# Re-tubing an amp that's biased "cold"....



## StevieMac (Mar 4, 2006)

I want to replace power tubes in my '63 RI Vibroverb and these amps are notorious for being biased "cold" from the factory. Indeed, my bias meter indicates that the set of 21 rated JJs in it are idling at a mere 20mA with 430 volts on the plates. IMO, the amp doesn't sound nearly as strong as it should and would be optimized around 35-40mA. Of course there's no bias adjust on these amps so...

Does anyone know if thetubestore.com can find me a set for this amp based on the info I have (i.e. idling current and plate voltages)?


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## epis (Feb 25, 2012)

Hi StevieMac, your amp is biased "cold" on purpose, because of nature of tremolo (vibrato) circuit. It's varying bias current, you may check out the bias when vibrato is engaged, you will see how much bias current is increasing. Biasing it hotter would probably damage output tubes.


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## nonreverb (Sep 19, 2006)

The easy route is to find tubes that bias hotter although modding the amp with a bias adjust won't hurt it either. I've done the mod for a couple of owners of Vibroverbs who requested it with no ill effects. The problem is it's a bit of PITA to mount the pot etc.


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## StevieMac (Mar 4, 2006)

epis said:


> Hi StevieMac, your amp is biased "cold" on purpose, because of nature of tremolo (vibrato) circuit. It's varying bias current, you may check out the bias when vibrato is engaged, you will see how much bias current is increasing. Biasing it hotter would probably damage output tubes.


Hi and thanks for the response. I'm confused however...which is not uncommon *;^) *According to bias charts I've consulted, 20mA at that voltage is only about 25-30% of max late dissapation for 6L6GCs. Considering 50% is considered "cold", why on earth would Fender bias THAT low? I'm not questioning your reasoning but the numbers aren't making sense to me.


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## keto (May 23, 2006)

Stevie, not sure if you caught what he said - run the vibrato, and check the bias with it running.


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## StevieMac (Mar 4, 2006)

keto said:


> Stevie, not sure if you caught what he said - run the vibrato, and check the bias with it running.


Got ya. I'd be interested to see how much it varies with the effect on.

UPDATE: Just tried it and...wow...it really does vary considerably. The readings I got were as low as 14 and as high as 49! I still think there's room to move upwards however. A previous set in there idled at 35mA, without incident, for instance.


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## dtsaudio (Apr 15, 2009)

> why on earth would Fender bias THAT low


The original output tubes were probably a lot hotter than the JJ's you have now. 
Is the set a Tubestore rating of 21 that you have. If so contact the Tubestore tell them what you have and the readings you're getting. They can make a pretty good estimate of what will get you in a better range.

Best solution though is nonreverb's. But that costs money, and he's right, it is a PITA.


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## StevieMac (Mar 4, 2006)

dtsaudio said:


> The original output tubes were probably a lot hotter than the JJ's you have now.
> Is the set a Tubestore rating of 21 that you have. If so contact the Tubestore tell them what you have and the readings you're getting. They can make a pretty good estimate of what will get you in a better range.
> 
> Best solution though is nonreverb's. But that costs money, and he's right, it is a PITA.



Thanks. That definitely answers the original question.

While we're on the topic of this amp however, there is another issue that I simply can not resolve:

There's a persistant and unpleasant buzz, primarily on the low notes, followed by a decaying "fizz" and what seems like some slight microphonics. If it was just a buzz, I'd suspect the baffle but the speaker bolts and baffle screws were checked and are properly torqued. Tried a different cab and the same noise is there. The buzz also sounds more "electronic" in nature anyway, as opposed to being a physical vibration issue. Anyway, I've swapped out every preamp tube one-at-a-time to eliminate a bad/microphonic tube as well. When I checked the power tubes with my bias meter, one was WAY low (i.e. 13 mA vs 35mA on the other) and thus I assume it must have somehow become damaged. Naturally I thought the extreme bias mismatch might account for the sound I've been chasing but a different set of matched tubes (the JJs mentioned earlier) also did not eliminate the problem. Finally, I wondered if "crossover distortion" (?) was present from the extreme mismatch or from too cold a bias on the other set of tubes. A visual and "chopstick" inspection inside yielded nothing obvious either. 

Anyway, I suppose that all of this is a long way of saying that I've run out of things to look for. Any other suggestions? My bias meter allows me to check output AND preamp tube plate voltages and current draw. Should I be checking anything in the preamp stage for instance?

