I'm going to run a pair of Sylvania 6V6gta at about 420V on plates, basically a Fender Deluxe Reverb power section (about 22W 6V6 pp power amp). The point is I wanted to use diode rectification, but it seems it's not possible doing this (even with right bias). It can be done with tube rectification, but I've read it's bad for the 6V6's with diodes. Must be something to do with max current through output tubes at max power, don't know well, really.
Any suggestions?
Any suggestions?
The only issue I can see is if you have a tube rectifier design and you switch to SS without compensating in the power transformer, you will not be running the 6V6s on 420V B+, but something higher as the tube rectifier has tens of Volts of diode drop.
If you choose the proper transformer to set your B+ at 420V, it should not matter. You amy get more rectifier noise with the SS recifiers, but snubbers and proper filtering should take care of that.
If you choose the proper transformer to set your B+ at 420V, it should not matter. You amy get more rectifier noise with the SS recifiers, but snubbers and proper filtering should take care of that.
Slideman: Model up your power supply both ways in PSUDII; that will show you the difference between SS and tube rectification, and you'll be able to quickly determine what transformer secondary voltage you need for SS rectification vs. tube rectification.
One other difference between the two is that SS rectification will be instant B+ to the tubes (no slow warm up as with tube rectification), and the SS rectification will provide lower PS output impedance, which in a guitar amp, may or may not be a good thing if you want voltage sag at high output.
One other difference between the two is that SS rectification will be instant B+ to the tubes (no slow warm up as with tube rectification), and the SS rectification will provide lower PS output impedance, which in a guitar amp, may or may not be a good thing if you want voltage sag at high output.
Could it also be why guitarists believe that it is normal to change tubes every year?
In those old days (well perhaps today also) the amps with the biggest number sold easier than the one with lower numbers. So squizing 22-25watts out of a pair of 6V6s ment more sale. It may also be the it gives preferable tone, but certainly it is possible to design with tube's life in mind in these days of mic'ed cabs and megawatt PAs.
So my suggestion is why not make it into a cathode biased version and live safe?
For a given B+, cathode bias is a little easier on the tubes since there is a voltage drop across the cathode bias R; this makes the voltage drop across the tube less than the same B+ with fixed bias. That's also why you get more power out of fixed bias for a given B+, since all the volttage drop is across the tube.
+1 to what toyboy said...Also the tube's bias automatically adjusts for an unatentional increase in plate current, preventing thermal runaway.
Another thing about pentodes/beam tubes, I know for HiFi the screens should be fixed bias since it supposedly sounds best, but this risks high screen current. Having individual 500ohm to 1kohms (preferably carbon comp) screen resistors both give nicer compression and prevents the runaway posibility you have with fixed.
According to Tomer ('getting the most out of vacuumtubes') fixed bias should not be used, neither at the grid nor the screen, if tube life is a concern.
Another thing about pentodes/beam tubes, I know for HiFi the screens should be fixed bias since it supposedly sounds best, but this risks high screen current. Having individual 500ohm to 1kohms (preferably carbon comp) screen resistors both give nicer compression and prevents the runaway posibility you have with fixed.
According to Tomer ('getting the most out of vacuumtubes') fixed bias should not be used, neither at the grid nor the screen, if tube life is a concern.
You can get as much power out as you like (within reason) with either fixed bias, cathode bias, SS rectification or tube rectification.............any combo you want.
SS rectification will provide a higher B+ (for a given transformer) than tube rectification, and fixed bias will use more of that B+ than cathode bias to make power. So pick your bias and rectification scheme to determine what voltage you need from a power transformer.
Look at typical operating conditions for 6V6 at Duncan's TDSL:
TDSL Tube data [6V6-GTA]
looks like 18-22W is pushing it a bit........
...its pushing it but lots of guitar amps get up to 30watts out of two 6V6s. Like I said, must be why guitarists are used to changing tubes so often.
I get 18watts out of cathode biased 6V6s with slightly less than 400v. Plenty of power for most situations unless using inefficient Jensen alnicos.
Hey, that caught my attention! Would you send me all the info about that amp? That's the point, using high efficiency speakers.
The point was getting about 20-25W from a couple of 6V6's, fixed bias (because I thought it was more efficient) with diode rectification, with something like 430VDC supply (between 415 and 425V plate voltage) but that doesn't matter at all. I don't know how to use curves graphics, that's why I'm asking!
Guitarists change tubes that often for the same reason race car drivers change tires a lot. If race drivers motored sedately down the street to the store and back, their tires would last longer too. RUnning the tubes that way results in a sound we like, and amps work hard for a living. We don;t think changing tubes annually is normal because we are stupid, it is because they naturally wear out being used that way. We could build a similar output power stage of much more robust tubes, but then it would not sound the same.
First off, you would be hard pressed to tell the difference between 18W and 22W. We are talking a fraction of a dB. Stand one foot closer to your speaker.
I think you are missing the distinction made earlier between cathode and fixed bias Given the same voltage, say 400v the fixed bias amp will deliver more power into the same load as the cathode biased amp is using part of that 400v across the resistor, say 30v. Now if you had a cathode biased amp that ran on 430v you would still have 400v across the tube as you would a fixed bias amp but 30v would be on the resistor.
Another factor is that fixed bias amps are usually biased around 70% of the tube's dissipation while cathode biased amps run around 100%. This is partly due to the bias shifting colder in a cathode biased amp in clipping. Rather than worrying which amp design gives you an extra watt or two (which really does not matter much sound pressure wise) think more in terms of the type of amp sound you want. That is more important and is a better reason to select one design over the other.
I doubt you would be getting 25-30w out of some 6V6's unless they were in Class B and you liked the sound of crossover distortion (it does have its place).
I think you are missing the distinction made earlier between cathode and fixed bias Given the same voltage, say 400v the fixed bias amp will deliver more power into the same load as the cathode biased amp is using part of that 400v across the resistor, say 30v. Now if you had a cathode biased amp that ran on 430v you would still have 400v across the tube as you would a fixed bias amp but 30v would be on the resistor.
Another factor is that fixed bias amps are usually biased around 70% of the tube's dissipation while cathode biased amps run around 100%. This is partly due to the bias shifting colder in a cathode biased amp in clipping. Rather than worrying which amp design gives you an extra watt or two (which really does not matter much sound pressure wise) think more in terms of the type of amp sound you want. That is more important and is a better reason to select one design over the other.
I doubt you would be getting 25-30w out of some 6V6's unless they were in Class B and you liked the sound of crossover distortion (it does have its place).
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