• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Tube amps: Self Biasing vs. Manual?

StevenZ

StevenZ

Texas or nothin'
Hey guys,

I'm curious to know what if any is the drawback of self biasing tube amps. Is there a reason there are so many fewer self biasing tube amps than manual biasing? As far as I can see, it's a no brainer to use a self biasing amp as they don't require matched tubes, and are pretty much plug and play. If anyone could shed some light on the situation I'd be greatly appreciative.


Cheers,
Steven
 
Register to hide this ad
You need some knowledge and understanding to go with manual biasing. That's why average consumer grade amps/consoles were cathode-self biasing and didn't require the need for balanced tubes however not always optimal.
 
Thanks Marty, but what are the sonic differences between each? Also, is one superior to the other other than just being adjustable?
 
I like self biasing amps....less chances of me screwing anything up...then its a time consuming and expensive proposition to ship for repair.

I had great amps and most were self biasing.
 
I wouldn't consider either necessarily a better design than the other.

There are pro's an can's for both, typically you'll get less power with
cathode bias(self bias) verse manual bias(fixed bias) as you loose an
bit of voltage at the cathode and the load is generally higher, But as
an designer you can modify Any of the Parameters to get the output
your after, and either can be "needing" matched tubes as it depends
on how the cathode or fixed bias is setup, be it individual or multiples.
(so neither one says you "need" or don't "need" matched tubes there)

Generally speaking cathode bias is the easier of the 2 for the end user.
 
Steven -- Both "fixed" bias (what I believe you are possibly calling manual bias) and "cathode" bias (or self bias) configurations can be used to create wonderful sounding amplifiers. Positron mentioned the potential differences in power output between the two approaches. Additionally, fixed bias designs also typically develop less distortion, as well as greater power output.

However, while the presence of bias controls typically indicates a fixed bias design and the lack of them typically represents a cathode bias design, understand that the presence -- or the lack -- of these controls does not automatically guarantee that a given amplifier is definitely a fixed bias design, or definitely a cathode bias design. A number of cathode biased amplifiers provide an adjustment pot for adjusting the quiescent current of the output tubes. A Fisher X-100B is an excellent example of this. Conversely, a number of fixed biased amplifiers provide no means for adjusting the quiescent current at all. An early Scott LK-72 is an excellent example of this type of design.

The bottom line is that there are likely two different issues at play here within your question:

From a design engineering standpoint, an output stage either operates with cathode bias, or fixed bias.

From a design features standpoint, BOTH design approaches may or may not have user controls provided to adjust circuit parameters, but fixed bias designs typically do have user controls, and cathode biased designs typically do not.

The problem comes in when a unit is judged as being one type of engineering design, based solely on whether it has user controls or not. As you can now see, that can lead to some very wrong assumptions.

This may or may not be the answer you were looking for. But it is necessary information to know, so that when you are listening to various different designs, you can make your assessment secure in the knowledge of just what the heck it is you're listening to!!

Dave
 
as Kegger said....not all manual biased tube amps require 'matched pairs' of output tubes....case in point Rogue 'M' series monoblocks employ manual biasing for each tube, thus optimizing current flow through each tube.

BTW, it's a piece of cake to do
 
dcgillespie said:
A number of cathode biased amplifiers provide an adjustment pot for adjusting the quiescent current of the output tubes. A Fisher X-100B is an excellent example of this. Conversely, a number of fixed biased amplifiers provide no means for adjusting the quiescent current at all. An early Scott LK-72 is an excellent example of this type of design.
Click to expand...


Most Williamson type amps have a single adjustment pot to balance out the tube current, but its a cathode bias system. The adjustment simply allows you to make sure both tubes are pulling the same amount of plate current. My Fisher 290T (from a Philharmonic console) is fixed bias, but has no adjustment.
 
dcgillespie said:
Steven -- Both "fixed" bias (what I believe you are possibly calling manual bias) and "cathode" bias (or self bias) configurations can be used to create wonderful sounding amplifiers. Positron mentioned the potential differences in power output between the two approaches. Additionally, fixed bias designs also typically develop less distortion, as well as greater power output.

However, while the presence of bias controls typically indicates a fixed bias design and the lack of them typically represents a cathode bias design, understand that the presence -- or the lack -- of these controls does not automatically guarantee that a given amplifier is definitely a fixed bias design, or definitely a cathode bias design. A number of cathode biased amplifiers provide an adjustment pot for adjusting the quiescent current of the output tubes. A Fisher X-100B is an excellent example of this. Conversely, a number of fixed biased amplifiers provide no means for adjusting the quiescent current at all. An early Scott LK-72 is an excellent example of this type of design.

