Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Jaymz77 on December 20, 2016, 05:26:35 am
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Hi all,
I have just finished installing a turret board into my 1987x reissue.
When I first powered it up there was a faint hum (normal) then a hissing/ white noise type sound came in. I could hear signal (touching the end of a cable) but after a minute or two the hiss faded out (yes slowly faded) and I could no longer hear signal.
Any ideas what that could be? Nothing popped and I didn't let any smoke out but this weird phenomenon freaked me out a bit.
All help appreciated.
Cheers
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did you check DC volts while it faded, anything on the DC side look/act weird?
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To help troubleshoot it really helps us if you can give pictures, and a schematic. Otherwise it's just wild guessing. Do you have a listening probe or an oscilloscope to track down where the signal is disappearing? It does sound possible that the output tubes are biased to shutoff, possibly, but they slowly get there as they warm up? If not, it could be a miswired section of the board, but since we can't see that and don't know the exact schematic you're using, we've got nothing to go on.
~Phil
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Shooter: no i switched it off straight away. I didn't want to let the smoke out. Plus it was late and my usefulness had run out for the night. Thats something ill check.
Here is the layout i have used. SDM layouts are pretty reliable.
One thing i noticed is that the bias pot is maxed in one direction. I may have to change the resistor value. Im using the stock marshall TX's not the metro ones in the layout.
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I'd just validate that the negative bias voltage is in range for what is expected. Do you have pictures? The layout seems right for the schematic I find for the 1987, just do a double check that you get 25k across the end terminals of the pot and 47k at the other, (meaning it should be max 72k).
~Phil
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Cheers.
Sorry it's early here... what do you mean by the 47k and 72k bit? The trim pot is 25k, what are the other numbers for?
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To validate the bias values, the schematic has a 47k resistor and then the pot works as an additional 25k variable resistor. Total at max should be that 72 (47 + 25) or thereabouts. Just making sure you have the variance you'd need for that circuit. If the voltages are out of tolerance as well, then that could be a problem. If the - voltage is too low (too far negative) or too high you'll be out of bias tolerances and either at shutoff or in red plating territory. The first request was to just make sure that you're getting the right resistance ranges between 47 and 72... Then to make sure the voltages are in range too.
I'm not sure what the expected negative bias is as there's still no schematic to look at.
~Phil
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Yeah right, of course. Like I said, it was early. 😊
I don't have a schematic that represents the layout. I do have a schematic for the stock 1987x, but I'm at work right now. I have compared the two and they are basically the same with some minor differences. The layout shows a 150k resistor in the bias circuit but the schem shows a 220k. I always knew that 150k seemed like an odd value and expected to have to change it.
As far as the hiss goes, I'm leaning towards a dry solder joint or 6. It was the way it faded out, along with any signal I had, that had me worried.
This could be a job for my new scope meter if redoing the joints doesn't work.
Cheers
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Good luck, let us know how it turns out.
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D.C. voltage measurements, throughout the amp.
I have no good explanation, but wonder if the bias on some tube goes from normal (passes sound) to very cut-off/high negative bias (no sound).
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Are all the tubes lit when it fades out? May need to turn the lights off to tell.
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Thanks guys.
I'll check all these things and report back.
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Guys I'm as frustrated as hell right now. Its my birthday today, I'm getting old, i got stuck somewhere i didn't want to be today that ate into my tinkering time, i had about an hour on the amp, nothing achieved and now i have to get ready to go to dinner and won't have time to touch the damn thing until boxing day at least.
FRIGGEN HELL.
I tried playing with the bias circuit and got nowhere. After wrestling with damn components to try and get them out without messing anything else up i still can't get the right bias. Ive tried swapping the 150k for 220k and swapping the 47k for 56k, nothing seems to want to give me the right range. Its like it needs a 64k. I noticed that the bias pot on the original is 20k and I'm using a 25k. that will obviously have some effect but i should still be able to get the right bias but maybe at one end of the pots travel.
The hissing has me stumped. It does it on standby and volume has no effect.
Now i have to wait 5 days before i can do a damn thing. AARRGGHH!!!
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That sucks. Yeah you should be able to dial the resistance to maximum or minimum and have a significant effect.
