Peavey Mace Bias Mod?

[iwantmypie]

Hello all. I was wondering if anyone had any information on performing an adjustable bias mod to an old Peavey Mace. I've found information of the internet on people modding Butchers, 5150s, and Classics to give them an adjustable bias pot and get them working a little hotter than the cold factory settings Peavey seems to give most of their amp.

I was also wondering if doing something like this would mean I should change the screen grid resistors. A few months ago my 400 ohm power supply resistor burned up on me and I had to replace it. Just the other day one of my 100 ohm screen grid resistors gave out on me. Do all these heating issues have any relation to each other or even the bias for that matter?

Thank you.

[Enzo]

The screen resistor burnt up because that tube failed.

The 400 ohm resistor - which is between the plate and screen nodes - burnt up because a tube failed.

I have to think you have a Mace VT instead of a plain old MAce. The original Mace already had a bias adjuster inside.

To convert this or any fixed bias amp to adjustable fixed bias, all yopu do is convert the fixed resistors that form the voltage divider into variable ones.

In the case of the VT, the 68k 1R9 would be replaced with a pot and resistor in series. Maybe a 47k resistor and a 25k pot, or whatever.

[iwantmypie]

Yeah I forgot to mention mine was a VT combo model. Thanks so much for the info though. I had no idea I had a failed tube, I always just kinda looked at them and figured if they were glowing they were working.

Now that these problems seem to be answered, is it safe to say that the failed tube is the same one that is connected to the failed screen resistor?

[Enzo]

The orange glow is the heater (also called filament), and when you see the heater glowing, it tells you one thing and one thing only - that the heater works. Tubes can totally short out and still have a working heater.

ANy birnt screen resistor means the tube associated with it is bad.

I realize it is your money I am spending, and there are 6 of the tubes after all, but really. Why cheap oout on this monster amp? How about a fresh set of 6L6s in there? How old are the ones that didn't burn something up...yet?

[iwantmypie]

I bought the amp used about 3 years ago and I'm pretty sure the tubes were new when I got it as I remember they use to make clinking sounds like they were getting broken in or something. I don't really know if that's what it means or not but I figured so since they haven't done it for a while. The tubes are JJ and I really like(d) the sound of them. Only problem is I've been having trouble trying to find a matching sextet of them. I don't know how important matched tubes really are but I figured it'd be the best. This is my first tube amp and I'm still learning after a few years.

[Enzo]

Matched isn't critical, I just prefer them that way for hum balance. You won't usually see sextets on the shelf, but any tube selling company will make up a set for you.

[defaced]

Check out eurotubes.com. Bob imports JJs from the factory. If you tell him the amp you have, and he can get you a matched set of 6 without any problem.

[iwantmypie]

Awesome. Thanks a bunch for the help Enzo. Not to get too annoying but I have one last question

Would you happen to know an easy way to set up test points for when I want to adjust the bias? I've seen that some amps already have these installed and it seems to make it a lot easier. If not could you recommend the easiest method to get the readings I need on my multimeter? Thanks again.

[Enzo]

Not at all easy on this amp. The sockets are on a pc board, and all the pins 8 are tied together along with the pins 1 and 7. (The heaters have one side grounded) The normal lifting of the cathode for a 1 ohm resistor is terribly incomvenient here. it would require grinding away the copper around each pin 8. Nothing technically daunting about it, you could do it, but really, a pain in the ass.

I generally use the shunt method anyway, but I don't recommend that to a novice since it requires current measurements around the output transformer.

The easiest thing to do is use a bias probe. There are several versions on the market. it uses an adaptor you plug into a socket, then plug the tube into the adaptor. Bob not only sells sets of JJ tubes, he also sells bias probes. And then you could check any amp you encounter with it. There are simple adaptors used with your own meter, and there are more fancy ones that include their own meter, and some even fancier ones that do calculations for you.

