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Fixed Bias: How to build it conveniently to use?
Fixed Bias: How to build it conveniently to use?
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Old 8th February 2018, 01:48 PM   #21
Lingwendil is offline Lingwendil  United States
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Fixed Bias: How to build it conveniently to use?
Quote:
Originally Posted by PRR View Post
Use Self-Bias Whenever Possible!

When fixed-bias works you can get some/lots more power out of a pair of tubes. But when it does not work things can get very ugly. A 250 Ohm 10W cathode bias resistor is very reliable. After decades of blowing-up tubes (I used to do that a lot in my job) I have mellowed and will accept a reliable 25 Watts rather than 50 Watts on good days and major repairs on other days. (My last major P-P project was down-rating an unreliable 60W amp to a 20W which may out-live me.)
Fixed bias is for sure a nice engineer's solution to getting the most out of a design, but nothing beats self-bias for shear reliability and simplicity. My experience is that individual cathode resistors is even better balance-wise, and cross-coupled garter bias is even better still, but garter bias can waste twice the bias voltage, so isn't practical in all instances. Lower volt applications it works fantastic, and cathode bias in general is a nice way to reduce dissipation on older guitar amps that are running lots of voltage across the finals and making you nervous

When building an amp that isn't living with someone likely to pay attention to such things, I use self-bias or garter bias to make it more or less foolproof.

Quote:
Originally Posted by Merlinb View Post
Transients can demand a hundred times more power than the average level. A 10W amplifier will clip fairly often.
Yup. Knowing this can open your eyes to a lot of things, and also can influence designs... A nice source follower can work wonders in letting finals "push" a little harder on transient higher signal, but the cathodes need to let the extra current through somehow! Combination or fixed bias is the way to go here. Or, better yet, more sensitive speakers or box design.

Also, running of of a better source (like a nicer DAC with better headroom/dynamic range) can make things more apparent due to lack of compression.

Interestingly enough, my 6SN7 push pull is garter biased, and probably does just a smidge over a watt at "full blast" without any audible clipping behavior, and at very high (more than would be comfortable!) listening volumes. I have it hooked up to some DIY karlsonator (with vifa/tymphany TC9FD) speakers that are reasonably sensitive. Hooking this amplifier up to some home theatre speakers will give you much less audible volume for the same signal before sounding questionable...
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Old 8th February 2018, 02:37 PM   #22
goldenbeer is offline goldenbeer  Germany
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Quote:
Originally Posted by kodabmx View Post
I've never seen a 100V peak on a 1V RMS signal [...] Considering in most music, the RMS is only a couple of db less than peak
100V peak vs 1Vrms is 5000x more power...

Today, or during the height of the loudness war, cd recordings regularly peaked out at only 8dB vs rms or even less, but there was also a time where the average recording level was at -18dB, which is a factor of ~63 in power... Now, if you like to listen to music which also has quieter passages, a factor of 100 in power is not too uncommon.
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Old 8th February 2018, 03:51 PM   #23
Blitz is offline Blitz  Germany
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Many different concepts ... any impressions on the sound differences they actually provide ? I guess that there are for sure different effects audible like:

- Really the lowested saturation of the core by matching the currents per PP-Output tube (the explaination given by Menno for the improvement on details with the autobias module) vs.
- Autobias of the current - source setup (which is only really the same current if both tubes behave identically, no ?)
- Classic fixed-bias vs. cathode-resistor
- mixture of the last two concepts ?
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Old 8th February 2018, 05:02 PM   #24
petertub is offline petertub  Sweden
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If implementing "classic fixed bias" : do not stabilize the bias source unless B+ is also
stabilized. Reason is that if both are unstabilized they will follow mains fluctuations
and to large degree be a stable combination ( B+ increases, bias will also increase).
This is both simple and cheap and needs no advanced solutions.
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Old 8th February 2018, 07:28 PM   #25
kodabmx is offline kodabmx  Canada
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Quote:
Originally Posted by goldenbeer View Post
100V peak vs 1Vrms is 5000x more power...

Today, or during the height of the loudness war, cd recordings regularly peaked out at only 8dB vs rms or even less, but there was also a time where the average recording level was at -18dB, which is a factor of ~63 in power... Now, if you like to listen to music which also has quieter passages, a factor of 100 in power is not too uncommon.

Thanks for the correction, I thought my math was off. Now I understand what you mean. If, for instance, you listen to a classical piece with those extremely quiet parts that you need to turn up to hear, then there is a cresendo and it's shaking the walls. I feel like building a compressor just to deal with that, and movies.
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Old 8th February 2018, 08:20 PM   #26
Lingwendil is offline Lingwendil  United States
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Fixed Bias: How to build it conveniently to use?
Quote:
Originally Posted by Blitz View Post
Many different concepts ... any impressions on the sound differences they actually provide ? I guess that there are for sure different effects audible like:

- Really the lowested saturation of the core by matching the currents per PP-Output tube (the explaination given by Menno for the improvement on details with the autobias module)
Reducing core saturation at idle is definitely a benefit of autobias, but individual cathode loads or garter bias can accomplish this in an acceptable manner too.

