Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: NSB_Chris on May 01, 2019, 08:44:54 pm
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Hello All,
I have been researching/planning for my next build and came across something interesting. While searching for channel switching schematics I came across the following preamp circuit:
https://drtube.com/schematics/marshall/2000pre3.gif
I am not interested in building this particular circuit, but some aspects gave me pause. It looks like for the “mix” position, the footswith can toggle on either channel. Rather than just switching the channel connection on and off, they ground the channel that they want to be off, so they can have either channel on by themselves or mix the two. They use 10k mixing resistors between the two channels. I think fender usually used 220K mixing resistors. Seems the lower mixing resistor value would bleed a lot of the signal to ground when only one channel was engaged. Comments?
I would be interested in how others prefer to achieve simple channel switching. My thought for this amp right now is a simple Bassman 5F6-A amp but be able to switch between the 5F6-A preamp and something like the JMP2204 preamp. I would also put master volumes on both preamp sections. I don’t think I would mix the two, only switch between them.
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It looks like for the “mix” position, the footswith can toggle on either channel. Rather than just switching the channel connection on and off, they ground the channel that they want to be off, so they can have either channel on by themselves or mix the two.
There is a "mix" input jack and a "mix" effects loop that I'm seeing. I presume you mean the "mix" input jack. You can wire a two-button switch for one, the other, or both. You can also wire a one-button to do one or the other. Or a two-button with each button acting on its own channel independently so that the end user is perpetually confused.
They use 10k mixing resistors between the two channels. I think fender usually used 220K mixing resistors.
The Marshall circuit in question is in the low impedance realm whereas the classic Fender 220K/220K mixer is in the high impedance realm.
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Thanks. Yes, I was referring to the “mix” input jack. Interesting configuration, but I would just have the input go to both channels an have a single button switch that toggles between the two channels. The mixing resistiors on this circuit being right after the cathode follower did not occur to me. Makes sense!
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They gain-up to >60V signal out of V2a V6a. They want to get down to <1V signal with low impedance at the Send/Return jacks. So a tone-stack, buffer, 22k:5k divider/pot, and some 10k mix resistors.
> Seems the lower mixing resistor value would bleed a lot of the signal to ground
Signal is CHEAP. You need two gain stages to get from guitar to power tube grid. Maybe another with heavy tone-stack. Here we have five gain stages from guitar to power tubes. The idea is to gain-up until the small tubes cry, then "bleed signal" so the following stages can be worked clean (or not, if the player desires).
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Good info. Very cool!
In looking around a bit more, I realize that Rob Rob has added a JCM800 6V6 circuit which is EXACTLY what I was going for.
https://robrobinette.com/RR2104_Master_Volume_Micro.htm
This saves me a huge amount of effort comparing circuits and agonizing over decisions. I will probably start another thread when I get further along, but what I am considering is this circuit with essentially something close to a Bassman 5F6-A as a clean channel. Looks like I could use the RR2104_Master_Volume_Micro circuit but after the coupling cap following the first gain stage, have a relay diversion to the Bassman circuit which would enter at the volume pot (probably put the bright cap on a switch). With this the first gain stage would be shared by both the JCM800 and 5F6-A circuits, and the rest of the preamp circuit would honor pretty closely the originals.
I think I would probably put master volumes on both after the tone stacks and mix before the phase inverter. Maybe not necessary on the Bassman side, but it is easier to defeat it later than drill another hole after things are already assembled.
Maybe it makes more sense to have the first gain stage be directly shared by both the JCM800 and 5F6-A preamp circuits and have the relay just switch in before the phase inverter (no mixing required).
Thoughts?
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Wow, I am not finding a lot of reference material for channel switching, let alone channel switching with relays.
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Just found this link, which has some discussion on the topic:
https://el34world.com/Forum/index.php?topic=14799.0
Easy enough to have the relays switch between preamp circuits at both the input and output of the preamp, but I question the benefit. Seems simple enough to have both preamps active all the time and just have the relay control which signal goes to the power amp section.
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I switch only the preamp outputs in my Marshall Dual 50... http://sluckeyamps.com/misc/Marshall_Dual_50.pdf
And I switch only the preamp inputs on my Dual Lite... http://sluckeyamps.com/dual_lite/dual_lite.pdf
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Thanks! Big help seeing what others have done successfully or at least have reviewed with experience and deemed acceptable.
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Another question... Below is my current draft which routes to the two channel signal paths after the first gain stage and relay switches between preamps at the end. Some of the channel switching references that I have seen included a capacitor between the first gain stage plate and the “y” connection to the two gain stages. With coupling capacitors on both channels I don’t see the advantage of another capacitor before the “Y”. Thoughts?
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No need for additional caps ***BETWEEN*** the preamps and point Y. However, there must be a cap between point Y and the LTP PI grid.
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Your circuit looks like it will operate, as drawn.
No, the C? After the first triode is not needed.
One thing you may not have considered, your two volume controls are now in parallel, so are effectively 500K pots.
Without having a relay to disengage one channel from the other, you are getting half the output of the first triode into the grid of each of the 2nd stage triodes.
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* luckily, 2Meg audio taper pots are easy to find on Ebay.
If you don't want to add another relay, replacing the two 1M volume pots with 2Meg pots will allow the 2204 style channel to go further into distortion.
The bassman-esque channel wouldn't change as dramatically, however, both pots need to be changed.
With both 1M pots switched to 2M, the anode of the first triode "sees" 1M to ground, not 500K.
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Haha! I missed this one big time. At the mention of "Y" I immediately jumped to the output of the relay. Never even saw the red circle that you were talking about. :embarrassed:
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....another capacitor before the “Y”.
I think this would be fine: keep C? but double-value, lose C? and C? because it's all ground after the C? at the plate.
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Thanks everyone! I think I will just relay on the input side after the first gain stage. I will either leave the relay on the back side of the preamp sections or just use mixing resistors and bring them back together there.
For the relay on the input side, I am guessing I don’t have to ground the inactive lead because the volume pot leaks to ground. On the back side, I am guessing the same is true because the MV pot leaks the inactive side to ground. I will update the schematic and post it.