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
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
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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
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
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.
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.
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!
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!
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.
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.
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
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
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
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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.
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?
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?
I noticed these links!
http://www.diyaudio.com/forums/tubes-valves/22154-wurlitzer-schematic-needed-6420-amp.html
http://www.diyaudio.com/forums/tubes-valves/168724-wurlitzer-6420-audio-amplifiers.html
The last one has a schematic, however I do not know how close they are to your amp!
Just wondered if the guy ever completed his drawing?
Might be worth a quick PM!
Regards
M. Gregg
http://www.diyaudio.com/forums/tubes-valves/22154-wurlitzer-schematic-needed-6420-amp.html
http://www.diyaudio.com/forums/tubes-valves/168724-wurlitzer-6420-audio-amplifiers.html
The last one has a schematic, however I do not know how close they are to your amp!
Just wondered if the guy ever completed his drawing?
Might be worth a quick PM!
Regards
M. Gregg
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Thanks for the links M Gregg,I actually have been following that schematic when it comes to recapping this amplifier. They do seem to be quite close to identical in layout. It's a little bit hard to read at times but it is much better than nothing.
Just on a whim I hooked up a speaker directly to the red and black leads exiting the amplifier and it does sound quite nice and balanced without the crossover. I'll rebuild the crossover after I have the amp 100% restored. Even playing through a cheap floorstanding speaker(I don't want to run it through one of my nice speakers just yet)I'm really impressed with the power and clarity of this unit. Hmm,maybe I should try and find another and make a monoblock setup.
Just on a whim I hooked up a speaker directly to the red and black leads exiting the amplifier and it does sound quite nice and balanced without the crossover. I'll rebuild the crossover after I have the amp 100% restored. Even playing through a cheap floorstanding speaker(I don't want to run it through one of my nice speakers just yet)I'm really impressed with the power and clarity of this unit. Hmm,maybe I should try and find another and make a monoblock setup.
928GTS,
A chinese link shows a few mods and Schematic I did try translating it however the auto translation is very poor! If you want it in word format just send me your E-mail as I said it is very poor!
http://www.wowhififever.com/Wurlitzer/Wurlitzer.htm
Regards
M. Gregg
A chinese link shows a few mods and Schematic I did try translating it however the auto translation is very poor! If you want it in word format just send me your E-mail as I said it is very poor!
http://www.wowhififever.com/Wurlitzer/Wurlitzer.htm
Regards
M. Gregg
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A note about field coil speakers. The field coil takes the place of the permanent magnet on modern speakers. It is totally unrelated to the voice coil. The amplifier output drives a voice coil and has nothing to do with the field coil. The only concern when switching from field coil to PM speaker is what role the field coil MIGHT have played in the amp power supply.
Some field coils are wired in series with the B+ in place of where a choke would have been. The current through the coil energizes it. SOme others are wired across the B+, acting as a load for that supply. In the first case, series, obviously if the field coil is removed, the circuit will not operate. But conversion is easily done by the insertion of a choke or a power resistor. In the case of the shunt wiring, if the field coil is removed, the circuit still works, but the B+ voltages wil rise due to the missing bottom end of the voltage divider action of the whole circuit.
So any amplifier can drive either kind of speaker as long as one way or another the voice coil gets a magnetic field to push against.
I don;t see the Wurlitzers as much, but I have rebuilt TONS of old Seeburg tube amps and I keep a power resistor with clip leads attached in my drawer just for the purpose of bypassing the need for a field coil during service.
Some field coils are wired in series with the B+ in place of where a choke would have been. The current through the coil energizes it. SOme others are wired across the B+, acting as a load for that supply. In the first case, series, obviously if the field coil is removed, the circuit will not operate. But conversion is easily done by the insertion of a choke or a power resistor. In the case of the shunt wiring, if the field coil is removed, the circuit still works, but the B+ voltages wil rise due to the missing bottom end of the voltage divider action of the whole circuit.
So any amplifier can drive either kind of speaker as long as one way or another the voice coil gets a magnetic field to push against.
I don;t see the Wurlitzers as much, but I have rebuilt TONS of old Seeburg tube amps and I keep a power resistor with clip leads attached in my drawer just for the purpose of bypassing the need for a field coil during service.
No need to use the crossover at all if you are using a full range speaker or a speaker system with it's own crossover. The chokes (A.K.A coils or inductors) present a high impedance to high frequencies while the capacitors present a high impedance to low frequencies. They are used in combination to route the desired frequencies to the desired drivers.
Thanks for the explanation mashaffer,I was reading up on chokes and couldn't quite understand where they were coming from but your explanation makes a lot of sense.
I'll make a video of the amp playing and post it on YouTube for you folks.
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