Quote:
- Autobias of the current - source setup (which is only really the same current if both tubes behave identically, no ?)
A current mirror on top of the current sink can help the balance, as will individual current sinks. There are ways to allow the current to change with signal level (I think John Broskie called his scheme "sliding bias" in one of his blog writings) but I've not had much experience with them

Quote:
- Classic fixed-bias vs. cathode-resistor
They can sound different for sure, as cathode bias adds some degeneration to the signal, even when bypassed. Large signal excursion will cause blocking distortion. Cathode bias will have less gain and more even harmonics too... In my opinion fixed bias is one of the best ways to prevent blocking distortion, especially when you run followers to feed the grids. Fixed bias will often sound a bit more clean, especially when pushed hard. It will often measure a bit nicer too.

Quote:
- mixture of the last two concepts ?
Depends on how much you skew the control... more fixed bias with around 10-15% of the bias being developed across a cathode resistor can work pretty well and sound great, but unless parallel connecting a bunch of tubes I don't like to use it... you could throw a current mirror under the cathode and run fixed bias, but that needs a little bit of fooling around to make work well, not that it's all that difficult...


Do I sound a little vague, like I'm withholding a bit of info? Sure. Part of that is because I've not found any form of autobias that I truly like yet, and I'm trying to inspire you do dig deep into some older literature and do some research. I find looking at older patents from the sixties and earlier can be quite enlightening and helpful.
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Old 10th February 2018, 05:09 AM   #27
PRR is offline PRR  United States
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Quote:
Originally Posted by Merlinb View Post
Transients can demand a hundred times more power than the average level...
Quote:
Originally Posted by kodabmx View Post
I've never seen a 100V peak on a 1V RMS signal
Merlin said "power", not "voltage".

So 10V peaks on this "1V signal".

Merlin has engineered recording circuits, and may know something.

Quote:
Originally Posted by kodabmx View Post
...in most music, the RMS is only a couple of db less than peak...
"RMS" can be defined many ways. In speech/music a 50mS time-constant is fairly common. On that time-scale, 50ms RMS does tend to be more than a few dB below peak. (I've run many dozens of hours of live recordings, and more than a few processed recordings, through this computation to know if my masters were "loud enough".)
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Old 11th February 2018, 06:27 AM   #28
Miles Prower is offline Miles Prower  United States
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Quote:
Originally Posted by Blitz View Post
I want to build a PP-Amp with fixed bias. I tried different autobias modules like tentlabs, but they smoked after a while tgem self to death. So, in a perfect world, a switch beteen the measuring resistors, a stationary voltmeter and round knobs to be used without screwdriver would be cool...or is there a better way ? It would be nice to see some pictures ...?
Just implement it like this (attached). Doesn't get much simpler. Metering consists of 10R/0.25W current sense resistors in the cathodes. These should be sized small so they act as fuses of last resort.

The metering is with a 100uA meter with a series resistor to set the full scale. I calibrated it by setting up a 100mA CCS with a power transistor to set the current. Used a 100K pot in series with the meter and adjust to 100uA. Note the series resistance. In this case, it was a little over 7K, and within a couple of ohms of a combination of 12K + 1K. A small on-off-on switch selects each final for bias adjust.
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Old 11th February 2018, 11:51 AM   #29
kodabmx is offline kodabmx  Canada
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Quote:
Originally Posted by PRR View Post
Merlin said "power", not "voltage".

So 10V peaks on this "1V signal".

Merlin has engineered recording circuits, and may know something.



"RMS" can be defined many ways. In speech/music a 50mS time-constant is fairly common. On that time-scale, 50ms RMS does tend to be more than a few dB below peak. (I've run many dozens of hours of live recordings, and more than a few processed recordings, through this computation to know if my masters were "loud enough".)

Thanks for the information. Agreed my math was wrong as I wasn't thinking of it as dynamic range. I guess a good example of needing good peak reserve would be playing an old scratched record and having the scratches not clip?
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Old 12th February 2018, 09:11 AM   #30
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by kodabmx View Post
I guess a good example of needing good peak reserve would be playing an old scratched record and having the scratches not clip?
Just some info: For a 5mVrms (5cm/s) pickup you can expect rare transient peaks up to 16dB above the 5cm/s level, which is 31.5cm/s. Clicks due to dust and scratches may reach 22dB above the 5cm/s level, which would amount to 89mVpk using that pickup.
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