A tube running at 45% of dis is likely not to be in the sweet spot of operating conditions. Stay at 60% or more to get the tube to act as it should in most cases.
it’s not the heater that makes the tube run hot if it is within 10% of spec voltage, it is the current running through the plate at idle. A hot tube is biased at a point where lots of current is flowing with no signal applied. Its symptoms are many but the most obvious is red plating! The higher a tube is biased the shorter the life over 85% or so. It’s like revving a cars engine at 6000 rpm all the time but instead of rings and bearings your boiling off the emissive coating that creates the electron cloud.
it’s not the heater that makes the tube run hot if it is within 10% of spec voltage, it is the current running through the plate at idle. A hot tube is biased at a point where lots of current is flowing with no signal applied. Its symptoms are many but the most obvious is red plating! The higher a tube is biased the shorter the life over 85% or so. It’s like revving a cars engine at 6000 rpm all the time but instead of rings and bearings your boiling off the emissive coating that creates the electron cloud.
plate dissipation rating + screen grid dissipation rating will be your tube dissipation rating, for class AB1 you can go 70% and still expect reasonable tube life...
cathode resistors are chosen to put your tubes in this range...and for power tubes like the kt88, individual resistors are better for stability of bias...
cathode resistors are chosen to put your tubes in this range...and for power tubes like the kt88, individual resistors are better for stability of bias...
My latest BEWITCH 6550 configuration:
Matched quad of SHUGUANG ELECTRON TUBE KT88-98 (42W max. dissipation)
2 x NOS 6SL7GT in preamp position 1.
2 x NOS 6SN7GT in preamp position 2.
KT88 shared Cathode resistor at a value of 186R. Voltage drop across the 186R measures 32V.
KT88 Plate to Cathode measures 341V.
Bias current per KT88 tube calculates to ~81mA, which equates to ~66% of KT88-98 max. dissipation. I feel that this is close enough to my original 70% tube bias goal.
Listening tests will begin today and I shall report.
If I do NOT discern a material difference in sound quality using this 186R Cathode resistor, I will return to a 250R Cathode resistor - which (subjectively) provided me with the best sound quality of the BEWITCH, to date.
I shall report on how 186R Cathode resistor fares.
Matched quad of SHUGUANG ELECTRON TUBE KT88-98 (42W max. dissipation)
2 x NOS 6SL7GT in preamp position 1.
2 x NOS 6SN7GT in preamp position 2.
KT88 shared Cathode resistor at a value of 186R. Voltage drop across the 186R measures 32V.
KT88 Plate to Cathode measures 341V.
Bias current per KT88 tube calculates to ~81mA, which equates to ~66% of KT88-98 max. dissipation. I feel that this is close enough to my original 70% tube bias goal.
Listening tests will begin today and I shall report.
If I do NOT discern a material difference in sound quality using this 186R Cathode resistor, I will return to a 250R Cathode resistor - which (subjectively) provided me with the best sound quality of the BEWITCH, to date.
I shall report on how 186R Cathode resistor fares.
take note that increasing cathode currents lowered plate voltages....
depending on your speakers, the higher plate volts can swing more output voltage as opposed to higher cathode currents and lowered plate voltage, this will be what you will be listening for, you may or may not hear the difference if at all...
depending on your speakers, the higher plate volts can swing more output voltage as opposed to higher cathode currents and lowered plate voltage, this will be what you will be listening for, you may or may not hear the difference if at all...
I would still go with individual resistors and bypass caps. I’ve used from 390 to 470 ohm 12 watt Mills resistors with good luck on my KT88 amps. That roughly correlates to your 186 and 250 ohm for the pair of tubes. Your V+ will determine which is safe dissipation wise. A higher V+ leans toward the 470 ohm.
Well Tony, once again, you were right on the money.
After finding satisfaction by completely optimizing the normal BEWITCH configuration, my curiosity got the better of me.
Yesterday, I lifted the NFB completely and installed 6SN7GT in all 4 preamp positions.
Subjectively speaking, the resulting sound is as good as I have been able to conjure, from 100Hz and above. Actually, the sound has the intimacy and insight that is not too dissimilar to my old CARY 300B monoblocks - but with better extension at the tops. Loving it!
The bass is still deep and weighty, but less good, however. Not terrible, just a bit plodding and spongey. Gone is the tight and snappy bass that I am used to.
