Biasing at 85-100% in class AB. (Rivera/Cameron/KT77 cont.)

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Nigel

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So, to get KT-77's cooking, I'm running one at 74ma/432v (31.5watts!) and the other at 60ma. I find the mismatch more rokken than close matching. This is in a Rivera M60, all 5 preamp tubes are Mullard long plates. #corksniffinFTW

So, I don't seem to like these tubes biased less than 26 watts or so at idle.

Should I just get a THD Bivalve or some cathode bias/class A amp?

Did rockers of yesteryear bias their marshalls at 100% and sacrifice glass to the Gods of tone?

What say ye my brethren?
 
Nigel":b963gywe said:
So, to get KT-77's cooking, I'm running one at 74ma/432v (31.5watts!) and the other at 60ma. I find the mismatch more rokken than close matching. This is in a Rivera M60, all 5 preamp tubes are Mullard long plates. #corksniffinFTW

So, I don't seem to like these tubes biased less than 26 watts or so at idle.

Should I just get a THD Bivalve or some cathode bias/class A amp?

Did rockers of yesteryear bias their marshalls at 100% and sacrifice glass to the Gods of tone?

What say ye my brethren?
Click to expand...
KT77's have a maximum Plate dissapation of 25 watts. Theres a balance for me between tone andrisking your amps components which is what I think you are doing. You are putting a whole lot of trust in your fuses. Me, I don't trust my fuses that much. :no:
 
Just found this thread: viewtopic.php?f=25&t=102473&hilit=100%25+bias

CCV's are like my old Zinky with 545v of B+. I modded that amp with 2K5W screen resistors but it still ate KT-77's for lunch. Looks like I need a CCV!

Mark sounds like my kinda bro!

"I set the Bias hot because it sounds best to me. I designed the amp to run a hot bias on high power and that contributes to the tone I like. I have had no problems with premature tube failure with the hotter settings."
 
So is this a request for input from your RT brethren, or is this a statement of purpose and rationale?

I'm not keen on biasing that hot - period. There's a keen balance somewhere in that mix, and the upper levels of that balance are fine with me. I've got a quad of KT77s biased at around 33mA for the quad. They sound terrific.

And they'll last :D

Good luck though, whatever floats yer boat :salute:
 
The downside is not in failed glass, it is frying the screen resistors and having the rest of the amp circuit exposed to that. No matter how you are balanced at the PI, you are always going to have a tube that takes more than the other, whether it is 2 or 4 tube set ups and the tubes pulse with every chord. So, at times, you may have a tube seeing in excess of 100%. I always find the amp (with exception of a CCV) gets noisy and you can tell when running the bias up excessively and this is usually a cut off point that I wouldn't exceed. I don't necessarily stick to the recommended 70% and run most of mine around 80%, but this would be with SED's, that I trust. No way I would do it with anything else, especially JJ's. Some tubes run a little better cooler than hotter, with regards to tone. So yes, in the end you are shortening tube life, that is a given and logical. How much, who knows? But, if it was an amp that I am particularly fond of, I wouldn't subject the screen supply side to the dangers of a meltdown.

My 2 cents, which is about all that it is worth.....

Steve
 
Thanks for the feedback. With the KT77, it's tough to say what 100% REALLY IS. Also, driving a tube above 100% is not something I fear, as the "safe limit" isn't necessarily rock and roll. The official 25 watts doesn't seem true to me, more like a 30-32 watt tube. Some say it's 28 with screen, I don't know. What I do know is that tube whipped with high bias in the right circuit configuration is the most stellar tube sound I've ever heard. Peter Diezel is shipping everything with it now so there must be magic in that bottle.
 
Nigel":1mpvge0s said:
Thanks for the feedback. With the KT77, it's tough to say what 100% REALLY IS. Also, driving a tube above 100% is not something I fear, as the "safe limit" isn't necessarily rock and roll. The official 25 watts doesn't seem true to me, more like a 30-32 watt tube. Some say it's 28 with screen, I don't know. What I do know is that tube whipped with high bias in the right circuit configuration is the most stellar tube sound I've ever heard. Peter Diezel is shipping everything with it now so there must be magic in that bottle.
Click to expand...

Which 77's are you using?
 
Going to try some 1K5 and 2K screen resistors in the Rivera this weekend. The KT's are stable at 26-32 watts at idle, but MAN those screens are bright and with a slight variance of speaker resistance (a 6 ohm vs 9 ohm "8 ohm" speaker in the TAC), tone is real different.

