I had put out a wanted ad on Craigslist for various vintage electronic bits and bobs and was pleasantly surprised when a fellow e-mailed me saying that he had an amplifier that had come from an old Wurlitzer speaker cabinet. When I went to take a gander at it I had noticed that it was a powered extension cabinet for a Wurlitzer organ. The fellow who owned it wanted to keep the extension cabinet for a chest or some sort of storage which struck me as a bit odd.

When I brought the amp home I spent some time and at least replaced the paper capacitors as they looked well past it. I then decided for laughs to bring it up on a variac and see how it would cope. To my surprise the amp woke up from its slumber and worked well enough. The amp was original supposed to use 6L6/6L6G tubes but this one was equipped with Tung Sol 5881's. I was surprised that the 50 year old can electrolytics were still doing their job. After playing around with it for a while I decided to see if I could find a matching unit for it so that I could make a mono block setup.

After much searching I figured that this model, the 6420, was quite difficult to come by. I then had the fortune of coming across a post on a forum where a fellow mentioned that he had 6 or so of them sitting around. I sent him an e-mail and, after having negotiated a price, had one sent to my address. Upon arriving it seemed that the courier service had decided to use the package as a football for a pickup game as the large can capacitor had been broken off of its mounting. Fortunately the other ground straps were still soldered to the chassis and intact.

The amp had already been recapped though I wish it could have been a neater job. This amp worked straight away and compared to the other amp it seemed to have quite a bit more punch to it. I had figured that the electrolytics in that amp were quite tired and needed to be replaced. I decided to perform a full recap and evaluation of the first amp.

A fair amount of time had passed since I had recapped the first amp and I had done a lot of soldering in that time so my ability had improved. I spent a considerable amount of time making sure the component dress was neat and orderly. As well as using these amps for personal use I'll be using them as demonstration units of my work for my "on the side" hobby electronic repair business. I wanted to make the underside of these amps look like art and to show that real care, time and consideration had gone into them.

My first stage was to replace all of the electrolytics in the amp and so I used a JJ 40/20/20/20mfd to replace the main can cap. I bridged two of the 20mfd sections to create the 40mfd I needed to replace the can capacitor as it originally was 50/40/20mfd. I used an F&T 30mfd to replace the single section capacitor and Sprague TVA 25mfd/25V to replace the two 20mfd/25v electrolytics. The coupling capacitors used were old stock Cornell Dubilier WMF's and one F-Dyne polyester.

After completing this I made voltage measurements on the good amplifier and then compared them to the good amplifier. I noticed that some of the voltages on both the preamp were down and that I was 80v down on the screen supply of the 5881. I decided to go through the amp and test each resistor and found that some were out of tolerance. To test my theory I did some quick substitution of some modern metal film resistors into the circuit. Upon powering up the amp I found no real significant improvement.

At this point I was a bit confused as if the resistors were good and the capacitors were fresh then what could be the problem? I then decided to compare the two amps and see if I noticed any differences. The second amplifier seemed to be of an earlier build date due to the presence of dog bone resistors as well as there being more cloth covered wire used. I had remembered that some of these units had been setup for use with field coil speakers hence there being two speaker out plugs. One is used with regular speakers whereas the other is used with field coil units. The speaker plugs are 7 pin plugs. Two of these pins are connected to the OPT whereas the other pins are used to carry voltages back to the organ.

In doing some testing of the voltages originating from the filter capacitors I had noticed that a section of the main filter capacitor connected to one of the 7 pin plugs and then to a large Ohmite power resistor. I realized that I had yet to check the resistances of both power resistors. I also noticed that the second amp only had one of these power resistors where as the amp at fault had two.

I measured this odd power resistor on the first amp and it gave a bit of a peculiar reading. I then decided to flip the resistor around as Ohmite usually put the ohmage in plain writing right on the resistor body. When I flipped it around I had noticed that some of the paint covering the resistor had turned black and some was flaking off! Perhaps this resistor was loading down the filter capacitor and so for laughs I disconnected this resistor from the circuit and fired up the amp. To my relief the voltages on both the preamp and the power amplifier came up to match those of the second amplifier.

Next on the equation is to drill mounting holes on the second amplifier for an RCA jack to match that on the first amp. I also need to install new strain reliefs and grounded plugs on both. I then need to go through and recap the second amplifier as well as clean up the lead dress on the resistors since they were wired in a more carefree manner. Once I receive my second speaker plug I'll be ready to do some stereo testing! These will most likely being powering a set of Altec Lansing Santana II's.

