Smoking Pot

[jordan86]

Yes, I live in Colorado. But not that kind of smoking pot. That's not my style. 
(I apologize if that terrible joke has been used here before. I guess I should have searched first.)

Yesterday I pulled an old Valve Art KT88 out of the storage box and popped it into my SE amp. Had the chassis out on the bench already so I pushed the bias pot up to the ballpark of where I usually set it for a KT88, maybe a touch over (foolish me). Power on. Wait 10 sec. Standby on. And almost immediately I see smoke. So I power the amp off. Turned the bias pot WAY down. Give it a min to chill and then try again. Almost immediately, seeing smoke again. I notice this time its coming from the bias pot itself. Its a 1K 2W PEC Allen Bradley. Comes directly off the cathode of the KT88.

At this point I am thinking that this tube must just be pulling crazy current. Swapped the tube and the smoke is no more. Measured the pot and amp voltages and those seem fine. Amp sounds fine. But the pot feels pretty awful on the rotation now. Gritty and catchy in one spot. I assume that's where the wiper was during the smoke sesh.

The one abnormality is that toward the start of the rotation the resistance climbs smoothly up to about 30R then drops to about 15R(in the gritty spot), then climbs back up smoothly the rest of the way. I'm guessing that's the cooked spot.

Is it toast? Need replacing? I presume I could just avoid the gritty spot and/or carefully bias around it. I wouldn't normally hesitate to replace a simple pot but this one is $25+. :(

[acheld]

I would think that a smoked pot with damaged media as you describe would no longer be acceptable. 

But what is special about an Allen Bradley pot?  I'm not familiar with this . . .

[jordan86]

Nothing special about Allen Bradley I don't think. That is just the manufacturer. It's just a higher wattage PEC type of pot. Pricey.

[shooter]

put a 1k 2W standard resistor in, does it like to smoke?
if not what's the math for it?

[sluckey]

2 watt pot ain't enough! You need a much higher wattage resistor on the cathode of a KT88.

[shooter]

ah, and I wanted more smoke 

[DummyLoad]

you need a rheostat, not a pot for a power valve - 25W should work for a KT88 - they are NOT inexpensive. avoid the asian garbage on ebay and amazon. 


--pete

[jordan86]

I will look at the rheostat. Here's the flow of it on the schematic. It's an adjustable bias circuit for all octal tubes.

Right off the cathode is a 1 ohm resistor for bias checking, then the pot, then 100R in series.

[shooter]

take your potsmoker out, replace with something close to 400 ohms, measure the 1 ohm, then the math tells you the answer for no smokin


EDIT: 10W or more

[sluckey]

I'm sure you've read and understand this...

     https://ax84.com/archive/ax84.com/static/sel/AX84_SEL_Options_101004.pdf

[jordan86]

I'm sure you've read and understand this...   https://ax84.com/archive/ax84.com/static/sel/AX84_SEL_Options_101004.pdf

I have, yes. Though I did not do the install. My friend who built it many years ago did. He went with a 100R resistor instead of 220R. Practically it works out in terms of a good sweep. I can bias a 6V6 at one end of the pot, and a KT88 at the other.

[sluckey]

I can bias a 6V6 at one end of the pot, and a KT88 at the other.

But can you do it without smokin' pot?   

[jordan86]

I can bias a 6V6 at one end of the pot, and a KT88 at the other.

But can you do it without smokin' pot?   

Haha!

It’s worked fine for 10 years. I think it’s more so this tube. Which doesn’t necessarily make a ton of logical sense since I recall using it before.

[sluckey]

Sounds like your luck has run out. Better get a higher power pot or heed the warnings in that pdf I linked.

[jordan86]

Would a 5W wirewound pot work out? Found this for a reasonable price. https://www.digikey.com/en/products/detail/cts-electrocomponents/026TB32R102B1A1/203778

I also have a 150R 10W ohmite on hand that I could drop in place of the 100R.

[sluckey]

Much better. In fact, two and a half times better than ole smokey.

