Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: proaudioguy on January 15, 2021, 11:59:25 pm
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I have noticeable 60/120 cycle hum when the Intensity control is at 1. 10 has no hum. I did not record that but I did record the other noises.
I’m chasing my tail here guys.
&feature=youtu.be
Let me know what you think please.
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To check amp noise, never plug guitar. Test amp alone.
Tubes amp; alway begin by trying know good tubes.
Reverb ; Clean RCA jacks amp back plate and at reverb tank. At cable end too.
If you don't know how, we can explain.
You can try new cable. Standard stereo cable work fine.
Try reverb tank outside the cab.
Did filter caps are originals ? It is a must to replace them. Don't forget bias circuit filter cap.
Old filter caps can cause strange problems that one would not guess. I saw it.
Good luck .
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This is a brand new amp. All the connectors and cables are all new. Its a hoffman AB763. The only thing retained was the original transformers, except the reverb transformer, and the chassis. The original cabinet was re-covered. I did a bunch of the work in 2004, then the rest 8-10 years ago and did things off and on to it until maybe 2013. It was done and was good but the tremolo was weak so I was still dicking around with it trying different things for a while. There are ghosts in this thing. Circuit looks great. I’ll take a gut shot and post it up. Maybe someone will see something.
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Most of that noise is there whether there is a guitar plugged in or not as I demonstrated. There is the additional ground related RIZZ noise when the guitar is plugged in and not being touched. Touching the chassis makes that go away too. Doesn’t seem right to me. I would like to know hot to address that. The other issue is that massive HISHHH. The only noise I want to hear is the hum and buzz my guitar pickups put through. I have several high gain fender amps and on the clean channels, they are clean as can be. No pickup noise at all. On the high gain, the noise is about the same when I am on this amp, on the non-humbucking position. With the humbucking position on the other amps on the clean channel you wouldn’t know they were on unless I really cranked them.
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Video
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Do you also measure plate voltage with your fingers? I got nervous watching this video
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This is a brand new amp. All the connectors and cables are all new. Its a hoffman AB763.
I read only AB763 and to me it is a vintage, Sorry
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Don't work with your finger, lethal voltage there :BangHead:
I do a better wire routing.
How to explain ?
All yellow wires must run close to the chassis, never side by side.
If wire must cross another wire it mut be in 90 degrees.
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"There is the additional ground related RIZZ noise when the guitar is plugged in and not being touched. Touching the chassis makes that go away too."
-Proaudioguy
Look a bad ground somewhere. What about the ground wire of the power cord ?
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I don't like wires routing at the tubes's sockets.
They are not close to the chassis, to say one thing, they may act like antenna for noise.
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I don't like wires routing at the tubes's sockets.
They are not close to the chassis, to say one thing, they may act like antenna for noise.
I am open to suggestions. I have moved them all around. This is an ongong process. When I built it they were all against the steel. There is no way to make them not run in the same direction so I have them separated. If you have photographic examples of how they should be run that will reduce the hiss, cool.
Do you also measure plate voltage with your fingers? I got nervous watching this video
As for putting my fingers in there, I routinely tie in 400A 3 phase for work. Sometimes its hot (not under load). I am very well aware of what is there and where my hand is at all times. Nothing is willy nilly even if it looks that way. Like watching roofers or tree service guys. They are like monkeys. I haven’t had a shock since I was 18 working for a plumber, doing what I was told, and before that I was a small child, almost 3. I learned real fast. I took apart TV sets as a kid and never got shocked. I know what not to touch. Even so, your answer provided no information about my issue with that cap collecting buzz like a buzz antenna. I would prefer it was isolated by a shield. I didn’t think that type of cap was at all directional but I’m open to suggestions.
Here is me at work. Hope we can go back to work soon.
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This is a brand new amp. All the connectors and cables are all new. Its a hoffman AB763.
