Adding Individual Bias Controls?

[jbefumo]

THOUGHT I saw a topic dealing with this, but several searches didn't turn it up, so ...


About to move my "black faced" Super Reverb into a somewhat more manageable Twin Reverb cabinet (The 70W Supers are the same width as an older Twin).  Anyway, while I've got it open I put back a Master Vol control -- cross-line, post PI, push-pull, with normal state being disengaged, and pull-to-engage -- mainly because I thought a control would look way better than those black plugs, and anyway, I thing they are frequently useful, so long as you have the choice.  Ditto returning the Mid control on the Normal channel -- might not bother with a "bull to activate," since I can't see how that's a particular tone problem anyway.


So, the bias .... was thinking it might be beneficial to add an ability to check/set each of the two (KT66, now) power tubes individually -- or not? 


Is there any reason an approach like that shown in the attachment, but doubling the pot and resistor values so the resistance to ground remains the same, wouldn't work?  [I seem to recall having tried something like this before and changing one influenced both tubes, but it was a long time ago and I may have messed something else up.]


Also going to install a variation on the "RMS" control Dennis Kager used in his Sundown amps (second attachment).


Am thinking, though, that I will likely used a dual-ganged 500R pot so as to keep the cathode paths to ground separated, since both KT66 and KT88 sheets advise separate cathode loads when cathode biasing. 

[sluckey]

Your bias circuit should work just fine with no interaction. You shouldn't have to double all resistor values either. As long as you're 'improving' the bias circuit, you may consider adding a 10µF cap between the wiper and ground of each bias pot.

[jbefumo]

Thanks!  Actually, just realized that my cross-line MV will be problematical with individual bias, unless I add a few more capacitors .... how these 'little tweaks' can snowball ....

Your bias circuit should work just fine with no interaction. You shouldn't have to double all resistor values either. As long as you're 'improving' the bias circuit, you may consider adding a 10µF cap between the wiper and ground of each bias pot.

[sluckey]

Thanks!  Actually, just realized that my cross-line MV will be problematical with individual bias, unless I add a few more capacitors .... how these 'little tweaks' can snowball ....

Most people put the crossline MV across the plates of the PI tube so there will be no interaction with bias. I've never seen one on the output tube grid side of the coupling caps.

[jbefumo]

I usually try to avoid any DC to pots ... if designing from scratch I just sandwich each MV lead between two caps.  Never thought of it before, but I just noticed that Kager's Sundown MV (which he calls "Gov") is between the bias resistors and the grid resistors.  Seems like that might have some impact on what the power tube grids are seeing ....

Thanks!  Actually, just realized that my cross-line MV will be problematical with individual bias, unless I add a few more capacitors .... how these 'little tweaks' can snowball ....

Most people put the crossline MV across the plates of the PI tube so there will be no interaction with bias. I've never seen one on the output tube grid side of the coupling caps.

[sluckey]

Most people put the crossline MV across the plates of the PI tube

That's wrong. Sorry. It was a senior moment.

[jbefumo]

Just turned ... 64 ... I can empathize!

Most people put the crossline MV across the plates of the PI tube

That's wrong. Sorry. It was a senior moment.

[Paul1453]

I am just starting out with this biasing stuff,
so I put these on my watch list.

What do you guys think of these?

http://www.ebay.com/itm/201444907997

http://www.ebay.com/itm/181860973282

I don't think I could assemble these components for the price the Chinese are willing to ship a complete board to me for.   

[jbefumo]

I've had generally good luck ordering stuff from China via Ebay. One got lost, but the seller refunded without question, and otherwise have been remarkably fast. Quality is a mixed bag, but for non-critical stuff, it's pretty hard to go wrong. Of course, I try to avoid PC boards in my builds ... just because ... I'm old and stubborn and usually need it in as much time as I care to devote to rooting around the parts bin ...


I've decided to leave the existing single bias control, just to keep the mods under control.  Also, whoever did this AB767 retro build was a consummate craftsperson, and i'm being shamed into a higher-than-usual standard of conduct ...

I am just starting out with this biasing stuff,
so I put these on my watch list.

What do you guys think of these?

http://www.ebay.com/itm/201444907997

http://www.ebay.com/itm/181860973282

I don't think I could assemble these components for the price the Chinese are willing to ship a complete board to me for.   

[Paul1453]

The dual control board shows a schematic for the circuit.

Is there anything objectionable in that design?

I have been reading that if your control pot fails you want your bias voltage to go extreme negative to protect your tubes.

I'm not sure that the design shown would do that.

It appears to me that if the wiper in this design opens, you would get no negative voltage to your tubes which would cause them to melt down.   

[jbefumo]

I never delved that deeply into the topic, but think a pot failing in that kind of environment is pretty unlikely.  Schematic looks pretty standard, assuming you have a separate tap for bias. 


There's an outfit called TentLabs that has some super sophisticated components, amongst which is a gizmo that constantly monitors and corrects power tube biasing, correcting for uneven wear, etc.  They seem to be popular in the high-end audiophile circles, but they're pretty expensive and probably make no audible difference in a raging guitar amp.

The dual control board shows a schematic for the circuit.

Is there anything objectionable in that design?

I have been reading that if your control pot fails you want your bias voltage to go extreme negative to protect your tubes.

I'm not sure that the design shown would do that.

It appears to me that if the wiper in this design opens, you would get no negative voltage to your tubes which would cause them to melt down.   

