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(2) 6AV6 instead of 12ax7

Dlav

Member
2014-02-18 12:28 pm
Louisiana
Hello, I'm thinking about splitting up a 12ax7 into 2 6av6's due to the fact in my Eico st70 the left and right channel share a 12ax7 in a gain stage, I'm not getting good seperation of channels when I turn my balance from left to right. Could this work? I checked the data on the 6av6 it does appear to be one half of a 12ax7. I do understand about the extra heater load. If it's not a 12ax7 problem, where else could I look to find this balance issue. When I turn it all the way left or right the other channel still has quite a bit of volume coming through. Any advise would help...

Thanks David
 

DAK808

Member
2012-05-27 7:34 pm
Hi David, it would be way easier to try and track down the problem with your imbalance than two install 2 6av6s. I would try and clean the balance control first and after that test the amp to see if it made a difference. Also I checked the ST70 schematic and noticed that the tape monitor switch is right before the balance control so you also need to clean that switch too. You can spray some contact cleaner and move the switch, then spritz again to flush any contaminants, then follow with some caig de-oxit. good luck
 
An EBC91 (6Av6) is a duplex diode triode. What will you do with the control diodes, pins 5&6?

Connect them to the cathode and make them an internal shield.

Hello, I'm thinking about splitting up a 12ax7 into 2 6av6's due to the fact in my Eico st70 the left and right channel share a 12ax7 in a gain stage, I'm not getting good seperation of channels when I turn my balance from left to right. Could this work? I checked the data on the 6av6 it does appear to be one half of a 12ax7. I do understand about the extra heater load. If it's not a 12ax7 problem, where else could I look to find this balance issue. When I turn it all the way left or right the other channel still has quite a bit of volume coming through. Any advise would help...

Thanks David

Your channel separation problem isn't because you have two triodes in one bottle. This isn't RF, and the capacitance between sections is much too small to matter at AF. Though the 6AV6 isn't quite the same as 1/2 a 12AX7, it's close enough. I'd looki for other causes of the problem, and I'd suspect that balance control. I don't use them in my designs as I prefer separate volume controls for each channel. That way, I can get it right even if it's a bit less user friendly.
 
Hello guys thanks for the responses, the amp sounds wonderful,I've rebuilt the unit, except the things I haven't got to yet ,the wire, switches, and pots ,all new caps every single one, new resistors metal films, and hi end dale hi wattage in power supply, I've also taken apart the balance and volume pots, very carefully and de-oxit the resistance strip and it all went back together nicely. I have not done any thing with the tape monitor switch, will clean that. To Mr Snell, I was hoping just to leave pins 5&6 alone, can I do this? Is there another tube that's 1/2 of a 12ax7? I'm also wondering if something is going on with that balance or phase rev switches that would cause this problem, I do want to get rid of all that extra mixing of left and right channel with center speaker, it's a wired set up. Thank you guys any advise will be awesome .
David
 
6AV6/EBC91 is the only valve I am aware of which is essentially half a 12AX7/ECC83. The same triode appears in the ECL86, but I would not suggest using that!

You need to ground the diode anodes. No metal inside the valve should be left floating.

As others have said, it is most unlikely that your problem is caused by the valves.
 
Hello guys thanks for the responses, the amp sounds wonderful,I've rebuilt the unit, except the things I haven't got to yet ,the wire, switches, and pots ,all new caps every single one, new resistors metal films, and hi end dale hi wattage in power supply, I've also taken apart the balance and volume pots, very carefully and de-oxit the resistance strip and it all went back together nicely. I have not done any thing with the tape monitor switch, will clean that. To Mr Snell, I was hoping just to leave pins 5&6 alone, can I do this? Is there another tube that's 1/2 of a 12ax7? I'm also wondering if something is going on with that balance or phase rev switches that would cause this problem, I do want to get rid of all that extra mixing of left and right channel with center speaker, it's a wired set up. Thank you guys any advise will be awesome .
David
Pins 5 & 6 must be returned to the negative point beneath the cathode resistor to avoid spurious things happening.
 
6AV6/EBC91
You need to ground the diode anodes. No metal inside the valve should be left floating.

I’ve thought about using only one half of a dual triode in an audio circuit, simply because of present-day and future avilability of quality tubes. I’ve done this with short term test circuits, with 12**7 types, leaving the unused section’s plate, grid and cathode unattached to the circuit, or “floating”. I’ve have not seen issues that I’m aware of, but the testing was minimal, i.e., gain or distortion measurements at a fixed single frequency.

