Single-Parallel-Single switch to amp input

[harryk]

I am starting a a new amp project which is based on Plexi Marshall. I would like to have a switch in it`s input where I could pick either normal or bright input or both inputs in the same time. I do not want to use 4 input jacks and use a jumper to connect both inputs together. Does anybody know if there is a proven solution exsisting somewhere for this kind of need? Harry

[Colossal]

Hi Harry,

I am building a Mark Huss 6V6 Plexi. I originally had planned to incorporate such a switch in my amp but ultimately decided against it, though I still think it would be quite interesting to hear the differences in tone. Anyway, I ran the idea by the experienced builders on ppwatt.com and was told that while it was possible to do, much of the magic in the Plexi was to be found in blending both channels. I also wanted to incorporate a Parallel/Cascade switch. So, to get both channel switching and the Plexi/2204 mod to work correctly, the switching scheme would get more complex. Since I would probably just end up running it with both channels most, if not all, of the time, it seemed like a lot of complexity for something that might go largely unused. I ended up skipping it and using one input jack, I jumpered the Bright and Dark channels permanently, and just went with the cascade switch instead. Would there be any difference between a channel select switch and just turning the volume down for each channel?

I'm certainly not try to convince you one way or another, just pointing out my own experiences. I've attached a drawing I made when I was thinking about it. Perhaps it will be of some use to you. My skills are pretty basic and somewhat limited so perhaps there is a more efficient way to do it. That's just what I came up with :); the design is unproven. Oh, and for the grid resistors, I show 10k but you can use whatever you deem appropriate. On my build I do have a switch that selects between 34k (68k + 68k) and direct (no resistor) input.

For the switch in the drawing, check out Mouser:

http://www.mouser.com/search/ProductDet ... M202401-RO

If you do go for it, please report back as I would be very interested to know how you did it and what it sounded like and all that.


Colossal

[harryk]

Hi Colossal! Thank you very much of your help with this problem. I know that Plexi sounds best with both inputs jumpered, but because I am an experimental guy, I always want to have alternatives to find the best tone for different purposes and situations. I was heavily considering to do it permanently parallel like you propose, but let us see where I am ending with this case. This Plexi project is waiting until I get my present project Bluetone Express Baby ready. It has a Twreck Express pre amp and a 2xECL86 PI+powerstage in push-pull giving some 10-13 W power. I also add a VVR to it to control overall volume level. I have already done one amp with VVR in cathode bias mode and it works well. I am also putting VVR to my next Plexi project. I ordered a pair of those kits from Dana and really like to see do they help to tame 2xEL34 tubes power to reasonable level. Harry (My amps can be seen in www.bluetone.fi)

[Colossal]

Harry,

I'm glad it was of use. I too like the idea of switching options just to satisfy my tonal curiosity and experimental nature. My amp has a number of switches to change voicing, bright, fat, cascade, etc. A lot of this interest comes from my lack of experience so I wanted to have a platform that at its root sounds killer, but has some flexibility built into it. Along the way I've seemed to find that the best amps and ampbuilders select for simpler models (!) so I reign myself in by saying, "well, if [a certain mod] was worth it, it would have already been standard on these great amplifiers":lol:. But yes, I suppose if you wanted to have as many possible tones the channel switching option could be quite interesting. Maybe others here can add something? I also incorporated a VVR2 into my 6V6 Plexi, will post some build photos soon. By the way, I've seen your site before. Your amps look and sound great!

Colossal

[villems]

Hey guys!

Here is maybe another way to skin a cat. This way you ground the input of the unused channel and save two resistors.

Colossal, your drawing was so good that I took the liberty to use that as template in my example. I hope you don't mind.

