70's Fender Bassman 100 bias question

[christpncher]

I have a '77 Bassman 100 and it has mismatched tubes (2 and 2).

one set burns brighter and heats up faster than the other set. cant remember which brands but i believe 2 are Fender branded and 2 are Sylvania? i can check on that later since thats probably an important detail.

does this mean it needs to be biased or are they just not a good matched set? i read something about these amps being fixed bias, mine has a "hum balance" pot.

ive been using it for low-volume band practice once a week for about 2-3 hrs, am i wrecking them by using it as-is?

it sounds fine, no issues other than the mismatched tube glow.

thanks

[BassmanPaul]

If you are talking about the glow of the heater some manufacturers tube are more visible than others.

If you mean the blue glow inside the tube then same answer, some glow more than others.

If you are saying that the grey metal plates are actually glowing red then your amp needs to be serviced.

[christpncher]

im not sure what you mean about the grey plates and the heater. anyone have a pic or diagram showing the different parts?

i dont think there is really anything wrong as it sounds great, and i can get a good loud, clean sound out of it

im just concerned because they are not a matched set. 2 are Fender brand and the other 2 are Sylvania with the halo getters. the Sylvania burn brighter. its an orange glow, from near the bottom and from the tips. same on both sets, just different intensity

can these amps even be biased?

[billfitzmaurice]

Originally Posted by christpncher

i dont think there is really anything wrong as it sounds great, and i can get a good loud, clean sound out of it

If it ain't broke don't fix it.

[T-Bird]

Hi.

When was the last time the amp was serviced?

Tube amps do require periodical maintenance, whether that's performed by a tech or by the owner, that's another thing.

Running "mismatched" tubes without the said person being a tech type with a tube tester, is never recommended. Very little to gain vs. quite a bit (the amp) to lose.

Fixed bias means that it has to and can be adjusted. Fenders choice was to do it with a fixed value resistor, as was Mesa Boogies. Mesa did that to rip off ignorant musicians, Fender did that because it simplified the circuit and there was no need to change the bias as they always got an ample supply of tubes that biased correctly on their amps.

Originally Posted by billfitzmaurice

If it ain't broke don't fix it.

^This.

Kind of .

If You fry the amp as a result of the lack of maintenance, the cause of that happening stares back at ya from the mirror.

Get the amp looked at by a professional.
IMHO anyway.

Regards
Sam

[LowEZ]

Originally Posted by christpncher

does this mean it needs to be biased or are they just not a good matched set? i read something about these amps being fixed bias, mine has a "hum balance" pot.

That is not a hum balance pot, it's a bias balance pot which is found in many CBS-era Fender amps. If you've touched it at all, it may be the cause of your issue. Do yourself a favor and take it to a tech. Have him convert the pot to good ol' bias adjust (a common mod) and splurge for a new matched quad of 6L6's.

[beans-on-toast]

The Bassman 100 has both a hum balance pot (100 ohm) in the heater circuit and a bias pot (10K ohm) for the power tubes.

The first thing to do is to put the amp in standby and look at the power tubes. If the glow that you are referring to is present, it is the heater windings glowing brighter on one set of tubes. This isn't a problem. Depending on the tube, there can be varying amounts of heater glow, from nothing visible to a bright glow.

Let's assume that the glow is not from the heaters. Turn the amp off standby. If there is a stronger glow in one pair of tubes, it may or may not be a problem. It could be that one pair just glows more than the other.

Now let's assume that there is a bias issue. Your amp has a single bias pot for all four 6L6GC power tubes. If one pair of tubes is running too hot, adjusting the bias might make the other pair run too cold. You might need to install another matched pair or quad of power tubes. The other alternative is to modify the amp and install a second bias pot (or a fixed resistor but this is a hack), one for each pair of 6L6GC's. This will allow you to bias and balance each pair.

If one pair of power tubes is simply running a little hotter than the other, that pair will wear down faster and eventually will match the other pair. No big deal. Some people install mismatched pairs because they like the tone. A tech can tell you if this is the case.

I think that it is important to verify bias settings by confirming with readings but I always set the bias by ear. It has to sound good. As you said, it sounds good without any distortion so there probably isn't a problem. If it isn't a heater glow and you are concerned, I'd take it to a tech to find out what the story is.

[christpncher]

thanks for the responses!

i havent messed with anything yet. i think it would probably be a good idea to have it checked out just in case since i am going to be using it on a regular, weekly basis now.

supposedly one set of tubes are the original tubes. would it have come with the Fender branded or the Sylvania tubes? its a 1977 silverface CBS era

[beans-on-toast]

Fender used US manufactured tubes in their amps. They would buy whatever was most cost effective. By the tail end of the 70's the Fender branded 6L6's were mostly from GE but there were some from Sylvania as well.

To complicate matters, manufacturers branded tubes for each other. For instance, if Sylvania had a big contract, they would buy RCA tubes and put their own brand on them. RCA didn't manufacture 5AR4 rectifier tubes. They would buy them from other companies and add their own brand.

It is hard to say which pair in your amp is original. Fender was branding their tubes in 77. There are tube image libraries out there like this one tubemongerlib.com, you can search the library and see if there is a 6L6GC with the same structure as your Fender branded ones. It least this would help you identify their origin.