I've been running a pair of 6L6s in a 20 watt 1-12" PP combo (fender princeton reverb ll) without any issues for over a year (the amp was designed for 6v6s but I rebuilt the bias to suit the change to 6L6s.
Upon changing to a fresh pair of 6L6s one of the tubes shorted out internally...nice lightshow!
Seems it was a bad tube. I did however just read on another forum about the potential for something called flyback voltage from the OT due to running a higher impedance load (as is the case with my 2 6L6s going into a 8 ohm load)
I replaced the shorted tube with a fresh set and so far no trouble yet.
Should I adjust the load? How? There is not really for another 8 ohm spk to run parallel)
Output impedance and flyback voltage
Moderators: pompeiisneaks, Colossal
Re: Output impedance and flyback voltage
Yeah that's one of the hazards of mismatching.
That's what I have read too, you can get flyback on running a higher load than the amp is set to.
If you run a lower load it is harder on the power tubes.
What cha gonna do?
Most high quality OT's can handle a mismatch of one step either way.
In fact some prefer the tone one way or the other.
Me? I'm too scared to blow an OT so I just match 'em up.
That's what I have read too, you can get flyback on running a higher load than the amp is set to.
If you run a lower load it is harder on the power tubes.
What cha gonna do?
Most high quality OT's can handle a mismatch of one step either way.
In fact some prefer the tone one way or the other.
Me? I'm too scared to blow an OT so I just match 'em up.
Tom
Don't let that smoke out!
Don't let that smoke out!
Re: Output impedance and flyback voltage
If the arcing occurred before playing the amp, the impeadance had nothing to do with it. This because impeadance is related only to AC (signal) current.
You could buy a Weber "Z-Matcher" to resolve the mis-match if you want to.
You could buy a Weber "Z-Matcher" to resolve the mis-match if you want to.
If it says "Vintage" on it, -it isn't.
Re: Output impedance and flyback voltage
The primary impedance of an output transformer is not static. Impedance is not the same thing as DC resistance. Impedance (Z) = SQRT(R^2 + X^2), where "SQRT" is the square root operator, "^" means raised to the power of, R = DC resistance, and X = inductive reactance (XL) - capacitive reactance (Xc) (most people who have had trigonometry will recognize that the formula for computing impedance is an application of the Pythagorean theorem). As speakers are reactive loads and XL = 2πfL and Xc = 1 / (2πfc), it is easily to see that the reflected primary impedance that the tube sees changes with respect to frequency.
With the above said, the critical thing to remember is to not operate the tubes with a primary load that is too low, as doing so will shorten tube life due to over dissipation. Operating with a higher reflected primary impedance generally results in less power being delivered to the load.
In closing, we need to remember that an output transformer is an impedance matching device. Primary impedance is function of secondary impedance. A transformer has an impedance ratio, which the square of the voltage ratio (also the turns ratio). An output transformer that matches an 8K primary to an 8-Ohm secondary has an impedance ratio of 1000:1 and a voltage ratio of 31.6:1. Additionally, there are ~31.6 turns on the primary winding for every turn on the secondary winding.
With the above said, the critical thing to remember is to not operate the tubes with a primary load that is too low, as doing so will shorten tube life due to over dissipation. Operating with a higher reflected primary impedance generally results in less power being delivered to the load.
In closing, we need to remember that an output transformer is an impedance matching device. Primary impedance is function of secondary impedance. A transformer has an impedance ratio, which the square of the voltage ratio (also the turns ratio). An output transformer that matches an 8K primary to an 8-Ohm secondary has an impedance ratio of 1000:1 and a voltage ratio of 31.6:1. Additionally, there are ~31.6 turns on the primary winding for every turn on the secondary winding.
Re: Output impedance and flyback voltage
Thats right about the mismatch - higher Zs make for generally higher voltages and increase the probability of arcing. Often the socket or tube base is the weak link.
This is just about words, but the culprits are voltage spikes -- not flyback. Spikes in the couple kV range are normal in tube power stages. You get them because when you try to change the current in a reactive load some charge tends to build up at the door.
This is just about words, but the culprits are voltage spikes -- not flyback. Spikes in the couple kV range are normal in tube power stages. You get them because when you try to change the current in a reactive load some charge tends to build up at the door.
Re: Output impedance and flyback voltage
Ok... so the arcing happened within the tube itself while playing.... just a bad tube. Doesn't GT test these things? Isn't that why we pay the big bux?
Re: Output impedance and flyback voltage
Welcome to the wonderful world of current production tubes. With that said, most current production tubes are at least an order of magnitude better than they were ten years ago. Back then, the importers were basically just relabeling NOS Soviet-bloc and Chinese designs that were close matches to U.S. and Western European designs (e.g., the Sovtek 5881WXT is a relabeled Soviet servo tube). Today, most tubes are still just relabeled close matches, but the selection process seems to be better.
Re: Output impedance and flyback voltage
One last thing: a good phrase to remember when dealing with reactive loads is "ELI the ICE man." The acronym "ELI" is used to denote that voltage (E) leads current (I) through an inductive circuit. The acronym "ICE" is used to denote that current leads voltage through a capacitive circuit.