| Author |
Topic: dickerson amp |
Michael Butler
From:
California, USA
|
Posted 21 Jul 2015 4:16 pm |
|
a friend of mine purchased a dickerson amp. it is one put out by fators prior to magnatone. i've been unable to find the exact schematic but on the dickerson/magnatone site i've found a close resemblance. a lot of the parts look old, some parts seem to not be attached and even tho there is a cap can with two 20mf/450v and one 20mf/25v, there are also a lot of other electrolytics connected.
i'm enclosing a ton of pictures and the closest schematic i could find, so i hope i'm not inundating anyone but i would like some recommendations as to how to approach this. oh, and i can't figure out why the holes don't align with the chassis and cab.
thanks in advance.
mike B.
[img]]http://www.magnatoneamps.com/schematics/magna_M195-4J.jpg[/img]
_________________
please see my Snakeskin's Virtual Music Museum below.
http://muscmp.wordpress.com/
Last edited by Michael Butler on 21 Jul 2015 6:40 pm; edited 1 time in total |
|
|
|
Blake Hawkins
From:
Florida
|
Posted 21 Jul 2015 6:23 pm
|
|
Did not see the schematic. However, I can tell you what I'd do with it. I would first do a continuity check on that field coil speaker and make sure the field coil is good. Ditto for the voice coil.
I would do continuity checks on each winding of each transformer.
If you want the amp playable, you will need to replace every capacitor both tubular and electrolytic
in the amp. Also you will need to check every resistor to make sure it is within tolerance.
Would be a good idea to spray the pots with a cleaner/lube,
If you have a tube tester, check the tubes.
Otherwise just put them in and slowly power up the amp witching the voltages after you have done all the above.
The result will be a restored, usable, amplifier. |
|
|
|
Michael Butler
From:
California, USA
|
Posted 21 Jul 2015 6:37 pm
|
|
| Blake Hawkins wrote: |
Did not see the schematic. However, I can tell you what I'd do with it. I would first do a continuity check on that field coil speaker and make sure the field coil is good. Ditto for the voice coil.
I would do continuity checks on each winding of each transformer.
If you want the amp playable, you will need to replace every capacitor both tubular and electrolytic
in the amp. Also you will need to check every resistor to make sure it is within tolerance.
Would be a good idea to spray the pots with a cleaner/lube,
If you have a tube tester, check the tubes.
Otherwise just put them in and slowly power up the amp witching the voltages after you have done all the above.
The result will be a restored, usable, amplifier. |
here is the schematic again. hope it works this time.
thanks for the info, blake!
http://www.magnatoneamps.com/schematics/magna_M195-4J.jpg
_________________
please see my Snakeskin's Virtual Music Museum below.
http://muscmp.wordpress.com/ |
|
|
|
Dave Campbell
From:
Nova Scotia, Canada
|
Posted 22 Jul 2015 3:14 am
|
|
| i would absolutely replace those caps before you do anything else. blisters on them means that they're ready to blow, which will cause a pretty big mess. |
|
|
|
Blake Hawkins
From:
Florida
|
Posted 22 Jul 2015 4:45 am
|
|
Ok, Michael, I've got the schematic.
If the tube line up in your amp is the same,
I'd go through compare each connection and component value with the way the amp is actually wired and note down any variations. That will give an accurate
picture of what you have.
Then you can start doing things.
Are you going to do the work.
Since I don't know your level of electronic skill,
I don't want to be overly simple or too complicated to make sense.
It is a nice amp and certainly worth restoring.
Your photos are excellent and make a great reference.
My guess is that those 450 volt electrolytic were wired in to replace the caps in the can which are also surely bad. Cans are available now, but it has been practice for many years to leave the old can in place and just wire in replacements. |
|
|
|
Michael Butler
From:
California, USA
|
Posted 22 Jul 2015 12:08 pm
|
|
| Blake Hawkins wrote: |
Ok, Michael, I've got the schematic.
If the tube line up in your amp is the same,
I'd go through compare each connection and component value with the way the amp is actually wired and note down any variations. That will give an accurate
picture of what you have.
Then you can start doing things.
Are you going to do the work.
Since I don't know your level of electronic skill,
I don't want to be overly simple or too complicated to make sense.
It is a nice amp and certainly worth restoring.
Your photos are excellent and make a great reference.
