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Post subject: Re: Bassman 100 bias problem ?
Posted: Thu Aug 15, 2013 11:05 am
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It's not the tubes. It's not the bias supply. It's not the grid stoppers or screen resistors. It's not the voltage on the anodes. It's not the sockets. One last thing to check. The AC voltage on the heaters. If they are ok... (which I bet they are)...

After dinner, try the amp out. My bet it's going to sound as good as ever.

If that is the case, there must be something wrong with the method of reading the idle bias.


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Post subject: Re: Bassman 100 bias problem ?
Posted: Thu Aug 15, 2013 11:35 am
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BMW2002Ti wrote:
It's not the tubes. It's not the bias supply. It's not the grid stoppers or screen resistors. It's not the voltage on the anodes. It's not the sockets. One last thing to check. The AC voltage on the heaters. If they are ok... (which I bet they are)...

After dinner, try the amp out. My bet it's going to sound as good as ever.

If that is the case, there must be something wrong with the method of reading the idle bias.


+1

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Bassman 100 bias problem ?
Posted: Fri Aug 16, 2013 8:29 am
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After the mental gymnastics, I hope subway posts a successful response.

:)


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Post subject: Re: Bassman 100 bias problem ?
Posted: Fri Aug 16, 2013 8:43 am
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AC voltages around the heaters assuming i measured them correctly (pins 7 and 2) are 7.4v for all tubes

:) i think it sounds just fine, since this is my first tube amp ever so i'm not sure what sounds good or if it can sound better ?

i'm curious about the tube readings, maybe i can check via transformer shunt method to confirm if my readings were off before or if they are actually colder than they should be, maybe then i can tweak here n there.

i just need a couple of alligator clips, because i don't want to put my hands in there around those voltages :shock:


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Post subject: Re: Bassman 100 bias problem ?
Posted: Fri Aug 16, 2013 9:14 am
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Thanks, for the response! :D

7.4VAC is fine for the 6L6GCs. A tad high (to be expected with today's outlet voltages). And they will read higher, if the tubes are not in the sockets.

You know, I wouldn't even bother with the shunt method. You can't adjust the bias anyhow. If those tubes are brand new, matched, & strong testing -- at 500VDC on the plates -- they must pass around 40mA, by definition of a Class AB1 push-pull amp.

The only way they all pass 25-30mA is if they are all matched & poor testing or the wrong tubes. Or the bias supply voltage is way too negative -- which you measured as ok.

Just try out the amp. Check the tubes for any signs of over-heating. Just in case your readings are really way off. Tone will tell you all you need to know. If it sound cold and sterile --- it truly is passing the current you measured.

If the amp sounds close to what the amp sounded like before or it sounds like what your Fender should sound like --- the tubes must be passing the nominal current.

I'd recommend getting a BIAS KIT or some such device that goes between the tube and the socket. They are not that $$$. And can be read without pulling the chassis.

One note... if you do get a BIAS ADAPTER, you must reinstall the ground straps (and remove those 1-ohm resistors).

Good luck! Keep us posted.


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Post subject: Re: Bassman 100 bias problem ?
Posted: Fri Aug 16, 2013 9:27 am
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the last time this amp was working before i got into it was around 13 years ago give or take, it belonged to my brother's band, one tube starting glowing so bright, my brother jumped over the drums and pulled the plug off before anything bad happens :lol: , they were too broke to buy a new tube where i live at that time LOL (sold for about 100$). so i took over this project of restoring it now because i didn't want it to just rot in the storeroom where its very humid here most of the year. that's the story ! bringing an old amp back to a decent life :D.


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Post subject: Re: Bassman 100 bias problem ?
Posted: Fri Aug 16, 2013 9:33 am
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A worthy project!

Going out to help a friend install speakers into his AV-computer-digital studio system. He's a complete Luddite. :lol:

Good luck with the amp. I think you'll be ok.

http://www.tubedepot.com/bt-bias-scout.html


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Post subject: Re: Bassman 100 bias problem ?
Posted: Sat Aug 17, 2013 8:41 am
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ok i have good and bad news, the good news is that the amp is ok, the bad news is that im a total dumbass :oops:

i measured the resistance of my multimeter it reads 1 ohm (it doesn't have a set to 0 function), so basically it added a 1 ohm resistance to whatever the resistor i installed was, so that's why whenever i get a 1 ohm resistor and read it, it reads 2 ohms, i based my calculations according the to measured resistance, so if it reads 15ma, i multiply it with 2ohms and that should give me the correct current ? right or wrong ? or does the multimeter act as a resistor in parralel when measuring the current from pin#8 ?? in that case two 1 ohm resistors give 1 ohm in parrallel. if thats the case then my tubes are way too cold ? :shock: am i over complicating this ? :lol:


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Post subject: Re: Bassman 100 bias problem ?
Posted: Sat Aug 17, 2013 9:16 am
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Subway_savage,

No, the reason you are getting one-half the reading --- from what is the actual value, is because the one-ohm of your meter is somehow being routed in parallel with the one-ohm cathode-to-ground resistor.
If the resistance were in series, the resultant resistance = two ohms. This will give you TWICE the actual mA. Not 1/2.




