One or two bias test points?

[skyboltone]

I've often wondered about the wisdom of two test points for fixed bias in a push pull amp. Even with the best of "matched" tubes there is always a few ma difference. So you set it so one's a little hot and the others a little cool compared to your goal. What's the point of that? After all is said and done (redplateing aside) don't we just set bias so it "sounds right"?

Why not put a single one ohm resistor between a common tie point for both cathodes and ground, then measure the combined current as a starting point; then set it where you like it without having the pair over limits?

Comments appreciated

Dan

[gearhead]

Separate bias points should allow you to bias two tubes that are mismatched to the same bias value. Especially useful if you have two mismatched "unobtanium" tubes.

One thing I've speculated is that there has to be some limit to the mismatch that can be rectified (no pun intended) by the use of separate bias. Although I have never seen any empirical data, if they are off by too much, would imagine their dynamic characteristics (ie with an input) could be enogh to make a sonic difference. Such as one clipping and the other not when pushed (as with an Express!).

Just my thoughts.

[Phil_S]

Using the 1-Ohm resistor method is not all that accurate. As you know, it includes screen current. So, the number is relevant up to a point. It is also a relative reference between the two tubes and lets you know if you've got a pair that can or should be used together, or "not a pair" that don't belong together because they are too far apart.

As long as you are drilling holes, what's another tiny hole for the second TP, and what's a second measurement take?

I suppose the single TP is OK if you are sure the person who matched the tubes actually matched them appropriately. There is some controversy, or at least difference of opinion, about what can and should be done to match tubes. At the very least, two TP's help you to stay on top of this.

Call me nuts, but I put a pile of TP's on my last build, for both plates and the OT CT. I figure this lets me measure by the shunt method if I'm feeling up to it, or by getting the voltage drop over the OT winding. For the latter, I need to know the resistance of the winding, and have a pre-printed table to translate voltage drop to mA. It actually works very nicely.

I like this because (this is on my 'wreck variant) I've got 6AQ5's in a circuit that runs plate voltages that are probably higher than they should be for this tube (~330-335). As long as I'm living on the edge, I like to know what's up without having to guess at some allowance for screen current.

[skyboltone]

Separate bias points should allow you to bias two tubes that are mismatched to the same bias value. Especially useful if you have two mismatched "unobtanium" tubes.

Only if you have separate bias supplys for each tube.

[gearhead]

Oh, I thought that was a given. My mistake.

[Bob-I]

IMHO all of this is way overboard. I put 2 TP's on my amps, then adjust by ear anyway. Bottom line, you won't kiil your tubes unless you're way outta whack so why not use the best method possible, the ear.

[gearhead]

If you have reliable tubes, then it's probably redundant. On the other hand, some of the new tubes have been poorly build/tested. Have been known to drift by a significant amount in the first few hours, re:Allyns experience with some of the new EL-34 Mullard reissues.

[Allynmey]

If you have reliable tubes, then it's probably redundant. On the other hand, some of the new tubes have been poorly build/tested. Have been known to drift by a significant amount in the first few hours, re:Allyns experience with some of the new EL-34 Mullard reissues.

Actually those were the GT XF2 copies they now sell. Two bad sets out of two. not good. I think they are the GTel34M

[gearhead]

Yeah, I should have put the Mullard in (mullard) brackets. New Sensor "Mullard" are XF4 and the GT (mullard) EL-34M are XF2.

[skyboltone]

IMHO all of this is way overboard. I put 2 TP's on my amps, then adjust by ear anyway. Bottom line, you won't kiil your tubes unless you're way outta whack so why not use the best method possible, the ear.

Exactly, and on top of that we run more or less unbalanced PI's also. In the express, which runs very little Global NFB KF chose to keep the PI plate load resistors identical which given the characteristics of the Long Tailed Pair equals unbalanced output. One can play with the tail resistor to maintain balance at the expense of headroom if you want, but again, in the final analysis I'm not sure how all this "matched pair" stuff works out in the long run anyway. When I've had tubes bad out of the box, they've always been "matched" by the distributor. I imagine in some cases "matched" means "these look about the same".

[gearhead]

KF chose to keep the PI plate load resistors identical

Aren't they 82k and 100k?

[Phil_S]

...I'm not sure how all this "matched pair" stuff works out in the long run anyway. When I've had tubes bad out of the box, they've always been "matched" by the distributor. I imagine in some cases "matched" means "these look about the same".

Not to over value the business about tube matching ... I think it depends on who you buy your tubes from. I feel quite certain I know at least two tube dealers who burn in and match their tubes under probable operating conditions. Further, if you find they aren't matched, an exchange can be made.

Also, just about any fixed bias amp can be used as a tube matcher. That is, as long as you have two test points

[skyboltone]

KF chose to keep the PI plate load resistors identical

Aren't they 82k and 100k?

Sorry, Liverpool. 100k/100k, with cathode bias anyway so it wouldn't matter if the tubes are matched or not the PI is unbalanced top to bottom.

[Andy Le Blanc]

push-pull means phase invertion that results it two signals of
equal magnitude that are 180 degrees out of phase
most early methods of tube invertion also specify identical tubes to closely
realize an idealized lab model...... most tubed consumer products including
guitar amps have always been mediocre devices at best
tubes are a 10-20% tolerance device and there are nearly intolerable design
flaws in even the most celebrated amplifiers....etc

but they sound good

matched tubes help.... keeps things from being REALLY unbalanced
the long tailed chathod coupled inverter uses feed back to linearize its output
reguardless of component and voltage fluctuation, the larger "tail" resistor
reduces the effect of voltage fluctuation more than balance.......
the problem with it is theres only half the gain (still better than a splitload)
all the feedback squashes harmonic derivatives and you cannot at the same
time balance DC draw of the inverter tube and AC signal driving the power section
so you either end up with an amp of poor band width or an amp with
intermodulation issues so you have to apply feedback
balance thru a push pull stage really makes a difference..... its what your
ear hears and you can measure for at the same time....
keep the test points on the inside.

Id love to see Phil_s table to translate voltage drop ... 1ohm works ok
but its one more part

[Phil_S]

Id love to see Phil_s table to translate voltage drop ... 1ohm works ok but its one more part

Andy, test points on the plates and OT CT are also "one more part". The math is no biggie, see attached. I am working from a collection of about 50 untested 6AQ5's, hence the perceived need to match them in the amp.