Adding 4th preamp to plexi, channel switching questions.

[TimG]

Hi everyone,

I've tried the search feature here, but it says searching isn't allowed and I'm not finding specifics when browsing the many, many threads. Anyway, I've had a 100 watt MV '69 plexi MetroAmp kit from 9 years ago sitting around that I've only managed to get time to build it the last week, after finding most of the parts between several (house) moves. Crazy, I know! The amp has a great clean tone, and while I may swap values for resistors and caps to shape specific tones, I'm hungry to get the perfect gain I have in my mind as well. The gain (in my experience, and to my ears) sounded best with adding a fourth preamp tube, and I know exactly how to do this, but I don't know how to do the channel switching by kicking in/out an actual preamp tube when I want the existing clean or the gain (which I'll have separate gain/volume pots for, and be footswitchable as well).

I badly wish to retain the exact tone and characteristics that are in place now in this plexi clone, for a clean channel, but I wish to add the extra preamp tube to attain the quality I know I like best (I don't like the sound of drive pedals pushing the front end or via the effects loop). I know there are many ways to get extra gain out of an amp and channel switch with some of those methods, but I know the extra tube added in a specific manner gets me what my ears like best. So, I am trying to figure out how this will all go together.

I can add the second (I call a boost) volume with a relay, independent 24v transformer/diode/resistor/cap and extra 1M pot I have sitting around. I know I can achieve a boost in the effects loop (I'll be adding a tube buffered effects loop, probably with mixer pots for in/out db), but I prefer the natural tone of the amp with only a volume increase.

I'm just getting back into this and have about 100 books on tube amp mods and designs, theory, etc., along with forums like this, and the entire Internet at my disposal, but the amount of information is intimidating at this time and is way too much information for my goals. After I complete this, I do plan to delve into all the many years of information and experiment more. For now, questions... I hope some of you will be kind enough to offer. (sorry for the long story):

The footswitchable relay method used for the second volume control worked well, but it was just to switch the pots used (for the most part), so would this same method work the same way for kicking in/out a 4th preamp tube, or with some variation on the idea? In other words, I'm not quite sure how to completely bypass the 4th tube when I want a clean sound and kick it in when I want gain. Also, again, I wish to have this be footswitchable between the channels. But, could the relay handle the voltage passing through the tube for the scenario I need, or am I looking at this entirely wrong and making it too complex? I also want to avoid any delays, pops, etc. when switching, of course.

I feel like I'm missing something (besides the knowledge of how to actually do this), like it can't be simple, so I'm wondering about how this affects the overall voltage across the stages, or if it would even matter? I understand this is the preamp stage regarding voltage and methods that can be used, but I want to ensure the 4th preamp isn't adding or taking away anything from the current clean sound of the three preamp tubes. I wish to have it remain clear and tight even when rich with gain overdrive, but that itself (for the one channel amp mod) I did before seemed to be perfect. I just need a means to switch between them. I have so many ideas about how it can work, but it's pretty crude in my head and I don't want to do something stupid, pointless, or damaging. I'm happy to tweak and learn (that's part of the fun), but I want it to be with merit and with a sound principal.

I plan on a gain pot and volume, while the clean has it's same master & volume as it currently is. I'd add the boost volume to probably only work on the gain channel due to the volume differences between clean/gain. I've also been thinking that my bias for the clean sounds really good, and I may need or want to have a different bias set for the gain to really dial it in for that channel; I assume a dual bias then, where when one channel (clean) is switched to, it has the right bias for that sound quality, and when switching to gain, it has the relevant/best bias for the gain channel. I may actually keep the presence, bass, mid and treble to work across both channels, but I figure it wouldn't be too difficult to separate out the EQ for the two channels by adding extra pots. If I do this, is there anything that I'd need to know? Does the dual bias option sound feasible/right? I'd assume it would work, but the difference in bias for the same power tubes seems like a potential issue, or is that a solid idea if applied properly?

So, in summary, I can do these things individually and I can do some of these things already, but I'm unsure about a few areas, especially how to achieve channel switching to kick in/out the fourth preamp tube (so it doesn't affect the 3tube default design that sounds so great for clean):

Here is what I'm aiming to do (some of this I've done in the past):
1) Tube buffered effects loop (can do this) (possibly with mixer pots; also can do)
2) Add fourth preamp tube for gain (can do this, but unsure how to make it two channel to get back the current clean)
3) Add extra volume pot for footswitchable boost for gain channel (can do this)
4) Line-out option (I think I can do this)
5) Footswitchable gain/clean channel by kicking in/out 4th preamp tube (unsure how do go about this) * This is why I'm posting, ignore the rest of my long post, if you like.

