Wurlitzer 660. I got the Wurlitzer bug.

[Carbsandcaps]

So I got bored waiting for parts and figured since I’m down to only one capacitor in the signal path I’d look into using an interstage transformer there too. The only one I have was the replacement for the 701 that I built. I know it’s not right for this instance but wanted to see if it would work. It’s going from the #30 plate to the 45 driver grid.

When I hooked it up it sounded like it was playing through headphones very little volume and no bass whatsoever. I decided to try putting a 470k resistor between leg and ground on the opposite side of the 45 grid (again just experimenting) which loaded the grid enough to make decent volume but still no bass. I understand that you can load the grid with resistance or inductance but I’m not sure how much inductance I would need for this to work. This transformer only has about 40hy. Most interstage transformers are being used for output stages. I found a few posts that touch on the subject but I can’t find any definite formulas or instructions to match inductance to certain tubes.

 

[Carbsandcaps]

I came across these two really cool looking thordarson interstage 2:1 transformers. I already hooked on up temporarily and it sounded great so I’ll go ahead and mount them and see how they end up.

 

[Carbsandcaps]

Well I threw in the towel on the interstage transformers (for now) in the preamp stage and went with some military grade pio sprague caps instead. I was barely getting any bass from the interstage transformers. The caps measured .051uf and .052uf after 70 years so that’s pretty good.

I spent quite a bit of time getting the voltages right. Either they changed the schematic at some point or one of them has a weak transformer because the voltages were way off from each other. Trying different resistors in different spots over a few days at one point one of them was motorboating so bad I thought it was going to blow my speakers. I ended up with a 1k ohm in one amp and a 20k ohm at R33. I also had to change the grounding resistor on the negative side. All said and done I got the voltages and biasing within 2% of each other and I’m happy with that. I ditched the adjustable biasing pots for an L setup for reliability. I also found a gold hammered paint which I think looks a lot closer to the original coloring than the copper paint. I also left the 1B5 in just to keep the voltages correct without yet another power resistor. It also keeps the look correct.

I’m super happy with the results. They sound amazing and very well balanced. They have such an interesting bass response it’s hard to explain. It’s bassy but soft at the same time. The overall sound is silky smooth. It’s amazing how much work, weight, heat, and power these things use for making 4wpc but that is some of the best 4 watts I’ve ever heard!

 

[Carbsandcaps]

The newest and probably final update for these because they just sound great. I decided to look into fixed biasing the other tubes in the amp. I was originally going to try the 30 preamp tube but the. Realized that there would then be negative voltage on the input jack which can’t be good for anything so I just stuck with filament bias with a 220ohm resistor from the negative filament to ground. After working with the dc voltage supply I got a nice 2v on the heaters and -13.5v on the grids. This also eliminated the 1b5 from the circuit completely so it’s just in there as a dummy for looks which is fine it’s not like you could see the 2v heater glowing anyway.

The. I moved onto the 45 driver tube. I wanted to do this so I could get rid of the cathode biasing and the subsequent bypass capacitor for a clean sound. Using the same negative supply for the output tubes. I used an L setup with a 220k resistor from the negative supply to the interstage transformer for the output and the grid of the driver tube. I used a high resistor to eliminate any crosstalk from driver grid to output grids and vice versa. I then used a potentiometer from grid to ground to get the -52v bias voltage. The driver and outputs use the same full voltage at the plates so the biasing voltages are the same. I ended up with 300k to ground on one amp and 220k to ground on the other. I replaced the potentiometer with resistors and my bias supplies were set. All voltages were good and now it’s time for a test run. Holy moly did they sound good. So much more bass and midrange, just all around a big improvement. These may be the best sounding amps I’ve ever put together. That 4wpc goes such a long way. I will definitely be on the lookout for more of these!

 

[moozical]

Hi, I'm a little late to this party, but I'm wondering if anyone can offer some help with my Model 660. It has been nicely converted to a push-pull 45 amp and sounded great... until the power transformer shorted and is now frying the oil inside. Nasty stuff, but I can't be mad at an 80 year old relic. The part no. is 21549. Does anyone have any ideas of where I can get a replacement or if there are any modern options? I'd like to stay with the vintage one since that would look nicer than grafting on something that is ill fitting.

Thanks in advance for suggestions.

