A fun project.

[KC2TAU]

Since it's been so nice out during the day time I'm usually riding around town on my bicycle and so by the time I get inside it's late and 40 watts or so past 7:30pm doesn't play very well on AM. However during the evening hours when everyone has gone to bed I still have my fix of working on old odds and ends. I've recently been restoring a pair of Wurlitzer 6420 amplifiers that came out of powered tone cabinets for their line of organs. They give about 22 watts from a pair of 6L6's and have a 6SN7 preamp and 5U4 rectifier.

I plan on using them as a mono block stereo setup for my listening room downstairs. I've been paying a lot of attention to my soldering joints and component dress as I want these to look *neat* underneath. The wires themselves are a bit spaghetti and I'm still unsure if I want to work on shortening those. I don't want Mr. Oscillation to pay a visit, though. I also plan on using them as a sort of "working portfolio" for my "on the side" electronics repair business. They're not done quite yet but I thought you folks might like to see a few pictures.

[KC2TAU]

A few more pictures...

[KC2TAU]

And one more picture!

[Opcom]

Those are the output transformers under the chassis there. Nice units. Cathode bias.. How is the cathode bypass cap? was it good enough to retain?

[W7TFO]

I'e seen & heard these before up in Pocatello ID, and were very nice sounding amps for music reproduction.

They always looked like Wurlitzer was in a hurry building them, but used quality parts.

Kudos!

73DG

[KC2TAU]

Those are the output transformers under the chassis there. Nice units. Cathode bias.. How is the cathode bypass cap? was it good enough to retain?

I've always been of the idea that when you're dealing with bias circuits to not take any chances and change any electrolytics. I don't want one going south and taking out a tube. I still have the originals and I made sure to not chop the leads off too close to the body so when I eventually buy a cap tester such as one of the Sprague TO series I'll see how they measure up.

Those are indeed the OPT's underneath the chassis. There is plenty of room on top so I'm unsure why they put them underneath there. It makes the whole unit look neater from the top, though.

When delivering the second amp the courier folks must have used it for a pickup game of football as the larger of the two can caps was broken off of its mounting. I'm going to replace all of the filter capacitors but I do want to reattach that just so that everything looks factory from the outside.

I'e seen & heard these before up in Pocatello ID, and were very nice sounding amps for music reproduction.

They always looked like Wurlitzer was in a hurry building them, but used quality parts.

Kudos!

73DG

The parts seem to be of fine quality though a few of the resistors had to go but what do you expect for something that is 60 years old? Terminal strip construction makes it as little of a hassle as it'll ever be. I had to trim down all of the leads of the resistors in the amp in the last two photos as they were all quite long. As you surmised it certainly seemed like someone wanted to head home that day!

I'm excited to see how they sound. I have a pair of Altec Lansing Santana II's that I rescued that will most likely be paired with these.

[KC2TAU]

Both amps are now done. This was an extremely fun project but it was also very challenging to keep myself constantly reminded of the high standard that I want to achieve with all of my work. It takes a lot of energy out of you but when you're done with the project you look at what you've done and you can barely realize that you did that. The pride is worth the effort most certainly.

[W3GMS]

Congratulation Mike, it looks like you did a fine job!  Now play some, Iron Butterfly - In A Gadda Da Vida‏ through them and enjoy the sound! 

Joe, W3GMS

[KC2TAU]

Joe,

That'd actually be a good idea for giving them a good airing out. I'm waiting on the 7 pin plug that Wurlitzer used as their version of a multi pin plug so that I can hook up the second amplifier to a speaker. Once that arrives in the mail I'll take some video footage, upload it to YouTube and post the link here.

[W3GMS]

I have that one in vinyl !!  I think the last time it was played was at my birthday party when I was in my early 20's....a long long time ago    Speakers were Rectlinear 12's, the preamp was home brew driving a Dynaco Stereo 120.  The TT was a Pioneer I think.     

