Bias problem

dcribbs1412 · Post by **dcribbs1412** » Mon Oct 29, 2012 2:43 pm

Was tweaking my 183 style over the weekend ...put the FET on a relay
and for some reason 2 of the sockets now bias really high..
before I had(KT77) biased all around 35-37ma, now for some reason both 1 and 2 sockets are at 55-57ma... swapped tubes and its still happens. Still searching for ideas. Appreciate any suggestions.

Darin

ic-racer · Post by **ic-racer** » Mon Oct 29, 2012 4:02 pm

Check the voltages at all the pins on the 4 sockets. Maybe the wire to the screen resistor on those two tubes got damaged.

dcribbs1412 · Post by **dcribbs1412** » Mon Oct 29, 2012 4:09 pm

ic-racer wrote:Check the voltages at all the pins on the 4 sockets. Maybe the wire to the screen resistor on those two tubes got damaged.

Thanks ic-racer
I will check again and report back.

Darin

ToneMerc · Post by **ToneMerc** » Mon Oct 29, 2012 4:18 pm

For those two sockets; if you have the two 1 ohm bias resistors to ground junction, verify that you didn't interupt the integrity of the ground connection for the cathodes.

TM

dcribbs1412 · Post by **dcribbs1412** » Mon Oct 29, 2012 5:18 pm

ToneMerc wrote:For those two sockets; if you have the two 1 ohm bias resistors to ground junction, verify that you didn't interupt the integrity of the ground connection for the cathodes.

TM

Will do TM
thanks

Darin

JazzGuitarGimp · Post by **JazzGuitarGimp** » Mon Oct 29, 2012 5:29 pm

I'm sure you've already done this, but retrace the mods your performed this weekend. Makes sense that if the amp was working fine before the mods, then something went south during the mods. Keep in mind that the bias circuit is a relatively high-impoedance circuit, and it wouldn't take much to influence the bias voltage. Would I be correct in guessing you have two bias pots in the amp?

dcribbs1412 · Post by **dcribbs1412** » Mon Oct 29, 2012 8:59 pm

ToneMerc wrote:For those two sockets; if you have the two 1 ohm bias resistors to ground junction, verify that you didn't interrupt the integrity of the ground connection for the cathodes.

TM

Checked/tightened grounds and still high bias
....will replace the 1 ohm resistors asap
Voltages seemed ok also...

JazzGuitarGimp wrote:I'm sure you've already done this, but retrace the mods your performed this weekend. Makes sense that if the amp was working fine before the mods, then something went south during the mods. Keep in mind that the bias circuit is a relatively high-impedance circuit, and it wouldn't take much to influence the bias voltage. Would I be correct in guessing you have two bias pots in the amp?

Thanks Jazz
just 1 bias pot

Darin

ToneMerc · Post by **ToneMerc** » Mon Oct 29, 2012 9:35 pm

dcribbs1412 wrote:
ToneMerc wrote:For those two sockets; if you have the two 1 ohm bias resistors to ground junction, verify that you didn't interrupt the integrity of the ground connection for the cathodes.

TM
Checked/tightened grounds and still high bias
....will replace the 1 ohm resistors asap
Voltages seemed ok also...

JazzGuitarGimp wrote:I'm sure you've already done this, but retrace the mods your performed this weekend. Makes sense that if the amp was working fine before the mods, then something went south during the mods. Keep in mind that the bias circuit is a relatively high-impedance circuit, and it wouldn't take much to influence the bias voltage. Would I be correct in guessing you have two bias pots in the amp?
Thanks Jazz
just 1 bias pot

Darin

Darin, I don't think you need to replace those 1R resistors just yet.
Mike

talbany · Post by **talbany** » Mon Oct 29, 2012 9:47 pm

Darin
First try another PI tube and check the PI voltages..If it still does it pull the output tubes and make sure your getting the same negative voltage from both sides of the PI couplers and pin 5 on each tube..
Tony

dcribbs1412 · Post by **dcribbs1412** » Mon Oct 29, 2012 11:52 pm

talbany wrote:Darin
First try another PI tube and check the PI voltages..If it still does it pull the output tubes and make sure your getting the same negative voltage from both sides of the PI couplers and pin 5 on each tube..
Tony

Thanks Tony
will do asap and report back

Darin

dcribbs1412 · Post by **dcribbs1412** » Tue Oct 30, 2012 4:01 am

Update
took out tubes to blow out any particles and reinstalled
and bias problem IS following the 2 tubes. Sorry my mistake...
I have 2 at 53ma and 2 at 15ma...bummer(lowest bias setting)
replaced PI tube...no go
replaced PI resistors 270k and 100k and 110k also PI trimmer...no go

No way could this be 2 month old JJ KT77 tubes going bad?
Put together a set of JJ 6CA7 and they biased right up (within 3ma)
Its cool the amp is up and running, bummer 2 tubes are bad after maybe 40 hours or so.
Not sure what caused this. Seems strange 2 tubes went out. Not sure
if that is common or not?

