#1
yea it sounds stupid, and it is. but when i got my amp kit from Ted Weber (from whom i will never use again for anything other than a speaker.

he doesn't match his tubes. there are people on his forum that have one KT66 redplating while the other one is fine. obviously i will pick up a better pair of power tubes in the next week or so anyways. i am just really nervous about the initial power up.

i am hoping to fire the amp up tomorrow. probably about 1 hour of work left to do and double checking things.

i do not have a bias probe and am going to build one i just dont have the socket to do so.
WTLT 2014 GG&A

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#3
Checking the bias current of each tube will tell you how far from matched they are.
What Min said is a good idea. There are two ways to approach it, both require a 1 ohm resistor (there's a third way but it isn't as good). A 1 ohm resistor on the plate isn't going to affect the sound very much, especially if you use good quality things like Dales (I expect Min to chime in here with his favourite power resistors ), but you can also add a switch to bypass it for normal use if you wanted to.
Extend both sides of the resistor to some recessed banana plug sockets. Make the end connected to the plate a red one. Now measuring across the two terminals will give you plate current (1 volt = 1 amp) and between the red one and ground will be the plate voltage.
I say recessed because the terminals will have high voltage on them so you have to make it safe. An inspection cover wouldn't be a bad idea. Putting the bias pot somewhere that you can get to it without removing the chassis wouldn't be a bad idea either.
It would have to be pretty bad to redplate bad enough before you get a chance to turn it off. All you risk damaging is the tube itself anyway.
Is this a JTM45 clone?
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#4
Quote by Cathbard
Checking the bias current of each tube will tell you how far from matched they are.
What Min said is a good idea. There are two ways to approach it, both require a 1 ohm resistor (there's a third way but it isn't as good). A 1 ohm resistor on the plate isn't going to affect the sound very much, especially if you use good quality things like Dales (I expect Min to chime in here with his favourite power resistors ), but you can also add a switch to bypass it for normal use if you wanted to.
Extend both sides of the resistor to some recessed banana plug sockets. Make the end connected to the plate a red one. Now measuring across the two terminals will give you plate current (1 volt = 1 amp) and between the red one and ground will be the plate voltage.
I say recessed because the terminals will have high voltage on them so you have to make it safe. An inspection cover wouldn't be a bad idea. Putting the bias pot somewhere that you can get to it without removing the chassis wouldn't be a bad idea either.
It would have to be pretty bad to redplate bad enough before you get a chance to turn it off. All you risk damaging is the tube itself anyway.
Is this a JTM45 clone?


yea its a JTM45.

for some reason i am slightly confused

I say recessed because the terminals will have high voltage on them so you have to make it safe



A 1 ohm resistor on the plate isn't going to affect the sound very much, especially if you use good quality things like Dales (I expect Min to chime in here with his favourite power resistors ), but you can also add a switch to bypass it for normal use if you wanted to.
Extend both sides of the resistor to some recessed banana plug sockets. Make the end connected to the plate a red one. Now measuring across the two terminals will give you plate current (1 volt = 1 amp) and between the red one and ground will be the plate voltage.


whats a banana plug socket and where would i put one? what does the 1ohm resistor do? and why would i want to switch it on and off?

could you elaborate a little bit more?


________________________


i am still new to this so any help is appreciated. i spend like 6 hours on the build today and got really far. even that PITA switch that Weber uses and having to relocate it and having to relocate one fuse holder, the chassis was laid out very poorly. basically all i have to do is connect the speaker jacks and the output transformer.

also while i am on topic, who would you recommend for some kt66 tubes? i know there aren't a whole lot of them out there like EL34, etc. which tubes would be best for this (i will not spend more than $60 on the set).
WTLT 2014 GG&A

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alright "king of the guitar forum"


Quote by trashedlostfdup
nope i am "GOD of the guitar forum" i think that fits me better.


Quote by andersondb7
youre just being a jerk man.



