re-tube question

[T_red7882]

So i recently retubed my twin for the first time, and i am happy as a clam... until moments ago when i hit my EQ pedal (which i kinda use a clean boost) and my twin broke up nicely when i hit a big low E chord... but the volume is on 4. Now, i do have the controls dimed and higher output pups on my tele, but i don't think this should be happening right? Before i popped in the the new power tubes (matched quatet GT6L6S), the old ones would only glow faintly at the top of the tube. these new ones glow brightly from the top of the tube to the bottom. I'm guessing i do need a bias adjustment? my tech buddy said the new tubes would probably be fine without a bias adjustment, but now i am starting to wonder. With the EQ pedal off, it doesn't break up... but i am kind to my neighbors and i won't go any louder to see where this sucker would breakup with no help.

[Guitarslinger1]

Originally Posted by T_red7882

So i recently retubed my twin for the first time, and i am happy as a clam... until moments ago when i hit my EQ pedal (which i kinda use a clean boost) and my twin broke up nicely when i hit a big low E chord... but the volume is on 4. Now, i do have the controls dimed and higher output pups on my tele, but i don't think this should be happening right? Before i popped in the the new power tubes (matched quatet GT6L6S), the old ones would only glow faintly at the top of the tube. these new ones glow brightly from the top of the tube to the bottom. I'm guessing i do need a bias adjustment? my tech buddy said the new tubes would probably be fine without a bias adjustment, but now i am starting to wonder. With the EQ pedal off, it doesn't break up... but i am kind to my neighbors and i won't go any louder to see where this sucker would breakup with no help.

The bias should be checked, and adjusted if necessary, every time you change power tubes in any amp, fixed bias or cathode biased. With the new manufacture tubes made these days, that can simply be all over the map, this is more important than ever.

[GDStone]

"Now, i do have the controls dimed"...WHY ?

I allways thought control nobs where there for a reason...old thinkn I guess..

[Scott S]

Are the plates themselves glowing? That would be bad.

[ASATmarty]

Isn't anyone going to challenge the "must bias, must bias, must bias" chanting?

It's simply not true.
It doesn't HURT and you can get some change in distortion characteristics but it's NOT necessary unless changing tube TYPES.

The proof of the pudding is in the eating and all my Fenders take different brands/vintages (even sub-groups like 6L6GB, 5881) and simply using the bias balance pot and/or hum adjust pot, sound great and run fine at gig volume.

Bias adjustment is fine (I put a variable resistor in my Pro Jr.) but let's stop the hysterical hype about it.

[T_red7882]

Originally Posted by Scott S

Are the plates themselves glowing? That would be bad.

what are the plates?

[Murphy Slaw]

I've played tube amps for 39 years.

I was gigging for money at age 16 with a Super Reverb. Getting paid and drinking beer. (those were the days)

I've never had an amp biased. Ever.

I had a beer fall over on a Peavey Deuce and blow up 2 6L6's that almost gave the drummer a heart attack, (it was in front of him on a chair, he actually witnessed the lightning), and slapped in 2 spares that were from the 50's I had in a bag, a fuse, and kept going. Played the amp with 2 old tubes, 2 original tubes, for a few years without a problem.

Bias. Shmias.

YMMV.

Murph.

[T_red7882]

Well, i'm loving how it sounds, even if it is breaking up a little earlier. I will get a multimeter for biasing or somethign soon, but for the time being, it sounds wonderful

[Dacious]

Originally Posted by ASATmarty

Isn't anyone going to challenge the "must bias, must bias, must bias" chanting?

It's simply not true.
It doesn't HURT and you can get some change in distortion characteristics but it's NOT necessary unless changing tube TYPES.

The proof of the pudding is in the eating and all my Fenders take different brands/vintages (even sub-groups like 6L6GB, 5881) and simply using the bias balance pot and/or hum adjust pot, sound great and run fine at gig volume.

Bias adjustment is fine (I put a variable resistor in my Pro Jr.) but let's stop the hysterical hype about it.

