Randall RG100 Head ... power loss

[joecool85]

I'd go with the 0.27 and 0.62 ohm 10w resistors from mouser.com and be done with.  Should work perfect.

[kernalflagg]

I'd go with the 0.27 and 0.62 ohm 10w resistors from mouser.com and be done with.  Should work perfect.

You don't think the extra 0.02 ohms (+3.33% resistance) will make a difference? I think this would increase the rail voltage (+/- 48V) to ~49.6V. (48*1.033 = ~49.6 ... Is this correct?)

[joecool85]

I'd go with the 0.27 and 0.62 ohm 10w resistors from mouser.com and be done with.  Should work perfect.

You don't think the extra 0.02 ohms (+3.33% resistance) will make a difference? I think this would increase the rail voltage (+/- 48V) to ~49.6V. (48*1.033 = ~49.6 ... Is this correct?)

That shouldn't matter what so ever.  That's a very small change.

[phatt]

I'm with Joe :tu:
In my limited understanding R3 and R13 would be around 100 ohm, 1/2watt.
and R2 and R12 are the high current 5Watt Wire Wound main emitter resistors where anything from .33 ohm to .5 ohm would work.

This looks like a straight forward DC power stage with bootstraped output. I doubt a bit either way will effect it,, that said read your DC on the output when up and running to make sure it's close to Zero.
Phil.

[kernalflagg]

I'm with Joe :tu:
In my limited understanding R3 and R13 would be around 100 ohm, 1/2watt.
and R2 and R12 are the high current 5Watt Wire Wound main emitter resistors where anything from .33 ohm to .5 ohm would work.

This looks like a straight forward DC power stage with bootstraped output. I doubt a bit either way will effect it,, that said read your DC on the output when up and running to make sure it's close to Zero.
Phil.

From looking at the schematic, R4 and R14 are the 100 ohm, 1/2 watt resistors. R2 and R12 are 0.6 ohm/7watt power resistors, and R3 and R13 are 0.27/7watt power resistors. However, the actual schematic that matches MY amp has no resistor values marked on it. For the values, I am going off of ANOTHER scematic with the same type of power output circuit. However, other parts, such as the input stage are totally different as what is actually on my amp.

I can get the 0.27 ohm resistors no problem. But I cannot find 0.6 ohm ones without having to order umpteen thousand from Ohmite.

Getting back to the Mouser resistors:
For the 0.6 ohm resistors, I think 0.62 ohm is better because it will DECREASE the amps of current flow; thus reducing the chances of frying something. On the other hand, they do have 0.56 ohm resistors, but those will allow more amps to flow through the circuit, and will increase the chance of frying something.

I realize that it may not affect it much either way under most operating conditions... However ... what about at HIGHER volume levels?

Am I correct for assuming this? Or is it still not going to matter too much?

[phatt]

Hi again,

I understand your concern but really I doubt there would be any difference between .47/.56/.62
Let's put it this way if it did then something else is very wrong with that circuit and it would be considered unstable and bad design and would have likely died an early death.

Lets assume the amp WAS quite stable and may have run for many years without issue.
just throw in some .47 ohm 5 watt WW or close and test the thing.
Phil.

[joecool85]

I'd still go with the .62 ohm myself, but Phil is right, most anything close should work just fine.

[phatt]

Whoops?
            I should read before I open my mouth. LOL.

Those 4 output transistors are connected in a rather different configuration. (headscratch)
Normally they run them in parralel but this is not the case here. xP

If you are worried then you could just piggyback two 5watt units of double the ohms.
i.e. 2x 0R47 will reap 0R23,,close enough to .22 ohms, wink.

I just checked the output devices in my comparison chart book and found 2N6254 are compatible with 2N3055. (a common device)

Anyway having just re read the first posting again to refresh my memeory I'm struggling to understand that you said the amp worked but had issues then you say it stopped.


I can't see how the amp could have possibly worked with no emmiter resistors? :loco

Although the VAS stage may have been able to push a little wiggle out to the speaker even with those output devices removed from circuit but that would be very low output.