Here a few pictures. I'll try and post some more over the coming days. I hope you enjoyed the story.


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I've done some work on the second amplifier. I still need to drill out the rivets holding in the old caps. I also need to install the filter capacitors and do a few other odds and ends. Enjoy the pictures!



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I'd recommend checking the resistors and replacing at least all that are out of tolerance.

On both amplifiers I removed each resistor, tested it, shortened its leads and replaced it if necessary. If there were shortcuts to be taken they were not taken with this build.

The amps are now done! I'm extremely excited to complete this large project. I'm really pleased with how they came out. I'm happy to have been able to take two old soldiers and bring them back to glory.

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Is there some way you could tell me where the voltages are on your amplifier? I am rebuilding one of these and though it powers up and there is sound, the sound is quite distorted and low in volume. I just changed all the caps and electrolytics and checked the voltages. I know it is not my tubes and I've tried multiple input sources with the same sound issue. Unlike in your situation I don't have a good working amp to reference voltages with! Here is what I am getting voltagewise:

6SN7 TUBE
pin 2 = 142v
pin 3 = 4
pin 4 = 150
pin 5 = 310

6L6 (V1)
pin 3 = 430
pin 4 = 430
pin 5 = 320
pin 8 = 23

6L6 (V2)
pin 2 = 430
pin 3 = 320
pin 8 = 25

where pin 3 of the 6L6s go the cap C5 (.0005mfd / 1600vdc I am getting 430v on both legs of the cap

5U4 Tube
pin 2 = 440
pins 4 and 6 meter bounced around into negatives then jumped high, couldn't get reading
pin 8 = 440

ELECTROLYTICS
positive end of C8 (30mfd / 500vdc = 438v
C6 = 23v
C7 and C8 = 320v

Can anybody tell me where the problem might be based on these voltages? Without knowing where they should be (why aren't they on the schematic?) I have no way of diagnosing the issue. Any help appreciated.

Resistor values drift too. Sometimes very badly. Have you tried to input a 1khz or so, sine wave and follow it stage by stage to see where your distortion is coming from? Have you checked the plate current of the power tubes during idle? Perhaps the amp is oscillating ultrasonically. Just a couple of ideas.

I went in and checked the resistors and as one might suspect they have drifted a bit all a bit upwards, I know the wall socket voltage in 1949 was a bit less back then when they made these amps so that is a factor too. While checking I found one of the R7s was not functioning so I swapped out the R7 from my other amp and fired it up and it works great now. No distortion, 10x the volume (though I'm not pushing it in that area at all considering the age of the equipment and the fact that it hasn't been used for over 10 years!). Wonderful sounding speakers, the amp powers 2 at a time, either a pair of 15" magnavox or a pair of 12" Jensen A12 field coil speakers, both pairs sound excellent and unique. Currently using original tubes that came with it - all RCA except for 1 Tung Sol 6L6, probably will experiment with tubes see what sounds best.

Going to start rebuilding the 2nd amp, have to order some parts: new R7 and R10 resistor, new 5U4G tube socket which got cooked when the amp overheated - apparently the transformer that got so hot it melted the wax which oozed out and dripped onto other parts of the chassy (see photo), these amps are mounted vertical inside the tone cab which is why there is wax on some of the resistors and the far wall of the chassy. Looking forward to having them both up and running.

I take it you have both pairs of speakers? If so would it be possible to run one 15" and one 12" per amp then add a couple tweeters or maybe a pair of Altec horns would be more at home here?

I bet you that would sound interesting! How would you power the tweeters though if both amps were used up powering the field coils? would you use a third amp???

Right now I'm exploring preamp options. I can currently power one amp at a time using a Trek UC1A pedal which is very clean, has adjustable tone and permits the input of an instrument with a 1/4" input. It is nice but what I'm really looking to do is get a tube stereo preamp of some type so I can run both amps. I don't have a LOT of money to spend but want to get something that brings out the best in these really nice speakers. If I could find something vintage that needs some attention for a good price I'd do that. Any suggestions? I'm eyeing things like a Bell 2418 stereo tube pre, audible illusions modulus 1, a couple old tube heathkit stereo pres, still learning to discern the differences between them.

I suppose the other option is run one speaker through the Trek and then buy a mono preamp and run the other with that...

You could do a three way setup using the two amps. Just build a passive crossover. You could use a low pass on the 15" at 500 Hz then a 500-2,500 Hz band pass on the 12" and a 2,500 high pass on the tweeter.

Or just do a two way with a 2,500 Hz low pass on the 15" and 12" then a 2,500 Hz high pass on the tweeter.

