Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: EL34 on February 04, 2015, 07:10:47 am
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Waking up this morning, I'm going over my Plexi 6V6 design in my brain and was wishing there was a way to eliminate two separate volume controls and just have a mix control to blend the two channels.
Then I started to wake up some more and thought, well how would you control the volume of each channel if you only had a mix control?
Then I got to the shop and looked at the schematic and realized that this would not be possible with simple pots
A concentric pot could eliminate one pot, but I am not a big fan of those
On the odd chance that someone has seen this done before in some other fancy way I thought I would throw this out to you guys
The front panel of the Plexi 6V6 would have 7 pots if you include two volume controls and a Master volume
That means the face panel is full all the way across and some of the pots are down by the power tranny
Here's the pre amp schematic and the current face plate mock up
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What about a balance/blend pot as used on many bass guitars to blend pickups, or similarly, as used on stereos use to balance signal from left to right?
Perhaps each channel (bright/normal) would go to the balance pot first, then into a single volume pot.
(http://www.strat-talk.com/forum/attachments/stratocaster-discussion-forum/58410d1358393898-would-wiring-work-blend_pot4.jpg)
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Here is my proposal. 1MA dual pot for both volumes and 1MB linear pot for the mix. This most likely won't be any better than the original design.
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yeah, both ideas plus the original still use two pots
It's a puzzle that can't be solved I believe
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...and some of the pots are down by the power tranny
I was a little nervous about that too. But it turned out to be a non-issue. My presence pot is closest to the PT, but the NFB circuit is low impedance and seems to be unaffected by the close proximity to the PT or power switch. My amp is very quiet. I don't think you'll have a problem.
You could drop the Master, but I find it very useful. I just use the two volume controls to blend the mix that I like (kinda set it and forget it) and use the Master to set the overall volume.
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EL34, The timing of your post is uncanny, I've been tinkering around with this very idea, and just spoke to Silvergun about it yesterday.
My dilemma deals with i do not have face plate space for the two knobs, nor can i use a double gang pot, because I'm an idiot, and choose the most difficult chassis to build with. (see the proof: http://instagram.com/llamatone (http://instagram.com/llamatone) -not an advert, but a reference)
Last night I modified a very similar parallel stage to go right to a 250Kb pot after the .022 and .0022 caps. Each cap to an end solder tab, on the pot, and the output going out the center to a standard 1MA volume pot with a proper ground reference, as Silvergun put it.. Being that I am still a student of electron activity, I tend to just try it and see if it works.
The resultant sound was a lower gain that had great cleans, but breakup was a on the ugly side, nor did the balance pot have any affect. I guess that the mixed signals are interfering with each other. I'm sure the solution is much more elegent than what I tried, but I had to start with a baseline.
I'm looking forward to see what great minds come up with. :worthy1:
-Scot
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You could drop the Master, but I find it very useful. I just use the two volume controls to blend the mix that I like (kinda set it and forget it) and use the Master to set the overall volume.
yeah, I think the master is a good idea
I always find them useful
So instead of two volume pots, I am leaning towards a mix control pot and then a pre amp volume pot
I think this will make blending the two channels much easier than fiddling with two knobs
So now what value for the Mix pot?
Pre volume is a 1ma of course
And since the original channel separator resistors were 470k, I drew a 1ml mix pot in the schematic below
But I use 250kl pots for mix controls on other devices and I don't stock a 1ml pot
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I still think using two volume controls is more versatile and gives you complete control. The mix pot will never allow you to totally kill one or the other. There will always be some signal from each channel getting thru.
But I'm curious to see how you like the mix control. Still the same number of pots so it's easy to change back if you desire.
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I can see that, but on the revibe and other mix controls I have used, it's not really an issue
With the mix all the way one way or the other you would still get a little bit of that channel but I don't see that as a negative because the other channel would definitely be way louder
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With the mix all the way one way or the other you would still get a little bit of that channel
I'm thinking it's gonna be much more than a little bit. Still curious to see how it actually works out.
On my plexi preamps the bright channel is too brittle sounding and the normal (dark) channel sounds like mud. I'd never use either channel by itself. Blending them together makes the sound very pleasing though. I think my two volume controls are set to blend in almost equal portions, so the mix idea is probably gonna work out just fine. A 1M-LIN pot will probably work better than a 250K.
Let us know how you like it.
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Just an idea so far.
Like you say, easy enough to go back to two volume controls
I will probably shape that mud channel better once I hear it in person
No need really for a 330uf cap on that cathode
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I have not played a real Plexi, so I don't know which of those channels is used the most??
However, IF you used the "bright" channel 80% time (for example) .......... then you could have a FAT control that dials in more of the "fatter" channel.