The fact that you have a burnt resistor on the board says the amp may have had a major problem and was never solved.

Anyway I'll just add,

The reality for the novice when it comes to discrete power Amp circuits is that 90% of them are high current DC coupled opamps and as such are hard to fix as when one device blows it often takes out several other devices.
(The DC current is very large so if it's got DC issues then it's going to blow something eventually)

Without the right gear (and a fairly good handle on what to do they can bend your brain but speaking from *my novice* expiditions into building many poweramp circuits that went up in smoke I did learn what to basically look for.

With a light bulb limiter wired in the mains or a couple of 47 ohm 5 watt resistors in place of DC rail fuses, Then With your meter set to read DCV, read the DCV on the 3 major points that will indicate the *DC conditions* of the Amp.

The basic aim of these designs is basically keeping the bases of Q8 and Q9*** as close to Zero DCV as can be done,, while Also keeping the output terminal very close to Zero VDC.
( *** Q8/9 on JM Faheys pics)

Those are the 3 points to test,, some designs will tolerate even 500mV offset at base of Q2 and at the output but if it reads over 1 volt then there might be a problem lurking.

The AC wiggle (your guitar) can then ride on this DC and wiggle up and down between the 2 voltage rails. (that's how Amplification works)
BUT if those DC paremeters are not set up correctly then all hell breaks loose and no doubt something will get mighty hot from DC Current draw.
That is a very simplified explanation so if you want to understand more about it then google like mad. 0:)
You might find plenty to read at ESP site (Rod Elliot) He does write his webpages with the novice in mind and he certainly helped me out with some hard stuff.
http://sound.westhost.com/amp-basics.htm
Phil.

[kernalflagg]

...            

I can't see how the amp could have possibly worked with no emmiter resistors? :loco

...

The fact that you have a burnt resistor on the board says the amp may have had a major problem and was never solved.

I had the amp at 3 different amp repair shops, and NONE of the techs spotted the missing emitter resistors!  

The burned one is a new development since I initially briefly looked at it a few years ago. I did not notice the missing ballast resistors then either, because I am indeed a novice amp tech. That is why I took it to the 3 different amp techs to begin with becuase I knew I did not know what I was doing really. None of them noticed the resistors missing and one of them charged me $80 AND COULDN'T FIND ANYTHING WRONG!    :trouble

That is why I have took it on myself, even though fixing amplifiers is not something I normally deal with. Nevertheless, I do have experience troublshooting electronic and electrical circuits ... on automobiles.  

But YES the amp STILL works with 4 missing emitter resistors and a burnt R4.  :loco

It even sounds half way decent, but has that horrible buzz, which was not there so much when I first got it!

[kernalflagg]

OK, I ran into a few snags again. The available length area for each power resistor on the main board is ~25 to 30mm. This was plenty for the original 7 watt resistors, which were ~ 22 mm in length. The Mouser 10 watt resistors, which are the ONLY ones available, are 49mm in length. Width and height are not an issue.

How could I make this work? Can I ...
1) Fold the leads of each resistor underneath the resistor and solder them in place? Or...
2) Mount the resistors at an angle (like 45 degrees) with one end off the PCB board? Or...
3) Use 5 watt power resistors from Mouser that are only 22mm in length, even though the originals were 7 watts each?

My concern with options 1 and 2 are:
What if the leads heat up, and the resistors "fall" over and short out against the board?

My concern with option 3 is:
Using a lower power rating resistor may fry the amp.

Any ideas/suggestions? (What about using vertical mount resistors instead?)

[substratus]

None of the options you sugested sound like a good one.

1 & 2:  Mounting components floating off of the board just isn't a good idea.  I wasn't thinking of it falling over but that might be possible.  I was thinking of it breaking off.  

3: I'm pretty sure that 5 watts will be too little, this is probably the worst option.

Vertical mounting might work, but your going to have to figure out how to mount it, I'm not sure the existing contact points will work without extra wire.  You'll also have make sure they will fit like this inside the case without bumping into anything.

