I've been chasing an oscillation for a few weeks now and finally have an answer
I have in effect 2 identical amps and both of them will present with this oscillation if fitted with 6L6s and they see a reactive load (again, I have 2 types available)
Remove the reactive load and plug direct to the cab = no oscillation
Alternatively, fit EL34s in either amp and run via the reactive load = no oscillation
Having spoken to the builder, he tells me that the OT was specced to sit in between the ideal primary impedance for both valve types and that the reactive load is weighing down the 6L6s to the point where it's causing a problem
I've read around this and I still don't quite get the relationship between the power valve resistance, the OT primary and how this is 'mis-loaded' (not sure if it's too high/low?) with the reactive load?
Could anyone explain the relationship and why one is bad and the other fine, in idiot speak and without the need to resort to algebra?
Finally, what value am I looking for on the valve data sheets? ..I'm ultimately trying to understand if KT66s, 6550s or KT77/88 might not cause the problem I have with the 6L6s?
Assuming it is resistance, then I see confusing data from different manufacturers
Eg
SED
EL34 plate resistance 2k
6550 plate resistance 3k
6L6 plate resistance 5.6k
JJ
EL34 - R(tiny k/tiny f) of 20K ..WTF?
Cheers
..a very confused Baz
The answer was never 42 - it's 1/137 (..ish)
Comments
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
The two reactive loads are a Fryette Power Station and a Two Notes Torpedo Reload - very different re-amp, but I suspect both may use the same basic reactive load circuit in the Aiken Amps site
And the valve type definately makes the difference..
I do not pretend any greater technical than ICBM but would like to make a few points?
The "Anode slope resistance" of a valve is RELATED to the "optimum load" but is not the same as it. The load specified (usually a-a for ppull) is one which give the best balance between output power and distortion. Remember ALWAYS, valves were designed and the ratings given for QUALITY reproduction!
Then, this OL will depend upon the particular circuit conditions: HT V, G2 V anode current. AFAICT guitar amp designers, sucked it and saw and if it sounded ok and did not eat valves (too badly!) Rock n Roll!
I strongly suspect those amplifiers have negative feed back ? If so I am not surprised that under SOME conditions they oscillate. It is likely that the happy-go-lucky, catch all traff loading gives slightly more overall gain with the 6L6 than the EL34.
Usual design fixes for a screamer? A pole in the PI, that is 100puff or so across one or both PI anode loads. Or a cap between the two anodes (needs to be 1kV rating) or you could try a shunt cap across the series feedback resistor.
But at the end of the day amplifiers are designed to drive "usual" loads. Had this been one of "ours" I would just say "tough"!
Dave.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
If you really want to get into this Baz you could try a "Zobel" network across the input of the load.
Start with a resistor, chunky one, equal to the working Z and a capacitor of 100nF then slap on more 100nFs and you Might find a value that turns the phase shift back to goodliness. I am assuming the "reactivity" is inductive?
IC I take you point about G2 overload. The fact is that if an amp is putting out say 20V rms (50W >8R) into a big reactance no power is dissipated IN that reactance but it has to go somewhere and is lost in the active OP devices.
The Quad electrostatic speaker claimed the lives of many power transistors because it was almost a pure capacitance. To be fair many of the amps blew because they were not stable!
Note! The Zobel C and R are in series!
Dave.
So I'm ok to run the amp at 8 ohms into a 4 ohm load ..thought that was bad for the OT?
I'm picking up an old scope next week, so I may have pictures (..bring popcorn)
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
The oscillation issue is a strange one. Competently designed amps are "unconditionally stable" and most valve guitar amps use fairly low feedback anyway.... The idea above from "ecc83" to try a Zobel network is a good one but should not really be necessary...
Jez.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
"Resistive loads are indeed safer than inductive loads for higher impedance loads, because inductive loads can rise much higher than the nominal impedance at the resonant point or at the upper frequencies. A resistive load at a 2:1 mismatch will remain mismatched at 2:1 over all frequencies of interest, while a 2:1 nominal mismatch with a reactive load will vary from 2:1 up to possibly 12:1 or even 24:1 at the resonant point. Higher than normal impedances are usually responsible for transformer, tube, or tube socket arcing failures.
In addition, resistive loads are much less likely to cause instability problems, like parasitic oscillations, because they don't have the phase shift associated with reactive loads. A purely resistive load will not have the same level of overshoot/ringing on square wave edges, either, so it won't have the higher-voltage "spikes" on these edges that can cause arc-over.
Typically, it is not the just the higher resistance load that causes problems. A 2x, 3x, or even 10x resistive load is not really a problem for the amp, because the tube voltage is limited by the plate voltage - the output can only swing to a max of twice the plate voltage on either side of the output transformer. For example, if you had a 500V supply, the max you could ever get is 1000v on each plate, or 2000V p-p across the entire output transformer primary. The transformer is usually designed to handle this. In reality, the output tube cannot drive to 0V at full saturation, especially in class A1/AB1/B1, and the transformer winding resistance also limits the upper range, so you wouldn't even get the full 1000V swing on each plate, more like 950V or so. If you drive the amp to full clip, you'd never get more than around 1900Vp-p across the transformer primary, no matter whether you had it connected to the nominal 8 ohm load or a 32 ohm load.
The problem comes in because transformers are not ideal, and they have leakage inductances and things that can cause the output waveform to not be a pure, flat, square wave. It will have "ringing" or overshoots at the edges, because of the fast transition of the edge, and this overshoot can be very high, depending on the transformer design. Here is where the load impedance characteristics come into play. A reactive load, and especially a mismatched one, can cause this overshoot to be very large, and the spikes can exceed the voltage breakdown ratings of the output transformer or tube socket insulation. The same will happen with a resistive load, but not to as great an extent, but there will still be higher voltage spikes if you run into an impedance that is 2x or 3x the nominal value than you would see if the amp was loaded with the correct impedance."
It's not though likely to cause continuous oscillation on its own unless there is a "perfect storm" of other factors....
As jpfamps says above, a capacitive load would be much more likely to cause a problem than a pure inductance... but a combination of various reactances (+ & - j) can get pretty nasty!
This is one of those interesting ones that if I had it on the bench here I could probably quickly solve, but is virtually impossible to analyse beyond speculation at a distance. I would wager a few beers it ends up being a decoupling or grounding issue though! :-)
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
Actually a negative feedback amp always has a load: the feedback network.