Eurotubes site says 35watt dissipation for KT-88 not 25watts. What gives?

[Sidewinder]

The title say it all.

In the thread I started on biasing my JCM 800 2203kk members are giving me formulas saying to figure 25 watts for dissipation.

I just stumbled onto a different suggested dissipation for KT-88 tubes which is what the JCM800kk uses.

Is it 35 watts or 25 watts?

Quote from Eurotubes"

Just to illustrate how little difference your plate voltage makes lets use 450 volts. So 25 watts divided by 450 plate volts equals .055 or 55 milliamps. So .055 X .7 = .0385 or 38.5mA. You can see that 20 volts only makes a difference of 1.5mA in where you would set the bias.
For KT88's and 6550's use 35 watts dissipation."​

 

[MartyStrat54]

If Wilder was here he would be like ape on shit over that statement.

I think I gave you some advice on your prior thread.

Your Kerry King amp only has a 100 watt power supply. Dig? Biasing the tubes to 35 watts will cause havoc. I've said this many times, when you are running 6550's or KT-88's in a Marshall amp, they run cold, because they are not in their ideal circuit.

If you put a KT-88 in a dual rail amp with near max plate voltage, you can get 50 watts per tube. In most amps "designed" for KT-88's they reduced the plate voltage and the rated dissipation is 35 watts.

Eurotubes is wrong "unless" they were talking about amps designed for these tubes. What they stated is the spec dissipation for a KT-88 and 6550. However, this is not applicable in 50 and 100 watt Marshall amps.

 

[Sidewinder]

With the tubes out at pin 3 I'm getting 513 VDC.

With 25watt dissipation:

25/513x700(70%) = 34.1 for bias setting.

With 35 watt dissipation:

35/513x700(70%) = 47.7 for bias setting.

I measured plate voltage with tubes out, black to chassis, but I stuck the red into the topside pin 3 hole. Didn't measure underneath. Does it matter?

The first time I measured underneath I had my meter set incorrectly for 500 DCA. When I checked the voltage at pin 3 there was a tiny arch and the speakers made a little noise. Is that because the meter was set wrong? I'm sure I had it on pin 3 and I didn't bump into anything else.

 

[Sidewinder]

Thanks! Ya, I can Dig lol!

This is why I'm asking, I want to be sure at what I'm doing.

I read in a different thread at this forum today that for the most accurate plate voltage measurement, one should pull all power tubes and measure at pin 3 with power and standby on.

I'm not clear if this can be done on top where the tubes plug in with the tubes out.

Or is it actually better to do it with the tubes in, power and standby on, and at pin 3, checking at the underside.

BTW I'm not trying to bypass advice given here. I did a search on Google because I was confused by a thread I read earlyier about calculation not at 70% but at .65 and then adding .5ma for screen voltage.

I should be good if I use 25/plate V x70 =bias. Yes yes?


Here is the Eurotube article:

http://www.eurotubes.com/eurotubes-Generic-How-To-Bias-Video-Marshall.htm

 

[MartyStrat54]

I just went over to look at the Eurotubes website. I found the page.

What Eurotubes states "is wrong." They are talking about biasing Marshall amps. They are not talking about other amps.

Eurotubes - Generic "How To" Bias Video

You really need to be careful checking voltages. One mistake could fry something and you can get hurt. High voltage is very unforgiving.

Your plate voltage is based on a loaded amp. Your actual plate voltage will be under 500VDC with the four KT's installed.

Again, check your meter setting before sticking probes on high voltage. What is your screen voltage (Pin 4)?

 

[MartyStrat54]

Well if you take all your power tubes out, this is your "unloaded" plate voltage.

With all the tubes in, this is your "loaded" PV. This is the voltage your amp is going to run at when you play so it makes since that this is the number you want to use when figuring out your bias.

Now I will say this. The difference between your unloaded and loaded plate voltage is not going to make a very big difference in your bias figure.

If you can, load the tubes and measure underneath the chassis and see what you come up with.

Please be careful!

 

[soundboy57]

Bob's not a tech, plus he's a keyboard player.

Sometimes I take what he says with a grain of salt.

Read his tube reviews...do you think he's biased towards JJ a bit??

 

[Sidewinder]

Okay, tubes are in, warmed up.

Pin 3 @ V101 tube is 470vdc pin4 @ v101 tube is 466vdc

Pin 3 @ v102 tube is 470vdc pin4 @ v102 tube is 461vdc

I'm running a digital bias probe. I checked bias just to see where they were with the new tubes @ the old bias setting. V102 was way high 67 and the other was 47. I figured I better adjust them down in the mean time before I get them set. Two bias adjust pots here.

