Cascade Mod for a 50w Plexi clone

[Mxzx]

Can someone give me the idiots version of the cascade or one-wire mod for this amp?

I have searched and can't find much that I can use.

Here are a couple got shots for direction.

Thanks

 

[damienbeale]

Misc Mods and Info

 

[Mxzx]

Cool, thanks!

I'll give it a try. I also have a PPIMV installed if that makes any difference.

 

[Mxzx]

Well that didn't work!!

Bright channel squealing when vol turned up, normal channel sounded same. Yuk!!

 

[damienbeale]

Then you have oscillation and need to use shielded cable on the input grids. You'll also need the chassis bottom shielded i.e. in the headcab.

 

[Mxzx]

Okay, I might need a little more explanation.

The only thing I did different was to unsolder the wire at lug 2 on V1 and tape, then ran a new wire from that lug to the lifted end of the 470k resistor. I just did this so I didn't have to lift the board up to unsolder underneath the 68k resistor terminal. Would that make any difference?

Thanks

 

[damienbeale]

It will if your lead dress is crappy. Even as stock these circuits can occasionally suffer from oscillation and rfi. Since you have added a shit ton of gain you have exacerbated the issue hugely. Bad lead dress will make this 10 times worse and plates should be nicely seperated from the grids even when the grids are shielded. There are certain practises that should be followed when building high gain amps.

From what little I can see, your lead dress needed a lot of work to start with.

 

[Mxzx]

Thanks for the info, I didn't build the amp, but it was a mess when I got it!!
Before


I cleaned it up some:


What exactly is lead dress, and what do i need to fix?

Thanks

 

[damienbeale]

Lead dress is the routing of the wires, in lieu of other components, and other wires. Some are more sensitive than others. Some spew out crap that other sensitive ones can easily pick up. This is why it is very important to follow certain rules, and often very important to shield the grids when cascading the front end like you have. You have just found out from your modding just how important it is.

This is why it is important to have good knowledge of what you are doing before building an amp. Whoever built that one, obviously did not. I would suggest that the metroamp instructions are some of the best that could be followed, but the forum is currently down, so you're out of luck there for the time being.

Most importantly you need to add shielded cable to both grids on v1. Preferably moving the plate resistors directly to the tube sockets, and adding the shielded cable core to the grid resistor, heatshrinking the shield off at the socket end, and grounding at the other end.

Also, if trying to use that amp without putting it in the headcab and therefore not shielding the underside, it will undoubtedly howl like a bitch with that lead dress and a cascaded front end. Personally I find the one wire mod way too much.

Add:
If you do want to rewire that thing all over again (even those heaters look dreadful) I've just found a copy of the 1987/1986 metroamp kit instructions on my hard drive...

 

[RG31]

I would take your mainboard out, remove all that God awful wiring, order a new wiring harness from 50 Watt Build Checklist | ValveStorm and just redo the whole wiring. I also have the Metro Amp instructions in a pdf file.

 

[Mxzx]

Thanks for the info. I was indeed playing it without the headcab, so maybe that contributed?

Anyways, I put it back to stock and it actually sounds fine the way it is. It is quiet, so I don't see any need to totally rewire it.

I do have the Metro instructions downloaded, maybe someday I will rebuild from beginning.

 

[damienbeale]

Thanks for the info. I was indeed playing it without the headcab, so maybe that contributed?

Not just maybe, that was probably about 75% of the problem.

 

[Mxzx]

Really, I didn't realize it made that much difference?

I think the head box on that one is a Weber, would that be shielded inside? I can't remember and already put it back together and put it on the stack!

 

[damienbeale]

Not sure. I tend to avoid weber stuff, having had to build one of their chassis kits for somebody. Was NOT impressed.

But cascading the front end via a one-wire mod adds a huge gain factor that is very unstable if not properly shielded, and completing the chassis is absolutely essential if not shielding the grids. It will be almost guaranteed to squeal like a bitch without a shielding plate underneath.

Remember, the extra distortion you get with these mods is a by-product of increasing the amplification factor by massive amounts. That includes all the crap that is riding on top of the signal along with it, and it can get to the stage where this unwanted signal overrides the signal you want, as it is often at frequencies that overwhelm the amp, making it unstable, and causing it to oscillate.

 

[RG31]

Damien, not to derail the OP's thread, But I am about to buy a 1959 HW reissue. What is the difference between the Cascaded Gain Mod over the practice of just jumpering the channels?

 

[damienbeale]

Jumping the channels does not cascade, it doesn't even add gain. It merely puts the two channels in parallel so you can mix the flavour of the two inputs together. Although the volumes on each channel are still somewhat interactive even when plugged into one channel only.

Don't go thinking you can achieve high-gain at anything less than neighbourhood destroying tones by jumping channels, because you cannot.

Jumping just places the two gain stages side by side, so you are blending the character of both. Cascading puts one stage in front of the other, which increases the gain factor hugely.

 

[RG31]

Jumping the channels does not cascade, it doesn't even add gain. It merely puts the two channels in parallel so you can mix the flavour of the two inputs together. Although the volumes on each channel are still somewhat interactive even when plugged into one channel only.

Don't go thinking you can achieve high-gain at anything less than neighbourhood destroying tones by jumping channels, because you cannot.

Jumping just places the two gain stages side by side, so you are blending the character of both. Cascading puts one stage in front of the other, which increases the gain factor hugely.

Don't want the high gain, just the Ritchie Blackmore grind with Deep Purple and Rainbow. To me, this is the tone that gave birth to Heavy Metal as a genre, but was taken to far to the extreme after Metallica released the Black Album. Today's high gain amps simply sound like a trash can full of a swarm of very pi$$ed of bee's The Marshall JVM series is the only high gain amp that still captures the heavy grind that Ritchie brought to us many years ago.

 

[damienbeale]

You'll still need it ridiculously loud to get there...
Jumping is not the order of the day for Blackmore tone anyway, imo.