6L6 spitfire/creme brulee variant : phase inverter question

[airfrankenstein]

Hi all,

I've built a spitfire/creme brulee using a 6L6 output stage and am looking for advice regarding the phase inverter. Currently it's the vox long-tail design : 1meg plates, 1K2 cathode, and 47k tail.
I reduced the tail resistor to 10k.

Would there be any reason to switch to a fender long-tail design, will this change harmonics, gain?
amp is cathode biased.

thanks for any insight, learning hands on.

[M Fowler]

The 1k2 makes the amp brighter and gong to 10k tail added gain.

Changing out those two resistors can really change the nature of your amp, maybe 820R and 24k.

[airfrankenstein]

thanks !

[Jana]

1 meg plates?

[M Fowler]

1 meg plates?

Phase inverter grids, thanks for keeping components labelled correctly Jana.

[airfrankenstein]

sorry you're absolutely right

[ampfab]

I had built a spitfire type amp and i installed a switch and a few extra components so that i could switch between the vox style PI and the fender/marshall style PI. there was a noticable tone/harmonics change and a gain change. the fender/marshall style PI had more gain and a thicker deeper tone. it was useful when tayloring the tone to different guitars.

[airfrankenstein]

I think I might try the same to toggle between a 470cathode/10k tail and a 1K2 cathode/47k tail.
Did you use a 2pdt, 3pdt ?
Would you happen to have a layout/schematic of how that get's wired up?

[ampfab]

I believe that i used a DPDT switch to parallel a 1.0k resistor onto the 1.2k resistor to obtain a resistance close to 470 ohms. the opposite side of the switch I paralleled a 15k or 12k resistor onto the 47k resistor to drop the resistance to near 10k.

[airfrankenstein]

what power tubes did you use ? el84?

[ampfab]

El84.

[Gaz]

The reason that a Vox, Fender, or Marshall have different cathode resistors is not arbitrary, it's to achieve a particular bias point for the LTP depending on the B+ voltage and tube type. The Vox has a larger bias resistor to achieve roughly a center bias point with the its lower B+ voltage (compared to say a Marshall.

[ampfab]

Marshall, vox, fender, Gibson,matchless, and countless others seem to use the similar values in all different situations. I get the idea that putting a vox LTP in front of a marshall or fender power tube configuration is not "wrong". Many would say that using a 470k plate resistor on a 12ax7 is "wrong", but I've definitely seen it done. Balanced isn't necessarily best. Am I right or wrong? What are the values on the marshal 18w LTP? I believe that they are typical marshall, the marshall 18w has similar b+ as the vox.

[Gaz]

You're correct that there is no wrong or right (as long as nothing goes up in smoke), but I was just pointing out that things weren't always tweaked for tonal reasons, especially when it comes to phase inverters. . T

You're wrong about the 18W. The values are much different than a typical Marshall (why didn't you just Google the schematic?). Bias has nothing to do with balance of the splitter either.

[ampfab]

i guess i cant rely on my memory about the 18w. certainly changing the tail and cathode resistor with not cause the circuit to go up in smoke. i would still encourage the OP to experiment.