[DummyLoad]

if you built with the B+ as per schema you posted, then Ibias as designed is 30.5V ÷ 430Ω = 71mA - so calculating power dissipated by Rk is E x I = P. so then 30.5V x .071A = 2.16W. as such we typically use 2x power rating for safety margin. a 5W resistor is viable but will run VERY hot. a 10W resistor is better.

personally prefer use 15W to keep the FR4 from getting discolored over time. in this case if you want a safer solution, a 350Ω 10W fixed R in series with a 100R 5W pot would work well and give a respectable amount of variance for biasing.

install a 1Ω 1W 1% R under the 430Ω & 100uF cap for bias monitor.


--pete

[DummyLoad]

schema of original... 

[jordan86]

Thanks, all. I do have all the AX84 documentation and all the variants. I'm completely a self learner in all this so bear with me on a few final questions...

1) Does a larger value resistor in series cause the pot to see less voltage/wattage?
2) Does it matter which comes first in series, the pot vs fixed R?
3) As the resistance of the pot increases, the voltage across would increase as well, yes?
4) So running the pot at lower resistances would cause less stress on it?

I'm not clear on how plate current affects the pot here, so that's what I am trying to figure out. It seems like a lower cathode resistance produces less voltage. Easier on the pot? Or is that negated by more plate current? Does that stress the pot harder?

I presume at the end of the day I just need a higher wattage pot and a higher fixed R to carry more fo the load? I plan to do that, just trying to understand.

[jordan86]

you need a rheostat, not a pot for a power valve - 25W should work for a KT88 - they are NOT inexpensive. avoid the asian garbage on ebay and amazon. 

Hey Pete....all the 25W rheostats I've seen on like mouser and DigiKey are like $40+. Maybe I'm not shopping the right stuff?

[shooter]

"I'm not clear on how plate current affects the pot here"


you have a series circuit, so Itotal =Vtotal / Rtotal

[jordan86]

While I am in there and rewiring, is one of these schema better than another, or are they all practically the same?

1) Which resistor is immediately attached the the cathode
2) Where the test points are inserted?
3) Does the bypass cap need to be in parallel with the test points as well?

Three seems most right, per my thinking, and DummyLoad's input.

[shooter]

3 is close, put G2's cap to ground

[jordan86]

3 is close, put G2's cap to ground

Yes, G2 has its own path to ground in the layout. I'll go with #3

[sluckey]

You seem to be focused on voltage. But voltage did not smoke that pot. EXCESSIVE CURRENT smoked that pot. Let me say this first... The value of the cathode resistor in that amp determines the amount of current flowing through the tube. The smaller the resistor, the more tube current (hotter bias). And the larger the resistor, the less tube current (cooler bias).

Of course, when you increase the pot resistance, the voltage across the pot increases and biases the tube cooler and current through the pot decreases. But when you decrease the resistance of the pot, the voltage across the pot decreases and biases the tube hotter and current through the pot increases. This is all normal operation, except... your 2 watt pot just cannot handle the increased current of this circuit.

Most likely... That 2 watt pot has a small metal contact (the wiper) that rides on a circular carbon track. The excessive current traveled through the small metal contact and arced to the carbon track. That arc burned the carbon track causing the smoke you saw and now the spot on the track that burned is rough. You felt that later.

I think that 5W WW pot you found will be sufficient. For sure the expensive heavy duty rheostats would be bullet proof. They are also considerably larger than your Allen Bradley pot.

The thing that puzzles me the most about all this is that AX84.com let it slide. Those guys are much too sharp to use a 2 watt cathode bias resistor on a KT88.

[sluckey]

The stock drawing is the best because the test points serve two purposes... Measure current through the tube ***AND*** measure cathode voltage. You need both of those to calculate tube dissipation.

[DummyLoad]

you need a rheostat, not a pot for a power valve - 25W should work for a KT88 - they are NOT inexpensive. avoid the asian garbage on ebay and amazon. 

Hey Pete....all the 25W rheostats I've seen on like mouser and DigiKey are like $40+. Maybe I'm not shopping the right stuff?

rheostats in that wattage range are fairly expensive, typically ≈$50.00 or more.

sometimes you can snag a used ohmite on fleabay for ≈$25.

a 10W 350Ω from mouser is typically under $1-$2 depending on the make, tolerance, etc. - mouser sells clarostat, alpha and CTS 5W parts - sample here


--pete

[PRR]

Equivalent Rk from 300 to 600 using 1/16W pot.