I see the Cameron has 7K per pair of EL34's. The Rivera's OT is 3K5 for two, and KT-77's like a higher value than EL-34's, but the B+ is around 435 vs 545 on the Cameron so the lower value gets a pass I suppose. They sound good with 3K5, 7K is a bit choked. Still trying to wrap my awareness around these relationships...
 
You keep on biasing them that hot at idle, you're going to cook an output transformer primary. The reason they are biased at idle into cutoff is so that power dissipation off or on is at maximum or minimum - the voltage difference between grid to cathode and reference ground is always at a large potential and heat dissipation is kept to a minimum. Current flow is controlled by the source AC signal on the grid that of which a tube is happy. biasing the tube at max plate dissipation stresses the plate designs internally, especially if they are not coated, because you will get transient DC leakage current flow from the plate to cathode even with no signal applied. This AC controlled valve is now sinking DC current to ground through the OT primary and your power source. since the tube is always on and never off at idle - AKA grid leak bias of zero of a cathode bias setup, This can further lead to bias runaway because of the heat being generated with the tube always being on. Think logarithmic increase.

I suggest you stop increasing screen voltages and bias without further knowledge of what is really going on and why they are class AB biased in the first place. I would also suggest stop biasing to datasheets and get yourself a tube tester and a curve tracer, modify it for 8 pin vaccum tubes, and bias based on individual characteristics instead of shooting two sails to the wind without a clue what you are up against.

Just my .02. Take it for what its worth.
 
Thank you ALL for the time and effort to give me feedback on this. I love this board, it's fertile ground for reason and intellect where passionate feelings abound and lower emotional noise is kept to a minimum. One could say the supply is filtered right!

How is it that the CCV is biased over 100% and runs this way with no problem for years? Is it cathode bias? I'm installing larger screen resistors to lower screen voltage and therefore current this weekend.

The KT-77 gets to magic land when whipped with high bias...at this point I mod the Rivera, sell it and get a Bivalve, or sell my soul for a Cameron CCV. Hep me lawd!

:lol: :LOL:
 
Well, for starters, there is no such thing as over 100% - conductivity does not work that way, last i knew mark did not throw any superconductors in the CCV.

My bet would be in the transformer design and a few safety measures heeded, both after the phase inverter, in the power supply, and in the transformer design itself. Non-of-which are in the scope of what you're really doing, it is more how the amplifier is designed and not directly tube related.

Mark has a reputation of building very unsafe amplifiers in his past. With trial and error comes the knowledge that goes with it.
 
Nigel":1353ooz0 said:
...at this point I mod the Rivera, sell it and get a Bivalve, or sell my soul for a Cameron CCV.
Click to expand...

As I read all this stuff, I knew selling your Rivera was going to come into it sooner or later. Sounds to me like you simply bought the wrong amp. Remind me not to buy your M60 from you. (and yes, I am looking at used Riveras, that's how I found this thread)

glpg80 is quite right......no offense, but your ears aren't doing your M60 any favors. You are a prime candidate for electronics school so you can learn to fix your own amps, and maybe design the one you really want.

Go with the Flexi 50, not the Bivalve.

Nigel":1353ooz0 said:
...Hep me lawd!
Click to expand...
Lawdy don't unburn your bridges for you. Better save your money instead.
 
soundchaser59":599jhtbj said:
Nigel":599jhtbj said:
...at this point I mod the Rivera, sell it and get a Bivalve, or sell my soul for a Cameron CCV.
Click to expand...

As I read all this stuff, I knew selling your Rivera was going to come into it sooner or later. Sounds to me like you simply bought the wrong amp. Remind me not to buy your M60 from you. (and yes, I am looking at used Riveras, that's how I found this thread)

glpg80 is quite right......no offense, but your ears aren't doing your M60 any favors. You are a prime candidate for electronics school so you can learn to fix your own amps, and maybe design the one you really want.

Go with the Flexi 50, not the Bivalve.

Nigel":599jhtbj said:
...Hep me lawd!
Click to expand...
Lawdy don't unburn your bridges for you. Better save your money instead.
Click to expand...

I've noticed the Rivera sounds best biased slightly hot, as there is a balance between SL and M channels. Too hot and the clean channel doesn't distort properly. That, and it doesn't have sufficient B+ to open up the KT-77's properly.

I did get a Bi-Valve, and it is a wonderful amp with the 77's. Sold it though. They really need 500v+ to open up. May 'speriment with an Uber when I get the chance.
 
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