One question, will the resistors be lying down, or hanging off the side/upside down when the amp is in the normal position?  If they are on top then if you can find away to support them being off the board then #1 should definitely work.  I was thinking of something like standoffs glued to the resistors, they will likely need to be ceramic as I'm unsure plastics will take the heat and the metals will be conductive and there appears to be traces where they might need to go.  This might also work if they aren't on top but I'd be reluctant to try it unless it was my only option. (edit: you'll need glue that can take the heat, not really sure what the best type would be)

[JHow]

Maybe these 7Watts from mouser, wirewound, 22.2 mm length?:

66-W22R270JRLF

66-W22R470JRLF

[kernalflagg]

None of the options you sugested sound like a good one.

1 & 2:  Mounting components floating off of the board just isn't a good idea.  I wasn't thinking of it falling over but that might be possible.  I was thinking of it breaking off.  

3: I'm pretty sure that 5 watts will be too little, this is probably the worst option.

Vertical mounting might work, but your going to have to figure out how to mount it, I'm not sure the existing contact points will work without extra wire.  You'll also have make sure they will fit like this inside the case without bumping into anything.

One question, will the resistors be lying down, or hanging off the side/upside down when the amp is in the normal position?  If they are on top then if you can find away to support them being off the board then #1 should definitely work.  I was thinking of something like standoffs glued to the resistors, they will likely need to be ceramic as I'm unsure plastics will take the heat and the metals will be conductive and there appears to be traces where they might need to go.  This might also work if they aren't on top but I'd be reluctant to try it unless it was my only option. (edit: you'll need glue that can take the heat, not really sure what the best type would be)

You know what ... YOU just brought up a very good point:
These will be hanging upside down the way that the amp normally sits! This means that I don't really have to worry so much about them falling over onto each other or onto the board. I will defeinately take another look at this.

Maybe these 7Watts from mouser, wirewound, 22.2 mm length?:

66-W22R270JRLF

66-W22R470JRLF

Despite the fact they are vitreous enamel, and not ceramic like the originals, I think they should still work. I believe the enamel will make the amp run quieter but their heat dissipation characteristics are not as good as ceramic. That shouldn't be a problem as far as I can see.

Resistance-wise:
The 0.27 ohm ones are correct, but the 0.47 ohm ones may be a little far from the originals that were 0.60 ohm. since these are in the power output stage of the power amp circuit, I'm not sure if the smaller resistance will have a detrimental effect when the amp is cranked up to higher volume levels...?

According to Ohm's law:
less resistance = more amperage, which may possibly = expensive compressed smoke

[J M Fahey]

That Randall amp needs those .27/.62 ohm resistors as shown in the schematic.
It was an old trick (also used by others such as Acoustic) to even the current sharing.
You have a length problem here.
Those 49mm long ones are way too much.
In 22 mm/5W:
1) you can use a 5W .27 ohm without a problem because it dissipates less than one half what the .62 ohm does.
Why did they use a 5W then? because buying a smaller one costs practically the same and simplifies bulk ordering (as from Ohmite)
2) substitute each .62 ohm with two .33 ohm ones in series.
Mount them this way:
In your hand, put two of them vertical.
Each of them will have an upper lead and a lower one.
Twist upper leads together, solder and clip them.
Now you have a nice vertical .66 ohm, 10W resistor.
Open it slightly into an inverted "V" shape and insert free legs into the PCB.
Done.

[joecool85]

2) substitute each .62 ohm with two .33 ohm ones in series.
Mount them this way:
In your hand, put two of them vertical.
Each of them will have an upper lead and a lower one.
Twist upper leads together, solder and clip them.
Now you have a nice vertical .66 ohm, 10W resistor.
Open it slightly into an inverted "V" shape and insert free legs into the PCB.
Done.

Juan, wouldn't that give him a .66ohm 5w resistor?  I think he would need two 1.32ohm resistors in parallel to give him a .66ohm 10w resistor.