Pulling tubes

[DanKraw]

Hey guys,
During a discussion about amps with a co-worker he mentioned that if I want to lower my output and achieve tube saturation at a lower volume I can remove two of the tubes from my 6260's power section. I don't doubt this is true, but I'm wondering if there are specific power tubes to remove and is it potentially harmful for my amp? Thanks in advance.

 

[TheWinterSnow]

yes it will work however you have remember, pulling out the tubes to get half power also double the impedance and as a result halves the power dissipation for each tube, so you will have to rebias your amp hotter (when you pull two of the tubes out, each tube will be running at half power, reducing your amps total output to a quarter power (and tons of crossover distortion)) and go down a notch on the speakers impedance (the amp selector switch) If you are using the 8 ohm tap, it is now 16 ohms which means you need to use the 4 ohm tap into an 8 ohm load to truly get 8 ohms.

To properly do the mod, check your plate voltage on your power section then take two of the tubes (either the inner or outer tubes) and then place a bias probe on one of the remaining tubes. Select the correct impedance for your cabinet, remember your going down to the next lower tap. Fire up the amp and get the current reading, the not to exceed current will be the wattage per tube (6L6 is 30 watts) divided by the plate current (I = P/V) where I is current, P is power (of the tube) and V is voltage (of the plate). Then take 70% of that and that is where your current should be biased at. A 500v amp (like a 5150) is 60mA Do Not Exceed @ 100% and 42mA @ 70%, so most likely it should be the same with the 6260, but I am not sure.

The bias current will change depending on the load you have on it (the OT impedance tap and cabinet load) so make sure the Impedance is set correctly or your bias will be incorrect regardless of your readings.

 

[DanKraw]

Thanks for the info. Unfortunately I don't have a voltmeter, so I can't really adjust my bias at the moment. You were saying that I have to change the impedance that I'm using, so should I just set my cab and head to 4 ohms? My mistake, the cab isn't switchable between impedances, it was a Mono/Stereo switch. :S

 

[BLUElightCory]

Not to mention that pulling two tubes really doesn't reduce the volume by very much. IMHO it's not worth the trouble.

 

[TheWinterSnow]

yeah I was saying that when you take two of the tubes out, due to the nature of parallel components, the OT provides the total impedance to only two of the tubes (when they normally would be supplied to all four tubes equally) making the output current from the tube half of what they would normally be. To compensate, you need to drop the secondary OT winding down to the next tap so that each tube will see the same amount of resistance as if all 4 tube where installed.

For example, if your cabinet is 8 ohms total and you have your impedance selector set to 8 ohms, you are balanced only when you have all 4 tubes installed, when you remove two of them, you have to go to the next lowest tap. Like in the first example, if you have an 8 ohm cabinet, in order to make your amp (or tubes) think it is seeing 8 ohms at the cabinet, you need to set the impedance selector to 4 ohms.

Not to mention that pulling two tubes really doesn't reduce the volume by very much. IMHO it's not worth the trouble.

yes it only lower the maximum volume by 3db, however it dramatically reduces the perceived volume before you start hearing power tube saturation. So if you want crank sounding tone at lower volumes, removing tubes is a cheaper alternative than getting an attenuator.

 

[DanKraw]

Cool, thanks for the help guys. I just tried the head with 2 power tubes and then went back up to 4, it was a noticeable difference, but it wasn't all that much quieter. I think I'll stay with 4 power amp tubes and down the line maybe I'll pull a couple out or just get an attentuator. Thanks again, your information answered my questions.

 

[TheWinterSnow]

Cool, thanks for the help guys. I just tried the head with 2 power tubes and then went back up to 4, it was a noticeable difference, but it wasn't all that much quieter. I think I'll stay with 4 power amp tubes and down the line maybe I'll pull a couple out or just get an attentuator. Thanks again, your information answered my questions.

the sound difference is the result of the tubes being biased colder (you are getting crossover distortion) a 100 watt amp with all 4 tubes will only get 25 watts with only 2 of the tubes.

As you said the volume has not changed, however, the volume when you begin to reach power tube saturation is greatly diminished and you can get that cranked tone at much lower volumes.

If you want the tone to stay the same regardless if you are using 2 or 4 tubes, you need to bias each setup correctly.