Ideally, I would like to apply NFB to 100Hz and below only - and leave 100hZ and above at ZERO NFB.
QUESTION: With this in mind, how does this idea sound: Bypass the NFB resistor with a (say) .22uF poly cap, creating a high-pass filter for all frequencies above (say) 100Hz. By doing so, it is my hope that the higher frequencies would NOT be subject to NFB - and I will achieve the best of both worlds. That is NFB on the bass and ZERO NFB on the mids and the tops.
Thoughts?
the low frequency deficiency that you noticed is more in part to the limitations of the output transformer and its core...
an OPT that can put out say 50 watts at 1khz will only be half that at 30hz and at 16khz...
this is your limitation...
the next move is suppose is to use separate cathode resistors for biasing...
i read somewhere that 3rd harmonics as a result of output tube slight imbalance can help tha bass...
but if you ask me, i would rather go for a bigger output iron...
an OPT that can put out say 50 watts at 1khz will only be half that at 30hz and at 16khz...
this is your limitation...
the next move is suppose is to use separate cathode resistors for biasing...
i read somewhere that 3rd harmonics as a result of output tube slight imbalance can help tha bass...
but if you ask me, i would rather go for a bigger output iron...
@TonyTecson
Thanks for these alternative ideas, Tony.
Do you have any thoughts to share on the idea above - with (say) a .22uF bypass cap across NFB resistor, to help tighten-up the low-end only?
Have you tried such a workaround - or similar?
Thanks for these alternative ideas, Tony.
Do you have any thoughts to share on the idea above - with (say) a .22uF bypass cap across NFB resistor, to help tighten-up the low-end only?
Have you tried such a workaround - or similar?
Generalizations:
If the push and pull quiescent DC currents are the same, but the transconductance of the output tubes are mismatched, it results in more 2nd harmonic distortion.
If the push and pull quiescent DC currents are un-balanced, that results in early core saturation with low frequency signals.
Take a look at the last page of the Heathkit W-5M user's manual.
It shows the increase of harmonic distortion at low frequencies, if the DC push and pull currents are not equal . . . Even Though there is plenty of global negative feedback around the output transformer.
It is known that global negative feedback around the output transformer does not fix low frequency saturation, it makes it Worse.
That is true for both push pull, and for single ended.
If the push and pull quiescent DC currents are the same, but the transconductance of the output tubes are mismatched, it results in more 2nd harmonic distortion.
If the push and pull quiescent DC currents are un-balanced, that results in early core saturation with low frequency signals.
Take a look at the last page of the Heathkit W-5M user's manual.
It shows the increase of harmonic distortion at low frequencies, if the DC push and pull currents are not equal . . . Even Though there is plenty of global negative feedback around the output transformer.
It is known that global negative feedback around the output transformer does not fix low frequency saturation, it makes it Worse.
That is true for both push pull, and for single ended.
a 0.22uF bypass cap across NFB resistor will cut the high frequency response well into the midrange you want to do that?
the caps across the NFB resistor are chosen to get a nice looking square wave at say 10khz, nothing more nothing less...
i try to avoid NFB if i can and i have done it with an all pentode mode single ended tube amp using the 12gt5 and 6bn11...look at attached scheme for schade feedback....
Attachments
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a reason to avoid GNFB...
there is also the OTL which gets rid of the output transformer altogether....
Tony,
How true.
I would also point out that if you use Global Negative Feedback taken from the output transformer's primary, if the transformer core saturates, the negative feedback will make the saturation worse. de ja vu all over again.
A transformer with a core that is in saturation, will have less primary inductance. The negative feedback will just send more current into the primary, and guess what happens to the saturated core?
Negative feedback signal sample taken from the primary?
Yes, Schade, Ultra Linear, even Triode wired Pentodes and Triode wired Beam Power tubes.
All but one of my tube amplifiers use Triode wired Beam Power output tubes.
The lessons:
Use a transformer that is big enough so it will not be in saturation.
Be sure to have equal push and pull DC quiescent current in the primary.
Or, just turn the Volume control down a little.
Just my opinions, and just my experience.
Have fun listening, I know you do.
I loved my time in the Philippines!
How true.
I would also point out that if you use Global Negative Feedback taken from the output transformer's primary, if the transformer core saturates, the negative feedback will make the saturation worse. de ja vu all over again.
A transformer with a core that is in saturation, will have less primary inductance. The negative feedback will just send more current into the primary, and guess what happens to the saturated core?
