Got a Bias Meter, now what!? Calling all experts...

[vortex]

I just tried out an Allesandro bias meter on a few of my amps and got some surprising readings (to me...)

I don't know what to do about the results. I am hoping some experts can give me an evaluation and some recommendations and let me know what sonic improvements I might be able to achieve.

First amp; Tweed Deluxe '59 -(6V6 #1-32 ma), (6V6 #2- 41 ma.) Damn,this amp sounds killer with this mismatch ,can it sound better!?

Second amp;Fender Princeton II- (6V6 #1-18 ma), (6V6 #2-19 ma) seems cold on the bias, the sound doesn't completely suck, what to do about it? Can it sound better?I like this amp but it seems a little wimpy in a live context... and doesn't have the beef of my SF Deluxe Reverb.

Third amp; 1978 Deluxe Reverb (6V6 #1-42 ma), (6V6 #2- 24 ma.) This seems like a mismatched pair of tubes.How do I approach this?

I am trying to understand how biasing affects the amps sound and am quite confused. My tweed Deluxe has always sounded fantastic no matter what but now I have it scoped out the bias the figures look wrong. Shoot at one point the OT was dieing on the Tweed Deluxe but it still sounded great, even though it was only putting out about 2 watts!

Any help appreciated. Many thanks,
Fuzzca

[oc disorder]

Interesting that matching and correct bias don't always equate to killer tone.
It is possible to provide individual bias adjustments for each valve but maybe a bit scientific for rock and roll!
I suppose having several valves and going through them to find ones with a similar current draw would be advantageous... was just going to suggest you d/l these ball park bias charts.
They are for one valve and if you know the configuration and ht voltage
give a suggested current reading.

Power Tube Bias Charts

Until someone posts with experience with these circuits I suggest you measure the HT and compare your figures with the charts.

[Enzo]

The amp sounded killer, but now that you have a meter to measure something in it, you think it might be wrong. The meter is a useful and interesting tool, but it is just a tool. If I get a new screwdriver, that doesn;t mean all the screws in my house are loose. The third baseman is an important member of a baseball team, but he is not the whole team. Getting a new one won't solve a pitching problem. (Geez how I love dumb analogies)

it sounds good because it sounds good. Bias is just one of a million things in an amp. it contributes what it contributes, and nothing more nor less.

We like to buy matched sets of tubes, but there is no requirement for them. I like them for the hum cancelling they provide. Some people purposely mismatch tubes for tonal reasons. When you buy a new amp, Fender or Peavey or Marshall never installed matched tubes.

To me the bias is there to set an average idle current designed to provide good tone without stressing the tubes. 32ma and 41ma is an average of about 36ma. If I were to set the bias on that amp, and the tubes were that far apart, I'd set the amp so they average the bias current I am targeting. If you replaced these tubes with a pair that both sat right at 36ma, who knows, they might not sound as good to you.

Second amp. You have the bias meter in your hand, and you have the amp right there. Turn the bias control and FIND OUT what difference it makes... or doesn;t make. You can watch and make sure you don;t set it to 100ma, so you won;t burn anything up. You may liven up the sound or might not, so try it. DOn;t compare it to the other amp, unless the speakers are the same, the cabinets are the same, the circuits are the same, the B+ voltage is the same, the rectifier is the same, and everything else. COmpare the amp to itself. You will find out how much of any difference is bias related, and how much isn;t.

Third amp. Maybe mismatched tubes, maybe a bad tube, maybe a weak tube, maybe a screen resistor gone way high in value, maybe a control grid coupling cap starting to go leaky. Swap the tubes places. Does the same tube draw twice the current of the other in the new socket home?

In some amps, even perfectly matched tubes won;t balance the same. The resistance of the output transformer winding enters into the current calculation. SOme transformers are not wound even, so one side has a higher resistance than the other. Power off the amp and you can measure that resistance with your ohm meter even.

[Robert M. Martinelli]

Here we go again......

A point many people don't seem to grasp is that, if everything in a guitar amp was perfectly and evenly balanced, that amp would not be a guitar amp anymore, but rather an Hi-Fi amp.

First point :
As Enzo said a million times, back in the old days the component's tolerances were rather loose ( 10 or 20% were fairly common ) and the voltages inside the amps varied considerably ( Old Fender schematics called for +/- 20 % ).
The "magic" these old amps seem to have is there to prove you don't need everything to be SO perfect.

Second point :
Most guitar amps out there are push-pull AB1 designs, and this means that, in case of a perfectly balanced PP stage ( PI, tubes, OT ) we would have a hum-canceling design ( good ) which, unfortunately would kill all even harmonics ( extremely bad ).

As long as the mismatch is not severe, I wouldn't worry that much about it.

The ( not severe ) mismatch in your first amp is most likely one of the reasons for its "killer" tone. If you don't believe me, you'd probably trust your ears; get a perfectly matched set, swap the tubes in that amp, re-bias if necessary and hear for yourself, the amp will most likely sound thinner and less warm, because it will behave more linearly.

( Not severely ) mismatched tubes help some even harmonics to survive through the output stage and to make it to the speaker. All in all I would say that a 4-5 mAmps difference ( or, say, some 10 % ) is to be considered a reasonable match to achieve a good compromise between the need for hum-canceling and the need to help preserve the amp's harmonic content.

Stop worrying too much about matching tubes!

Hope this helps

Best regards

Bob

[vortex]

Thanks for the replies everyone. You are right Enzo!, I want to check all the screws in the house now!
I am still wrapping my head around this and will try some tube substitutions and check and see if any of the part values have drifted etc. It's time to do a cap job on the Deluxe Reverb so it might well be that time has taken it's toll on other components.
Any other insights appreciated.
Thanks again!

