Pulling Power Tubes

Animosity · August 17th, 2006, 08:28 PM

A question for Pro Junior operators out there. I'm currently unable to use my PJ above 3 or 4 due to my close proximity to neighbours and was searching the 'Net for info on attenuators. I happened upon this piece of advice regarding the pulling of power tubes to reduce volume.

Q: In a magazine Q-A, a player wanted to pull tubes to reduce power, but the "expert" said this would cause a meltdown of the remaining tubes. Of course, it was suggested that the expert's attenuator product was the preferred way to go. Is any of this true?

A: This is a person who should know better!

Removing tubes from a multi-tube fixed-bias output stage is never a problem. You can remove any number of tubes, and yes, that means you can take one tube out of a two-tube amp; one, two, or three out of a four tube stage, et cetera. This sounds heretical to techs stuck in the mire of convention, but it is something that has been known since tubes were invented.

The even-number tube extractions reduce power symmetrically. Neither the tubes nor the transformer will be damaged. Power will be reduced and so will frequency bandwidth - you will lose some bass and some treble. This is the point that switching the impedance selector to a less-than-load setting is supposed to correct, but it is completely subjective whether you should. The only 'should' of the matter, is do I like it this way, or do I like it that way?

In the uneven tube extractions, asymmetric power reduction occurs. Conventional thought says "the one tube on one side of the circuit will be trying to match the output of the two tubes on the other circuit half". This is wrong. The single tube can only produce so much power, and that's all it does. It doesn't melt down. The transformer does not blow up.

So, what's missing from conventional thought? The realization that tubes are "self-limiting power governors", which was stated in The Ultimate Tone (TUT), and explored in more detail in TUT2 and TUT3. TUT4 explores all of this in great detail. Our "expert" should get a copy.

In the end, you can pull tubes to reduce power, unless the amp is cathode biased - then you have to split the bias resistor. In any case, you do not have to worry about the impedance selector either.

Has anyone actually tried this, and if so, is it recommended ?? Any help or good advice would be appreciated as I'd like to drive the amp a bit more but without the corresponding volume increase.

Thanks.

Tremo · August 17th, 2006, 09:04 PM

Well, I see some errors in the above. Let's look at a typical BF/SF era Fender. First off, the output transformer is a fixed ratio impedance matcher. If you start pulling tubes on the primary side, you change the primary driving impedance, it goes UP. That will then require a higher load impedance on the secondary side in order to match the new primary impedance. Unless you are OK with running into a mismatch and sacrificing power that way.

Secondly, pulling one tube in a 2-tube push-pull stage really upsets the apple cart. The remaining tube is not really biased for class A, so you will run out of headroom real fast and get a real lopsided output waveform. also, since you no longer have cancelling bias currents in the OT primary, you are now putting a healthy magnetic bias into the OT, which will tend to make it saturate early in one direction.

the problem with leting the OT saturate, is when that happens, the peak currents through the tube go through the roof. Very hard on it.

Bottom line, it's your amp, your money, do as you wish. But you have been warned.

JohnnyCrash · August 17th, 2006, 09:05 PM

I'm not too sure on this anymore (it's been a while since I got rid of my BJR), BUT I thought the PJ was a cathode biased amp.

As far as the rest of what that dude's saying I always thought it was always bad to pull one tube in a two tube setup...

I'd go with Tremo's judgement on this one, he knows a ton about amps... he always helps me out (thanks again, as always, Tremo).

----

HOMEMADE ATTENUATOR FOR ABOUT $20:

Get yourself a 50watt (or 100w) 8ohm L-Pad at Radio Shack (it's meant for home stereo systems, some are clicking stepped, some are a large knob/pot types), then order a non-polarized electrolytic 5uF or 10uF capacitor (or both and a switch) and two 1/4" jacks from somewhere (like Mouser.com).

Wire it up and you've got a variable 8ohm 50 watt attenuator with caps that'll let the high frequencies pass unattenuated for about $20.

