Smoke

G9, matched tubes and autobiasing
« on: November 22, 2004, 06:21:52 AM »
I searched around but I couldn't find anything on this here.

Regarding the G9, and possibly other Gyraf projects, do the preamp tubes have to be matched?
Can someone please explain auto bias , and whether that feature is included in the G9. Which are the bias resistors?

As far as I understand this if there is no autobiasing one channel on the G9 may sound different from the other, is that correct?

What about power tubes, as far as I know they should be matched to eliminate hums and buzzes. Can they be autobiased?

Thanks


mcs

Re: G9, matched tubes and autobiasing
« Reply #1 on: November 22, 2004, 06:28:43 AM »
Quote from: "Smoke"
I searched around but I couldn't find anything on this here.

You should have searched some more - it has been asked before :wink:

Quote
Regarding the G9, and possibly other Gyraf projects, do the preamp tubes have to be matched?

No, the tubes don't have to be matched. Just use the same kind of tubes for both channels.

Quote
Can someone please explain auto bias , and whether that feature is included in the G9. Which are the bias resistors?

As far as I understand this if there is no autobiasing one channel on the G9 may sound different from the other, is that correct?

You have been reading about poweramps. The G9 is a preamp, so you don't have to worry about biasing at all.

Best regards,

Mikkel C. Simonsen

flyingfader

Re: G9, matched tubes and autobiasing
« Reply #2 on: November 22, 2004, 01:12:21 PM »
Quote
You have been reading about poweramps. The G9 is a preamp, so you don't have to worry about biasing at all.

Best regards,

Mikkel C. Simonsen


 :shock:  I do not agree, what are R8/R7, R24, R29/30 made for? just wondering mmm.... :cool:

best regards,

Fly
"Any instrument when dropped will roll into the least accessible corner."

mcs

Re: G9, matched tubes and autobiasing
« Reply #3 on: November 22, 2004, 01:27:23 PM »
Quote from: "flyingfader"
:shock:  I do not agree, what are R8/R7, R24, R29/30 made for?

Yes, of course there are biasing resistors - I just said you don't have to worry about them.

In poweramps you often have to check/set the bias, which is not the case here.

Best regards,

Mikkel C. Simonsen

Smoke

G9, matched tubes and autobiasing
« Reply #4 on: November 22, 2004, 03:28:37 PM »
I guess that means the auto biasing feature is included here.

It's just that I got told in a guitar shop that the tubes might change after a month or so and "disbalance", and if my circuit doesn't have autobiasing i would run in to many many problems.

mcs

G9, matched tubes and autobiasing
« Reply #5 on: November 22, 2004, 04:54:40 PM »
Quote from: "Smoke"
It's just that I got told in a guitar shop that the tubes might change after a month or so and "disbalance", and if my circuit doesn't have autobiasing i would run in to many many problems.

They're talking about power tubes (EL34, KT88, 6L6 etc.) - not preamp tubes like the ECC8x. And don't believe it when the people in the guitar shop tell you to change the tubes every 6 months... :wink:

Best regards,

Mikkel C. Simonsen

Smoke

G9, matched tubes and autobiasing
« Reply #6 on: November 24, 2004, 01:01:41 AM »
yeah but the wierd thing is that i was purchasing 12AU7's from him. I asked him are they matched just so i know, and he started talking about autobiasing on preamp tubes.

I know powertubes in general should be matched.

what will happen if you replace the 12au7 with 12ax7 in the G9?

gyraf

G9, matched tubes and autobiasing
« Reply #7 on: November 24, 2004, 03:28:07 AM »
Quote from: "Smoke"
what will happen if you replace the 12au7 with 12ax7 in the G9?


It is a common misunderstanding from the guitar-amp business that 12AT7/12AU7/12AX7 (ECC81/82/83) are interchangable. They are not. They have the same pin configuration, but are otherwise very different beasts.

There is two ECC82 tubes in a G9 channel.

