Yes that is correct, though I have no idea why you want to use individual biasing 🤷♂️
Thanks Merlinb. I asked if there was any upside to using individual biasing, but have not received a answer yet. If there is no upside, I won’t.
tizman,
A. Using a 12AX7 as an example:
Why use individual bias when you parallel the triodes?
1. A few are not well balanced, the currents are un-equal.
Result is distortion and reduction of dynamic range.
The effects are generally more severe when the triodes are used as a voltage gain stage, versus as a cathode follower.
2. You can easily check the balance of the currents, both new and years later.
B. Using a pair of 6L6GC in push pull as an example:
Why use individual bias when you parallel the tubes?
1. A few are not well balanced, the currents are un-equal.
The output transformer laminations will saturate early.
Result is distortion and reduction of dynamic range, especially for low frequencies.
2. You can easily check the balance of the currents, both new and years later.
The 12AX7 problems above are less frequent.
6L6GC and other power output tubes that are used in a push pull output circuit more often have the above listed problems.
If tube currents are badly balanced, the tube life of the higher current tube may be shortened.
In any case, it is good to use very well matched tubes.
Just my opinions.
Your Mileage May Vary
A. Using a 12AX7 as an example:
Why use individual bias when you parallel the triodes?
1. A few are not well balanced, the currents are un-equal.
Result is distortion and reduction of dynamic range.
The effects are generally more severe when the triodes are used as a voltage gain stage, versus as a cathode follower.
2. You can easily check the balance of the currents, both new and years later.
B. Using a pair of 6L6GC in push pull as an example:
Why use individual bias when you parallel the tubes?
1. A few are not well balanced, the currents are un-equal.
The output transformer laminations will saturate early.
Result is distortion and reduction of dynamic range, especially for low frequencies.
2. You can easily check the balance of the currents, both new and years later.
The 12AX7 problems above are less frequent.
6L6GC and other power output tubes that are used in a push pull output circuit more often have the above listed problems.
If tube currents are badly balanced, the tube life of the higher current tube may be shortened.
In any case, it is good to use very well matched tubes.
Just my opinions.
Your Mileage May Vary
Thanks 6A3sUMMER. I understand the benefits of individual bias resistors in power tube output stages. My question is specifically with respect to parallel triode cathode followers and individual cathode resistors on their triodes. Thanks for your reply.
Not much related to this thread but an important application of seperate cathode resistors of say 100R as used on
tubes such as the 6AS7G/6080 to assist in regulated power supplies. More cathodes requires higher R values.
Forces better sharing of the load current.
Schematics I've been seeing don't show a "build-out" or isolating resistor between cathode(s) and output jack. This is often OK, but can sometimes cause parasitic instability, with the output stage's inductive output impedance resonating with the load's capacitive impedance. The heavy feedback of a cathode follower can make this into a resonant tank that needs damping for best stability, in some cases.
One easy way to get some damping is to take signal from the bottom of the cathode bias resistors, and then call them cathode stops. The resistance adds to output impedance, and that may or may not matter, but so would a separate build-out resistor, so...
Doing it that way, only a single, common, load resistor and output coupling cap are used, but current balancing is maintained.
All good fortune,
Chris
One easy way to get some damping is to take signal from the bottom of the cathode bias resistors, and then call them cathode stops. The resistance adds to output impedance, and that may or may not matter, but so would a separate build-out resistor, so...
Doing it that way, only a single, common, load resistor and output coupling cap are used, but current balancing is maintained.
All good fortune,
Chris
Some tube vendors do balance testing on dual triode tubes, and some are very close matched dual triodes.
That costs a little extra, but it is worth it.
With that extra cost, very often the parallel triodes circuit can be simplified, and still be very well balanced.
That costs a little extra, but it is worth it.
With that extra cost, very often the parallel triodes circuit can be simplified, and still be very well balanced.
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Separate biasing encourages more equal current sharing, but this really only applies to decives with a very low internal cathode impedance, e.g. big bottles or transistors. Preamp triodes -especially triodes in the same bottle- generally share current quite satisfactorily even with shared biasing.
Quick question. Assuming I use the Tube Society buffer as my base schematic, but parallel the triodes, what should be the new value for the resistor marked R8 on my hand drawn schematic? I am not sure if I should double R8, divide it by 2, or leave it be. Thanks in advance for your replies. I have attached both schematics.
Thanks. Given that I have a lot of current available in this case, what is best practice?
Sorry, I was thinking of the load resistor! R8 is the grid leak (names are easier than numbers). Any value from 100k to 1Meg is fine.
Interesting thread . . . and I can understand why you might want to find some use for a preamp that you never use.
That said, it seems to me that modding it into a CF just so you can use it with an amp that simply needs a clean sounding method of controlling the volume is somewhat of a convoluted approach.
Is controlling the volume digitally an option? What's your source? I suspect that most any source should work fine driving a 250k input.
I don't understand what the advantage of adding all the CF circuitry would be unless you're using super long interconnects. And you'll still need some type of volume control.
