5F4 build issues

ok_state_blues

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Hi I am working on a pseudo 5F4 build using Hammond organ transformers. Due to not having a bias tap I took the RobRob rout and tapped it from my rectifier socket at pin 6 as stated on his website. But now for some reason, I am only getting voltage at pin 4 and not pin 6. I am suspicious of the diode being open. Am I on the right track in my sleuthing? Also should the ground and tip of the output jack for the speaker show continuity to ground?
 

Phrygian77

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The OT secondary DCR is very very low, so assuming you're not measuring a shunt to ground, that's why. I'd be more suspicious of however you've done the bias than a diode. You're going to have to provide more details on what was done ... and, pics of course.
 

2L man

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Other probe on PT CT you should see few hundred volts AC to both rectifier pins 4 and 6. And double when measuring between 4 and 6. Open burn diode won't effect but even functioning diode should not effect to AC. Shorted diode will burn bias circuit. When power is off using multimeter you should have about 100 ohm and up resistance for both side of PT secondary coils on rectifier sockets.

Loudspeaker jack "hot" show short against ground because OT secondary coil resistance is very low if you have connected "neutral" to ground.

I think you won't see proper negative bias voltage until bias circuit resistors, trimmer and filter are connected. Check two times that you connect electrolyt right way!!!
 
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ok_state_blues

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Can do on the pictures. @Phrygian77 have you ever used Robs method of stealing bias voltage from the rectifier? @2L man that was the method I was using as my meter is only rated for 600 volts, but it seems odd to me that pin 4 is seeing ~ 356 volts AC but pin 6 shows zero.

Also as a side note, the measurements were taken through the bulb current limiter. 6.3 measure across 12A_7 sockets show ~3.2 volts and 5v at rectifier shows ~2.5 volts across pins. It's almost like half of the transformer is being grounded out.
 

ok_state_blues

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Have you installed rectifier back up diodes? If so, Tap bias circuit directly to HT lead terminal. VAC before diode... Pulsing DC after diode. Check you meter settings.
I don't have those installed. I would like to once I get everything up and running though.
 

King Fan

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But now for some reason, I am only getting voltage at pin 4 and not pin 6.
measurements were taken through the bulb current limiter. 6.3 measure across 12A_7 sockets show ~3.2 volts and 5v at rectifier shows ~2.5 volts across pins. It's almost like half of the transformer is being grounded out

So all of the PT secondaries are at half mast? Somebody here may have a ready insight on how this could happen.

BTW you don’t need or want to measure voltages on the limiter. Just use it first to make sure bulb goes dim (no short), then remove for testing.

Was the PT tested and working before you hitched up the bias?

What PT? And yes. Pics of all the PT connections will be helpful.
 

ok_state_blues

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@King Fan 10-4 on the pictures will do. The PT was tested unloaded. It is a hammond AO-26577. I made sure to test this one as I had the first PT installed (AO-20994) and tidy but found the 6.3 tap dead. Frustrated I pulled it and installed the new unit. Here is the schematic of the unit it was pulled from. All taps were tested per Rob's troubleshooting page and did fine. It's just when that bias set up is connected, the light stays bright vs dim when not.
 

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Phrygian77

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@ok_state_blues I am not familiar with @robrob circuit/diagram for this, but what you are talking about doing is exactly what is done on a Princeton Reverb. It is a half wave rectifier circuit using one side of the primary, and it doesn't matter which side you use as long as that diode is rectifying the negative half of the waveform. What does matter, is that you're not trying to get the negative half of the wave from an already rectified positive half, which is what @D'tar was alluding to. The rest of the circuit is just a voltage divider and a cap for filtering the DC.

You can get away with using half of the secondary because the bias circuit doesn't, or rather should not, load down and imbalance the total load.
 

Lowerleftcoast

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Also should the ground and tip of the output jack for the speaker show continuity to ground?
The speaker impedance and the secondary coil of the OT are in parallel. The secondary is probably less than 2 Ohms. That low resistance will often look like continuity on a MM.

Only use the light bulb limiter to determine a fault. Once there is no fault detected remove the LBL. It may be dragging your readings down.
 

andrewRneumann

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Only use the light bulb limiter to determine a fault. Once there is no fault detected remove the LBL. It may be dragging your readings down.

It's just when that bias set up is connected, the light stays bright vs dim when not.

OP is saying that the LBL is burning bright with the bias circuit connected. Suggest leaving the LBL on for the time being. The absurdly low readings on the other secondaries, I assume, are caused by the large voltage drop across the LBL. To me, all the symptoms sound like a short in the bias circuit.

Score another win for LBL. Smart of you to have one.
 

King Fan

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OP is saying that the LBL is burning bright with the bias circuit connected.
Good eye, Andrew. To be fair to folks reading the initial posts, I don't think that was mentioned early on. Your diagnosis is most likely. I hope the pics will clarify -- yes, OK_state, keep the LBL in the circuit if it's glowing bright.
 

ok_state_blues

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ok_state_blues

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ok_state_blues

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Ok please excuse the mess but I hope to resolve this issue then tidy up wiring. Just for reference, I am getting HT volts of approximately 357 on pin 4. Nothing on pin 6, the animation shows the gist of what I am shooting for.

Blue wire is from pin 6
Red wire is bias adjustment
The upper left solder joint is headed to the 2x220k resistors per enclosed drawing where bias once tied in.
 

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Lowerleftcoast

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should I leave all other connections in place in the meantime?
First, I would disconnect the bias wire from the rectifier pin6 and measure to see if it has voltage when the bias is disconnected.

I am not sure my suggestion is accurate because I can't see where everything is connected. That is why I want you to *check my work*. I am not seeing a wire from the diode to the bias pot. If there is one it may be wired correctly as you have it. Although it may need a ground wire.

What are the values of the resistors on the bias board? The first resistor, the high voltage from the rectifier socket pin6 hits, should probably be around 100k. The smaller values you see on the schematics are for a much lower voltage from a PT bias tap.
 
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