How And When To Tube Roll...

[saggy]

so i just got my first tube amp and I’m already curious about how to change tubes. I bought it used, and while the tubes still have life they’re about 2-3 years old. I have been playing it non-stop for a week and notice the bias drifting a touch. I don’t have a tube-tester but the unit has the bias switch which I’ve had to reset once. (Is that normal or should the tubes stay “biased” longer?)

I’m curious about how and when it’s necessary to change the tubes out and if you change just a few at a time or do them all at once. And to be clear, I’m the same form of idiot as the rest of you when it comes to sitting still with gear!!


Just for reference it uses the following tubes:

EL34 (6CA7) x 4
ECC83 (12AX7) x 2
ECC82 (12AU7) x 1

I have a feeling this is going to get expensive pretty quickly...

 

[triode17]

Sag - First off, the bias current will drift with changes in AC line voltage. Being summer, tons of A/Cs are turning on and off so the line is variable. What amp is it? How much does it drift?
The tubes should be tested to determine their condition. If you can't, preamp tubes last a LONG time, about 5000-10000 hours so don't worry about them unless they make a ping when you tap them turned on or make clicking or popping noises. The power tubes only last about 1000-2000 hours and do wear down over time.

 

[Hyfi]

When & How? For me, I wanted to alter the sound, which can be done by changing tubes. The AX and AU Pre tubes may change the sound more. For me, I wanted more bass and ended up with Golden Lion Gold Pins for my Pre. My amp is Hybrid and I ended up using NOS RCA Cleartops in 6SN7s.

Some people just buy lots of different tubes and play around with the sound. If you talk to someone like Brent Jesse and tell him your amp and the sound you are looking for, he can direct you to what flavors to try.

http://www.audiotubes.com/bjtubes.htm

 

[KlipschFan61]

I got into lots of tube swaps at first but now I've gotten over the initial buzz of owning tube amps and allow whatever my current selection of tubes to live a little. Hard to form any opinion if I don't leave them alone for a few months.

 

[saggy]

. What amp is it? How much does it drift?
.

It’s the Unison Research Triode 25. The bias doesn’t drift too much. Maybe from noon to 11:15 on the dial. Of course, that could be a lot and i wouldn’t know.

 

[saggy]

I got into lots of tube swaps at first but now I've gotten over the initial buzz of owning tube amps and allow whatever my current selection of tubes to live a little. Hard to form any opinion if I don't leave them alone for a few months.

Excellent advice. I’m thinking of just getting some newer tubes and letting it marinate. I think it gets pretty costly too quick for me if i keep changing.

 

[Hyfi]

Look at the site I linked, click on each tube style to see a list and description of the different characteristics of each one. Think about the current sound and what you might like to change, then pick your tube. Rolling just to say you rolled is useless unless you are attempting to tailor the sound.

 

[Palustris]

Look at the site I linked, click on each tube style to see a list and description of the different characteristics of each one.

That site is designed to sell tubes. Unless the "characteristic" sound includes the precise operating point for the specific tube it, is generally smoke and mirrors. For instance, does your 6SN7 sound too "lush and romantic?" Change the operating point to eliminate 2nd harmonic distortion. Try 9mA @ 250V with a grid voltage of -8V; that will clean up the sound nicely and make it much more clean and open.

 

[Paully]

I got into lots of tube swaps at first but now I've gotten over the initial buzz of owning tube amps and allow whatever my current selection of tubes to live a little. Hard to form any opinion if I don't leave them alone for a few months.

I hear that. When I first got into tubes, I was constantly buying and swapping tubes. It was fun. 15 years later, I know what I like so if it sounds good I leave it and let it play.

To the original poster: people often have a back-up set of tubes. If the tubes in the amp start to sound a little dull or lifeless, or if you find yourself just not listening as much, pop in the new tubes. If it makes a noticeable difference (after some burn-in) leave the new tubes. If it doesn't, put the old ones back in and carry on. As noted above, small signal tubes last a very long time while output tubes wear sooner. I don't know what you do and don't know, but the tubes you listed are quite ubiquitous and tons of threads on good options exist. So do some research and play around for fun. Once you settle, get some back-up tubes to roll in as needed. New production EL34 are quite inexpensive for a quad and very good. Old stock 12au7 aren't all that expensive and picking up unused and used/tested singles shouldn't set you back too much. Old stock 12ax7 can be quite pricey, but you only need two. Hit the archives, do a google search, check the classifieds and wherever else and have a good time!

 

[primosounds]

It sounds like your amp has fixed bias. And contradictory to its name, it means that there usually is a means to adjust the negative bias voltage, which you have indicated.
You need to realize that from the point you turn on your amp until it reaches a stable operating point the bias will slowly drift. So, if you are expecting the bias voltage to be at say, .70 vdc, after setting it just the evening before, and you check the bias when you first power on the amp, it will be at a different voltage, maybe .8vdc. If you adjust it to .7 maybe an hour or 2 later it maybe at .65vdc. So, you need to remember to check it after maybe an hour, and try to keep to that schedule. Most amp manuals suggest a schedule as to how often you need to check it.
Tube rolling is a fun and easy thing to do although it can seriously impact your wallet.
Like others mentioned power tubes wear out fastest and need to be changed like tires when they wear out.

