Hoffman Princeton Reverb bias circuit problem

[mresistor]

Hi guys. Since I gotta stay inside here in CO I have been working on a conversion of a PRRI to a Hoffman PR turret board. Got it all wired up and the bias circuit doesn't work right.


At first the - voltage went up to -22mv then ramped back down to 0 and then to + 200mv and then back down to near 0v. 


Now I have +232 on the cathode of the diode and nothing on the anode. 


Things I have done


replaced the diode (had a 1N4007 in there and now its a 1N5408, I know it's overkill)
replaced the 47uf cap (the original checks good with my tester, within tolerance)
checked the 22K resistor good
checked the 10k bias trim pot and then replaced it with another
checked the resistance of the intensity pot- within specs
checked continuity of the intensity pot to the grid pins of the power tubes.   checked perfect
reflowed the ground solder joints on the turrets and at the main transformer ground
checked all the jumpers on the turrets.. checked the whole thing against the layout (several times)  it's right
Checked the 100K range resistor


The original PRRI circuit had a blown filter capacitor and a shorted out GZ34 both diodes. There are safety diodes on the rectifier socket from the factory.
The current GZ34 is a JJ from Dougs Tubes.  6V6s not installed (of course) V1 2 &3 are installed V4 is not installed.
The HV Nodes A through D have appropriate voltages that are reading high because the power tubes aren't installed.
The filames are all reading 3.3vac per side.

I'm stumped    - there isn't anything under the board in that area and not directly underneath except the HV lead from the rectifier to Node A
The OT lead to Node A is also routed under the board but it's not underneath the bias circuit.
It's just not putting out negative voltage. It's like there is no ground on the cap and 22K resistor  but  there is. Hope you guys can help.

[mresistor]

I've built several of these Hoffman boards and all work perfectly.   I just built a standard AA1164 PR on an eyelet board and it works great.

[mresistor]

I just turned it on again ..  bias v went to -44 then ramped back down and stayed at 0v . 
There is 177 volts on the cathode of the diode. 

[shooter]

back down and stayed at 0v . 

0 is UP from -44!  0vdc kills tubes

[mresistor]

Thank you Shooter - I am aware of what lack of neg bias does to power tubes.

[Latole]

Thank you Shooter - I am aware of what lack of neg bias does to power tubes.

Don't put Power Tubes in the amp until you fix bias.
You don't need Power Tube to operate bias.

Look you may have a short in bias circuit. It could be a power tube.
Remove all Power Tubes and read bias voltage.  Report

[mresistor]

OK  this is for anyone that ever encounters this..   I found the problem.  its the safety diodes are bad.


I bypassed them and the - bias V is steady at -40.   Mods you can delete this topic if you wish.



So back to my work.  You'all stay safe out there.   

[Latole]

Where are these diodes ? At tutput tubes plate ?
They where short ?

Where is this amp shematic ?

Update :

 I find schematic , it is a mod you do, on rectifier tube ?

[mresistor]

Latole   the PRRI is a Fender Reissue and the safety diodes, also called the Dynaco mod I believe, are on the rectifier socket stock from the factory, I think. 
I believe they were opening up.

https://robrobinette.com/5e3_Modifications.htm#Backup_Diodes    Called backup diodes I guess.


The Hoffman PR schematic is on his Turret Board page, go to the store and turrt boards and click on Princeton Reverb that will take you to the documention.
Many people here are very familiar with the Hoffman circuits.  Sorry I didn't post them for you.

[Latole]

Thank you mresistor, I know very well what are these diodes on rectifier tube.

IMO, Issu with these diodes is a lack of voltage everywhere not only on bias circuit

[mresistor]

IMO, Issu with these diodes is a lack of voltage everywhere not only on bias circuit

You are suggesting that the Power Transformer HV winding is failing?

[Latole]

No I mean defective diodes may drop voltages everywhere in the amp, not only for bias

[mresistor]

Amp seems to work fine ,  thanks. 

[2deaf]

The symptoms suggest the possibility that the bias circuit was taken from V7 Pin 4 instead of from V7 Pin 3.  The Service Manual has Diode 6 connected from Pin 3 to Pin 4 and the bias circuit taken from Pin 3.

