Peavey Valveking Modifications

[jazbo8]

I plan to modify a Peavey Valveking 212 that I got in trade recently, for those that are interested, you can read about my plans on my blog. Your suggestions and comments are welcomed.

[jazbo8]

Here are two "JCM" mods I found on the net (Ultimate Guitar and Peavey Forum):

Mod 1 (by ilya-v):


Mod 2 (by 65 SG, gain matching mod not shown):


Both of these mods bypass the V2a gain stage when the "JCM" mode is selected, and retain the anode driven tonestack. I wonder if keeping V2a in the loop and converting V2b to a CF to drive the tonestack would make the amp more "JCM"-like? Perhaps something like this:

Mod 3 (tonestack mods not shown):

[MarkusBass]

I doubt if any of these modifications will work - who develops such mods? The last one definitely needs at least 100k resistor instead of 1.5k. With the first two you will get huge pop when switched on for the first time. The R107 resistor should be grounded with a high value resistor - maybe 10 Meg will work.
If you want to work on such modifications I do not recommend applying them blindly. Instead, you can buy a book like for example "Designing Tube Preamps for Guitar and Bass" by Merlin Blencowe. In the book all components of typical preamp are explained. Without it you will be very frustrated after doing such mods.

Mark

[jazbo8]

Some possible VK212 phase inverter mods are shown here.

[jazbo8]

Wow! Thanks for for the confidence... The sources for the mods are shown, and they do seem to work just fine judging from the comments found at those sites, although I have not tried them myself yet.

[J M Fahey]

Mods 1 and 2 will work, sort of, they are bypassing a gain stage after all, but will thump/pop like crazy when engaged, unusable as sound changers in a live situation.

#1 shorts noise reduction diodes so welcome hiss to the party.
Although tinnitus will mask some of it, at least to amp owner; audience will notice it anyway.
#2 bypasses the R106-C106-R112 net which is an important part of the Marshall sound so now it will be muddier.

Mod 3 is nonsense, plus they don´t even show switching ... am I missing something?

Note: vague references to 2 HUGE Forums without linking to the specific comments which might back these Mods is not exactly realistic.

Should we browse thousands of posts?

By the way, they are famous for cutthroat diverging opinion wars, so searching you will definitely find opinion backing *anything*.

EDIT: wow !!! in

Some possible VK212 phase inverter mods are shown here.

you are actually quoting yourself as proof

Mod 1 might work or not, it´s heavily dependent on actual plate voltage at the first triode, unless it sits exactly at 1/4 the rail voltage available second (cathodyne) triode will be polarized wrong and may even be fully saturated or at least lose a lot of signal excursion ... meeded to properly drive the following power tubes.

In typical gain stages 20% or even 30% variation idle voltages are acceptable; here it´s *critical.
And by the way, with values shown, I very much doubt first triode plate voltage will sit where it´s needed.

Besides, the 47K+diode net seems to insure that grid stays more positive than cathode ... a surefire method to saturate the second triode.

EDIT2: you mention a few forums and find words of praise to each of them:

Blogroll

AX84 "Very informative site for amp builders"
Ayumi's Lab "Nakabayashi-Son’s great site, full of information on tube amp design and Spice models…"
diyAudio "The best forum for audio DIY"
EL34 World "One of my favorite amp building sites"
Preservation Sound "Vintage professional audio gear, history, and ads!"
retro vintage hi-fi "now we are talking!"
TubeCAD "John Broskie's Guide to Tube Circuit Analysis & Design , nothing beats it!"
Valve Wizard "Merlin’s informative site"
VinylSavor "Thomas Mayer’s awesome blog"

I *guess* you forgot:
MEF "the no nonsense, no BS Forum !!!! "

By the way, thanks for the confidence vote too

[jazbo8]

It seems I have ruffled some feathers... I shall correct my wrongful ways...

Mods 1 and 2 will work, sort of, they are bypassing a gain stage after all, but will thump/pop like crazy when engaged, unusable as sound changers in a live situation.