I read only AB763 and to me it is a vintage, Sorry
My bad! I was on the old forum. I’ve been on this one since 2012. I just assume the hand full of people that usually answer me, know the project. That’s on me. I think I have 3 posts up on this page. I am sorting one issue at a time. The story is this. I bought the amp in the mid 1990s. It was a basket case AA270. The tolex was torn up, the amp had lots of problems. It smelled amazing. Still had great tone even with lots of nasty noise. I wanted it for a project and I wasnted channel switching. I bought one of those slimey Torres Super Texan kits, thinking I was going to be “rebuilding the amp into a 2 channel blue monster” or something like that. What I got were piss poor instructions and hardly any components. If I saw the guy today, I’d punch him in the face. I called them and told them it was as if a child wrote the instructions and they didn’t like that. It was years before I realized the guy doesn’t speak english natively and his grammer is HORRIBLE, but he’s a marketing genius. I installed the kit and it sucked plus it didn’t solve the problems I had. I got side tracked but I was reading a lot of tube amp books and such as I could afford to buy them. Then in 2004 I found this or another website and joined and asked some questions and a nice guy contacted me directly and offered to build me boards and I traded him some brand new Kendrick Brown frame speakers I had bought for the amp. He maybe got a way better deal than me but he hand wrote me explicit instructions which walked me through the entire process. I had bought a black face reissue panel in the mid 1990s from my fender dealer cause the old one was beat. I stripped the cabinet and used wood filler as needed and I had bought the tolex and grill cloth in the 90s and never used it. I finally used the tolex and had lost the grill clothe so I had to buy that again. I gutted the chassis totally, then we had 4 hurricanes in 6 weeks and our lives were upended and actually we have never gotten back to normal since then. So finally several years later, I lost track I ordered the under $100 worth of parts from Mr Hoffman to build the turret boards. I lived with it for a while, still had a few issues but was mostly good. I bought some nice RCA 6L6GCs and biased them and there was no temolo. THe tubes also sounded a bit dark for my tastes. I made a bunch of posts then. I ended up pulling the bias vary and installing the roach circuit. This was a bad idea. I hated it. The tremolo worked but it killed the tone. The tremolo didn’t have that deep cycling and the 50k pot to ground really sucked down the tone. I then sold those tubes and ripped that circuit out and had the intention of putting the bias vary right back but life side tracked me again. I lost my music room due to mice taking up residents in there. Once I had the whole room empty and cleaned up, my wife moved in and I never got it back. :dontknow: Then my dog died and I literally quit playing guitar for 6 years. I really only noodle anyway but I totally quit. It was super depressing every time I picked up the guitar. My pup was my blues buddy and would sing along. So now I am just working on finishing projects and trying to get things done.Knowing what I know now, I would have replaced all the old Electrolytics and left the old beat up tolex, etc and played the old amp with its funny smells. It had character. During the rebuild I replaced all the tube sockets, the boards with turret boards, all the pots, all the knobs, all the jacks, all the switches. THe chassis and transformers were original. Back about 8-10 years ago I replaced the reverb transformer. I had some bad microphonics and that solved it. That’s why I say its way easier to start with all new parts for a new build than to try and make old parts into a new amp. I’m chasing gremlins throughout this build.Knowing what I know now, I would have heat shrink around the ends of every wire to keep them from fraying.
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I don't like wires routing at the tubes's sockets.
They are not close to the chassis, to say one thing, they may act like antenna for noise.
I am open to suggestions.
Answer #7
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Easier to read for me like that : :occasion14:
My bad! I was on the old forum. I’ve been on this one since 2012.
I just assume the hand full of people that usually answer me, know the project. That’s on me. I think I have 3 posts up on this page.
I am sorting one issue at a time.
The story is this.
I bought the amp in the mid 1990s. It was a basket case AA270. The tolex was torn up, the amp had lots of problems. It smelled amazing. Still had great tone even with lots of nasty noise. I wanted it for a project and I wasnted channel switching. I bought one of those slimey Torres Super Texan kits, thinking I was going to be “rebuilding the amp into a 2 channel blue monster” or something like that.
What I got were piss poor instructions and hardly any components. If I saw the guy today, I’d punch him in the face. I called them and told them it was as if a child wrote the instructions and they didn’t like that. It was years before I realized the guy doesn’t speak english natively and his grammer is HORRIBLE, but he’s a marketing genius. I installed the kit and it sucked plus it didn’t solve the problems I had. I got side tracked but I was reading a lot of tube amp books and such as I could afford to buy them.
Then in 2004 I found this or another website and joined and asked some questions and a nice guy contacted me directly and offered to build me boards and I traded him some brand new Kendrick Brown frame speakers I had bought for the amp. He maybe got a way better deal than me but he hand wrote me explicit instructions which walked me through the entire process.
