Completed 80's funk chassis

[ayan]

OK I see it, it's not a trimmer, just 15Ks before the 220K and from 100K trimmer to gnd.

Funk, I do have a few dumb questions:
How did you arrive at determining a 15K was needed before the 220K and at the gnd side of the trimmer? Is this to unload CL2 a little more and/or the OD entrance or exit? Did you determine this with your ears or waveforms. I stumped.

No doubt Brandon will answer. But, if your network needs to be 320K and it is 290K instead, you need a make-up 30K somewhere. I believe most people set their trimmers between maybe 15-10K to ground and 50%, so Brandon surely split the difference and put a 15K tail to the trimmer and the other 25K before. C2 will be loaded identially to what it would have been if the amp had a 100K trimmer in series with a 220K resistor. You may want to draw yourself a little sketch to see what's really going on in the amp.

Cheers,

Gil

[Funkalicousgroove]

OK I see it, it's not a trimmer, just 15Ks before the 220K and from 100K trimmer to gnd.

Funk, I do have a few dumb questions:
How did you arrive at determining a 15K was needed before the 220K and at the gnd side of the trimmer? Is this to unload CL2 a little more and/or the OD entrance or exit? Did you determine this with your ears or waveforms. I stumped.

I used a DMM!! My trimmer was about 85K and my resistor was about 205K

[BobW]

Gil,
That makes perfect sense now, I didn't realize Brandon's trimmer and his 220k were reading low, but simply assumed the values were close to 220k and 100k. Thanks

Brandon, I assumed you were adding additional resistance in the network to tweak to an optimum tone. That's what happens when the whole thread isn't read.

I understand the goal is to keep the values as close to tolerance as possible, hence measuring resistors and trimmers in the critical areas, such as OD entrance. This now makes sense why Gary mentioned trimming the pots. In addition to the OD entrance, bias, and plate voltages, are there any other critical areas to consider that require close tolerance?

Seems after two builds, I'm still learning fundamentals of HADs design.

[Funkalicousgroove]

In the Amps(dumble) I've been in the parts all seem to measure almost exactly their marked value, of course I'm not daring enough to go de soldering coupling caps, but resistors are all within .1%

[ampdork]

of course I'm not daring enough to go de soldering coupling caps

Can't they be tested in circuit?

[glasman]

of course I'm not daring enough to go de soldering coupling caps

Can't they be tested in circuit?

Only if you have fancy piece of equipment called an impedance bridge that allows you to measure complex RC, RCL circuits and break it up into the components. Good ones start at $1500.00. Ones you really want to use $10K+.

Gary

[gcenker]

I'm really spoiled, we 2 of these things in my lab(amongst other state of the art test instruments):

http://www.andeen-hagerling.com/ah2700a.htm

Makes life really easy!

[Funkalicousgroove]

WOW!! If anyone was struggling with Ideas for a X-mas present for ol' Funk one of those would do fine!!

[gcenker]

Yeah, I'm very fortunate. I work in the field of metrology(science of measurements) and we are in the business of proving things down to the parts per million range e.g. ±0.0001% = ±1ppm.

Needless to say parts I use in my builds are very accurate with respect to their rated values. I have this nasty habit of measuring everything

[BobW]

Brandon, I don't want to beat a dead horse here, and mean no disrespect, but what is the real purpose of measuring resistors and marking the caps and pots? I assume it is to have a known value to tweak the knees not based on the formulas alone but for simply using your ears and determine if you need more or less based on the real measured / degooped values.

btw, I appreciate all your efforts here on this forum.

[Funkalicousgroove]

Brandon, I don't want to beat a dead horse here, and mean no disrespect, but what is the real purpose of measuring resistors and marking the caps and pots? I assume it is to have a known value to tweak the knees not based on the formulas alone but for simply using your ears and determine if you need more or less based on the real measured / degooped values.

btw, I appreciate all your efforts here on this forum.

you mean me or dumble?

[BobW]

you mean me or dumble?

You or anyone else. What is gained from measuring for precise values?

[Funkalicousgroove]

A control, in my case since I build at least 2 of these a month for sale; it allows me to have some degree of consistency. For DIYers it's a known good starting point, I read alot of threads from folks that are less than in love with their amps, they change all kinds of things, and the issue could simply be one or two off spec resistors.

I don't think it's totally necessary, with the amp pictured I used the first setup to make something a bit brighter with a bit lower gain, I didn't like it, so I added the 15K resistors, had I not known what I was starting with a 10 minute tweak could have been weeks and weeks of tweaking.

[gcenker]

What is gained from measuring for precise values?

I TOTALLY agree with Brandon! I'm in the measurement business. If you don't measure, you can't possibly have consistency. Don't confuse a tolerance for accuracy. If you're using 10K resistors with a "tolerance" of ±5% and the majority of them "measured" 6.5% low(i.e. 9,350 ohms), they are Out Of Tolerance - OOT in my field - and deemed unusable. They are "accurate" between each other but not precise with respect to their nominal value (your shooting for 10K remember?).

By measuring the components and ensuring they both meet their tolerance AND are precise, you've now successfully built in repeatability when making several amplifiers. The reason you hear people bitch about "this particular ______ amp sounds fantastic but the other ones, not so much" - that's mainly due to the inconsistency between chosen components in their builds.

FWIW, I had to return a small boat load of electrolytics to the supplier because they were +150% of their nominal values So instead of getting a 25µf values, they "measured" 37.5µf. Since the caps were being used in a parallel RC configuration to set up a bias for a preamp tube, that just changed the way that circuit functions considerably.

Hope this helps,
-Greg

[BobW]

Thanks Brandon and gcenker. OK, having a known base line does make the tweaking easier.

FWIW, I have tried using a scope to create a baseline of good vs. bad tone / waveforms when single components have been changed, and have come to the conclusion, it doesn't give you much to work with. I have yet to try a spectrum analyizer. thanks, I believe the horse is now dead.