Perhaps they have anti-pop resistors installed in the actual amps, or they don't switch the settings live, I don't know...

Mod 3 is nonsense, plus they don´t even show switching ... am I missing something?

hey, that one is mine, it is just an idea to change the configuration of V2 so there is no switching of the Lead mode like the other two mods.

Note: vague references to 2 HUGE Forums without linking to the specific comments which might back these Mods is not exactly realistic.

Should we browse thousands of posts?

Very in-considerate of me... here are the links:
Mod 1: V212 Mod Wiki (see middle of the page)
Mod 2: 65 SG Mod

By the way, they are famous for cutthroat diverging opinion wars, so searching you will definitely find opinion backing *anything*.

I had no idea about how those sites operate, I will take your word for it... But both of these mods are on the "Wiki", so perhaps some concensus was reached by the motley crew...

EDIT: wow !!! in you are actually quoting yourself as proof

Not as proof, but just a follow on to the first post in the thread - I am trying to document my research on the blog, that's all.

Mod 1 might work or not, it´s heavily dependent on actual plate voltage at the first triode, unless it sits exactly at 1/4 the rail voltage available second (cathodyne) triode will be polarized wrong and may even be fully saturated or at least lose a lot of signal excursion ... meeded to properly drive the following power tubes.

In typical gain stages 20% or even 30% variation idle voltages are acceptable; here it´s *critical.
And by the way, with values shown, I very much doubt first triode plate voltage will sit where it´s needed.

Besides, the 47K+diode net seems to insure that grid stays more positive than cathode ... a surefire method to saturate the second triode.

This is what I was looking for, some constructive critism and dialog, so, here we go - Mod 1 is taken directly from Merlin's page, and I have use this exact circuit in another amp, and it worked well, in fact, I upped the 470k coupling resistor to 1Meg.

EDIT2: you mention a few forums and find words of praise to each of them:

I *guess* you forgot:
MEF "the no nonsense, no BS Forum !!!! "

By the way, thanks for the confidence vote too

Ha ha, you guys are already so well known, and hardly require a referral from me! But you are right, it should go on my "blog roll"... And if you don't mind, I will steal your tagline! Very apt description of MEF.

[jazbo8]

JM was right, the Mod 3 I posted above was total nonsense... I was trying to change it to the standard CF driven tonestack and forgot to change the cathode resistor value for V2b, somehow I completely overlooked it... Here is the corrected schematic:

[aaronsg71]

Hey did you find out how to do the jcm800 mod? I can't find it and want to know exactly what to do, I have the 1st series Valveking 100 head, thx

[jazbo8]

Hey did you find out how to do the jcm800 mod? I can't find it and want to know exactly what to do, I have the 1st series Valveking 100 head, thx

Which one are you referring to?

[aaronsg71]

Jcm800

Which one are you referring to?

Just the basic one, without the added switch

[jazbo8]

The one without the switch would be my Mod 3, which is just an idea I have and it's has not been tested yet - but on paper, it should be more JCM800 like than the other two mods, since the cathode driven tonestack is one of the more important design features of the Marshall.

[aaronsg71]

I've never done any mods before but I do have friend who can, did you do anything yet to your Valveking? Is it difficult? How would you bypass the gain stage? A diode?

[jazbo8]

No, I haven't had a chance to work on the amp yet, tied up with other projects at the moment... I have seen pictures of the other mods, which invloved cutting some traces on the PCB and running some jumpers - it did not look that hard to work on the amp, especailly if I can the PCB out the chassis. I do not want to bypass V2a, but instead re-wire V2b as a CF and of course change the parts for the tonestack. I still need to see if I want to keep the Volume Boost and Gain Bosst switch functions, since those were not in the JCM800, they will most likely be disabled. But we will see...

[Enzo]

Has anyone read through teh thread over at PV forum? has pictures for the text challenged.