Geezer and I did this on the Tweed Overdrive Special and I thought it was a really useful tool. This is essentially just another variation of what has already been suggested.
This would still maintain the two volume control idea but allow a mix. Just an alternative and not necessarily a "better" one. Just thought I'd throw this in the mix.
With respect, Tubenit
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That's another good idea
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...and some of the pots are down by the power tranny
I was a little nervous about that too. But it turned out to be a non-issue. My presence pot is closest to the PT, but the NFB circuit is low impedance and seems to be unaffected by the close proximity to the PT or power switch. My amp is very quiet. I don't think you'll have a problem.
On my GA77 I have the PPIMV on the far end, closest to the PT then the presence control. I used shielded cable on both. It's very quit, but I wish I would have put the presence pot on the back next to the speaker jacks. Would be easer to wire up, no need for shielded cable, very short hook up wires.
Trade off is a little harder to adjust it but I think it would be better.
Brad :dontknow:
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I am not even sure the presence is needed
It's very subtle on my old Hoffman Plexi 50
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I am not even sure the presence is needed
It's very subtle on my old Hoffman Plexi 50
If you drop that feedback resistor down to 22K then your presence control will have a lot of presence. :icon_biggrin:
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yeah, that's just copied over from my plexi 50 schematic
That value is bound to change once I fire it up
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I tried to figure out how to semplify this thing
and I think I got a more complicated version :icon_biggrin:
(http://i.imgur.com/WRy7rgm.jpg)
Franco
p.s.: As I can see Topbrent has think to this thing before of me :think1:
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Kagliostro,
That is a GREAT idea! I like it.
with respect, Tubenit
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Maybe replace R1 with a pot sending variable signal to either channel.
EDIT: I was traveling earlier & posting on my phone. To me the issue you pose can be re-stated: 2 or more pre-amps > MIXER > Power Amp
You seem to want to build all this into the same chassis.
Simplified: The Bogen CHB-20A has a tapped 2M pot for this purpose, located on the AUX input. http://el34world.com/charts/Schematics/files/bogen/BOGEN_CHB-20A.pdf (http://el34world.com/charts/Schematics/files/bogen/BOGEN_CHB-20A.pdf)
EDIT 2: I think this is similar to topbrent's post, Reply #1 above.
More complex: The Bogen MXM, http://el34world.com/charts/Schematics/files/bogen/bogen_mxm-a.pdf (http://el34world.com/charts/Schematics/files/bogen/bogen_mxm-a.pdf) , and the Traynor YSR-1, http://el34world.com/charts/Schematics/files/traynor/Traynor_YSR-1.pdf (http://el34world.com/charts/Schematics/files/traynor/Traynor_YSR-1.pdf) , also mix multiple inputs.
However these MXM & YSR-1 types lack true individual channels. Instead they mix into a master tonestack. However, a tonestack could be built into each channel, and mixed after their tone recovery stages.
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Waking up this morning, I'm going over my Plexi 6V6 design in my brain and was wishing there was a way to eliminate two separate volume controls and just have a mix control to blend the two channels.
Then I started to wake up some more and thought, well how would you control the volume of each channel if you only had a mix control?
Then I got to the shop and looked at the schematic and realized that this would not be possible with simple pots
A concentric pot could eliminate one pot, but I am not a big fan of those
On the odd chance that someone has seen this done before in some other fancy way I thought I would throw this out to you guys
The front panel of the Plexi 6V6 would have 7 pots if you include two volume controls and a Master volume
That means the face panel is full all the way across and some of the pots are down by the power tranny
Here's the pre amp schematic and the current face plate mock up
How about moving the input/inputs to the side panel ?no blend,but frees up some space..blend is interesting though.
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put the presence control on the rear next to the Z selector: if there is one.
make sure that you specify coax runs to and from the MV since it will be hovering over the power tubes. shouldn't be a problem if you keep the power tube signal and filaments down low and the MV coax up in the air.
--pete
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This is an old mod published years ago but you still have to use a mix pot and a volume pot.
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Here are examples that show a dual gang balance pot. Doug's example shows a different variety of mix pot.
Pretty simple implementation.
It would require adding two turrets to the board for the 470k-470k/500pf mixing resistors, (which may very well be redundant.)
Looks like it should work, but if there are glaring problems with my drawing, please let me know.
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I've think a lot about this thing
and I'm sure I've the right solution who gives the best performance
with it you can have the mix of each channel from 0% till 100%
so you can get 50% + 50% or also 100% + 100% (and this can't be performed with a blend control)
the secret solution is here
(http://el34world.com/Forum/index.php?action=dlattach;topic=18208.0;attach=49010;image)
:icon_biggrin: :icon_biggrin: :icon_biggrin:
Really, I convinced me that this is the best solution
Franco
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Really, I convinced me that this is the best solution
me too!