I had bought a black face reissue panel in the mid 1990s from my fender dealer cause the old one was beat. I stripped the cabinet and used wood filler as needed and I had bought the tolex and grill cloth in the 90s and never used it. I finally used the tolex and had lost the grill clothe so I had to buy that again. I gutted the chassis totally, then we had 4 hurricanes in 6 weeks and our lives were upended and actually we have never gotten back to normal since then.
So finally several years later, I lost track I ordered the under $100 worth of parts from Mr Hoffman to build the turret boards. I lived with it for a while, still had a few issues but was mostly good. I bought some nice RCA 6L6GCs and biased them and there was no temolo. THe tubes also sounded a bit dark for my tastes. I made a bunch of posts then. I ended up pulling the bias vary and installing the roach circuit. This was a bad idea. I hated it. The tremolo worked but it killed the tone. The tremolo didn’t have that deep cycling and the 50k pot to ground really sucked down the tone. I then sold those tubes and ripped that circuit out and had the intention of putting the bias vary right back but life side tracked me again.
I lost my music room due to mice taking up residents in there. Once I had the whole room empty and cleaned up, my wife moved in and I never got it back. Then my dog died and I literally quit playing guitar for 6 years. I really only noodle anyway but I totally quit. It was super depressing every time I picked up the guitar. My pup was my blues buddy and would sing along.
So now I am just working on finishing projects and trying to get things done.Knowing what I know now, I would have replaced all the old Electrolytics and left the old beat up tolex, etc and played the old amp with its funny smells. It had character. During the rebuild I replaced all the tube sockets, the boards with turret boards, all the pots, all the knobs, all the jacks, all the switches. THe chassis and transformers were original. Back about 8-10 years ago I replaced the reverb transformer. I had some bad microphonics and that solved it.
That’s why I say its way easier to start with all new parts for a new build than to try and make old parts into a new amp.
I’m chasing gremlins throughout this build.Knowing what I know now, I would have heat shrink around the ends of every wire to keep them from fraying.
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Hey there. Your project will take some time to fix, BUT DO NOT give up. That circuit is lovely and as you have noted it can be dead quiet when everything is copacetic.
I'm not one of the stars on this platform, but at least some of your hissing noise may be from carbon comp resistors in the first stage of both the Normal and Reverb circuits. My own preference is to use metal film resistors in that position. This may or may not work for you, but it would be a relatively simple start to working on your amp. These are R3,4,5,6 on the Hoffman AB763 schematic.
This is a nice write-up regarding grid stoppers (which I had a tough time understanding at first):
https://www.aikenamps.com/index.php/grid-resistors-why-are-they-used
It is worth viewing sluckey's build photos to see how seriously he takes the role of these resistors.
I do have one comment about your video: I totally trust your judgment about what is safe for you -- that's your beeswax, not mine. Just remember that there are a lot of folks who travel this forum who have NO high voltage experience whatsoever, and that's why you will get some pushback.
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That makes perfect sense.
I will order metal film for all the signal path resisters.
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Hi Proaudioguy,
....and don't forget Metal Oxides resistors for Plate loads and Output tube screens. More robust at higher volts and potentially better noise performance than Carbon Comp. :icon_biggrin:
(by the way, I've had great success with Hoffman's AB763 boards :icon_biggrin:)
Kind regards
Mirek
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Talking about lead dress.....
I saw THIS on a Mojo blog post. I intentionally did NOT route my stuff like this because there is so much parrallel routing here that would cause magnetic coupling noise. Have I been misunderstanding this issue all along? My routing was intentionally more random to avoid having any wire “close” to any other, as well as only crossing at right angles to avoid coupling. This routing on Mojotone’s amp seems counter intuitive to me. I love the way it looks though. This is how we do our racks but every cable is balanced and shielded and running typically at line levels.
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Does the hum vary with various combinations of intensity pot rotation with rotation of other pots?
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Does the hum vary with various combinations of intensity pot rotation with rotation of other pots?