With the ideal tested qualities I've had in the past with the gain, fx loop, etc., I think the majority of the work/ideas are there, but without channel switching it's not as valuable, and I don't want to give up one quality for another. I'm unsure how feasible this is, but I'd like to avoid two amp heads and switching between actual amps. I'm hopeful it's not too complex (it doesn't seem like it should be?) Any insight, ideas, tips, layouts, etc. would be appreciated.

PS: Unrelated inquiry; I've considered the option to switch down from 100 watts to 50 watts, and the information in books and online seem to show this is pretty easy, but really, how close do these methods get to actual 1/2 power operation and sound quality? If the 100 watt OT isn't being saturated like a 50 watt OT would be, and so on. I'd add that to my list of goals for this amp, but that's not really too important; nor is the fx loop or second/independent volume for a lead boost (just to be clear, I only outlined any detail to outline that I've done the switching in the past, but that may work different). Wow, sorry for such a long post filled with a lot of irrelevant info!

[herbvis]

I modded my metro 2203 to have 3 channels with 2 master volumes, and an fx loop using the relay kit from colossal amps

Go here, Dave will sort you out
http://www.colossalamps.com/boards.html

[TimG]

Thanks a lot for the link. I don't mind building my own relays, I'm just not sure how to hook it up for the desired effect. The site shows the PDF is being revised and isn't currently available. Do you have any details on the method(s) you used to channel switch, if it kicks in/out another tube itself, or if you just did cascading with the existing tube stages? Any layouts/blueprints/ideas to help would be appreciated. I've got to say, those relay boards look nice and clean and I might go with one or two to save time, but I'm still unsure how to get the design done for switching. Not only for the additional preamp tube to be kicked in/out, but for the different bias settings (depending on the channel), the entire EQ section swapped out, being able to use different caps and resistors for the clean vs. gain channel. I figure everything I want to use will just go through the relay to engage or disengage as needed, but I'm concerned about the voltage/amps passing through, any tips/tricks to do it right, and exactly how to kick in/out that fourth preamp. I have a bit to learn to get this right. I'm just doing a lot of reading, hopeful to find the layout/schematic to have this work, then I'll go crazy with tweaks after.

[TimG]

Oh, also if anyone has any brand/type of relay switch they recommend. If I have a lot of variations between the channels, then the more connection points, the better. I don't want to get too crazy (I don't know if I'd have to anyway), but I feel like if I can find the right info/link online or in a book, I should be okay to figure out the rest... or I hope anyway. Thanks again!

[d95err]

Study the schematics for Bogner Ecstacy. It has a lot of relay switching, including switching in and out gain stages.

[Thiez]

It's easiest to give advice when you post a schematic of you amp now. Switching in and out an extra tube isn't that hard. In my build i've made and 1987X with switchable 2204 stage.
Easiest way is to put the first switch right behind the coupling cap of the stage after which you would like to add the 4th stage. Then you can build the whole 4th stage and after the coupling cap of the 4th stage you put the second relay switch. This way you are swithing the whole 4th stage out of the signal chain.
You do have to keep in mind that you need to load down the screen of the following stage, so you haven't got a 'floating' screen in between switching.

For example this schematic http://home.comcast.net/~jbjdav26/2in1version1.GIF

To prevent the relay from popping, most of the time people add an diode over the switching coil. If this isn't enough you can also add a 0.1uF in parallel with this diode. I've done this aswell and no popping what so ever.

[Thiez]

Here's a link to my schematic. It's as close to 1987x and jcm800 as i could get. Only the 470K of the normal channel was put behind the relais, to load down the screen of the tube. In the left corner you can see my channelswitching schematic.

https://www.dropbox.com/s/3d1z73d9lz0vs ... x.pdf?dl=0

[TimG]

Thanks a lot, guys! I've taken a look at the Bogner schematics a long time ago and didn't consider that. It was always a question of how accurate they were. I used to have the Bogner Ecstacy 101B, but I never blueprinted the circuit at the time (I feel a little dumb about not doing that for study purposes). Anyway, I didn't expect an actual schematic, but with that and the other tips/details, that will surely be a huge help to me, so I really appreciate it. I'll look it over as soon as I get some time this weekend. Thanks!

[TimG]

Oh, I just wanted to say thanks again, and the reason why I didn't post a schematic, is because at this time the plexi build is the stock build for the MetroAmp kit from 2006, with nothing changed yet. I had previously done the mods on a 70s Marshall JMP I had, adding the boost volume that was footswitchable, the tube buffered effects loop, and the 4th tube stage, but it was just a one channel hot rodded beast and I didn't retain the clean channel and had always regretted it. I later sold the amp head, so I want to recreate a similar result, but add the channel switching. It's not that I have to kick out the 4th stage to get the clean and I can go about it in different ways, but knowing how good the clean sounds on this build as is, and how much I liked the overdrive on the hot rod Marshall with an added 4th stage from before, I think I'd be extremely happy if I can pull it off (and I know it can be done), so this information is greatly appreciated.