Soft Start Board Goof

[Grainger49]

There is a regulator, three pin semiconductor regulator, that is in the driver tube cathode circuit.  Look at the schematic for the driver board.  This could be a problem but someone more familiar with this circuit should point you to a possible problem in the cathode area.  I'm pretty sure it is black magic!

Try this, remove the 2A3 and see if it still stops at 21.8V.  I'm just trying to cut the circuit in half to see which half it affecting your voltage. I doubt this will do anything but it is worth a minute to check.

[InfernoSTi]

Grainger,

Without the 2A3 tube, the OB is 251v (at the 300K resistor on the OB side). 

I also took the effort to change out the parafeed cap (3.3uf) and that didn't make a difference.  Just wanted to make sure it wasn't the cause since things are right around that point. 

John

[Grainger49]

John, 

We are chasing two things.  The OB voltage should be the 2A3 voltage divided by three.  There should be ~460V on the 2A3 plate so 153V would be the target.  251V available says it should be tunable.

Did you check the voltage on the power supply side of the 2A3 plate choke?  I keep asking because the plate choke might be a problem.

[InfernoSTi]

Oh, THAT voltage!  

The power supply side of the PC-1 is 564v, the same as T16.  The T1 side of PC-2 is a falling voltage that started about 9v and fell to 7v when I stopped measuring...it was continuously falling.  This was WITH the 2A3 installed.

John

EDIT: And without the 2A3 in, I get the same power supply side voltage on the PC-2 and I tried adjusting the OB voltage and was able to reduce it to the correct 150v.  

I also checked the "in" and "out" sides of the PC-2 on the good amp:  494v at the power supply side and 476v at the parafeed cap side (measurements taken on the PC-2 terminals themselves.

And I swapped 2A3 tubes again (this time listening to the one I swapped out and it is fine...it plays nicely on my other amp so I didn't inadvertantly swap a bad tube for a bad tube).

[Grainger49]

For some reason you are not getting the voltage on A2 that should see.  If the tube just wasn't conducting the high voltage would all be at the plate, A2. 

Right now the only thing I can think of is a bad plate choke or a bad connection.  You have the voltage coming out of the power supply but it doesn't get to the plate of the 2A3.  The only thing between there are two solder joints and the plate choke.

With the amps off measure resistance on both plate chokes, red wire to black wire.  Then check continuity from the lug with the black wire on PC-2 to Terminal 10 and from the lug with the red wire to Terminal 16.

[Paul Birkeland]

If you can't dial the driver tube voltage close to the target value, then you can be sure that the 2A3 voltages will be off, but that's not where you need to look for issues.

I'd double check the transistors, jumpers, and wiring of the new driver board to be sure everything is as it needs to be.  If changing the driver tubes doesn't remedy the issue, then it's most likely a minor wiring issue around the driver socket. 

[Grainger49]

Paul,

Are you saying that there can't be anything wrong in the plate choke circuit?

[InfernoSTi]

The PC-2 resistance is 327 ohms on the "good" amp.

The PC-2 resistance is 0 ohms on the "bad" amp.

[Grainger49]

John,

Zero, is not right.  But the PC-2 would still pass on the voltage to the plate if it were shorted out completely.  Did you trace the connections to the rest of the circuit and were they good? 

[Doc B.]

Yeah that tends to indicate that something else that is in parallel with the plate choke is shorted. Check the filter cap that is in the circuit just before the plate choke and also check the solder pads and terminals where the wires connecting to the plate choke are connected at their other ends.

[InfernoSTi]

Sorry, Grainger, yes, here is the complete info:

With the amps off measure resistance on both plate chokes, red wire to black wire.

 

The PC-2 resistance is 327 ohms on the "good" amp.

The PC-2 resistance is 0 ohms on the "bad" amp.

Then check continuity from the lug with the black wire on PC-2 to Terminal 10 and from the lug with the red wire to Terminal 16.

I have continuity from the lug with the red wire to T16 on both amps.

I do not have continuity from the lug with the black wire on either amp to T10 BUT I do have continuity to T1 on both amps (which is what I think you were wanting me to check, actually).

John

[InfernoSTi]

Yeah that tends to indicate that something else that is in parallel with the plate choke is shorted. Check the filter cap that is in the circuit just before the plate choke and also check the solder pads and terminals where the wires connecting to the plate choke are connected at their other ends.

This should be interesting.  The filter cap of which you speak is on the power supply board, correct?  That is the one that is soldered onto the base of the power transformer, right?  Is there a good way to check the filter capacitor?  I'm looking at the amp and wondering how to do this....sorry to be a bit slow on this.

I will retouch the solder at T16 and T1 just to be sure there are good connections there. 

John

[Grainger49]

Often, but not always a "blown" cap has a rounded end.  Do any of the filter caps look odd?

[InfernoSTi]

All the caps look fine.  I'm going to try resoldering T16.  I sure hope that is it!

[Doc B.]

A bad solder joint is more likely to create an open reading than a short. I would look for miswires or bridged solder pads and terminals. It seems that the power supply itself is OK as you get OK readings on the B+ supplying the small tube. You could try disconnecting the plate choke from the circuit, right at the plate choke terminals, and measuring the across the disconnected wires by themselves to see if you still read a short. If not then measure the plate choke terminals relative to the chassis and see if you measure a short.