No. Just comes UP when I turn the intensity down, regardless of other controls. In fact I have another noise that is driving me nuts. Sounds like a storm. Real quiet but there. I have V4 pulled out right now and the HUM is still there when intensity is turned down and the storm is still there. If I pull out V5 there is no change on the storm. Didn’t check the hum with V5 out. If I pull V6, the amp is dead quiet. That would be the LTP PI.
In regards to the static storm noise....
V1 makes no difference.
V2 makes no difference.
V3 makes no difference.
V4 makes no difference.
V5 makes no difference.
V6 makes the entire amp dead quiet when removed.
There is something before V6 and after V4B that is getting through the phase invertor and making a racket. Its very low but I’m listening in a quiet room. Its not “ok”.
Additionally, my CH 2 (vib) Volume pot has a bit of a crackle (all tubes in) when turned and as I work it back and forth it is getting stiffer. All other pots are super smooth. These pots are brand new, albeit I purchased them like 10 years ago. They have little to no use on them. As much of a hassle as it is, I think I will replace that POT. When I go into the amp again, I would like to tackle whatever is making this noise. I’m not sure I can record it with my phone but its very audible to my ear.
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I should note, the storm noise appears after the amp is hot. When its first turned on, that is not there. Again, if I pull V6, it goes away. I thought about the resisters on the power tubes getting hot but I would think they would make noise even if V6 was out. Also I don’t have them across the tube socket like Fender does. Mine are off to one side so they won’t get that direct heat. Also they are metal oxide and pretty big. Also, they aren’t that used. Everything other than the transformers in this amp are new (purchased around 8-10 years ago). The amp has only a few hours of play on it and most of that is between making tweaks.
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Here is a video of the noise. You will need to wear headphones and crank it up to hear it. I used the phone and it doesn’t record that great.
One thing the mic picks up that I cannot hear over the refrigerator in the next room is that 120Hz hum that is going along with the noise. Later in the video I turn down the intensity pot and you can hear the 60cycle hum.
&feature=youtu.be
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Just occurred to me that V1B is still in the circuit even if the volume is turned all the way down. That tube has been swapped before but I’m going to pull it out and see if that makes a difference for the bacon frying noise. It won’t affect the HUM though.
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Progress. Pulling V1 makes the bacon frying go away. I put a different tube in V1 and the bacon frying comes back. Doesn’t appear to me to be a bad tube. What do you all think? Volume pot is down so has to be between the volume pot and the 220K mix resister. I could hear the 120Hz hum when I got close after pulling out V1. The intensity 60Hz hum is still there. This is why I posted that question about DC heaters. I would like the amp to have nothing but resister SSSS when the gain is wide open.
So, any ideas?
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Hi there, sorry you are having amp problems. Regarding the crackle, since it goes away when you remove/change V1 then check or replace the plate resistor(s) for that tube. They should be 2W metal film 100K or RND65 at the least. Even if they are correct in type I would still look there first since the problem appears after warm-up. If it's not that look for a leaky coupling cap that is connected to either stage of the tube. It could also be a cold solder joint.
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Any kind of crackle you get from a pot that is known good and clean is likely to be a DC leak from somewhere, most likely a coupling cap, and this can hold true with a leaky cap for the entire circuit - not just when you turn a knob.
A quick test for the reverb hum would be to lift the ground leg of the reverb pot and attach it to another ground point, ideally to the cathode of the the recovery tube (V5?). I have had this occur in lots of amps I have built... The Hoffman layout shows a ground buss that appears to connect to the chassis and if yours is that way AND you have connections to ground via the chassis at other places, like the input jacks, you are inviting ground loop problems - I know fender did it, but you have replaced their original design, and as a general rule you should have only one chassis ground connection, and that should be at the input jack. But that's another topic.
Lastly, if you get a reduction in noise when you touch the amp chassis with no guitar plugged in, then check continuity between the chassis and the AC ground - or round bottom connection of the AC plug. The AC cord should have the green (ground) wire connected directly to the chassis somewhere near the power transformer, but definitely not to the the amp ground buss.
Good luck!
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GREAT ideas. I am pretty sure those plate resisters are not likely 2W. They are carbon comp and they aren’t huge. I have an order in for 1/2W 100K Metal Film to replace them but if they should be 2W then I’ll need to source them. Mojotone doesn’t appear to sell them so I won’t be able to get them on that order.
