Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Captain chunkulus on February 08, 2018, 09:34:35 pm
-
Hey guys, can't seem to figure out what I've done wrong with this build. It's based loosely on a marshall jvm topology. I have been over every resister, capacitor, every wire, every pinout many many times and still cannot figure out what's wrong with the amp. I'm posting a link to a video of the issues. Any help would be great.
-
Reverse the wires on your output transformer.
Jim
-
Just tried that. No difference.
-
Hard to see on the video, what configuration are your input jacks? I thought I saw some holes. Are they mounted up? Sorry I did not have the volume up enough on the first listen. Almost sounds like a jack needs to be retentioned on the shorting lug. Easy stuff first. Could also be lead dress. Is this a proven design? Layout?
Jim
-
Well, I had previously built this circuit and couldn't ever get rid of the hum so I rebuilt the preamp circuit and now it's doing what it's doing. When I get a chance, I will draw a schematic for the circuit and post it. I have tried several different input options. It sounds somewhay like grounding issues but I haven't been able to find anything wrong. I have used a continuity meter and traced Eveything from point to point and Eveything is wired to the places they should be. Minus a few circuit tweets, this is pretty much a copy of the high gain marshall jvm circuit if that helps. I have tried using chop sticks while the amp is on to move signal wires around to see if something changed but little to no difference.
-
How many input jacks do you have?
Jim
-
Just one. One Meg to ground, 1k, .047uf, 100pf to ground, 1 Meg to ground, 1k to input.
-
FX loop?
Jim
-
Input picture
-
No effects loop yet. I wonder if the fact that I built the input circuit on a daughter board and ran wires to the fist stage has any bearing on this. I used shielded guitat wire first but it doesn't seem to make any difference.
-
Shouldn't be a problem. I'm sure you checked your wiring on that board? No bridges when you cut the board? Are you going to a good ground? I'm hoping someone else will jump in here! :help:
Jim
-
Sorry, I thought that was a strip board.
Jim
-
No worries.lol. I'm sure my lead dress could be better but, this seems so much worse that just a lead dress problem. Like I said, I'll try to draw up a schematic for the circuit and post it in the next day or two.
-
Input picture
Just one. One Meg to ground, 1k, .047uf, 100pf to ground, 1 Meg to ground, 1k to input.
So, you brought the cathode resistor, cap, grid stopper(s)? from the preamp gain stage(s) over to that little board where your input jack is??
If so, where did you come up with that idea?
Ok, I found this in the schematics library....now I get it
-
Yeah, that's the one.
-
Like I said, I had it working before but could never completely get rid of the hum so, I rebuilt the preamp. The power amp is exactly the same as it has always been since I first built it.
-
Here's a little more info. I'm using a non center tapped tsl100 power transformer, a mercury magnetics super stack output transformer. The bias circuit is from a jcm900 amp. Using 22uf500v caps and 10k 2 watt resister on the power rail from the preamp except for the phase inverter which is tapped separately from the power supply with a 2 watt 22k resister and a 500v 47uf cap on its power rail. Pretty much just like the jvm. Since I'm only using 4 preamp tubes and not 5 like the jvm, there is one less cap and resister on the preamp power rail that the jvm has. The guitar input, preamp are grounded to a ground buss that connects to the speaker ground then to the ground lug on the power jack. The power supply is grounded via the center lug where the 2 100ohm resister on the heater circuit goes to ground. Don't know if any of this helps or not.
-
The guitar input, preamp are grounded to a ground buss that connects to the speaker ground
take the speaker outta the loop, ground the preamp buss to the input jack ground tab, which should be chassis ground also, just further away than HT ground
-
The guitar input, preamp are grounded to a ground buss that connects to the speaker ground then to the ground lug on the power jack.
This sounds squirl'y. Try to isolate the preamp ground.
The power jack ground should only be connected to the chassis at a point that is very close to where it enters the chassis with it's own separate connection. (in theory)
It is a safety ground only and should not be confused with the circuit ground.
Take a look at the library for some grounding schemes and listen to shooter
-
It looks like you've got some serious wiring faux pas going on there
Whats happening with all of those green wires coming off of the cliff jacks?
-
The green wire from the board is the -feedback wire going to the 16 ohm tap. The other great wires are going back to the output transformer 4 8 and 16 ohm taps. I just chose green for those runs because I had a lot of green wire laying around.
-
Some good old hi rez pics all around the inside of the chassis will be very helpful to anyone who is playing along,,,,even Jim
-
Currently it looks like a rats nest because I've been rewiring stuff. However, I used shielding guitar cable from the input grounded on one end and the high pitched oscillation stoped.
-
Okay here are some pics. One thing, the sound fades back in when you hit a chord. In other words sounds right but when you stop the hum fades back in almost like a compressor sound.
