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Post subject: Re: Bassman 100 bias problem ?
Posted: Thu Aug 15, 2013 6:14 am
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ok looks like i found the problem, someone at sometime not sure who decided to put a wire from the transformer ground point to pin#8 while still keeping the original ground for pin#8, so when i installed the resistor i only disconnected the original ground point from pin#8, so basically the resistor was grounded from both ends hence the low readings, i will cut that additional wire and remove it, its not even in the schematic.

now i'm getting a sensible reading, though the tubes are a kinda cold, they are running at around 22-25ma


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Post subject: Re: Bassman 100 bias problem ?
Posted: Thu Aug 15, 2013 6:32 am
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Ground from the PT or OPT (output tranny)? Doesn't really matter, as the resultant resistance of anything + straight line to ground = zero. Path of least resistance and all that jazz.

Good detective work!

How old are those 6L6GCs? Could they possibly be half used up?


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Post subject: Re: Bassman 100 bias problem ?
Posted: Thu Aug 15, 2013 6:39 am
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PT

schematic:
http://imageshack.us/photo/my-images/24/edbl.jpg/

picture:
http://imageshack.us/photo/my-images/51/g6xa.jpg/

the tubes are a new matched set from TAD

im guessing some resistors have gone up in rating like old ones usually do ?


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Post subject: Re: Bassman 100 bias problem ?
Posted: Thu Aug 15, 2013 6:57 am
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Subway_savage,

OK.. brand new matched TAD must read above 22-25mA at idle. If the plate voltages are around nominal value, you should have a very terrible sounding amp with these kinda readings.

JOC, what kinda DVM are you using to measure the mV across the one-ohm resistors?

Maybe a photo of how you are rigging these resistors in the amp may help. You may still have another parallel path to ground which it changing the absolute value of the resistance.

BTW.. do you have another way to measure the bias? Like a BIAS PROBE or some such aftermarket tool?


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Post subject: Re: Bassman 100 bias problem ?
Posted: Thu Aug 15, 2013 7:55 am
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a more accurate reading is: 28.12 ma, 28.12 ma, 24.51 ma and 29.6 ma (most balanced i got)

using a digital multimeter set on the lowest DCV scale, i got the ground end of the resistor connected to the ground terminal through a small terminal block (cuz that crap wont stick), ground lead stuck into the chasis between the rear switch plate and the chassis and positive lead to pin#8.

voltage at pin#3 reads 506v for all four tubes.

so this gives dissipation wattage of 14 watts, 14 watts, 12.47 watts, 15 watts

three tubes can be matched but one tube will either be higher or lower than the rest (3rd and 4rth)


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Post subject: Re: Bassman 100 bias problem ?
Posted: Thu Aug 15, 2013 8:04 am
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You know, it almost sounds like you have one 1-ohm resistor for each pair of 6L6GCs. What was the voltage reading, again, at the point before the 1500-ohm grid stoppers? With no input signal.

Thanks!


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Post subject: Re: Bassman 100 bias problem ?
Posted: Thu Aug 15, 2013 8:49 am
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-61v, -61v, -62v, -62v across the 1500k grid stoppers


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Post subject: Re: Bassman 100 bias problem ?
Posted: Thu Aug 15, 2013 9:04 am
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Not across the 1500-ohm resistor, but at pin 5 of each 6L6GC socket. If this is what you mean...

That's a bit too much negative bias voltage. Should be in the minus 40-45VDC range. Pull the leg off those 68k-ohm dropping resistors and measure them. If they are ok... read all the other resistors in the bias power supply network (3.3k-ohm, 15k-ohm, 470-ohm) -- remember to pull one leg of each resistor before reading them.

Good luck! Let's see if we can hunt this thing down. :D


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Post subject: Re: Bassman 100 bias problem ?
Posted: Thu Aug 15, 2013 9:11 am
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Just for S&G's, what's your plate voltages at V3A and V3B. You should be seeing around 320 volts at pins 1 and 6.

Your bias-supply voltage seems a bit high but not overly so (spec calls for -52 volts) as is the observed plate voltage. I'm wondering if the problem is actually centered at the phase inverter......

Arjay

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Post subject: Re: Bassman 100 bias problem ?
Posted: Thu Aug 15, 2013 9:25 am
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You know what? He should read the voltages on pin 4 (screen). If they are way too low, maybe getting poor electron flow through the tube.

They should be a nominal 5-10VDC below the plate voltages. Way too low maybe a choke issue.


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Post subject: Re: Bassman 100 bias problem ?
Posted: Thu Aug 15, 2013 10:05 am
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yes sorry i meant pin 5.

resistor values:
456Ω, 3.4kΩ, 15.5kΩ, 64.4kΩ, 64.8kΩ

V3 phase inverter:
pin#1 332v
pin#6 330v.


V4,V5,V6 and V7:
pin#4 reads same as pin#3 @ 509v


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Post subject: Re: Bassman 100 bias problem ?
Posted: Thu Aug 15, 2013 10:37 am
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Pin 4 should read 5-10VDC less than pin 3. But, if it pins 3 and 4 read the same -- then the screen is not the issue (causing the low bias readings).

And all of your resistor readings in the bias supply circuit are within tolerance.

Ok, it's time to either 1.) try another method to read the idle bias or 2.) simply try out the amp. If the tubes are truly running in the 25-29mA range (at 500VDC), you will not harm them. As they would be running way too cold.

But, the tone of the amp will be sterile, with a grating odd-order harmonics component. It may sound just as loud. As you maybe placing the output tubes into Class B range.

If the amp sounds good. Your method of reading the idle bias needs attention. Whether it's the 1-ohm resistors, DVM, or wiring of the resistors. Good tone prolly means the true idle bias is in the usual 35-42mA range per tube.

Check the tubes for any signs of overheating (red plating or early distortion). Running the tubes too hot.

BTW... measuring the 1-ohm resistors with a standard DVM may not give correct values. As the precision is tainted due to lead lengths, etc of the DVM, itself.

PS: Just outta curiosity, how long are you waiting to read the voltage drop across those 1-ohm resistors? You should wait a good 5 minutes after hitting the Standby Switch. To assure the tubes are warmed up to operating temps.


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Post subject: Re: Bassman 100 bias problem ?
Posted: Thu Aug 15, 2013 10:49 am
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One other thing... these are 6L6GC and not 6L6WGB, right?

These:

http://www.thetubestore.com/Tubes/6L6-5 ... -6L6GC-STR



Not these:

http://www.thetubestore.com/Tubes/6L6-5881-Tube-Types


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Post subject: Re: Bassman 100 bias problem ?
Posted: Thu Aug 15, 2013 10:50 am
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BMW2002Ti wrote:
If the amp sounds good. Your method of reading the idle bias needs attention. Whether it's the 1-ohm resistors, DVM, or wiring of the resistors. Good tone prolly means the true idle bias is in the usual 35-42mA range per tube.


+1

I'm all out of ideas at this point. Possibly it is the procedure being used to check (and set) the idle bias.

A Bias-Rite (or similar device) is far superior to adding the customary 1-ohm resistor to each tube socket and using a multimeter.

Arjay

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Post subject: Re: Bassman 100 bias problem ?
Posted: Thu Aug 15, 2013 10:58 am
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the tubes i have are TAD 6L6WGC-STR

ill check the readings again later on and ill play around with it and see, for now ill go have dinner ! (i just want to throw it back into its cab and let it enjoy a good life)

thank you guys for the help, i appreciate it !


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