Electrolytic caps and measuring them?

[Mark]

A funny thing happened, I’m in the midst of repairing a Mesa Boogie 295 dual power amp.

In another thread I talk about my findings and update the circuit diagram.

https://ampgarage.com/forum/viewtopic.php?f=6&t=36878

I didn’t replace the filter caps as there is so many of them and I’m not experiencing hum. The only caps I decided to replace were the IC filter caps in the bias.

Firstly, I consider these to be the most important caps in the circuit and secondly they are relatively cheap.

I wasn’t expecting any audible difference and typically I would put this down to conformational bias. However, the amp sounded better and I heard a greater volume difference between “class A” and simuclass (better known as extended class A).

I don’t think I should have heard any difference at all, particularly as these caps are used in the bias supply. I also measured the old IC caps in circuit with my ESR70 and they measured fine.

https://www.peakelec.co.uk/acatalog/esr ... meter.html

In the past I used an old Sprague cap tester which measured cap value, ESR, power factor, leakage, and insulation, while the ESR70 measures value and ESR.

https://youtu.be/vaQptqb-KNM?si=ynSevzC-xTyLxaoU

https://people.ohio.edu/postr/bapix/CapChkrs.htm

What do you think would account for the difference in timbre before and after replacing the caps or do you think my original thought that it’s conformational bias sound right?

The amp was markedly different brighter and more lively, it wasn’t something subtle.

Thanks for your time and input.

[ViperDoc]

Did you happen to compare your pre- and post-replacement bias voltages/cathode currents? I'd bet you altered the bias setting when you changed the caps.

[WhopperPlate]

Did you happen to compare your pre- and post-replacement bias voltages/cathode currents? I'd bet you altered the bias setting when you changed the caps.

Definitely a point if consideration …but this all echoes another thread currently happening discussing the sonic qualities of different filter caps . I find the effects of these changes not subtle at all, in fact I consider the bias caps as very important for defining the entire feel and timbre of any amplifier .

What brand did you replace the IC cap with ?

[Mark]

I replaced them with Vishay caps. Vishay bought up a lot of companies and makes components for many brands. I would have preferred Nichicon, but Vishay was all that I could get locally.

IMG_9320.jpeg

https://au.rs-online.com/web/p/aluminiu ... 45379?gb=s

[WhopperPlate]

Those Vishay are noticeably upper midrange forward sounding , good for Marshall crunch , lots of harmonics . I really like that brand for the bias specifically.

[Roe]

Those Vishay are noticeably upper midrange forward sounding , good for Marshall crunch , lots of harmonics . I really like that brand for the bias specifically.

I also like them for bias (and many other things). the 10uf bias cap affects sound. they do have that british sound that is perfect for marshalls, Voxes, wreck expresses etc

[Mark]

Those Vishay are noticeably upper midrange forward sounding , good for Marshall crunch , lots of harmonics . I really like that brand for the bias specifically.

How did you determine that Charlie?

I had issues with the highs in my Mesa Boogie SOB which I converted to a Mark 1. The fix was to get rid of silver mica caps and use ceramic caps. In a lot of amps I would prefer silver mica, but in this amp ceramic sounded better.

[Mark]

Did you happen to compare your pre- and post-replacement bias voltages/cathode currents? I'd bet you altered the bias setting when you changed the caps.

I checked out the bias today and it was much as muchness. I found the EL-34’s were conducting less and the 6L6’s were conducting more. So it’s a bit of a mystery.

I adjusted the bias and it made little difference to the timbre of the amp.

[WhopperPlate]

Those Vishay are noticeably upper midrange forward sounding , good for Marshall crunch , lots of harmonics . I really like that brand for the bias specifically.

How did you determine that Charlie?

My ears

I have swapped these capacitors in many amps many times for fun , for repair , and for plenty of tone tail chasing.

I do remember the first time I noticed when I made such a change about 15 years ago. I wasn’t expecting anything but a properly functioning amplifier, but when I tested the thing suddenly I had an amplifier that was far more musical and easier to play than before the swap. My band mate was playing the amp at the time , and I suddenly became jealous. Dots became connected , and further experimentation was inevitable .

I had issues with the highs in my Mesa Boogie SOB which I converted to a Mark 1. The fix was to get rid of silver mica caps and use ceramic caps. In a lot of amps I would prefer silver mica, but in this amp ceramic sounded better.

That will often do it . I prefer ceramic generally for fender and silver mica for Marshall . The sozo caps work well , but so do old Lemco and RS branded caps

[Stevem]

Different idle bias points will produce different levels of class A wattage and the change over point out of class A and into class A/B can be noticed.