This may be a stupid thing to bring up, but I just want some opinions. What do people here think about whether cathode biased amplifiers are "self biasing" or should be biased when changing brands of the same tube type. I have seen it both ways and done it both ways, but finding a real consensus and a real rule of thumb concerning it has eluded me, although I haven't really researched it much.
I just was reminded of it when looking at the schematic for the newer Kustom Defender 5, MOD version. It calls for a 270 Ohm resistor when using a Shuguang tube and a 220 Ohm resistor for a Sovtek tube. Do you all think the different tubes necessitate biasing? I stole the amp, unused for a song and haven't even plugged it in yet, but I have heard it's a pretty good little practice amp, and at least Kustom is nice enough to send one a diagram...so I checked it out.
I just was reminded of it when looking at the schematic for the newer Kustom Defender 5, MOD version. It calls for a 270 Ohm resistor when using a Shuguang tube and a 220 Ohm resistor for a Sovtek tube. Do you all think the different tubes necessitate biasing? I stole the amp, unused for a song and haven't even plugged it in yet, but I have heard it's a pretty good little practice amp, and at least Kustom is nice enough to send one a diagram...so I checked it out.
For my own amps: If cathode biased, or for fixed non-adjustable bias, I always check afrer a re-valve as a bare minimum. If the bias is within an acceptable range, then I call it good and leave it be.
If it is a paying job, then I go by what the customer wants, and charge accordingly for the any extra work if parts need changing.
For fixed adjustable bias, then I just go ahead and set it.
Cheers, and regards,
Ant
If it is a paying job, then I go by what the customer wants, and charge accordingly for the any extra work if parts need changing.
For fixed adjustable bias, then I just go ahead and set it.
Cheers, and regards,
Ant
Hi Ant Moore. That's an intelligent way to look at it. I check too just to get the experience, but really never thought of asking a customer what they wanted. The customer is always right as far as I am concerned though and I think that I will do just that when I do any work for someone else. I only do the occasional repairs so far, but wish to do more as in my area there isn't really anyone doing it locally.
Have you ever found the bias on a re-tube job which was off enough to cause a problem? In my limited experience I have not ever even Seen a red plate situation, nor have I had a time when performance seemed compromised. Of course a fixed bias amp is another situation. I don't have as much time on those as most of my own amps have been lower powered amps of the cathode biased type.
Do you perform your biasing with any set up other than using a meter in the usual fashion? I actually have a BiasEasy probe/meter "in the cart" at Studio Sound Electronics waiting until they reopen in February to buy. They cannot be used on the 7881 tube but I have yet to run into that one anyway. I thought it might be worth a try. For my own use, I like to over bias just as I find I get a little tone and I don't mind replacing tubes more often. However, for anybody else I have always tried for right on dead center.
Thanks for the reply. It is appreciated. I try to soak up as much as I can here from the experience of others and just like what you mention about checking with the customer about something as simple as re-biasing, it was something that had not come to mind. I absolutely hate it when I, as a customer, do not get good service in any way, and in todays world it happens way too much!
Have you ever found the bias on a re-tube job which was off enough to cause a problem? In my limited experience I have not ever even Seen a red plate situation, nor have I had a time when performance seemed compromised. Of course a fixed bias amp is another situation. I don't have as much time on those as most of my own amps have been lower powered amps of the cathode biased type.
Do you perform your biasing with any set up other than using a meter in the usual fashion? I actually have a BiasEasy probe/meter "in the cart" at Studio Sound Electronics waiting until they reopen in February to buy. They cannot be used on the 7881 tube but I have yet to run into that one anyway. I thought it might be worth a try. For my own use, I like to over bias just as I find I get a little tone and I don't mind replacing tubes more often. However, for anybody else I have always tried for right on dead center.
Thanks for the reply. It is appreciated. I try to soak up as much as I can here from the experience of others and just like what you mention about checking with the customer about something as simple as re-biasing, it was something that had not come to mind. I absolutely hate it when I, as a customer, do not get good service in any way, and in todays world it happens way too much!
To be fair, I've not seen any great problems in my time on cathode-biased amps. They are pretty forgiving, giving the the negative feedback that cathode biasing results in.
The truth is, it depends what I have.
I have biased across a nominal cathode resistor. I have biased across a "real" cathode resistor. I have biased across the resistance of the output transformer DCR - which is scary with both probes are at too much voltage! I quite like this, bit it does mean I need to be very careful. The cathode resistor is generall perfectly adequate, In my experience,
Goldilocks.
Bias is a massively subjective thing, There is no right answer. As long as you are between the point where the amp burns up from too hot bias, and where it just sounds like **** from too cold bias, you're good,.
I've had a customer specify "cold bias as far as possible so my tubes last ,longer". Grinds my gears. But what can you do?
Cheers, and regards,
Ant
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