machting output tubes

[francesco]

Hi , I have a twin reverb (ultralinear) and some doubt
I replaced them recently but they are not completely machted, in fact their reading is;
V 7: 23ma
V 8; 29 ma
V 9: 26 ma
V 10 : 27 ma
so, what position should i put them to be the "most machted" possible?
I mean should I place, side by side, ( for example)the one with 23 ma with the one with 27 ma. or what?
Thanks:Francesco.
P.S.the tubes are 6L6

[Bruce / Mission Amps]

Originally Posted by francesco

Hi , I have a twin reverb (ultralinear) and some doubt
I replaced them recently but they are not completely machted, in fact their reading is;
V 7: 23ma
V 8; 29 ma
V 9: 26 ma
V 10 : 27 ma
so, what position should i put them to be the "most machted" possible?
I mean should I place, side by side, ( for example)the one with 23 ma with the one with 27 ma. or what?
Thanks:Francesco.
P.S.the tubes are 6L6

IN reality, those four tubes are all pretty close anyhow but I would put the 23ma with the 29ma and the 26 and 27ma tubes together.
Then I would set the idle current by measuring the 29ma tube side and set it at around 32 watts to 36 watts on that side.

[francesco]

[QUOTE=Bruce / Mission Amps;79273]IN reality, those four tubes are all pretty close anyhow but I would put the 23ma with the 29ma and the 26 and 27ma tubes together.
Then I would set the idle current by measuring the 29ma tube side and set it at around 32 watts to 36 watts on that side.[/QUOTE
so i sould leave them in the position they are now.

What do you mean by "set it at arournd 32 to 36 Watts on that side"?
I remind you that it is a silverface twin reverb( ultralinear 135 Watts) and there is no way to set any pot or tim but the "tube output machting" and the "hum balance" on the rear panel.
Thanks,ciao. Francesco

[Robert M. Martinelli]

The power Bruce was referring to is the quiescent dissipated plate power.
When idle ( bias ) current flows, you have to multiply it by the plate voltage to obtain the plate dissipated power in quiescent conditions - e.g. if your plate voltage is, say, 450 VDC and your bias current on one tube is , say, 35 mAmps, then that tube's plate is dissipating 15.75 Watts in quiescent conditions.

Your tubes work in pairs, so the quiescent dissipated power on one side is the sum of the two bias currents on that side multiplied by the plate voltage.

Hope this helps

Best regards

Bob

[pdf64]

To adjust the bias, need to fit different resistor value in the bias voltage supply, see schematic.
http://www.schematicheaven.com/fende..._135_schem.pdf
Which one you change is up to you, but I would alter the 1k2 on the bias winding.
To re-enforce voxrules point, you need to measure your plate voltage, it's likely to be over 500V on that model, then derive what quiescent plate current you need for a reasonable quiescent plate dissipation (quiescent = no signal), then set the bias voltage accordingly. Reasonable is usually taken to mean between 50 and 70% of max dissipation (30 watts for a 6L6GC, 23 to 25 Watts for a 6L6GB / 5881). Peter.

[Enzo]

If you don;t want to modify your amp for bias adjustment, then just do what Bruce suggests. Put the 23 and 29 on one side - they average 26 apiece - and the 26 and 27 on the other side. Then turn the output tube matching control to the point of least hum.


The amp maybe didn;t have bias level adjusting when built, but it is a simple modification and it has been done to many of them.

[francesco]

Originally Posted by Enzo

If you don;t want to modify your amp for bias adjustment, then just do what Bruce suggests. Put the 23 and 29 on one side - they average 26 apiece - and the 26 and 27 on the other side. Then turn the output tube matching control to the point of least hum.


The amp maybe didn;t have bias level adjusting when built, but it is a simple modification and it has been done to many of them.

can you,please, indicate me a link in which i can find a schematic to mod bias.
I mean from fixed bias to adjustable bias?
Thanks.Francesco

[Robert M. Martinelli]

Hi Francesco,
I googled for " ultralinear twin reverb" and found this schematic.

http://www.schematicheaven.com/fende..._135_schem.pdf

Is this your amp's schematic ?

( just to be sure we' re talking about the same thing before we all go any further ).

