KT88 Bias Adjustment

[batinal]

I have the problem with my KT88 PP stereo integrated amp to set bias voltages of KT88 of which scheme as below;

If I set Vk to the same value by adjusting the pots for both tubes, I have audible hum so I cannot set them to the same value but different values, I.e. 0.410v and 0.435v, etc. Can anyone tell me what I have done wrong? Those tubes are said to be pairs.

Thank you very much in advance.

Kay

[anatech]

Hi Kay,
Two questions.
Are the tubes semi-new (good quality) at least and matched?

Have you checked the resistors to make sure they are the same value (measure them)?

-Chris

[gingertube]

This one I think is simple - The 10 Ohm cathode resistros are NOT equal. The NO HUM condition is when the idle currents are matched.
The fact that one side reads 0.41V abd the other reads 0.435 just means that the resistors are 5% different in actual value.

Might I suggest that you replace the 10 Ohm resistors with 1% 0.5W metal film. This will mean that the voltage readings for the same current will be within 1%, Plus the low power rating of the resistors means they will act as a fuse and blow up in the event of a tube short, thus protecting your output transformer.

Lesson: Check the tolerance of the components you are using and be aware that even if they start out at 1% tolerance they may move ourside that tolerance value if subjected to abuse.

Cheers,
Ian

[richwalters]

Sounds like tubes may be matched on DC conditions but not on AC signal gm. I found this out with 6550B's.
Don't overlook the hum may originate in the previous stages to phasesplitter, i.e unequal ripple rejection in either phasesplitter output to to rough B+.
Is the neg bias clean and C16/17 in good condition ?
You don't show any neg feedback loop. I presume you don't have any , so this will make the output stage matching doubly sensitive to mismatch and even a mains transformer in proximity will be noticeable.

Perhaps you should post the rest of the circuit...

richj

[anatech]

Hi Ian,
I had a customer come in with the same problem. One Chinese tube was defective. I was thinking tubes because the elements were aligned poorly. Some new outputs solved it.

So it can happen!

-Chris

[batinal]

Thanks for great tips!

I will check on all the possibilities you mentioned.

Sorry, I am still working on making full schematic of the amp which came to me from China w/o it.


Best regards,
Kay

[kt88music]

Originally posted by gingertube
This one I think is simple - The 10 Ohm cathode resistros are NOT equal. The NO HUM condition is when the idle currents are matched.
The fact that one side reads 0.41V abd the other reads 0.435 just means that the resistors are 5% different in actual value.

Might I suggest that you replace the 10 Ohm resistors with 1% 0.5W metal film. This will mean that the voltage readings for the same current will be within 1%, Plus the low power rating of the resistors means they will act as a fuse and blow up in the event of a tube short, thus protecting your output transformer.

Lesson: Check the tolerance of the components you are using and be aware that even if they start out at 1% tolerance they may move ourside that tolerance value if subjected to abuse.

Cheers,
Ian

Good one Ian,

I think i will tri that on mine too

[richwalters]

Why use 1% tolerance stuff for tubes ? In earlier days even 20% was quite justified and only a few resistors used at 5% at critical locations. looking at the other posts regarding matched tubes from vendors, tube checks are usually done at 250V. At the more usual 400V in amps, variables will result. One might notice in AB, the tube quiescent conditions and low power class A may be well matched but when running on demand into class B there can be quite a cathode current mismatch between pairs.

With output transformers, the standard EI stampings provide a very small but inherent magnetic gap between laminations. Compare the inductance values with a toroid of same performance and with a continuous iron sheet trany how little current it takes to saturate the core. Various current balancing circuits have appeared but I found them all effected the sound quality. All in all I find the basic transformer the best solution to tube variations and that little inherent magnetic gap keeps the iron B/H curve more linear and touchhigher to saturate.. to the advantage of tube operation.

richj