Got bored last night.....bias readings

[R_ADKINS80]

I just got my new bias probe in the mail yesterday from Eurotubes and decided to test it out last night on my Triple. Keep in mind I was not able to do any tone testing becuase the wife and baby were asleep not 10 feet away from my full stack.

My TAD 6L6GC-STR's were running and a frigid 17mA draw in Spongy/Tube Rec (how I normally run it) and a still cold 27mA in Bold/Diodes. I knew these things ran 6L6's rather cold but DAM!!

I have been doing alot of reading on here and also back and forth emails with Bob from Eurotubes. I came across a really old thread from '05 or '06 of a guy running his stock Mesa 6L6's in EL34 bias setting on Spongy/Tube rec and they were pulling a decent 33mA draw which is quite good for a 6L6. I decided to try it so I powered it down and switched it over to EL34 bias and warmed it back up. No explosions...no funny looking tubes.....no funny noises coming through the amp aside from the normal "waterfall" hiss that I normally get from lots of gain dialed in. I let them idle like this for about 20-30 minutes or so and they were steady at 34mA draw which is quite a good setting for a 6L6.

I can't see any ill effects from doing this. I know people here have done it and that Bob recomends this for better cleans and more dynamics as long as the tubes don't run too hot...which 34mA is not too hot for a 6L6. The big question for me tonight is going to be: how does it sound and feel now?

 

[fleeced]

and what was the answer?

 

[R_ADKINS80]

And the answer is: The absolute biggest, hairiest balls I have ever heard out of a recto. PERIOD!

 

[Jacko123]

Very cool. Big hairy balls are good when it comes to tone. Dunno if my wife would agree for other applications, though.

I should get a reader when I get my tubes from Eurotubes. Is it the "Eurotubes Bias Probe" for $25?

 

[Benraldo]

if you look at the schematic all the 6l6/ el34 switch does is switch a resistor in and out of the bias circuit.
no harm in running your 6l6's on the "el34" side providing your within safe spec of the tube.
I've replaced one of the resistors in the bias circuit with a pot and now have an adjustable bias circuit. I've ran my 6l6's hotter than where you've got them and have seen no ill effects

 

[R_ADKINS80]

Very cool. Big hairy balls are good when it comes to tone. Dunno if my wife would agree for other applications, though.

I should get a reader when I get my tubes from Eurotubes. Is it the "Eurotubes Bias Probe" for $25?

Yeah thats the one I got.

 

[fleeced]

so ... definitely safe for me to try ... ?!

Don't mind a shortened tube life ... just don't want any burn outs on the boards

Is the worst thing that could happen that the power tubes may not last as long?

 

[R_ADKINS80]

Bob at Eurotubes told me that as long as you check the plate current and it's between 32-40mA then it is pefectly fine.

 

[groovebelly]

so EL 34s draw more current than 6L6s? I thought it was the opposite... don't know why

 

[mikey383]

so EL 34s draw more current than 6L6s? I thought it was the opposite... don't know why

6L6s draw more current based on this page: http://www.webervst.com/tubes1/calcbias.htm

I've replaced the resistor in the bias circuit with a pot so I have adjustable bias. I checked the bias on the set of E34Ls I've been running in my Dual for 2 years....45mA @ 486V :shock: !!!

It's still running strong, so I guess they can handle that much.

 

[155]

hey how bout some pix of that mod that would be good to do!

 

[mikey383]

hey how bout some pix of that mod that would be good to do!

Ask and ye shall receive.

This is for a 2 channel. It's the lower left corner of the board, looking from the back. I removed the 22K resistor and replaced it with a 50K trim pot. Bourns part #3252W-1-503. It's a little pricey, $18 from Mouser. Mouser part #652-3252W-1-503. The leads used are the center (wiper), and the one directly under the screw (can't remember if it's 1 or 3...the other is not used, it's for CCW rotation). Install it, and see if the range is good enough. If it's not, you'll need to add a resistor in series with the trimpot.

Since you're ordering parts, you may want to order some 1W 1 ohms 1% resistors to put on the tube sockets in place of the ground wire so you can get an accurate bias reading. I didn't do this, as I bias using the OT shunt method. Also, you may want to check this capacitor to see if it's cooked like mine was. It's directly north of the bias section, above the blue cap. It's C28, .02uF, 400V.

You may want to read up on biasing before going about with this. Also, make sure you drain the caps prior to doing the mod. Only do this if you're confident you can do it. Here are some sites about biasing, and a handy bias calculator.

http://www.duncanamps.com/technical/lvbias.html

http://www.diyguitarist.com/GuitarAmps/PT-Biasing.htm

http://www.webervst.com/tubes1/calcbias.htm

 

[droptrd]

And the answer is: The absolute biggest, hairiest balls I have ever heard out of a recto. PERIOD!