Best regards

Bob

Edit : OOOPS - I now see Peter has posted the same link before - Sorry Peter - ehm, can't we consider this a double-check ?

[francesco]

Yes Bob, it's just that one,I know it well.
I was thinking to replace the 33k(close to"output tubes machting") with a 15k resistor and then put,in series, a pot of 47k.
Do you think it would be all right?
Thanks, ciao. Francesco.

[Robert M. Martinelli]

Hi Francesco,
no, a simple variable resistor in series won't help, because a voltage drop across a resistor only occurs when current is flowing. Control grids don't draw any current in an AB1 design, so you would not have any voltage variation across the variable resistor.

The only way to do it right is to modify the biasing network from fixed to variable, adding one potentiometer ( acting like a variable voltage divider ), but if you are concerned about biasing each tube exactly with the same current, then you will have to put 4 potentiometers ( one for each tube ) between the -58VDC line and the tubes.

Usually to make the bias variable the potentiometer is connected with one of the outer lugs to the -bias voltage, the wiper to the grids ( in your case to the 1,5 K grid resistor ) and the other outer lug to ground, but it's advisable to add a resistor in series between this last lug and the ground, so the grids won't be to 0 V in case you fully turn the pot ( thus endangering the tube ).

With a 47K potentiometer and a 68K series resistor to ground you should be able to vary the bias voltage from -58VDC to about -35VDC.

One last thing, I haven't asked you the method you're using to determine the bias current : Are you using the 1-Ohm resistor between cathode and ground or are you measuring current directly in series with each plate ?

( I'm asking because there is a difference in the results you get ).

Hope this helps

Best regards

Bob

[francesco]

Originally Posted by voxrules!

Hi Francesco,
no, a simple variable resistor in series won't help, because a voltage drop across a resistor only occurs when current is flowing. Control grids don't draw any current in an AB1 design, so you would not have any voltage variation across the variable resistor.

The only way to do it right is to modify the biasing network from fixed to variable, adding one potentiometer ( acting like a variable voltage divider ), but if you are concerned about biasing each tube exactly with the same current, then you will have to put 4 potentiometers ( one for each tube ) between the -58VDC line and the tubes.

Usually to make the bias variable the potentiometer is connected with one of the outer lugs to the -bias voltage, the wiper to the grids ( in your case to the 1,5 K grid resistor ) and the other outer lug to ground, but it's advisable to add a resistor in series between this last lug and the ground, so the grids won't be to 0 V in case you fully turn the pot ( thus endangering the tube ).

With a 47K potentiometer and a 68K series resistor to ground you should be able to vary the bias voltage from -58VDC to about -35VDC.

One last thing, I haven't asked you the method you're using to determine the bias current : Are you using the 1-Ohm resistor between cathode and ground or are you measuring current directly in series with each plate ?

( I'm asking because there is a difference in the results you get ).

Hope this helps

Best regards

Bob

Forgive me Voxrules, I forgot to say that I own a bias prober with 4 octal probes. That's why I coud tell exactly what the miliampers were on every single tube in a previous post of mine.
Anyway why could i not do a mod like that one on the super reverb black face-AB 763(not reissue) I mean a pot to control the overal voltage and the balance to macht the two couple of tubes.
By the way, what if I insert in the super reverb a pot(post resident trim) like the one on the twin so to make the two 6L6 really "machted in pair"?
As they never seels you tubes truly machted, so I could macht them by Myself and then bias them.
Let me know Your opinion,please.
Ciao,Francesco.

[francesco]

adding one potentiometer ( acting like a variable voltage divider )

That's the way I wanted to do it!
Francesco

[Robert M. Martinelli]

Hi Francesco,
yep, in fact I was suggesting both solutions, the first if you're happy with the possibility to bias the tubes in pairs, the second if you want to bias the tubes one by one. I admit the latter to be more costly, but it gives you the possibility to compensate for poorly matched tubes.

If you' re going with a single pot, you' ll have to measure and "select" the tubes in pairs, taking care that the sum of the bias currents of the two pairs
is more or less the same ( e.g. 24+27=51 mAmps for the first pair and 22+28=50 for the second...well....you got the picture.... ).