I built one (a 100w one) and put it in a small aluminum Hammond chassis. I use it to test for oscillation in high powered amps that are cranked (without having the neighbors call the cops on me) - and more importantly, I use it on my '50s tweed Twin head clone I built to get the coolest overdrive I've heard at manageable volumes :)

I learned about the DIY attenuator from some of the guys on 18watt.com.

You can see how "ghetto" my attenuator looks in my Photo Gallery, but you can really make yours cooler looking if you want!

3StringGuitar · August 17th, 2006, 09:36 PM

Wow Johnny Crash, that's awesome, I'be been really getting into building pedals and modding them and this seems right up my ally for my Sovtek Midget head. I'll have to give it a go one of these weekends.

JohnnyCrash · August 17th, 2006, 09:39 PM

Quote:

Originally Posted by 3StringGuitar

Wow Johnny Crash, that's awesome, I'be been really getting into building pedals and modding them and this seems right up my ally for my Sovtek Midget head. I'll have to give it a go one of these weekends.

Just be sure you wire it up right (you don't want that OT having no load on it). If your L-Pad doesn't come with a diagram PM me and I'll email you a diagram.

3StringGuitar · August 17th, 2006, 10:12 PM

Awesome thanks.

Tremo · August 18th, 2006, 02:21 AM

The Pro Jr. is fixed bias.

Animosity · August 18th, 2006, 06:46 AM

Thanks Tremo and JohnnyCrash for the advice.

Now, bare with me as I'm not overly amp knowledgable; until now I just turn 'em on and turn 'em up.

I also have a Marshall JTM45 half stack at home that's collecting dust due to the fact that I just can't quieten the damn thing. If I were to make a 100w L-Pad gizmo, is it going to be able to attenuate the Marshall as well or will it go into melt down due to the power output of the JTM ?

One more stupid question before I sign off, JohnnyCrash, can you please fill me in on using 5uF and 10uF caps in this thing ? I understand the L-Pad concept, but can't quite see where the caps come into it. A wiring diagram would help if you have one.

Thanks !

August 17th, 2006, 08:28 PM	#1 (permalink)
Animosity TDPRI Member Join Date: May 2006 Location: Australia Posts: 67 41 Photos	Pulling Power Tubes A question for Pro Junior operators out there. I'm currently unable to use my PJ above 3 or 4 due to my close proximity to neighbours and was searching the 'Net for info on attenuators. I happened upon this piece of advice regarding the pulling of power tubes to reduce volume. Q: In a magazine Q-A, a player wanted to pull tubes to reduce power, but the "expert" said this would cause a meltdown of the remaining tubes. Of course, it was suggested that the expert's attenuator product was the preferred way to go. Is any of this true? A: This is a person who should know better! Removing tubes from a multi-tube fixed-bias output stage is never a problem. You can remove any number of tubes, and yes, that means you can take one tube out of a two-tube amp; one, two, or three out of a four tube stage, et cetera. This sounds heretical to techs stuck in the mire of convention, but it is something that has been known since tubes were invented. The even-number tube extractions reduce power symmetrically. Neither the tubes nor the transformer will be damaged. Power will be reduced and so will frequency bandwidth - you will lose some bass and some treble. This is the point that switching the impedance selector to a less-than-load setting is supposed to correct, but it is completely subjective whether you should. The only 'should' of the matter, is do I like it this way, or do I like it that way? In the uneven tube extractions, asymmetric power reduction occurs. Conventional thought says "the one tube on one side of the circuit will be trying to match the output of the two tubes on the other circuit half". This is wrong. The single tube can only produce so much power, and that's all it does. It doesn't melt down. The transformer does not blow up. So, what's missing from conventional thought? The realization that tubes are "self-limiting power governors", which was stated in The Ultimate Tone (TUT), and explored in more detail in TUT2 and TUT3. TUT4 explores all of this in great detail. Our "expert" should get a copy. In the end, you can pull tubes to reduce power, unless the amp is cathode biased - then you have to split the bias resistor. In any case, you do not have to worry about the impedance selector either. Has anyone actually tried this, and if so, is it recommended ?? Any help or good advice would be appreciated as I'd like to drive the amp a bit more but without the corresponding volume increase. Thanks.

