Bluesmaster CLEAN optimized for STRAT (Blackface cap values)

[erwin_ve]

Claus, that's a cool take on the bluesmaster; interesting about the NFB thing; can you hear the influence of the cap across the NFB resistor?

[bluesfendermanblues]

Yes I can. Its seems like the top frequencies get more relaxed and less stressfull - hard to describe really - but its feel like your shoulders lowers so to speak.

The HiFi guys knows all about how to find the correct cap value.
Its done with a scope and a 10kHz square wave generator. You can see the amount of ringing in the OT. We are talking about ringing in the Mhz range around 150-300 Mhz. off course you can't hear these freqs, but you can hear the consequence of OT high freq ringing all the way down in the in the guitar range of frequencies.

[llemtt]

..

The HiFi guys knows All about how to find the correct cap value.
Its done with a scope and a 10kHz square wave generator. You Can see the amount of ringing in the OT. We are talking about ringing in the Mhz range around 150-300 Mhz. Off course you cant Hear these freqs, but you Can Hear The consequence of ringing in The guitar range of frequencies.

++1 on this matter

I always say that. If you are experiencing a bad OD sound probably your problems lie in your power amp...

cheers
teo

[bluesfendermanblues]

Here are some links if you want to know more about tracing ringing

http://www.audiophiletalk.com/cgi-bin/y ... 228937/695

http://www.siteswithstyle.com/VoltSecon ... xfmrs.html

The ringing phenomenon is relevant for all audio transformers even the ones we use in Neve preamps:

http://www.audioxpress.com/reviews/media/hansen2872.pdf


If you check out old HiFi schematics of QuadII, Leak, Mcintosh 275 etc. you will see that all of these amps use a cap in parallel to the feedback resistor.

[greiswig]

The HiFi guys knows all about how to find the correct cap value.

I'd love to hear more about this. How did you calculate your cap value? I Googled, but wasn't able to find very much that offered pointers for people new to "loop stability theory."

[bluesfendermanblues]

How did you calculate your cap value?

You don't - you need a scope and a 10 kHz square wave generator (or downloaded test tone).

You want a square reading on you scope that is not 'overshot'. And it depends on the OT, you need trial and error with various caps - e.g. 10pf, 15pf, 20pf etc.

Select the one that gives the best square curve.

Check more info at the links I posted above or make a google search for "10 kHz square tube amp scope"

[talbany]

Blues
Doesn't that work as a kind of bypass cap around the NFB resistor allowing those higher frequency's to be fed back in return canceling them out..If this is the case then this mod would be more prominent on a higher value say 100k than it would on a 4.7K..Do I have this right..

Thanks!!

Tony

[greiswig]

Tony,

That's what I was able to glean elsewhere: the cap shunts high(est) frequencies around the resistor, so they DO become part of the GNFB. So you're right: for amps that have lower R values on the feedback resistor, it may not be necessary at all. Good point.

Claus, I hope you'll chime in here, too.

[bluesfendermanblues]

Blues
Doesn't that work as a kind of bypass cap around the NFB resistor allowing those higher frequency's to be fed back in return canceling them out..If this is the case then this mod would be more prominent on a higher value say 100k than it would on a 4.7K..Do I have this right..

Thanks!!

Tony

Hi Tony,

The value cap parallel to the feedback resistor determines the cutoff frequency. Just like any other RC filter

E.g. take a look at the attached Carvin X-100 schemo, where 120pf is paralled to a 10k resistor, giving a cutoff freq off 132 kHz.


On the Bluesmaster, 100k paralleled with a 10pf ~132 kHz

On #102 and #183, 4.7k feedback paralleled with a 120pf ~159kHz.

[talbany]

Claus
Thanks for the reply.. Got it..

Tony

[bluesfendermanblues]

Claus
Thanks for the reply.. Got it..

Tony

Great, I'm looking forward to hearing about your experience with this small mod, if you decide to give it a try on one of your amps.

All the best.

[greiswig]

I got confused somewhere...

How did you calculate your cap value?

You don't - you need a scope and a 10 kHz square wave generator (or downloaded test tone).

then...

Hi Tony,

The value cap parallel to the feedback resistor determines the cutoff frequency. Just like any other RC filter
(SNIP)
On the Bluesmaster, 100k paralleled with a 10pf ~132 kHz

On #102 and #183, 4.7k feedback paralleled with a 120pf ~159kHz.

At one point, you seem to be saying it can't be calculated, but must be measured with a test tone and scope. Then you give guidelines for sizing the cap that seem to come from a calculation.

Can you clarify a bit?

[stevlech]

I'm no authority on this by any stretch, but I think what bluesfendermanblues was saying is that the frequency of the ringing is determined by using a scope and then a cap size is calculated to filter it.

Hope this helps, and sorry if I'm intruding.

[bluesfendermanblues]

Hi G and stevlech

You use the scope/10KHz square wave to monitor OT ringing

And you use trial and error to find a pf value that gives the lowest amount of ringing on the scope.

The actually RC frequency is not important as such, since its in the MHz range - usually around 150 mHz - 300 mHz.

I'm no expert either. I just perform the test on my scope and my ears like the result of using a cap - it seems to give a more relaxed high end sound.

BTW - I always wanted to build a HiFi tube amp and have looked at a lot of old HiFi schemo's and always wondered why they used and still use a cap in parallel with the feedback res. Turns out that back in the 50' and 60' when tube amps were the only amps around. It was common practice to make the square wave + scope test in order to choose a suitable capacitor for the feedback loop.

Looks like we have lost a little of the tube amp knowledge the former generations of amp builders had. We need to shape up

[Structo]

Do all output transformers have the artifacts known as ringing?

And if they are above human hearing range, (20KHz) how does one perceive it?

Does it sound like a harshness on top of the signal?