MV60 bias lights always on after installing KT-88 tubes, please help

[Henry2022]

I got this MV60 with el34 tubes for few months now, been happy with it but always heard the KT-88 tubes are good, so I ordered 4 matched KT-88 tubes and install them, I did turn the bias screws counter-clockwise first. After they were installed, I turn it on and warm up for 2-3 minutes and then play some music, all 4 bias LEDs are on, and I continued to play it for over 6 hours, same thing.
Next day, I play it again for hours, for the first time I saw the V5 bias light (lower left) went off, and few hours later v7 bias (upper left) was off too, but it came back on again later (1-2 hours later). Now I have been play it for 3 days, everyday over 10 hours, the most I saw was v5, v7, v8 bias lights went off, v6 (lower right) never off, and v8 only off for a short time, almost on as long as the v6 light.
I didn’t switch the tubes at all, and the music seems like can play alright, no high pitch sound and no weird noise.
Can I continue to play it like this until it work out by itself? Or I was hurting the amp and should put the el34 tubes back immediately? The el34 tubes are not old, but I want to try KT-88 (they are by gold lion) even KT-120 on my MV60, hopefully I can get better sound (or something different) out of it.
Please help if anyone experienced something like this before, thanks in advance

[AnotherJohnson]

You are not able to get them to turn off by adjusting?

The procedure is to shut the amps off. Install the new tubes. With speakers hooked up, but no volume applied, turn each light on and then back to just off. Play music for half an hour. Turn off volume again. Readjust to turn each light to on, and then back to just off.

It is normal to have to do this. It is normal to have the lights flash while dynamic music is playing, but it is not normal for the lights to just stay on.

If you are unable to adjust, then either a tube is bad, or one or more of the bias circuits isn’t right. Once properly biased, the tube should run for 1000+ hours if it’s a good tube.

I rebias once or twice every year.

[AnotherJohnson]

I suppose it is also possible that the MV60 isn’t able to bias the KT88s, but you say you’ve read that they’re better than the EL34s.

I dunno …. I know this is part of the fun of tube rolling … make it you’re own …

There are several who may give you a more useful response than mine. Best of luck!

[AnotherJohnson]

I just looked at the owner’s manuals for the 60 and 60SE.

*********
ADJUSTING OUTPUT TUBE BIAS
Biasing the amplifier is a simple procedure that can easily be done by the owner. Only a plastic screwdriver is required. A red plastic screw- driver has been included with your amplifier for this purpose. After replacing the tubes, the amplifier should be connected to a load (your loudspeakers will work fine) and there should be no signal applied to the amp. It should be connected to you preamplifier with the volume control turned fully down. Turn the amplifier on. After about one minute, turn each of the four screwdriver adjustable controls counterclockwise until the associated indicator LED just goes off. Clockwise rotation may be necessary first to turn the LED on; do not turn the adjusting screw any further in the clockwise direction once the LED has come on.
After the amplifier has warmed up thirty minutes, the procedure just described should be repeated. NOTE: It is normal for these LEDs to flash or flicker when the amplifier is in use.
********

The 60 uses EL34s. The 60 SE uses 6550s.

KT88s aren’t mentioned. Is this a mod that CJ suggests?

[AnotherJohnson]

Digging further, while the 6550 and KT88 look similar enough to allow direct swapping, and the socket geometry allows many tubes to “fit, the EL34 pin voltage needs are not as close. Maybe this is a difference in the 60 vs 60SE model.

Someone who rolls tubes will probably know. If it were mine, I’d be running EL34s - they are a delightful tube, and CJ’s original choice for thr 60.

Meanwhile, if you’ve properly attempted biasing without success, I’d go back to the EL34s (properly biased). If that works, you’ll know that the problem is with the KT88s.

[Truth71]

Someone please correct me if I am wrong, but I thought one of the baic differences between the MV60 and MV60SE was that one ran EL34 and the other ran 6550?

I do not think in this model switching tubes form El34 to KT88 ( or voice versa) is "proper" unless the factory makes modifications?

[AnotherJohnson]

Someone please correct me if I am wrong, but I thought one of the baic differences between the MV60 and MV60SE was that one ran EL34 and the other ran 6550?