August 17th, 2006, 09:04 PM	#2 (permalink)
Tremo Banned Friend of Leo's Join Date: Mar 2003 Location: Silicon Valley, CA, USA Posts: 3,803	Well, I see some errors in the above. Let's look at a typical BF/SF era Fender. First off, the output transformer is a fixed ratio impedance matcher. If you start pulling tubes on the primary side, you change the primary driving impedance, it goes UP. That will then require a higher load impedance on the secondary side in order to match the new primary impedance. Unless you are OK with running into a mismatch and sacrificing power that way. Secondly, pulling one tube in a 2-tube push-pull stage really upsets the apple cart. The remaining tube is not really biased for class A, so you will run out of headroom real fast and get a real lopsided output waveform. also, since you no longer have cancelling bias currents in the OT primary, you are now putting a healthy magnetic bias into the OT, which will tend to make it saturate early in one direction. the problem with leting the OT saturate, is when that happens, the peak currents through the tube go through the roof. Very hard on it. Bottom line, it's your amp, your money, do as you wish. But you have been warned.

August 17th, 2006, 09:05 PM	#3 (permalink)
JohnnyCrash Poster Extraordinaire Join Date: Mar 2005 Location: Lost Angeles and Orange County Posts: 7,128	I'm not too sure on this anymore (it's been a while since I got rid of my BJR), BUT I thought the PJ was a cathode biased amp. As far as the rest of what that dude's saying I always thought it was always bad to pull one tube in a two tube setup... I'd go with Tremo's judgement on this one, he knows a ton about amps... he always helps me out (thanks again, as always, Tremo). ---- HOMEMADE ATTENUATOR FOR ABOUT $20: Get yourself a 50watt (or 100w) 8ohm L-Pad at Radio Shack (it's meant for home stereo systems, some are clicking stepped, some are a large knob/pot types), then order a non-polarized electrolytic 5uF or 10uF capacitor (or both and a switch) and two 1/4" jacks from somewhere (like Mouser.com). Wire it up and you've got a variable 8ohm 50 watt attenuator with caps that'll let the high frequencies pass unattenuated for about $20. I built one (a 100w one) and put it in a small aluminum Hammond chassis. I use it to test for oscillation in high powered amps that are cranked (without having the neighbors call the cops on me) - and more importantly, I use it on my '50s tweed Twin head clone I built to get the coolest overdrive I've heard at manageable volumes :) I learned about the DIY attenuator from some of the guys on 18watt.com. You can see how "ghetto" my attenuator looks in my Photo Gallery, but you can really make yours cooler looking if you want!

August 17th, 2006, 09:36 PM	#4 (permalink)
3StringGuitar Tele-Holic Join Date: Sep 2005 Posts: 623	Wow Johnny Crash, that's awesome, I'be been really getting into building pedals and modding them and this seems right up my ally for my Sovtek Midget head. I'll have to give it a go one of these weekends.

August 17th, 2006, 10:12 PM	#6 (permalink)
3StringGuitar Tele-Holic Join Date: Sep 2005 Posts: 623	Awesome thanks.

August 18th, 2006, 02:21 AM	#7 (permalink)
Tremo Banned Friend of Leo's Join Date: Mar 2003 Location: Silicon Valley, CA, USA Posts: 3,803	The Pro Jr. is fixed bias.

August 18th, 2006, 06:46 AM	#8 (permalink)
Animosity TDPRI Member Join Date: May 2006 Location: Australia Posts: 67 41 Photos	Thanks Tremo and JohnnyCrash for the advice. Now, bare with me as I'm not overly amp knowledgable; until now I just turn 'em on and turn 'em up. I also have a Marshall JTM45 half stack at home that's collecting dust due to the fact that I just can't quieten the damn thing. If I were to make a 100w L-Pad gizmo, is it going to be able to attenuate the Marshall as well or will it go into melt down due to the power output of the JTM ? One more stupid question before I sign off, JohnnyCrash, can you please fill me in on using 5uF and 10uF caps in this thing ? I understand the L-Pad concept, but can't quite see where the caps come into it. A wiring diagram would help if you have one. Thanks !