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SVT-CL: Mixing Power Tubes
- Thread starter willeb
- Start date
For some reason i was not thinking clearly about what the bias adjustment actually does in the circuit.
So that means that replacing a tube on both sides of the svt would actually just have the effect of throwing both sides of the amp off of being matched and causing the OP to end up needing to buy 6 new tubes rather than just 3.
No that is not a guarantee. It could work just fine for years. It could also run fine for years just replacing one. There could also be problems from doing either (besides the possibility of infant failure of the new tube(s) ). Maximum power and minimum distortion comes from all six tubes being able to run as close as possible.
IMO this almost off topic discussion is more academic than practical. Proper testing by a good amp tech should be the first order after a tube failure. Only that way can an HONEST tech give the best advise as it pertains to your unit.
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I do understand that disclaimer about the amp tech bit, but i am trying to glean as much knowledge about the subject as possible and it's discussions like these that help me along the way. I'm just not the kind of guy that is willing to take my gear to a tech unless I'm absolutely certain i can't take care of it myself. Thanks a lot for your insight.
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May be time to PM you some basics? There are some good links in the stickies for beginning amp repair already. No need to tie up posting space.
There are many safety concerns to learn if you want to dig into tube amps. Most lessons are not best learned "the hard way"!
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There is a good collection of technical/troubleshooting information in the portaflex wiki that can help you get started on understanding some of this if you want to check it out. Much of it is written with the ampeg B15 in mind, but it can be applied more broadly.
http://wiki.talkbass.com/index.php?title=Category:Ampeg_Portaflex_wiki
Figuring out how to service your own equipment and being less dependent on a tech is a very good thing. You learn by trying.
There's some good basic info in the portaflex wiki that Corey referred you to. There's a thread here that you might find interesting.
I don't know where you are in terms of experience but building a pedal either from a kit or from scratch is a great way to start. You can modify it and see what effect changing various components has. What affects the low end and how to make it sound optimal. It helps you to gain experience with soldering as well. At a higher level, buying an old amp or preamp and restoring it is also a great learning experience. It allows you to look at all the different parts of the amp. That includes electronics as well as the cabinet and retolexing work.
When you feel comfortable with what you can do, you can move on and tackle your more valuable gear. There are a lot of folks here that can offer support so feel free to ask.
For the record in any of the tube SVTs there is no such thing as a pair. bias is adjusted, in most, by adjusting one side and then the other. If a single tube goes down you need to match a tube to the others on that side and adjust the bias appropriately. If you cannot match then you NEED to replace the triple on that side and redo the bias to suit.
Also for the record I went to engineer school when tubes were current technology and transistors were a science that would never last!!
Ha! Well, by the mid 70's course focus at my school was designing IC's (not designing with, but designing the actual IC's themselves). Tubes were never mentioned, which one of my older professors lamented, as he believed learning about tubes first would give a good foundation in electronics.
So, not being a guy who plays around much with "glowFETs" but who is very comfortable with BJTs, I would agree: match to the side. If there are three tubes on a side, I would match their cathode currents, to ensure that DC power dissipation in each tube is the same (or close). Otherwise, you could have one of the three tubes bearing the brunt of the total plate current (and thus operating near its limits) but not the other two.
But what about matching tube transconductance?
If this were a transistor amp I wouldn't worry about transconductance at all, because with emitter feedback the gain equation is no longer dependent upon it (voltage gain no longer is gm*Rl but becomes Rl/Rc).
Now, I'm assuming that in a tube amp with cathode feedback (i.e. no cap across the cathode resistor), transconductance again becomes less important, and thus you select tubes with matching catchode currents, rather than transconductance. But here's my question: is this true, and if so, how less important does gm become?
(Let me add -- I'm wondering about this because I'm unsure how much of a factor the small-signal plate resistance of a tube (the value of which might be related to gm?) is in a tube-amp's gain equation. In a transistor amp, small-signal collector resistance can usually be ignored.)
