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Long time reader, new member. Have built a few fairly low wattage amps in the last 8 or 9 years and use them to gig with. For the most part they sound pretty good. I seem to have just enough knoweledge to keep myself out of major trouble. My question this evening is: what would be the best (safest) way to disable one output tube in a parallel single ended output stage?
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Common method - 'float' the cathode on the tube you want to disable by disconnecting it from ground.
-- disadvantage - screws with your OT impedance ratios. Not in a dangerous way.
Safest methods
- put a disconnect switch between the PI/driver (Yes, I know ParallelSE has no PI, but you know what I mean) coupling cap and the bias injector (if fixed bias). Make SURE the bias network stays connected
- if cathode biased, add a 220k resistor from grid to ground on the tube you want to disable. Pretend that's the bias injector and proceed as above.
I wonder if you could find an nP3T switch that would let you alternate the quiescent tube.
BTW - remember that cutting the watts in half only takes 3dB out of your volume. A less-efficient speaker/cab could do better than that.
Hope this helps! -
to disable one output tube
Well, the way I would do it, connect the screen to a toggle switch. Toggle the screen to ground to disable, and probably would not have that annoying "pop" if you tried to toggle the cathode..
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Hmmm... let's think about that for second, that's a potential way to blow your B+ rail or at the very least kill the screen voltage to the other tube at the same time and make a mess of everything.Originally posted by mooreamps View PostComment
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Why not simply give each power tube it's own cathode resistor & cap? Turn the amp off and pull a tube.
If you were being anal you could use a multitap OT to correct impedances but a 100% mismatch shouldn't be a problem, match the Z for 2 tubes, as you're expecting a loss of efficiency when using one tube anyway.Comment
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Huh, daft question, why not cuting the B+ to the tube??? Route the trany wire to teh first tube's plate, then to a switch, then to the second tube's plate.Comment
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"Huh, daft question, why not cuting the B+ to the tube???"
What will happen to the remaing tube's plate current, if they share a cathode resistor?Comment
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Originally posted by MWJB View Post
I admit i didn't think of this one, NR1 tube would dissipate twice the current? But for my defense, i would have two cathode resistors if it was mine, so i can use two different tubes
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I should probably expound on this a little. I'm building a cathode biased single ended el84 / 12ax7 amp not too unlike the P-1. I have prototyped it in a junk chassis and it sounds great; voxy and chimey at low to mid volumes and nice and creamy (but not gritty) when cranked. I was thinking about the parallel SE option for a little more headroom at slightly higher volumes and yes, I was planning on seperate cathode resistors /bypass caps for each of the el84s. Most of the rooms I play are smaller blues bars or bar / restaurant type places and I know I will get enough out of a rig like this. I'm using a 2.5K SE output tranny and it sounds fine with 1 el84 so it should be even better with 2. All of the replies had valid approaches to what I wish to accomplish, I guess I'll sort through them and determine which one fits my needs the best. The old "yank one tube" is no doubt the quickest and easiest but I would like to be able to switch it. Many thanks to all,
SlideComment
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Explain please!Comment
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screen voltage disconnect
As I understand it if you ground the screen it will turn off the tube. So as Gary was explaining you would disconnect the B+ from the screen and leave the B+ to the other tube unaffected and ground the screen.
Rickard Bergland (in performer's audio mag.) and Kevin O'Connor have also suggested using a pot after the PI to the input of one power tube to turn off the input signal to one tube but leave the voltages to the tube in place so it will act as a current generator. Kind of like half a post PI MV. This pot would allow varying amounts for signal to the tube all the way to a simulated SE. I tried it and didn't care much for it.
Marc
MarcComment
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Thanks Marc.
I do understand what's been said in this thread. I was being somewhat facetious, since I tend to agree with Bruce's reply. My first thought on this was "that would be a great way to dump the screen voltage on BOTH tubes".
Maybe I'm thinking about this wrong so, I kind of thought Gary might expand a little.
Actually, upon second look. I think I see what he means. The toggle would disconnect the screen from the supply and ground it. I was reading as "just ground the whole connection", as opposed to "break the + connection and ground it".Comment
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Removing the B+ from the screen and grounding the screen (with no B+ supplied) should be OK, but that isn't quite what Gary said.Originally posted by Jag View Post
Here is what was written so as not to confuse anyone:
"Well, the way I would do it, connect the screen to a toggle switch. Toggle the screen to ground to disable",
I was just pointing out that the B+ screen supply, grounded with the screen, could lead to a minor catastrophe and shut the entire B+ supply off to the rest of the preamp... even if it didn't blow something up.
I guess you could use a SPDT with the screen connected in the switch's middle lug, B+ on one end and ground the other or a DPDT switch where one side removes B+ and the other grounds the screen if needed...etc.
Still you'd need to address the unloading of the B+ rail and the impedance mismatch of the single power tube.
Neither of those two problems would be a big issue though.Comment
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AgreedComment
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