RickshawRecords
Joined: Mar 07, 2006
Posts: 80
San Diego, CA
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Posted on Sep 04 2010 08:59 PM
A have a help question for Tube-Gurus:
I'm un-modding a hot-rodded Silvertone 1484. Stock schematic here:
http://www.freeinfosociety.com/electronics/schematics/audio/silvertone1484.pdf
Question: Shouldn't the cathodes of the output tubes be tied to ground?
There is an adjacent error in the schematic - The 330K bias resistors do NOT go to ground in the middle. I found that out the hard way. (Goodbye D5!)
However, with everything back, all the power supply voltages are good, but there is ZERO output.
It seems like a simple mistake to make: The guy drawing the circuit transposes the dot at ground from the cathodes to the bias resistors... It could happen!
It also looks like both pin 8's once had another wire soldered to each of them... Which could have been cut off and discarded by the hot-rodder.
Finally, most Fender Push-Pull amps have grounded cathodes, a-La the Bandmaster here:
http://www.schematicheaven.com/fenderamps/bandmaster_ab763_schem.pdf
Many thanks in advance.
— Complete DIY Ribbon Microphone plans available from:
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Last edited: Sep 04, 2010 23:39:08
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jimmyjazz
Joined: Apr 20, 2007
Posts: 47
Twin Cities
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Posted on Sep 04 2010 09:24 PM
I didn't spend any time looking at the schematic but for fixed bias, the cathodes are tied to ground.
You could also throw in a little resistor between the cathodes and ground if you want to measure the bias current. Like a 10 ohm 1/2w.
<a> you don't want the bias voltage tied to ground as you found out.
— Looking for some jazz and a little libations - js
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RickshawRecords
Joined: Mar 07, 2006
Posts: 80
San Diego, CA
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Posted on Sep 04 2010 09:58 PM
Thank you Jimmy Jazz!
I was 90% sure this was the case, and I'm gonna go try it now.
Edit: (30 min later) That was it. This amp sounds really great now.
So, for anyone in the future searching the internet for help with the Silvertone Twin Twelve 1484 schematic, here are the schematic errors I found in my early 1963 model, Chassis # 185.11040:
- The 330K bias resistors on the power tubes DO NOT go to ground.
- The cathodes of the output tubes are connected together, and they DO go to ground. (The circuit artist mistakenly placed the ground connection dot on the bias resistors, not on the Cathodes.)
- On V6, the first stage of the tremolo driver, Pins 6 and 8 are reversed. Pin 6 is the Plate, 8 is the Cathode
- The coupling capacitor C12 between V2 and V3 is .02uF like all the others, not .01uF as shown.
- On some internet schematics, a line has been drawn under R6 (V1), which appears to connect the bottom of R6 to ground. This is not a connection, just a stray line.
- In the original Sears schematics floating around online, the top of the 1MEG Reverb Depth pot (R37) goes to the grid (pin 7) of V2 (which is also the wiper of R21). <u>This is wrong.</u> R37 should not connect there, but rather to the bottom of R21 (the 1MEG Volume Pot) and the wiper of R19 (Channel 2's 1MEG Treble pot).
After troubleshooting for many, many hours, I am now intimately familiar with this amp, so if you have any questions, please email me at RickshawRecords@yahoo.com
EDIT: Feb 2011>
Clear, corrected schematics and a reproduction owner's manual now posted on my website:
http://diyribbonmic.com/sears_silvertone_1484_twin_twelve/
— Complete DIY Ribbon Microphone plans available from:
www.RickshawRecords.com
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