Steve


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## jb welder (Sep 14, 2010)

Firstly, don't get too hung up on the 70% figure. It's largely (questionable) internet wisdom. At higher plate voltages some say 60% (or even 55%) MAX. Many classic Fender amps were biased close to 50% and not considered biased "cold". Indeed, if those amps are used as a reference we could call 50% "luke warm". In later years, the red-knob "The Twin" was one of the first common amps with test points on the back for a multi-meter, along with rear mounted bias and balance adjust pots. The spec. was .04 volts, which was 40mA per PAIR of output tubes, or 20mA per tube. Even at 500 plate volts that is not much hotter bias than what you have. I don't remember hearing any complaints about them being biased too cold. However, a few years later they adjusted their spec: 20mA for best reliability, 40mA for best tone, 60mA for compromise.

With all that being said, the symptoms you are describing ('persistant and unpleasant buzz, primarily on the low notes, followed by a decaying "fizz" ') does sound like crossover distortion.
So that brings us full circle around to my personal preference for bias - as cold as possible (for longevity) without audible crossover distortion. Then I check the numbers just to make sure I have stayed within reasonable limits. Once you have a feel for your particular model you can simply reset for the number you know works for you.
Sorry for all the talk about adjustable bias, I realize you have no adjustment. However you mentioned a previous pair at 35mA worked for you. That would just happen to be about 50% (@430 plate volts), which I don't think you found to be "cold". Another pair like that should hopefully solve your buzz & fizz.


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## StevieMac (Mar 4, 2006)

Thanks for all that. Honestly, at this point, I'm HOPING it's crossover distortion because I've gone down every other path I can think of with no results. Based on everything I've read so far, crossover distortion appears to be the likely culprit so, yes, it appears we've come full circle. I'll contact thetubestore and see what they can offer.

Thanks everyone and I'll update this after I sort out the bias issue wit ha fresh set of tubes...


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## StevieMac (Mar 4, 2006)

Found a set of 5881s I had that measured 33mA each at idle in this amp. Buzz is STILL there however. Spoke to my tech over the phone and he thinks the extreme mismatch with an earlier set of tubes (mentioned previously) could have damaged the OT, even if only used briefly. Although I understand virtually nothing about how an OT works, his thoughts made some intuitive sense to me. The fizzy electric sound is present at ALL volume levels when I hit bass notes so maybe the OT is unable to reproduce low frequencies properly because of some internal damage? Ahhhh...what the heck do I know?


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## nonreverb (Sep 19, 2006)

StevieMac said:


> Found a set of 5881s I had that measured 33mA each at idle in this amp. Buzz is STILL there however. Spoke to my tech over the phone and he thinks the extreme mismatch with an earlier set of tubes (mentioned previously) could have damaged the OT, even if only used briefly. Although I understand virtually nothing about how an OT works, his thoughts made some intuitive sense to me. The fizzy electric sound is present at ALL volume levels when I hit bass notes so maybe the OT is unable to reproduce low frequencies properly because of some internal damage? Ahhhh...what the heck do I know?


Bias mismatch would be the last thing to consider if the OT is cooked. Usually that happens when you have a catastrophic failure with one of the output tubes that burned for a extended period or you cranked the amp with high impedance/no load or maybe it's defective. Normal range bias mismatch as you mentioned would not hurt the tranny. It still may be the tranny however, it might be something else. Don't condemn the OT right away.


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## jb welder (Sep 14, 2010)

Completely agree with nonreverb, way to early to even suspect the OT. The OT is something to look at when you have exhausted every other possibility. 
Sorry to sound rude but for a tech to jump straight to suspecting the OT makes me question their ability (unless you were insisting you thought it was the problem and they were just agreeing to be polite  ). Of thousands of broken down tube amps I may have replaced 20 OT's (or less). 
As far as mismatched tubes damaging the OT, I've seen amps that have been used with one of a pair of output tubes fallen out laying in the bottom of the amp, or one power tube not even lit up because it was in the socket wrong because the guide pin was broken. The amps were used with such a serious mismatch till the customer could get around to bringing them in for repair, complaining of excess distortion lol. Never were any OT's damaged.
Not saying it is impossible, just highly improbable.


