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Post subject: Re: Tube related question(s)
Posted: Wed Jun 18, 2014 6:51 am
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Aspiring Musician
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Joined: Sun Jan 27, 2013 3:44 pm
Posts: 366
BMW2002Ti wrote:
KC9SYJ wrote:
Thanks for helping us out there,Arjay. In response to the biasing question,...only the power tubes are biased...Arjay will probably correct me on this...but the preamp tubes are self biasing(if there is such a thing),therefore you need not worry about biasing those,...only the power tubes!

Again,if you are not sure how to do that,...by all means,have a good reputable amp tech take care of it,especially if you are new to tube amps! There's a LOT of dangerous voltages lurking around in that amp,...even if it has been turned off,unplugged,and has sat a while. Filter capacitors act like storage batteries and contain deadly voltages that can injure you,...or worse,kill you! Turn off and unplug that amp BEFORE pulling tubes,...and let those tubes cool down before touching them.

73
Dewey
KC9SYJ




Dewey, tubes in 99% of guitar amps have an idle bias. Output tubes, like 6L6GC or 6V6GT. And smaller 9-pin pre-amp, gain stage, EFX, phase inverter stage tubes (12AX7, 12AY7 or 12AT7 --- AKA signal tubes).

The two main topologies used are FIXED and CATHODE biased. Fender's Class AB1 amps have FIXED biased output tubes. And CATHODE biased signal tubes.

FIXED bias puts a set, or fixed, negative voltage onto the signal grid. This voltage is often adjustable via a bias pot.

CATHODE bias puts an idle bias onto the signal grid via a cathode-to-ground resistor. Many times a bypass capacitor parallels this C2G resistor. In this style of setting, an idle bias is not easily adjustable. But, this is not a worry. Since tube current draw and the C2G resistor can work in a range of plate voltages and attached component values. Kind of a self-leveling effect to an extent.

This is why most CATHODE biased tubes do not need bias adjustment. And replacing the OEM tube with a wide range of similar pinout and spec'ed tubes is fine. Like putting a 12AY7 into a position which normally uses a 12AX7.

The only caveat is if the socket's parameters are far from the nominal operating mode of the tube. Such as the reverb driver tube, in many Fender amps. A healthy amp (reverb tranny and tank) can demand a lot of current swing to work. Between 10-20mA. The 12AX7 nominal operating conditions should not exceed 3mA. Substituting a 12AX7 into the reverb driver position = short & most possibly noisy 12AX7 life.

HTH! :D


I have been reading about all that in the Tom Wheeler book "The Soul Of Tone; Celebrating 60 years of Fender Amps" Chapter 6 on "Tube Performance" discusses this very subject on biasing and other tube related subjects. I'm learning as I go! It's interesting reading about this stuff in order to get a better understanding of how tube amps function. This book is a 'must read' for all Fender amp fans!

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Post subject: Re: Tube related question(s)
Posted: Wed Jun 18, 2014 12:45 pm
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Aspiring Musician
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BMW - yes, we're very proud of him :) I see new articles about him almost every day :)

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Post subject: Re: Tube related question(s)
Posted: Thu Jun 19, 2014 6:22 pm
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Aspiring Musician
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If I had that amp and mine is a 60 W like it, I would put RCA 6L6GC in Power section and the 12AT7s would be NOS or ANOS as well as the 12AX7s! I have a Sylvania in V1 a real Mullard in V2 various RCAs and the two 12At7s are different brand ANOS. Next keep JJs for emergency or target practice!

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Post subject: Re: Tube related question(s)
Posted: Thu Jun 19, 2014 6:47 pm
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Neimenljivi,

Guess you'll see the Cup, again, this Summer. Slovenia seems to be a beautiful country. At least, what I saw of it after 2012 Cup.


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Post subject: Re: Tube related question(s)
Posted: Fri Jun 20, 2014 8:16 pm
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Aspiring Musician
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Lots of good info already given.

Hiss is very common when using high gain in most amps.
It's simply the pre tubes cascading into each other amplifying the signal, so that when nothing is coming into the input, like no volume from the guitar or you're not playing, hiss is very common and not indicative of a problem per se.

If the hiss is really too much, then you may want to look into a noise gate of some kind.
A noise gate simply "shuts" the "gate" of the input signal so that the guitars output is not adding to the hiss.

Biasing is not typically associated with the hiss from higher gain.
It's more the pre tubes giving that hot signal to the power section.
Biasing affects the overall amps tone. Some like a colder bias, some want a hotter bias, some leave it as per spec.

You may have a pre tube that's noisier than others might be.
You could try "tube rolling", which is simply swapping the same pre tube type into the different V/tube positions and noting any changes.
V1 is the tube that sets the base signal and is a very important tube. I put my best pre tube into that position. The other very important pre tube is the phase inverter/PI.
That's the tube the provides the final signal to the power section.
It's an often overlooked tube for tone tuning. Some say that tube should be balanced to provide even/equal signal level to the power section.
Others say to use an unbalanced tube in the PI position as the variance gives better harmonic quality. I used to only do balanced PI tubes, but I've since gone to unbalanced and I like it.

V1 is normally the tube closest to the amps input. PI is the tube closest to the power tubes. The reverb pre tube is usually a 12at7/ECC81 as Fender tends to prefer that lower gain tube to keep the signal cleaner.
If you want a cleaner tone with more headroom, you can try a 12at7/ECC81 in the V1 position, it can work very nicely to clean things up. That tube type can even work well in high gain channels as the lower gain pre drives the higher gain tubes and can smooth out the distortion.
It may even help keep the hiss down to a more acceptable level.

I would play around with the pre tubes for now, and do the power tubes later if you really need it.
Some amp designs are just noisier by design in the high gain channels, some much less so.
I have an Egnater Vengeance head that has a very high gain channel, but even when the gain is cranked to 11 the amp stays nicely quiet. Again, that's it's nature by design.

Also, it's ok to use different brand pre tubes in different positions and roll them around to hear what they do to the tone in different positions.
JJ's tubes are good quality, but I prefer Tung Sol 12ax7's for V1 and high gain stages.
Sovtek LPS 12ax7's have a great tone in V1 and PI positions, but they might ring in high power combo amps. Mullards are great in higher gain stages, but I don't like them in V1 or PI. My favorite for great tone, at the moment, are Jan Phillips 5751's, not the lower gain versions, but the one's that are very close to 12ax7 gain.
It's printed with green lettering. I have a pair that are older, not sure how old, but man they have a sweet tone

I hope some of that helps a bit.
Have fun tube swapping and tone chasing. :D


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