Peavey Ultra 60 bias mod

[chunkitup]

Hey I want to bias mod my ultra and I am including a hand drawing of the circuit. Should I just replace R99 with a trimmer in series with a resistor that will roughly add up to 47k across C66?

[Enzo]

No.

With this supply, you cannot make the bias voltage any higher without building a new power supply for it. Yuo can however easily reduce the bias voltage, which I assume is your goal anway


The bias supply appears across C65. The voltage at BIAS is going to be just about the same. R98, R99 form a voltage divider, but due to the low resistance of R98, not much dividing takes place.

The voltage at BIAS will be a percentage of the supply determined by the ratio of R49 to R99+R98. In this example then BIAS would be 47,000/(47,000+470) times the supply. Well, 47,000/47,470 = .99, so BIAS will be .99 what supply is.

Now if I for example cut R99 in half - 22k maybe - the story changes to 22,000/22,470. That fraction is .98. .98 times supply is not much different from .99 times supply. If the supply voltage at C65 was 60 volts, the difference between the two would be about half a volt.

For proper voltage division, you need to increase R98. I use about 3k as a starting point. Now we have BIAS = supply x 47,000/50,000. 47/50 is .94, so if supply is 60v, then Bias will be 56 volts. If we then drop R99 to 22k, the story becomes BIAS = supply x 22,000/25,000. Starting with 60v we then get 53v BIAS

MAke R98 4.7k and leave R99 47k. Assume 60v supply. BIAS = 47,000/51,700 x 60v = 54v. Now drop R99 to 22k. BIAS = 22,000/26,700 x 60 = 49v. That change in R98 cave us a couple extra volts range

The idea of replacing R99 with a resistor and pot is sound, but R98 has to change to make it do anything.

My examples are still limited range. What if we made R99 a 10k resistor and a 50k pot in series? Then R99 can vary from 10k to 60k, depending on the pot setting. Still using 60v (obviously you would use whatever is across C65 in your own work) and R98 as 4.7k:

BIAS = 60k/64.7k x 60v = 56v.

BIAS = 10k/14.7k x 60v = 41v.

Now we are starting to get some range here.

You can experiment with values to get the range you want. Just understand that the BIAS voltage is the result of the R98/R99 voltage divider.

[chunkitup]

Just printed everything you wrote and I am on my way to the bench. Great stuff. Thanks

[Gtr_tech]

This should be made sticky due to the FAQs about converting non adjustable fixed bias circuits to adjustable.

[Enzo]

Aw shucks...

One of my pet peeves is the Gerald Weber approach to describing mods. He tells you to replace this and that resistor and move some wire here to there. Never once explaining what this does in a circuit or otherwise increasing anyone's understanding.

So it is a personal mission to spread the word that electronic fundamentals answer so many questions. Fundamentals like Ohm's Law and voltage dividers.

[voxrules!]

Aw shucks...

One of my pet peeves is the Gerald Weber approach to describing mods. He tells you to replace this and that resistor and move some wire here to there. Never once explaining what this does in a circuit or otherwise increasing anyone's understanding.

So it is a personal mission to spread the word that electronic fundamentals answer so many questions. Fundamentals like Ohm's Law and voltage dividers.

I am COMPLETELY with you Enzo, to me this is much the same as the old Chinese proverb " give a man a fish and you' ll feed him for one day, teach him to fish and you' ll feed him for a lifetime!". Your attitude is THE attitude.
Regards
Bob

[chunkitup]

What does it mean to "make it sticky"? I read all the time about converting 5150's.

[Enzo]

He refers to the online forum software here. Certain threads can be made to stick to the top of the list, instead of moving down as they age.

[chunkitup]

Here are some pics of where I am. The 10k in series with the 50k trimmer is under the heat shrink and I changed R98 to 4.7k. I had no room so I pushed the cap to the side just enough to get the leads through. The bias voltage read -50.7 before the mod so I used ohms law (now I'm fishing) and figured a supply voltage of 51.2 So my high bias voltage is 47.5 and my low bias is 34.8 but this not actual measured voltage. I want to get my 1R's in there.
My question is this: Viewing the pic of the power amp board how do I get two 1R2W resistors in there? Do I assume the tubes are matched and put one 1ohm resistor between the post(male) and wire(female)going to ground and divide by two? Suggestions please.

[Enzo]

Having done it a number of times, the circuit board does not make it easy to insert 1 ohm resistors. Puting a single resistor in series with that black ground wire seems the easiest route to me. Otherwise you are cutting a lot of traces and adding jumper wires to reconnect orphaned smaller parts to their ground connections.

[Gtr_tech]

I have a little gizmo I built that is an octal plug with the socket on the other end wired straight through (except for plate) and has the plate connections and voltage check brought out to banana jacks. I can read plate current directly through one meter while monitoring voltage on another. I don't like the cathode current/ 1r resistor method and there's alot of amps that make shunting one side of the output transformer nearly impossible. Really speeds up pwr tube changes.

[chunkitup]

Alright the bias range was right on but I am not seeing any change across my 1ohm resistor which I put from the post to the black wire running to the speaker jack. It reads constantly at 7.4mv. I hear a change in the sound for the better but I need to know the cathode current so I dont burn up the tubes. What could be the problem?

[Enzo]

Pull the black wire off the post. Now measure resistance from the post to chassis. If it shows low resistance, then there is still another ground connection on the board.