Ohm's Law:

(voltage, E) = (current, I) times (resistance, R).

if R = 1 ohm, then

E = I. Or read voltage drop across the resistor = mA of idle bias.



But, if R = 2... then,

E = I times 2, or

E/2 = I. So, reading 40mV = 20mA.



If R = 0.5 ohms (two 1-ohm resistors in parallel), then...

E = I times (0.5), or

2E = I, 20mV read = 40mA.


If the amp sounds good and not red-plating or OD too soon in volume range --- then it must be running properly. This amp is designed to run Class AB1, push-pull. With 500VDC on the plates, it must be passing around 40mA of current per tube, at idle.

What I would do is replace the 1-ohm resistors with grounding straps. And get a BIAS DEVICE, like the kit from Tube Depot. This will give you more accurate readings. And you don't need to pull the chassis out of the cab.

In the meantime, just play the amp. Visually check the tubes, every now-&-then,for overheating. It really sounds like you'll be ok.

Hey! Thanks, for posting your results. It's good to see the complete picture.

http://www.tubedepot.com/bt-bias-scout.html

:D


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Post subject: Re: Bassman 100 bias problem ?
Posted: Sat Aug 17, 2013 9:29 am
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BMW2002Ti wrote:
Subway_savage,

No, the reason you are getting one-half the reading --- from what is the actual value, is because the one-ohm of your meter is somehow being routed in parallel with the one-ohm cathode-to-ground resistor.
If the resistance were in series, the resultant resistance = two ohms. This will give you TWICE the actual mA. Not 1/2.




Ohm's Law:

(voltage, E) = (current, I) times (resistance, R).

if R = 1 ohm, then

E = I. Or read voltage drop across the resistor = mA of idle bias.



But, if R = 2... then,

E = I times 2, or

E/2 = I. So, reading 40mV = 20mA.



If R = 0.5 ohms (two 1-ohm resistors in parallel), then...

E = I times (0.5), or

2E = I, 20mV read = 40mA.


If the amp sounds good and not red-plating or OD too soon in volume range --- then it must be running properly. This amp is designed to run Class AB1, push-pull. With 500VDC on the plates, it must be passing around 40mA of current per tube, at idle.

What I would do is replace the 1-ohm resistors with grounding straps. And get a BIAS DEVICE, like the kit from Tube Depot. This will give you more accurate readings. And you don't need to pull the chassis out of the cab.

In the meantime, just play the amp. Visually check the tubes, every now-&-then,for overheating. It really sounds like you'll be ok.

Hey! Thanks, for posting your results. It's good to see the complete picture.

http://www.tubedepot.com/bt-bias-scout.html

:D


+1 on all counts.

(edited since apparently the OP is bound and determined to ignore sage advice)

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


Last edited by Retroverbial on Sat Aug 17, 2013 10:57 am, edited 1 time in total.

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Post subject: Re: Bassman 100 bias problem ?
Posted: Sat Aug 17, 2013 9:38 am
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i added 4 10 ohm 5 watt resistors and the readings are 14,14,12,14ma (roughly after dividing by 10). i think it sounds a bit dull compared to other amps, might give that bias scout a try. thanks alot !


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Post subject: Re: Bassman 100 bias problem ?
Posted: Sat Aug 17, 2013 10:07 am
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Subway_savage,

OK. I think you need to step back and look at Ohm's Law and what you are doing to attach those resistors onto the amp & how they are being read.

10-ohm resistors on the cathode-to-ground will give you TEN times the actual mA, not 1/4th. In other words, if all is hooked up correctly and you are reading ACROSS the resistors --- then you should be getting ~400mV readings. In other words, there is something really wrong here. Your readings are far from the actual current flow.

I'd take the simple route. Forget those resistors. Tie the cathode-to-ground with a strap per tube. And get a BIAS KIT, or some such device.


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Post subject: Re: Bassman 100 bias problem ?
Posted: Sat Aug 17, 2013 10:44 am
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yeah i was getting around 140ish mv readings, ill order that bias scout thing and test it.


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Post subject: Re: Bassman 100 bias problem ?
Posted: Sat Aug 17, 2013 11:57 am
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How does the amp sound? Esp the midrange? Is it ok or kinda cold? Any oddball harmonics that shouldn't be there?

Be sure to ground the cathodes with a straight connection. Or you'll be measuring across the 1-ohm resistor in the BIAS KIT and the one-ohm (or 10-ohm) resistor in the amp.

Good luck!


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Post subject: Re: Bassman 100 bias problem ?
Posted: Sat Aug 17, 2013 1:17 pm
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It feels like it needs to be brighter, I got the middle pot broken and I need to replace I (someone decided to install a push in type knob and apparently glued it there, and when he tried to pull out the knob he broke the pot and left it as is).


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