~Phil
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So over the next 5 days when I can't work on the amp I should devise a plan of attack.
What could be possible causes of such a bad hiss when the amp is on standby? I mean, on standby there isn't even any HT flowing in the amp, so how the hell...?
I just don't get it.
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What could be possible causes of such a bad hiss when the amp is on standby? I mean, on standby there isn't even any HT flowing in the amp, so how the hell...?
I've never heard of such. I suspect a wiring error. Show us your work, layout, and schematic.
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I'll see what can do but being such a small chassis will make it hard.
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Here is the 1987x schematic. There are some differences between this and the layout but nothing huge, or so I thought.
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Ok gents, I found a small amount of time today, luckily I had been studying schems all day and had a plan.
Turns out I had the ground for the bias circuit connected to the 47/56k turret instead of the junction of the two 10u caps. Silly mistake. However I was forced to change the bias layout on the fly because I actually left out a turret so I can see how it happened.
Anyway the horrible noise has gone. I still have a small amount of noise but is a lot less. Maybe bad solder joints.
The main issue I have to sort out now is that the inputs seem to be cascaded. Both inputs won't work unless I open both volumes. Not sure how I did that.
The whole point of this exercise was to do a switchable cascade mod, I seem to have inadvertently made it like that before I have even done the mod ( I thought it best to just swap the boards and get the new one working before I did any mods).
I'm nearly there, just got to find this issue now
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*Update
The tiny amount of noise I have left seems to have something to do with the presence pot. When maxed there is no noise at all.
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Ok, well the noise is basically a non issue now. I guess it seemed worse through a set of headphones from a dummy load.
The issue now is that the volumes don't behave as normal. I can plug into either input and crank that volume to 10 while standing a foot away, not much volume really. But if I then crack the other volume that I am NOT plugged into, it then gives me decent volume. And it's also like I have 2 bright channels.
Any ideas? Cheers
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That's got to be another simple wiring error. Check the wires that connect the volume pot wipers to the 470K resistors on the board.
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Well I'll check again but the wiring looks fine to me.
Any other possibilities?
Cheers
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If you wire it like the schematic or layout you posted it will work as expected.
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I traced the wiring last night. But I'll check again
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I have that exact preamp circuit in two amps. I can play just fine through either channel with the unused channel volume control set to zero. That's the way it's supposed to be.
With the symptoms you describe the most likely suspect is a wiring error. Keep looking. You'll find it. Post some big hi-rez pics and we'll help you look.
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Yeah I'll do that tonight. I'm confident it is something simple too.
I spent yesterday with the drawings open on my pc and kept going back every now and then to study. It was doing that that triggered me to what was causing the hiss, checked it when I got home and I was right. Hopefully this one will dawn on me the same way today.
Cheers
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Looking at the schem vs the layout (already noted that there are some differences) I can't get my head around how the circuit works around v1a. I don't know if this has anything to do with the issue or not.
Let's see if I can explain what I see correctly.
The schem shows a 100k plate resistor between v1b pin 6 and the 10k filtering resistors. This is not what I see on the layout. The layout shows no resistor between pin 6 and that 10k, instead it appears to me to be on pin 1.
I do remember when I pulled the stock amp apart that resistor was not connected the way the layout shows. I pulled the factory one off so I could replace it with a cc resistor (part of my experimentation process).
I guess in essence I have the plates of the 2 halves of v2 reversed, by the other pins are correctly wired.
Am I seeing this right?
Do you think this might be my issue?
Cheers
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The schem shows a 100k plate resistor between v1b pin 6 and the 10k filtering resistors. This is not what I see on the layout. The layout shows no resistor between pin 6 and that 10k, instead it appears to me to be on pin 1.
I don't know how you can see a 100K connected to pin 1 and not see another 100K connected to pin 6. The two resistors are side by side with the bottom lead of each connected together. Plain as day.
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Sorry my bad, V2 I'm talking about, not v1
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Change the pin #s on the schematic. Pins 1,2,3 become pins 6,7,8. Pins 6,7,8 become pins 1,2,3. That layout is based on an old 1987. The 1987X schematic has a lot of differences! If you built this amp with the SDM layout you should also get the SDM schematic. But none of this is relevant to your problem.