 

[saggy]

It sounds like your amp has fixed bias. And contradictory to its name, it means that there usually is a means to adjust the negative bias voltage, which you have indicated.
You need to realize that from the point you turn on your amp until it reaches a stable operating point the bias will slowly drift. So, if you are expecting the bias voltage to be at say, .70 vdc, after setting it just the evening before, and you check the bias when you first power on the amp, it will be at a different voltage, maybe .8vdc. If you adjust it to .7 maybe an hour or 2 later it maybe at .65vdc. So, you need to remember to check it after maybe an hour, and try to keep to that schedule. Most amp manuals suggest a schedule as to how often you need to check it.
Tube rolling is a fun and easy thing to do although it can seriously impact your wallet.
Like others mentioned power tubes wear out fastest and need to be changed like tires when they wear out.

Thanks for explaining that. I have no problem checking bias to make sure tubes last and play properly. In general, if I check the bias daily, do i check it at turn-on or do i wait for the amp to warm-up a bit?

to @Paully I'm brand new at the tube stuff so everything is new to me. I've ordered some inexpensive Sovtek 12AXLPS tubes to start. I'll be doing some research on the EL34s and 12au7. Seems that, as you said, they're everywhere. I'll probably put something up on BT, too, just to see if anyone has some that they'd stand behind.

 

[primosounds]

Thanks for explaining that. I have no problem checking bias to make sure tubes last and play properly. In general, if I check the bias daily, do i check it at turn-on or do i wait for the amp to warm-up a bit?
.

Usually the amp maker has a recommendation on how often to do that. Generally it depends on the amount of time the amp is in use. Something like every 200 hours, or if you can notice a significant decrease in sound quality or if it seems like the plates have a slight reddish glow to them which may indicate that they are "under biased" and need more negative voltage.

 

[trobbins]

Without the measurement nous to confirm appropriate operating voltage and current levels within the amp, then you can easily run the risk of rolling a tube without appreciating if it is not appropriately operating in your amp. To my mind any such basic level measurements should also confirm that amplifier distortion has not gone awry somehow, as only then would you be 'listening' to your amp operating with nominal performance and hence be able to make comparisons that are not misleading.

It could be quite easy for the output stage to be idle current unbalanced, or push-pull valves to have quite different dynamic gain, and that sort of issue could grossly swamp distortion from any other aspect or tube rolling within your amp.

 

[triode17]

if it seems like the plates have a slight reddish glow to them which may indicate that they are "under biased" and need more negative voltage.

Primo-That's not correct. They are over-biased if the plates are starting to glow red. But DO need more negative bias voltage.

 

[primosounds]

Wow, i am sure that makes a big difference on the problem. I have read that over biased refers to excessive negative voltage causing the tube to go into cut off

 

[RogerMod]

Primo-That's not correct. They are over-biased if the plates are starting to glow red. But DO need more negative bias voltage.

Actually Primo is correct. Ive been reading his posts and he knows what he is talking about. Unfortunately the term BIAS has been turned upside down, by using the solid state definiton of bias. In a SS amp we measure idle current across a small resistor, around 0.2 ohms. Then we divide our voltage reading by whatever that resistor is. Its usually not something the consumer does. But remember in SS amps we do talk about idle current never bias. The actual bias on a BJT is always around positive 0.6-0.7 and is not easily controlled by voltage. In electronic terms BIAS always means the voltage on the control element (base, gate or grid) that controls the idle current which should not be called the bias. Typical values, all negative measured to ground are 50 V for KT88, 35 for EL34, 15 for EL84 and 80 for 300B. There is quite a range determined by the characteristics of the tube.

If you had a chat with a well informed tube guy, a ham radio guy or engineer and said the bias on my KT88 amplifier is 50 mV he would think you don't know what you are talking about.

Unfortunately the loose language of current audio has created confusion with accepted electronic terms. For example now we call unbalanced inputs single ended, a term which belongs to a type of ampifier typology. Can we just start using the right terms please. Lets start to clean up this situation.

 

[RogerMod]

Wow, i am sure that makes a big difference on the problem. I have read that over biased refers to excessive negative voltage causing the tube to go into cut off

You are correct, overbiased means the tube is running cold or turned off. Unfortunately 99% of the tube owners have been misinformed. What's worse it that they don't seem to care to get it right.

 

[E-Stat]

If you had a chat with a well informed tube guy, a ham radio guy or engineer and said the bias on my KT88 amplifier is 50 mV he would think you don't know what you are talking about.

And yet, the designated setting for my VTL MB-450s is 270-300 mV using 6550Cs - which also use KT-88s as well.

Go figure.

 

[primosounds]

And yet, the designated setting for my VTL MB-450s is 270-300 mV using 6550Cs - which also use KT-88s as well.

Go figure.

That is the voltage reading not the current reading. Not all amp makers use a 10 ohm voltage sensing resistor on the cathode. Dynaco uses 15.6 ohm resistors, and i have no idea what it is in your amp. So, best to follow the instruction manual. Also the VTL is PPP, parallel push pull, what is it 8 or 12 tubes per channel? So, the per tube setting of the cathode current maybe different from a simpler 2 tube per channel amp. Anyway, being an owner of that vtl 450, must bring a smile to the face of your tube dealer when you walk in the door.

 

[E-Stat]

That is the voltage reading not the current reading.

And the parameter on which bias is set!

Also the VTL is PPP, parallel push pull, what is it 8 or 12 tubes per channel? So, the per tube setting of the cathode current maybe different from a simpler 2 tube per channel amp.

While there are eight outputs, each is biased separately by its own trimmer pot.

Anyway, being an owner of that vtl 450, must bring a smile to the face of your tube dealer when you walk in the door.

Walk in the door?