The hum is when I turn down the intensity on the tremolo. The reverb is pretty good. V4 is the recovery tube for the reverb. V5 is the oscillator for the trem. One thing I noticed on the hoffman layout is there is a cap on the bias pot. I have a cap on the bias board. Mine is a larger value I think. I assumed they serve the same purpose and omitted the one on the bias pot. Mine is in a slightly different location in the circuit though. It appears to me it is doing the same job.
Your point about the grounds is well taken and has always been a concern. That being said this IS a real 1969 Twin reverb chassis and transformer set and cabinet and I am extremely hesitant to drill the pot holes out to lift the ground. Same for the jacks. If I could get jack isolators that fit the holes and allowed the jacks to tighten up, I would be willing to remove the back shell grounding I have across all the pots. This amp also has the bress plate. You could consider this to be 90% original other than the turret boards and components, but I did replace all the tube sockets cause the old ones had too many issues and I replaced all the pots cause its just a lot easier than rebuilding the old ones. I have the original boards. They are all warped. The amp is WAY WAY cleaner than it was with the old stuff on it and that was in the 1990s. Some would say I am nit picking.
Another question. If I have it biased too low and it sounds totally fine up to about 7 or 8 (and real loud) and then falls apart when the volume is dimed, is that what happens with crossover distortion? For some reason I thought it happened at low volume. I have to bias it low to get the tremolo to work. After all the “noise” is gone I’ll figure out what I want to do about the tremolo circuit. I would love to have it working on another tube instead of on the bias of the power tubes. I am considering making a 15” baffle and trying out my EV 15L instead of the 2 12Ls and taking out 2 of the tubes. I assume (perhaps incorectly) that doin this will affect the bias setting and the tremolo as well so I figured I’ll wait and see. Just trying to tackle each item one at a time. This is by far the best sounding amp I have, and I have several. I still want a Deluxe Reverb though.
Hi there, sorry you are having amp problems. Regarding the crackle, since it goes away when you remove/change V1 then check or replace the plate resistor(s) for that tube. They should be 2W metal film 100K or RND65 at the least. Even if they are correct in type I would still look there first since the problem appears after warm-up. If it's not that look for a leaky coupling cap that is connected to either stage of the tube. It could also be a cold solder joint.
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Any kind of crackle you get from a pot that is known good and clean is likely to be a DC leak from somewhere, most likely a coupling cap, and this can hold true with a leaky cap for the entire circuit - not just when you turn a knob.
A quick test for the reverb hum would be to lift the ground leg of the reverb pot and attach it to another ground point, ideally to the cathode of the the recovery tube (V5?). I have had this occur in lots of amps I have built... The Hoffman layout shows a ground buss that appears to connect to the chassis and if yours is that way AND you have connections to ground via the chassis at other places, like the input jacks, you are inviting ground loop problems - I know fender did it, but you have replaced their original design, and as a general rule you should have only one chassis ground connection, and that should be at the input jack. But that's another topic.
Lastly, if you get a reduction in noise when you touch the amp chassis with no guitar plugged in, then check continuity between the chassis and the AC ground - or round bottom connection of the AC plug. The AC cord should have the green (ground) wire connected directly to the chassis somewhere near the power transformer, but definitely not to the the amp ground buss.
Good luck!
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Hello Friends!
I have an update.
I removed the carbon comp 68k resisters from all 4 inputs and replaced them with metal film at the tube pins. I had to run an individual shielded cable from each jack to the resisters then tied them together at the grid V1A and V2A.
I replaced the carbon comp 100K plate resisters on V1 and V2. I will probably do V4 as well but I figured that was enough to check for noise cause I could just turn off the reverb.
I removed the back of POT ground bus and all the solder on the pots and made a floating ground bus.
The result, the storm noise is still there. The hum/ buzz appears to be increased slightly. No change to the hum that comes up when I turn the tremolo intensity all the way down.
I pulled each tube starting with V1 and the noise remained until I pulled V6. Previously I was sure the bulk of the noise was coming from V1 and V2.
I’m open to suggestions!!!
Questions about the ground bus.
Some drawings show different ways of doing this. This is how it IS though.
