I Need Help With A Wurlitzer Juke Box Tube Amp

[Grainger49]

Paul,

Thanks for digging into this for me.  You have a good grasp of the circuit.  The fast heat doesn't seem to be working.  The relay doesn't ever pick.  I think of it as a kick-in-the-pants heater circuit.  I'm guessing without it the first bar of a record didn't come through the speaker.  However, I have measured and the heaters seem to have a constant 6V AC on them.

So I am not connecting the B+ without the 4000 ohm field coil in the circuit.  How incredibly inconvenient!  Maybe I'll just lug the speaker to my house.

There were some changes in the circuit, changes in all the high voltage cap values.  One smaller by 1uF, one larger by 10uF.  I'll dig out which is which later after getting some coffee. (it's 6 AM now)

I see that the center tap is grounded through the field coil.  So a 4000 ohm coil to ground.  I know I don't have a choke like that but might cobble together a 4k resistor.

Somehow the B+ bleeds off.  Well it never gets to the caps so it just dies when turned off.  Power is supplied from the AC plug which plugs into the sequencer box inside the Jukebox.  It cycles on and off with every 78 played. 

When it was cleaned up for sale the 5U4 was replaced by a 5Z3.  I have no idea if that is more robust.  They got the octal to four pin swap done precisely.

My first comment to David was that he should have a set of spare tubes in the bottom of the box at all times.  Maybe what the Wurlitzer service men did as well.

Edit:

I see that the center tap is grounded through the field coil.  So a 4000 ohm coil to ground.  I know I don't have a choke like that but might cobble together a 4k resistor.

 

[Paul Joppa]

The center tap is grounded by the wire from pin 1 to pin 3. The field coil runs from B+ to ground, independently. At least that's how I read the circuit - though I need reading glasses and some guesswork!

 

[Grainger49]

I agree on your read of the circuit.  If P=V2/R=(320)2/4000= 25.6Watts

I'm not going to be able to cobble together a 25W 4k resistor.  I guess I'll bring the speaker here.

I'm assuming that applying a ground reference will help the B+ appear on the cathodes and C15.

 

[Paul Joppa]

...
I'm assuming that applying a ground reference will help the B+ appear on the cathodes and C15.

Yes; that is the connection normally used to switch guitar amps to standby (heaters but no B+).

You could use a cliplead to make the ground connection, but without the heavy current drain of the field coil, the voltage will be significantly higher. And the PSU caps won't drain unless you put something across the power supply!

 

[Grainger49]

Ok, I have got it.  I will use a big 10k resistor to bleed the voltage off.  I don't want to get hold of that much voltage again. 

The power supply caps are all rated higher than the stock caps were.  I might have enough headroom there. 

 

[Grainger49]

BTW, Saturday the Wurlitzer man from Florida came up to work on the amp.  Capacitor #14, across the dead "Quick Start" relay was shorted.  So even if the relay was good it would never have seen voltage.  The shorted electrolytic was in parallel with a relay coil winding.

The problem solving went like this, first he swapped out the quick start relay, but voltages were still bad.  Then he discovered the electrolytic was shorted. 

Since the electrolytic was going bad for some time the Jukebox came up and played better than ever.  Meaning it never sounded so good.  And my Brother-In-Law, eye surgeon, can afford the service call.  My work was free.