Red Plate Problem w/ Simul-Satellite

[dmt]

I've noticed something funny here and I dont know what it is from. had my Boogie Simul-Satellite retubed and checked out at the Mesa Store in Hollywood a few months ago because it would Red Plate in Class A mode. I thought it was somrthing to do with the older tubes and maybe with the amp. So they changed the tubes out and replaced them with a brand new set of Sovtek KT66's (same as what was in it before) and the problem went away. They also replaced the screen resistors...

Even now, if I put it in Class A mode it doesn't really redplate, the center is almost Red. But today I put in a pair of JJ EL34's and threw it in Class A as to not piss off my roomates since it was late and both tubes were Red Plating prety good. The whole plate wasnt red, but a good portion of it...if I put it in "Tweed" it got better, but I want to know what the problem is. I run the amp with the following settings through a Hotplate:

Volume (Pulled) 4
Tone 8
Master 5.5
Presance 8

I have an EQ and Compressor in the FX Loop, with the compressor boosting the output level quite a bit.

 

[ursinus]

It might be the tube sockets or thin pins on the tubes. I noticed that JJ's have thinner than normal pins and do not make good contact when plugged in the sockets. The red plating is occurring because the bias voltage is not getting to the tubes in all likelyhood. Try a set of Mesa or Groove tubes to check if the problem goes away.....

 

[Boogiebabies]

I hate hot plates. They just scare me.

If your tubes are turning from gray to cherry red plates they are drawing too much negative voltage. I'm surprised the KT-66's made it at all as they are a 25 watt tube and even a midrange 5 rating at -39V will draw -45ma and red plate quickly. The JJ's would not do this unless they have a very large natural plate current and are drawing too hot. Without a bias meter and a tube dealer who understands what Boogies draw, you are the the mercy of vendors who may sell you crap that no one else wants.
Mesa sells only tubes that they know will not draw over 17-25ma. so you should never have the problem your having. It sucks, but unless you have the ability to check your bias, plate voltage and run through a few dozen tubes to see what they bias up at. Everyone has thier own rating system and none of them are really alike. A problem with the tube vendors is they only test to 450V and a limited range of negative voltage.
Once the tubes get 490V and a hotter bias they dont match like the vendor stated. Some go 8-10ma off and some with high natural plate current just go nuclear. It's a crapshoot. In my other amps, I have no problem with adjusting the bias, but with the Boogie's, I have had the best luck with Groove Tubes in a # 5 rating ( NO KT-66's in the outer sockets) and NOS Svetlana/SED's from Ned at Triode Electronics. It may be a fluke, but I have been running two quads of Svet 6L6's and EL-34's for over two years in two of my C+'s. Even at 483V they all draw to the exact milliamp of each other. I use a # 32 6L6 and a # 34 EL-34.

 

[dmt]

Ok, so ijust got done playing with the KT66's and again, in Class A mode the center of the plates turns red. And to a lesser extent if I put it in Tweed. Its fine in Simul-Class (AB).

Its weird, I guess I just can't use Class A to make things quiter, either that or I just have to roll down the volume and not put those tubes as hard. I Just liked using the Class A mode since I don't have to play with the amp settigns, I can just leave everything as is. But that must not be the case.

The only real reason I tried this yesterday with the EL34's was to clean up the tone in Class A. I typically play in Simul-Class with all KT66's with no problem at all.

One other thing I was wondering about is if I am using a compressor and boosting the signal in the FX loop would that have an effect on the power tubes? It is ALOT quiter if I turn off the compressor. And I typically run the SD9 into the SM9, one boosting the other. The SD9 colume is cranked, with a tiny bit of gain, and the SM9 has the Gain up with the volume a bit lower. Sort of a doulbe boost.

According to Boogie, KT66 tubes should be able to run just fine in the amp with out a re-bias. I have no tube tester handy, nor the money to buy one, and I don't feel brave enough to dive into the guts of the amp with all the high voltages in there. I think I will try and get a hold of Mike B tmr on the phone. Maybe he has some ideas. Anyone know his direct line over there?

 

[Boogiebabies]

Yes, KT-66's in a GT, Sovtek # 5 will draw -35ma at -51V with 460V to the plates at 65% MPD. This is not so at -39V. At -39V they could draw -48ma at 460V or over 90% Max Plate Dissipation. If your running a tube at an idle of 90%, there is not much room to rise to full power. It's like setting the idle on a car to 6000 RPM with a 7000 RPM engine.

 

[dmt]

So basically, running the tubes in Simul-Class is already close to the tubes limit, and in running it at high volumes in Class A will cause it to redplate, and this is due to the nature of the tube as opposed to some problem with the amp. Same applies for the EL34's?

Will running in Class A at a lower volume alleviate the red plating?

 

[Boogiebabies]

At 90%, even a few minutes of any use can cause issues. When you switch to Class A, it will only get worse. When you cut the inner 6L6's out, the plate voltage to the outer sockets goes up as the voltage needs to find it's new path to ground. With this, it finds the outer sockets as the outlet and can raise the plate voltage 10+ volts. When that happens, the tube draws more bias so your tubes may now be running at 95% at idle. From there, you will probably red plate soon after warm up. You have to get those tube out before they blow something. EL-34's are a 25 watt tube as well, but they bias differently, draw different amounts of idle current and can handle higher plate voltage.

If your red plating in tweed, something more sinister could be wrong in the tube or bias supply. You can actually run a bad tube for a long time in tweed and not know it. When dropping the plate voltage, the bias goes way down as well.

 

[ursinus]

As always, Boogiebabies you are great wealth of info. Thank you.

 

[Boogiebabies]

I run GT KT-66HP's in my SLO 100. Musicians Friend made an error and listed them at $ 49.99 a quad so I nabbed three. I biased them at 75% MPD and the plates glowed after a few minutes and by the time I hit the standby, one blew. I called GT and got the bias specs and true wattage and have had no problem with them biased as a 25 Watt tube. I was interested in hearing them in the C+ Class A sockets so I hooked the bias meter up, I knew the plate voltage was 483V exactly like the other two, so after I hit the standby switch the digital meter went to 49ma. I have never hit a standby so fast in my life. They are perfectly fine in the 6L6 spots. The three quads are # 5's and also sound phenominal in the DR. They draw 35ma and sound huge.

 

[dmt]

How do i read the bias voltage? Multimeter? just ho dangerous is it?

 

[Boogiebabies]

http://www.biasking.com/index.html

 

[dmt]

i think i meant is there a way to read the voltage/current draw of the tubes in amp