Really? Someone put a pair of new JJs in a Princeton Reverb I bought recently. The first time I tried to play it on more than '5' it blew a fuse. When I plugged in my bias checker it was trying to pull 47ma per tube.

Fortunately because it's an underspecced power supply it sagged to 407 vdc on tbe B+ - which meant the amp was still trying to produce more than 20 watts. The bias resistor, bias circuit and everything checked out. Because they're JJs and seem unusually resilient they appeared to have suffered no harm.

But the power transformer was running hot enough to burn. When I changed the 27K bias resistor for a 40K and fitted a 50K trim pot I eventually got the current down to 16ma per tube, at which point the tranny was putting 445v on the plates and running cool enough to comfortably touch. At that it is running comfortably.

No doubt these tubes are an anomaly in terms of current draw, but if I'd left it as was, sooner or later it'd either burn out the power tranny, the tubes or both.

In a cathode biased amp it is somewhat different, as the tube pulling more current tends to cause the cathode resistor to drop B+ so the amp is somewhat self-regulating. But a faulty bias circuit connection will still toast things.

I put new tubes in Fenders before without biasing too - but to make a blanket statement that it will always be Ok is misleading at best and could leave people well pissed off with you at worst, if their amp toasts something.

On some amps I'd always check - a 100w Laney I worked on that blew it's tubes and an output tranny had a microphonic preamp tube that if you turned it up produced 60ma draw @ 440 volts on an EL34 and created severe ultrasonic oscillations. it took out a new replacement tube by frying it's grid. I only picked it up because turning the controls up on that channel caused plate current to skyrocket.

[dbmck]

Hi folks (newbie here),

I recently acquired a 1977 135 watt twin for my pedal steel (these are prefered by many steel players for all the clean headroom). The amp sounds fine, but it has clearly not been babied (from the looks of the cabinet), and many of the mismatched tubes could even be originals. I took it straight to a respected local tube tech for cap job and tubes. He called me back and said it didn't need it: filter and bias caps are fine and tubes all check out. Makes me nervous, but he came highly recommended, so why insist on work he says it doesn't need?

Can't help myself completely, so I'm planning to put all new JJ tubes in it. Just seems like it couldnlt hurt to have some new blood in this old girl. I mentioned this to him and asked whether it needs to be be biased for the new tubes. He says no, just use the hum balance pot on the back.

The comments I'm reading on this thread seem to affirm his advice. Anyone want to grab my mouse trigger finger and yell "Wait!! Stop!!" before I order these tubes and plop them in?

Thanks!

Dan

[vjf1968]

Originally Posted by dbmck

Hi folks (newbie here),

I recently acquired a 1977 135 watt twin for my pedal steel (these are prefered by many steel players for all the clean headroom). The amp sounds fine, but it has clearly not been babied (from the looks of the cabinet), and many of the mismatched tubes could even be originals. I took it straight to a respected local tube tech for cap job and tubes. He called me back and said it didn't need it: filter and bias caps are fine and tubes all check out. Makes me nervous, but he came highly recommended, so why insist on work he says it doesn't need?

Can't help myself completely, so I'm planning to put all new JJ tubes in it. Just seems like it couldnlt hurt to have some new blood in this old girl. I mentioned this to him and asked whether it needs to be be biased for the new tubes. He says no, just use the hum balance pot on the back.

The comments I'm reading on this thread seem to affirm his advice. Anyone want to grab my mouse trigger finger and yell "Wait!! Stop!!" before I order these tubes and plop them in?

Thanks!

Dan

I wouldn't re-tube an amp without checking the bias. You can order a bias probe from Weber and check the current state of the power tubes. If they are within spec then you should be fine but on the other hand its a crapshoot. You would better serve your own peice of mind by re-tubing the amp, checking the bias, and make adjustments if necessary. If you are going to order tubes I recommend www.eurotubes.com. They offer great service and they get your order out to you fast. They have re-tube kits for almost every Fender amp as well as videos on how to bias your own amp.