Whatever solution you decide to implement, when biasing remember to keep the quiescent power dissipation within reasonable limits ( 60-70 percent of the max. plate dissipation of the tubes you' re using ).

Congrats for your octal bias tester, can you give some details about that?
Does it measure the current(s) at the cathode or at the plates? If measuring at the cathode(s), does it measure and keep into account screen grid current(s)?

Best regards

Bob

[francesco]

Hi Bob,I was just think such a mod as the attached, or you think is it a wrong thing?
The "output tubes machting"pot it's a very useful tool I think, because when you rotate it til you don't hear a HUM anymore. it means the tubes are perfectly machted.In fact as for the light of the pilot light you could not tell that it's on(on the twin) That's why I'd liked to do such a mod on the Super Reverb too.
Here's the link to the bias prober I bought
http://www.tubeampdoctor.com/product...eb8a2dd4df8baf
Ciao. Francesco

[Robert M. Martinelli]

Hi Francesco,
nope, I think it will be very useful, and only one bias pot will suffice provided that you "match" the tubes in pairs so that the sum of the currents is more or less the same on the two "legs", that's all.
Best regards
Bob

[francesco]

Sorry something went wrong uploading the files, so I try again.

[Robert M. Martinelli]

Hi again Francesco,
This is the way I would do it,
the series 68K between the bias pot and GND is there to keep the voltage from going "too positive" ( actually less negative ) which could make the tubes "redplating", with the values shown the variable voltage divider should allow you to go from ( 68/(68+47) )*(-58)=-34.3 VDC to -58 VDC.
( actual values might be slightly different due to components' tolerance ).
Hope this helps

Best regards

Bob

[francesco]

Originally Posted by voxrules!

Hi again Francesco,
This is the way I would do it,
the series 68K between the bias pot and GND is there to keep the voltage from going "too positive" ( actually less negative ) which could make the tubes "redplating", with the values shown the variable voltage divider should allow you to go from ( 68/(68+47) )*(-58)=-34.3 VDC to -58 VDC.
( actual values might be slightly different due to components' tolerance ).
Hope this helps

Best regards

Bob

Well, in that way I think I'll be able to only lower the negative voltage.
What if I want to be be higher? say -65?
I think it's not possible,that's why I thought to replace the 22k with a 4,7k resistor. am i wrong?.
What do you think about adding that mod on my super reverb? that is, making a balance bias control on it?
I'd like you have a look at it and let me know your opinion.
Thanks. Ciao. Francesco

[Robert M. Martinelli]

Yep, it's a variable voltage divider, so it can only divide whatever voltage appears at its input, it can't elevate it.

If you need a "more negative" voltage, you need a different tap on the transformer, or a separate transformer - I think this is not the case, as setting the bias voltage so negative would bias the tubes waaaaaaaay tooooooo cold and you would have cross-over distortion caused by tubes switching on and off.

Bias is there to keep cross-over distortion from occurring, and -58V is already "cold", I doubt you will ever need to go lower than that.
OTOH going towards "less negative" voltages increases the amount of bias current, at the point that, if the control grid approaches 0V electrons would no longer be rejected by the control grid and the tube would merely act as a diode.

To keep this from happening I added the 68K resistor that narrows the range from the maximum ( -58VDC ) to about -35VDC.

As I told you, I would do it exactly the way I shown you.

Hope this makes it clearer.

Best regards

Bob

[Robert M. Martinelli]

Oh, one last thing,
reducing the series resistor's value as you would do would have no effect, because you get a voltage drop across a resistor only if you have a current flowing through it ( Ohm's law ). This does not happen with control grids which are voltage-operated and don't draw any current in A, B and AB1 designs.

Regards

Bob

[Gtr_tech]

Originally Posted by francesco

Hi , I have a twin reverb (ultralinear) and some doubt
I replaced them recently but they are not completely machted, in fact their reading is;
V 7: 23ma
V 8; 29 ma
V 9: 26 ma
V 10 : 27 ma
so, what position should i put them to be the "most machted" possible?
I mean should I place, side by side, ( for example)the one with 23 ma with the one with 27 ma. or what?
Thanks:Francesco.
P.S.the tubes are 6L6

I wouldn't lose any sleep over a couple mA.....they're close enough to play nice.