Switchable fourth preamp stage on 100 Watt plexi

[JB59MODEL]

Hello fellow amp heads. New to the forum here. Planning to build a 100 watt plexi based on on the Granger chassis. This chassis is already equipped with the punch-out for an extra preamp stage and holes for an external bias pot and test points.
Looking for some advice on how add the fourth preamp stage for the additional gain and make it switchable so that I can run either three stages or four depending on what kind of tone I want, I plan to use classictone transformers, F&T filter caps, and the amp will be modded with an FX loop and PPIMV. I will build the cabinet myself. My background: I have about 20 years experience as tool & die machinist, I have done carpentry work, gunsmithing, and was an instructor at an aviation maintenance school (including basic electricity, so I know the difference between a henry and a microfarad. Once I get my shop set up, I would like to start building cabinets. 100 watt plexi cabs to begin with, and later, perhaps cabs for bluesbreaker combos and 5E3 fender tweeds. I figure after materials and labor I should be able to sell the cabs for about $185, But, all that is down the road somewhat. Any suggestions?http://forum.metroamp.com/posting.php?mode=post&f=7#

[bbaug14]

Depends. If you just want a switch its not all that complicated. If you want to use relays and a footswitch for on-the-fly switching it gets a bit more tricky.

[JB59MODEL]

No, a simple switch would be fine, if I need a foot switch, I have a nice JCM 2000 DSL 100 with mercury magnetic transformers in it (not going anywhere!) and a 2466 Vintage modern 100 watt (love those KT66's!) but I have been wanting to build a plexi for a while now.

[herbvis]

its easy I have a schematic that will switch the added stage in or out. It is designed for a relay but can easily be adapted for a dpdt switch. Shoot me your email.

[JB59MODEL]

Does the turret board need to be extended and additional components added for the extra stage?

[neikeel]

Does the turret board need to be extended and additional components added for the extra stage?

Ideally yes, you can add a small board like the Trems (and original SIR amp) had. It can also be done with board stand offs mounted to the extra tube socket bolts in a PTP sort of way if you plan it carefully, but it never looks quite as neat in my opinion.

I think Herbvis went for a clean plexi and a #36 options only. It gets more cluttered and more interaction between channels if you go for plexi, 2203 and 2203 + a tube options without relays (at least it did whne I tried it). George seems to have cracked it with his Metro-Plex using relays, and I think diodes for one mode but of course he will be selling these so a commerically sensitive area!

When I get more time I am going to go the relay route

[herbvis]

I had to ground out the #36 stage via relay when in 1959 mode to avoid "channel bleed interactions". Worked out rather well.... I did have solder bridging 2 of the traces on the bottom of my relay board that was causing all sorts of issues. After after hours and hours of failure due to this unseen nusiance ....pcba

Since I got it all sorted out I decided to continue on and I just ordered another relay....Im going to add that 2203 option after all.

[VelvetGeorge]

Thanks Neil, plexi and modded modes are tube gain only. The boost is a FET of course. No diode clipping in the Metro-Plex.

Switching is a combination of relays and Jfets. A relay for switching the loop in/out. Also relay for switching master volumes. Jfets for guitar level routing. Also a jfet controlling parts in the second gain stage.

Whatever kind of switch you use make to maintain DC reference on both sides to avoid a pop. Just a resistor to ground is sufficient and can usually be very high value. 2.2M 10M or 22M.

george

[JB59MODEL]

I'm beginning to think it will be simpler to just build a normal plexi and just buy a tube driven overdrive pedalhttp://forum.metroamp.com/images/smilies/confused2.gif

[Thiez]

Buying an OD pedal will be simpler indeed. But modding your amp is much more fun then buying an OD

It won't have to be very difficult. You can buy standard PCB boards for relayswitching (http://www.tube-town.net/ttstore/Kits/N ... :5910.html)
Only connecting the wires to the right place and you will have ampswitch and footswitch. Also room for two relays, so you can even make your FXloop switchable.

[JB59MODEL]

slightly off subject, are there labels available that can be applied to the back panel to identify the various jacks and switches?

[herbvis]

slightly off subject, are there labels available that can be applied to the back panel to identify the various jacks and switches?

I 2nd this. I would love to be able to label my additional add ons

[Thiez]

Any labelwriter or stickerprinter? It's not the most beautifull solution maybe, but works mighty fine.