Another Asteroid/K60 Build

[deuce42]

Thanks! I like it quite a lot as well.

MK mod is described here:
http://ampgarage.com/forum/viewtopic.php?t=7665
and it basically consists of:
"Replace the two .02uF PI Caps with .047uF OD's
Replace the two 220K PI>power tube grid/bias resistors to 100K
Change the V2 Plate resistor from 100K to 220K

There you go. Bias it a little hot."
Power tubes used are 5881.

Niki

Apologies for digging up an old thread but I am wondering about the effects of the power grid/bias resistors if using a PPIMV pot for volume changes? As these resistors are removed and replaced with the pot and 2M2 resistors, can someone explain to me how this may affect this "Knopfler mod"? As I understand it, the 220k resistors are to be changed to 100k for this mod normally but with a PPIMV these resistors don't exist.

How would this affect the mod and the amps tone?

Cheers guys

[deuce42]

Am I right in guessing the 2m2 resistors are varied somewhat along with the 250k pot that attenuates?

[Synchu]

Correct.
2.2M with 250K in parallel are approximating the standard 220K (about 224K to be more exact).

I would look into a VVR of some sort to attenuate the beast with a built-in solution, while I am actually using an external attenuator with mine.
There's simply too much interaction going on between all stages of this design that I feel breaks if you mess with inserting something in between.
That's just me, though.

Niki

[deuce42]

Yes I take your point. Somewhere here I remember reading that VVR was a bit of a no no with this circuit although I cannot ascertain why. Maybe the power scaling Kevin Occonner method may be a better idea....

[deuce42]

I'm also wondering about the mod to reduce the flabby bass - so called 0.1uf cap after the gradual/fast switch being reduced to 0.01uf etc. Whilst I accept it may be beneficial at high gain settings, does it compromise the tone at lower settings I wonder?

I am debating whether it adds a benefit to one end of the gain but ruins another end of tone at lower overdrive? I'm confused somewhat...

[andrew]

Sorry for this brief interruption but is there a schematic for the power amp section or could someone tell me the capacitor values for the Asteroid? I can't seem to figure it out from the layout. Thanks for any help.

[Synchu]

Yes I take your point. Somewhere here I remember reading that VVR was a bit of a no no with this circuit although I cannot ascertain why. Maybe the power scaling Kevin Occonner method may be a better idea....

Yup. Some sort of power scaling solution it would be.
However, an external attenuator might be a better solution.

As for the bass - the Knopfler mode tightens the bass as well and you may definitely play with the tone stack values.
I got it back to the "original" specs as it sounds better to me.

@andrew
Which power amp capacitors?

Niki

[Roe]

I'm also wondering about the mod to reduce the flabby bass - so called 0.1uf cap after the gradual/fast switch being reduced to 0.01uf etc. Whilst I accept it may be beneficial at high gain settings, does it compromise the tone at lower settings I wonder?

I am debating whether it adds a benefit to one end of the gain but ruins another end of tone at lower overdrive? I'm confused somewhat...

you can use once big cap for the gradual mode and one small cap for the fast mode. caps in front of the switch prevent DC across the switch

[deuce42]

I'm also wondering about the mod to reduce the flabby bass - so called 0.1uf cap after the gradual/fast switch being reduced to 0.01uf etc. Whilst I accept it may be beneficial at high gain settings, does it compromise the tone at lower settings I wonder?

I am debating whether it adds a benefit to one end of the gain but ruins another end of tone at lower overdrive? I'm confused somewhat...

you can use once big cap for the gradual mode and one small cap for the fast mode. caps in front of the switch prevent DC across the switch

Looking at the schematic, which side is fast and which side is gradual? I'm sorry I'm confused? One point is between the 82k/22k junction and the other side is between the tube plate and the 82k. Just unsure which is which...

[deuce42]

I'm also wondering about the mod to reduce the flabby bass - so called 0.1uf cap after the gradual/fast switch being reduced to 0.01uf etc. Whilst I accept it may be beneficial at high gain settings, does it compromise the tone at lower settings I wonder?

I am debating whether it adds a benefit to one end of the gain but ruins another end of tone at lower overdrive? I'm confused somewhat...

you can use once big cap for the gradual mode and one small cap for the fast mode. caps in front of the switch prevent DC across the switch

Looking at the schematic, which side is fast and which side is gradual? I'm sorry I'm confused? One point is between the 82k/22k junction and the other side is between the tube plate and the 82k. Just unsure which is which - ie which side you'd put the big cap and which side you'd put the smaller cap so that the bass can be tightened in the fast mode and remain the same for gradual mode.

[Roe]

I'm also wondering about the mod to reduce the flabby bass - so called 0.1uf cap after the gradual/fast switch being reduced to 0.01uf etc. Whilst I accept it may be beneficial at high gain settings, does it compromise the tone at lower settings I wonder?

I am debating whether it adds a benefit to one end of the gain but ruins another end of tone at lower overdrive? I'm confused somewhat...

you can use once big cap for the gradual mode and one small cap for the fast mode. caps in front of the switch prevent DC across the switch

Looking at the schematic, which side is fast and which side is gradual? I'm sorry I'm confused? One point is between the 82k/22k junction and the other side is between the tube plate and the 82k. Just unsure which is which...

104k is fast (100%), 22k is slow indeed

[deuce42]

Ah I understand - so the schematic with Colossal's switcher has those two caps in the right configuration anyway!

[deuce42]

Hi guys, as for the "Knopfler mod" for this build, with 5881's I seem to be getting all sorts of crazy bias readings. I cant seem to lower the bias enough and would presume with a 5881 it would need a bias milliamp reading in the low 30's. I'm guessing I'd need to change out the bias resistors to ground before the trim pot? I'm a bit confused what to lower it to, or whether I'm completely wrong in my assessment?

As usual, any help appreciated:)

[martin manning]

How are you measuring idle current? Are you seeing unstable readings on both power tubes?

Remove the power tubes and measure voltage on pin 5's. What range of voltage do you have using the bias trimmer? If you set it to -40V or so is it stable? Do you measure any voltage across the bias feed resistors (measuring from bias supply voltage across the resistors to pin 5's)?

[deuce42]

Thanks Martin, its crazy unstable at -40v and the trimmer won't do much to change this