KT120's and Fender (Sunn)300T- blows tube protection fuses

[66Strat]

Also, looking at the schematic the combined plate current and screen grid current for 3 tubes is 100 milliamps per side.

[pbman1953]

TC Tubes, St. Paul

[66Strat]

I have no experience with that vendor. Looking at their posted tube testing methodology, there is no mention of tube burn-in. Tubes can drift significantly in the first 100 hours or so. Most premature failures due to manufacturing defects occur during this time period.

[pbman1953]

http://tctubes.com/

[pbman1953]

I took the amp to the bench and pulled it out to test with my Bias rite-




Here are the first ,before tweaking results, based on the Fender manual-Settings are supposed to be  bias at .1  and  balance at 0mv




Voltage and cathode readings from tubes 1-6-




749/32      757/ 68     757/ 30      773/ 64.6       768/ 39.7    780/ 67.8






Then after using the balance and bias control to try and smooth things out




781/ 44      770/ 41.3       774/40       780/ 33       773/ 40.8      781/  33.1






Retested factory points from the the back panel controls-


Bias-  113.7mv


Balance-  .4mv   



For the first part of this the OT was cold and the bell top of the PT was slightly warm. At this point the OT is still cold and the PT is warm all over, with the bell top warmer than the sides. I'd estimate with a playing session the PT would be hot but not burn you.



Overall I see the balance playing a big part to try and close the gap.  But tubes 4 & 6 are just going along for the ride.

[sluckey]

Here's a little functional diagram that shows how to quickly adjust bias and balance and also which tubes are affected by each pot. While desirable, it's not so important for each tube to conduct the same amount of current. The important thing is for the top triplet of tubes to conduct the same ***total*** current as the bottom triplet.

I also included the screen fuses so you can quickly tell which tubes will be affected if a particular fuse blows.

[pbman1953]

Very cool diagram, thanks!




I just finished at least 45 minutes here at home playing along with some tunes at a pretty good clip.  I can probably 60-70% of a gig level volume and all the fuses haven't popped and the amps sounds killer.


So I need some opinion on the factory settings vs. what I came up with at the end and what I should do with the 2 tubes that are at around 40% of dissipation.






OT is slightly warm, slightly


PT sides are twice as warm of the OT, the bell top is slightly less warm than the sides.


.'

[sluckey]

While desirable, it's not so important for each tube to conduct the same amount of current. The important thing is for the top triplet of tubes to conduct the same ***total*** current as the bottom triplet.

Set the bias according to the factory setting. 100mV per side. Your settings are kinda hot.

[pbman1953]

If II do that I'll have 3 tubes at almost full spec dissipation and fuses will

[sluckey]

Retested factory points from the the back panel controls-

Bias-  113.7mv

Balance-  .4mv   

According to what you said earlier, you're running over spec. Back it down to 100mV per side!

[pbman1953]

The retested was after the tweaking to bring down the current.


If look back at  my post, the first part was the factory settings. The second set was after i played with the bias and balance to get the cathode close together.

[sluckey]

You're not listening. You just said earlier that the bias is set for 113mV. THAT'S TOO HOT!.

Connect your meter probes to J9 and J10. Adjust the bias pot until the meter reads 100mV. That's the factory setup.

Now connect the meter probes to J10 and J11. Adjust the balance pot until the meter reads 0mV. That's the factory setup. (And just for shits and grins, connect your meter to J9 and J11. I bet it will read 100mV just like J9 and J10 reads.)

Your amp will now be running cooler than it is while set to 113mV.

[pbman1953]

Just to be sure that we're on the same page. Do you know what the Bias Rite meter is from Weber VST, Ted Weber's company

[sluckey]

YES I DO. NOW PUT IT UNDER THE BENCH, GRAB YOUR METER, AND SET THE BIAS AND BALANCE THE WAY THE FACTORY SAYS.

Do you understand that functional drawing I made for you?

[pbman1953]

I see the bias side and the balance side.




Last week I installed the fifth and sixth tube  then factory balanced. The bias would not go to 100 but to . 98. The balance would go to .1. After that the fuse blew.


If i put it back LIKE YOU WANTED,  I fear the fuse might blow again.

[sluckey]

Just set it the way you want it.

[shooter]

Bias Rite meter

If I'm not mistaking, the meter works with1 tube at a time, but the pot controls 3 tubes, and the TP's monitor 3 tubes.

If you have 113mA and you can't adjust to 100mA, you have a problem, since 100 < 113.

[66Strat]

The 100 millivolt reading is a combined measurement for each bank of 3 tubes. These readings should be taken using a Volt/Ohm meter as Sluckey recommended. Also, you should measure the bias voltage at TP42 (Pins 3 and 8 of tube V4) as indicated in the schematic. This should be at least -49.4 volts. It may need to be more negative, -50 volts or more to get the KT120 tubes to idle in the correct bias range.