Joe, W3GMS

 

[KC2TAU]

I think I might have it in vinyl as well. I definitely have "Ball", though. Both are fantastic albums. The completion of this project will give me an excuse to bring the record player downstairs and go through the vinyl I have bought but have yet to listen to.

[K6JEK]

Beautiful work.  Nice amps.  I saw Iron Butterfly perform Ina Gadda Da Vida. Dum dum dadadadum dum dum dum. Still every time I hear it (not that often) I can't help but think of this:

A scientific expedition disembarks from its plane at the final outpost of civilisation in the deepest Amazon rain forest. They immediately notice the ceaseless thrumming of native drums. As they venture further into the bush, the drums never stop, day or night, for weeks. The lead scientist asks one of the natives about this, and the native's only reply is "Drums good. Drums never stop. Very bad if drums stop." The drumming continues, night and day, until one night, six weeks into the trip, when the jungle is suddenly silent. Immediately the natives run screaming from their huts, covering their ears. The scientists grab one boy and demand "What is it? The drums have stopped!" The terror-stricken youth replies "Yes! Drums stop! Very bad!" The scientists ask "Why? Why? What will happen?" Wide-eyed, the boy responds, " . . .bass solo!!!"

[W3GMS]

I think I might have it in vinyl as well. I definitely have "Ball", though. Both are fantastic albums. The completion of this project will give me an excuse to bring the record player downstairs and go through the vinyl I have bought but have yet to listen to.

I remember when I listened to it in the early 70's the TT had a Stanton cartridge in it.  It always sounded pretty good to me.  I remember one manufacture made a non magnetic cartridge which was touted to be good since it did not have any magnetic BH saturation curve to worry about.  I forget the name of the company that made that cartridge.  It may have been Micro Acostics or something like that.  I use to build all my loudspeaker enclosures back before I saved enough to buy the Rectilinears and had a lot of fun.  I did like the Rectlinears since they always sounded very natural to me, although those Altec Lansing tweeter horns were not to shabby.  I found some when I was 13 years old and built a cabinet for them along with some hefty woofers.  I had no idea about the need for cross over networks so I proceeded to blow them up real fast!  Live and learn....

Have fun,
Joe, W3GMS         

[KC2TAU]

I think I might have it in vinyl as well. I definitely have "Ball", though. Both are fantastic albums. The completion of this project will give me an excuse to bring the record player downstairs and go through the vinyl I have bought but have yet to listen to.

I remember when I listened to it in the early 70's the TT had a Stanton cartridge in it.  It always sounded pretty good to me.  I remember one manufacture made a non magnetic cartridge which was touted to be good since it did not have any magnetic BH saturation curve to worry about.  I forget the name of the company that made that cartridge.  It may have been Micro Acostics or something like that.  I use to build all my loudspeaker enclosures back before I saved enough to buy the Rectilinears and had a lot of fun.  I did like the Rectlinears since they always sounded very natural to me, although those Altec Lansing tweeter horns were not to shabby.  I found some when I was 13 years old and built a cabinet for them along with some hefty woofers.  I had no idea about the need for cross over networks so I proceeded to blow them up real fast!  Live and learn....

Have fun,
Joe, W3GMS         

Yes, I've been eyeing a Dual 1219 turntable that a fellow has for sale at a local audio shop. He wants far too much for it but I think I'll give him a "silly" (i.e realistic) offer and let that settle for a while. Perhaps after he sees that an offer has been made and that the table has been sitting there for a while he'll be more likely to agree.

The tweeters on the Santana line are a known weak point and, although they sound good, are prone to failure if too many watts are pushed through them. These amps make about 22 watts and so I doubt the tweeters will lift an eyebrow at that.

Crossover networks?! What do you mean that tweeter won't pass 50hz?!?!

