Kt88 triode strapped

[eschertron]

I thought it was an interesting question when it came up, and I thought the responses so far were particularly un-enlightening as none of them took me beyond the data sheet.

From Catalin's posts over the years, I've come to respect his experience and knowledge level. (greater than mine!)
So while in response to the posted question of reliability the answer may well be "no, not likely", the answer I see fairly often when it comes to trying specific configurations is "let us know how it comes out". If Catalin builds the PA and it melts down gloriously, then we'll have an answer that supports the conservative, practical data sheets. If it doesn't burn up (at least not right away), then we'll have another design example of a circuit that flouts the design center suggested values.

[Thoriated Tungsten]

If we strictly followed the data sheets to the letter, we wouldn't have 6V6 operating at > 410V instead of at the data sheet's recommended 285V in Deluxe Reverbs; 6L6 operating at > 525V instead of 450V in Super Twins; or EL34 being pounded on at more than their screen limits in Plexis.

FWIW then in my world many guitar amps were designed to intentionally abuse tubes solely for economic reasons. A tube doesn't sound any different if you 'pound on it', it just dies prematurely.

The classic guitar tube amps were designed back when product longevity and consumer protection were all but non-existent. Imagine intentionally designing a new car, where key components in the engine lasts a few hundred hours, if that.

Tubes are not inherently unreliable. NASA's Voyager I & II space probes haven't needed a house call yet to have their transmitter output tubes replaced. Similarly, high quality domestic radios from the fifties generally don't need any tubes replaced when you restore them. You frequently find them with the factory 'seals' on the tubes (yes, this is a thing among good quality European tube radio manufacturers), so the radio has never had any tubes replaced.

Most guitar amps just treat tubes like disposable dirt. Sure, the tubes don't die as fast as if you broke them with a hammer, but in my world it isn't far off. I feel compelled to build my own amp, because I would feel ripped off at having to pay for the physical implementation you mostly get with a commercial amp.

I mean, the only reason we still see people put the tubes *below* the main chassis in guitar amps is because Leo Fender was too cheap to put shaft extensions on his switches and potentiometers. A practice, which I notice in passing was a standard thing to do in the fifties. This was usually done whenever you needed the controls anywhere but near the bottom where the chassis is. I struggle to think of any manufacturer of consumer grade tube equipment from back in the day, who was dumb enough to deliberately make a design error ('choice') like that, regardless of product type or industry.

Additionally, how do you expect *anybody* on this forum to have the resources to say that a tube run beyond specs is reliable? Do you expect some of us purposefully destroy hundreds of tubes in our personal engineering departments in order to gather sufficient statistical data to back up any such statement? A sample size of a few from any one of us would tell you *exactly* nothing.

You might want to start by telling us what 'reliable' means in your world? In the late nineties some friends asked me to help design a 1KW (output) RF tube amp, which was supposed to be able to stand up to any unreasonable level of abuse by inexperienced or tired operators. We did buy spare a spare tube for the project, but to this day the amp is running on the original one. Which was a tested 'pull', Ie. second hand tube, with 5000 of hours on the tube in a commercial setting (Read: Had been operated in a controlled environment.)

I keep a box in my lab with all the failed (all glass receiving type) tubes I have found over the years from various sources - excluding Hi-Fi and guitar amps. While I haven't tested all of them, then I do own thousands of tubes. I have 19 - nineteen - tubes in the 'fail' box and yes, I do also have a proper transconductance tester with H/K leakage test and everything.

Personally I don't like to replace tubes, so I strive to design like the German engineers of the fifties did. Others may see this differently, of course.

[J M Fahey]

I think that's the wrong way around. In triode mode the tubes have lower output impedance and therefore give higher damping.

Oh, sorry, slip of the tongue.
Yes, triodes have lower internal resistance, which translates at the other end again as lower source impedance, which means *higher* damping factor.
Will try to correct my post but if Forum does not allow it, I stand corrected here.
Thanks.

[J M Fahey]

Quote Originally Posted by catalin gramada View Post
No global nfb was used.Why so dramatical changes in bass reponse.please?

The couple of times I've tried triode mode in an amplifier with no nfb I thought it sounded dull and lifeless. I put this down to the frequency response being flatter at higher frequencies because of the lower output impedance driving the inductive speaker load.

Triodes have *internal* NFB, always, unless you use them in grounded grid mode.

[Mick Bailey]

Triode strapping causes a greater plate current excursion and at high plate voltages the tube is easily pushed over the maximum plate dissipation with increasing signal levels. So you can bias the amp and it plays fine at low-ish volume, but get it cranked continuously and it red-plates. There's also the possibility of oscillation if a KT88 is hard-wired in triode mode; it's best strapped using a resistor (100R to 220R).

[catalin gramada]

Hi. Put some KT90 and KT120 in triode mode straped for a bass guitar sound check. Dramatical changes. Get a not usable, damped, muffled,wooly, no definiton tone and hights almost passed away, no matter changing speaker load. Scoped with sine and square the signal looks allright through the whole bandwidth. Everything was setted absolute flat in front relative to power tube grids from 40cps to 10kc/s. Two speaker cabinets was used showed the same type of manifestation.