Need some help figuring out a Wurlitzer 6240. - diyAudio
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Old 16th August 2010, 08:59 PM   #1
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Default Need some help figuring out a Wurlitzer 6240.

Hello,

I recently picked up what seems to be a Wurlitzer 6420 that I plan to turn into a guitar amplifier or perhaps a nice mono power amp. Before I get into this,though,I just need a bit of clarification.

So the input goes through pins 2 and 3,right? In the second picture you'll see a silver braided strap that is attached to the chassis,this is a ground strap,correct? I'm going to end up converting this to a three prong plug either way.

Another question I have I wonder if the item in the third picture a crossover unit? The terminals are labeled "woofer","mid range" and "treble". I know the 6420's were used with field coil speakers but does this crossover allow the use of permanent magnet speakers with this?

Any tips with these? I've heard the gain produced from the 6SN7 is quite low(too low for guitar)and that a preamp will be required.

Thanks!


Images:

http://img24.imageshack.us/img24/6609/dsc06474p.jpg

http://img217.imageshack.us/img217/3919/dsc06476s.jpg

http://img6.imageshack.us/img6/198/dsc06477g.jpg

Last edited by 928GTS; 16th August 2010 at 09:06 PM.
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Old 16th August 2010, 09:35 PM   #2
M Gregg is offline M Gregg  United Kingdom
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So the input goes through pins 2 and 3,right? In the second picture you'll see a silver braided strap that is attached to the chassis,this is a ground strap,correct? I'm going to end up converting this to a three prong plug either way.

928GTS,

Please slow down,

A few things I have not seen a Wurlitzer 6420.
However I do not see a mains transformer? This means it is going to be dangerous if you connect the live connection to chassis!

I would not advise you to use a non isolated mains supply amp (you may get an electric shock) Also if the input is not capacitor coupled then mechanical damage to the input tube could make the grid to come into contact with the plate HV.

I appologise if it has a mains transformer, just thinking of your safety!

I am sure others may be able to give you more information!

Regards
M. Gregg
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Old 17th August 2010, 12:09 AM   #3
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The PT is sitting on top of the chassis and the OT is located underneath. The input is also coupled via a capacitor.
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Old 17th August 2010, 12:48 AM   #4
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First the easy ones. Yes that is a crossover. The only things off of it worth salvaging are the chokes and the hardware. The caps are long past their useful life. I can't tell for sure but it looks like the choke on the right might have gotten a bit warm but it may just be some lacquer.

Yep, looks like a ground strap.

Now for a bit of bad news. One 6SN7 isn't going to be nearly enough gain for a guitar amp. Now it could make for the start of a nice mono system.

It doesn't look like it is set up for a FC speaker unless the FC(s) are energized through a separate connector or PS as there is no power connection visible at the speaker leads connection to the crossover.

Some shots up her skirt might shed some light on that aspect.
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Old 17th August 2010, 01:20 AM   #5
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Yes,those caps look ancient and they,as well as the the caps underneath,will be replaced.

I was planning on probably just getting it running and using it as a mono system and then methodically work out a good preamp for it while enjoying it as is.

I'll take a picture of the chassis this evening and post it. Thanks for your information!
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Old 17th August 2010, 03:16 AM   #6
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Here are some chassis photos for you.


http://img375.imageshack.us/img375/1443/dsc06487i.jpg

http://img375.imageshack.us/img375/7453/dsc06488e.jpg

http://img713.imageshack.us/img713/4829/dsc06490z.jpg

http://img13.imageshack.us/img13/9703/dsc06491p.jpg


Don't worry,those old caps will be replaced and the resistors will be checked for value.
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Old 17th August 2010, 09:06 AM   #7
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by 928GTS View Post
The PT is sitting on top of the chassis and the OT is located underneath. The input is also coupled via a capacitor.
Nice to see the underneath!

Now you have the drama of any components you change will change the tone of the amp. When I rebuilt some old valve radios this was very evident. Also a fender champ I built from Aspen's Tube Book. I managed to find some new old stock paper caps to hear the difference!

Hope it goes well!

Regards
M. Gregg
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Old 17th August 2010, 10:02 AM   #8
M Gregg is offline M Gregg  United Kingdom
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Just for information,

I always fit a 2W resistor value 1M - 500K across the supply caps to ensure they discharge after switch off. Old components will discharge depending upon how good or bad the insulation is!

We all make mistakes including me! And after going back to an amp the next morning assuming it to be dead started soldering a resistor and as I touched the solder to the leg of the resistor got a shock I will always remember!

So now as custom and practice I fit discharge resistors and test before I touch!

Regards
M. Gregg

Last edited by M Gregg; 17th August 2010 at 10:04 AM.
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Old 17th August 2010, 05:28 PM   #9
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Quote:
Originally Posted by M Gregg View Post
Nice to see the underneath!

Now you have the drama of any components you change will change the tone of the amp. When I rebuilt some old valve radios this was very evident. Also a fender champ I built from Aspen's Tube Book. I managed to find some new old stock paper caps to hear the difference!

Hope it goes well!

Regards
M. Gregg
Thanks for the well wishes. The caps underneath really needed to go. They're the usual wax fare that you see in amps of this vintage and they were literally crumbling when I'd touch them with a screwdriver. Right now I'm just going to replace the caps to get it up and running and then I'll play around with the values and brands to have some fun.

Thanks for reminding me about the bleeder resistors,most of the items I work on already have them installed and it really does help give you a bit of calm when working on them. Before I work on an electrical project I always probe around with my DVM and see if there is any charge stored in any of the caps or somewhere in the circuit before starting work.

My tough lesson in the power of electricity came when I had finished playing guitar through an old Lafayette tube amplifier I went over to turn it off and instead of hitting the on/off switch my hand touched one of the filter caps. I certainly received a "nice" wake up.
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Old 18th August 2010, 04:29 AM   #10
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So I finished up replacing a few of the caps(I just have the two electrolytics and the filter caps left)and I thought I'd have a bit of fun and see if it would work. It does and it sounds quite nice! I had my walkman running into it and a spare floorstanding speaker was used for output.

I didn't see any redplating nor did I hear any undue noises. I checked the PT afterward and it was only slightly warm which was good to know it wasn't drawing too much current. I still need to install bleeder resistors so I was quite careful to check the voltage around the circuit before I proceeded to handle it. It was really weird when very little residual voltage was present even across the filter caps. Even so this isn't an excuse to not install bleeder resistors...

So it does seem that it is set up for permanent magnet operation. The slight problem was the fact that with the crossover setup I was able to get good volume out of the posts labeled "woofer" and rather reduced output out of the posts labeled "mid range". I'm going to guess and say that this is because of how the capacitors in the crossover network are probably quite perished.

Those choke transformers,what are they used for in this circuit? Is it absolutely necessary to use the crossover or could one theoretically just bypass the crossover if they wanted to use just one speaker and hook up the speaker to the red and black leads leaving the amplifier?
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