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Biasing . . .

#22
Tubes and Caps will both hold a charge. If you put yer fingers in there you better know what's up or your mos-def down..nuff said.
Otherwise, if the test points and the adjustments are both on the outside and you don't need to open the chassis you are probably allright following instructions.
If you have any doubts or uncertanties AT ALL give the amp to a tech and pay him to put his fingers in the hot zone, that is his job..

I do my own work but when I was first starting out a Marshall Super Lead through me across the room after discharging the outputs thru my left elbow innocently leaning on the chassis and providing a path for my right index finger when it slipped....oops.

I lived to tell but I was lucky..my head and chest/arm muscles hurt for days...Marshall plate voltage is somewhere in the neighborhood of 700 volts...ouch.

Don't be a dumb-ass like me.....
 
#23
Most of the discussion of what's hot and what's not in teh chassis is moot.... since you don't have to gain access to the internals in order to do the biasing and balancing for the units having the controls and external measurment jacks on the chassis.

But the person who said capacitors hold the charge is correct. The capacitance of tubes is miniscule and unimportant to the issue (except for maybe a TV).

There is a theoretical possibility of a high voltage on one bias terminal if the biasing resistor is open.... If that were the case, it should already be very obvious from a no sound condition, blown fuses or horrible distortion.

In any case, you need a meter which can read accurately in the millivolt area. If you do as instructed, biasing should be quick and easy.

The balance adjustment requires more equipment, you need a load and a signal source. Use of a load resistor is better and certainly quieter than a speaker.

It may be possible to use a speaker if you don't mind a LOT of noise, and have no other load, but I have never done it that way.
 
#25
Jerrold Tiers said:
Most of the discussion of what's hot and what's not in teh chassis is moot.... since you don't have to gain access to the internals in order to do the biasing and balancing for the units having the controls and external measurment jacks on the chassis.

But the person who said capacitors hold the charge is correct. The capacitance of tubes is miniscule and unimportant to the issue (except for maybe a TV).

There is a theoretical possibility of a high voltage on one bias terminal if the biasing resistor is open.... If that were the case, it should already be very obvious from a no sound condition, blown fuses or horrible distortion.

In any case, you need a meter which can read accurately in the millivolt area. If you do as instructed, biasing should be quick and easy.

The balance adjustment requires more equipment, you need a load and a signal source. Use of a load resistor is better and certainly quieter than a speaker.

It may be possible to use a speaker if you don't mind a LOT of noise, and have no other load, but I have never done it that way.
Click to expand...
again, something i never thought about at all, gah

would having a bass plugged in and just lying there be fine to use as the signal source?

so all i need to look into is a meter and a load resistor
 
#27
i_got_a_mohawk said:
would having a bass plugged in and just lying there be fine to use as the signal source?
Click to expand...

Only if you don't mind potentially electrocuting yourself. You really need a separate tone generator that can do a wide range of tones and waveforms at controlled amplitudes. A tone generator also has the nice quality of being grounded through something OTHER than you.

so all i need to look into is a meter and a load resistor
Click to expand...

That MAY work sometimes, BUT it doesn't take the differences in tube emmision into account. You really need an oscilliscope to monitor the waveform (which you would need using meter biasing anyway-otherwise you'd never really know the ture onset of clipping) and a LOT more training. Biasing is easy once you know how, BUT it's the "knowing how and why" that's the most important.
 
#30
Psycho Bass Guy said:
All that tester does is measure quiescent plate current, which is fine for most guitar amps, but will leave a bass underpowered most of the time. You can get the SAME reading just by putting a 1 ohm resistor from the tube's grids to ground and reading the current.
Click to expand...

For us tube noobs, what's the easiest, safest way to do that? Or does it vary by amp?

Thanks, Tom.
 
#31
...one can do a bais adjustment if one is careful and learned in the ways. My question is then, when does one perform (or in my case have performed..) said ritual? I know that it's done when the tubes are changed, but does it apply to all tubes or just power tubes or just pre tubes? Which then leads to the question if one tube goes do it pay to change em all? I have about 5 or six in the pre section of my SVT3 Pro.

Thinking also that in light of the comments regarding the the "quiet nature" of the SVT3 Pro and given it's Groove Tube (which were not refered to as "all that" earlier in ths thread..) powered, when cash flow allows, a new set of tubes might make this head a little bolder shall we say? Not that this is a major issue since it's plenty loud for my use.

thanks for letting me play.
 
#34
tombowlus said:
For us tube noobs, what's the easiest, safest way to do that? Or does it vary by amp?

Thanks, Tom.
Click to expand...

Scope biasing is actually VERY easy and safe IF you have the proper equipment and your amp has external bias pots. It IS a good idea to CHECK current draw, but don't regard it as golden gospel. This is what happens when you try to bias a big bass amp using plate current.
 
#35
If you do the bias procedure as directed, with correct equipment, on an amp that doesn't have some other problem, you won't get that type failure..... You also may not get the absolute best possible sonic performance, but you won't pop tubes, and it won't sound horrible.

If, however, you don't know how to determine the problem / don't have the equipment / are just assuming that nasty noise is "because of bad bias", well, then you could. Particularly on an SVT, which runs close to the limits of the tubes.

That picture of the tube is a typical result of total lack of bias. I have seen it (once) from a loose grid terminal in the socket, but never actually from badly performed biasing. That was on an old H-K tube hifi amp, and it was NOT the only resulting problem.... a large ceramic resistor was also slagged into glass...... etc, etc.

That's not to say that it might not happen / could not happen that way.......

Yes, it is best to use an oscilloscope as well as the other equipment. You potentially could do the job without, but I would regard it as for emergency work only. There is a reason why ANY electronics technician should have at least one decent 'scope.

There is also a reason why there are electronics technicians..... to translate the problem the customer describes (how about "hey, this box is shorted out"?) to the actual problem and perform the fix.
 

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