Kt88 triode strapped

[catalin gramada]

Hello. Does it reliable to use a 5k Ra-a OT for Kt88 triode strapped at 570v. fixed bias. pp configuration. Please? Thanks

[Thoriated Tungsten]

I would say that is the worst idea ever, considering the *original* datasheet says 'Separate bias resistors are essential' for triode strapped P-P operation. This in addition to you wanting to run the tubes at an anode voltage of 100 V beyond that suggested in the datasheet. Even at 485 V the total, resting dissipation of each tube will be run at 100.0% of maximum, total allowed dissipation for anode plus screen grid. So even this lower voltage will be highly questionable when it comes to reliability and tube longevity. Even more so considering current production tubes.

So no.

[J M Fahey]

WHY triode strapped?

With pentodes you can feed thin wire screens some reasonable voltage (say, around 400V ) and let robust plates handle a higher one; you canīt do that with triode strapped connection.

That alone shows itīs a bad idea under high voltage supply conditions.

[bob p]

KT = Kinkless Tetrode

[Jazz P Bass]

Not too sure if that is a pun
KT tubes are beam power tubes.
The KT stands for Kinkless Tetrode because it is a four electrode device.

[bob p]

Yes, "KT" is the giveaway that it's not a real pentode.

Back to the OP's question, I don't think it's right for us to tell him that he doesn't want to run a KT-88 at high voltage in triode mode -- maybe he does. There are lots of HiFi types who preach the virtues of triode operation, and lots of HiFi type builders who like to use high power tetrodes or pentodes in triode mode to get an amp that will produce more watts than the anemic direct-heated triode offerings.

[Thoriated Tungsten]

Yes, "KT" is the giveaway that it's not a real pentode.

Back to the OP's question, I don't think it's right for us to tell him that he doesn't want to run a KT-88 at high voltage in triode mode -- maybe he does. There are lots of HiFi types who preach the virtues of triode operation, and lots of HiFi type builders who like to use high power tetrodes or pentodes in triode mode to get an amp that will produce more watts than the anemic direct-heated triode offerings.

He asked whether the suggested configuration would be reliable. To which the objective answer should be a clear 'No' due to the reasons provided.

[Enzo]

There are lots of HiFi types who preach the virtues of triode operation

Sure and there are hifi types who swear by $500 power cords.

I think it is incumbent upon us to give the best advice WE have, and not worry about what someone ELSE thinks is right.

[bob p]

Sure and there are hifi types who swear by $500 power cords.

I think it is incumbent upon us to give the best advice WE have, and not worry about what someone ELSE thinks is right.

We're like to pull out the "best engineering practices" argument whenever it suits us, but we fail to make those arguments far too often, such that we're pretty selective about when we decide to use them. Usually that's when it suits us. If we were consistently concerned about giving the best advice and not worrying about what someone else thinks is right, then we wouldn't condone dangerous ideas like using acrylic sheet as circuit boards in tube amps.

The OP never asked about $500 power cords, or any other sort of HiFi malarkey. The guy wants to run a KT-88 in triode mode at high power. That is a legitimate concern that is far from the domain of HiFi fantasy mojo. I'm completely missing why the hyperbolic reference to a $500 power cord is part of this conversation. There's nothing absolutely wrong with running a KT-88 or a 6550 in tiode mode at high voltage, if you take into account all of the relevant parameters.

[Enzo]

I brought the $500 cord up as a counter to "a lot of hifi guys...". He didn't ask about cords, just as he didn't ask about hifi guys. All I meant was the advice we hand out ought not be held up against what the hifi community thinks. Or NASCAR, or fishermen. If the hifi guys want to beef our forum for approving acrylic circuit boards, they are welcome to do so with no argument from me. Note that we as a group never gave that advice, only some individuals. Acrylic boards is not part of our canon.

So the bottom line was I was simply disagreeing with you about the hifi guys as a supporting basis. I didn't see it as supporting that some other area of interest does something.

[catalin gramada]

There is nothing about hi- fi but half power mode. Using on my bench a kt90 pair.independent bias switch to set power dissipation so anode current set as same.Measured current in line with screen grids without limiter resistors shows 6w dissipation arround.I get 18.5v into 7.5ohm load clear sine.Is a little bit less than half than normal pentode connection for the same load.same supply voltage under load. Speaker test shows dramatical differences- excesive bass reponse in triode mode. Power grids are feed from very low impedance source. No global nfb was used.Why so dramatical changes in bass reponse.please? It is smply of output coupling matter?How changes of primary OT impedance will affect it?

[J M Fahey]

Starting with triode strapping, datasheet never ever suggests anyhing even approaching 560V +V for that connection, maximum mentioned is 400V or 485V , no signal, dropping to 349 and 422V respectively at full output.

And for tetrode connection, they mention 560V for plates, but keeping screens at 300V.

As of bass response, triode connection offers way lower damping than tetrode (or pentode) so cabinet response will definitely change.
Funny thing is that probably you will not see it on a resistive test load.

[Malcolm Irving]

...As of bass response, triode connection offers way lower damping than tetrode (or pentode) ...

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

[Dave H]

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.

[bob p]

Starting with triode strapping, datasheet never ever suggests anyhing even approaching 560V +V for that connection, maximum mentioned is 400V or 485V , no signal, dropping to 349 and 422V respectively at full output.

And for tetrode connection, they mention 560V for plates, but keeping screens at 300V...

I'm all in favor of keeping our recommendations focused on best engineering practices and targeting de novo builds using design center values, and I think your advice is good. But at the same time, I have to think about how Fender and Marshall have purposefully ignored the data sheets in producing some of the best sounding amps ever built.

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.

These amps are famous for doing exactly what the data sheets say that you shouldn't do with their tubes. Some of their greatest traits are derived from their characteristics as Intentional BEP Violators. Sure, going along with BEP doesn't allow us to recommend exceeding recommended values, but real world data clearly shows that it can be done.

This makes me think that anyone who's designing a circuit shouldn't feel totally constrained by the tube charts when it comes to deciding whether or not a given set of parameters will work. In that respect, if I were asking the OP's question I would hope for a more pragmatic response, based upon real world experience, rather than a verbatim quote from a data sheet. Anyone can read a data sheet but real world knowledge about feasibility needs to come from real people with real world experiences.

One of the great virtues of this site is that it is populated by many people with practical knowledge that goes far beyond the data sheets. I love to read those types of answers because I learn something every time. In that respect I think that answers that use words like "worst idea ever", and then parrot a data sheet, are particularly disappointing because they are stifling toward innovation.

FWIW I've always had a question in the back of my mind about how far a set of 6550 could be taken in push-pull triode mode. 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.