1987 turret board rewire- Hiss on start up that fades

[Jaymz77]

Plugged into either input with both volumes on zero I can still hear bleed when I touch the input lead tip.

[Jaymz77]

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?

[Jaymz77]

Merry Christmas to all.

[pompeiisneaks]

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

[Jaymz77]

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.

[pompeiisneaks]

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.

[Jaymz77]

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

[pompeiisneaks]

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

[Jaymz77]

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.

[pompeiisneaks]

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

[Jaymz77]

Could it be an internal v1 tube issue?

[Jaymz77]

I replaced both of those 470r/ 470k resistors at the same time and just checked them. They are correct.

[Jaymz77]

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

[pompeiisneaks]

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

[Jaymz77]

When you say behind, do you mean after them in the circuit?
Like should I take it from the presence end of the bus?

[Willabe]

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?   

[Jaymz77]

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.

[Willabe]

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.     

[Jaymz77]

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.

[Willabe]

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.   

[Jaymz77]

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

[Jaymz77]

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?

[sluckey]

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

[Jaymz77]

Cheers.
I'll have to work out how to apply that to a 4 holer

[sluckey]

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.

[Jaymz77]

The drawings you linked show 2 with one removed for the switch

[sluckey]

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?

[Jaymz77]

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.

[pompeiisneaks]

You're not looking at the bottom of the PDF, it's there.  The hotswitch mod version if on the bottom.

[Jaymz77]

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.

[sluckey]

Look at the schematic! Hoffman's layout is of no use to you since you followed the Metro layout.

[Jaymz77]

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

[sluckey]

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.

[Jaymz77]

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?

[sluckey]

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.

[Willabe]

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. 

[Jaymz77]

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?

[Jaymz77]

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.

[Jaymz77]

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.

[Jaymz77]

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

[pompeiisneaks]

Outstanding,

I'm sure I'm not the only one that would love hearing a clip or two... ;)

~Phil

[Jaymz77]

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?

[pompeiisneaks]

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

[Jaymz77]

Yeah I hear you. I've had my bat set pretty high by those same guys.