 

[Carbsandcaps]

That’s hard one because they have a hv output with a center tap, a 5v output, a 2.5v output. And a 2.5v output with a center tap. Have you measured the voltages with all the leads removed? Are you sure it’s shorted and not something else dragging it down? There’s a ton of resistors and capacitors in there that could be shorted.

 

[Carbsandcaps]

Also the 45’s are already ran over the recommended plate voltages and using them with 120v line voltage only makes it worse making everything work harder so im thinking if anything shorted its going to be multiplied.

 

[moozical]

Thanks for the reply, I do appreciate the help. I haven't measured anything yet. It scares me a bit since last night I was able to bring it to life using a variac and adding a CL60 to the line in wire (it was immediately blowing fuses upon startup) when I heard the transformer start to make frying noise and drops of oil leaked out of the bottom of the casing where the wires exit. This confused me since I didn't think the PT was potted, so maybe it's a capacitor leaking? I'm unsure of exactly how this was assembled to work, but it was done by a well-respected builder and I think it was No. 3 for him. I tried to reach out with no luck (unfortunately we're getting old and gray ). It worked fine for over a decade, but the PT did get plenty warm after an hour.

I can remove the PT and see about the voltages. Until I read this thread, I didn't know about it having caps/resistors inside the cover (if I read that right).

Thanks again, I hope to get this one back after 9 years of languishing in the "closet of technology". It's a special amp with Electraprint output and interstage transformers, and other goodies.

 

[gadget73]

Probably looking at either a custom job or a re-wind on the original if its done. In either case I'd get the primary specified for 120-125v to fix the voltage problems.

Heyboer is the usually-suggested source for re-winds. I'm having a set of output transformers in a semi-custom job made up by musicalpowersupplies. I think he also does power transformers, might be able to contact him with your transformer's specs and see if he can make one for you if Heyboer isn't available.

 

[Carbsandcaps]

Thanks for the reply, I do appreciate the help. I haven't measured anything yet. It scares me a bit since last night I was able to bring it to life using a variac and adding a CL60 to the line in wire (it was immediately blowing fuses upon startup) when I heard the transformer start to make frying noise and drops of oil leaked out of the bottom of the casing where the wires exit. This confused me since I didn't think the PT was potted, so maybe it's a capacitor leaking? I'm unsure of exactly how this was assembled to work, but it was done by a well-respected builder and I think it was No. 3 for him. I tried to reach out with no luck (unfortunately we're getting old and gray ). It worked fine for over a decade, but the PT did get plenty warm after an hour.

I can remove the PT and see about the voltages. Until I read this thread, I didn't know about it having caps/resistors inside the cover (if I read that right).

Thanks again, I hope to get this one back after 9 years of languishing in the "closet of technology". It's a special amp with Electraprint output and interstage transformers, and other goodies.

The goo coming out of the transformer could also be normal. All of the wiring coming out of mine was covered in a waxy gooey substance from the building process I’m sure. I would bet that the transformer isn’t the problem but I could be wrong. The stand alone wiring terminals in the underside are close to the chassis and since they are mounted only by a single spot on the middle of the strip they tend to rock like a seesaw. I’ve had globs of solder short to the chassis at least 3 times and drag down the b+. As a matter of fact it happened today while I was working on them.

 

[tubeactive]

No goo or leaking substance is normal. Wax insulation from inside melts, wires short eventually, inevitable if the tranny also runs hot...
Your re-painted 660s are gorgeous !
Keep up the good work....

 

[Carbsandcaps]

I said I was done but that was a lie because I can leave anything alone. I decided to go globe style so I started collecting them, at least the ones I could afford. I also decided I didn’t want to waste the open tube sockets because you can never get enough tubes. I had some 260 globe tubes that I could use in the 4 pin spot but I had to to find something for the odd 5 pin spot so I went to the internet and looked through as many pictures of tubes as I could and found that the 27 tube being a pentode would have the pin arraignment that I needed. Luckily they were only $8 for 2 on eBay so I ordered them up. Well you can’t have tubes that aren’t lit up can you? The 26 is a 1.5v filament and the 27 is a 2.5v filament so there’s no easy solution. I decided to try a 5v switching power supply from Amazon that was on sale for $9. Ordered 2. After trying different resistors I finally got them to light at their correct voltages. Even at 2.5v the 27 was extremely bright so I decided to lower it to about 1.7v and it gave it a nice orange glow.