If one wanted to use only half of a dual triode, what would be the best way?
1/ Leave the unused section “floating”? (I’m assuming not - the heater is powered.)
2/ Tie all three elements to ground?
3/ Or possibly: bias the unused section greatly negative, and apply a low plate voltage?

Any thoughts?
 

Koonw

Member
2013-04-09 9:37 pm
I’ve thought about using only one half of a dual triode in an audio circuit, simply because of present-day and future avilability of quality tubes. I’ve done this with short term test circuits, with 12**7 types, leaving the unused section’s plate, grid and cathode unattached to the circuit, or “floating”. I’ve have not seen issues that I’m aware of, but the testing was minimal, i.e., gain or distortion measurements at a fixed single frequency.

If one wanted to use only half of a dual triode, what would be the best way?
1/ Leave the unused section “floating”? (I’m assuming not - the heater is powered.)
2/ Tie all three elements to ground?
3/ Or possibly: bias the unused section greatly negative, and apply a low plate voltage?

Any thoughts?

Greeting.

You can wire 12AX7 type heater either parallel (6.3V) or series (12.6V). If series then both heater will be on it's no good as the heater will worn out in long run. So you have to wire them in parallel, using a jumper or A/B switch to connect pin 4-9 or 5-9. You can wire both sections in parallel, and switch on when needed. I have done this for output tube each with their own bias and connect to same OPT and to turn on with heater control switch to on either one of 6V6/6L6/EL84. there is no problem. So I don't see why there should be a problem in twin sections.
 
KoonW--thanks for the reply!

I've thought of just powering half the heater, and a resistor could be used in place of the unused heater half, so that an additional 6.3 V source would not be necessary. However, there are a few tubes in this "group", such as the 12FQ7, that do not have a center tap.

I'm still wondering, even if the heater is un-powered, would there be detrimental effects of allowing the unused half to float, or would it be even worse to ground the elements?
 

Koonw

Member
2013-04-09 9:37 pm
KoonW--thanks for the reply!

I've thought of just powering half the heater, and a resistor could be used in place of the unused heater half, so that an additional 6.3 V source would not be necessary. However, there are a few tubes in this "group", such as the 12FQ7, that do not have a center tap.

I'm still wondering, even if the heater is un-powered, would there be detrimental effects of allowing the unused half to float, or would it be even worse to ground the elements?

Right, for really unused section, to prevent accidental introduce high voltages, magnetic induction etc, it's safer to ground all unused elements. But if you intend for A/B use, and as you wire the unused plate, grid, cathode to the used one, the danger would be avoided, except may add some inter-electrode capacitance/ inductance.
 
For best results the unused half should have a cold heater, with the electrodes connected to either the cathode of the used half or some other point which is more negative than the cathode.

The cold heater prevents 'sleeping sickness' (a cathode interface layer).

Setting a low or slightly negative potential for the unused electrodes ensures that they don't pick up any stray electrons. Stray electrons hitting a floating electrode can cause a large potential to build up. This can be negative or positive, depending on how many secondary electrons are emitted for each incoming primary electron. The large potential can then affect operation of the used half.

Some Chinese '12AX7' appear to have a floating 'screen/shield' between the sections. This could cause trouble.
 
If one wanted to use only half of a dual triode, what would be the best way?

1/ Leave the unused section “floating”? (I’m assuming not - the heater is powered.)
2/ Tie all three elements to ground?
3/ Or possibly: bias the unused section greatly negative, and apply a low plate voltage?

Any thoughts?

If this is one of the 12**7 family, then what I'd do is punch another hole in the chassis, install another socket, and wire each socket so's to power up one half only, leaving the other dark and disconnected. That way, if the active section goes bad, switch sockets and use the good half. That way, you get twice the service life from each bottle, if the failure wasn't catastrophic like letting the vacuum out..

If I didn't want to do that, then I'd look for small signal pents that I could use as pseudotriodes. That's the halibut, there are very few small signal singleton triodes in the 7-pin and 9-pin mini formats, though no lack of same as metal envelope Octal types.

6AU6s make very linear singleton triodes with a u-factor that falls nicely between those of the 12AU7 and the 12AT7.
 
Some Chinese '12AX7' appear to have a floating 'screen/shield' between the sections. This could cause trouble.

The JJ ECC832 used as a replacement for a 12DW7/7249 also has a shield between the two sections. This odd looking dual triode appears to have been originally designed for RF application. It seems to work okay for audio, though.

Thanks for the replies.