Musicians Roadhouse

I guess I have to replace my power amp valves now, right?
I have no idea when and why it happend. Everything works and sounds just fine. There is light in all of 4 power amp valves. Power attenuator is ok. But when I turn on auto-bias I can hear quiet pops in cab.
Added little poll about which valves should I choose - 6550 or KT88?

P.S. There is no need to make any mods with amp to replace original 6550 with KT88?

Don't think you have to replace all of them, just the one in V3. I think what the light is telling you is the amp cannot bring that tube up in the range.

If the amp sounds good I would leave it alone. It is not uncommon to run like that. If you do get a new tube hopefully it will be matched well to the others. If you want to try a new quad I vote 6550s, I know those will sound good, not sure how 88s would sound in there but they are a nice tube in certain amps.

Thanks for a quick response, Surf!
In this case I'll buy new 6550 valve, replace it with the original and see what happens.

P.S. Is it ok that 3 valves will be original, and 1 JJ (or other manufacturer)?

Again it is all going to depend on how close the new tube is matched, the AFD should be able to bring it into range. I would try that first.

Ok, I understand. No more questions. 6550 is on the order now.
Thanks again for quick response! I thought I'll have to wait for at least one day to get the answer.


Wait a minute.. I've got Vintage Modern 2466 with four Shuguang KT-66's in it. So I can try to put them in my AFD100? Just set the bias little cooler..
Or even more.. I have matched quad of stock KT-66's that came with my VM (one tube is dead). Can I replace pair of 6550 with two KT-66, and use my AFD100 with 2x6550 and 2xKT-66??

*this is what I found in AFD100 manual...

By the way, can anyone recommend some basic books, websites or materials on guitar amp design to me? I am looking to better understand how they work. I'm not looking to become a repairman or anything but I'd really appreciate some tips. Thanks!

You can put a KT66 in the AFD, but I would guess the amp would not sound so good unless you replaced all four. I know Santiago said you can run any combination but he could not predict the tone results.

Still playing and everything is ok. But yesterday evening I noticed this:

STANDBY - ON



STANDBY - OFF



V3 is not blue.. Is it because AFD can't bring V3 into range?

Power tubes can develop a FLUORESCENCE GLOW. This is a very deep blue color. It can appear wherever the electrons from the cathode can strike a solid object. It is caused by minor impurities, such as cobalt, in the object. The fast-moving electrons strike the impurity molecules, excite them, and produce photons of light of a characteristic color. This is usually observed on the interior of the plate, on the surface of the mica spacers, or on the inside of the glass envelope.

THIS GLOW IS HARMLESS. It is normal and does not indicate a tube failure. Enjoy it.

I did enjoy it today but I guess that things are getting worse.
This happened after I played my AFD100 with these settings today: Master Volume - 10, Power - 3-5, Gain - 6-8..



Well, I didn't notice anything while playing, only when I finished playing and looked at the back of the amp. Still everything works and sounds great.
Anyway, I have ordered four JJ EL-34 already, and I'll get them at Saturday. But still it disappoints me to see this happening to the amp.

Hmm, it is odd but not impossible for all those tubes to start going bad all of a sudden, I am going to guess you simply do not have that many hours on them. Well I know its no help but this is what can happen to any of use that have tube amps. I have been pretty lucky with my tubes the last few years.

Slap the new ones in, bias it up and you will be back rockin, it happens.

Okay, I have just installed the set of JJ EL-34 valves, biased it (38 mA) and guess what... V3 indicator lights again.
All the same, while biasing I can hear quiet pops in the cab. But I can play it and it sounds awesome as always.
So it's the amp problem. Something with that auto-bias thing, right?





P.S. After V1 and V2 led lights appeared, I did auto-bias and it helped.

The amount of glow does not necessarily have to indicate how good or bad a tube is.
My 6550's look like this:





They are biased per pair but all good.


Gr,

Gerrit.

Yes, tubes are ok, but what's wrong with that V3 led light then??
What about warranty - I bought this amp in Germany, Koln.. It's 1400+ km far from me.

Yes your amp is under warranty, it needs to be checked out. I do not know enough about the auto bias system to tell you what is happening but the amp doesn't like that tube for some reason. Unfortunately it is time for service.