Ville

[harryk]

I took and ordered one on-on-on switch from my local supplier and hopefully have it next week. Anyhow I will make a field study with a multimeter and loose cables to check switching combinations in real life before proceeding with project. It is not nice if you make certain size holes to your amp chassis and then the component is not the right one. Harry

[mhuss]

You can also have three input jacks, with the third being "both." Here is an example of how it can be wired (just ignore all that Hiwatt stuff to the right )
http://mhuss.com/Hiwatt/Schematics/DR_P ... put_v1.pdf

FWIW, I always use 'both' nowadays.

--mark

[Colossal]

Hey guys!

Here is maybe another way to skin a cat. This way you ground the input of the unused channel and save two resistors.

Colossal, your drawing was so good that I took the liberty to use that as template in my example. I hope you don't mind.

Ville

No, I don't mind at all, and your method is far simpler! I knew there was a better way. Like I said...beginners mind...

[gearhead]

Before I was into amp building, a budding builder with massive electronics industry technical chops wanted to do the same thing. I was his guinea pig (for a significantly reduced price). A couple prototypes and 5 months later he decied to throw in the towel. Ended up just doing two entirely separate preamps; signal split at the input and rejoined going into the PI.

Having said that, I'd still give it a whirl. He is an extromophile when it comes to hum, so you might get there with an aceptable noise floor for you. He hasn't built any TWs. I thought of turning him on to them, but decided it might push him over the edge. LOL.

[tomrasdf]

Check out the Mystique Amps "Blue Star". It takes a different approach for a similar effect. Both channel's are hardwired in parallel, one input with independant volumes. The voicing is tweaked for greater differentiation between the two channels, and the more gainy one can grounded out via a footswitchable relay for "channel switching". It's a nice sounding amp. If you google it, you'll find soundclips on someone's personal webpage.

[davent]

Hello,
Could you build it with two inputs and then use an A/B/Y pedal to choose between the three channel modes?

Here's an example a quick search turned up. http://www.morleypedals.com/abyes.pdf

dave

[harryk]

Hi guys! Thank you of your kind support with this switching question. I will try that on-on-on switch first with multimeter and alligator clip cables to see if it works in real life. I do not prefer outside splitter cables in guitar inputs because they are always lost when you need them. Also I do not want to have many separate input jacks because there is lack of space in face plate. This is because I am aiming to have VVR etc. in this amp. I believe that parallel position will be the most usable way to play with plexi, but you never know before you try. I have never played any real Marshall, that is why I want to have all options soundwise. Harry Bluetone

[KT66]

When I build a Plexi I normally just wire a piece of shielded cable from 1 input jack over to the first 12AX7 and connect it to the junction of 2 resistors ( 33Ks make the most sense, but I have used 68Ks before I really thought about it and that worked fine too.) each of which goes to the grid of the first stage of each channel. This way the channels are always jumpered and the signal path is shorter than using a patch cable.

Just for fun I built this amp :
http://www.classictubeamps.com/6v6plexi.html with 2 switches on the rear panel that allow you to turn each channel on or off, but my conclusion is that there is no difference in sound between flipping the switch or just turning the volume knob all of the way down for the channel you don't want to be on.

Hope that helps.

[Colossal]

I ended up skipping it and using one input jack, I jumpered the Bright and Dark channels permanently, and just went with the cascade switch instead. Would there be any difference between a channel select switch and just turning the volume down for each channel?

Just for fun I built this amp :
http://www.classictubeamps.com/6v6plexi.html with 2 switches on the rear panel that allow you to turn each channel on or off, but my conclusion is that there is <b>no difference in sound between flipping the switch or just turning the volume knob all of the way down for the channel you don't want to be on.</b>

Hope that helps.

Thanks for answering this question KT66. I was really wondering about that!

[harryk]

I think that all best things in life are simple. It seems to be human nature that I make things at first too complicated. That solution with only one input jack and parallel connection to both halves of V1 and TURNING volume up or down in each channel is simple but seems to be really sensible and well functioning solution to my problem. Unfortunately I already ordred one on-on-on switch, but maybe I can use it somewhere else in future my projects. Harry