The bottom line is that there are likely two different issues at play here within your question:

From a design engineering standpoint, an output stage either operates with cathode bias, or fixed bias.

From a design features standpoint, BOTH design approaches may or may not have user controls provided to adjust circuit parameters, but fixed bias designs typically do have user controls, and cathode biased designs typically do not.

The problem comes in when a unit is judged as being one type of engineering design, based solely on whether it has user controls or not. As you can now see, that can lead to some very wrong assumptions.

This may or may not be the answer you were looking for. But it is necessary information to know, so that when you are listening to various different designs, you can make your assessment secure in the knowledge of just what the heck it is you're listening to!!

Dave
Click to expand...

Mr Gillespie thank you for your detailed and precise explanation, you are an asset to this forum. I was waiting for someone to broach this question, I have a pair of Eico HF-32's both rebuilt which I believe are cathode biased. Each came with a quad of great vintage tubes, I imagine the tubes have strayed in their conductance. Reading the above posts having a matched pair/quad is not necessary in a cathode biased amp. How important is this? Should I /could I run a 1000R resister from the cathode to ground to measure current with my multi-meter? Second question is it possible to preform a mod to change it to fixed bias and have individual bias pots for each tube? Thanks for you time.
Cheers,
Kirk
 
I was hoping that what was posted covered it pretty well but I did not.

Neither cathode bias nor fixed bias means you don't need matched tubes.
(each one depends how it's setup, either could or could not "need" them)
 
Some amps have a fixed bias that sets all tubes to the same grid voltage and
those amps should/need to use matched tubes for best operation.

Some amps have an cathode bias that shares the cathode resistor and sets all
tubes to the same cathode voltage an those amps should/need to use matched
tubes for best operation.

The same as above can be used for pairs of tubes, and pairs should be matched.
 
Last edited:
If you have individual bias per tube be it separate fixed pots or separate cathode
resistors per tube then you can get away with unmatched tubes, BUT with those
that use individual cathode bias it is still recommended to use tubes that do draw
similar current so as not to unbalance the primary of the output tranny. (in PP)
 
And the HF-32 uses a single cathode resistor for all 4 tubes.

So yes it should use a matched quad.

And it would take a fair amount of reworking to change it over.
(also at least 1 tubes heater uses the cathode voltage as well)
 
Hi Kirk -- Thank-you for the kind words! I just try to explain things as I wished they would have been explained to me so many years ago.

To Keg's comments I would add, that in the HF-32 you can likely install 4 "typical" EL84s, and the amplifier will operate just fine. However, if you want to achieve maximum performance from it, a quad of tubes matched not only for quiescent conditions, but also for large signal Gm conditions will deliver the best results.

No doubt the circuit can be modified to bias and balance all four tubes individually under quiescent (DC) conditions, and even under dynamic (AC) conditions. It can also be modified to operate with fixed bias. As Keg said however, it would take a significant amount of work to accomplish these things, but it could be done, and it would improve performance.

The real question comes down to how important the ultimate performance of your amplifiers is to you. For some people, having their car get them from point A to point B dependably is sufficient. For others however, knowing that it did so with maximum power and efficiency available at a moment's notice is just as important as basic dependability is. Similarly, your amplifier will certainly sound good with 4 reasonably matched output tubes. But if you are searching for that Nth degree of performance, then this issue may likely bug you until you know for certain that maximum performance has been (or can be) achieved.

Finally, if you want to measure the current draw of each tube in the existing design, you would want to place a precision 10 ohm resistor (.25 watt ideally) between pin #3 of each of each output tube, and all of the existing connections currently on these pins. Therefore, pin #3 of each tube would only have its own 10 ohm resistor connected to it, and that's all. The other side of these resistors would connect to the leads originally connected to pin #3 of their tube (which were all originally connected together in daisy chain fashion). Then, to measure the current flow of a particular tube, measure the voltage directly across each resistor -- NOT from pin #3 to ground. For example, a voltage reading of .35 vdc across a 10.0 ohm resistor equals 35 ma of total current draw for that tube.

I hope this helps!

Dave
 
Keg and Dave,
That did help.
Thank you for the clarification. I'm going to do the 10 ohm/pin #3 mod to get an idea of the current draw and proceed from there, I think the individual biasing of each tube is beyond my limits as of now. That said the Eico HF-32 is a great sounding amp, I don't use them as daily players but put them into rotation every few months of so and savor the goodness of their "smoky Mullards" for a weekend....a really sweet sound!
Cheers,
Kirk
 
You must log in or register to reply here.
Back
Top Bottom