Negative feedback signal sample taken from the primary?
Yes, Schade, Ultra Linear, even Triode wired Pentodes and Triode wired Beam Power tubes.
All but one of my tube amplifiers use Triode wired Beam Power output tubes.
The lessons:
Use a transformer that is big enough so it will not be in saturation.
Be sure to have equal push and pull DC quiescent current in the primary.
Or, just turn the Volume control down a little.
Just my opinions, and just my experience.
Have fun listening, I know you do.
I loved my time in the Philippines!
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Just unpacking this a bit as some of these ideas and comments seem to be conflicting with what I am hearing.
With factory levels of GNFB - tweaked with a tiny 11.5pF bypass cap - the BEWITCH 6550 amp produces some of the deepest, tightest and most pleasing bass that I have experienced.
Changing the 6SL7 (position 1) and 6SN7 (position 2) to ALL 6SN7 PLUS lifting GNFB completely, the amp remains utterly silent to full volume and produces wonderful mids and tops. But the (still) deep and weighty bass is now plodding and spongey.
Should I blame the OPT?
Perhaps the circuit just needs to be optimized for the 6SN7 now sitting in the (formerly) 6SL7 hole?
Here's the current schematic...
(Please note that I regularly switch back and forth between NOS 6550 (Russian) to new build SHUGUANG KT88-98 output tubes as they have their own distinct sonic signatures...)
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If the output transformer bass was super wonderful with the original tube types and tube placements,
and now the bass is not good anymore when you removed the global negative feedback.
In that case, I would not blame the output transformer.
You did remove the negative feedback. That raises the output impedance (lowers the damping factor).
I bet the lower damping factor changed the sound of your woofer.
and now the bass is not good anymore when you removed the global negative feedback.
In that case, I would not blame the output transformer.
You did remove the negative feedback. That raises the output impedance (lowers the damping factor).
I bet the lower damping factor changed the sound of your woofer.
no, i just want to point out that you have hit your bottleneck, and the output transformers were always the bottleneck..
you can run the next 6sn7 with a tad more plate current, but this helps the highs more....
get yourself a better output transformer....
Tony,
Post # 211 has Schade negative feedback from the output transformer's primary.
And it also has global negative feedback from the output transformer's secondary.
Post # 214 does not have any Schade negative feedback.
It used to have global negative feedback from the output transformer's secondary, but that was disconnected.
Now it does not have any negative feedback other than UL mode, or Triode wired mode (a switch to select the mode).
No global negative feedback, and no Schade negative feedback, lowers the damping factor (depending on the woofer and cabinet design, the woofer 'cares' a lot about damping factor).
The output transformer sounded good with the negative feedback, according to SONDEKNZ; but not as good with no negative feedback.
That sounds like the output transformer is not the problem.
However, SONDEKNZ noticed that it now 'produces wonderful mids and tops'.
Only the input tube was changed from 6SL7 to 6SN7, and the global negative feedback was removed.
Some amplifiers mid and highs are "opened up" by removing global negative feedback. Perhaps the mid and high frequency drivers, and their crossover elements sound more clear with an amplifier that has a slightly higher output impedance.
Food for thought?
Post # 211 has Schade negative feedback from the output transformer's primary.
And it also has global negative feedback from the output transformer's secondary.
Post # 214 does not have any Schade negative feedback.
It used to have global negative feedback from the output transformer's secondary, but that was disconnected.
Now it does not have any negative feedback other than UL mode, or Triode wired mode (a switch to select the mode).
No global negative feedback, and no Schade negative feedback, lowers the damping factor (depending on the woofer and cabinet design, the woofer 'cares' a lot about damping factor).
The output transformer sounded good with the negative feedback, according to SONDEKNZ; but not as good with no negative feedback.
That sounds like the output transformer is not the problem.
However, SONDEKNZ noticed that it now 'produces wonderful mids and tops'.
Only the input tube was changed from 6SL7 to 6SN7, and the global negative feedback was removed.
Some amplifiers mid and highs are "opened up" by removing global negative feedback. Perhaps the mid and high frequency drivers, and their crossover elements sound more clear with an amplifier that has a slightly higher output impedance.
Food for thought?
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Appreciate all the thoughts.
Plenty of food for thought…
Just a quick note that the UL/TRIODE switches were removed long ago.
This amp operates in TRIODE only!