[shooter]

 
Could I "cheat" by installing an Rk to "individually" tweak to taste

back to regular programing

thx

[66Strat]

Looking at the schematic, the Balance Control (R361) adjusts the bias voltage for tubes 6, 8, and 10. The Bias Control (R335) adjusts the bias voltage for tubes 5, 7, and 9. I can see where you might blow a few fuses if you were trying to get 100 millivolts between test points J10 and J11.

As a starting point, adjust the Bias Control to get the -49.4 volts on pin 3 of V4. Then adjust the Balance Control to get the -49.4 volts at pin 8 of V4.

Put each tube independently into the V5 socket and measure the current draw between J9 and J10. Do not make any Bias or Balance adjustments while doing this. Write down the current measurements for each tube. It should be in the neighborhood of 33 milliamps or so for each tube. On paper, sort the tubes by individual current draw and assign them into two triplets of tubes, the goal being to best balance the aggregate current draw between the two triplets. Let tubes V5, V7, and V9 be triplet A, and tubes V6, V8, and V10 be triplet B.

Put each triplet of the tubes into their respective tube sockets and measure the current draw between test points J10 and J9, and test points J9 and J11. These two measurements should be approximately equal.

[sluckey]

I know you are nervous about adjusting the bias IAW the factory instructions. I would be too if I had just bought 8 KT-120s to go into an amp designed for 6550s. So I wont ask you to turn either bias or balance pot.

I would like for you to simply check two voltages though just so we can get an idea how the amp is operating. Let the amp warm up a few minutes. Connect your black meter probe to J9. (This is chassis ground.) Connect the red probe to J10. We expect about +100mV. Now move the red probe to J11 (don't move the black probe). We also expect about +100mV.

What two voltage measurements did you get?

[pbman1953]

I did what you said and I'm getting to what I reported before,


Bias-  113.7mvBalance-  .4mv 



Then, I was able to bring it back to manual spec of bias @ 100 mv and balance at 0.


What's next?

[sluckey]

You're done. Tubes should last longer now that you are running them cooler. I still want you to do this and report back...

I would like for you to simply check two voltages though just so we can get an idea how the amp is operating. Let the amp warm up a few minutes. Connect your black meter probe to J9. (This is chassis ground.) Connect the red probe to J10. We expect about +100mV. Now move the red probe to J11 (don't move the black probe). We also expect about +100mV.

What two voltage measurements did you get?

[pbman1953]

Someone asked me to check my fuse type and I looked up what I had which was a Buss GMA100MA- It's a fast acting.


Seems I have to order the T version which is a slow blow.


Off to Amazon

[jjasilli]

That was me. 

[pbman1953]

This falls back on me because I didn't have the correct fuse. 




New fuses have been ordered.


Sorry for taking everyone down this rabbit hole. I learned a lot though.


Thanks!

[sluckey]

There was a simple point I wanted to make but you just would never answer me. I wanted you to know that having 100mV at J10 and 0mV between J10 and J11 is exactly the same as having 100mV at J10 and 100mV at J11.

The amp really has two identical and completely independent bias circuits. Calling one a balance circuit is kinda misleading. You adjust the bias pot to set the top three tubes to 100mV as monitored across a 1Ω cathode resistor at J10. Then you adjust the balance pot to set the bottom three tubes to 100mV as monitored across a 1Ω cathode resistor at J11. Now since you have the same voltage at J10 and J11, there must be zero volts between J10 and J11.

[pbman1953]

Thanks  but this was more of a matter of understanding not a non willing to cooperate.


I appreciate every time all of you spend the  time to help.

[sluckey]

It's usually easier to understand when you cooperate.

Really doesn't matter at this point. We all learned something.

[pbman1953]

Last question in this Bias/ Balance type setup-


At what percent of dissipation would the setting be?


Just curious, to know if this has a numbered answer


Thanks

[sluckey]

Percent of dissipation would be actual dissipation divided by maximum dissipation multiplied by 100.

[jjasilli]

Good to go! 

[pbman1953]

Percent of dissipation would be actual dissipation divided by maximum dissipation multiplied by 100.

Am I supposed to take an average of all 6 tubes then use your method above?

[sluckey]

Look at a data sheet for KT120. Note the maximum plate dissipation. It may be 60 watts. So, 60 watts x 6 tubes = 360 watts max.

Then measure the plate voltage for the top triplet of tubes. Maybe it's 750 volts. So, 750V x .1A = 75 watts for the top triplet of tubes.

Then measure the plate voltage for the bottom triplet of tubes. Maybe it's 750 volts. So, 750V x .1A = 75 watts for the bottom triplet of tubes.

And, 75 watts + 75 watts = 150 watts for total power of all six tubes.

Percent of dissipation would be actual dissipation divided by max dissipation times 100%. So, 150W/360W = 0.416 x 100% = 41.6%.

If you need to know the % dissipation for each individual tube use your bias rite to get the plate voltage and plate current for each tube and do the math.

[pbman1953]

Seems like I'm looking pretty good in the middle of the 60-70% range.


All 6 dissipations add up to 233


233 -/-  360 = . 647


.647x 100 = 64.7

[sluckey]

I'd love to see the numbers you used to get 233.