-
More pics.
-
Pics cont.
-
More.
-
Last note, all controls work as they should. Resonance, presence, eq. However, the gain pot seems strange, it should completely mute the sound if turned all the way down but it allows a completely clean guitar sound through when turned off.
-
That sounds like a wiring error but without a working schematic we can't tell for sure. There are different ways to make gain controls.
-
This is how it's wired.
-
Okay, so comming off the 2.2 Meg it goes into the gain circuit then comes back in at the 2nd tube stage.tried to make connections on my phone in red..
-
That sounds like a wiring error but without a working schematic we can't tell for sure. There are different ways to make gain controls.
I'm certain that I have stuff wrong.lol. any help from you amazing knowledgeable amp gurus is greatly appreciated.
-
Take some time, edit the schematic so it accurately reflects what you have in front of you, and if you can't stay away, just continue looking for wiring errors.
-
Is this your basis for your amp;
http://el34world.com/charts/Schematics/files/marshall/Marshall_jvm410%20sch.pdf
have you pulled tubes 1 at a time starting with V1, then 2 then 3 till you only have PA tubes in?
and move the speaker ground out of the preamp :laugh:
-
Already moved the speaker ground. Lol. I was using a jcm800 2204 schematic for the grounding schem originally. My original video post however is no longer relevant because I figured out the main issues. #1 shielded guitar cable to the1st stage grounded at 1 end. 2nd had to reverse the two wires on the output transformer. Now I'm left with just some sort of ground hum and the weird gain pot thing. Yes it's the jvm 410 or jvm210 schematic. I will try to get a modified schematic up over the weekend for what I changed. Haven't tried pulling tubes since after I found the other issues but will give it a try this evening.
-
it's the jvm 410
the schematic looks more like a EE grads final project trying to impress the teacher than a tube amp :icon_biggrin:
-
it's the jvm 410
the schematic looks more like a EE grads final project trying to impress the teacher than a tube amp :icon_biggrin:
Right, lol. That's why I'm doing some things different.
-
Some good old hi rez pics all around the inside of the chassis will be very helpful to anyone who is playing along,,,,even Jim
Hey Mr. Latetotheparty.... We were deep into it before you came waltzing along... Real techs don't need no hi-rez pics!
Jim
-
it's the jvm 410
the schematic looks more like a EE grads final project trying to impress the teacher than a tube amp :icon_biggrin:
Man, I was thinking the same thing! Although connectors and other details relevant to the layout is kind of nice - but does get cluttered.
Jim
-
Captain
Just for giggles, try lifting your feedback.
Jim
-
Captain
Just for giggles, try lifting your feedback.
Jim
I will when I get home this evening. Btw Jim, thanks so much for being patient with me. I love learning about this stuff and building stuff. The first build I did was with my Dad. We built a delux reverb with a full eq. This is my first real serious build by myself. I taught myself how to read schematics and have been reading lots of books. I have a copy of the tube amp book ( that's where I started) mullard circuits for tube amps and several others. Haven't been doing this very long. I just love it! Tube technology fascinates me.
-
Hey I learn something new every time I come here! This is a cool hobby and this forum is the best place on the planet to learn. Other than Silvergun and Jojo, and... we are all nice people! :icon_biggrin:
The first modification I ever did was with my dad converting a mono hi-fi tube amp for guitar back in the 60's. Great memories. Don't forget to thank Pops for your current obsession!
Jim
-
Indeed! My dad got me into electronics when I was about 9 or 10 years old. He used to work for a company called Valley People here in nashville tn. I helped him populate the pc boards as a sub contractor. They didn't know it was a kid building the stuff at home, my dad would pay me like .25 a board. Gain brains, kepex's etc.
-
My dad got me into electronics
Same here, my dad was a recovering alcoholic , he was working with/for Goodwill fixing broke TV's n radios.
he taught me to solder, then shag tubes to the local shop to test, pretty soon I was running test patterns n tweaking color guns
Guess what you HAVE to do in the future :icon_biggrin:
-
Captain
Just for giggles, try lifting your feedback.
Jim
Tried the feedback wire, not that. It's really weird, I can turn the gain all the way down and the channel volune as well but I'm still getting signal at the phase inverter.
-
I think at this point we need you to draw up a schematic of exactly what you have. You mentioned that you checked and double checked your wiring and components. We need to see your design.
Jim
-
I think at this point we need you to draw up a schematic of exactly what you have. You mentioned that you checked and double checked your wiring and components. We need to see your design.
Jim
Well, it's not really my design. I'm working off of a jvm 410 schematic. Only real changes are plate resister values. Cathode bypass caps, bias voltage circuit and the addition of a dc heater circuit for the first two tubes. This is a one channel amp, no switching or reverb or effects loop as of yet. I can upload a schematic of the jvm with what values I changed and the bias circuit as well as the DC heater circuit.
