Engl Powerball II: what are these "mods"? Why can't I bias it?

[Andree88]

Hi, I've bought a used Engl Powerball II. It's fully working except two things:

1) The first "problem" I heard is that channel 3 (lead 1) and channel 4 (lead 2) have the same tone and the same gain. I've red (and heard online, even on Engl site) that channel 4 has more gain than channel 3 and is tonally different but in mine channel 3 and 4 has same sound and gain (if I put gain pot to max it's same gain on both). Why? Maybe last owner done something?

I though was "bad tubes" issue, so I replaced the stock Engl pretubes with TAD and JJ (that I used on my old amp, so I know they are good) and bought a new quartet of JJ 6L6 (replacing the quartet of TAD) to see if was a tube problem. But I had a second problem.

2) I cannot bias the 4 JJ as I want. Done some research online and I saw where I have to put the multimeter to test the current. I got a low value (108 mA on tube socket). So 108/4 = 27 mA each tube. I wanted 6L6 at least 32 mA each but I cannot go higher with bias pot. It stops at 108. Is it right to measure 108 and then divide per 4 (final tubes) in order to obtain the mA of each tube (approssimatively) or am I doing something wrong?
I measure in 6L6 socket, like in the image below.

I only have a multimeter, not bias probe or anything else. I can bias amps, done a lot with Marshall, Fender etc (except Mesa), but I cannot understand why JJ can't go more than 108 mA (even stock TADs were going to 130+ mA with pot). Am I doing something wrong or I have to change bias pot to bias my JJ 32mA each? Or am I counting this wrong?

I saw another thing inside amp, maybe a "mod"? Why are these resistors etc. here? It seems that they are added after. I though.. is it a "mod" to reduce "gain" on channel 3 and 4 or what?
The last owner said that he didn't change anything.. bah. I see thing changed, Here are the images.
RED circles is where I put my multimeter (ground, socket of tube and bias pot). YELLOW circles are the "mods" I saw.



And here the "mods" I see.









If anyone has an idea of what are these resistors etc and what maybe they do can please say it to me?
I'd like to have schematic of Powerball II (I only find the schematic of Powerball 1), Engl didn't sent shcematic to me even if I asked..

Thanks! Sorry for mistakes, I'm italian!

[J M Fahey]

Caro Amico, lei é pazzo?

I would never ever mess with that NASA level ultra complex PCB , the possibilities of damaging it beyond repair are HUGE.
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$ $$$$$
Unless it were absolutely necessary and in that case I would send it straight to an Engl Authorized Service Station.

In any case, you do NOT have a real problem, you just read somewhere on Internet about tube biasing and want to do what "they" did ... .

Why do you think that 32 mA is better than 28mA?

As of the unexpected volume situation, plus the pictures you show, yes, that is a crudely made Mod, basically they added a diode bridge, very similar to what is used in the infamous JCM900 and suppose for the same end: increase distortion - compression, but at the same time losing some volume (diodes can only clip signal level down)

Just look at the hair thin tracks on that PCB, EASY to break and HARD to repair, if at all possible.
Donīt mess with that.

In theory Iīd suggest removing it and sending it back to Factory spec (which usually is the best "Mod" ) but I donīt know if they pulled some component and now circuit is incomplete, so if it works, leave as is.

[alexradium]

Engl amps are all biased cold,by design.
Do they sound bad?No
Case closed.
As JM stated,the board is for advanced techs with proper tools,leave it alone if its not broken.

[Andree88]

Thanks for all your answers

JM I always used the 6L6 at 32mA and I was happy with the sound! I cannot say if PB with 32mA has a better sound because I can only go to 28mA at max

The only "problem" is that channel 3 and 4 have the same gain and tone (Engl website wrote that "lead 1 and 2 have tonally difference" but mine nope) and I just wondered if the mods could have reduce the gain and change the tone of channels 3/4. And I am very curious about mods!

Alexradium you say that Engl amps are all biased cold by design but why TAD tubes can be biased at 130 mA and JJ max at 110 mA? Are related to specs of tubes or maybe I've bought a quartet JJ with problems?

I will leave the amp as it is, I hope that the mods will don't damage the amp!

Can I ask if it's right the count that I do with bias? I mean number of mA/4 = mA of each tube?
And, last question, if I understood well the diode bridge is for volume/compression/gain... and the resistor above the bias pot? What should it do?

Thanks really really really! Sorry my bad english!

[J M Fahey]

Sounds crazy but I would take a couple closeup board pictures,send them straight to Engl, and tell them what happens and that you want to restore it to Factory specs.
That the visible Mod looks simple and accessible, what do they suggest?

Say you will NOT disassemble the amp, nor remove the PCB (as a Factory they will frown on both, and for good cause) but you simply will unsolder and remove what was clumsily added and you fear you might have to bridge 2 pads with a piece of wire or something similar if "Modder" cut a track or removed some resistor.

As of the added resistor near the bias pot, it might increase or decrease bias range and removing it might restore it to normal but as you see all answers include: maybe -if - perhaps - could - etc.

[Pedro Vecino]

I have never worked with Powerball II but with many Powerball I.

Channel 4 has more overdrive than channel 3, with slightly higher low content and also more compression. But this can not be seen with the maximum gain (in fact I have never raised it above 5). In Powerball I (with same gain control for both channels) the ideal point is about 4.