BTW, the layout you posted lost a lot of detail when you resized and converted to gif format. Here's a properly resized layout with all the missing detail.
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I had to resize it cos it was too big to upload.
I don't have an sdm schem. I could never find one.
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I'm not at my pc at the moment so I won't disagree, but what I thought I saw earlier was that only pins 1 and 6 were swapped, the other were the same
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Yeah you're right. The two halves are completely reversed. No issue there.
I read something somewhere about the two 470k mixer restistors creating a voltage divider that bleeds signal to earth via the unused volume pot. I would think this would result in lower volume on the used channel, as I'm experiencing.
Why I can't work out is what differences prevented this from happening in the factory 1987x.
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Yeah you're right. The two halves are completely reversed. No issue there.
I read something somewhere about the two 470k mixer restistors creating a voltage divider that bleeds signal to earth via the unused volume pot. I would think this would result in lower volume on the used channel, as I'm experiencing.
Why I can't work out is what differences prevented this from happening in the factory 1987x.
Sure, those two 470K mixer resistors will form a 2:1 voltage divider if the unused volume pot is set to zero. Works that way for every 1987 and 1987X ever built. Your amp is the only one that is misbehaving. It's really a simple circuit. The answer is hiding right before you. Keep looking. It ain't wired right.
I don't think anyone can do anything more without seeing your amp.
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Well I'm stumped. I'm out of ideas
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So every 1987 has volumes that both controll both inputs?
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Well I'm stumped. I'm out of ideas
Well then, take some pics.
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So every 1987 has volumes that both controll both inputs?
That's not what I said. How do you jump to that conclusion? Using simple mixing resistors to join two channels together is simple, cheap, and provides a lot of isolation between the two channels. But it's not a perfect mixer. There will always be a tiny bit of interaction between the two channels. But 470Ks provide plenty of isolation so volume control interaction is very minimal.
But the symptoms you describe indicate a problem. Under no circumstances should you be able to kill the signal in one channel by turning the other channel volume control to zero. THAT'S NOT RIGHT!
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No that's not what's happening.
I'm effectively getting 10% volume out of the channel I'm plugged into and the other 90% from the unused volume. Only the volume of the channel I'm plugged into will completely kill the signal.
I'm getting some pics, but all my leads are connected underneath so you won't be able to see them.
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Here
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Another
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The issue now is that the volumes don't behave as normal. I can plug into either input and crank that volume to 10 while standing a foot away, not much volume really. But if I then crack the other volume that I am NOT plugged into, it then gives me decent volume.
Maybe your mixing resistors are 470Ω rather than 470KΩ. Check it.
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Anoher
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Oh my god, you could be onto something, give me a minute
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Yep, 470ohm. Friggen idiot
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You aint the first. I've seen at least a half dozen problems with doing just the opposite, ie, using a 470K rather than a 470Ω on the cathode of a LTP PI. I knew it had to be something simple.
BTW, nice work on that amp.
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Thanks for the compliment.
This is weird. Volume 2 now has no effect on input 1 but I'm getting bleed through even when volume 1 is on zero. But if I'm in input 2 volume 1 still has an effect.
AARRRGGGHHH
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Temporarily remove the 470pF cap from the 470K mixing resistor. Do both channels behave the same now?
Bleed-thru with the volume on zero sounds more like a pot issue to me, ie, it's not really dialing down to zero ohms. Could also be a ground issue with the pot. Put a ground clip on the wiper of the pot. Does that kill the bleed-thru?
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Lifting the cap doesn't change anything.
Earthing the wipers does seem to make it better but not perfect.
Volume 1 wiper earthed while plugged into input 2 makes it bleed better. I can only hear bleed when vol 1 is cranked. It does let more hiss through tho.
Plugged into input 2 with vol 1 wiper earthed does the same but instead of hiss I have buzz, like I'm holding the tip of the input lead
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Plugged into either input with both volumes on zero I can still hear bleed when I touch the input lead tip.
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I'm thinking maybe a dodgy vol 2 as well as some earthing issues. If I touch the chassis while doing any of this it improves the situation.
Then again, I am doing this on a work bench using a patch lead in the input that I'm just touching to get signal, being listened to through a set of headphones plugged into a power soak/ dummy load that I made.