-
Don't know if this helps or not. :BangHead:
-
Pics
-
Pics
-
Pics
-
Pics
-
Pics
-
Pics
-
That is the most impressive cut and paste job I've ever seen. Ritchie has you covered now. :wink:
-
That is the most impressive cut and paste job I've ever seen. Ritchie has you covered now. :wink:
I know it's funny right! I don't know if it will help or give everyone a brain aneurysm. :l2:
-
WTF! With all due respect, I was hoping for something a little more traditional? :l2: Thanks Sluckey, my buddy.... :think1:
You may want to actually draw it out. Then revisit your layout and connections. Something might jump out at you.
Jim
-
I would try this change, probably won't help, but hey, I can color also :laugh:
-
You guys are killing me. Lol. :worthy1:
-
This is a working amp, just hums?
if yes;
can you post voltages, looking for bias, how hot you're running, I've seen tubes running hot hum, biased correctly they sing.
just re-looked and noticed the change I suggested "assumed" incorrectly :think1:, they were 2 halves of the same tube
-
2 separate tubes used for the directly coupled stage. Bias is at 35 milli amps per side. Or -45ish at the bias point.
-
This is a working amp, just hums?
if yes;
can you post voltages, looking for bias, how hot you're running, I've seen tubes running hot hum, biased correctly they sing.
just re-looked and noticed the change I suggested "assumed" incorrectly :think1:, they were 2 halves of the same tube
Okay, v1 tube reads 162v and 241v
V2 reads 312v and 128v
V3 reads 377v
Phase inverter reads 280v and 275v
Bias at the 220k resisters is -43v
Each tube in the power amp is reading individually between 32.6mv and 35mv
Also here is a video of the amp.
-
Also, thought i'd give a rundown of my grounding.
All preamp cathodes and preamp filter caps are grounded near the input on a buss wire with the exception of the phase inverter filter circuit which is grounded to the center of 2 100ohm resisters on the heater supply. All tone controls including first stage gain pot are grounded at the busswire as well. The presence control is also grounded there. The output transformer common is run to the speaker jack sleeves and then grounded at the phase inverter common ground which is also tied to the buss wire. This was done after reading info on Aikens sight about grounding. Doesn't seem to matter where I ground it, it makes no difference in hum. The power tubes are grounded to chassis through 2 watt 1ohm resisters. The main power supply is grounded to the center of the 2 100ohm resisters on the heater supply. The negative bias voltage supply is grounded also to the heater artificial Center tap.
-
WTF! With all due respect, I was hoping for something a little more traditional? :l2: Thanks Sluckey, my buddy.... :think1:
You may want to actually draw it out. Then revisit your layout and connections. Something might jump out at you.
Jim
Sorry, I thought since I have musicians short hand this would be more usefully but since it's not, I will try to do a hand drawn schematic as soon as I can.
-
since I have musicians short hand
I can't tell you an A from E :icon_biggrin:
look at your original schematic, it is drawn well, emulate that like you'd "cover" a song
If you're gonna do this long term, get yourself a drawing program, on the free side, expressSCH works fine, on the $side, Visio guys can do some amazing work.
As you're drawing, try and keep in mind which parts set up the tubes "operating" conditions, and which parts set up the tubes "tone or flavor". The DC and AC paths through the maze. China does well with mass quantity , you are striving for individual quality
-
since I have musicians short hand
I can't tell you an A from E :icon_biggrin:
look at your original schematic, it is drawn well, emulate that like you'd "cover" a song
If you're gonna do this long term, get yourself a drawing program, on the free side, expressSCH works fine, on the $side, Visio guys can do some amazing work.
As you're drawing, try and keep in mind which parts set up the tubes "operating" conditions, and which parts set up the tubes "tone or flavor". The DC and AC paths through the maze. China does well with mass quantity , you are striving for individual quality
Thanks for this info, I downloaded a copy of the express pcb. I'm going to try to figure it out and see if I can do a decent schematic. In the mean time, the only real issue I'm currently having is signal bleed when the gain pot and channel volume is down and hum. I searched the forum and the only thing that came up was this,
"A common high-resistance ground could cause some strange problems. The cathode of a triode has a low input impedance, but it also has a larger gain than the grid input. If there is a shared ground path that is poor quality (appreciably bigger than 0Ω), it is conceivable that the input signal develops a small voltage across the poor ground which serves to inject a signal into the cathode of a later stage. However, hum usually results from this cause, as power supply current are likely to be much bigger than signal currents.
So just a question, anyone ever have this problem? Any possible solutions?