I noticed that to get the highest bias down to 39 the other ended up at 23. Just screwing around with the two bias adjusters it does not appear possible to get them both set at 39 even though it might be temporary. Does this indicate a problem?

 

[Sidewinder]

25/470x70

37.2 bias is what I'm getting.

I'm starting the think one of the bias probes might be located incorrectly.

I was told in my other thread to locate them side by side and to one side because the two outside tubes are paired together and the two inside tubes are paired together.

My reasoning: The fact the the bias adjusters don't seem to be adjusting as anticipated. Also when I look at the board it just seems to be laid out that way, but I can't be sure.

What I'm saying is I think maybe the tube are paired as follows. The two outside left, and the two outside right.

 

[MartyStrat54]

Well your plate and screen values look like they should.

You can't always get the pairs to balance out perfectly, but if you are getting a big difference, more than 5mA, the tubes aren't matched very good.

You can try swapping the tubes around and see if it makes a difference in how it will bias.

 

[Sidewinder]

4 Brand new matched JJs from The tube store. They all said 44 on the tube boxes. I took it, that indicates matched, as I ordered them.

Do you think maybe the problem is the tube positions on the board are paired differently as mentioned above. Also thank you for all the help here!

 

[Sidewinder]

In other words bias pot 1 adjust position 1 and 2 and pot 2 adjusting position 3 and 4 as opposed to the standard JCM 800 positions IE 1&4 and 2&3?

 

[Sidewinder]

Photos: Notice the configuration relationship between the tube and board.

http://sidewinder.smugmug.com/Electronics/Misc/i-Ns7pnC4/0/X3/2013-03-17_19-46-31_447-X3.jpg



It must be paired the way I'm suggesting. Just look at the layout!

http://sidewinder.smugmug.com/Electronics/Misc/i-Dj673Cd/0/X3/2013-03-17_19-47-06_407-X3.jpg

 

[Alabama Thunderpussy]

If you put a KT-88 in a dual rail amp with near max plate voltage, you can get 50 watts per tube. In most amps "designed" for KT-88's they reduced the plate voltage and the rated dissipation is 35 watts.

Honestly I believe a dual rail guitar amp would probably feel stiff and dry as eskimo shit. You'd be better off running a distortion pedal through a Roland JC120.

 

[MartyStrat54]

Well back when these tubes came out it was all about horsepower. Everyone wanted the most wattage. Like I said, most KT-88 designs were well below max plate voltage for an AB1 push-pull amp.

I have an old Randall head from 1972 that uses four 8417's. These were the last power tubes made. They were of a new concept and they could also put out 50 watts on a single rail design. Randall however throttled the power down a bit to make it more stable and reliable. This Randall bench tested at 174 watts. It was designed like a Peavey MACE, solid state preamp and tube power, except the Randall uses a 12AT7 PI tube.

 

[Sidewinder]

I now have the two bias probes placed on the middle two tubes, and she's warming up. Hopefully my theory about where the pairs are placed is correct.

Any thoughts on this Martystrat54? Do you think I'm headed in the right direction? I guess I'll know soon enough.

I have to say I'm have an absolute blast working on this amp. I wish some day I could build my own, and actually understand what I'm doing, not just following directions.

I love it when a plan comes together!

 

[Sidewinder]

Here is where I got the idea to remove the tubes to check plate voltage. See second post.


http://www.marshallforum.com/marshall-amps/19826-checking-plate-voltage.html

 

[Sidewinder]

DING DING DING! We have a winner!

When biasing the JCM 800KK the probes have to be on the two center tubes. Now the adjustors are adjusting as would be expected!

I have the two sides dialed in at 37.2 mA. I'm going to check it again in 15 minutes to see if it holds.

I'm so happy right now!

Thanks so much for the help Martystrat54!

 

[MartyStrat54]

Way to go. I hadn't ever done much researching on the Kerry King model, but looking at your picture of the board, it made sense what you said.

 

[mickeydg5]

Sidewinder
Looks like you finally got it sorted.
I may have mentioned this to you on one of your other threads; most guys like the neighborhood of 36-38mV (mA) per tube on this amplifier.

You should have schematics for referencing. Here is a link. You can see how the bias pots are tied to each pair of power tubes.
MetroAmp.com Forum • View topic - Marshall 2203KK Kerry King schematics