But you know what? I wouldn;t bother. The power amp in this fine old Peavey beast is loud and clean, the tone of the amp comes from the preamp. If you simply install new tubes and play, it really will work just fine. It has weathered the last 25-30 years without adjustable bias. My opinion is that you could heat up the tubes some with a bias tweak, but wouldn't notice a lot of change.

[Gtr_tech]

You just may get away with just cramming in a new set of output tubes and getting on with your life, but proceed with caution.

After installing the replacement tubes, pwr up the amp and let it sit in stby for a couple minutes. Hit the stby switch, dim the lighting in the room and watch from the back of the amp for several minutes.The internal metal part of the tubes you can see through the glass (the plate) should not develop red/orange "hot spots". That is this metal part actually getting red hot due to too much current flowing through the tube. Normally if this happens it is near the area of the seam where the 2 halves of the plate are connected together by spot welds or crimping. If you see hot spots even on one tube shut it down....redplating tubes will eat themselves up in no time.

The other extreme is if they're running too cool the amp may sound bad. After being powered up or played for a few minutes, place your hand *near* the output tubes. You should feel a fair amount of heat radiated from them. If there's hardly any heat coming off them chances are they're overbiased and not really "waking up".

New production pwr tubes are so all over the map for bias requirements you really never know what to expect.

[Gtr_tech]

See this youtube video for an example of what to look for.
http://www.youtube.com/watch?v=qcy_5Wlr114

That is an extreme case. Even a small red spot indicates a problem.

[iwantmypie]

oh wow i never thought to check for that. from looking at that video it seems like red plating tubes are pretty noticeable in the dark. i will definitely have to check that out when i get the new ones. thanks!

[iwantmypie]

So I finally replaced the destroyed screen resistor in my amp and double check all of the other ones as well as caps and diodes and all that fun stuff to make sure they were still functioning. I figured if I do this mod I'll wait until I get some new tubes but I was wondering if it would be ok to run my amp without the two outer tubes for the moment? I recall reading somewhere a while back that the Mace's tubes run in pairs from the outside in. The suspected bad tube is one of the end tubes in the row of 6. Does doing this effect the amp in any specific way that I should know about before I power it back up again?

Thank you guys again for all the help you've been.

[Enzo]

There are no pairs. 6 tubes in a row. Three on the left are one side, three on the right are the other. The push and the pull if you like. You may remove one tube from each side and operate the amp. Doesn't matter which one. FOr that matter you could run on 5 tubes, but the output would be unbalanced. There is no way to step down the load by a third, so don;t try.

[iwantmypie]

so it's been a while but I finally got around to the mod that I asked about oh so long ago. I finally bought a bias probe since it was the easiest method for me to measure and bias correctly. Before I performed the bias adjustment mod I decided to take the probe and get reading for the stock setup.

Without any mods my plate voltage was around 519v. With the bias probe set up to my DMM I got readings of 5mA on 4 of the tubes, 9mA on one and 15mA on the last tube. From what I can gather this is extremely cold. I wish I knew more about tube amps when I bought this thing 3 years ago.

So I decided to do the trimmer mod and adjust it accordingly. As I assumed is the correct thing to happen. As I raised the bias, the plate voltage dropped. After much tweaking back and forth with the bias DMM and a second one to read the plate voltage, I got my plate voltage down around 490v and set the bias to about 35mA which was about 60% dissipation the max wattage for the tubes. I also decided to run the amp off the 4 tubes that were originally running around 5mA and ditched the other two. I know many people talk about biasing around 70% of the tube's max power but I figured I'd go a little colder for now. I know there probably isn't a correct number and 70% isn't a must live by number so I figured I'd just experiment to taste

It's already late here so I haven't had a chance to see how everything sounds so far but I left it on for a while and out of standby mode and I didn't notice any odd smells or insane heat. I did notice however that my tubes now have some blue glow to them which I did not have before. Not totally sure if that's good or bad, I'm just guessing its the gasses reacting differently to the hotter bias. I bought myself some new JJ 6L6GC s for XMas and I figured I'd get this new setup up and running before I stuck the new ones in.