The amp I have that schematic for is a suite case type with 4 8" speakers in each half.
If you PM me your email adress I will try to get the schematic out to you.
Montgomery Ward 35JDR8421
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Stevem
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Re: Montgomery Ward 35JDR8421
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Montgomery Ward 35JDR8421
Bumping an old thread since I have one of these on my bench.
Interesting amp, very similar to the Danelectro Commando
http://www.blueguitar.org/new/schem/mis ... mmando.gif
The vibrato uses a 12sj7 instead of the 6au6 in the Commando Schematic. I've worked on a Commando before that matched the linked schematic except the vibrato used a 6sj7. They both use an interesting bias setup for the output tubes. On the Commando, the output cathodes are connected to the filaments of the 2 12ax7's then to ground. The Montgomery Ward only has one 12ax7 so there is a 39R resistor in series with the 12ax7 filaments. Someone has added a 75R resistor in series for a total of ~113R. The bypass cap has also been replaced with a 12u/25v cap - not sure what the original was, the Commando used 10u.
So my first question: The 39R resistor has drifted and I want to replace it. Will also be replacing the cap. My (poorly educated) guess is that I want to shoot for ~25vdc on the cathodes of the 6v6's and ~12vdc between the resistor and the 12ax7. Does that seem right or am I completely missing the mark? That would also mean that the voltage rating of the cap should be upped to at least 50v.
Second question: The amp has 82R screen resistors on the 6v6's. Is there any reason not to up them to 100R since I have plenty of those on hand?
Interesting amp, very similar to the Danelectro Commando
http://www.blueguitar.org/new/schem/mis ... mmando.gif
The vibrato uses a 12sj7 instead of the 6au6 in the Commando Schematic. I've worked on a Commando before that matched the linked schematic except the vibrato used a 6sj7. They both use an interesting bias setup for the output tubes. On the Commando, the output cathodes are connected to the filaments of the 2 12ax7's then to ground. The Montgomery Ward only has one 12ax7 so there is a 39R resistor in series with the 12ax7 filaments. Someone has added a 75R resistor in series for a total of ~113R. The bypass cap has also been replaced with a 12u/25v cap - not sure what the original was, the Commando used 10u.
So my first question: The 39R resistor has drifted and I want to replace it. Will also be replacing the cap. My (poorly educated) guess is that I want to shoot for ~25vdc on the cathodes of the 6v6's and ~12vdc between the resistor and the 12ax7. Does that seem right or am I completely missing the mark? That would also mean that the voltage rating of the cap should be upped to at least 50v.
Second question: The amp has 82R screen resistors on the 6v6's. Is there any reason not to up them to 100R since I have plenty of those on hand?
Deric®
Re: Montgomery Ward 35JDR8421
FYI it looks like there's asbestos in the amp shown in these pics. If this material is in your amp you should seriously consider removing from the amp and properly disposing it.
TT
TT
Re: Montgomery Ward 35JDR8421
Thanks, will let the owner know.
For now, amp is up and running. Went with 100R for the 6v6 screens. Left the 75R/39R bias resistor in place and get ~7vdc across the 12ax7 filaments. Shorted the added 75R and get 12vdc. I'm guessing whoever worked on it before didn't realize the 12ax7 filaments are in series and tried to drop the voltage closer to the typical 6.3vac when the filaments are paralleled. Going to remove the extra 75R and replace the bias cap with a similar value.
Will post some pics when it's done.
For now, amp is up and running. Went with 100R for the 6v6 screens. Left the 75R/39R bias resistor in place and get ~7vdc across the 12ax7 filaments. Shorted the added 75R and get 12vdc. I'm guessing whoever worked on it before didn't realize the 12ax7 filaments are in series and tried to drop the voltage closer to the typical 6.3vac when the filaments are paralleled. Going to remove the extra 75R and replace the bias cap with a similar value.
Will post some pics when it's done.
Deric®