high gain lee jackson gp 1000

[Gorhrut]

thats what i figured on the cf. oh well, its just a couple extra components to wire up anyway.

and thanks for the clarification. idk why i was so confused by that, the way it was drawn....

[Rune]

Okay, so I tried rewiring the pot and simply remove the pot from the circuit just connecting the masters to C4, and the problem is still there. It is like a screeching frequency that totally overdubs the guitar sound, what I posted before was at the lowest possible noise setting at low trimpot level, at high level it is just noise when you play the guitar, not radio interference, and not squealing sounds, just HF buzz screeching sound.
So I guess the issue have to be somewhere before the trimpot in the circuit...
Now remember this is the clone so it might just have been built wrongly from the start... Any suggestions to what this could be? Could post soundclips of it tomorrow if required.

[Gorhrut]

Okay, so I tried rewiring the pot and simply remove the pot from the circuit just connecting the masters to C4, and the problem is still there. It is like a screeching frequency that totally overdubs the guitar sound, what I posted before was at the lowest possible noise setting at low trimpot level, at high level it is just noise when you play the guitar, not radio interference, and not squealing sounds, just HF buzz screeching sound.
So I guess the issue have to be somewhere before the trimpot in the circuit...
Now remember this is the clone so it might just have been built wrongly from the start... Any suggestions to what this could be? Could post soundclips of it tomorrow if required.

im not as smart as most of the guys on here so theyll probably have better info, but....

does the sound change as you adjust the input volume? if so the problem is before v1b. so either input wiring/components/etc or v1a wiring/components/bad tube?(what tubes are you using? if you went the chinese 12ax7 route those can be sketchy. have to go through a couple to find quiet ones)

also, you can try pulling tubes(starting with the third tube and working back) and see when the sound disappears. the tube where it disappears is the problem stage in the amp. then you can go in and do some more detailed troubleshooting

[Rune]

Tried several GT sets also with 12au7s and also a EH set. No difference, EH et actually made it worse due to a faulty tube.
Also there is no noise present when you mute the guitar/strings and it happens on all my 9 guitars with 5 different pickups/hookup methods.
It is not the tubes and not the guitars, might be a faulty input on the amp or might be that the switching system implemented on on V1a is fucking it up.

[Gorhrut]

finished redoing the preamp. billyz is (hopefully) going to give it a clean bill of health tomorrow....

[img3156]http://i.imgur.com/ZrOr47N.jpg[/img]
[img2340]http://i.imgur.com/0WNUhLm.jpg[/img]

[Rune]

Looks great and easily modified! Some wring yet, but a lot better than the original. I still crave double masters thought.

You know what I can't for the life of me find R101 (input resistor) on the clone! He might have hid it underneath some pcb at the back input. But if it isn't that would be a major issue, right? The R100 (1m resistor) is there clear as day, there are some really weird solutions used in the clone which obly makes it more unstabe if traces aren't perfect. Lots of small PCB islands Woth only a couple components....

[Gorhrut]

Looks great and easily modified! Some wring yet, but a lot better than the original. I still crave double masters thought.

You know what I can't for the life of me find R101 (input resistor) on the clone! He might have hid it underneath some pcb at the back input. But if it isn't that would be a major issue, right? The R100 (1m resistor) is there clear as day, there are some really weird solutions used in the clone which obly makes it more unstabe if traces aren't perfect. Lots of small PCB islands Woth only a couple components....

"It creates an input impedance for your signal. Check out "voltage dividers" online. Your pickups have impedance/resistance which forms a voltage divider with the 1M resistor. The higher the resistor, the more signal/voltage gets dumped at the input of the first stage allowing the amp to create more overall gain (which is amplification, not overdrive as per the common misunderstanding)."

found that online about the input resistor. so i dont think not having it would cause MAJOR issues, but it would behave differently.

its good twcc has us here to double and triple check his work...lol. he seems like a good engineer though. he seems to know what hes doing so i doubt he missed that.

