A relative has too much power in their amp and wants to run two tubes in the output section rather than four. The transformer has a tap to take care of the impedance change needed in only running two tubes but the output tubes are run in series with six volt heaters running on 12v. I figure I could remove the plate and screen pins on some old tubes he has and pop them in there to complete the heater circuit. I would like to keep the grid and cathode pins in in order to give the tubes some mechanical stability but am not sure if electrons will collect on the grid and cause trouble when the amp clips. So is it safe to leave the pins connected or not?
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I knew that was coming. Well given the case, any other answers?... how the hell can someone have too much power output from a tube amp?
Isn't there a volume pot somewhere in the circuit? I would rather turn the volume down a little than castrate tubes or circuit...
Greetings,
Andreas
Sorry, could not resist.
But I still do not understand the need for disabling two output tubes. If it's 'too loud', use the volume control. If there's too much gain, as SY suggested, this should be cured by making the appropriate changes to the driver stage.
Could you explain in more detail why there is 'too much power'?
Greetings,
Andreas
4 X KT88 in PPP will do it
I assume that this is a guitar amp and the player likes the sound of the amp as it enters clipping / compression or as it operates just short of meltdown ( my preference)......however his neighbors don't.
The common path is a resistive attenuator between the amp and speaker, but it just doesn't sound the same. Turning off two output tubes is also somewhat common. In my amps I just switch the screen grids to ground to disable 2 tubes. That seems to work, but as suggested doesn't make the amp that much quieter.
Another path that works well in lower pwered amps is the VVR. It is a variable voltage regulator usually made with a mosfet that allows turning down the B+ to reduce the power and the point at which the amp enters distortion. It can be done on a big amp, but it will be a big mosfet on a big heat sink.
Another way, other than just running "dummy tubes" is to regulate the 12vdc after rectification down to the required 6.3vdc... or use a dropping resistor (you could put it
on an octal plug in socket (assuming an octal tube).
I'd be surprised if the output transformer "has another tap" - that would be unusual on the primary side. One could use another secondary side tap, IF there is the right one.
A bit more information on precisely what the amp is and what the taps are could get you some much more precise answers.
_-_-bear
EDIT, actually none of that is required. You just have to use the proper two sockets, or rewire the filaments so that the pair of output tubes used are in series. I am assuming they put one primary "side" tube pair with filaments in series, rather than sharing between both sides. But there's really not a problem that i can see in running tubes on opposite sides of the primary with filaments in series (not for DHTs of course). These are indirectly heated tubes. So, rewire the filaments if need be.
on an octal plug in socket (assuming an octal tube).
I'd be surprised if the output transformer "has another tap" - that would be unusual on the primary side. One could use another secondary side tap, IF there is the right one.
A bit more information on precisely what the amp is and what the taps are could get you some much more precise answers.
_-_-bear
EDIT, actually none of that is required. You just have to use the proper two sockets, or rewire the filaments so that the pair of output tubes used are in series. I am assuming they put one primary "side" tube pair with filaments in series, rather than sharing between both sides. But there's really not a problem that i can see in running tubes on opposite sides of the primary with filaments in series (not for DHTs of course). These are indirectly heated tubes. So, rewire the filaments if need be.
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4 X KT88 in PPP will do it
I assume that this is a guitar amp and the player likes the sound of the amp as it enters clipping / compression or as it operates just short of meltdown ( my preference)......however his neighbors don't.
The common path is a resistive attenuator between the amp and speaker, but it just doesn't sound the same. Turning off two output tubes is also somewhat common. In my amps I just switch the screen grids to ground to disable 2 tubes. That seems to work, but as suggested doesn't make the amp that much quieter.
Another path that works well in lower pwered amps is the VVR. It is a variable voltage regulator usually made with a mosfet that allows turning down the B+ to reduce the power and the point at which the amp enters distortion. It can be done on a big amp, but it will be a big mosfet on a big heat sink.
Let's not assume it is a guitar amp for now
but rather a theoretical question on tubes operating in an amplifier. While there are many modifications that can be done to reduce the power of an amp this is a quick, no modification method of cutting the power in half. Pop in the dummy tubes and switch the impedance, put in working tubes when the need for more power. Only 3dB but better than nothing, also half the cost of re-tubing the output.Want to leave the amp as it is, if it were my amp I would rewire the 12v so that the two sides are not in series but that each push-pull pair was in series. The amp has a jack in the back that allows an extra speaker to be used and this switches the secondary impedance. Plugging in a dummy jack will change the output ratio allowing the two output tubes to be properly loaded.Another way, other than just running "dummy tubes" is to regulate the 12vdc after rectification down to the required 6.3vdc... or use a dropping resistor (you could put it
on an octal plug in socket (assuming an octal tube).
I'd be surprised if the output transformer "has another tap" - that would be unusual on the primary side. One could use another secondary side tap, IF there is the right one.
