Heater Center Tap resistors Popping

[Goldguitarguy]

Hey folks,

I recently finished my 124 ods build (which is my first ever build) and I’ve been loving it to pieces so far but I wanted to push the amp all the way up for a run test on a dummy load to see how it holds up (or doesn’t) at high volumes to before I drag it out somewhere to play.

The drive channel goes all the way to ten on volume, level, ratio and master with out any issues and sits there fine. Clean channel with the volume at 5 I got the master past 6 and it popped the heater center tap resistors. First time I thought maybe it was a bad tube so I swapped them and replaced the resistors. Turned the amp on and it worked fine all the way up to “annoy my rural neighbors volumes” with no noise/hum. Then, I tried to run test again... Drive channel no issues, clean channel u dear the same conditions popped the resistor again.

Could this be ultra sonic oscillation somewhere in the clean channel causing an issue with the power tubes? Or am I out to lunch? I’m not an amp builder by any means...lol

[brewdude]

Just grasping at straws, but...
Do you also have a real center tap on heater winding? In addition to the artificial center tap?

[Goldguitarguy]

No center tap for the filaments on the PT, I’m using a Hammond 290FX.

[sluckey]

Closely examine the output tube sockets (top side and bottom side). Maybe even use a magnifying glass. Do you see any evidence of arcing (carbon trace, crack, etc) between pins 2 and 3 of either socket?

[xtian]

Also check your power tubes' screen and grid resistors.

[Goldguitarguy]

I checked the screen and grid resistors already all were within tolerance. No burn marks or cracks on the tube sockets.

I got to thinking “it’s weird” because I can make my ears bleed with this thing cranked as far as would in a basement through a speaker so maybe the dummy load box is causing the issue. I’ll have to open it up tomorrow and see. Would a failed dummy load do that? Running it at zero load I know can burn the OT. Would it short the tubes maybe first?

[Charlie Wilson]

Hello, zero load with a signal going through the amp turned way would probably kill your OT real quick. A mismatched load turned up would do a number on your tubes first. The only way those resistors are opening up is somehow a high voltage is getting into your filament supply. What wattage are the resistors and what wattage is your dummy load?
CW

[talbany]

I just had an old Blackface Bassman on my bench with a similar issue. It was carbon deposits on the sockets that was arcing and taking out the fuse. These are sometimes difficult to detect (especially with black sockets) and I was able to spot it only with a high powered magnifying glass..Once that path to gnd was established you're better off replacing the socket..Was the transformer getting hot with the amp at idle?
1.Tubes
2.Sockets
3.Transformer
Tony

[pdf64]

I think that the most likely cause is a power valve with an intermittent short to its heater, eg g2-k.
Can the fault be provoked whilst the amp is running via your light bulb limiter?
Try a known good, alternative set of power valves.

[Goldguitarguy]

Hello, zero load with a signal going through the amp turned way would probably kill your OT real quick. A mismatched load turned up would do a number on your tubes first. The only way those resistors are opening up is somehow a high voltage is getting into your filament supply. What wattage are the resistors and what wattage is your dummy load?
CW

I replaced the resistors right after it happened the second time. I checked the grounds, turned the amp on with the dummy load, all the knobs down and no signal, I checked the voltages, put the tubes in, checked the voltages again, hooked up the speaker and all was good in the hood again at house volume so luckily no major transformer or tube damage.

Heater resistors are 120r 1/2w metal film and the dummy loads are two 16ohm 100w wire wound in parallel.

[Goldguitarguy]

It think that the most likely cause is a power valve with an intermittent short to its heater, eg g2-k.
Can the fault be provoked whilst the amp is running via your light bulb limiter?
Try a known good, alternative set of power valves.

I tried a different valve set and it produced the same results already. I now got myself convinced that there’s a mismatch with the dummy load. I’ll open the dummy box when I get home from work later today and see if there’s an issue internally. I did test a 150w bass amp last before this one so I might have cooked a resistor or connection unknowingly.

[talbany]

It think that the most likely cause is a power valve with an intermittent short to its heater, eg g2-k.
Can the fault be provoked whilst the amp is running via your light bulb limiter?
Try a known good, alternative set of power valves.

I tried a different valve set and it produced the same results already. I now got myself convinced that there’s a mismatch with the dummy load. I’ll open the dummy box when I get home from work later today and see if there’s an issue internally. I did test a 150w bass amp last before this one so I might have cooked a resistor or connection unknowingly.

Generally the chain of events that take place with a (higher than specified)"
mismatched load?
An incorrect load can commonly cause whats known as '''Back EMF". Where the energy being displaced during the speakers recoil This current then gets stepped up by the transformer and shows up at the tube in the form of flyback voltage. The excess negative energy usually searches for the nearest gnd potential (pin 2) hitting the balancing resistors and causing a large arc that leaves a carbon trace on the socket..Once it's established a path to ground (pin 2 and 3) you can still change tubes and match the load correctly however the slightest bit of back EMF in the future will cause it to intermittently arc out again generally when the driver reaches peak excursion.

In case you didn't know?

Tony

[Goldguitarguy]

So I opened up the dummy load box and as suspected the resistors are toast. I never had much faith in those considering I ordered them from Amazon. I ordered some new ones from mouser and when they get here in a few weeks I’ll try to test again. Until then I’ll just enjoy the amp.

And thanks Tony that actually explains a lot I didn’t know and makes sense why it happened.

I’ll follow up here in a few weeks when the resistors come in.

[sluckey]

Those center tap resistors saved you from a possibly much worse fate. That amp would probably benefit from a couple protection diodes connected from plate to cathode (or ground) on the output tubes. You can use a single 3KV diode or a series of three 1N4007s for each tube.

[Goldguitarguy]

Yeah honestly when they first popped I thought the OT failed. And then when I measured the heater voltages I got 90vac referenced to ground but when I lifted the pt heater wires I only got 7vac and I was like what the hell...lol