[Nigel Tufnel]

Hope I'm not pissing on the op's thread or any forum rules for that matter but I have one of these gp-1000's circa 88' sitting in the closet. I'm sure I could be persuaded to part with it if anyone is interested. No mas thread hijack, shoot me a pm

[Gorhrut]

dont need another unfortunately, just want to say...

the coolest thing about these amps is that they go to 20, not just 11, nigel tufnel


and i dont mind the thread being hi jacked. it might spark the convo on this thread again....

[Rune]

Double and triple checked today, R101 the 68k resistor going to tube is missing, it is not on the back input either, so it was left out. I don't know if it is something that could create noise, but worth checking.
Also... the whole TUBE PCB is kind of messy on the clone...Especially around V2, seems like I need to reopen my original 88 and check out the back of the original tube PCB, a lot seems to be not following schematic... Not just he mods, but the whole arrangement.

[Gorhrut]

Double and triple checked today, R101 the 68k resistor going to tube is missing, it is not on the back input either, so it was left out. I don't know if it is something that could create noise, but worth checking.
Also... the whole TUBE PCB is kind of messy on the clone...Especially around V2, seems like I need to reopen my original 88 and check out the back of the original tube PCB, a lot seems to be not following schematic... Not just he mods, but the whole arrangement.

in my last post i thought you were talking about the 1M on the input. my bad.
the 68k is called a grid stopper resistor. here's the info on these:

The grid resistor accomplishes the following things:

It helps prevent high frequency parasitic oscillation in the tube itself
It helps prevent radio frequencies from getting into the input stage, where they can be rectified and lowpass filtered (AM detection) and become audible at the amplifier output
It can limit grid current when the tube is driven into the positive grid region, which helps in preventing "blocking" distortion



its easy to add that resistor but you could also snap a ferrite bead on your guitar cable to accomplish the same high frequency and rf attenuation.
here's a cool vid on those
https://www.youtube.com/watch?v=81C4IfONt3o

[Rune]

It's a very weird thing to leave out and I'm 100% sure it is not there, so really really weird... Yeah that could be my whole problem right there.

Also seems like he switched the position of R202 and R204, but can't be sure without doing some readings with my multimeter. Can't read the stripes on the resistors yet, haven't gotten into that, yet, however R202 and R203 is the same value and they have the same look, so according to the schematic and the pictures provided by the guys who made that famous schematic which are all posted here TWCC's clone has R202 and R204 switched with the same lines/traces on the tube PCB....

Also the heater wires are connected to different heater connections on the tubes. Like on the original 88 it is connected to heater center tap and heater triode 2, but on the clone it is connected between heater triode 1 and 2. Now I haven't read up on that one either, and I don't know if it is the same or not?

[Gorhrut]

finished redoing the amp and its sounding good in general but there are a few issues im having. im willing to try all suggestions.....

the control that was a trimpot on the original does not behave in the same way as it did. it is just a second volume control now....may just remove it cuz i used it fully on anyways.

the master volume control works normal until i turn it all the way down and then it does this weird buzzing sound(like touching the tip of a guitar cable)

im also still having some hum issues. the amp itself is quiet with guitar/cable unplugged. with guitar plugged in i have a really loud hum/buzz. sound changes when i rotate the guitar.
the guitar is shielded with copper tape and ive checked several times for grounding issues. im thinking emi(living in a house from 1910, half the outlets are ungrounded. the one im using is grounded but is definitely on the same circuit as the microwave, maybe some other kitchen appliances too) or microphonic pickup(can hear if i blow on the pickup or snap my fingers near it)

[Structo]

Wow, those ferrite beads really do work.

I have a little amp that oscillates that I may have to try this on.

[Gorhrut]

yeah for rf and some general oscillations they are great. emi is totally unaffected though.



i recorded a vid of the problems im still having with my amp.
https://www.youtube.com/watch?v=MB94ONR ... e=youtu.be

would greatly appreciate any advice.