audio-talk

Hi guys. I need a little help please. I have a set of 4 Electro Harmonix KT88 valves. I noticed today a slight drop out on a song and slight bluring. Sounds almost like a small jump on vinyl. Could be my bluetooth receiver but the tubes are also getting on. Probably had 300hrs tops through them but about 8 years old.
What are the tell tale signs of needing new ones. I haven't got a tube tester.
I like the sound the electro harmonix give and listen to mainly Msle/female solo with piano and guitar. Mainly rock based. But modern stuff problem classed as easy listening. Definately something radio stuff.
If contemplating new kt88tubes and not wanting to spend silly money. What others would be contenders.
If my tubes are still good, it would be nice to compare

If the valves were dying off due to long usage rather than faults, then you will see lower transconductance. This means less current than expected, flowing, for a given anode to cathode voltage, and bias. So, without a tube tester, measure the anode, cathode, G2 and grid voltages, and measure ( if you can ) the voltage drop across the cathode resistor, noting its resistance value. Work out the current flowing, based on the voltage across the cathode resistor, using I = V/R
If you then look at a standard KT88 valve datasheet, find the graph that shows the curves for your G2 voltage ( relative to cathode ) . Then check using the anode voltage (relative to cathode ) and grid voltage ( relative to cathode ) and see what current should be flowing.
If it's less than about 70% of what's shown on the datasheet, the valve is too aged and should be replaced.
( Now having said that, I need to go away and check if the pentode graphs normally show anode current on the X-axis, or anode plus G2 current ! )

Having said all this, 300hrs is not that much, unless the valves are being run very hard and near/over their maximum dissipation.
If you get sudden drop-outs of sound, there are a lot of other possible faults.

Also check the "getter", the silver or black bit on the inside of the envelope, if a large part of it is transparent, that’s another sign they are getting on. It very much depends on how hard you amp works the valves.

The graphs show anode current when it's working as a pentode - annoying, so you have to add in the likely G2 current, since the cathode current ( through the resistor ) is the sum of the G2 and anode currents.
Are these valves working as pentodes ( or tetrodes ) or triode-wired ? It's easier to check if triode-wired.

+ to what Nick said, inside of glass going brown

Thank you very much for the replies my amp id a WAD kit88 running in pentode mode I believe. The testing skill is way above what I am capable of but the visual clues are very helpful. The tubes look okay and after reinserted them the problem seems to be resolved.
Other than upgrading the tubes, any other mods are probably beyond me.
The amp was upgraded before I got it though. With a capacitor change. Same transformers as original.

Ah, dirty socket connections then hopefully.

I just remembered, yes another sign is funny noises. Have had a bunch of 6SN7's go off with clicking crackling noises. Sounds like bad connections, but not. Also had Golden Dragon EL34's where the getter gradually disappeared. Didn't turn white, just disappeared! 300B's that made a soft pulsing noise - air had got in, so they made an effect like a guttering blue gas flame as well.

I read somewhere a valve starts deteriorating as soon as you turn it on ...

Well after some hum getting louder over previous months, I replaced all the kt88 tubes. I did this because I read that old tubes can let transformer noise through. After inserting the new tubes, the amp is silent again. It was only after buying the new tubes that I noticed the silver tops on the old tubes had diminished greatly. This was mentioned previously on this post and looks like a good indicator that tubes are on the way out to a novice like me

Life is a learning curve... Good result !