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In Reply to: Re: I wanna tube! Please advise posted by jenya on November 02, 2001 at 12:44:09:
"You can imagine that if you drive a second tube with an amplified signal, you will even further amplify that signal."I can see this being the case. It just seems that the first stage APM'd signal should go to the cathode and not the grid....the grid input just seems to be a modulation signal (thereby not needing to be a strong signal). This is what confuses me.
"Another thing is that not all tubes are designed for amplification. For example, output tubes usually don't have great amplification factor but are capable of conducting fairly high current that can be routed through an OPT. Therefore, all amplification has to be done before that stage."Now this sheds a LOT of light on things! This is something I can wrap my mind around. Is this what is happening with so many of the SET designs where there are only two tubes in the circuit?
"I am sorry, if I keep confusing you."
NO! hehe...you're doing great! I seem to be the one confusing myself, if anyone :)
Follow Ups:
"I can see this being the case. It just seems that the first stage APM'd signal should go to the cathode and not the grid....the grid input just seems to be a modulation signal (thereby not needing to be a strong signal). This is what confuses me."I think I understand the way you think about it. Amplifier doesn't produces power, it takes a power from a power supply and, as you point that out, modulates it with the signal. Try to think of a tube as a kitchen faucet valve. One end of it is connected to a high pressure water line (B+), another points out to the sink (ground). Now, you are putting your hand on the twisty part of the faucet (grid) and while listening to your favorite music, keep opening and closing faucet. There you got it, an amplified signal in the shape of the water stream. The same happens with electron stream. Valve is sitting between high potential (B+) and the ground and by adjusting the grid voltage more or less current goes through it. All the power, however, comes from the power supply. In multistage circuit, the last valve is the one that controls the output power of an amplifier. For it to conduct a lot of power, its grid voltage has to be swung over a very wide range. In order to achieve that range, the previous stages are used.
"Now this sheds a LOT of light on things! This is something I can wrap my mind around. Is this what is happening with so many of the SET designs where there are only two tubes in the circuit?"
I'd say yes.
Jenya
"In multistage circuit, the last valve is the one that controls the output power of an amplifier. For it to conduct a lot of power, its grid voltage has to be swung over a very wide range. In order to achieve that range, the previous stages are used."Ok...so would I be correct in saying: The higher B+ is (greater Voltage), the higher the modulating voltage MUST be on the grid?
If this is the case, then it would seem that it would be VERY near impossible to have a line level input modulating a SINGLE high(er) power amp (no in between stages)...you kinda have build up the modulated signal until it's big enuf to actually properly 'feed' the main power amp tube.
Groovy...I think I'm finally getting this!
"Ok...so would I be correct in saying: The higher B+ is (greater Voltage), the higher the modulating voltage MUST be on the grid?"Even though I understand your logic, I wouldn't call that assumption a rule. It depends on the bias voltage and limiting factors of the valve. So, let's say your tube's bias is set to -70V. Roughly speaking, you can swing grid plus-minus 70volts. You see, you can't allow grid to go positive, otherwise it will start sucking electrons on it and you'll get grid current. Anyways, I am pretty sure you can find two setups at which one of them may have lower B+, but use wider grid swing. The important thing is that you are getting the idea how this work. If you think of the kitchen faucet example, you can imagine that if you have a faucet open for a certain idle current, the amplitude of the swing around that point is determined by either the distance from idle current to a complete shutdown or the maximum available current. Beyond any of these points you will get what's called 'clipping'. This is a fairly loose analogy, but similar boundaries exist for valve. One is grid going positive, another is complete current shutdown with grid being very negative.
"...you kinda have build up the modulated signal until it's big enuf to actually properly 'feed' the main power amp tube."
Exactly!Jenya
I see where my logic was too generalized now.One thing that I've been pondering over recently is power supplies....
Am I mistaken or is NOT the purpose of a PS, in this case, to supply a clean DC signal? Some of the PS's I've been seeing are SO esoteric and EXPENSIVE to build! more than the amp itself many times!
I almost get the impression that different circuit topologies actually require a special type of PS. Please tell me this isn't the case ;)
I don't think you need to worry about esoteric power supplies right now. A power transformer, tube rectifier, a filter choke or two, few capacitors will do fine. Of course, you could go with SS rectifiers. Cheap, easy to make. Personally I have a strong preference towards tube rectifiers.Jenya
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