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Matching tubes for cathode current at the working plate voltage AND matching transconductance between tubes is important. You are correct, the SVT is not cathode biased, it is what is called fixed biased.
Although tube matching standards are available, tube resellers match the tubes themselves do not adhere to matching standards. They do whatever they want. Matching can very from laboratory grade instruments with stable power supplies to someone with a tube tester. Check out the standards used here. More details of how fixed and cathode biasing affect the tube matching are here. Their power tube matching process is described here. Their process is more the exception in the industry rather than the norm. There are other good resellers but they all tend to be vague on their process. Manufacturers tend to use test jigs and dedicated equipment and some even follow military tube testing standards. An overview of tube matching is discussed in the TB Portaflex Wiki here.
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Thanks very much! But this statement implies to me that cathode feedback doesn't significantly affect dependence on device gm. So I'm wondering why doesn't the cathode feedback make gain independent (or significantly less dependent) upon device transconductance? Or is there something else going on?
Thanks for the encouragement beans. I have done some modding and repair on guitar tube amps like replacing burnt out resistors, changing tone stack cap values to see what happened and even a full on mood of a peavey windsor head. I love doing stuff with electronics.
Cathode bias, not feedback. In cathode bias the tube self biases through a resistor and bypass cap. Lower power option with interesting distortion character, no feedback loop. Basic triode to a transistor; Plate-collector, grid (control)-base, cathode-emitter.
There is something else going on. In part it has to do with how the tubes interact with the transformer and noise. Check out this link at the end, the paragraph starts with "Transconductance matching is important for AC (signal) balance in the output stage." There are other useful points made in this document.
To quote the same source again, here is some additional information. Look for "Q: I was wondering if you can answer a question regarding balanced triodes? What would you consider more important a characteristic for determining balance between two triodes, transconductance or amplification factor? Or is there something else that is even more important?"
In the operation of the amp, there is what happens in the steady state but when you start playing and an ac signal is applied to the input, everything changes. The big cap (used in musical instrument amps) and parallel resistor help maintain the bias point. The circuit is dynamic and frequency dependent. How the circuit reacts depends on the size of the cap. Short the capacitor and you have a certain gain. The plate load impedance and cathode impedance, which are dynamic, provides the gain. There is the nonlinear nature of the cap and there is latency in how it responds. A 50uF, which is often what we see in small amps, has an infinite impedance at DC and it comes down as the frequency goes up over the useful range of the amp. The bias point is going to vary, the gain is going to vary. The cathode bias is not going to hold the gain rock solid. I didn't touch on shift in impedance at the plate as demands are made on the power supply. There are other factors that affect how the device functions.
It's very complicated. So much so that solid state designers still haven't been able to figure out the dynamics of what the tubes are doing to replicate them perfectly.
Geez Guys!! My heads too old for all this thinkering and trying to remember stuff!!
It doesn't half hurt does this sort of stuff. I can do with a cuppa.
Randy, your post about pairs above ignores what happens when three differing tubes are on each side. one of the tubes will pass more current than the other two in the triple. It does more work. That means that it will wear more than the others. In extreme cases it will blow first if driven too hard. That puts you right back at the point this thread started: one dead tube. Do yourself a favour and think triples not pairs. As I said earlier, there is no such thing as a pair in an SVT.
i agree which is why I'm supping my big cup o' Earl Grey! (What's good enough for Captain Picard is good enough for the likes o' moi.)
My head's got a hole the size of a lemon where brain should be. I hate MRIs!!
I'm lucky 'cos I can still function.
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Thanks for the feedback. I think i may have worded what i said a little over confidently. I meant it literally as a question as in "would it be more correct". I see now that i was way off. I was trying to match the sides to each other but forgetting that it is mostly important that the tubes within each group match each other and not as important if group a doesn't match group b.
okcrum
in your chest
Right. Older SVTs had an "AC balance" or "Balance" adjustment to compensate for this slight imbalance between the tube groups (and also their driver tubes). It disappeared in later SVTs (at least the SVT-CL) because it was not considered necessary IIRC.
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