The power supply is rated at 5 amps and the 2 tube and resistors were drawing about 3 so should be good there. I had to cut one leg off the 27 to fit then just wired up the sockets. Just enough room for the power supply.

Now you can’t have all globe tunes without a globe 80. He problem here is that I measured about 135ma with the amp on and the 80 is only rated for 125ma. I decided to try to cut as much amperage out as I could. And there was plenty. The majority of the resistance comes from making up for the lack of a field coil speaker. If I could manipulate the negative voltage from the back bias design than I should be able to lower the positive voltage without just dumping it to ground through a resistor. I got rid of all of the resistance to ground on the positive side and decreased the resistance to ground on the negative side. I found a perfect combination that gave me about -54v for bias and 270v for the plates. I did have to increase the resistor to the filament on the 30 tube since it runs right off b+. It all seems to settle in very nicely. I just put 2 220k resistors from the bias supply to the outputs and driver tubes and was in business with a lot amperage wasted on heat and more to the tubes. I ended up with 122ma draw from the 80 tube which is close but still within range. They still produce enough heat to keep you warm in the winter but they sound so sweet it’s worth it. (Still waiting on 1 more 230 globe tube to come in the mail)

 

[moozical]

Thanks for the replies guys. I'm more optimistic that I was two days ago. I hope to have time over the weekend to dismantle it and see what's happening. I agree with tubeactive that what I saw, smelled and heard isn't right. I have hope that the goo gods will smile upon me.

CandC, I'm very envious of your hard work and beautiful amps.

 

[moozical]

Oh well, it has become very obvious once I removed the transformer and peeked in the cover, that it's toasted. This poor thing had a very hard life with water damage and who knows what else. I'm surprised that it worked at all, let alone over a decade of intermittent use. I don't want to energize it, period. I'll be looking for a replacement. I may have one possible source, but it's a gamble. I'll let you guys know how that works out.

BTW, the goo was indeed the heavy waxy coating applied to the wires. There are no other electronic bits under the hood.

 

[Carbsandcaps]

I’m messing around online and it looks like the 771 amp uses a similar transformer may be even overkill. Just not sure if the high voltage tap is the same. There’s not much on eBay right now

 

[moozical]

I’m messing around online and it looks like the 771 amp uses a similar transformer may be even overkill. Just not sure if the high voltage tap is the same. There’s not much on eBay right now

Thanks, I saw those as well. As we say in the South, "slim pickin's!". I didn't realize these things have gotten more popular and depleted.

Thanks to the smart people here, I'm in contact with Mr. Butterfield and he may have something. I know I said that I wouldn't power mine up, but after a good night's sleep, I'm feeling less disgusted. If it doesn't erupt with noise and fumes, I may even measure a thing or two (totally against my nature since info and choices are bad).

 

[Carbsandcaps]

I’ve been listening to them on a table for long enough now. I decided to replace my fisher x-202b with them for now. They probably weigh twice as much and put out 1/10 the wattage but they just sound so good. Definitely hooked on triodes

 

[moozical]

Thanks to the advice from you guys, I got the old Wurlitzer running again. I was able to get a power transformer from Mr. Bill (mffe on epay). It was looking rough, but works well. Bill was nice and gracious, offering a partial refund if it was doa. It took me a while to sort out the connections, but I was fortunate to have no issues. It now sounds just like I remembered, great!

Mine is a stereo unit using 45's and we396 tubes. Being a stereo unit, there's a lot going on inside that I still don't quite understand. It has Electraprint interstage and output transformers. The PT still gets pretty warm after 90 minutes and I'm guessing it operates at around 120-140 degrees.

Here's a photo and thanks again for the help...

 

[Carbsandcaps]

Man it’s crowded in there I wonder where they stuffed the extra output transformer? Looks pretty though!

 

[moozical]

All four transformers are under the black hood. It is tight for sure, but they don't seem to mind. I put them in long ago back when you could phone Jack Elano and let him entertain you on transformer lore. He actually remembered the values I needed from a silver wound set ordered for another sister amp made by Willie Rivers (his personal version). I couldn't afford silver, but these did make a nice improvement.

This amp is heavy for an old man to hoist!