Maybe I should test it properly, plugged into my quad box using a guitar for signal?
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Merry Christmas to all.
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Merry Xmas!
I'd guess that you're onto something. Signal bleed usually is due to either a miswiring or a bad pot, the pot doesn't provide a solid connection to ground, or the pot itself just doesn't get to enough resistance rating to the next path compared to ground, so some of the signal still leaks past.
~Phil
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Maybe bleed is it the right word.
The volumes are interacting with the channel I am not plugged into.
I'm getting half volume out of each volume knob.
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Oh that kind of behavior implies the channels are blending out of phase from one another and cancelling out somewhat.
If you have a layout and schematic and pictures of the exact board we may be able to sort it out, but that sounds like an issue I had with a Gibson EH-185, I basically couldn't jumper channels due to it, they'd be destructive and lose volume.
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The layout, schematic and pics are posted earlier in the thread.
It's odd, because plugged into input 1 it seems to work properly.
Plugged into input 2 I get extra volume when I turn volume 1.
I get a squealing sort of noise when I turn it up a bit.
I'm at a loss. I can't find any wiring mistakes
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Oh duh sorry, somehow my brain didn't think to look at page 1 :P
That could mean that one of the caps there on input 1 output to the pot are leaking across to the second if I'm looking at the schematic right... Either that or the mixing resistors (470k) have one that's way off and letting signal back into the other side? not sure... seems weird.
~Phil
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You mean one of the yellow .022u/ 22nf caps? One of the dear as poison Sozo caps that based on their price should never fail? I'll be annoyed if it is.
I originally has a mistake where I put 470r resistors in there instead of 470k. If I remember correctly the bright channel started working properly after I changed them. But the normal channel is still not right.
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Double check that the other one isn't 470 as well. If so, it could be allowing too much to bleed back into the other circuit in some weird way.
As for the caps, C4 or C5 for the ones that could be or maybe C6. I think C4/C5 could only be problematic IF C6 is as well. The way the schematic shows those two channels they share the same first preamp stage so they'd be in phase for sure.
~Phil
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Could it be an internal v1 tube issue?
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I replaced both of those 470r/ 470k resistors at the same time and just checked them. They are correct.
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Ok I just tried lifting a leg of c5 (bright cap) no change.
I lifted a leg of c6 and it improved noticeably.
What reall makes the problem basically disappear it touching the chassis while I try these things. That obviously suggests an earthing issue. The bus rail across the back of the pots has no connection to earth and didn't from the factory before I replaced the board. Maybe I need to install one. Where should the earth connection be made? Is the earth lug for the preamp filter cap appropriate?
Cheers
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That is supposed to get earth from the pots themselves, but sometimes they dont' make good enough connection. You may want to take a ground off the buss bar behind those and take it to a transformer lug to see if that helps with that.
~Phil
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When you say behind, do you mean after them in the circuit?
Like should I take it from the presence end of the bus?
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I think some of what your chasing are ghosts.
Plugged into either input with both volumes on zero I can still hear bleed when I touch the input lead tip.
By 'bleed' in this 'test' you mean the amp buzzes when you touch the cords tip?
This is not a good test. IMO, it means nothing at all, as it's not a real life use of the amp. You don't use the amp that way, who cares if their amp does that?
(Someone just posted recently that when he turned up the amps volume with NO tubes in the amp started to oscillate. When he put the tubes in, real life use of the amp, the amp was fine.)
Plug in a guitar turn both of the guitars volume to 0 and both amp channels to 0. Still hear 'bleed' when you touch the guitar strings?
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That's a fair point and duely noted.
However what I'm more concerned about is that the opposite volume affects the channel that I'm plugged into.
Your point is a good one. I noticed that noise that I can hear through headphones is inaudible through a quad box. So you are right. But one volume should not affect the volume of the other channel.
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I noticed that noise that I can hear through headphones is inaudible through a quad box.
That's another test, using head phones, that is not how your going to use the amp.
However what I'm more concerned about is that the opposite volume affects the channel that I'm plugged into.
But one volume should not affect the volume of the other channel.