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## keto (May 23, 2006)

http://www.lynx.bc.ca/~jc/tremoloCircuits.html

Came across this night looking at Traynor stuff this guy has, thought might be of interest.


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## StevieMac (Mar 4, 2006)

> suspecting the OT makes me question their ability (unless you were insisting you thought it was the problem and they were just agreeing to be polite  ).


This is entirely possible - the being polite part - as I threw it out there initially. What he actually said was it could be worth checking if nothing obvious turns up. 

Which reminds me...there was one spot on the circuit board, where R52 & R53 reside I think, that looked "burnt". The resistors themselves looked just fine however (i.e. quite clean & not burnt) so I don't know what happened there. Maybe they were poorly replaced at some point and the board got burnt in the process? Anyway, I believe R52 & R53 are "ground reference resistors" if that makes any difference.


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## Nero (Jan 31, 2012)

I wonder if this fizzy sound as the notes decay is due to poor grounding within the amp, causing some coupling between stages where there shouldn't be any. What you hear may not be the notes distorting as they decay, but the distorted sound becoming audible because the clean sound level is low enough that you can hear the distorted sound. If the amp volume control is at zero, and the guitar is plugged in, with it's volume at full, is there any guitar signal present at the speaker? Just a guess on my part.


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## StevieMac (Mar 4, 2006)

Nero said:


> I wonder if this fizzy sound as the notes decay is due to poor grounding within the amp, causing some coupling between stages where there shouldn't be any. What you hear may not be the notes distorting as they decay, but the distorted sound becoming audible because the clean sound level is low enough that you can hear the distorted sound. If the amp volume control is at zero, and the guitar is plugged in, with it's volume at full, is there any guitar signal present at the speaker? Just a guess on my part.


If there is a signal at 0 volume, I simply can't hear it, even with my ear pressed right up to the speaker.

You mention grounding issues though and, it seems to me at least, I'm getting more 60cycle hum with my Strat than I'm used to hearing through other amps. There's only one position for instance that I can hold my guitar in that doesn't produce quite a bit of that familiar hum....


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## nonreverb (Sep 19, 2006)

That's because when you turn any Strat to that particular position in that room, the pickups line up with the radiating 60 cycle interference in the room causing the hum to decrease. Single coil pickups act like a rudimenary antenna.
The winding ratio is going to have an effect not only on the signal output but also the hum interference.
Amp ground and filter hum is a different matter and is usually constant regardless of the guitar.
Hope this helps


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## loudtubeamps (Feb 2, 2012)

As you're probably aware re-issue Fenders are notorious for minor and major problems because of the poor build quality.
I see more of these than anything else for what is usually a bad solder joint(s).
As most here will suggest,check the easy stuff first, cables, speaker, speaker lead and tubes etc.
Then, if you are comfortable doing so:
It's usually a case of "open er' up" , get out yer' chopstick and look for loose components or any resistors that look cooked.
good luck. Cheers, Doug
Any chance of posting a sound clip of the amp?


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## ampaholic (Sep 19, 2006)

For what it's worth I've spent hours trying to find solutions to weird noises in my vintage amps, stupidly convincing myself that it had to be caused by something hard to find or complicated to diagnose, only to discover that it was dirty tube sockets, a dirty pot or some other simple solution.
I've also created weird noises in Blues Juniors and Deluxes by moving wires and those ribbon cables out of place and not putting them back exactly where they were, again convincing myself that it had to be a bad part somewhere. Matter of fact in one Blues Junior the bias was all over the place and refused to stabilize, again caused by a wire that I had inadvertently moved too close to one of the power tubes.
Perhaps doesn't give you much faith in my technical abilities but I've learned the hard way to make certain I rule out the simple stuff first!


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## Wild Bill (May 3, 2006)

The level of idle current set by the bias voltage is not at all a sensitive, exact figure. Idle current can vary over a wide range before the human ear will notice any difference in tone. The 70% figure has been around for years as the best compromise between best power out and warm tone without having the tubes run so hot that they hurt themselves, reducing tube life.

It is true that the type of tremolo that varies the bias voltage will swing it far enough on half of the tremolo signal as to let the idle current run much higher. However, on the other half of the wave it reduces the idle current, making the output tubes run cooler!

It's all about the heat! Output tube circuits are designed to deliver big swings of POWER! While doing that, the elements within the tube will heat up. If the design parameters are not set correctly, the heat within the tube can damage it. In extreme cases, it can damage it very quickly.

