EL84 bass/treble - help me understand!

[RJ Guitars]

Looking at that OT it appears that it's going to be a 3.6K primary... lemme know if I am reading that incorrectly. I'm not overly informed or familiar with the 18 watt amps but my thought is that is quite a bit low in primary impedance for a pair of EL-84's... in fact even for a quad.

I had Edcor wind a custom OT (EM0830) using the GXPP15-8-8K core but with multiple secondaries and an 8.1K primary. These sound absolutely awesome in half power Trainwreck builds and I can pass one along to you very reasonably if you take a leaning in that direction.

Mercury Mag will also have some very good options for a primo OT that is right for an 18W and there are several of us that can get you a dealer discount from them.

[rp]

I had Edcor wind a custom OT (EM0830) using the GXPP15-8-8K core but with multiple secondaries and an 8.1K primary... I can pass one along to you very reasonably if you take a leaning in that direction.

And you won't have to wait 6 weeks or until after New Years if you tarry!

[Colossal]

I had Edcor wind a custom OT (EM0830) using the GXPP15-8-8K core but with multiple secondaries and an 8.1K primary. These sound absolutely awesome in half power Trainwreck builds and I can pass one along to you very reasonably if you take a leaning in that direction.

Mercury Mag will also have some very good options for a primo OT that is right for an 18W and there are several of us that can get you a dealer discount from them.

I would go with RJ's custom Edcor or a Mercury.

[Gainzilla]

PM sent.

[Gainzilla]

Hey, regarding the EL84 in relation to the LTPI, has anyone had any experience using a different PI tube (12AT7 or 5751, for instance)? If the goal of changing values in the Marshall PI is to not slam the tubes as hard, wouldn't that accomplish the same thing? How do the differences in current/transconductance come into play?

[RJ Guitars]

Lots of tubes have been tried in that spot but there tends to be some magic at work with the 12ax7 and the LTP. Can't say why but most of the great sounding amps, both the clean and high gain types use a 12ax7 in the phase inverter.

[Structo]

You can try other dual triodes like a 12AT7, it will cut gain and volume.

[RJ Guitars]

Upon using a different value triode in the PI slot, measure and compare your plate voltages. I think you'll find they are not simply scaling the gain... the operating point of the various tubes that will do this task can vary widely and you may want to tweak things if you permanently move to a different tube.

Have you considered a PIG (Phase Inverter Gain) control? Look for the details on page 2 of this thread... http://www.ampgarage.com/forum/viewtopic.php?t=6603 this is right out of Ken Fischers "TrainWreck" pages.

Using this you can vary the signal coming into and out of the PI and leave the magic of the 12ax7 and LTP alone.

[Gainzilla]

Thanks! Unless I'm missing something obvious, this is a fairly garden variety pre phase inverter master, correct? The attached image shows the master that currently exists in my amp. These are the same, right? I just want to make sure I'm not missing something.

In looking at the rocket, and a few other el84 designs and I'm thinking maybe I need to play with the output stage coupling cap and grid leak values. I think they're standard .022, but I see other values more commonly used. Don't the grid leak and coupling caps form a high pass filter? I'll need to confirm my current values, but I'm wondering if I got myself in a low-end roll-off kind of situation?

Different r/c network frequencies:

* 220k/.01uf = 72.343
* 220k/.022uf = 32.88
* 220k/.033uf = 21.922
* 470k/.01uf = 33.863
* 470k/.022uf = 15.393
* 470k/.033uf = 10.261