Beautiful work.  Nice amps.  I saw Iron Butterfly perform Ina Gadda Da Vida. Dum dum dadadadum dum dum dum. Still every time I hear it (not that often) I can't help but think of this:

A scientific expedition disembarks from its plane at the final outpost of civilisation in the deepest Amazon rain forest. They immediately notice the ceaseless thrumming of native drums. As they venture further into the bush, the drums never stop, day or night, for weeks. The lead scientist asks one of the natives about this, and the native's only reply is "Drums good. Drums never stop. Very bad if drums stop." The drumming continues, night and day, until one night, six weeks into the trip, when the jungle is suddenly silent. Immediately the natives run screaming from their huts, covering their ears. The scientists grab one boy and demand "What is it? The drums have stopped!" The terror-stricken youth replies "Yes! Drums stop! Very bad!" The scientists ask "Why? Why? What will happen?" Wide-eyed, the boy responds, " . . .bass solo!!!"

Instrument solos that really are solo (i.e no band backing) are tough to do. You can't just sit there and noodle or else you'll just sound lost. The smart folks are those who incorporate a rhythm into the solo. You saw them perform live? Lucky. That reminds me of my father and uncle telling me how they saw Jimi Hendrix live. Sheeeeesh. 

[Opcom]

Very tidy work there. Do those two large resistors get pretty warm?

[KC2TAU]

Very tidy work there. Do those two large resistors get pretty warm?

It's interesting as the two units are a bit different. The wiring on the unit with two large Ohmite resistors is messier than the other one but on the other one the resistor leads were extremely long. Also as you can see some of the resistors in the unit that is pictured in the last picture are 1W or 2W units whereas there are 1/2W units in the other amp. Go figure. I haven't had a chance to test the temperature of resistors I have a laser temp gun that I'll use since I don't want to stick my fingers in there while the B+ is flowing.


The top resistor in the unit in the last picture apparently got warm at one point. I was wondering why that amplifier was still low on power and distorting when it hit its limit. I was missing 80V on the screens of the 6L6's and I traced around to try and find where that line went to. I then noticed that one wire was leading to that power resistor. I decided to measure that resistor to see what its value was. When I received a value that didn't fit the picture I flipped the resistor over to see what its value was as Ohmite would usually list the value on the resistor body. When I did so I noticed that some of the paint was flaking off of the resistor. I then figured that this resistor was loading down the screen supply and so I disconnected it. As soon as I did that the screen voltage came right up to the value that I measured in the other amplifier.

The input on that amplifier is an octal connector and so it leads me to believe that power resistor might have dropped the voltage to feed some other part of the organ. I'll look at the circuit this evening and see if I can figure out its rhyme and reason.

[K1JJ]

Pretty cool looking amp, Mike.  There's something about using a tube amp for music.

Now, for your next project: A workbench! Your back will love it. 

T

[KC2TAU]

I know, I know. That requires convincing my father that he doesn't really need the table in the basement as he does all of his computer work upstairs anyway. I certainly have collected enough gear to make a decent bench. I agree, though, doing troubleshooting work on the floor is.....not fun.

[KC2TAU]

I couldn't stand waiting around for the 7 pin plug to come in so I wired up the second amplifier with alligator clips. I was wondering why I wasn't getting audio output as I had it wired exactly like the plug. I heard a very low hum from the speaker and so I knew that the speaker was getting signal and yet no audio was getting through. I decided to take apart the plug to see what was up and to my relief it seemed I was missing a connection. Someone at some point had jumpered two pins with some wire. I jumpered the same pins on the other amp and it came to life.

I hooked up one of my shortwave portables that does a good job on FM to test it out. I haven't done conclusive tests on how much AC is sitting on the input so I don't want to go hooking up an MP3 player until I'm absolutely certain. Most of the FM stations around here use incredible amounts of compression and so everything sounds like "Csshhhhhhh" however the local classical station seems to ignore that agenda. They had a really nice choir piece on and I really couldn't believe how lovely and smooth all of the voices sounded. Getting vocals sounding good without any grain in them is tough and these amps had absolutely no problem. They're just incredibly clean and powerful. I couldn't really turn it up as folks in the house were sleeping so that will be for tomorrow.

If you're looking for a fun and affordable amplifier project then I really would recommend a pair of these.