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#5
To bias a valve you have to know the plate current and the plate voltage. If you put a 1 ohm resistor in series with the plate you can just measure the voltage across it and know the plate current. Multiply that figure by the plate voltage and you have the idle power dissipation. It needs to be between 60% and 75% of the maximum power dissipation for the tube in question.
with a KT66 the nominal value is 25W and the absolute maximum is 30W. If you take it as 25W to be safe then you are looking at an idle power of between 15 and 20W.
Banana plugs are the terminals you see on a lot of meters. You just need something to touch your multimeter probes to so you can measure the voltages on them. You could always use some other type of terminal inside the amp if you aren't trying to make the things biasable without removing the chassis. You just need somewhere convenient and safe to stick your multimeter probes.
You only need the 1 ohm resistors in the circuit when biasing so there's no reason you couldn't have a switch across them so they aren't loading down the circuit in any way when you are actually using it. I doubt you'd be able to notice the difference so

Now having said all this - does that clone have a bias pot or is it like a real JTM45? If it doesn't have an adjuster all of this work would be pretty pointless. If you just want to check that tubes are matched you may as well just make a bias probe because there's nothing you can do about it anyway.

With KT66's it's the same as all other power tubes. Get SED if you can afford them otherwise get JJ. I think both of them make KT66's.
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#6
Quote by Cathbard


Now having said all this - does that clone have a bias pot or is it like a real JTM45? If it doesn't have an adjuster all of this work would be pretty pointless. If you just want to check that tubes are matched you may as well just make a bias probe because there's nothing you can do about it anyway.

With KT66's it's the same as all other power tubes. Get SED if you can afford them otherwise get JJ. I think both of them make KT66's.


it definitely has a bias pot. its a little trimpot (50k liniar)

https://taweber.powweb.com/store/6m45_layout.jpg

^that is the layout.

i am also assuming this should all be done with a load on? sorry really stupid questions.

again thanks for your patience as i am trying to learn as much as i can.
WTLT 2014 GG&A

Quote by andersondb7
alright "king of the guitar forum"


Quote by trashedlostfdup
nope i am "GOD of the guitar forum" i think that fits me better.


Quote by andersondb7
youre just being a jerk man.



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#7
Yes, always have a speaker plugged in.
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#8
so i would install the tubes, and check bias that way. would i start the trim pot at minimum or middle? or where?

then if the tubes are vastly different. say 20 and 30 where would i set my trim pot. run one overly hot or run the low one?
WTLT 2014 GG&A

Quote by andersondb7
alright "king of the guitar forum"


Quote by trashedlostfdup
nope i am "GOD of the guitar forum" i think that fits me better.


Quote by andersondb7
youre just being a jerk man.



****** NEW NEW NEW!
2017-07-07 2017-07-07 Update and a Chat On Noise Constraints *** NEW FRIDAY 7/7
2017-04-13 RUN AWAY from COMPUTERS!!! TCE? RANT ALERT!!!
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2017-02-21 How to Hot-Rod the Hell of your Stratocaster for $50! (Tuesday 2017-2-21)
Resentments and Rambling from a Guitar Junkie
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#9
Start with it down low and work it up. Bias it so neither of them have an idle dissipation of more than 20W. Better to have one a bit cold than one redplating.
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#10
Quote by Cathbard
Start with it down low and work it up. Bias it so neither of them have an idle dissipation of more than 20W. Better to have one a bit cold than one redplating.


okay that is what i was thinking.

is there anything else i should do turning it on for the first time? the other build i did i just flipped the switch and crossed my fingers and didn't hear a pop and luckily everything was (and still is fine). there has got to be a better way.
WTLT 2014 GG&A

Quote by andersondb7
alright "king of the guitar forum"


Quote by trashedlostfdup
nope i am "GOD of the guitar forum" i think that fits me better.


Quote by andersondb7
youre just being a jerk man.