Why not just add a passive preamp using a decent stepped attenuator to the new amp? Then save the preamp rebuild for use with an amp actually needs either gain or improved impedance matching such as a normal SS or Class D amp which has a low input impedance.
The Aleph J apparently needs neither gain nor an improved impedance match. In such a scenario wouldn't fewer parts / less circuitry sound cleaner than adding more complexity?
The original preamp was a 4S Universal that I built long ago and didn’t really use much. I then added an additional tube to add a cathode follower, and used it with other solid state power amps I have on hand. This added unneeded gain and the noise that comes with it, so I didn’t use it much either. I have now removed the signal circuit from the chassis, and turned the preamp into a free standing 250VDC power supply. I am putting the parallel cathode follower circuit into its own enclosure, and am using an umbilical to connect it. This way I can build other signal circuits into small enclosures and use the same power supply, should the need arise.
I need to reduce gain with the Aleph J. The gain structure with my source has me at 2 out of 10 on the volume to achieve ear blistering levels. There is also too much noise coming through the speakers, which are either Khorns or La Scala, both of which are highly sensitive. I’m hoping that the ability to reduce gain cleanly will also reduce noise. I could use a passive volume pot, and will try one as well, but I had the unused 4S, and figured turning it into a nice power supply would be fun.
I've never used a cathode follower but my understanding is that it doesn't add gain but it also doesn't reduce it either. Well, maybe technically it reduces it by a very tiny bit, but not enough to have any practical audible effect on your gain issue.
It would be far easier to add a simple voltage divider at the input of the Aleph J, using couple of fixed resistors and then put a "passive preamp" in front of it. That will solve your gain problem. You'll have a far wider range of adjustment and it will be transparent as long as you use high qualiy parts.
My understanding is that a cathode follower will provide a slight reduction in gain at full volume while also buffering the output to the amp by providing a low (ish) impedance source. There are quite a few people that dislike the idea of an unbuffered voltage divider feeding an amp. I haven’t compared, so I can’t comment on SQ.
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I'm sure the more technically minded here will chime in but my understanding is that the difference in gain between nothing and a cathode follower is something like 1 (unity gain) vs 0.9 for the CF. Solving your gain problem will require much more attenuation than that. And if, as you say, that's at full volume, at normal listening volumes I have to wonder if it would be audible at all.
In other words, I doubt that the addition of a CF will significantly alter the very limited range of your existing volume control. Adding a voltage divider to the amp will solve that issue if you choose the right combination of resistor values.
As for impedance . . . any modern source would qualify as a "low impedance source" wouldn't it?
The rule of thumb is that the source's output impedance should ideally be no more than 10% of the input impedance.
Where you run into problems is using a tube preamp whose output impedance is high - typically because the plate resistance of the tube is high - with a SS amp whose input impedance is low. I believe that many SS amps have input impedances between 10k and 20k. That's the scenario where the use of a CF is needed. That and if you're driving long interconnects.
In contrast, using a tube preamp with a tube amp, whose input impedance is typically 100k, is not a problem. So if your amp has an input impedance of 250k it seems like you have a lot of wiggle room even if you add a voltage divider.
I guess, in general, I find that simpler is often better and adding all the complexity that's being discussed seems like it's not really necessary in order to simply end up with a volume control whose range of adjustment is more practical.
Of course, everything has its trade-offs and "quite a few people" will always disagree on any method of solving a problem.
The amp is a DIY Aleph J with an input impedance of 250K. In addition there is another gainclone amp with an input impedance of 10K that I would like to use it with. The Aleph J doesn’t have a volume control. I could, as you suggest, just add a volume control, but this seams to be frowned upon by some in these forums. They say that it is best to buffer the volume control for optimal SQ. When I run the Aleph J at full gain, as is always the case without a volume control, the residual noise from upstream shows up in the speaker output. Again, a simple volume control at the input should help, but many say that it would be best to buffer that volume control rather than plunking it in on the input of the amp. As is, there is always residual noise on the speakers when I feed the Aleph J with a PC to DAC front end. This is the case with the higher gain DIY tube amps I own when the volume is on full. A simple volume pot may easily correct this issue but, again, I have heard several arguments for buffering this volume control. A paralleled 12AT7 cathode follower is supposed to be very transparent. We shall see if it fixes or adds to the problem.
The amp is a DIY Aleph J with an input impedance of 250K. In addition there is another gainclone amp with an input impedance of 10K that I would like to use it with. The Aleph J doesn’t have a volume control. I could, as you suggest, just add a volume control, but this seams to be frowned upon by some in these forums. They say that it is best to buffer the volume control for optimal SQ. When I run the Aleph J at full gain, as is always the case without a volume control, the residual noise from upstream shows up in the speaker output. Again, a simple volume control at the input should help, but many say that it would be best to buffer that volume control rather than plunking it in on the input of the amp. As is, there is always residual noise on the speakers when I feed the Aleph J with a PC to DAC front end. This is the case with the higher gain DIY tube amps I own when the volume is on full. A simple volume pot may easily correct this issue but, again, I have heard several arguments for buffering this volume control. A paralleled 12AT7 cathode follower is supposed to be very transparent. We shall see if it fixes or adds to the problem.