[billyz]

First of all you really need to know the Plate Voltages before you can know the proper range for the Bias current. My first thought is that your Tweed Deluxe is about to self destruct with the Bias current that high. But it is Cathode biased and that may be a factor. You need to know the Cathode voltage too and subtract that from the plate Voltage.

The princeton Reverb I like to run them around 25ma per tube so your may be a little on the low side. But they are weaker sounding than a Deluxe Reverb in any case. The PI is a Split load not a longtailed pair as in the Deluxe Rev.

Some tube mismatch is no big deal. But, There is plenty of mismatch going on the PI and even OT. I find that the better the Power Tubes are matched the better the amp sounds all around, better sustain, tone distortion etc.

Your other amp Is grossly mismatched, why don't you try to get a better pair of Tubes in it and see for yourself. Oh, And the tubes make a huge difference in the tone and power . Try a set of NOS RCA's ,matched for current and see for yourself what it could sound like. I have heard good things about the RI Tungsol 6v6's though, try a pair, just get a good warranty on them.

Check out his link to run the numbers.
Biasing Power Tubes

6v6gta tube are rated for 14watt plate dissapation. Older 6v6's were 12watts per tube.

[Euthymia]

On the subject of tube bias, you will get many opinions. Here are mine (and I've included some modding suggestions to be carried out by a tech):

The tweed Deluxe's tubes are not all that mismatched as far as idle current goes. If a client brought me such an amp, I would not recommend a new set of output tubes based on that difference in idle current if the tubes otherwise tested okay.

The Princeton II's not running all that cold.

As far as the tone, I like to clip the 500pF snubbers off V1 on these. Looks like the reverb mixer stage could use a 10uF cathode cap if you wanted to add some booty. To give the amp an adjustable bias, you can replace the 15K resistor going from the 220K/220K junction to ground with a 25K pot and a 5K resistor.

The Deluxe Reverb shows something out of whack. I would check the voltages at each pin of the power tubes. If they are close, then you've probably got tubes that I would not let out of my shop due to the difference in idle current. These amps also have snubbers on the power tubes that can come off.

[SatchmoeddieII]

I ran a Supro 6624 with a way mismatched pair of 6973s for years, and I think a new matched set sounds stiff, and not nearly as good. One was really weak, and the other was around 75-80% total, or half worn out, but the other was junk, and it sounded fantastic! I have bought some pulls to replace the old junk RCA. The other was a Sylvania, and in a different sized envelope (glass tube length). They looked identical as far as the plates. Some things just work, and if it aint broke.................These tubes were not even the same length, or brand. Someone hot rodded the amp, and stole 1/2 of the tube that ran the tremelo, and redid the attenuation to ground on the grid alsoplus added a .01 cap. I made a fat loud channel. I am putting it back to stock with a relay board, and pull pots to kick in the mod .

[GHRock]

Biasing in most all push-pull Class AB (almost all guitar amplifiers) is just about setting the idling current (amplifier voltage output at zero) so that transitions through zero voltage output don't cause a "notch" in the waveform which is unwanted distortion.

Too little current and the notch causes bad distortion in the output.
Too much current and the output tubes run hot and circuit quits working properly because of the unexpected extra current causes bad distortion in the output.

You are not going to "hot bias" and make your amp sound better. Your amp probably sounds best when the proper current bias make the crossover notch go away and not cause problems with the design of the output circuit.

FlukeAmpAndGuitarRepair@gmail.com

[Enzo]

Hi GH.

You will find a whole lot of people in the guitar amp world will disagree with that. Setting an amp up for zero crossover notch is perfect for hifi amps. But guitar amps often DO sound a lot better when biased "too hot."

Numerous people have found that their amp sounded the best it ever sounded - to them - when their tubes were cherry red and waiting to melt. I myself am not a proponent of biasing tubes to the brink of meltdown, but many guys are.

[Robert M. Martinelli]

GH,
No disrespect intended, but Enzo's right, and, though I own a '64 AC30 ( OMG class amp, thanks Steve ) which is biased at about 125% ( and known to be pretty rough on the tubes ) I'm by no means a "meltdown bias" lover, as I usually bias the amps I repair around 70-80%, which in most cases fits the bill without causing the tubes' premature death, because, as long as the tubes' average PD between "off" and "working" conditions stays around 100%, the tubes are still safe ( they swing to, say, 120% when on and they manage to cool down when "off" at 80% ).

OTH, as a rather experienced player/collector ( with a pretty good ear ) I CAN hear a difference when changing the same amp's bias from "cold" to "hot", and it's not about cross-over distortion, which has a fairly recognizable pattern/sound and disappears from my scope at relatively low bias levels already, but rather about the even/odd harmonics percentage the signal fed to the speaker(s) contents.

It would take you an FFT analysis to have a visual confirmation of the phenomenon, anyway, as the bias is made hotter, more even harmonics make it to the speaker, while, by making the bias colder, the amp tends to sound more sterile ( odd harmonics ). Too cold, and the cross-over distortion spikes re-appear on the o-scope's CRT ( way before the human ear spots 'em ).

By saying this, obviously, I am in no way saying that biasing "hot", "hotter" or "OMG" is good practice, I'm only confirming that setting the bias at different levels DOES produce a change in sound which should be pretty obvious to any player with a decent ear.

Sorry if I've been rather wordy....

Cheers

Bob