The bias voltage is like the idle speed adjustment in an old fashioned auto carburetor. It sets the no-signal idle current. When we apply a signal the grid, which is the element that controls the current flowing in the output circuit, will see that bias voltage swing high and low, following the AC signal wave which will add to and subtract from the fixed bias voltage. This makes the current on the output side of the tube swing much higher and lower, following that tiny input signal. This is what is called amplification.

With a push pull amplifier using two output tubes, the circuit takes advantage of this situation by having two tubes working together. Each one works only for a little more than half of the input wave cycle. That means that during the time it is NOT working it gets to cool off a bit! This opportunity to "rest" during part of the cycle means that the tube can deliver a bit more than just half the usual amount of power it could deliver. That's why a push-pull amp delivers much more than just twice the power of just one tube. A Class A Champ delivers about 5 watts from one 6V6. A Deluxe uses two 6V6's in push pull and delivers more like 15 or more watts, depending on how generous is the guy measuring the output power!lofu

Factories have traditionally set the bias on their output tubes low for a simple reason - they will last longer and are not likely to fail during warranty! Remember, most engineers are NOT players! They think in terms of an amp being very clean and reliable, There is no guarantee they prefer a great rock sound. For all we know, some of them are polka fans!

If you look at a tube hifi or PA amplifier from a techie perspective you will quickly see that the voltages and biasing are set such that the amp delivers much less power than the same pair of tubes in a guitar amp. That's because the amp is expected to be very clean and show the best tube life possible, not to let you sound like Brian May!

Lowering the bias percentage in amps with higher voltages doesn't make sense to me. The biasing figure comes from a formula that includes the plate voltage. If the plate voltage is higher the math will deliver a lower value anyway! So the tube will still be operating at 70% of the Plate Dissipation figure, or whatever percentage you use. The worry is that if you set things for greater than 70% the tubes will have insufficient time during the cycle to cool off and will get hot enough to start hurting themselves.

As far as crossover distortion, I doubt if that's your problem. More likely, something has been damaged from the extreme mismatch. Or, a failed part like a resistor or capacitor affected one tube, eventually damaging it to the point where you measured such an extreme mismatch between it and the other tube. Normally, tubes don't change that much unless something was wrong. You could have a leaky coupling capacitor feeding the bad tube, allowing the plate voltage from the driver tube to swamp out the bias voltage and make the tube conduct more like a short! There are a LOT of things that could have happened! I suspect the problem is deeper than just tube swapping will fix. People get fixated on changing tubes because unless they understand all the parts under the chassis, swapping tubes is all they can do!

Please don't over-react but there is a slim possibility that the output transformer has been damaged. The output tubes are expected to be reasonably well matched so that the current swings between the two sides of the transformer are balanced out. You can have a large difference but in very extreme cases those currents will NOT be balanced and the transformer will build up heat inside itself and damage a winding. This means one side of the output stage will not properly reproduce it's half of the signal, causing distortion like you describe.

A damaged output trannie would never be my first suspect and I am NOT telling you to spend a bundle and change it without FIRST having a real tech check it out! I'm just mentioning it as a possibility. I once had a new amp in my shop with a bad hum that at first I just couldn't track down. Everything was new and looked great! Finally, I checked the bias on each output tube and found a huge mismatch! Normally, I find a mismatch of a few ma. of idle current actually sounds better for rock guitar, allowing a bit more of a harmonic "swirl" but this was more like 15 ma of difference! There was insufficient cancellation of power supply ripple. A new set of output tubes cured the problem. 

I asked the owner where he got his last set of output tubes and he told me "From my local music store, man!". This is often a poor source. Most stores do not have much techie expertise. Why would a good tech work there for minimum wage? Also, they usually buy their tubes from the same source as their strings and guitar straps. These sources have no idea about the need for matching and don't really care anyway! So any pair of output tubes you buy there could be hugely mismatched. In this case, the amp owner had the bad luck to get an extreme mismatch. 90 % of the time the tubes will be close enough that any problems will not be obvious. Likely the only real problem will be shorter tube life.

Don't mean to write you a book! Just trying to give you a clearer picture of how things work and what could be going on. I don't think your problem is simple enough to be solved without crawling inside the amp and troubleshooting with instruments. 

Wild Bill/Busen Amps


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