Duplicated the same build but slightly different tone!?

[Decko]

Hi TAGers,

A recently duplicated a low wattage T wreck. Dual 6sn7s in the power section. Amp A has a great tone. Amp B generally sounds the same but it is a little constipated and has less highs and and lacks the quick response than amp A. Amp B simply does not sound as open or lively. There is also less response in the FB which uses 25 k pot and 100 k resistor tapped to the 8 ohm selector.

I spent a good 4 hours resoldering, cleaning the boards, I even changed out the output transformer to the same batch as Amp A! I double checked all the values of caps and resistors. All the voltages between the amps are the same within 3%. The layout is exactly the same.

All the components are the same with the exception of the pots. Amp A has Bournes, Amp B has Alphas. The caps on the preamplifier are PVC 612s. .022 on one and .02 on the other for coupling caps.

What can be the issue?

The pots?

The difference in .02 and .022 on the coupling caps?

Or is it just a simple reality that 2 bottle of the same wine can taste completely different?

Please help....

Thanks
Danny

[Phil_S]

Have you swapped the tubes from A to B to see if the tone follows the tubes?

[Decko]

Yep! Swapped the tubes. Amp B still lacks the articulation.

Any ideas?

[selloutrr]

all parts are +/- roughly 20% unless you meter each part and have matching transformers, identical Tubes, not just brand and type, and the same impedance at ever solder point. ... In short .. no two amps are 100% the same.

[Decko]

Okay it sounds like I need to adjust my approach. I need to tweak each amp individually rather than assume that the tone will sound the same based on copy exact...

Thanks!
Danny

[KindaFuzzy]

I always end up tweaking each one individually or just living with the fact that each amp is a different beast, exact circuit or not. Sometimes wire placement makes a big difference too, or if it's stiff sounding, the power supply could be a little more efficient than the other one.

[Decko]

Many thanks for your feedback gents.

I replaced the filter caps after the B+ from 80uF to 100uF. Big difference. Really cleaned up the muddiness and tightened up the bottom end. The amp is now more responsive, tighter and more in your face. Still needs more tweaking with regard to the higher frequencies.

I have concluded that Amp B has much more low end than amp A therefore it became increasingly boomy and bassy as I increased power causing it to sound awful!

Now I plan to work on elevating the mids and highs to round out the tone.

As it is my bass pot is rolled back to 9:00 position, mids are at 1:00 and treble is at 3:00. Sounds good here but still need a bit more tweakage.

REgards,
Danny

[Structo]

I find with pots especially a large difference in value between pots rated the same.
Like with Alpha pots which I like OK but the tolerance isn't that great.
A 1 Meg pot could read 845K to 1.3M which can drastically alter the tone.

Caps also can have a wide tolerance, especially electrolytics.
Most of the time those are 20% which can be a huge difference from cap to cap.

Lead dress, even differences between transformers all play a part.

I guess that is where the art part comes in to be able to tailor the tone to your liking or to match another amp of the same schematic.

With me, it's more of a happy accident when things sound good.

[Ian444]

I have concluded that Amp B has much more low end than amp A therefore it became increasingly boomy and bassy as I increased power causing it to sound awful!

Maybe double check the coupling cap values and grid resistor values, they generally create the low frequency cut-off filter, or they usually do this in one particular part of the circuit, if you get one of them out by a factor of ten, you will get the symptom you describe I would think.