****** NEW NEW NEW!
2017-07-07 2017-07-07 Update and a Chat On Noise Constraints *** NEW FRIDAY 7/7
2017-04-13 RUN AWAY from COMPUTERS!!! TCE? RANT ALERT!!!
2017-03-02 - Guitar Philosophy 1001- Be Prepared For the Situation (Thursday 2017-03-02)
2017-02-21 How to Hot-Rod the Hell of your Stratocaster for $50! (Tuesday 2017-2-21)
Resentments and Rambling from a Guitar Junkie
---> http://trashedengineering.blogspot.com/
#11
It's safer to put the 1ohm resistor between the cathode and ground and reading the voltage across that resistor.

I usually close my eyes and pray.

Actually, in all seriousness, I pop power to the amp like 10 or 20 times before I'm even done building.
Prs se Holcomb is the answer
Last edited by AcousticMirror at Oct 22, 2011,
#12
Offer up a prayer to Apollo and stand clear - close to the mains switch. Every time you switch on a new amp build it's scary, doesn't matter how many you've built. When it works it's like that feeling you get when your parachute opens.
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#13
Quote by Cathbard
Offer up a prayer to Apollo and stand clear - close to the mains switch. Every time you switch on a new amp build it's scary, doesn't matter how many you've built. When it works it's like that feeling you get when your parachute opens.


I want this feeling.
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#14
Quote by bj_squeelie
I want this feeling.


i do too.

mains on...pop! light flashes and dims. dont have any more fuses here so will pick them up tomorrow. meanwhile trying to find the short.
WTLT 2014 GG&A

Quote by andersondb7
alright "king of the guitar forum"


Quote by trashedlostfdup
nope i am "GOD of the guitar forum" i think that fits me better.


Quote by andersondb7
youre just being a jerk man.



****** NEW NEW NEW!
2017-07-07 2017-07-07 Update and a Chat On Noise Constraints *** NEW FRIDAY 7/7
2017-04-13 RUN AWAY from COMPUTERS!!! TCE? RANT ALERT!!!
2017-03-02 - Guitar Philosophy 1001- Be Prepared For the Situation (Thursday 2017-03-02)
2017-02-21 How to Hot-Rod the Hell of your Stratocaster for $50! (Tuesday 2017-2-21)
Resentments and Rambling from a Guitar Junkie
---> http://trashedengineering.blogspot.com/
#15
Quote by AcousticMirror
It's safer to put the 1ohm resistor between the cathode and ground and reading the voltage across that resistor.
Yes but if you stick it on the plate you've got access to the plate voltage too without having to run another wire. One way or another you've got to have plate voltage somewhere so you may as well kill two birds with one stone.
Myself, I'd stick the probe points inside the chassis instead of on the back just to be safe. If you use a nice big chunky Dale power resistor that gives you good probe points anyway so no real need to do anything else but fit them. Not as convenient but certainly safer. Of course an inspection plate covering any probe points you extend to the back would sort that out too. Depends on how easy you make it to remove the chassis I guess.
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Last edited by Cathbard at Oct 22, 2011,
#16
i dunno. plate shunt is definitely more accurate but i don't like being anywhere near the plates after I've verified it once.
Prs se Holcomb is the answer
#17
I've been zapped that many times I get a rush from it now.

Joking guys, joking - don't go touching high voltage - it hurts.
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#18
don't like the plate voltage.

man that hiwatt clone scared me so much to work on. after 500vdc i'm just not having fun anymore.
Prs se Holcomb is the answer
#19
You'll get used to it.
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#20
i don't like it. i've taken 220vac, 380 b+ whatever.

i'm pretty sure according to wikipedia anything close to 600 will melt my skin.
Prs se Holcomb is the answer
#21
I've been zapped with 240V more times that half the kids here have had hot dinners. The biggest voltage was about 2000V back EMF off a coil. That wasn't the one that hurt the most though, that was 240V from right hand to left knee. Never wear shorts when working on live circuits, doesn't matter how hot it is, just don't do it - EVER.
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