Another question...
On my imperial clone buil I wasn't happy with the way the OD channel sounded so I changed that part of the circuit.
Apparently , due to the design, the clean channel and the OD channel seem to interact. I noticed that when I turn down the gain knob ( A1M voltage divider ) the clean channel looses clarity. I think that because of the shared 1st triode there is some signal bleeding down via the voltage divider to ground.
I drew a resistor (blue) to try blocking the signal going in that direction. Is this a good approach? I haven't tried it yet, but I'd like to know what you guys think. Or is there another way to avoid too much interaction? Adding a relay between the plate load resistor and either circuit is an option, but that would be the least of my favorite options...
Unwanted interaction between channel 1+2
Moderators: pompeiisneaks, Colossal
Unwanted interaction between channel 1+2
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Re: Unwanted interaction between channel 1+2
Solved! Tried various values and settled for 100k in that spot.
Re: Unwanted interaction between channel 1+2
I dunno how that could help, but as it has, great!
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Re: Unwanted interaction between channel 1+2
If you look closer to the schematic you can see without the 220k in blue the signal has a bleeding path through the 1M voltage divider pot..
Well 220k was too large.. so the 100k is resisting that signal enough to follow the path into the clean channel
Well 220k was too large.. so the 100k is resisting that signal enough to follow the path into the clean channel
Re: Unwanted interaction between channel 1+2
I just don't see it 
If it happened at very high settings of channel 2 gain then yeah, I could see that the grid clipping, even via the 10k grid stopper, might affect the clean channel.
But not a low settings.
If it happened at very high settings of channel 2 gain then yeah, I could see that the grid clipping, even via the 10k grid stopper, might affect the clean channel.
But not a low settings.
https://www.justgiving.com/page/5-in-5-for-charlie This is my step son and his family. He is running 5 marathons in 5 days to support the research into STXBP1, the genetic condition my grandson Charlie has. Please consider supporting him!
Re: Unwanted interaction between channel 1+2
I'm far from being an expert.. but this is my explanation:
Since both channels share the 1st preamp stage and load resistor they always have to interact b/c the unused channel's signal gets grounded way further before the PI. In the original imperial's circuit there must have been enough resistance to avoid the signal entering the unchosen channel to an audible level.. and follows the part of the path into the tone circuit of the chosen channel.
But by altering the OD channel I must have disturbed that balance. Apparently when the 1M voltage divider is set to a low level the least resistive path is through that pot to ground. Let's say the pot is at 25%. The total resistance would be +- 60k from the pot + 47k from the resistor between pot and ground = +-- 107k. The signal gets easier through here and this results in a loss of volume and high frequencies on the clean channel path. When the voltage divider pot is set to more resistance like 50% , then the total resistance is +- 250k... enough to avoid signal going through this pot, or rather choose the least resistive path through the 250pf into the treble pot of the clean channel..
Does this make any sense? You are the expert... I'm just a noob
Since both channels share the 1st preamp stage and load resistor they always have to interact b/c the unused channel's signal gets grounded way further before the PI. In the original imperial's circuit there must have been enough resistance to avoid the signal entering the unchosen channel to an audible level.. and follows the part of the path into the tone circuit of the chosen channel.
But by altering the OD channel I must have disturbed that balance. Apparently when the 1M voltage divider is set to a low level the least resistive path is through that pot to ground. Let's say the pot is at 25%. The total resistance would be +- 60k from the pot + 47k from the resistor between pot and ground = +-- 107k. The signal gets easier through here and this results in a loss of volume and high frequencies on the clean channel path. When the voltage divider pot is set to more resistance like 50% , then the total resistance is +- 250k... enough to avoid signal going through this pot, or rather choose the least resistive path through the 250pf into the treble pot of the clean channel..
Does this make any sense? You are the expert... I'm just a noob
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Re: Unwanted interaction between channel 1+2
Let me try to explain why that should not be related.
