Tube Problem in Class A mode

[dmt]

I am running a Boogie Simul-Satellite amp with 4x KT66 tubes and a RCA 12AX7A and HYTRON 12AT7 preamp tubes. When I change from SimulClass to Class A one of the Outer Tubes starts glowing red hot. The actual grey inner plate starts turning red. You can watch it change colors. Usually this is when the volume is cranked. I am wondering if this is something normal or if this means that its not properly baised. Its fine in SimulClass mode, and I rarly use Class A but I want to know what is the cause of this.

 

[stokes]

It is not normal for a tube to redplate.I cant find a schematic for this amp but I can tell you that a KT66 is not exactly like a 6L6.It is very similar but it is a tetrode,not a pentode.It doesnt have a supressor grid,it requires a 1k screen resistor to keep the screen from drawing excess current.When switched into class A the current in the tubes is raised somewhat to bring it into class A.If the amp wasnt specifically designed for the KT66 and has the 470 ohm screen resistor that is found in most Mesa power amps it might be that tube is going over the edge and drawing too much current.Try switching the tube to a different socket and see if the problem follows the tube,this will eliminate other possible problems in the amp itself.If it redplates in another socket it is the tube and could be that the tube is biased hotter than the rest and drawing too much current.Are you sure the tubes are biased in the right range for that amp?Even matched tubes can be "off" by a few ma's and if they are biased on the hot side that one tube can be too hot or the screen resistor described earlier could be the problem.Could also just be the tube going bad which could also be caused by the screen resistor issue.I repaired a MKII for someone with the same problem with the KT66,the 1k resistor cured it.

 

[fatboy135]

how many watts runs for class A?

 

[stokes]

Dont know for sure but assuming this power amp operates like ,say the MKIV class A lifts the cathodes on the inner tubes I would assume 25 to 50 watts max.Like I said,I cant find a schem for this amp so I aint 100%sure.I do know the MKII I worked on had a problem with very short tube life with the KT66's because of the screen resistor issue.I suspect this is exactly what is going on here.A lot of people think that the KT66 is a direct drop in for the 6L6.and a lot of tube vendors will tell you just that.But it is not the case.If you use them in an older Fender that has no screen grid resistor at all,they dont last long at all.The 470ohm found in most 6L6 type amps just aint enough,you need the 1k or tube life is greatly shortened.I think the MKIV has 2k or so on the outer tubes on the older version that was designed to use EL34's in the outer position,but this amp I am not sure of since I cant find the schem for it.

 

[dmt]

I beleive the power section is very similar to a 2:90. it was designed as a stereo amp to a markIV. boogie decided to put in a clean preamp so you could play it standalone but that was really an after thought. pretty much they dont make these amps anymore becasue they began making stereo poweramps. i forget what mike at mesa boogie said about this amp, there isnt a whole lot known about it. theres no manual or any documentation. i beleive it drops to 25 watts in class A, maybe 15 watts. not sure. it was rebiased when the tubes were installed. i believe the tech dropped the plate voltage on the tubes. he did this becasue it was blowing fuses like popcorn. there is a 3 amp fuse in there where there should be a 2.5 i may have had it go as hig as 3.5 amps.

 

[stokes]

If an amp is "blowing fuses like popcorn" the absolute worst thing you can do is put a larger fuse in it.The fuse rating is selected to blow before the current draw can damage other components in the amp.Lowering the plate voltage is also not any kind of cure.You have to determine what is causing the excess current and correct that.In fact blindly lowering the plate volts is likely increasing current.For example if you lower the plate voltage your current draw on the tubes will go up,you must readjust your bias to go with the voltage drop.Try switching the 2 outer tubes and see if the redplating follows the tube or if the other tube in the new position redplates.I cant imagine why the tech would tell you to put a higher value fuse in there but that is asking for trouble.It is possible that something,such as a screen resistor in the tube socket that is redplating,blew and is not blowing the fuse because of this.Switch the tubes and see if the problem follows the tube or stays on that socket.Let me know and we'll take it from there.

 

[fatboy135]

ok, could you please swap the tubes????

 

[dmt]

I will switch tubes tonight. The tube only redplate when in Class A, and only when its cranked or has been running in SimulClass for a while. I tend to run the amp with the Master Volume at 6.5 and the Preamp Pulled and at 3.5 - 4. It has had no problems with fuses for over a year. It blew fuses before I had the tubes changed probably because the tubes were dying. After the swap to KT66 it blew the fuse because I had the amp cranked to the max and I had the tech adjust it. Its been fine for a long time, except for the redplate issue in Class A. I hardly use Class A mode since I have a THD Hotplate to reduce the volume.

Right now there is a 3 AMP 250V Fast Blow fuse, it is marked for a 2.5 AMP Slow Blow.

