It was recommended by my tech, it also allows me to swap tube without without manually biasing the amp.
Is sound quality affected by this mod?
Is sound quality affected by this mod?
What you're suggesting is changing the amp to run in Class A, which wasn't how the amp was designed to operate.
jeff
It's not difficult to install individual biasing per output tube, or else a DC balance control per channel.
The hardest part is deciding where to put the extra pots, which is probably in place of the front panel octal sockets.
Anyone with a tube amp should own a DVM anyway.
The hardest part is deciding where to put the extra pots, which is probably in place of the front panel octal sockets.
Anyone with a tube amp should own a DVM anyway.
Cathode resistor biasing trades (in this case) some performance at higher signal levels for some safety margin and (potentially, if using separate cathode resistors for each valve) some balancing of output transformer currents over the working lives of the output valves. It's a fairly major change, and your choice might depend on your expected use - if expected to often be used up near full output, you may not want to accept the trade-offs, but at lower level use, it is a net positive. At that point, you might even consider full triode operation too, or optionally.
All good fortune,
Chris
All good fortune,
Chris
You should not keep changing all kind of unknown tubes in your amp. It's bad for the wallet and potentially bad for the amp. A set of well matched new EL34 will last for many years to come.
@Chris Hornbeck already pointed out most of what it means. So, basically you'd be trading some power for convenience. Though it is a bit more complicated than that. Depending on how hard you tend to run your amplifiers you might or might not notice any of it by ear.
In general grid biasing will give you more power, and will behave better when the amplifier clips. Cathode biasing has the downside of the bias point moving when you run the amplifier heavily in to class B territory. Instantaneous clipping might not be that bad, but there is the risk of running in to crossover distortion during clipping.
The second downside of cathode biasing is that you'll have one or two more capacitors on the signal path, and it will be an electrolytic. Also, you'll have one more time constant in the amplifier, and since the ST-70 has negative feedback, there is a risk of low frequency instability. The latter isn't a serious problem, only means the capacitor value has to be appropriately sized and stability tested adequately before your tech hands the amp back.
FWIW my gut feeling after having designed, built and listened to both biasing options both in SE and PP form is that I do tend to prefer grid biasing, especially in push-pull amplifiers. That's not to say cathode biasing sounds horrible, rather there seems to be a slight tradeoff.
In general grid biasing will give you more power, and will behave better when the amplifier clips. Cathode biasing has the downside of the bias point moving when you run the amplifier heavily in to class B territory. Instantaneous clipping might not be that bad, but there is the risk of running in to crossover distortion during clipping.
The second downside of cathode biasing is that you'll have one or two more capacitors on the signal path, and it will be an electrolytic. Also, you'll have one more time constant in the amplifier, and since the ST-70 has negative feedback, there is a risk of low frequency instability. The latter isn't a serious problem, only means the capacitor value has to be appropriately sized and stability tested adequately before your tech hands the amp back.
FWIW my gut feeling after having designed, built and listened to both biasing options both in SE and PP form is that I do tend to prefer grid biasing, especially in push-pull amplifiers. That's not to say cathode biasing sounds horrible, rather there seems to be a slight tradeoff.
I'd still like to hear a little more of what this "tech" had to say. Maybe OP has a thing with tube rolling, and a lack of knowledge, and tech wanted to just cool bias the 6l6 family for him and let him play around.
That being said, I agree with everyone else. Take the time and set up individual bias. That will help you learn a little of what's going on anyhow. If you don't already, get a good DMM. There is 0 good reasons for not taking the minute to bias any tubes after any changes. And learn, look into what has been said. We're here to help, it's just a bogus idea.
Loren
That being said, I agree with everyone else. Take the time and set up individual bias. That will help you learn a little of what's going on anyhow. If you don't already, get a good DMM. There is 0 good reasons for not taking the minute to bias any tubes after any changes. And learn, look into what has been said. We're here to help, it's just a bogus idea.
Loren
Just some relevant data from the RCA 6L6GC data sheet, showing different OPT primary loads chosen for similar operating points, one with fixed bias, the other with cathode bias.
As you can see, push-pull 6L6s can be run in class AB1 using either cathode bias or fixed bias.
As you can see, push-pull 6L6s can be run in class AB1 using either cathode bias or fixed bias.