Power Amp Tubes

**White Lightning** · 06.02.06, 05:41 PM

I blew a fuse (well, actually more like four) today jamming and finally took my 5150 combo to a service professional. He said the problem was that the power amp tubes are blown. I just changed my pre-amp tubes two weeks ago, and inspected the power amp tubes which looked and sounded fine. I replaced the Electro-Harmonix 12AX7 tubes with JJ's-leading to a killer sound. I am wondering two things. One, could the switch in pre-amp tubes lead to the power amp tubes to blow. Two, was the employee at the dealer correct in saying bad power amp tubes could lead to blown fuses? The fuses I kept putting in the amp were powering the amp until I flicked the standby switch on the 5150, which lead to another blown fuse and complete shut-off of the amp. Also, if it is the tubes, what brand power amp tubes are you guys currently running-just looking for a few free reviews from peers.

**HipCat5** · 06.05.06, 04:48 AM

The blown fuses when you turned on the stndby switch were probably related to the bad tubes in the power amp section. If the tube is over-conducting or even collapsed on itself, there's a dead short there and when the B+ gets applied (when you hit the stndby switch) then the fuse will be overdrawn and its popped. When you hit the main power switch, what you're applying power to are the heaters of the tubes ONLY and any other Non-signal path components. stndby applies the B+ to the tubes.

As for the new preamp tubes, nah, its pretty doubtful that those had anything to do with the blown power tubes. coincidence, really.

The things to be concerned about with power tubes are the voltage rating (6L6GC; do not get 6L6GB or 6L6 b/c they'll fail ... the voltage rating is too low for the 450+ volts on the B+; must get 6L6GC) and the bias of the tube. The 5150 doesn't have a way for you to bias the amp yourself without pulling a resistor and installing a pot & safety resistor in place of it, so you need to find Power tubes which will drop in and be biased properly. Someone once posted "5150 amps are biased to take a medium tube (a rating of 4-6 if it's a groove tube)" so you can take that info and see where that leads you.

Good luck

**White Lightning** · 06.05.06, 11:14 AM

I went ahead and ordered some JJ 6l6GC tubes, but I was wondering whether or not the amp needs re-biased for these tubes. I've never replaced power tubes before, so I was wondering what the purpose of biasing the amp is for. I was confused from reading in a previous forum topic about whether the 5150 combo needs re-biased with every power tube switch or not.

**HipCat5** · 06.07.06, 05:32 AM

Your confusion is well justified because it depends on what tech you ask when talking about the 5150. The long & short of all tube amps is that if the power tubes are biased using a negative voltage applied to the grid of the power tubes respective to the cathode ... then its a FIXED BIAS amplifier. If the tubes are biased using a resistor on the cathode, this is called CATHODE BIAS. FIXED BIAS is what the Peavey 5150 uses on the power tubes and even though the name sounds like you can't adjust the bias, often you can and SHOULD. The issue with the Peavey 5150 is that some bright engineer at Peavey decided to set the bias FOR YOU using a fixed resistor instead of a variable resistor (commonly called a potentiometer). They often do this because it costs a lot to install a pot with minimal drift so instead they just put in a metal film resistor of proper value and they leave it at that.

The use of the fixed resistor in the Peavey 5150 causes us issue because now the exact bias point of the replacement power tubes needs to be known prior to installation. Not all tube sellers know this and its not easy for the average guitarist to figure this out either. The answer to the problem is either to contact a seller who can sell you a matched pair of 6L6GC tubes which are selected for being proper for the Peavey 5150 ... or ... you can get a mod done to your Peavey 5150 to allow for proper biasing of any tubes you happen to purchase of type 6L6GC. Personally, I am going to have my combo modded for the variable resistor because I don't want to be over-charged for power tubes if/when they go bad PLUS I want the ability to set that bias myself for good tone & proper tube life.

The 5150 combo is different from the 5150 head in regard to the bias, from what I've read. The HEAD version seems to be biased very cold from the factory and thus has a bit of crossover distortion in the power amp section causing some pretty nasty tone issues (I played a 6505 in a guitar shop once and I can tell you, its not that great straight from the factory). The 5150 combo, for some strange reason, is biased better and thus has better tone. A bias mod on the combo is still a good idea to get the fixed resistor replaced with a variable resistor (and safety resistor) ... but its not 100% necessary from a tonal standpoint, esp if the power tubes are hand selected for the amp.

Final answer: Yes, all tube amps need to be rebiased if you're replacing the power tubes because to not do so could cause issues. In the case of the 5150, you have a few options for a few good reasons. Purists will tell you that you must re-bias the amp. The working musician will tell you that unless you're hearing a lot of crossover distortion or going through tubes every year, don't worry about it too much. A set of power tubes *could* last a really long time if they're biased a touch cold, but not so cold to make the tone terrible. Also, its very important to use 6L6GC on the 5150 because any other 6L6 type of tube will fail in short order due to plate voltage tolerances. 6L6GB is only rated for 400v, from what I understand. Using those would probably mean tube death in months or less and if the death of the tube is horrific enough, it might take your output transformer with it (not good).

Good luck - hope this helps
http://www.aikenamps.com/WhatIsBiasing.htm

**White Lightning** · 06.07.06, 08:11 PM

HipCat, you've been a godsend! Thanks for your advice. I chose to have the combo biased for safety's sake, and thanks to the link you posted, I know so much more about the process and purpose of the action of biasing amps. Thank you so much for your advice and help in preventing me from making a possibly huge mistake in this process.