& here is the most insane mod I have ever seen done on a 5150. 6L6 & KT88 together
Peavey 5150 and 6505 amps.
The 5150 and the 6505 are the same amps and they have an insane amount of gain in the preamp section. I?m going to be very honest here and say right up front that I do not care for the 5150 heads in stock trim. The power tubes while running just under 500 plate volts are only drawing about 10 to 12 milliamps of plate current. This means the amp is producing what is called crossover distortion and can not produce a good warm clean tone to start with. It also causes the amp to be sluggish and lack dynamic response.
The 5150 combo and the Ultra combo are another story. For some reason PV got it right and although the combo amps are also fixed bias they are fixed MUCH warmer and I can supply you with power tubes that will get the bias between 38 to 42mA which is right where they belong with NO bias mod.
If you check a stock 5150 100 watt or an Ultra plus 100 watt head using an oscilloscope and a signal generator which is the classic old school method of biasing a tube amp, you can plainly see crossover distortion in the wave forms until you get to about 33 to 34 milliamps of plate current then the sine waves look perfect. I personally like them set at 40 to 42mA per tube. Now you 5150 players who have played or own the 5150 combo don't have this problem, for some reason the combo's are biased much warmer and with the correct grade of power tubes they will bias up to a very nice 38 to 40mA with no need to modify them. This is why most players who have tried both the head and the combo, like the clean tone and the combo's dynamic response better.
There are a lot of amp tech?s out there that have attended technical trade schools or have taken a couple college courses and have no real understanding of how and why vacuum tubes work. They for the most part have never read or studied any of the classic tube manuals but most of them will be very quick to say, "Bias? No, you don?t have to worry about that, it?s fixed bias." So if you?re looking for a tech to bias your amp properly and you get this answer say, "Gee thanks!" and quickly move on to the next tech because they obviously know nothing about tube amps.
In July of 1999 I retubed a 5150 for a local guy named Jeff Miesen, better known as Jeffytune on the Peavey forum. The Peavey amp forum used to be one of the only real open forums on the net, but unfortunately this changed the summer of 2001 and now if you post something that Peavey doesn't like your post will be deleted. This is unfortunate as I did enjoy this forum. Now back to that 5150. Jeff was real happy with the JJ Electronic 6L6?s and ECC83?s that we loaded his amp with, but I have to say I still didn?t care for it. I tried to convince Jeff to let one of the local tech?s I work with to bias his 5150 but he was leery and didn?t want to part with the $40.00. (He was probably saving for another Peavey amp!) A few weeks went by and I couldn?t take it any longer, I really wanted to hear for myself what a 5150 sounded like with the power tubes biased properly so I called Jeff and offered to pick up his 5150 and take it to one of my tech friends for a bias job, free of charge! Jeff said yes! Two days later I picked the amp up and brought it home. It now read about 34 to 35 mA on the plates which was just out of crossover distortion. I played his 5150 for about two hours at all different settings. I liked it! It now had a good warm clean tone and the dynamic response was much improved. I promptly called Jeff to come get his amp. After hearing it he was impressed, a total success!
In August of 2000 I decided to mod a 5150 using an idea I was going to use on a 100 watt Marshall. The mod consists of a split dual bias circuit allowing the amp to run all 6L6?s or a pair of KT88?s in the outside sockets and 6L6?s in the inside sockets or any combination of EL34?s, 6L6?s and KT88?s. I called it the 5150ex for experimental. I mounted the two bias pots on the back panel for easy access, reworked the front panel and hard wired in two milliamp meters, one for the two inner tubes and one for the two outer tubes.
I tried all of the above tube combinations at lots of different bias settings, this thing is a great test bed! My favorite combination is a pair of JJ E34L?s in the center sockets set at plate current draw of 44mA and a pair of JJ KT88?s drawing 65mA in the outer sockets. At these settings the amp makes 113 watts before any clipping. In stock trim a 5150 is well into clipping at 100 watts.
This amp gets run hard multiple times a week and has well over FOUR years on it now with NO problems what so ever.
The 5150 ex is a beast! It also exhibits a very fat, warm clean tone. The distortion in the clean channel alone is more than I need! The lead channel is just simply over the top, way over the top! With it?s Zebra wood front panel and snakeskin covering it?s not bad looking either! Just about every local player that comes in for a retube wants to play it and I never say no because it?s great to see and hear their reactions. The Marshall players seem to be most impressed.