 

[stokes]

You say it "only redplates in Class A only when it is cranked or has been running in SimulClass for a while".So it is not limited to class A.Of course it has had no problem with fuses,your fuse is too big,although to you .5 amp might not seem like a lot,to your amp it could be very bad.Consider this,lets say the normal bias point for your tubes were 40ma,running at 100ma would cause them to redplate and die.You havent even approached the .5 amp yet,the extra 60ma is only .060amp,do the math,.060amp vs .5 amps.If a 2.5 amp fuse blows there is something wrong,period.Putting the 3 amp fuse is going to eventually fry something in the amp.Just cranking the amp to the max should not blow the fuse,it was designed to handle it.You say it was blowing fuses"probably because the old tubes were dying".Unless the old tubes were redplating,that is not the case at all.Old tubes tend to get weak and if they arent redplating are delivering less power,not likely to blow a fuse.Until you determine what is causing the 2.5amp fuse to blow,you are just delaying an inevitable failure.Even tho the amp may sound great like this,tube amps always sound their best when they are driven beyond the design tolerances,unfortunately they quite often sound incredibly great just before they die.If you continue to use that amp with a 3amp fuse you are playing with fire.

 

[dmt]

The one tube is red plating only in Class A.

I am just following what the tech suggested. I am not exactly sure what he adjusted in the amp when he changed the tubes.

 

[stokes]

In your last post you said"The tube only redplate when in Class A, and only when its cranked or has been running in SimulClass for a while."Which is it?Switching the tube to see if it redplates in the other socket will tell us if the redplating is being caused because of a problem with that tube.Doesnt tell us if the problem with that tube was caused by something in the amp,on the other hand if the other tube redplates in that socket,we will know for sure something in the amp is the immediate cause.I know of no "mod" that would make you use a higher rated fuse,except some drastic changes to the entire circuit.Dont know what your tech did,but it doesnt sound right to me.In a pinch I have used a 2.5amp in place of a 3amp,to trouble shoot,but I would never recomend going the other way.

 

[dmt]

Ok here is the skinny:

The tubes are marked 1 2 3 4 from left to right. Tube 4 being the one that is directly above the fan and on the rightmost side when facing the back of the amp. The amp settings are:

Volume: 4 (pulled boost)
Tone: 8.5
Master: 5.5
Presence: 10

These are the settings that I usually keep the amp.

1. In SimulClass no tube redplates ever.

2. When in Class A:
a. inner pair visably dims a bit
b. tube #4 redplates slowly

3. When in Class A with tubes 1 and 4 reversed:
a. tube 1 (in position 4) redplates and has a visibly blue glow
b. tube 4 (in position 1) redplates

4.When in Class A with Inner pair and Outer Pair switched (2 1 4 3)
a. tube 3 (position 4) redplates
b. tube 2 does NOT redplate

My conclusions:
1. Tube 1 has alot more life due to its blueish glow, based on the fact that originally all tubes had a strong blue glow.

2. Tube 4 is really on its way out since it redplates in both position 1 and 4.
3. Position 4 has a haigh voltage or bias and is causeing the tube in it to redplate.
4. The amp may not have been properly biased for Class A, but seems to run fine in SimulClass.
5. I will need a new set of tubes in the near future.

Am I correct in my assumptions? Have I missed anything?

 

[dmt]

On a seperate note, If i turn the amp down in Class A mode, it seems that the tubes do not redplate, I did no test of this today, but that was my impression when I discovered the problem and turned down the amp.

 

[stokes]

There is something wrong with your amp.Lets start with Class A.In this mode only the 2 outer tubes are functioning,the switch lifts the cathodes of the inner tubes,taking them out of operation.When you are in class A socket 1 & 4 should be identical electronically i.e. same voltage, same bias current.When you put tube 4 in position 1 it is redplating because a malfunction in socket 4 has cooked it.The fact that tube 1 in position 4 redplates,where it didnt in position 1 tells me something is wrong with socket #4. The problem only seems to be arising in Class A mode,because to achieve Class A the current in the tubes has to be raised higher than it is in class AB,which is how the tubes are biased in Simul,although you dont see redplating in Simul whatever damage is in the amp is slowly taking its toll in this mode as well,it just isnt as severe as class A because of the lower current,but believe me,it is taking a toll.You seem to want to believe that the problem is solely tube related,it isnt.Putting a new set of tubes in is going to result in the same problem after time.Like I said earlier I dont know what the tech did that would call for a 3amp fuse in that amp,and the only conclusion I can come to on that subject is that he is ignoring the problem and forestalling an eventual failure.From what I read here,I dont think you are qualified to open the amp and probe any deeper,if I am wrong about your technical knowledge I apologize,but I would advise you to take the amp to someone who can.You need to find out why the amp was blowing the properly rated fuse (2.5amp),and fix that as well as any other damage caused by running the amp as you have been.If you ignore it and put new tubes in that amp they will eventually die,no doubt about it,and most likely kill the amp sooner or later.

 

[dmt]

Well what I can do for the time being is get a 2.5 Amp Slow Blow fuse and see if it blows. I doubt that it will. I beleive I was blowing fuses because I had old dead tubes, and then it was blowing fuses becuase it was still running hot. After the tech adjusted it a second time it was fine and I blew a fuse once or twice. I think even when I blew a fuse I was pushing the amp at max settings and also with a distortion box, prolly a Fuzz Factory, which is very loud to begin with. I will have the amp checked out soon. In the mean time I won't run the amp in Class A.

 

[stokes]

And what do you suppose is causing whatever tube you put in socket 4 to redplate?Doesnt matter what you put into the front of a tube amp or if you push it to the max,it shouldnt blow a fuse,if it wasnt designed to work with the volume on 10 the volume wouldnt go to 10.But hey,its your amp,go for it.Good luck.