Metropoulos Forum

Question: where’s the best place to buy new pots for this thing?? They’re pretty scratchy, and need to be replaced.

I’d like some high quality pots. Where to get em?? I’d like some that have the right size threads, so they fit the holes in the front of the chassis correctly.

Thanks in advance!!

bmwfreq wrote: ↑

Tue Aug 21, 2018 2:12 pm

Just a foot note for everyone here.

I replaced all of the resistors in my amp with 1% metal film resistors. Amp sounds awesome!!! Much more growl but...not very 34ish.

Going back to carbon comp.

When will I learn that if it ain’t broken...DON’T FIX IT!!!

When you put the resistors in, try a metal oxide as 100k load resistor for the cathode follower. It's interesting what it does.

bmwfreq wrote: ↑

Tue Aug 21, 2018 2:15 pm

Question: where’s the best place to buy new pots for this thing?? They’re pretty scratchy, and need to be replaced.

I’d like some high quality pots. Where to get em?? I’d like some that have the right size threads, so they fit the holes in the front of the chassis correctly.

Thanks in advance!!

You could replace them with standard but select Alpha. Tolerances can be pretty crappy. Otherwise grab some high dollar ones maybe from Valvestorm.

@CoffeeTones

I'm using a 12k tail resistor.

Of the various mica's I've tried, they all make the high-treble sound more pronounced and is the opposite of what I'm trying to achieve. With ceramics, the treble can be tailored more depending on the type of ceramic & voltage rating. I've got 1kV X5F's plate snubbers, 500V Y5P bright cap, 1kV X5F 1st stage voltage divider cap and a 3kV X5S 3rd stage voltage divider cap. Now I've tweaked the circuit to suit the TAD's, maybe the ceramics need to be adjusted as well. I also forgot to mention I reduced the 2nd stage grid stopper from 120k down to 68k as that was rolling off too much treble when using the TAD's.

One thing I haven't touched in my amp is the 1 ohm metal-film biasing resistors on the power tubes. Metal-film resistors tend to have that strident treble to them, so maybe they are contributing to the problem. I might have to change them out for some matched carbon-film's.

I would just omit the 1 ohm resistors and use the shunt method to bias. The shunt method is more accurate and the tone seems to improve because of it. With the small tolerance variable of the 1 ohm resistors, your actual bias can be off by 2mA or more. Just be aware that when using the shunt method, the disconnected meter lead will have full B+, so don't touch it. http://www.marstran.com/50W%20Bias.htm Which ceramic are you using for the fizz cap?

Hi Coffee!!

How will the metal oxide affect my 34 tone. Since that is my main objective here.

The top end will be more snappy. You may or may not like to use it to balance the carbon comp smoothness. It's a more simple thing to try.

Thanks. I've never tried the shunt method, but it looks fairly straight forward to do. I'll give it a go.

Fizz cap is a Vishay Draloric HBU Series 2kV U2J(N750) 47pF. It's the best cap I've heard in that position.

How exactly do the lower power rated caps make a difference in sound, Coffee?

I wouldn’t think this would have any baring on the tone influence.

BRMSlash should be the one to answer that because it takes having a spec settled for a while, record it a good bit and then make those type of changes. He has experimented with more different VOLTAGE RATING caps than I. There is more to it than voltage rating due to ESR, actual capacitance of the compared parts, and other characteristics of caps that change as ratings change, those make a difference.

All this taken into consideration, I will tell you that moving the correct wires in relation to others can make and amp sterile and harsh as well as making it muddy and loose. Find the right balance and you are rewarded with great tone and harmonics. You would swear that parts had been physically changed unless you saw it done in person. That is the reason I choose not to chase value changes as much as I did previously. I just do what I call "wire tuning" once a known, good circuit is in place. I have long ago found which cap brands I like for different reasons. For example, you could try two Xicon MPP .001uF caps in series or some other 500pF poly cap as the treble cap if you want to add another color to the palette.

Interesting.

I did play with my lead dress a bit, but didn’t notice any change in tone. Interesting concept though.

I am going to change the high voltage 2200pF ceramic cap I have installed to a lower rating and see what happens. If anything changes I’ll let ya know.

When using different voltage ratings of the same model capacitor & dielectric, the treble frequencies typically go one of two ways. The treble either gets more compressed/controlled or it gets looser/harsher. As the voltage rating is further increased or decreased, the result can reverse as well. For example, say a 500v cap has compressed sounding treble. A 1kV can be more open sounding and less compressed, but may be a bit harsh in the upper treble, but then changing to a 2kV cap the treble can be compressed again.

I'm not sure about the technical reasons why the voltage rating effects the treble, it might be something to do with dV/dt, dissipation factors or impedance vs frequency depending on the physical dimensions of the cap?

I changed the cap...it DID change the tone. Who wuda thunk?

And for the better. Brighter and a bit more wah. Same value, just a lower power rating...I’m really surprised. Wuda never thought it would make any difference.

Lesson learned.

By saying "same value", did you verify that with a meter? Same brand and composition?

I absolutely did.

Same brand. Both ceramic.

It read 2.23nF = 2230pF (new cap)
Previous cap reading 2.21nF = 2210pF