My guess is that those 450 volt electrolytic were wired in to replace the caps in the can which are also surely bad. Cans are available now, but it has been practice for many years to leave the old can in place and just wire in replacements. |
blake: i'm a hobbyist and certainly not a tech but i have been successful in maintaining my amps. i mostly have replaced electrolytics and added the 3 prong plugs.
note that this schematic is not perfect since it has the extra input and extra volume control but i'm pretty sure i can work around that. the tube complement is the same with 5y3, 6sj7, 6j5 and 6L6. i do see that the cap can as well as the outboard electrolytics are all connected which is one of the confusing things. i usually replace them with F&T from amplified parts and mark a red X on the cap can so everyone knows it is inoperable.
i have already used deoxit in the tube sockets and pots and i have a knight tube tester so i can check the tubes as well as it does it.
i would like to check the viability of the transformers but i am not sure of that procedure. primary vs. secondary, i guess.
thanks for your assistance, i really appreciate it.
mikeB.
_________________
please see my Snakeskin's Virtual Music Museum below.
http://muscmp.wordpress.com/ |
|
|
|
Blake Hawkins
From:
Florida
|
Posted 22 Jul 2015 1:29 pm
|
|
The safest way to check the action of the power transformer is to use a variac to apply voltage to the primary. Attach your Voltmeter on the AC scale with clip leads to the plate terminals of the rectifier tube.
Slowly bring the voltage up, watching the readings.
Also keep an eye and nose out for smoke.
If all goes well, when you reach full line voltage,
you will have 500 to 600 volts.
Shut it down and repeat the procedure with the clips across the rectifier filament pins where you should
have 5 volts and across the pilot light terminals where the voltage should be 6.3 volts. Of course, since the tubes are not in their sockets, the readings will be a bit higher.
You can verify the centertap ground with the power disconnected and the Ohmmeter reading between each rectifier filament pin and the chassis.
It will be a fun project and compliment your experience. Thanks for letting me know about your previous amp work. The Knight Tube Tester is handy to have and will do a good job for you. |
|
|
|
Blake Hawkins
From:
Florida
|
Posted 22 Jul 2015 1:44 pm
|
|
A note about the method of wiring the replacement filter caps. From your photo it looks as if the previous tech simply wired the new cap to the terminals of the can. In the "old days" this was frequently done and it worked as long as the can cap had failed "open" but there is most always some resistance left in the can and it could short in the future.
When you wire in your replacements, install them with a terminal strip and leave nothing connected to the can. If you can't do or don't have a terminal strip, let me know. I can either give you one
and suggest am alternate method. |
|
|
|
Michael Butler
From:
California, USA
|
Posted 22 Jul 2015 2:10 pm
|
|
| Blake Hawkins wrote: |
A note about the method of wiring the replacement filter caps. From your photo it looks as if the previous tech simply wired the new cap to the terminals of the can. In the "old days" this was frequently done and it worked as long as the can cap had failed "open" but there is most always some resistance left in the can and it could short in the future.
When you wire in your replacements, install them with a terminal strip and leave nothing connected to the can. If you can't do or don't have a terminal strip, let me know. I can either give you one
and suggest am alternate method. |
in other amps, when i replace the caps, i still use the cap can as ground for them. seems like it should be ok since the can is connected to the chassis. but, is it?
thanks,
_________________
please see my Snakeskin's Virtual Music Museum below.
http://muscmp.wordpress.com/ |
|
|
|
Blake Hawkins
From:
Florida
|
Posted 22 Jul 2015 9:32 pm
|
|
| Using the amp can for a ground is fine. No problem there. It is possible to obtain a replacement can. I think http://www.tubesandmore.com (Antique Electronic Supply) has them. There are also other sources for replacement cans. You should be able to find something very close to the original. Cost is about $15 to $20. |
|
|
|
Donny Hinson
From:
Glen Burnie, Md. U.S.A.
|
Posted 23 Jul 2015 1:23 am
|
|
| By looking at the resistors and caps, I'd say this is a very old amp. And, with older amps, it is common for people to replace the old can-type caps with axial-lead types, and just leave the old can-type caps mounted on the amp. Also, keep in mind that this amp was not designed for modern tubes. Most of those old amps that used a 6L6 came with either a metal tube, or a glass "coke-bottle" type. They're not the same as modern 6L6GC versions. |
|
|
|
Stephen Cowell
From:
Round Rock, Texas, USA
|
Posted 23 Jul 2015 4:56 am
|
|
New production can caps are available... I put one in my brown Princeton and it worked great.
Normally these amps take a pair of 6V6... new tubes work fine in this application.
_________________
Too much junk to list... always getting more. |
|
|
|