Tried it last night. Way better! Big Hairy Balls! :twisted: :twisted: :twisted:

hey how bout some pix of that mod that would be good to do!

Ask and ye shall receive.

This is for a 2 channel. It's the lower left corner of the board, looking from the back. I removed the 22K resistor and replaced it with a 50K trim pot. Bourns part #3252W-1-503. It's a little pricey, $18 from Mouser. Mouser part #652-3252W-1-503. The leads used are the center (wiper), and the one directly under the screw (can't remember if it's 1 or 3...the other is not used, it's for CCW rotation). Install it, and see if the range is good enough. If it's not, you'll need to add a resistor in series with the trimpot.

Thanks for the awesome tip. Ill have to try this mod too.

Since you're ordering parts, you may want to order some 1W 1 ohms 1% resistors to put on the tube sockets in place of the ground wire so you can get an accurate bias reading. I didn't do this, as I bias using the OT shunt method. Also, you may want to check this capacitor to see if it's cooked like mine was. It's directly north of the bias section, above the blue cap. It's C28, .02uF, 400V.

You may want to read up on biasing before going about with this. Also, make sure you drain the caps prior to doing the mod. Only do this if you're confident you can do it. Here are some sites about biasing, and a handy bias calculator.

http://www.duncanamps.com/technical/lvbias.html

http://www.diyguitarist.com/GuitarAmps/PT-Biasing.htm

http://www.webervst.com/tubes1/calcbias.htm

Cant wait to try this one too!

 

[fleeced]

Hi - help needed ...

My DR has lost all volume seemingly between the pre-amps and the power amp. The tones sound like they're still there in the pre amps, - the power amp is OK (volume fine through the effects return circuit) - as you guys sound you know your tech stuff .... I thought I'd ask for any ideas before I send my amp away?

I posted the first "help" message an hour ago here

http://forum.grailtone.com/viewtopic.php?f=24&t=33455

 

[mikey383]

Hi - help needed ...

My DR has lost all volume seemingly between the pre-amps and the power amp. The tones sound like they're still there in the pre amps, - the power amp is OK (volume fine through the effects return circuit) - as you guys sound you know your tech stuff .... I thought I'd ask for any ideas before I send my amp away?

I posted the first "help" message an hour ago here

http://forum.grailtone.com/viewtopic.php?f=24&t=33455

Try swapping out your phase inverter tube. V5. If that still doesn't help, try V4. Keep going up the line until you find which one it is. Most likely a preamp tube went bad.

 

[fleeced]

Hi - help needed ...

My DR has lost all volume seemingly between the pre-amps and the power amp. The tones sound like they're still there in the pre amps, - the power amp is OK (volume fine through the effects return circuit) - as you guys sound you know your tech stuff .... I thought I'd ask for any ideas before I send my amp away?

I posted the first "help" message an hour ago here

http://forum.grailtone.com/viewtopic.php?f=24&t=33455

Try swapping out your phase inverter tube. V5. If that still doesn't help, try V4. Keep going up the line until you find which one it is. Most likely a preamp tube went bad.

Tried that - swapped them all (more than twice!..) ...

 

[colemaneuclid]

so since all mesa 6l6's are tested for mesa's fixed bias range, it's safe to run them in el34 mode? they'll just run a bit hotter? even in bold/diode settings? or only safe in spongy/tube?

if you look at the schematic all the 6l6/ el34 switch does is switch a resistor in and out of the bias circuit.
no harm in running your 6l6's on the "el34" side providing your within safe spec of the tube.
I've replaced one of the resistors in the bias circuit with a pot and now have an adjustable bias circuit. I've ran my 6l6's hotter than where you've got them and have seen no ill effects

 

[R_ADKINS80]

You can run them in either bold or spongy but stick to the tube rectifiers when using them in EL34 mode. Thats what I was told but I run spongy/tube 100% of the time anyways.

 

[fleeced]

Hi - help needed ...

My DR has lost all volume seemingly between the pre-amps and the power amp. The tones sound like they're still there in the pre amps, - the power amp is OK (volume fine through the effects return circuit) - as you guys sound you know your tech stuff .... I thought I'd ask for any ideas before I send my amp away?

I posted the first "help" message an hour ago here

http://forum.grailtone.com/viewtopic.php?f=24&t=33455

Try swapping out your phase inverter tube. V5. If that still doesn't help, try V4. Keep going up the line until you find which one it is. Most likely a preamp tube went bad.

Tried that - swapped them all (more than twice!..) ...

Just heard back from the tech ... "3 pre amp tubes bad" ... amp cured.

What on earth could cause 3 pre amp tubes to go bad all at once though? ... Can one tube going bad cause others to ?

Can putting a Tung Sol in V1 (not sure of the bias rating) cause three other tubes to go bad?

I'm pleased the amp is cured but unsure if we've found the symptom and not the cause .......