Pull a POWER TUBE on your Deluxe...

[ESQUIREoholic]

Yes that's right - you can pull a Power tube on your Deluxe reverb and it is absolutely fine. You can run it like that forever if you want because of the way they are biased. Doesn't matter which power tube. Anyone who says you can't is probably thinking hi fi systems. You can pull a power tube on many of the original Fender line like a Twin Reverb - pull 1, 2 ,or 3 of the power tubes and it will work just fine without damage. You get more gain, lower volume and sounds cool for recording.

OK, now for the other takes on this .....

THIS POST was intended for starting a discussion. NOT an "apply these practices" doctrine. Sorry for any confusion I will be more careful in my future posts to arouse discussions.

 

[Colin Johnston]

Is there not a rule about pull an output valve (or 2 in a Twin) to cut headroom and you must double the speaker resistance?

 

[1293]

You should probably change the impedance of your speaker load appropriately, though.

 

[JohnnyCrash]

I've heard that too. Never tried it myself since I build amps at varying wattages for recording and don't want to play with fire (even if it is a myth).

Still, BE CAREFULL, it will change the impedance load you need... which can kill power tubes and/or output transformers if there is a bad mismatch.

 

[fezz parka]

No the bias will be way off one tube. It's not a single ended amp, it's a push-pull amp, and it's not designed to run that way. It's OK on Twins cuz you stil have a set pushing and pulling. Do not do this.

 

[ESQUIREoholic]

I knew this would spark up some opinions and discussion. I've asked this question to so many respected amp builders and many have told me that you can do this - while others not. I've heard the "pull 2 only" on the Twin but it is amazing how many thoughts and opinions there are on this. It would be great if we were all on the same page about this . Or it was written in a book well supported by evidence to the fact as a rule in using and working with tube amps. BUT NO, the advice and can do's and don't do's seem to ever be mixed up among the builders and musicians that use the tube amps...

I'll tell you what is kind of interesting are these Power Scaling boxes. Some builders are employing them in there amp designs. Any thoughts on these???

 

[BritishBluesBoy]

Here's an idea.... Why don't you pull a tube or two out of yours and let us know how you get on with it...??

Whaddya reckon??

BBB.

 

[Ben Harmless]

Umm... That second power tube handles half the signal. Not just half the power - but the "top" or "bottom" of the output wave. I've heard amps (not mine) accidentally run without one of two tubes, and they sounded pretty bad.

Maybe it won't destroy your amp, (I think it probably will over time) but your amp was designed to, and tries to use both tubes.

I have never heard an amp designer or technician reccomend this practice.

 

[ESQUIREoholic]

Nobody recommended it they just said I could. I personally haven't done it myself, but I thought it was a good topic for discussion. Sorry for any misleading....

 

[The Radium King]

i've done it. i have a 1/2 power switch (removes one tube on each side of the ot) and a single-ended to push-pull switch (removes both tubes on one side of the ot) on my twin (using cathode lifts). o'connor talks about it in tut1and tut2 and you are starting to see manufacturers use it on their amps (i think the orange tiny terror has a se-pp switch for 1/2 power). bias is unaffected unless you have a cathode bias situation with shared cathode resistors. in a fixed bias fender amp there's no issue; removing tubes will cause B+ and Vs to rise, but if you are biased at 70% it's no big deal. push-pull transformers can act as single-ended transformers no problem and with no effect on impedance (connected load on the primary side of the ot doesn't change). if you are removing the tubes on one side of the ot, common practice is to remove those on the nfb side.

removing one tube on each side of the ot in a 4 tube amp (the typical 1/2 power approach) will affect impedance. since the two tubes on each side of the ot are wired in parallel, removing one results in a doubling of impedance; your 4 ohm ot now wants to see 8 ohms (or 8 wants to see 16, etc.). ots can handle a 100% mismatch, so it's not a big deal. if you have a twin you can live with the mismatch, disconnect 1 speaker (you now have a matching 8 ohm load) or wire the speakers in parallel for 16 ohms (100% mismatch in the other direction).

power scaling is a bit different. this is an o'connor thing also, which he talks about in tut 4 and 5. i'm very unfamiliar with it, but i think that the gist is that you drop the B+ and Vs in the amp to achieve a power reduction. this is done at the power supply, so the voltage drop is seen in the phase inverter and the preamp also. the trick is keeping the power tubes in bias, as the the bias voltage doesn't drop linearly with the supply voltages.