The first tube is the preamplifier stage. If you substitute this for an ECC83 you will get higher gain, but somewhat poor'er frequency response and uneven gain steps. It is possible to recalculate the resistors around the tube to get even gain steps, but the high-end precision generally suffers from using ECC83's, even when carefully implemented.

The second tube is the SRPP output driver. An ECC83 does not have the output impedance needed to drive low-end through the output transformers, so here you will loose low end.

Jakob E.
..note to self: don't let Harman run your company..

Smoke

G9, matched tubes and autobiasing
« Reply #8 on: November 24, 2004, 06:23:07 AM »
oh ok i thought it would result in some smoke (:

thenovice

G9, matched tubes and autobiasing
« Reply #9 on: November 28, 2004, 08:29:59 PM »
Quote from: "gyraf"
Quote from: "Smoke"
what will happen if you replace the 12au7 with 12ax7 in the G9?


It is a common misunderstanding from the guitar-amp business that 12AT7/12AU7/12AX7 (ECC81/82/83) are interchangable. They are not. They have the same pin configuration, but are otherwise very different beasts.

There is two ECC82 tubes in a G9 channel.

The first tube is the preamplifier stage. If you substitute this for an ECC83 you will get higher gain, but somewhat poor'er frequency response and uneven gain steps. It is possible to recalculate the resistors around the tube to get even gain steps, but the high-end precision generally suffers from using ECC83's, even when carefully implemented.

The second tube is the SRPP output driver. An ECC83 does not have the output impedance needed to drive low-end through the output transformers, so here you will loose low end.

Jakob E.


not to change the subject,well

can you use a 12AU7 tube instead of the ECC83s?

and if so will the sound dynamics be different?
i.e. is that a mis-step?
"Jack of all trades, master of none, though ofttimes better than master of one"


thenovice

G9, matched tubes and autobiasing
« Reply #10 on: November 29, 2004, 02:25:11 AM »
?
?
"Jack of all trades, master of none, though ofttimes better than master of one"

thenovice

G9, matched tubes and autobiasing
« Reply #11 on: November 29, 2004, 02:25:22 AM »
?
? :sam:
"Jack of all trades, master of none, though ofttimes better than master of one"

zebra50

G9, matched tubes and autobiasing
« Reply #12 on: November 29, 2004, 04:57:34 AM »
I think your question is not so clear. If you mean 'can I use 12AU7 in the G9' then the answer is 'yes' .  12AU7 = ECC82.

But as Gyraf explained above, you can't really use the 12AX7/ECC83.

z50
Ribbon microphone services
http://www.xaudia.com
Microphone blog

Animatic

G9, matched tubes and autobiasing
« Reply #13 on: September 30, 2008, 11:59:48 PM »
Quote from: "zebra50"
I think your question is not so clear. If you mean 'can I use 12AU7 in the G9' then the answer is 'yes' .  12AU7 = ECC82.

But as Gyraf explained above, you can't really use the 12AX7/ECC83.

z50


12Axx and ECCxx are Europe and USA naming conventions for the same tubes.

If you pick your cathode resisters to give the same output voltage
per given input signal  for 2 tubes in matching circuits,
you are basically biased the same, so the tubes will basically act the same.

Care might be taken matching Cathode and Plate resisters for two side by side channels,
but tube variations even in 'matched pairs' will likely deviate a bit any way.
I have put cathode pots on to tune and then gotten the closest match.
If you really want to be fanatically finicky leave the pot in and periodically re-bias.

PRR is the guy to give the definitive explaination here.
But Gyraf got it right for sure.

I built an 6SN7 SRPP circuit into a guitar amp, parallel to a normal 12AX7 preamp,
to run a guitar synth through and it sounded good just the one tube.
No tone controls just the one SRPP stage.

You can get different 'effects' swapping tubes in a guitar amp,
but accurate reproduction isn't always what you want in a guitar amp.
It usually is in a  mic preamp.
Go with what ya know and work with what ya got.
Then build more.


 

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