[sluckey]

+1

[mresistor]

2Deaf   Thanks for that.    I don't recall where the bias circuit WAS on the PRRI as I took it all out a while back, maybe I have a picture.   But I do indeed have the bias tap now hooked to pin 4 of the recto and that is after the diode and is pulsed DC as I understand it..  I will move it to pin 3 and see how that works.

[mresistor]

2Deaf   that was it..  thank you very much.   I owe you one.     

[dude]

OK  this is for anyone that ever encounters this..   I found the problem.  its the safety diodes are bad.


I bypassed them and the - bias V is steady at -40.   Mods you can delete this topic if you wish.



So back to my work.  You'all stay safe out there.

I had the same problem on a 6V6 Plexi I built, I too used safety diodes off a/c supply to rectifier.  I don't think any of the diodes were bad but maybe the wrong diodes. I used 3000v, 1N5408, is this a wrong diode to use? or the right diode but one or both defective, my meter doesn't check these diodes.
I took them out and bias was right on...?

[sluckey]

I had the same problem on a 6V6 Plexi I built, I too used safety diodes off a/c supply to rectifier.  I don't think any of the diodes were bad but maybe the wrong diodes. I used 3000v, 1N5408, is this a wrong diode to use?
I took them out and bias was right on...?

1N5408 is NOT a 3000V diode. Didn't we recently have this same conversation? 

The problem is not the safety diodes but the point you pick off the AC to send to the bias circuit. You must connect the bias circuit directly to the transformer where there is AC present. If you connect after the safety diode all you have is positive pulses and they will be blocked by the bias diode which can only pass negative pulses, but there ain't any to pass so the bias voltage is zero.

[dude]

Sorry, if l asked a redundant question, at times l do. But are you saying the 1N5408 is not capable of a max of 3,000 volts, not that that's ever needed in a tube amp.  Or it's an incorrect diode to use in this application?
I just looked at the diagram l copied from  the correct way to wire this safety feature. I made a mistake copying that correct diagram. I attached the a/c wires from PT to pins "3" and 5 along with the diodes then diodes cathode to pins 4 and 6. Should have been pins 5 and 7 to a/c...? Not pin 3. But would it make no difference if l used pin 3 or 7 as both are empty pins. Forgive my ignorance if l have his wrong.

[sluckey]

I attached the a/c wires from PT to pins "3" and 5 along with the diodes then diodes cathode to pins 4 and 6.

That's fine. But this means that now you have to connect the bias circuit to pin 3 or 5.

Google 1N5408. You'll have your answer in under a minute and I'll quit pickin' on you.   

[dude]

Ha, well that's it, thank you. I probably attached the bias off dc. I'll never know as I took the diodes off and probably changed the bias supply to the correct lug. Ha, if I did l'm an ass. I should have paid more attention. Hey, l'm an old geaser, and losing it, glad l can still play 


On the 1N5408, 3 amps, not volts


The old saying you can't teach an old dog new tricks, may be true

[sluckey]

Don't beat yourself up. Easy mistake. You're not the only one.

[PRR]

> l'm an old geaser, and losing it,

This old geezer... Ampeg V-40 has a stunning 590V of B+. Tube short, PT burned. Used a generic PT of slightly lower voltage, but mis-wired. Habit to have a voltmeter on an amp after a major transplant. It was on the 1,000 Volt scale (pre-safety meter) and the needle was about to pin when I killed power. I had wired it for what would have been 1,180V!!

[Latole]

Ha, well that's it, thank you. I probably attached the bias off dc. I'll never know as I took the diodes off and probably changed the bias supply to the correct lug. Ha, if I did l'm an ass. I should have paid more attention. Hey, l'm an old geaser, and losing it, glad l can still play 


On the 1N5408, 3 amps, not volts


The old saying you can't teach an old dog new tricks, may be true

Google ;

Charactiristics 1N5408 diode
Maximum Recurrent Peak Reverse Voltage - 1000 V.
Maximum Average Forward Output Current - 3 A.
Maximum Forward Voltage Drop per element at 1.0A DC - 1.1 V.
__________

Diode ; we always talking about Peak reverse Voltage.
These are good diodes for your need.

To solder diode; always use heat sink if not you may damage diode.