Although I think that Sluckey's probably right, you have something wired wrong, I also think below is part of what your hearing and is normal.
Many amps have 'interactive' volume controls. If you plug into 1 channel and turn up the volume of the other channel it will effect the tone and/or the distortion/drive of the amp.
A Fender tweed deluxe, 5E3, is famous for this. Neil Young had a rig made with servo motors and memory settings to turn the volume controls live on stage during songs to use this. He has foot switches for the different pre programed settings he likes. He's been using this rig live for at least a few decades.
I attached a highlighted schematic of the 1 you posted showing this on your amp.
The red is the normal channel output going to the 2nd stage tubes gird input. But that pF cap/C6 across the 470K mixing R on the other has at least 3 different paths to ground depending on where that bright channels volume is set.
When the bright channels volume is off/0 then the signal that goes back through the 470pF/C6 cap goes to ground.
If you turn the bright channel volume up all the way now you will bypass C5 and go to the 22nF cap/C4 to the 100K plate R then through the B+ filter cap to ground.
This is why your having the problems plugging into the normal channel, input 2, because when you plug into the bright channel, input 1, there's no 470pF bypass cap across that channels 470K series mixing R. So it has only the 470K mixing R to bleed through and that 470K value is giving good isolation from the other channel.
The orange line shows where to break that 'bleed' path. That's what you did when you lifted 1 end of that cap.
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Wow, what an awesome reply. Thanks for that.
I took the amp upstairs to do a proper test and it appears to be working normally, that is the volumes are working independently, that is I get no volume increase from the opposite channel as I did before. I do get what is described above and that's fine. A couple of observations.
There is definitely some bleed on the bright volume pot. Set to zero I can hear guitar through speakers.
I seem to be getting a lot of hiss of the treble channel.
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I took the amp upstairs to do a proper test and it appears to be working normally, that is the volumes are working independently, that is I get no volume increase from the opposite channel as I did before.
So the amps working normally when testing the amp through a speaker cab and not with headphones and a dummy load?
There is definitely some bleed on the bright volume pot. Set to zero I can hear guitar through speakers.
Well, not really a problem as long as you don't play your amp on 0. (If it still bugs you buy some different pots and swap them in until you find 1 that is silent on 0.)
I seem to be getting a lot of hiss of the treble channel.
Bright channel will have more hiss than normal channels as hiss is hi end frequency. Could be a noisy 12AX7. Keep trying different 1's until you find a quite 1.
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Well, yes it does now appear to be functioning properly apart from that bleed on volume 1.
It does hiss on the bright channel more than I recall it doing before I installed the turret board, but that could also be from not being installed in the head cabinet yet. I've heard these things can be as noisy as hell when not in the cabinet.
I'm now gong to attempt to do the second thing behind this whole exercise. Install a push pull pot to cascade the stages. Wish me luck
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Well I performed the switchable one wire mod no problems, so it seems. However I'm not terribly impressed with it to be honest. It doesn't seem to be giving me too much extra gain. I definitely get some but not huge amounts. Perhaps I was expecting too much.
I read somewhere that to get the most out of the mod you need to change other things like the tone stack slope resistor.
Any feedback on this mod for a 1987?
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Take a look at Hoffman's hot switch mod. It certainly has plenty of gain if done properly.
http://el34world.com/Hoffman/files/Hoffman_Plexi50.pdf (http://el34world.com/Hoffman/files/Hoffman_Plexi50.pdf)
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Cheers.
I'll have to work out how to apply that to a 4 holer
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Cheers.
I'll have to work out how to apply that to a 4 holer
Huh??? Hoffman's Plexi is a 4 holer if you want it to be.
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The drawings you linked show 2 with one removed for the switch
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The drawings you linked show 2 with one removed for the switch
The drawings show either 2 or 4 inputs (your choice). But the hot switch mod has nothing to do with the input jacks so only 1 jack is shown on the hot switch page. Notice that the unused volume is now a master volume?
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Not that its a huge issue but the drawings clearly only show 2 input jacks. Bright high and normal high.
its easy enough to add the lows, but that is what the drawings show.
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You're not looking at the bottom of the PDF, it's there. The hotswitch mod version if on the bottom.
