Change the tonestack on an RXES?

[cap217]

I posted this on tgp and Ztalk but you guys here are the best around for amps. I had this question in the rxes iron thread but I think it might get attention on its own.



I like the RXES a lot but I want more bass more hight and less mids. The tonestack makes it very mid heavy. I talked to the old tech for DR Z yesterday and he mentioned that he could change the tonestack. He said it would give it a more scooped sound and kind of lean towards a wreck. That sounds appealing but I wanted to know if anyone has done this or considered this? The rxes is the most mid heavy Z amp.

[David Root]

Have you tried looking at tonestack design simulators? Here's one that lets you see what happens to the mids when you change components.

And there is always the one in Duncansamps.com, also very good, has a graphical presentation that lets you see how many dB the mids get cut.

[Firestorm]

I don't know what circuit is in that amp, but mids and bass are kind of joined in typical FVM stacks. You can shift their relationship by changing the slope resistor, but they'll still rise and fall together. Can you sketch out how the circuit is now?

[cap217]

I don't know what circuit is in that amp, but mids and bass are kind of joined in typical FVM stacks. You can shift their relationship by changing the slope resistor, but they'll still rise and fall together. Can you sketch out how the circuit is now?

I am not smart like you guys. If there is anyone that can provide this info then please do. Would a gut shot help?

[Firestorm]

Gut shot is better than nothing. Some guys here can read the layout like a book. I prefer schematics, but whatever you got would help.

[cap217]

Pics

[cap217]

I took this apart tonight. I havent taken it apart since I bought it but it was time to do a tube swap. It had:

GT EL84S
NO name 12ax7
AEL Ef86
Svetlana 5v4


The amp was super dusty so I sprayed out the sockets and replaced all tubes and installed tube dampeners. I went with:

Sovtek EL84M
JJ balanced V1 12ax7
GT
JJ 5ar4


The amp calls for a 5ar4. The 5v4 was working but I assume that the ar4 will give me a boost in the mids and clean up the sag. The amp does sound better and fuller but I didnt push it yet.

Just to be clear. I am correct that the 5v4 is ok in this amp. It just creats more sag and earlier breakup. The 5ar4 will give more headroom and a bigger bottom end?

[M Fowler]

I've seen a few different things people have put together regarding the RXES which is different from the original RX that doc provided the board layout to this forum.

1/2 12ax7 to 100k slope, 500pf into 250kB treble, .022 into 250kb bass and .022 into 25kb mid, 1mA volume after tone stack into EF86.

[pdf64]

It's a Sylvania 5V4, made in USA.
When replacing it, especially with a different type, the static dissipation of the power tubes is best checked..
There's a fair chance it puts out more voltage / less sag than the JJ 5AR4.

The photos have poor resolution - could you try again, perhaps in good daylight conditions?
Pete

[cap217]

It's a Sylvania 5V4, made in USA.
When replacing it, especially with a different type, the static dissipation of the power tubes is best checked..
There's a fair chance it puts out more voltage / less sag than the JJ 5AR4.

The photos have poor resolution - could you try again, perhaps in good daylight conditions?
Pete

The amp calls for the 5ar4. The 5v4 was in there by the previous owner.

[M Fowler]

Here is a better shot of the chassis.

[cap217]

Sorry my pics aren't good....

Here is one pic that looked good though. This one compared to the above shows a difference. There is a capacitor missing in mine.

Why would this be?

[cap217]

here it is circled.

And it looks like mine has an extra cap inserted between the 2 left orange drops on the bottom.

Hmmmmm?

[Gaz]

I don't know what the circle on the right is, but the small black silver mica Cap is across plates of the phase inverter. It is commonly called a "fizz cap" and is there to prevent supersonic oscillations by attenuating very high frequencies (mostly above human hearing) Almost every amp has one, copying the original tweed Bassmans. This cap usually only effects the amp audibly when overdriven where there at higher harmonics present, but even then you would have to make the cap unusually high to hear it.

[cap217]

Here is a reply from the guy who built this amp. He worked for Z back then.

The one on the right is a dropping resistor. It drops B+ voltage to the plate of the tube. It is there on your amp, just underneath the board attached to the blue multi can cap. That is why there is a piece of wire on yours. Same connection, just laid out differently. The other one is a silver mica capacitor that is in the phase inverter circuit. It is used to smooth out the top end a bit. Tough to say if they do much really. You would have to a/b it to see if you hear a difference. I never did on many of the amps. That cap isn't crucial to the circuit. Stuff like that could be left to the whim of Z. If he thought it made it better he would add the cap. That would last for a few months or less and it would go back to no cap. You would hear a bigger difference with a tube change than if that cap was in the circuit or not.