Most optimal location for 2 pairs of matched tubes.

[the boogie man]

I just bought two pairs of matched tubes for metal panel super lead. I am not certain as to where I should place the matched pairs in reference to each other. I was thinking (V4 and V6) (V5 and V7). But, maybe it doesn't matter, so long as one of each matched pair is on a separate end of the primary. Could someone shed some light for me?

[danman]

Are the two matched pairs fairly close to each other in current draw? Did the seller provide you with the numbers?

[the boogie man]

Not very close. The plate current is about 3mA apart. Transconductance is 1.5 mA/v apart.

[danman]

When you say plate current are you referring to the tubes idle current rating at a given bias voltage? I only ask this because some sellers (especially on Ebay) will list the ma numbers from their particular tester but this is not the true idle current number that you are looking for. Alot of these testers will also not test the tube at the correct voltages to give an accurate idea of their current draw in a tube amp. Most sets of matched tubes listed on Ebay are not anywhere near being a match. Now if you bought these from a reputable dealer with the proper testing methods then you should be fine. A 3ma difference in curent draw between pairs is really not to bad and I would try the pairs in a few different combinations to see which way runs the quietest and biases up the closest between sides.

[the boogie man]

Thank You For the advice! They were an ebay deal. I will do as you suggested and move them around to see what results I can get.

[Haze13]

Matched pairs are for the phase inverters, differential pairs, mu-followers and SRPP's. Phase Inverters in the marshals are not linear because there are different values of plate resistors (actually there must some miss-match in the values for the best linearity but it has to be trimmed), there is no diff pairs and definitely mu-followers with SRPP... So, I don't think that there is a reason to use matched tubes except the power tubes in the Push-Pull power amps.

[the boogie man]

You lost me, "Matched pairs are for the phase inverters, differential pairs, mu-followers and SRPP's". I understand that the long tail pair circuit isn't balanced but wouldn't you still want matched tubes for keeping it biased properly?

"I don't think that there is a reason to use matched tubes except the power tubes in the Push-Pull power amps". But I was talking about the power tubes in a Marshall(Push-Pull).

I am sorry if I missing your point. I still got a lot to learn.

[danman]

When you install the new power tubes it would be a good idea to set your bias pot for the highest negative voltage first. This will insure that the new set will not be over-dissipating when you fire up the amp. From this point you can get a reading on each one to see how much current each is drawing and then make your adjustment. Are you using a single bias probe or reading 1ohm resistors on pin 8 when you bias?

[the boogie man]

When you install the new power tubes it would be a good idea to set your bias pot for the highest negative voltage first. This will insure that the new set will not be over-dissipating when you fire up the amp.

Yes, thank you for the tip!

Are you using a single bias probe or reading 1ohm resistors on pin 8 when you bias?

I just divide the plate voltage into plate dissipation and then multiply by .65(or so). I'll check the wave form on a scope and see if it looks good and then use my ear. I have compared this method to current draw and shunt. They all get get close enough. The shunt method is usually off by like 1.3mA do to voltage burden(as Fluke calls it). The cathode current draw method requires you to subtract screen voltage 15% (RCA Manual).

I took your advice and moved them around to see what works the best for me. I have them on opposite primaries. It sounds good, low noise floor, and seems pretty much matched. No need for quad in my case.

[Haze13]

Dif. pair/Phase Inverter (splitter). With the tube matched and "same" resistors you get the same voltage on both plates but with 180 degrees out of phase from each other. Since the plate resistors on the PI in the marshall amps are different (100k and 82k) you are not getting the same voltages. So you have 2 amplitudes, and each one is being amplified buy a power tube => in other words it is and asymmetry. So if your amp is assymetric from the begging, why to use a matched pre and PI tubes?

Power tubes should be matched so that you woun't end up with a crossover distortion.
http://upload.wikimedia.org/wikipedia/c ... ortion.png

[Roe]

put the coldest pair in v4-5 and the hottest pair in v6-7. be careful if its a high-voltage amp. look for redplating when playing

[Carbia]

Dif. pair/Phase Inverter (splitter). With the tube matched and "same" resistors you get the same voltage on both plates but with 180 degrees out of phase from each other. Since the plate resistors on the PI in the marshall amps are different (100k and 82k) you are not getting the same voltages. So you have 2 amplitudes, and each one is being amplified buy a power tube => in other words it is and asymmetry. So if your amp is assymetric from the begging, why to use a matched pre and PI tubes?

That's wrong.

The resistors are different to match the gains of each phase, because due to PI desing circuit, the gain of each triode is different.

This is the explanation from valve-wizard:

Because this circuit does not use a constant-current sink in the tail, the two outputs will not be perfectly balanced if both triodes have the same value anode resistors (although balance will be pretty good all the same). The difference in gain between the two outputs is given by:
A1/A2 = 1 + [(Ra+ra)/(Rk(mu+1))]
Where Rk is the total tail resistance Rb+Rt.
In this case we can see from the characteristics graph that ra = 40k, mu=45:
A1/A2 = 1 + [(82k+40k)/(47k(45+1))]
= 1.06
So the inverting output will be 6% higher than the non-inverting output. This could be corrected by making Ra1 6% smaller in value, but in practice it is not necessary; a slightly unbalanced phase inverter is often quite benificial to guitar tone, due to the additional 2nd harmonic it introduces. Nevertheless, this is why the 'traditional' version of the circuit also uses mis-matched anode resistors.

[Haze13]

That's right... but there is more than 6% of miss-match between the resistors and that's why I wrote in the first post that they have to be trimmed. There is an impact of the feedback that has to be taken in to consideration.
The second harmonic is canceled in the power section because of the push-pull operation, and the third is increased... No?
The pre-amp is other story of course...

[Carbia]

In addition, both halves of output transformer are not usually matched.

In fact, if a power amp is perfectly matched, sounds awful for guitar purpose.

But, like everything, how much mismatch is acceptable? I don't know, let's try

[Haze13]

You just said that all Single Ended Fenders, Valcos, Gretchs and others + cathodyne type PI amps sounds bad