[jordan86]

Thanks, Pete. That CTS 5 watter is exactly the one I ordered last night. Also picked up an Ohmite 10W in 200R that's just like the cathode bias resistors in my Dr Z amps, and a new 1R 1% 1W.

[PRR]

When cut-off, the tube (6V6 6L6 EL34 6550 KT88) grid bias is about 1/10th of the B+; but being cut-off there is no heat here. The worst-case will be near (0.1)*(0.6) of B+. This is also the ratio of heat in the plate to heat in the pot. So for a 12W 6V6 we can have 0.72W in pot and for a 42W 6550 we can have 2.52W in the pot. (We also see that if the tube is worked nearer 34 Watts a "2W" pot may last for years.)

[jordan86]

PRR, your genius is next level sir. I did have the idea of a bias resistor in parallel, which I think you accomplished in a very complex way.

I decided to go the route of original design, upgraded components for now.   

[jordan86]

Well after all that I confirmed today that the pot smoking KT88 is definitely faulty. Tested some other tubes (all good) then tried the stoner Valveart again. Dialed the restistnace way up to ensure it wasn’t running away on me and fired it up. Arc city inside the tube. It was wild. Only left it on for a second or two but it was fun to watch. 

I also spent some time biasing different octal tubes and measuring the cathode resistances. KT88’s were in the 140-200 ohm range. KT66 was around 400R or so. 6V6 was in the ball park of 800R. Think I will do a 150R instead of 200R in series. I’d almost prefer a 500R pot but the 1K will give more flexibility with 6V6.

Thanks for the help all. Learned quite a bit through this. 

[HotBluePlates]

... I think it’s more so this tube. Which doesn’t necessarily make a ton of logical sense since I recall using it before.

Well after all that I confirmed today that the pot smoking KT88 is definitely faulty. Tested some other tubes (all good) then tried the stoner Valveart again. ... Arc city inside the tube. It was wild. Only left it on for a second or two but it was fun to watch.  ...

It's worth thinking about how a build will handle the high current that comes with a tube failure.

I have also encountered tubes that draw FAR too much current, without being obviously-failed (as when the tube arcs).  I've got a pair of 6L6WGBs labeled in the 80s by an off-brand (who might have bought/re-sold marginal tubes discarded by the original manufacturer).  Given the same supply voltage & bias, they draw double the plate current of normal 6L6s.  These tubes require a much larger bias voltage to pull the plate current back to normal levels.

What this implies is we can't rely on "normal bias voltage" or "normal cathode resistance" (resulting in a bias voltage due to cathode current) when plugging in "any tube."  Unless there is a way to pre-screen the tube, it pays to at least monitor idle current when installing a new tube (and I guess we found out even when installing a "good tube" that hasn't been used in a while).

[shooter]

for fun do a "sound check" based on bias
when I did mine i picked 60% 80% 100%, I started with 80%, dialed "my sound" then left all knobs alone and just changed bias, re-played the same riff each time.
then for more fun;
once bias is set to your happy place, screw up impedance
leave knobs the same, same riff, (assuming 8 ohms cab now) change it to 4ohms, play, 16 ohms play
Now you have an amp dialed for the player vs the masses


EDIT: re-verify bias if you like screwed up impedance

[jordan86]

New 5w CTS pot is fantastic. Smooth feel. Put a knob on it too since the shaft is so much longer. Super convenient. No more trying to get a guitar pick down into the slotted shaft.

Accordingly to my math the most wattage I would ever be pulling would be about 2.8-3watts.

That being the case, does my 1 ohm bias check resistor need to be 3 watts as well? There’s a BIG ol carbon comp in there that’s 1R/5%. I have no idea what the wattage rating is though. I imagine it’s only 2w though.

[sluckey]

That being the case, does my 1 ohm bias check resistor need to be 3 watts as well? There’s a BIG ol carbon comp in there that’s 1R/5%. I have no ideas what the wattage rating is though. I imagine it’s only 2w though.

1/2 watt is fine. You would have to measure 707mV across a 1Ω resistor to equal 1/2W dissipation! The % tolerance is important. I suggest 1%. Hoffman sells them for $0.17.

I prefer 1Ω/1W/1% but you have to look harder to find them and they cost more.