 

[ESQUIREoholic]

Thanks
The Radium King,
That was interesting. Although I didn't follow all of it, I get most of it.
Thanks for the reply.

 

[Tremo]

Man, a DR running on only one 6V6 would have some nasty ass x-over disto.

More gain?? I'd love to hear how you came to that conclusion. Pray tell.

I'll pass, thanks.

 

[Paul G.]

I love the fact that we live in a world where anybody can make stuff up and if one person believes it, then we have a "controversy" or "discussion".


You can pull a tube from a 2-tube push-pull amp. If other modifications are not made, you WILL hurt the amp and it WILL sound awful.



You can stick you tongue on a frozen lamppost. Somebody told me it tastes like lobster.

Discuss.

p.

 

[1293]

I love the fact that we live in a world where anybody can make stuff up and if one person believes it, then we have a "controversy" or "discussion".


You can pull a tube from a 2-tube push-pull amp. If other modifications are not made, you WILL hurt the amp and it WILL sound awful.



You can stick you tongue on a frozen lamppost. Somebody told me it tastes like lobster.

Discuss.

p.

I just spent 4 1/2 very cold hours with my tongue stuck to a lampost. It DID NOT taste like lobster. Whoever told you that is a jerk.

 

[Mike Simpson]

Yes that's right - you can pull a Power tube on your Deluxe reverb and it is absolutely fine. You can run it like that forever if you want because of the way they are biased. Doesn't matter which power tube. Anyone who says you can't is probably thinking hi fi systems. You can pull a power tube on many of the original Fender line like a Twin Reverb - pull 1, 2 ,or 3 of the power tubes and it will work just fine without damage. You get more gain, lower volume and sounds cool for recording.

OK, now for the other takes on this .....

On a Twin Reverb or Dual Showman you can pull 2 tubes but it does matter which 2... You should pull the outside pair or the inside pair

I have never known anyone who would reccomend pulling one of a pair of tubes, the idea seems to be asking for problems.

 

[EMan]

To clear up a couple of things - P/P output circuits are in parallel (draw the equivalent circuit), and output impedance for any given tube in Class AB1 P/P configuration is given as PLATE to PLATE. This would indicate that the speaker load requirement would certainly be affected if a tube was removed.

Fundamentally, each tube in a P/P output stage runs independantly with a common Plate supply. Depending on the particular amp, removing a tube is generally not an issue. The OT is not designed for SE, and there are some limitations (no air gap in the core), and no power supply ripple cancellation, but this isn't cause for electrical damage.

MPO from reading a lot of statements from these type amp boards, is that most folks run their output sections closer to Class A than AB1. In that light, I'd prognosticate that the actual cross-over distortion isn't that bad.

It can also be a good tool for amp trouble shooting.

I have done this myself, on purpose and by accident, and have never found any sound reasoning why it would hurt anything. Would I run an amp that way? No, can't see any reason to do such a thing.

 

[ESQUIREoholic]

I hope no one thinks this whole thread was a Can Do discussion...

I merely was stating what I've heard or was told you could do, Not recommeneded, but could do with some of the Fender amps.

I personally have not done it with any of my amps over the years or my Deluxe Reverb, but I thought I could shed some light or different takes on this by bringing up the subject. That's why I wrote :

OK, now for the other takes on this .....

at the bottom of my original post.
Thanks for the reply's, infornmative results of pulling tubes , and opinions.

 

[Rob DiStefano]

I also think that pulling a power valve in the average two valve A/B amp is just asking for trouble. What greatness is to be gained at what expense? I'll stick with my stable of 5 to 40 watt tube amps and choose the appropriate one for the task at hand .... YMMV.

 

[Tim Armstrong]

I understand that a guy could also remove a sock and shoe from one foot and walk around in the snow. Yep, it sure is possible!



Cheers, Tim

 

[EMan]

Just an example - a friend of mine had an outdoor gig, and brought his TR. Some stage person dropped a push broom, and busted a output tube in his Twin. I told him to use two of the tubes that weren't busted in either the outside or inside slots. He did the gig...and never noticed the difference.

I've also had guy tell me that he was packing up his DR after a gig, and found 'a' 6V6 in the bottom of the amp. He didn't notice the tube missing either.

Just some real life tales....