What would cause bias resistors to blow?

[Mason]

I just finished up a Soldano SLO 100 build. All the voltages were spot on and I was able to fire it up and get sound immediately. I set the tubes at a pretty low bias just to get things checked out, they were at about 40% PD.

As I was playing, going through the controls, and seeing how everything worked the ht fuse popped. This was after about maybe 15 - 20 minutes of play. I put another fuse in and it popped immediately after taking it off standby. As I was probing around in the amp I noticed both bias resistors were open, with a chunk of the side of the resistors burned up. These are the 1 ohm resistors for checking bias. In this build, they're between pins 1 and 8 with pin 1 tied to ground.

I jumped that connection between 1 and 8 and was able to turn the amp on without blowing a fuse, but after 10 minutes of testing one of the tubes started red plating so I shut down the amp.

I'm going to go through all the power tube socket connections and resistors to see if anythings up, I can't see anything shorting and my meter confirms that. Any ideas what the issue could be?

[pdf64]

That the cathode current sensing resistors blow is a symptom, not a cause. The power valves are passing far too much current.
Maybe the bias supply to the control grids is wonky?

[sluckey]

What kind of power tubes? What voltage do you have on pin 5 of all power tubes?

[Mason]

Sovtek 5881 power tubes, all new and matched, these to be exact https://www.tubedepot.com/products/sovtek-5881-6l6wgc-power-vacuum-tube. The bias pot turns from -98 volts to -33 volts when the reading is taken right before 2.2k grid resistors. Last night when this happened I had it set to about -57 volts.

When the power tubes are pulled everything seems fine. I get sound out of the fx loop send and things seem normal.

This is the schematic I'm following, https://el34world.com/charts/Schematics/files/Soldano/Soldano_slo100.pdf. I'm also using this build log as a guide, http://www.c3amps.com/SLO%20Head%20Build.html, and following the same wiring scheme with the parallel bus wires on the power tubes.

[Mason]

One other quick note. The PT I'm using has a bias tap, it's 110 ac and 140 dc after being rectified. The caps I'm using in the bias network are rated at 160v. Could it be possible that the cap fried? It seems like it should withstand 140 dc, but it is close.

[pdf64]

On the sosckets that have blown their 1ohm resistors, clip a meter on to the socket terminal #5, ie after the 2k2 grid stopper. Ensure that the socket contact is suitably tensioned to grip the valve pin.

Bias taps are usually more like 50VAC. 140VDC is far more than could ever be needed, so use a dropper resistor between the tap and the rectifier to drop it to a more suitable level. Maybe start at a trial valve of 47k. Don't fit the power valves until the bias supply is good.

Note that new tested burnt in valves can be / go bad, hence the importance of a good warranty and testing valves well before it expires. Consider that they might have been bashed during shipping. Use valves that you know to be good for checking the build.
Valves that have redplated are suspect and may be damaged, if not before the incident then certainly after; they won't redplate until the anode dissipation is significantly above their limit. Mark them up as such.

[sluckey]

The bias pot turns from -98 volts to -33 volts when the reading is taken right before 2.2k grid resistors. Last night when this happened I had it set to about -57 volts.

MEASURE BIAS VOLTAGE DIRECTLY ON PIN 5 OF ALL OUTPUT TUBES!

[Mason]

Measured on pin 5 of each tube, with tubes installed and the amp in standby, I get the same reading as I do before the grid resistors. The voltage's for all tubes pin 5 was within half a volt of -57 volts.

[shooter]

with tubes installed

on fixed bias amps, the bias can and should be measured WITHOUT tubes 1st.  once you verify it's stable DC (and adjustable if there's a pot), then add tubes

[Mason]

Should I do this in or out of standby?

[pdf64]

To avoid confusion, just take standby switch out of circuit, link its terminals together or connect the wires or whatever.

[mresistor]

Should I do this in or out of standby?

here's a schematic


https://el34world.com/charts/Schematics/files/Soldano/Soldano_slo100.pdf

[Mason]

I've had the amp running with the power tubes pulled for about 30 minutes and haven't noticed much fluctuation in the pin 5 voltage. It starts at -57 volts and settles in at -59.2 volts. This is consistent across all sockets. One thing I did notice was that pin 5 for one of the tube sockets seems loose, I'm testing from the top of the socket where the tube would sit and I have to wiggle the meter a bit to make contact with the pin. Part of this build requires bending the pins around to run the parallel lengths of bus wire between sockets, so I suppose this could have loosened some of the pins and made intermittent contact with the tubes?

I'm not sure if that alone would be enough to cause my problems. I only have one bias resistor per pair of tubes so I wasn't able to narrow down to any specific tube socket, and both resistors blew so they're all potential suspects.

[mresistor]

Can you post up some hi res pics of the power tube area and bias area of the amp? Might assist in helping you find the problem.

[HotBluePlates]

... pin 5 for one of the tube sockets seems loose ... I have to wiggle the meter a bit to make contact with the pin. Part of this build requires bending the pins around to run the parallel lengths of bus wire between sockets, so I suppose this could have loosened some of the pins and made intermittent contact with the tubes? ...

That is very likely the amp's problem: Pin 5 lost contact with one or both output tubes, removing bias voltage from the tube and causing plate current to skyrocket.