[jbefumo]

This looks pretty interesting as well -- from the same seller:


http://www.ebay.com/itm/Tube-Amplifier-Filament-LM317-Adjustable-Regulator-Soft-Start-Power-Supply-Board/171988571929?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D35694%26meid%3Daaaebe290f56405d94ecbb7731592997%26pid%3D100005%26rk%3D1%26rkt%3D6%26sd%3D181860973282

The dual control board shows a schematic for the circuit.

Is there anything objectionable in that design?

I have been reading that if your control pot fails you want your bias voltage to go extreme negative to protect your tubes.

I'm not sure that the design shown would do that.

It appears to me that if the wiper in this design opens, you would get no negative voltage to your tubes which would cause them to melt down.   

[Paul1453]

I never delved that deeply into the topic, but think a pot failing in that kind of environment is pretty unlikely.  Schematic looks pretty standard, assuming you have a separate tap for bias. 

OK, if you are done with this topic I understand.

But for anyone that wants to explore further.

Would a separate bias tap actually be required?

If you sent that little board rectified and unfiltered DC at say +70 VDC, would that be a problem?   

Or could you just tap your HT secondary, and put a resistor in there to knock your AC supplied to the board down to what it is looking for?   

[jbefumo]

Well, the components might not be up to the kinds of voltages you'd get from your mains. You COULD mess around with the FWB diodes -- just use one, off one leg of your PT secondary, but then you're back to modifying their board, which might not stand up well to rework.


Pretty interesting store, though -- they have a bunch of interesting looking stuff ...

I never delved that deeply into the topic, but think a pot failing in that kind of environment is pretty unlikely.  Schematic looks pretty standard, assuming you have a separate tap for bias. 

OK, if you are done with this topic I understand.

But for anyone that wants to explore further.

Would a separate bias tap actually be required?

If you sent that little board rectified and unfiltered DC at say +70 VDC, would that be a problem?   

Or could you just tap your HT secondary, and put a resistor in there to knock your AC supplied to the board down to what it is looking for?   

[2deaf]

The dual control board shows a schematic for the circuit.
Is there anything objectionable in that design?

R2 and R3 need to be an order of magnitude larger.  At 30K, changing W1 affects the voltage on W2 by changing the voltage drop across R1 and by changing the load on the transformer secondary.  At 300K, the changes would be negligible.  jberfumo's original post has 220K resistors on the wipers so that changing one pot has very little effect on the other pot.

R2 and R3 also put the tubes at their coldest bias should the wiper fail whether they are 30K or 300K.     

[2deaf]

Would a separate bias tap actually be required?

A separate bias winding would be necessary.  You cannot use the bias tap on a HT winding because it would constitute a direct short.

If you sent that little board rectified and unfiltered DC at say +70 VDC, would that be a problem?

Yes.  If you snip all of the diodes off and give it -70V unfiltered DC across capacitor C1, it will work.

Or could you just tap your HT secondary, and put a resistor in there to knock your AC supplied to the board down to what it is looking for?

No.  If you have a FWB, you cannot use this board on the HT at all.  If you have the standard FW with CT that only uses half the winding at a time, you can snip D3 and D4 and hook D1 and D2 to their own ends of the HT secondary via their own resistors.  Try 220K first and work from there.  Or you could snip D2, D3, and D4 and hook D1 up to one end of the HT secondary via a resistor.  Try 100K first and work from there.

[Paul1453]

Would a separate bias tap actually be required?

A separate bias winding would be necessary.  You cannot use the bias tap on a HT winding because it would constitute a direct short.

If you sent that little board rectified and unfiltered DC at say +70 VDC, would that be a problem?

Yes.  If you snip all of the diodes off and give it -70V unfiltered DC across capacitor C1, it will work.

Or could you just tap your HT secondary, and put a resistor in there to knock your AC supplied to the board down to what it is looking for?

No.  If you have a FWB, you cannot use this board on the HT at all.  If you have the standard FW with CT that only uses half the winding at a time, you can snip D3 and D4 and hook D1 and D2 to their own ends of the HT secondary via their own resistors.  Try 220K first and work from there.  Or you could snip D2, D3, and D4 and hook D1 up to one end of the HT secondary via a resistor.  Try 100K first and work from there.

1.  I am just learning, but I was told different on a separate thread.
I don't see any direct short path.

2.  Why snip the diodes?  The resistor to lower AC to the board should give it the say 70 VAC it is looking for.

Like I said, I'm still learning about this negative grid biasing.   

[2deaf]

1.  I am just learning, but I was told different on a separate thread.
I don't see any direct short path.

The bias tap and the CT on the HT winding would be the AC inputs to the D1, D2, D3, D4 bridge.  The CT is grounded and so is the junction of D3 and D4.  Draw a line from the CT to the junction of D3 and D4 and there is a direct short through D3.

2.  Why snip the diodes?  The resistor to lower AC to the board should give it the say 70 VAC it is looking for

Where are you getting this +70V? 

[Jack_Hester]

THOUGHT I saw a topic dealing with this, but several searches didn't turn it up, so ...

This may be the link that you were thinking of: 

http://el34world.com/Forum/index.php?topic=19695.msg205355#msg205355

Jack

[jbefumo]

Thanks -- like DC filament supplies, one of my unfounded fixations.  as it stands, after checking and rechecking the bias on each of my two power tubes, I concluded that I was fretting over a problem that didn't exist and simply did a final check of everything and put it back together.... Easy fix!

THOUGHT I saw a topic dealing with this, but several searches didn't turn it up, so ...

This may be the link that you were thinking of: 

http://el34world.com/Forum/index.php?topic=19695.msg205355#msg205355

Jack