Single Rectifier problems @ high volumes. Help? :)

[ericalto]

I've noticed weird things happening at high volumes with my single rec ever since i got it (used). When the master and output are at 5 or higher, there is a significant volume drop when playing the lower strings or palm muting. Basically whenever the amp receives a decent amount of bass, it just gets sucked back, and then fades back in. As well, regardless of the output level, when the master is above half, it begins to sound similar to an amp with a blown speaker. It starts to sound thin, trebly, and very non-musical sounding.

Has anyone else experienced this? Is it normal? I know most amps aren't meant to have the knobs cranked, but i was expecting to squeeze more power out of it. The tubes are JJ kt66s and JJ 12AX7s. Any help or insight would be great thanks

 

[YellowJacket]

If you want more horsepower, you might want to get a 4 x 12 with high efficiency speakers in it. (Celestion G12H 30s, V30s, c90, etc or the WGS equivalent) Check the power rating on a speaker and if it pushes 100db / watt / metre or more, you'll get more volume out of a single.

That being said, it sounds like that amp needs servicing. Have you checked the preamp and power tubes lately? How old are they? Definitely test the tubes FIRST before sending the amp to a tech to check over the rest of it.

 

[domct203]

Is this a series I or II Single Rectifier? The Series II will have a bias select switch on the back of the chassis with the choice of 6L6 or EL34.

Your problem may be the KT-66's. The KT66 is not a 6L6 sub, while their bias points are the same the KT66 has almost double the filament current of a 6L6 (closer to an EL34), and normally draw a lot more plate current so they are usually out of the bias range of a 6L6. Cranking the amp with alot of bass may be pushing the power tranny to it's limits, especially with the series I that was not designed for that high a filament current.

I'll echo the advise of replacing all the tubes, pre and power amp. You have no idea of the condition of the tubes, or where the powertube bias is running. Get powertubes selected to run in your amp.

Is this your first Mesa, or your first tube amp?

Dom

 

[ericalto]

thanks for the replies guys. Heres more info: Its a series 2 head going into a 2x12 rectifier cab. The tubes are about 6 months old, and i bought them from eurotubes, so i was under the impression that the tubes were picked to be compatible with the amp on the 6L6 setting. I just threw in the 6L6 tubes that the amp came with and I had the same problem. I also noticed that the blue haze on the tubes would flicker in unison with the volume dropout. I did my research on kt66s before i bought them, and have read that many people have had great results (with bias switch on 6L6). I will replace the preamp tubes later today.

 

[YellowJacket]

thanks for the replies guys. Heres more info: Its a series 2 head going into a 2x12 rectifier cab. The tubes are about 6 months old, and i bought them from eurotubes, so i was under the impression that the tubes were picked to be compatible with the amp on the 6L6 setting. I just threw in the 6L6 tubes that the amp came with and I had the same problem. I also noticed that the blue haze on the tubes would flicker in unison with the volume dropout.

REPLACE THOSE TUBES!!!

Oh, and you just have a 2 x 12? Do you put it up on anything when playing? You may just be blasting your ankles. If you want more presence, get another 2 x 12 and use them both for big gigs. While running 4 speakers doesn't raise the decibel level, it definitely moves more air which gives the amp more cut and presence. This may be just the solution you are looking for.

I did my research on kt66s before i bought them, and have read that many people have had great results (with bias switch on 6L6). I will replace the preamp tubes later today.

Well, if the filament current is closer to an EL-34, you should be running the bias in the EL-34 position so you don't cook your tubes prematurely. (Hence the flickering blue light. This light means your tubes are gassing)

 

[ericalto]

alright thanks so much for the help. I suppose the actual volume isn't too much of an issue for me. I was just curious if anyone else is able to turn the knobs past 5. Now, before i change the bias switch, is there any possible change that the amp could be damaged with the kt66s?

p.s. the blue flicker occured with the original 6L6's, so i don't think it has anything to do with the bias switch

 

[domct203]

I recommend you leave the bias switch where it is. If the amp is designed to handle a high current draw heater it does not matter where the bias switch is. Flipping it to EL34 will lower your bias voltage which in turn will make those KT-66 run even hotter. You'll risk redplating the tubes. You really need to measure the idle current draw (bias) so you know where those tubes are running. Chances are if Bob at Eurotubes picked those KT-66 to run in 6L6 mode they are running warm already. A flip to EL34 bias would most likely be the death of them, and possibly a screen grid resistor or two.

Dom

 

[domct203]

alright thanks so much for the help. I suppose the actual volume isn't too much of an issue for me. I was just curious if anyone else is able to turn the knobs past 5. Now, before i change the bias switch, is there any possible change that the amp could be damaged with the kt66s?

p.s. the blue flicker occured with the original 6L6's, so i don't think it has anything to do with the bias switch

If the amp will run EL34's don't worry about it. See my previous post and leave the bias on 6L6 until you can get a bias measurement.

I've had my master volume at 3:00 with none of the issues you describe.

Dom