A bit more information on precisely what the amp is and what the taps are could get you some much more precise answers.
_-_-bear
EDIT, actually none of that is required. You just have to use the proper two sockets, or rewire the filaments so that the pair of output tubes used are in series. I am assuming they put one primary "side" tube pair with filaments in series, rather than sharing between both sides. But there's really not a problem that i can see in running tubes on opposite sides of the primary with filaments in series (not for DHTs of course). These are indirectly heated tubes. So, rewire the filaments if need be.
The real issue is with removing a number of pins from the tube. I would like to leave as many in as I can and the screen and plate pretty much have to go. Not sure what the effect of retaining the grid and cathode will have on the amp, specifically when the grids are driven to 0v.
Rewire the heaters - the filaments.
Do not remove anything from the pins.
This leaves the amp FAR more original than removing connections to the pins.
As I said and the previous poster said, wire the fils so that each pair of PP tubes uses the same filament series string.
Done.
The grids and screens will NOT be driven if there is NOTHING in the tube socket.
No need to leave the tubes in the sockets. Remove them.
Otherwise the amp will be very non stock, wires will be removed from the sockets when none need be.
This way the other tubes can just be plugged back in whenever the owner wants.
Ok?
_-_-bear
Do not remove anything from the pins.
This leaves the amp FAR more original than removing connections to the pins.
As I said and the previous poster said, wire the fils so that each pair of PP tubes uses the same filament series string.
Done.
The grids and screens will NOT be driven if there is NOTHING in the tube socket.
No need to leave the tubes in the sockets. Remove them.
Otherwise the amp will be very non stock, wires will be removed from the sockets when none need be.
This way the other tubes can just be plugged back in whenever the owner wants.
Ok?
_-_-bear
I AM NOT REWIRING THE AMP THE OWNER DOES NOT WANT IT. As I said, the 'pins' will be removed from the worn out tubes the owner is supplying. Nothing internal to the amp will be changed! The owner pops in the modified tubes, plugs a jack into the external speaker socket, when he wants to go back to four tubes he removes the jack and replaces the modified tubes and installs regular tubes.
If it was my amp I would put in a VVR. Not going to happen. My original question was about the grids and cathodes. Heck, I'll just cut them off and be done with it.
Hmm, you still have not really answered the question that he guys have asked. You have said it is not guitar amp. What is the specific problem with 'too much power'. Is it too much gain? is it too much heat in the chassis? Is it trying to limit the volume the kids are trying to extract from the unit? What kind/type of amp is it?
We all love a puzzle, but the better information you give the better chance of getting an answer that is relevant to your problem.
Cheers,
C
We all love a puzzle, but the better information you give the better chance of getting an answer that is relevant to your problem.
Cheers,
C
Dude, he will NOT know that you moved 4 heater wires.
That way he can UNPLUG the unused output pair.
ASK HIM.
Unless it is some sort of major expensive collectible amp, it makes no difference.
You are not doing a serious mod, just moving the wires going to the heaters from
one place to another...
IF it is a major collectible amp, then leave it ******* alone.
And if you do the trick you say, DO NOT leave the plate or screen pins in place.
Seems silly to me, but I can't see the amp or your friend.
_-_-
PS the next guy who gets the amp will be scratching his head about the wierd tubes with all the pins cut off!! HA HA
That way he can UNPLUG the unused output pair.
ASK HIM.
Unless it is some sort of major expensive collectible amp, it makes no difference.
You are not doing a serious mod, just moving the wires going to the heaters from
one place to another...
IF it is a major collectible amp, then leave it ******* alone.
And if you do the trick you say, DO NOT leave the plate or screen pins in place.
Seems silly to me, but I can't see the amp or your friend.
_-_-
PS the next guy who gets the amp will be scratching his head about the wierd tubes with all the pins cut off!! HA HA
Guess you missed the little smiley. Yes it was for a non-hifi amp used with the intent to modify the signal. The problem is too much volume was produced as a byproduct of the signal modification. I was looking for an answer to a general question of what would happen when tubes were operated in that condition. How and when someone uses them that way is up to the user.
Found out the amp had tube retainers so the loss of a couple of pins was no problem. In the end there were two NC so four pins seemed enough for me. Thanks for trying it, I did not have time to throw together a circuit myself and we were meeting at my sisters for supper. Cut them with side cutters and filed them flat before supper.
First off he did not bring the amp. Second, I would not mod a piece of equipment without the approval of the owner even if I thought it would be a good idea and thought that he would not find out. That is just wrong. And it is ironic you telling me not to mod the amp if it is collectable after saying I should mod it behind the owner's back.
Please go back and re-read from the start. I said I wanted to remove the screen and plate pins from the tube but was wondering if I should remove the grid and cathode. I am sure the owner will either put in a full working compliment of tubes if he decides to sell it and may even throw in the modified set of tubes as a selling feature.
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