Really appreciate everyones time.

Darin

ic-racer · Post by **ic-racer** » Tue Oct 30, 2012 12:59 pm

dcribbs1412 wrote:Update
took out tubes to blow out any particles and reinstalled
and bias problem IS following the 2 tubes. Sorry my mistake...
I have 2 at 53ma and 2 at 15ma...bummer(lowest bias setting)
replaced PI tube...no go
replaced PI resistors 270k and 100k and 110k also PI trimmer...no go

No way could this be 2 month old JJ KT77 tubes going bad?
Put together a set of JJ 6CA7 and they biased right up (within 3ma)
Its cool the amp is up and running, bummer 2 tubes are bad after maybe 40 hours or so.
Not sure what caused this. Seems strange 2 tubes went out. Not sure
if that is common or not?

Really appreciate everyones time.

Darin

One thing I like about the bias trim on my Fender Super Twin, it would even those out.

I don't have a lot of experience with 'new manufacture' tubes, but maybe this is why some places are selling tubes that are matched after being 'burned in' ; to avoid this type of problem.

Structo · Post by **Structo** » Tue Oct 30, 2012 3:16 pm

Yeah, it sucks when new tubes go bad.

Among online tube dealers I found Doug's Tubes to be among the few that state they
burn in their tubes before rating them.
This makes a much more stable tube in an amp as it has already settled down.
I don't find any bias drift with his tubes.
Currently I am using the TAD 6L6GC STR's. (Tall Bottle)

They use a Maxi-Matcher for power tubes and a Vacuum Tube Valley dual triode characterizer for preamps tubes.

No affiliation but I have bought like three sets of power tubes over the last couple years with very good results.

http://www.dougstubes.com/

67plexi · Post by **67plexi** » Tue Oct 30, 2012 5:06 pm

Darin, Do you have the 1/2 power switch ?
You may not be out of the woods with your bias issue.
#183 has pulled the same problem with me on 3 builds all fixed now I hope.
The last #183 power amp section used a 1966 black face twin 85 watt PT with a NOS quad of 6l5GC.
worked great 200+ hours then changed the PT to a heybour spec. for EL34 worked well for around 40
hours then the bias went out of control 45-129 mA red plate city. The FET was intermittent
Replaced the FET works well now but after a few hours the bias would act up again.
Changed the bias board 3 times no change then went to a classic style bias board 3k3 2 watt resistor
on the 10k pot had to use a 47k tail resistor and switched to a quad of RFT el34 biased at 34 mA.
Bench tested the amp 4 hours a day for 8 days so far so good.
One thing all my voltages were and are spot on with the heybour PT. B+1 465 volts as per #183 spec.
If this amp goes wacko again I'm going with a quad of KT88's.
let's hope you just got a bad batch of JJ el34's.
If I recall the first #183 build I had to change the 3k9 bias resistor to 4k7 and 27k tail resistor.

Steve.

dcribbs1412 · Post by **dcribbs1412** » Tue Oct 30, 2012 6:32 pm

Thanks ic-racer and Tom
need to check out Doug's Tubes for future tube buys

67plexi wrote:Darin, Do you have the 1/2 power switch ?
You may not be out of the woods with your bias issue.
#183 has pulled the same problem with me on 3 builds all fixed now I hope.
The last #183 power amp section used a 1966 black face twin 85 watt PT with a NOS quad of 6l5GC.
worked great 200+ hours then changed the PT to a heybour spec. for EL34 worked well for around 40
hours then the bias went out of control 45-129 mA red plate city. The FET was intermittent
Replaced the FET works well now but after a few hours the bias would act up again.
Changed the bias board 3 times no change then went to a classic style bias board 3k3 2 watt resistor
on the 10k pot had to use a 47k tail resistor and switched to a quad of RFT el34 biased at 34 mA.
Bench tested the amp 4 hours a day for 8 days so far so good.
One thing all my voltages were and are spot on with the heybour PT. B+1 465 volts as per #183 spec.
If this amp goes wacko again I'm going with a quad of KT88's.
let's hope you just got a bad batch of JJ el34's.
If I recall the first #183 build I had to change the 3k9 bias resistor to 4k7 and 27k tail resistor.

Steve.

Thanks Steve
great info...I'm gonna keep on eye on the JJ 6CA7's biased at 32ma
in there for now...my PT is about 10V less at 455V.The 6CA7's seem to create a lot more heat than the KT 77's. Would love to try the RFT's someday when I have the cash.
I will keep the classic bias board idea close in case something else happens.
Appreciate your time

Darin

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