I've run the input from the line out of an MP3 player and it actually drives the 6SN7 just fine. You can build a preamp and you will most likely improve the sound quality but you can do quite well without it for the time being.

I actually need to take voltage measurements for someone else who is having an issue with their 6420 so I'll post them here as well when I'm done. How does one amp power two speakers? The right speaker connector has connections on, if I remember correctly, pins 4 and 5 to the output transformer.

Chances are the speakers are wired in parallel with maybe the field coils in series like the two 12" speakers in a Magnavox CR-198C are wired.

I noticed inside of my 7 pin speaker out cables that pins 1 and 3 are jumpered together. By jumpering pins 1 and 3 on the AMP speaker outputs would that then ground the voltage that would power the field coils, thus allowing them to use normal permanent magnet speakers?

The speaker output then would be coming from pins 4 and 6 right? When first powering these amps up after I rebuilt them I tried going this route (sacrificial speaker route) but there was no output - wasn't sure if I did this wrong or not. Here is what I did:

Jumpered pins 1 and 3 on both speaker outputs on the amp, then alligator clipped speaker wires to pins 4 and 6 on the first speaker out. Did I do this right? It is possible my sacrificial speakers had been previously sacrificed henceforth thwarting my initial efforts, I'll have to check to make sure they actually work! I'd be curious to know though in the event that some day I do want to connect permanet magnet speakers to these amps.

Heres the schematic for reference, are the speakers wired in parallel? Both speakers are 8 ohm and have field coil readings of 5200 ohms.

Which pins does the field coil connect to on the plug?

Field is powered by pins 1 and 7. And like I mentioned pin 1 is jumpered to pin 3 inside the speaker plug.

You would have to leave both field coils connected or replace them with suitable resistors which then could require larger value filter capacitors as those field coils usually perform the function of a choke as well.

Here are the voltages that I measured from my amp.

6SN7:

Pin 1: 0vdc
Pin 2: 147.1vdc
Pin 3: 5.54vdc
Pin 4: 67.5vdc
Pin 5: 262.5vdc
Pin 6: 106.1vdc
Pin 7 & 8: Heater voltage

6L6 Left:

Pin 1: 0vdc
Pin 2: Heater
Pin 3: 403.2vdc
Pin 4: 396.5vdc
Pin 5: .016vdc
Pin 6: 0vdc
Pin 7: Heater
Pin 8: 31.73vdc

6L6 Right:

Pin 1: 0vdc
Pin 2: Heater
Pin 3: 400vdc
Pin 4: 395vdc
Pin 5: 0vdc
Pin 6: 0vdc
Pin 7: Heater
Pin 8: 31.72vdc

5U4:

Pin 1: 415vdc
Pin 2: 0vdc
Pin 3: 393vac
Pin 4: 0vdc
Pin 5: 395vac
Pin 7: 413vdc
Pin 8: 0vdc

Hi,

I've just run across this thread. I have (4) of these sitting on my shelf. Maybe I should say I have 3 complete and the output and power transformer out of the 4th one. These can be run with regular 8 ohm speakers by hooking up the 8 ohm speaker (in my case a Altec Model 9) to pin 4 common and pin 5 the 8ohm out. You will need to put a 6K resistor large wattage between pins 1 and 7 and a jumper between pin 1 and pin 3. There will be a little hum present on the output. The amps are capable of 18 watt output.

As is they sound ok not a lot of bass. In my case I find the circuit less than what I consider musical so I'm in the process of gutting several of these in order to build something that will sound better. I have a nice driver circuit that is made up of a 6Sl7 input and a long tail 6sn7. I've used this in the Dynaco stereo 70 I have and it sounds great. Much better than the single 6SN7 the Wurlitzer has. I think I will run the output stage triode.

I do have a question concerning the C5 and R14 the .0005 cap in series with the 56K resistor on the secondary of the transformer attached to the plates of the 6L6.

Do I need this cap and resistor?

Oh, I have a different schematic mine is of the Model 400 tone cabinet. It has different operating voltages than shown.

burned you should get a triode amp and send the 6L6 amp to me

Yes you could use a high wattage resistor if you wanted to use with modern speakers, but it would be easier to use a choke in series with the resistor. That way you still have the filtering that was provided by the field coil.

Would be nice to pair the speakers with a couple tweeters (Altec 811B or similar horns would work good) and a proper pair of boxes which could even be made to hold the amps as well. Could even buy an audiophile grade remote controlled volume control/input module to control the volume to the speakers or a regular preamp.