[chunkitup]

No, the resistance is quite high although it does not show as open. There is another ground for the filaments that goes through the ribbon to the main board but IS open til the black wire is connected to the post on the power amp board. I was reading about measuring across each half of the pri on the OT. It does not seem like alot of people like doing it this way. Suggestions on how to get this baby biased?

[Enzo]

Make or get a bias probe.


If your mod now makes the bias voltage adjust through a range, then the tube current should also sweep. What you need to do in my view is read the traces to make sure nothing is grounded in a way to interfere with the readings, and that your 1 ohm resistor is in fact between the cathodes and ground.

These pcbs were not designed with adding sense resistors in mind.

[chunkitup]

I have been contemplating making a probe. For now I didnt go under -45v and it sounds real good. Although the tubes get rather hot. I noticed on the schematic that there was a fan on this model but is not there anymore. I might start looking for a fan, if anyone knows where to get one. The tubes dont glow any brighter than any of my other amps that have the same JJ 6l6's. I wonder if you could explain how the ultra channel eq works. I can upload the schem if needed but Enzo I bet you got one. The top end was just nail biting so I changed the two 100pf on the edge channel to 470pf's and holy crap it sounds so much better. I also changed the 1meg grid resistor for the 2nd stage to 470k and it really tamed this beast down but still with over the top gain.

[Enzo]

It was designed as a gain monster. You can mod it to tame it, but don't be surprised if it fights back. Why you think they call it Ultra?

-45 is pretty hot for a Peavey, they usually spec at -55, so I don't doubt it runs hotter than usual.

There is a fan SUPPLY terminal on the schematic, but I don't recall fans on the Ultras. If there is one, I suspect it is on the 120 watt model, they used the same circuit board. These are the same 6L6s running at the same voltages and dissipations as every other 60 watt amp on the planet, and darn few have fans.

If you have to have a fan, look at the schematic, the fan terminal is just a tap off the +24v rail, so the fan needs to be a 24VDC model of a size to suit. Where you'd put it is another story.

Look up the schematic for the PV Triumph PAG60. You will note it is just about the same schematic as the Ultra. The power supply is the same, and the fan is from that. I suspect they just transferred the drawing and the fan came along. The Triumph PAG - the PAG stood for Parallel Axial Geometry or something like that - had the tubes sticking straight out the back horizontally, so they needed more help cooling. I don't think the fan came along for the ride when it became the Ultra. I could be wrong or course.

[Enzo]

I wonder if you could explain how the ultra channel eq works. I can upload the schem if needed

I have them all.

The clean channel EQ on these is the standard passive tone stack used all over. The Ultra channel EQ is an "active EQ" stage. Active EQ is more versatile, has more range, but can be touchier to dial in. Once you get it, it is real cool.

Active EQ involves a gain stage, in this case V3A. The output from the stage is from the plate, down through C24, and on down to relay K5B, where it exits the preamp. But after C24, it also taps through the three EQ pots. The wipers of the pots fedd through some RCs to set the freq bands and are then gathered together and fed back to the grid of the tube. The other end of the pots are tied together, and that point is where the signal from the gain channel post pots is injected. The pots then serve as controllable feedback elements for that stage. The RCs make them frequency specific.

[chunkitup]

Enzo, thank you so much. I have this amp sounding so good that my bro, lead guitarist in a band that was picked to play at the Harley Davidson 105th anniversary a few weeks ago has decided to use this head as his main amp. The only thing is I need to get the channel switching right. We cant find the footswitch which uses a 7 pin connector. The thing is he took off the reverb tank and only uses the clean and ultra channel. I want to put in a simple on off switch that goes between the clean and ultra channel. Can you help?

[Enzo]

You have several options.

Peavey may still have that footswitch - call PV parts.

Peavey, bless their hearts, will make up a footswitch for you for any obsolete model - call and ask price on that.

It is just a 7-pin DIN connector, get a plug and wire it to your own pedal. It is just simple relay control.

You can only get at the Crunch channel with a footswitch, so making one up or buying one would allow all three channels to be used.

[chunkitup]

I am ordering the 7pin DIN and I will do it myself with help. Why hand the ball off now!

[chunkitup]

alright so I have this thing sounding real good. I am going through and replacing the coupling caps with nice 715p's and I replaced the edge caps with 470p mica and that got rid of alot of the piercing high end. My question is this:
What purpose does having C20(.0022) and C21(.001) in the Body of the eq? What is happening when you roll thru the pot? Why doesnt the Bottom have any caps? Yes I am still learning and all experience has been with Marshall tonestacks. I do understand how the feedback works but the RC/freq specific portion is boggling me.

[Enzo]

I never thought about it. My expertise is troubleshooting. I never parsed that circuit out. You might call Gene Ford at Peavey ( or gford@peavey.com) or one of the guys in service and ask about it. I don't recall whether DUncan or AIken cover active EQ on their wonderful web sites. Add amps dot com to either one and study the materials there anyway. Look up active EQ circuits in general.

Chapter 15 in the RDH has some coverage - look under boosting with feedback

RDH can be found online various places. I downloaded mine from here:
http://headfonz.rutgers.edu/RDH4/ If you are not aware of this book, it belongs on your "shelf."

I have no idea what the center bands of each segment are, and I would expect them to be interactive anyway


There is no cap specifically next to the bottom control, but there is a cap in the circuit, C24. This is an active EQ circuit. The "standard" Marshall or Fender TMB tone stack is a passive stack. Regardless of the knobs, they are cut only. Active EQ can boost and cut. Active EQ requires a gain element in the stage. (Otherwise, how could it boost anything?)