help with some peavey XXX mods

[Vengeance078]

I have a peavey XXX I just acquired and I am trying to do a few mods to it. First I am wanting to change the screen grid resistors in it and bias it with 2 kt88 power tubes. Then I would like to install a mercury magnetics choke in it and be ready to gig with her.

I have done many mods on amps using other people's guides and learned quite a bit but there aren't any clear cut guides on these so I drew up the mods on the schematic and would like to run them past yall to make sure that I am correct in what I am attempting to do.



here are the screen grid resistors. I know I need to change these four 1k5w. I believe if I do that I should be fine to install kt88 tubes and bias. Am I correct in this thinking or is there something I am missing like increasing sweep of the bias pot or something. Currently the amp is running 4 el34 tubes and I would like to use 2 kt88s.

Next is the choke mod.



I believe the choke goes where that resistor is and the. I have to modify the amps standby switch so it won't pop when I switch it on or switch channels. I believe that will be done with this next step.



I believe I have this all correct but I have never done any mods without a guide from someone else so I am trying to make sure I'm not crazy when planning them out myself lol. Thank you in advanced.

[Jazz P Bass]

Not too sure were we are going with these questions.

The amp came with 6L6 tubes installed from the factory.

Was the bias circuit modded for the EL34's?

Two KT 88's will require a change of the speaker impedance.

[Vengeance078]

With 2 tubes I just change the impedance selector so for my 8ohm cab I would switch it to 4 ohms. The amp already has a switchable bias circuit for 6l6 or el34 from the factory...

[Jazz P Bass]

Oops.
I forgot that the XXX is 'bias selectable'.

So then, why do you feel that the 1K/ 5 watt screen resistors should be bumped up to 1.5K?

[tedmich]

I think he wants to swap the 100 ohm/ 5w SGR to 1k ohm/ 5w which is pretty standard and prudent. Supposedly PV started using 700 ohm SGRs in 2004.

The more conventional route would be to run KT77s in this amp. Were you planning on doing the odd 2x JJ KT88's in the outside sockets and 2x JJ 6L6GC's in the inside sockets with a dual bias mod like Eurotubes suggests for the 5150? Or just running 1/2 the tubes with KT88s as 4 x KT88s won't fit, I believe. The bias circuit may not be up to it but either way a real bias probe would probably be required to dial them in.

The choke will be ok but isn't worth the bother IMHO, whats with the "cut" and "move" in the third picture? That makes no sense to me.

[Enzo]

The standby may pop a little when turned on, but so what. ANy pops from channel switching will not have anything to do with the standby switch wiring. The PV standby kills the screen voltage, turning off the tubes. it works well and reliably to standby the amp, and has done so since the 1970s. I see no reason to change it. In fact, to move the switch to the AC side means you are switching the entire B+ string off and on, the main reservoir caps have to charge at that time. I bet you get an even larger pop. The PV way has the main reservoirs always charged, so only the downstream caps have to charge at switch-on.

I agree with tedmich, I think the choke is a waste of time. What do you expect it to do for you? it will have a lower resistance than the 400 ohm resistor, and so your screen voltages will be higher as a result.

The adjustable bias has a limited range, on purpose. PV didn't want people be able to adjust it out of range. So while it does have the 6L6/EL34 switch, you will still likely want to make changes in the resistor vales to increase the range. Pull the power tubes, measuere the bias voltage at pin 5 in the sockets, and see what range you have. Then decide the voltage range you want, and deal accordingly.

[Vengeance078]

I want to run just 2 kt88 tubes. Chokes tighten up the low end and improve the harmonics on the 6505+ I played with one so I was thinking it would do the same here.

when you add a choke to the 6505+ they say you are supposed to move the standby switch wiring that's why I had it cut and moved.

[tedmich]

so like Dave's post here:
Page 12: The Mother Of All 5150/6505 Threads!!!! @ Ultimate-Guitar.Com Forum Archive

good photos, I'd go slow and careful or risk trashing a decent amp. Fixing the pop in the standby is nothing I've ever contemplated, as I avoid standby (cathode poisoning!)

You'll need a real bias probe to see if the Kt88s will work, it might sound good with them and the choke probably won't sound worse I think your money would be better spent on decent 12AX7s but I've never added a choke.

My favorite XXX mod is upping C3 to 2.2nF, losing some of the shrill, try it you might like it.

[Vengeance078]

Yes I was making sure I drew it up properly basically I've been reading up a lot on different XXX mods. One of my favorite amps. I'll try the C3 mod also.

I have a bias probe so that won't be an issue for testing the kt88s they should bias fine though you can get the el34s really hot if you try I know a lot of people like e34l in the XXX.

I have boxes of 12ax7s I love so that's already done I have my favorite set of JJs in there now.

[tedmich]

Good luck, I like my XXX almost as much as my modded Ultra+. I recently swapped all of the filter caps on my XXX, putting in the largest value caps that would fit in each position, it has about 3x the filtering uF and is a very stiff and quiet amp now. I also upped the cathode bypass cap C21 from 0.47uF to 1uF, also thickening things up.

I may also add a JSX dual resonance/presence pot in the place of the 3 way dampening switch in the XXXs negative feedback, as I love the tone it adds to the Ultra+. Keep in mind that my tone mods are largely because I play at very low volumes with no pick so my sound is very different from someone playing at 10 through a 412 cab with lots of pick attack and harmonics. My gain is always MAX though.

I wish the XXX has a reverb tank but use an Alesis Picoverb in the FX loop instead.

Do try tube rolling, it was a sad day when my NOS Amperex Bugle Boy died in V1, it sounded killer...

[Vengeance078]

I have an old CMI branded RCA 12ax7 in v1 then the rest are JJs and it currently has tungsol el34 in it.