5F4 build issues

[ok_state_blues]

10-4 I followed the diagram provided by rob from his website. It's the animation in post #16 if that helps. I know it for sure has voltage at both pins when the bias board is disconnected.

 

[Lowerleftcoast]

I know it for sure has voltage at both pins when the bias board is disconnected.

Good.

If the bias board is just like Rob's layout, that should work. Is there a ground wire from the positive end of the two bias caps? (You can just measure for continuity to ground from that location.)

 

[andrewRneumann]

With a 220K resistor first thing on that board, there could be a dead short to ground directly after that and it wouldn’t matter. The short, if it exists, would seemingly have to be somewhere between the rectifier socket and that 220K resistor.

Let’s see a photo of the rectifier socket.

What’s the bias board resting on? You are absolutely sure it isn’t shorting out against something under the board?

With the amp off, measure resistance to chassis from pin 6 with the bias connected. Should be 50 ohms or something.

Interesting puzzle we have here.

 

[ok_state_blues]

@Lowerleftcoast in current set up pictures both points connected to the copper jumper are showing continuity to ground.

@andrewRneumann resistance shows .9 ohms. However pin 4 shows 88.5 ohms using the same technique

 

[ok_state_blues]

Closer shots

 

[ok_state_blues]

More

 

[andrewRneumann]

Ok, this might sound bonkers, but is it possible the board you used for the bias circuit is conductive? I have never seen one that looks like that before, but there's a lot I haven't seen... What material is that?

 

[ok_state_blues]

Ok, this might sound bonkers, but is it possible the board you used for the bias circuit is conductive? I have never seen one that looks like that before, but there's a lot I haven't seen... What material is that?

It is a scrap piece of countertop laminate. I normally use it as a backing material in the chassis of builds to keep the wires/boards from shorting to the chassis. Maybe I should reuse some of the original organ board to replicate the bias circuit?

 

[andrewRneumann]

It is a scrap piece of countertop laminate. I normally use it as a backing material in the chassis of builds to keep the wires/boards from shorting to the chassis. Maybe I should reuse some of the original organ board to replicate the bias circuit?

Hmmm... I'm not sure.

I guess I would unsolder the bias circuit from the rectifier and then go around the board with an ohmmeter and see if there is a short somewhere. You seem to have a lot of excess solder on pin 6--do you have a solder-sucker to clean some of that up? While you have it disconnected, check the resistance of pin 6 to chassis and verify it comes up to match pin 4.

There is a simple wire (blue) from the rectifier to the board. It looks intact, so the short has to be right there at the rectifier socket pin 6, or right on the board before the 220K resistor.

 

[ok_state_blues]

Hmmm... I'm not sure.

I guess I would unsolder the bias circuit from the rectifier and then go around the board with an ohmmeter and see if there is a short somewhere. You seem to have a lot of excess solder on pin 6--do you have a solder-sucker to clean some of that up? While you have it disconnected, check the resistance of pin 6 to chassis and verify it comes up to match pin 4.

There is a simple wire (blue) from the rectifier to the board. It looks intact, so the short has to be right there at the rectifier socket pin 6, or right on the board before the 220K resistor.

Yes I can clean that connection and check resistance this evening.

 

[andrewRneumann]

You got me chasing a wild rabbit. Electrostatic Dissipative laminates (ESD)

Should be easy to determine if this is the problem with an ohmmeter.

 

[ok_state_blues]

How about along with pulling the existing bias board and replacing with the Hammond organ board. Maybe that will give us a good clean slate to start from. I think you are on to something @andrewRneumann. It makes sense that the board could in theory do that as that would make a very easy path to ground. More research will enlighten.

 

[ok_state_blues]

Ok gang updates from the bench side. Per cleaning up that pin 6 per @andrewRneumann I found a small solder bridge from that pin to the old ground ring at the base of these sockets. Doh! Lesson learned. Go into troubleshooting with a magnifying glass and bright light! After cleaning up the bridge and pin, we now have a passing score per the LBL test! Now as a side note, where I was once getting ~350 v at pin 4 and nothing at pin 6, now I am at 378 on my pins 4 and 6. The original schematic shows the final output being ~ 325 to 330 volts. I am hoping that as I add more tubes, that voltage will come down.

 

[andrewRneumann]

Ok gang updates from the bench side. Per cleaning up that pin 6 per @andrewRneumann I found a small solder bridge from that pin to the old ground ring at the base of these sockets. Doh! Lesson learned. Go into troubleshooting with a magnifying glass and bright light! After cleaning up the bridge and pin, we now have a passing score per the LBL test! Now as a side note, where I was once getting ~350 v at pin 4 and nothing at pin 6, now I am at 378 on my pins 4 and 6. The original schematic shows the final output being ~ 325 to 330 volts. I am hoping that as I add more tubes, that voltage will come down.

Alright, great! Lesson learned makes it all worth it. I was just reminded of the proverb, “Failure is the mother of success.”

We’ll leave the light on for ‘ya.

 

[D'tar]

Glad you found issue!!!! What do you have now for -ve bias voltage at the control grid?

 

[ok_state_blues]

I didn't get to far into testing last night and hope to continue going through the rob Robinette start up procedure this evening. But all in all no smoke or fire thus far

 

[ok_state_blues]

Ok so buzzing along. I have negative voltage on pin 5 of one tube and no negative voltage on the other sockets pin 5. I found this just as I was getting ready to install tubes in the output section. Also I have noticed my incoming B+ has reduced significantly after adding 3 preamp tubes. I believe this has to do with me replicating the field coil through the center tap. Testing continues but just FYI if you plan on using Hammond Organ iron for a build, it is a major labor of love/science experiment!

 

[King Fan]

I have negative voltage on pin 5 of one tube and no negative voltage on the other sockets pin 5.

Sounds like you’re doing things right on stepwise testing. How much negative voltage on the 'live' one? Can you trace it backwards to the bias board on the dead one?

 

[ok_state_blues]

-29.5 on the live one. I did some more research and found the original called for a 700R load on the CT to ground for the field coil. In mine, I have a 1k 50 watt chassis mount that I think is contributing to lower overall voltage. So if I keep it in place, in theory I am actually building a 5E4a! Kinda neat to me as you don't see many baby bandmaster/supers out in the wild.

@King Fan I followed the circuit and found 1 resistor that should have been 220k is measuring ~50R so off to the parts box!

 

[ok_state_blues]

@andrewRneumann @Lowerleftcoast @King Fan @D'tar will you look over my latest notes. After fiddling with the bias connection and swapping to 6v6GT's, these are my current numbers plugged into the wall (no LBL). I know the pre is not true to the schematic and can adjust if that helps. I am just unfamiliar with this part of the build for spotting mistakes.