5F1 Scratch Build - Preamp Plate Voltage Differences

[420Jazzerciser]

Put together a 5F1 in an old Accutronics reverb tank I pulled from a Hammond organ.  It sounds good but I am wondering about the plate voltage differences at the pre amp. 

B+ for the plate load resistors at the filter cap node is 278VDC.  I follow that to the node of the plate load resistors and get the same reading.  But when I read it across the 100k resistors Pin 1 reads ~180 and Pin 6 reads ~150.  Why the difference?

I have check the resistance on the plate load resistors and they both read right at 100k.  I have tried 3 different 12AX7s, with similar voltage readings.  I even changed the battery in my meter.  Also, I am seeing ~1.5 VDC on the Grid at pin 7. 

Any thoughts would be appreciated.

[sluckey]

Also, I am seeing ~1.5 VDC on the Grid at pin 7. 

This probably explains the lower voltage on pin 6. If you set the volume control to zero (max CCW) does the grid voltage go to zero volts? And does the plate voltage increase?

A leaky .02µF coupling cap connected to the volume pot is the most logical suspect for putting 1.5v on pin 7. Volume pot faulty ground connection is another. What are the voltages on pins 3 and 8?

Also disconnect the NFB wire from the speaker jack. Any better?

[420Jazzerciser]

I turned it on and the 10k/2 watt dropping resistor started smoking??!!  Not sure why it would do that now.  All voltages checked out fine and I played it several times over the last few days.  Turned it off and now I will change out that resistor. 

Now my plan is to pull the 12ax7 and 6v6 and start it up with a light bulb limiter with just the rectifier tube to see if there are problems in that stage.  If all seems ok, I will turn it on with the limiter with the 6v6 installed, then the 12ax7.  Is that a good way to troubleshoot the problem?

[sluckey]

Did you do anything I suggested???

I don't know that you have a problem. When troubleshooting plate voltage differences your amp should be plugged directly into the wall outlet.

[420Jazzerciser]

When I went to troubleshoot with your suggestions I fired up the amp and that is when the resistor started smoking, so I shut it down.  Can't check out your suggestions with that resistor smoking, right?

When troubleshooting a smoking resistor in the past on another project, I ran it through a current limiter with tubes removed then the rectifier, then the power tube, which is where I found the problem.  Does that make sense?  How would you deal with the smoking resistor? 

Thanks for the help!

[sluckey]

That smoking resistor has nothing to do with your original complaint or anything I've suggested.

[tubeswell]

10k/2 watt dropping resistor smoking

This is caused by a short-to-ground on the end of the supply resistor. The Resistor is dropping several hundred volts across it, causing lots of current (volts/resistance = current).
Look for the short. Will either be a shorted filter cap or a blob of solder or wire touching the ground (I.e. the chassis behind the board - depending on how your amp is constructed)
Or you maybe missed something - like the wire between the coupling cap and the vol pot is connected to the wrong side of the coupling cap or something (putting High VDC onto the pot, but also providing a shorting path from the High Tension supply to ground through the pot (more so when the pot is cut)


- not saying that ^this^ is exactly the case because you haven’t posted any pictures of what you built. But there is definitely a DC short to ground from the high tension rail to cause the supply resistor to smoke.

[420Jazzerciser]

Thanks for the input...

No ground connection at the B+3 node.  It was connected to the volume pot but I didn't ground the pot.  Fixed that issue and now voltages look great.  No volts on the grids of the 12ax7 and plate voltages are right at 188 VDC each.

I replaced the 10k/2w resistor and it is running fine now, no smoke when I leave it on and play it for awhile.  Could not having that ground connection have been the cause for the high current through that resistor?  I looked all over for a short and could not find one...

[sluckey]

I replaced the 10k/2w resistor and it is running fine now, no smoke when I leave it on and play it for awhile.  Could not having that ground connection have been the cause for the high current through that resistor?

Very unlikely. The volume pot is a very unusual place to ground a filter cap. Why did you choose to do so?

[420Jazzerciser]

I jumped the cathode of the 6v6 to that node of the filter cap as per the 5f1 layout, then because of space constrictions I couldn't just run a bus wire to ground as the layout does.  So I ran a wire from that node's turret on the underside of the board over to the volume pot ground lug then ran a bus wire from that to the chassis. 

I just added the bus wire and the voltage problems were corrected.  But still trying to figure out why that resistor smoked before.  I played it and left it on for about an hour and the replaced resistor was just fine.