Octave effects and amp damage?

[GregS]

' The average cathode current rose slightly over idle when playing but nothing major'

Unless it was just at low gain setting, that seems strange.
What happens to the cathode currents at full power (compared to static conditions)? With and without the octaver?

Are you referring to the peak signal current? For sure that would be way higher than the idle current. The way I understand how the digital multimeter works is that it averages the readings so you don't see the individual instantaneous readings. It was not at a low gain setting during my test. I would see the current rise by about 8 mA over idle when I plucked a string hard.

Greg

[Mick Bailey]

Some octave effects can damage the speaker, though - mainly transistor amps where I've seen this; quite a few sub-octave pedals generate a square wave and in an amp where the speaker is closely matched to the amp's output. Pushing the amp too hard with the sub engaged and turned up pushes a lot more thermal energy into the speaker than a regular bass waveform.

[GregS]

I've seen tubes redplate, but never turn orange... that's pretty darn hot if the plates were orange! Was there any increase in hum before the tube blew?

Same with me. This was different and more dramatic than anything I've seen before. Yes, the background buzz noise got louder as the signal level faded. I recognize that symptom which is why I immediately looked in the amp.

About the bias - don't THINK "unlikely because it's new & high quality." KNOW for sure! Even if the bias is reaching the solder tab, are you sure it's making it to the tube pin?

I am sure the bias is making it to the tube pin. Inside the tube I can't be sure. The outer plate structure doesn't look damaged at all on the one side I can still see clearly through. From what I can see of the other side it looks ok too. I can't see right in the middle because of the stuff on the glass.

Can you post a pic of the dead tube? I'm just curious.

I'll do my best with my cell phone.

Greg

[Alan0354]

My (sketchy!) understanding is that as frequency is moved down, the magnetic circuit 'headroom' decreases, and that when it saturates, the primary inductance (which facilitates the primary impedance) collapses. At that point the impedance transformer action will probably fail and primary impedance becomes the primary resistance, eg for this amp, a few ohms rather than a few hundred ohms.
That might push the power tube plates into overdissipation and bring forward an impending failure mode?
Perhaps repeating that current measurement test comparing 20Hz and 40Hz at full power would indicate whether the above was the case?

This makes sense, if the primary impedance decrease and the tube draw the same current, the tube dissipate a lot more power. You basically change the load line and working outside of the power limit.

I don't know this particular amp, but from looking at My Marshall, I think people really push the power dissipation of the tubes already. I remember I had a thread here about using my Marshall and a signal generator to get 60Hz to break in the speaker, I ran no more than 30Wrms on a pair of 6L6. I reach down to 50Hz on and off, I blew 2 tubes in 2 hours!!! That's a guitar amp, I did reach below guitar frequency too. I ended up buying an 18V transformer and using a variac to finish the breaking in.

Ha ha, I've been in the DIY audio forum looking at building an audiophile amp. I learn that DC imbalance between the two half of the push pull really becomes very critical at low frequency. When you use an Octive, you reach into that region, if you have DC imbalance the efficiency goes down, in turn you crank up the amp.....and the tube goes poof!!!!

[pdf64]

OK, sorry I'm more used to measuring things with a sine wave as the signal, rather than an instrument.
With the meter on dc, what happens to the cathode current when there's a continuous full power sine wave?

[J M Fahey]

^^^^^^^ I remember that.

I think I commented that using an amp, and a tube one at that, to get important and continuous power at 50/60 Hz was, at least, inefficient , let alone making the poor guitar amp work very hard (a transistor amp would have suffered much less) and to boot, any cheap mains transformer can do that (and nothing else) all day long with no sweat

EDIT: simulpost, I was answering alan

[GregS]

To see it from another point of view (although it leads to a similar conclusion) , post mortem autopsy or not, there's something we already know: the tube was glowing orange.

As justin says, it is a lot of heat ... which in due time means a lot of power is being dissipated.

And power is V * I .

Since voltage is predefined by the PSU, only variable left is intensity.

Tube current (at a given plate voltage) depends straight on grid voltage so everything points to either gross lack of bias or destruction of grid function (internal shorting, wire breaking, loss of wire to pin contact, etc.)

I fail to find other plausible explanations.

That the tube afterwards does not pass much current, if any at all, may come from destruction of internal structures ...... but while glowing orange it was passing a lot.

One thing to note is that nothing around the tube heated up much which indicates that it occurred very suddenly rather than a gradual ramp up of current. Probably a sudden catastrophic failure.

One other thing I haven't mentioned is that the grid leak resistors I used are a little high for a 6550 at 220K. With the lower value grid leaks the output of the phase inverter was getting loaded down to much and the amp wasn't making full power.

Greg