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I don't know what you are looking at but on the top, unmodified pdf it has 2 jacks. The bottom, modified pdf has 1 jack and a switch.
I don't want to start a brawl over it because ultimately it doesn't matter, but thats is what is on the drawings.
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Look at the schematic! Hoffman's layout is of no use to you since you followed the Metro layout.
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Ok sorry, the little box in the corner.
Still only one of the bottom schem.
What is the difference between this and the "one wire mod" I can see that one 470k mix resistor comes off the coupling cap from v1b and goes to ground
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Still only one of the bottom schem.
Well, you got the full schematic on page 3 then you got just the HS mod later on. The HS schematic only shows the stuff that's relevant to the mod.
You got the schematics. Look and see. Hoffman's Hotswitch basically changes the Plexi into a 2204 except you use a dedicated toggle switch rather than a switching jack that requires you to move the guitar cable from one jack to another.
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I thought thats what the one wire mod did, made the 1987 a 2204. Im figured i must have been wrong about that when i did it and heard the result. That being said, i haven't spent much time on it. Just an operational check really. Ill have to spend some more time on it for an accurate assessment.
I was looking over this "Neil Special" layout i have, and it seems they all do similar things.
In the attached picture (one wire mod), what does the 100k on on the switch do? Whats it there for?
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I thought thats what the one wire mod did, made the 1987 a 2204.
It does. All those mods will cascade the two triodes in V1. The one wire mod is more involved than just one wire. Sure, one wire will cascade the stages but there are several other things that need to happen to make it sound as good as a 2204. It's all documented on the net.
Hoffman's hotswitch will be very usable. That Metro looks pretty good too.
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In the attached picture (one wire mod), what does the 100k on on the switch do? Whats it there for?
Can you post a schematic instead of a layout drawing?
Sooooooo much easier to see what's going on with a schematic than looking at a layout drawing.
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Willabe, sorry i don't have one.
Here is the layout of the "Neil Special". Im liking the looks of this one. Obviously this is on a 100w'er. The preamps are almost the same though, apart from the mods to the normal channel.
It says it uses the 2203 low input. Would making that i high input be as simple as putting a 1M across the jack?
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For the record, I spent some more time evaluating the mod.
It's ok. A bit wild and feedback prone. There is more gain there than I first thought. But you need to use it sparingly in conjunction with the power tube breakup. On its own it sounds crappy.
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Disregard the question above about making the 2203 low input a high one.
After studying the layout i can see whats going on. The high input works on both settings.
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Well gents, I think I've found my happy place.
All those mods I was doing at the start of this thread I have now removed and redone the way of the "Neil special" above, minus the switchable PT.
All I can say is HELL YEAH! That's what I'm talking about!
The main difference from what I can see is that this way turns v1a into the cold clipper of the 2203/4, the previous way didn't. Of course this means you lose the normal channel in 1987 mode, but who uses that anyway? You can still get a nice clean out of the 2204 low input (high input is common to both modes).
I'm very happy with it now. I can get the roaring 1987 bright channel plus a 2204 in one amp, happy days.
I used the earthing scheme from the "Neil special" and it has quietened the amp down noticeably. You do get some noise in 2204 mode when the gain is cranked, but I feel the level of noise is normal for that circuit. I might have a play to see if it can improve that at some stage.
Someone with a 2204 might be able to chime in and let me know how much noise to expect out of that circuit.
Anyway, thanks for everyone's help it is much appreciated.
Rock on
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Outstanding,
I'm sure I'm not the only one that would love hearing a clip or two... ;)
~Phil
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I'm sure I could manage that. I might see if I can borrow a mic and do a decent one, otherwise I'm t will just be on my phone.
Promise to ignore the playing tho?
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Hey Uncle Doug isn't the best player I've ever heard, and I still like listening to his demos, so you can't be too bad ;) (don't know if you've watched his videos, but he does a lot of demos on them, some great others so-so).
On top of that, I do videos and my demos are pretty horrific if you asked me, but I'm just trying to let people hear what it sounds like.
Some people think I play pretty well, but I don't :D I've listened too too much Page, Hendrix, Clapton, etc to feel very confident.
~Phil
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Yeah I hear you. I've had my bat set pretty high by those same guys.