The signal is already 100% shared at the point you put a resistor. That means all you're doing is limiting current for the V1B triode. It shouldn't impact the bleed.
The output from that place of equal potential is the tone stack that then leads to the grid of the V2B triode, and the other half of that signal goes to the grid of the V1B after going through a cap and a pot. their respective outputs go to the relay 1 switch which chooses which signal path is grounded and which is allowed through, then into mixing resistors that go off to X which I believe must be the tone stack? What's not clear to me is if relay 1 and relay 2 are tied somehow, i.e. when you choose the channel with relay1 does taht also automatically choose the reverb signal path between one channel and another?
The locations I can see on this schematic that 'might' allow some bleed are the mixing resistor network, if you don't have a full outstanding ground on that relay, some signal will bleed through to that other side... ONLY then.
There could also be some bleeding through the reverb?
How do you know for sure there's signal bleed if the input signal is identical? just the amount of gain on one vs the other? Are you on the cleaner one and still getting gain?
~Phil
The signal is already 100% shared at the point you put a resistor. That means all you're doing is limiting current for the V1B triode. It shouldn't impact the bleed.
The output from that place of equal potential is the tone stack that then leads to the grid of the V2B triode, and the other half of that signal goes to the grid of the V1B after going through a cap and a pot. their respective outputs go to the relay 1 switch which chooses which signal path is grounded and which is allowed through, then into mixing resistors that go off to X which I believe must be the tone stack? What's not clear to me is if relay 1 and relay 2 are tied somehow, i.e. when you choose the channel with relay1 does taht also automatically choose the reverb signal path between one channel and another?
The locations I can see on this schematic that 'might' allow some bleed are the mixing resistor network, if you don't have a full outstanding ground on that relay, some signal will bleed through to that other side... ONLY then.
There could also be some bleeding through the reverb?
How do you know for sure there's signal bleed if the input signal is identical? just the amount of gain on one vs the other? Are you on the cleaner one and still getting gain?
~Phil
tUber Nerd!
Re: Unwanted interaction between channel 1+2
I hear you... but by adding that 100k between the plate of V1B and the coupling cap to V1A the tone and volume loss on the clean channel went away. So there must be interaction somehow?
The relays 1 and 2 are switching simultaneously in order to let the reverb switch along with the required channel.
I get no OD on the clean channel unless the volume is turned past noon. Normal behaviour of a 20W amp...
To let you in on why I did all these changes:
After the build I noticed fizz on the OD channel. Not pleasant... on the clean channel too when dimed.
I went through a whole process of chopsticking, resoldering, grid stoppers in various values, lead dress, changing tubes, ruby mod, ... all to no avail. Then I changed the paraphase PI into LPT PI.. this helped some.. but still that annoying fizz. I was thinking: even without the fizz I wouldn't like the OD.. that midbite overrides the entire tone setting.. so I changed the preamp into an 18Watt TMB, but with a single tone knob and a gain control.
There is still a little fizz, but I fear it will always have it. Now the tone control is at least useable and the little fizz is neglectable...
The relays 1 and 2 are switching simultaneously in order to let the reverb switch along with the required channel.
I get no OD on the clean channel unless the volume is turned past noon. Normal behaviour of a 20W amp...
To let you in on why I did all these changes:
After the build I noticed fizz on the OD channel. Not pleasant... on the clean channel too when dimed.
I went through a whole process of chopsticking, resoldering, grid stoppers in various values, lead dress, changing tubes, ruby mod, ... all to no avail. Then I changed the paraphase PI into LPT PI.. this helped some.. but still that annoying fizz. I was thinking: even without the fizz I wouldn't like the OD.. that midbite overrides the entire tone setting.. so I changed the preamp into an 18Watt TMB, but with a single tone knob and a gain control.
There is still a little fizz, but I fear it will always have it. Now the tone control is at least useable and the little fizz is neglectable...