. So it would be no good. I had parts with different values from what was shown on the schematic. And it came from the factory that way. Link below to SEARS data.

Bill J.

https://app.box.com/s/cqt834sv4gqsfl1yb6o5r0ecbdm5e3mc


Well... yes and no. See R49 on the schematic (my schematic matches what you linked to)?

Image

That's not how it's wired. You can see the 2.2MΩ resistor in the top center of the photo below. One side of R49 ties to ground, with the reverb tank, and one side of R49 ties to a terminal lug with the other reverb tank wire (green) and a red shielded wire that goes to the Reverb Depth switch. The other side of the Reverb Depth switch is wired to the foot switch jack with a shielded red wire and then another shielded red wire connects from the same point on the foot switch jack to the point between R60 and R61. That's how it was wired when I got it. I have pics of how the amp was wired before I started working on it and that part hasn't changed... The wiring around V5, V7, and V8 isn't complete because a lot of it is on the power supply board. Pin 7 of V5 is wired to the power supply board. I'll look at it with fresh eyes in the morning

Image

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 Post subject: Re: Silvertone Model 1484 Guitar Amp
PostPosted: Nov Tue 20, 2018 5:52 pm 
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Joined: Dec Wed 24, 2014 7:34 pm
Posts: 2017
Location: Weimar, Texas
I dug into the power supply board and the schematic in general. There is only one 2.2MΩ resistor on the parts list and schematic BUT there are two 2.2MΩ resistors in the amp. One is right where it's supposed to be (R49) and one is added (the resistor in the reverb tank circuit). R49 is on the power supply board and is in the circuit as it shows.

Much to my dismay it looks like I have some of the caps installed wrong on the PS board. I thought I replaced them one at a time so I am not 100% on that. It doesn't look right compared to the schematic though... More studying is required.

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 Post subject: Re: Silvertone Model 1484 Guitar Amp
PostPosted: Nov Wed 21, 2018 5:47 pm 
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Location: Vancouver Island, Canada
I'm glad you found that second resistor. Now you know, if the reverb tank doesn't work, you can always try going back to the original circuit as shown in the schematic.
Your question was why add that resistor?
I can't answer that question, but imagine there was a noise issue that they were trying to control.

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 Post subject: Re: Silvertone Model 1484 Guitar Amp
PostPosted: Nov Thu 22, 2018 5:34 am 
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Joined: Dec Wed 24, 2014 7:34 pm
Posts: 2017
Location: Weimar, Texas
I spent a few hours working through the power supply board today. Technically speaking it's not just a power supply board, it also contains part of the tremolo circuit. I mentioned finding some caps installed wrong on the tremolo circuit. I fixed that... and then put it back like it was. I thought I was wrong once and found out that I was wrong about being wrong :oops: :lol: Most of what I did today involved tracing out the off board connections plus fining some missing parts.

I found where I had pulled a pair of 25V 25µF electrolytics and not replaced them. And the 5µF 450V negative bias electrolytic was missing. While checking resistors for being in tolerance I found an oddball. R41, by the schematic and parts list, is a 1/2W 68kΩ resistor. My amp has a 1/2W 150kΩ resistor in it's place. The screwdriver in this photo is pointed at the resistor in question. I'm starting to understand the reverb circuit but not 100% confident.

Image

Schematic section. There's a pen pointing at R41. I almost cut the bottom off but you can see the neg buss (gnd) highlighted in green at the bottom:

Image

The original amp had a 20µF/10µf/5µf 450V electrolytic can. I found a 20µF/20µF can that fit nicely on top and worked for the 20µF/10MF service but I still needed to replace the 5µF section. All I have in hand are 10µF 450V electrolytics so that's what I went with. I tucked under the choke mounted to the back like so (choke has been removed to make it easier to wire this area)

Image

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 Post subject: Re: Silvertone Model 1484 Guitar Amp
PostPosted: Nov Fri 23, 2018 4:02 am 
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That resistor appears to be setting the bias for that side of the 12AX7, which as you know, is a duo-triode with the cathodes separated. It seems the engineers decided to increase the bias.

With that grade of carbon comp resistor, anywhere within 20% is similar to the specs when the amp was built.

In a reverb tank, the signal is fed thru long springs which vibrate at the same tempo as the music. Quite often there is an amplifier circuit included to compensate for loss.
But the best explanation for how it works is that there is magic mojo in there. :lol:

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 Post subject: Re: Silvertone Model 1484 Guitar Amp
PostPosted: Nov Fri 23, 2018 4:30 am 
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Joined: Dec Wed 24, 2014 7:34 pm
Posts: 2017
Location: Weimar, Texas
westcoastjohn wrote:
That resistor appears to be setting the bias for that side of the 12AX7, which as you know, is a duo-triode with the cathodes separated. It seems the engineers decided to increase the bias.

With that grade of carbon comp resistor, anywhere within 20% is similar to the specs when the amp was built.

In a reverb tank, the signal is fed thru long springs which vibrate at the same tempo as the music. Quite often there is an amplifier circuit included to compensate for loss.
But the best explanation for how it works is that there is magic mojo in there. :lol:


My genius self pointed the pen at the wrong resistor. They doubled the value of R41, more than doubled actually, which is on the grid. I finally chased the signal around and around (not the best schematic imho) and figured out the reverb circuit (not including magic mojo, some things should remain a mystery.

I replaced about a dozen resistors. About 1/2 were out of tolerance ( the amp wasn't stored very well) and about 1/2 were at the upper end of their tolerance range. I replaced in kind, ie did not correct to the schematic. The amp had never been worked on and if not for poor storage it would have been pristine.

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 Post subject: Re: Silvertone Model 1484 Guitar Amp
PostPosted: Nov Fri 23, 2018 5:35 am 
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Yes, I remember now that a typical reverb circuit has a driver stage or stages ahead of the springs. Then sometimes there will be another triode to amplify the reverbed signal.

Tubes are the most fun to work with for that, but transistors can be used and work as well with much less voltage and current draw. Ages ago, I built a reverb pedal that uses a small reverb can and 3 transistors that works with a 15 volt printer power supply.

There is a stand alone reverb unit in a cabinet that was built by Fender. It would be enlightening to find a schematic for one of those.
If you listen to Surfer tunes, you will hear occasionally a surf hooligan beating on a priceless Fender Reverb cabinet with drum sticks. 8)

This amp is going to sound great when you get it finished. I guarantee it will be too loud.

I found this schematic that shows how the Fender amps were wired, for comparison.
There is an electromagnet at either end of the reverb spring, (transducers) that send and receive the pulsations. The clean signal is mixed with the reverb signal, and there is also a feedback loop for echo effect or depth.


Attachments:
Reverb_Schematic.png
Reverb_Schematic.png [ 235.63 KiB | Viewed 617 times ]

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