I do not think in this model switching tubes form El34 to KT88 ( or voice versa) is "proper" unless the factory makes modifications?

I think you are 100% right .

[AnotherJohnson]

The el34 tubes are not old, but I want to try KT-88 (they are by gold lion) even KT-120 on my MV60, hopefully I can get better sound (or something different) out of it.

KT120s too?

Arrrrgh …..

The key is “or something different”

[AnotherJohnson]

FWIW, converting an MV60 to an MV60SE is one of the standard model upgrades that CJ is willing to do (according to their web site).

So if you really want to get out of the EL34s and into 6550 territory, having the upgrade would be a good starting point. And while you’re at it, you could get their direct advice about the KT120 idea…

[roberto]

Also the Special Edition version has Teflon Capacitors and Metal Foil resistors. These parts make to my ears a very nice difference. The unit is quieter and the quality sound is more transparent. The music flows nicer and has the distinguishing Conrad Johnson signature quality more evident.

The SE version is to my ears the best.

Happy listening!

[roberto]

You can not use as direct replacement the EL-34/6CA7 to a 6550. You must HAVE TO change the circuit inside the amp.

You can change the 6550 for the KT-120 as a direct replacement. Or the KT-88 direct replacement for the 6550.

[roberto]

The 60SE VERSION is for the 6550/KT-88/KT-120.

The 60 Version is for EL34/6CAS7 only.

[roberto]

The difference in price between the model 60 and the 60SE (Special Edition) is significant. Not only the use of the 6550s, the electronic circuit has different parts.

[roberto]

The electronic circuit is for the SE 6550 tube. EL34 is internal wired different. They are not interchangeable. There are amplifiers that are capable to accept both, moving switches. These switches make to allow you to use EL34 or 6550. They do change the wiring and the right circuit operation for each type of tube.

[Henry2022]

Thank you so much for all the replies. Now I understand my MV60 is for el34 only, I will swap it back now.
But I can’t stop wondering, since I am playing the KT88 with no problem now, except the bias lights stay on almost all the time.
What can happen if I keep running it ?
It does sound better than el34 I think.

[admin]

If the tubes you are using are lower resistance, you can potentially burn through them much faster and cause premature failure. I don't know how much the sound would be affected with an unbiased tube. I suppose that would vary based on the unit, the tube, and how far it's off the recommended bias setting.

[AnotherJohnson]

Biasing is done to avoid asymmetrical clipping, which sounds harsh.

If you’re not challenging the amp, you may be avoiding clipping entirely and it may sound fine.

It’s your system, so run it as suits you.

It is also possible that you’re easy to please and that “something different = something better.”

I would not worry so much about tube life. I would worry about killing caps or resistors, or about fires.

[AnotherJohnson]

https://learnabout-electronics.org/Ampl ... iers12.php

Much about transistors here, but many principles are the same.

[Big Dog RJ]

Yes! That's absolutely correct! The KT88 output tubes can only be used on the MV60SE version and NOT on the standard MV60 version. That's a definite No No!

In order for the MV60 to be modded to the SE version, there are a few internal adjustments required; resistors, and other settings to accommodate the higher current required to facilitate the much larger KT series tubes.

When the wrong type of tubes are used, as in the OP's case, what is taking place is that the required idle current bias is not lined with that particular tube type, simply because it's the wrong tube!
Hence, the idle current will never settle and this will cause premature output tube failure, plus a few other elements damaging inside. Definitely something you don't want.

Changing the 60 to a SE version is not difficult though, and can be done by any trained service tech. You just need to check with CJ which areas need adjustments. (Also provided CJ divulges this info).

Once you have changed over to the SE version then the following tubes can safely be used:
6550, KT90, KT120 & KT88

If you are not planning on the SE version then only one type of output tube can be use. The MV60 has been designed based on a particular circuit using the EL34 and the only type of tube that is designed for this amplifier version.
The MV60 is a wonderful amplifier and when upgraded to the SE version, provides finer detail and a fuller sound.

However, if you're happy with the EL34 presentation and type of sound, just sit back and enjoy those fine tunes!
Cheers, RJ

[roberto]

Agreed 100% with RJ.