I used these numbers that you provided when you had the bias set at 113mV and I come up with a total of 180.25 watts.

Then after using the balance and bias control to try and smooth things out

781/ 44      770/ 41.3       774/40       780/ 33       773/ 40.8      781/  33.1

Retested factory points from the the back panel controls-

Bias-  113.7mv

Balance-  .4mv   

So, now that you have properly set the bias test point to 100mV your numbers should be less than 180.25 watts.

Would you post the numbers you used to calculate 233 watts?

[pbman1953]

I think there is some confusion here, or at least I am, ha.




1)
This set below was taken from the start of all this. Taken after I performed the factory instructed settings-  Bias at 100mv and balance at .0


That's how I came up with 233


Formula used and I'll round up each number :  voltage x cathode-  Ex-  749 x .032 = 24


749/32      757/ 68     757/ 30      773/ 64.6       768/ 39.7    780/ 67.8


24 .               52          23                 50 .               31 .             53 .          - 233

2)
The first test above (factory book setting) . had every other tube off from each other. So I decided , with using both pots to get them closer to each other.





This set was taken after I played/ tweaked the bias and balance pots to get 179. I used the combination of the 2 pots to level out the readings.


781/ 44      770/ 41.3       774/40       780/ 33       773/ 40.8      781/  33.1




35                  32                 31               26                32                26              182(adjusted number)

[sluckey]

I used the combination of the 2 pots to level out the readings.

What voltage is on J10 and J11?

[pbman1953]

At this point J10 is 100 and J11 is 0.




Just a point to mention- When I was tweaking the the bias /balance for the second set of measurements,  I moved some of the tubes to different positions. Maybe that could of helped. Because in the very beginning I couldn't get 100 at the bias . The pot was maxed out and only went to .098 on my lowest meter scale. Now I seem to have more swing and the pot is not maxing. 


 

[sluckey]

What voltage do you measure between J11 and chassis ground?

[pbman1953]

Chassis ground as in J9?


If yes, 0

[sluckey]

Chassis ground as in J9?

If yes, 0

Yes, J9 is chassis ground. And no, J11 cannot be zero. Humor me. Just stick your red meter lead into J11 and touch your black lead to chassis ground. It better not be zero. What does your meter read?

[pbman1953]

I warmed up the amp for a least 15-20 min.




The bias reads- J10- 100.4


The balance reads- J11-  .4




DC scale set at 200m, my lowest setting

[jjasilli]

Chassis ground as in J9?

Chassis ground literally means the metal body of the chassis.  So you can put, or clip, your ground probe to the chassis.


Some circuit point might be grounded to the chassis, so you could use that.  But you don't know how perfect that other ground point is, unless you do a continuity check between it > chassis body.

[pbman1953]

Ok, I get that, so I clipped to one of the spring holders that hold down the tubes.


Very different results-

Bias- J10-  5.2

Balance- J11-  103.5

Like it's backwards compared to using the book method


Another thing I tested- The spring mounts totally ground out to each other and zero out on my meter. The J9 to chassis ground has a measuruament of 3.6 ohms. Not a total ground out. Hmm, what's with that?

[sluckey]

Bias- J10-  5.2

Can't be so.

You just don't get what I'm driving at. I know it. That's OK. We wont talk about it.

[Ed_Chambley]

Keep trying to take measurements Sluckey is telling you. I watched you use a meter and begin to rely on or at least try. Sure we are a bunch of old hound dogs that don't get to run anymore. Makes us grumpy.


It is clicking, but sometimes we get a block and cannot understand. Just ask specifically and Sluckey being the fine gentleman, tends to be direct. Wanna know a secret? He loves teaching this.


I am glad you got them running, that is a lot of dollars.

[pbman1953]

OK, I admit it , I may have seemed like a block head but I think , with some help, and you know who you are, that I've figured it out


Here's the setting after setting to the manual-


J10 to J9-  100mv


J11 to J9-   99.6mv




------------------------------




J10 to chassis ground- 107.6


J11 to chassis ground-  105

[jjasilli]

Yes, there's some resistance from test point J9 to to actual chassis ground. This would explain for the slightly different voltage readings.  Presumably the manufacturer knows this and has accounted for that in the instructions to use J9 as a test point @ 100mV.  The difference in mV between J9 vs. the actual chassis is small; only a few mV which = mA due to the 1 Ohm bias sense resistors.  The bias is reasonably within balance. Again this is the bias specified for 6X 6550's -- 3 tubes per side; ea bias sense R serving 3 tubes.  The 100mV bias specification is not necessarily correct for KT-120's; though supposedly KT-120's should be plug & play (except maybe for heater current demands as someone said earlier).

[pbman1953]

Last night , with the new fuses, the amp performed like a champ. It kicked butt!




A couple questions come to mind-


If 100 is set for 6550's, what would be the test range for KT120's?




Is higher than 100 a hotter or cooler bias?




I remembered to get a feel for the PT last night before I shut it down. The bell top was hot and it stung and I couldn't leave my hand on it. The sides were less in temp than the top.




Thanks to all! I do appreciate the help.