Mod in 2203 - what is this??
- Thread starter peterc52
- Start date
glpg80
Well-known member
It’s two mods in one. It’s a 200k plate resistor on the last 820 stage, and it looks like it’s also a footswitchable bypass of that stage that lets you bypass that stage directly to the tonestack.
The two smaller resistors on the top left are also not stock and also both 200k. Looks like every plate resistor is 200k in that amp.
Why they didn’t just put the cap on the Jack instead of fly leading it like that is beyond me. Total hack job but it works. The 200k plates across the amp would be preference for a pretty significant gain increase. I can see it went unstable, because with a gain increase like that they had to solder the grid stop resistor directly to the first stage.
Plate resistor increases are a different kind of gain increase though, as it shifts your bias point for the tube colder. I’m not a fan of going ham on increasing plate resistors IMO.
The two smaller resistors on the top left are also not stock and also both 200k. Looks like every plate resistor is 200k in that amp.
Why they didn’t just put the cap on the Jack instead of fly leading it like that is beyond me. Total hack job but it works. The 200k plates across the amp would be preference for a pretty significant gain increase. I can see it went unstable, because with a gain increase like that they had to solder the grid stop resistor directly to the first stage.
Plate resistor increases are a different kind of gain increase though, as it shifts your bias point for the tube colder. I’m not a fan of going ham on increasing plate resistors IMO.
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glpg80
Well-known member
The higher setting is safer on tubes and bias. It’s better to undervoltage a primary than over voltage.
glpg80
Well-known member
It looks like they short tip to ring, and sleeve is footswitch ground. Its not your typical control type of footswitch - your guitar signal is passing through the switch and back into the amp. It’s a single button you need. It may have to be built. It’s not the way things are normally done FYI.
harddriver
Well-known member
Splawn used a stereo jack for his two button footswitch in the early Marshall mods and Quickrods. One footswitch function for the rhthym/lead and one footswitch function for the solo boost volume engagement. If there are two functions to that mod then a two button footswitch would be needed maybe that's why he used the stereo jack instead of a regular jack.
Maybe the one function is like a tonestack lift selection aside from lowering the plate voltage to the stage?
My guess is they are using the large electrolytic cap to dampen the footswitch from clicking when engaged I looked it up and Splawn used a 1 uf cap in line before the signal goes through the switch.
If this byapasses V2A gains stage then is this some form of crude clean channel maybe?
Maybe the one function is like a tonestack lift selection aside from lowering the plate voltage to the stage?
My guess is they are using the large electrolytic cap to dampen the footswitch from clicking when engaged I looked it up and Splawn used a 1 uf cap in line before the signal goes through the switch.
If this byapasses V2A gains stage then is this some form of crude clean channel maybe?
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SpiderWars
Well-known member
I'm not seeing any 200k plate resistors? They're all Brown-Black-Yellow but not the stock 100k.
To me it looks like the footswitch adds that elytic cap in series with the existing treble cap. But that would serve to REDUCE the total capacitance there which I can't wrap my head around why you would want to do that. I could almost see increasing the treble cap to serve as a mid boost of some sort but not reducing it. And I agree about the hack job.
How does the amp sound when you footswitch it?
EDIT: Now that I look closer, it almost looks like a mute or something. The big elytic looks to be in series with the existing treble cap but is so big relative to the treble cap that it's negligible. The tip and ring are soldered together at the jack so it appears the red-white wires are always common so that elytic cap is always in circuit. But when you press the footswitch it grounds out the signal (thru the big elytic cap). Does that sound reasonable? And what if there was a pot in the footswitch? Footswitchable boost?
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SpiderWars
Well-known member
Didn't see this until after my edit above. I think that's what it is. It's either a mute or...I'm wondering if there was a pot in the footswitch if it would give you a footswitchable "boost". It actually wouldn't be 'boosting' per se just attenuating but the end result is the same...more volume vs less volume. But I'm not sure if that would work or be unstable.
It looks like a TRS insert loop/preamp out to me.
The electrolytic capacitor is for DC blocking of the high-voltage on the cathode. It is a large value so there is no low-frequency rolloff when driving a low-impedance (10k-100k). The cathode follower output signal goes to the tip, the return comes from the ring and goes back to the tone stack.
If you use a mono 1/4" cable the return side will be muted so it acts as a preamp out, if you use a stereo TRS cable, it becomes a send/return for a series TRS insert FX loop, or for a volume pedal. When the cable is unplugged, the switching connections tie the tip to the ring for normal operation.
The electrolytic capacitor is for DC blocking of the high-voltage on the cathode. It is a large value so there is no low-frequency rolloff when driving a low-impedance (10k-100k). The cathode follower output signal goes to the tip, the return comes from the ring and goes back to the tone stack.
If you use a mono 1/4" cable the return side will be muted so it acts as a preamp out, if you use a stereo TRS cable, it becomes a send/return for a series TRS insert FX loop, or for a volume pedal. When the cable is unplugged, the switching connections tie the tip to the ring for normal operation.
harddriver
Well-known member
And that's why you are the amp guru Randall......
So he basically has an effects loop. I wonder why he didn't just use separate regular jacks with one for the red wire as the send and the white wire as the return as he could have used the upper unused input jack hole that's above the already installed stereo jack.
So with the stereo jack setup one would have to have another remote box that then splits the red send and white return separate again to for a send signal that can be routed through effects mono then recombine back that remote box with the white wire return to form a complete loop?
SpiderWars
Well-known member
So the tip and ring tabs on the jack are not soldered together? It's hard to tell but it looks like they are.
EDIT: nevermind I see it. the tabs on the unseen side are not soldered together, the tabs on top make the connection when unplugged.
A TRS effects loop allows use of a standard TRS insert cable that has a 1/4" stereo jack on one end and two 1/4" mono jacks on the other end. It saves one cable. I used single-jack TRS insert FX loops on my Sabre and Intruder amps because I hated having to run two cables for each loop.
https://www.sweetwater.com/store/de...able-1-4-inch-trs-to-dual-1-4-inch-ts-12-foot
https://www.sweetwater.com/store/de...able-1-4-inch-trs-to-dual-1-4-inch-ts-12-foot
peterc52
Well-known member
It's not a great loop. It has a buffered send, but no buffered return. The signal is high-level coming off the cathode follower, so it will be too hot for pedals, but might be okay for line-level rack unit. Since it doesn't have a buffered return amplifier, the drive to the output will be low, especially after it goes through the tone stack, so you likely won't be able to get full volume, unless the rack effects has a high output level setting (some do).
I'd say try it, and if you don't like it, rip it out and replace it with one of the high-voltage LND150 mosfet-based loops on the market.
I'd say try it, and if you don't like it, rip it out and replace it with one of the high-voltage LND150 mosfet-based loops on the market.
glpg80
Well-known member
A common 2 button switch will not work as I mentioned before - you’d have to wire the switch to short tip to ring and not tip to ground and ring to ground like most switches and correct designs.
Yes I am colourblind and brown and red are close - the resistors are 100k (brown is 1, red is 2, easy mistake). They still have been changed though.
If I were the OP I’d rip out whatever that is and remove it. It’s not the common method to install a loop and as mentioned before tonestacks need a buffer beforehand otherwise they are quite an impedance load vs frequency to whatever is driving it in the loop.
Better to just get a metroloop installed. I hope no traces have been cut on the board, it looks like they may have underneath. You’ll have to drill tiny holes to attach tinned wire to then solder to permanently short it back together.