I don't know what that artifact is, but an added clipping system it doesn't make sense in that amp. Itīs possible that on these wide tracks being such a bulky rectifier and with large resistors is related to the filament voltage in DC. With some measures it could be determined.

Normally the 6L6 are set to 25mA measured through the anodes and you can raise that setting. But: exceeding a certain limit itīs convenient to treat the back cover (remove it or adapt it) so that it acquires ventilation capacity. Keep it in mind.

If the range of the bias circuit is not sufficient, the adjustable resistor has a series resistor on one side. By measuring its value and putting another resistor in parallel you can lower the value to accommodate it by moving the bias range upwards (more dissipation). In this image it has a 68K added in parallel. I do not remember the value of the bottom resistor but it could be 22K/27K or something similar.

This is about Powerball I but I imagine that in Powerball II itīs similar.

[vintagekiki]

maybe yes, maybe no

http://web.archive.org/web/20070212211038/http://www.aznaur.spb.ru/azgschemes/azg-engl-mod2.gif

https://guitar-gear.ru/forum/topic/4085-c%D1%85%D0%B5%D0%BC%D0%B0-engl-powerball/

Edit
https://www.mylespaul.com/threads/diy-engl-preamp.38320/

[Andree88]

http://web.archive.org/web/20070212211038/http://www.aznaur.spb.ru/azgschemes/azg-engl-mod2.gif

What is this mod2? D:

Normally the 6L6 are set to 25mA measured through the anodes and you can raise that setting. But: exceeding a certain limit itīs convenient to treat the back cover (remove it or adapt it) so that it acquires ventilation capacity. Keep it in mind.

Yeah, now in summer I have removed the back cover But is it right to measure the mA and read, for example, 130 and then divide per 4 (so I get the amount of mA per tube)?

Channel 4 has more overdrive than channel 3, with slightly higher low content and also more compression. But this can not be seen with the maximum gain (in fact I have never raised it above 5). In Powerball I (with same gain control for both channels) the ideal point is about 4.

Yeah, the fact is that even with gain at 12 o'clock the tone is the same in channel 3 and 4.. no more low end and compression on channel 4.. is the same as channel 3

[Pedro Vecino]

I don't know how you get those 130mA. I always measure the tubes individually to know what intensity flows through each one.

In relation to the sound of channels 3 and 4, in Powerball I the second triode of the second tube (pins 6, 7, 8) is active only on channels 2 and 4. I usually use a method in unknown or very complex amplifiers to identify the assignment of each triode in the circuit:

In this case you can take a used 12Ax7 and cut off pin 6.

Inserting it into the second socket channels 2 and 4 should stop sounding. If this happens and channel 3 sounds you have the assurance that channel 4 not sound identical to channel 3 because there is a extra triode in its path.

- If with that tube installed channel 4 still sounds itīs obvious that it has a switching problem. Very rare.

- There would be another possibility: that when installing that "special" tube, channels 3 and 4 would not sound. It would also be a problem in the switching circuit. Equally very rare.

[g1]

But is it right to measure the mA and read, for example, 130 and then divide per 4 (so I get the amount of mA per tube)?

Where are you measuring? As far as I know, each power tube has a 4.7ohm cathode resistor. So you measure millivolts at the cathode and divide by 4.7 to figure out the cathode current.

For the gain, try using very low guitar volume, so it just starts to distort. Then see if there is any difference between ch.3 and 4.

[Andree88]

In relation to the sound of channels 3 and 4, in Powerball I the second triode of the second tube (pins 6, 7, 8) is active only on channels 2 and 4. I usually use a method in unknown or very complex amplifiers to identify the assignment of each triode in the circuit:

In this case you can take a used 12Ax7 and cut off pin 6.
[ATTACH=CONFIG]54706[/ATTACH]

Thanks, I will try it tomorrow!

Where are you measuring? As far as I know, each power tube has a 4.7ohm cathode resistor. So you measure millivolts at the cathode and divide by 4.7 to figure out the cathode current..

I'm using a normal multimeter with black wire to ground and the red one on this pin of a power tube socket, I think is V1 (red circle on image). I only have a normal multimeter.. I've followed a YouTube video called "bias Powerball II".. I'd like to know every tube how much mA has..

[Andree88]

In this case you can take a used 12Ax7 and cut off pin 6.

- There would be another possibility: that when installing that "special" tube, channels 3 and 4 would not sound. It would also be a problem in the switching circuit. Equally very rare.

Ok, cut the pin 6 of my used 12ax7 and put it in second tube socket. Clean channel and crunch channel works, channel 3 and 4 are muted.
So is there a switching problem? Or PB 2 is different from PB 1 so I have to do other tryies? I have 1 more 12ax7 where I can cut a leg...

[J M Fahey]

Please donīt

Cutting 1 tube leg is an extreme measure to do a test, it solves nothing and of course why would you chop two tubes?

[Andree88]

Please donīt

Cutting 1 tube leg is an extreme measure to do a test, it solves nothing and of course why would you chop two tubes?

Oh no no, I mean that I have another used tube (a China one that I doesn't use), maybe cut pin 6 on PB II doesn't works, so I could cut another pin in the other used tube and try it on second socket instead of the one without pin 6!!!

[Pedro Vecino]

It could have a different assignment than in Powerball I, even with that triode assigned in another socket... If you want to try cutting pin 1 in other tube, go ahead with it.
Juan Manuel: itīs not a question of new tubes to make them useless but to give another use to the old or defective ones.
What strikes me the most in that amp is the function of the bridge rectifier and these resistors.