wurlitzer 6420

[Ndirish147]

Getting ready to do the rebuild on these. I will try them with the field coil speakers first and then my choke. I do have a question about the schematic. what is the purpose of the cap and resistor that ties the plates of the 2 6l6's together?

 

[cademan]

It may be there to prevent oscillations.

 

[Ndirish147]

I looked at some other schematics and the only other place i could find it was in a guitar amp. In that case the resistor is a pot used as a brightness control for the amp. Would it be safe to remove this?

 

[RWood]

That looks like a zobel network and like cademan said it is there for stability. Yes, Dr Z uses these in his guitar amps. The brightness, or more typically "presence" controls come off the output transformer secondary, not primary as this one is.

I would keep it as-is, but you can certainly try it without that network. Then again, like century tek I would have kept the field coil speakers. I have a 7020 and that's what I plan to do....one day

 

[Ndirish147]

Thank you for the replies.... here is another question. It seems like the b+ goes through the field coil on pin 7 and comes out pins 1 and 3 but go to ground? assuming the the center tap of the rectifier circuit is grounded somehow off of the speaker 1 socket? sorry if this is not making sense

 

[cademan]

Center tap is chassis ground. The B+ is running through the field coil like it would a resistor. Have a look at the can capacitors. Which is negative? Center tap/chassis ground.

When those speaker plugs are pulled, they kill the B+ so you need them connected when testing of the original field coil speakers.

 

[Ndirish147]

ok i am having trouble wrapping my brain around this and i really want to understand it. the b+ goes through the field coil but where does it go after that? in the schematic b+ goes to the output transformer before it hits the field coil.

 

[s-petersen]

It looks like it leaves the field coil, and goes to the screens of the output tubes, and on to power the high voltage on the other tubes. I have worked on several jukebox amps like this, and to work on them without the speaker, I had to bypass the field coil with a resistor while testing.

Edit: I am having second thoughts, the more I look at it, the more I am starting to think it is just across the B+ and ground. It may not be used as a choke in this amp, I have to study it a little more.

Edit again,
yes, it looks like the field coil of the speaker is in series with the 1000 ohm resistor across the b+ and ground of the power supply, so not really a choke after all I think.

 

[Ndirish147]

Really appreciate the replies on this....It looks like all i would have to do is ground my center tap from the rectifier circuit in place of the field coil speaker.

 

[s-petersen]

Yes,It looks that way. According to the schematic the lead to the speaker socket pin 1 is, or gets grounded, there are 2 speaker sockets on the amp?

 

[Ndirish147]

yes 2 sockets. i wonder if speaker 1 gets less dc current than speaker 2 so they both grab it from different spots in the circuit

 

[s-petersen]

It looks like only the low frequency speaker has the field coil., according to the schematic there is only 1

 

[cademan]

Take a look at the end of that speaker plug. It should have a jumper wire on it at pins 1 & 3. I think that's what it is but I'll have to study this loop a little more. This is where it disconnects the center tap to the chassis when the speakers are unplugged. It might not be used a choke as its after the first filter capacitor and the primary B+ for the output transformer is connected before the choke. Most chokes are before the main filter. This one is after it.

When unplugged, center tap is lifted. When plugged back in, center tap is then connected back to the chassis which is ground and also chassis ground.

The treble speaker is a permanent magnet speaker and is just connected to the secondary side same as the bass while going through that divider or crossover network.

 

[century tek]

Now I have to have a second look at some of my field coil amplifier schematics. I thought all field coil speakers doubled as chokes. One of my amps uses a field coil speaker and is also the choke so now I'm going to have to look.

Usually a choke smooths out the ripple in rectified DC but if the field coil (choke) is after the output transformer center tap, then this design must not suffer from ripple??

 

[s-petersen]

I thought the same thing. The jukebox amps I worked on used the field coil for a choke, but when I looked at the schematic, I was surprised.

 

[Ndirish147]

Brought this amp up on a dbt last night and it seems to be working correctly. There is a little bit of hum but not too bad. I did have to jumper my center tap to ground in order to get this to work because i was not using the field coil speakers. All 4 speakers in the original cabinet are field coil speakers. This first amp that i am working on now was used to drive the 2 12 inch speakers and the other amp drives the 2 15 inch speakers. I will try to take some measurements tonight but i can already tell i am going to need more preamp gain

 

[s-petersen]

B+ may be a little high without the speaker loads, Check the B+ against the can cap voltage rating, if it is too high, you could install a resistor in place of your jumper. or you could insert your choke between the rectifier and first cap to reduce hum

 

[Ndirish147]

took a few measurements last night. I am getting 475 volts on the plates of the output tubes...pretty high luckily i am only letting this run for a minute or 2 at a time so i don't cook anything. I will try to figure out what my plate current is this weekend.

 

[cademan]

took a few measurements last night. I am getting 475 volts on the plates of the output tubes.

With or without the field coil speakers?

 

[Ndirish147]

Without... Only hooked up to a permanent magnet speaker