The negative from the 2 PSU caps in the can grounds to the ground bus behind the pots. Hoffman shows all the caps on the same ground all the way to the main grounding lug. The original Twin reverb has this grounded at the brass ground bus near input of channel 2, then the wire continues under the board presumably back to the main ground. I have a grounding bus on the turret board. Channel 1 input ground is the start of the floating bus and also is the attachment point to the ground bus on the board at the other side of the cathod resister cap pair for V1 just like the hoffman layout. This is how it was before as well except I had the bus on the backs of the pots, and I figured that was causing possible additional paths to ground through the chassis. The backs of the pots are not grounded to the bus anymore. On the hoffman layout at the far right of the ground bus behind the pots, there is a ground symbol and it says ground bus. Should that also be attached to the main ground? That would create a loop would it not? I believe my ground from the bus to the cathode/ resister pair, is grounded to the main lug with an under board wire, but its been so long, I guess I should double check that. Looks like the current layout does NOT have that wire. Maybe that is part of my problem? So it is possible the path to ground is the wrong way but this was how I was told to do it at the time.
Any ideas about the storm? What about the hum when the intensity is turned down all the way?
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Of course the chassis is in contact with the input jack so there are 3 paths to ground there. The input jacks for Normal chan through the chassis and through the bus wire to the board and to the input jacks of channel 2 which also make contact with the chassis. There is no way around this unless I drill the chassis jack holes out for isolators but its a 68 or 69 chassis and I’ve managed not to make any new holes. I’ve even got the turret board mounted to stand offs using the original screw holes that screws the eyelet board down.
I forgot to mention, I sprayed Deoxit Fader lube F5 into all the pots and they don’t appear to be making any noise now at least not in regards to dust.
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https://el34world.com/charts/grounds.htm
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The hum is when I turn down the intensity on the tremolo.
No-hum with Intensity full-up, Hum with Intensity full-down. So look at what is at either end of the Intensity pot: a bias-supply (hummy side), and a cap-to-ground (no-hum side; tremolo circuit).
The above suggests, "the hum might be from the bias-supply, where there should only be d.c. volts." So set your meter for Volts AC, and measure the output of the bias supply. If you have significant a.c. voltage there, you need to fix your bias supply which is not filtering properly.
It's easier to see what I'm talking about if you look at the schematic (on Page 7) of the Hoffman AB763 (https://el34world.com/Hoffman/files/Hoffman_AB763_2.pdf) rather than just gazing at a layout. I know Doug originally never supplied schematics (because "it's same-as the AB763"), except the connection from Trem Intensity to the bias-supply is not-same-as-AB763.
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https://el34world.com/charts/grounds.htm (https://el34world.com/charts/grounds.htm)
OK so I have 2 channels of input jacks. Should I break the ground between them?
Also channel 1 is connected to the preamp ground and then the preamp ground has an under board wire back to the chassis ground point. I should eliminate that?
The other confusing thing is you should 3 caps grounded by the preamp and 2 grounded on the chassis ground, but on the Twin reverb, its the other way around. I even have gut shots of an original unmolested TR and it is that way as well. On the twin reverb there are 2 large and 1 smaller that ground to the chassis ground and 2 smaller that ground near the preamps. I’ll look at it again in the next couple days. Now its off to see the grands.
Do ya’ll have any idea where the “storm” might be coming from given that its there even with all but the PI tube pulled out? Its got a JAN Phillips 12at7 and I’ve had several different ones in there. All NOS.
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The hum is when I turn down the intensity on the tremolo.
No-hum with Intensity full-up, Hum with Intensity full-down. So look at what is at either end of the Intensity pot: a bias-supply (hummy side), and a cap-to-ground (no-hum side; tremolo circuit).
The above suggests, "the hum might be from the bias-supply, where there should only be d.c. volts." So set your meter for Volts AC, and measure the output of the bias supply. If you have significant a.c. voltage there, you need to fix your bias supply which is not filtering properly.
It's easier to see what I'm talking about if you look at the schematic (on Page 7) of the Hoffman AB763 (https://el34world.com/Hoffman/files/Hoffman_AB763_2.pdf) rather than just gazing at a layout. I know Doug originally never supplied schematics (because "it's same-as the AB763"), except the connection from Trem Intensity to the bias-supply is not-same-as-AB763.
Yes perfect logic. Problem is my bias supply doesn’t match the hoffman and I can’t find anything with all the detail in one place. My supply is for a twin reverb. The filter cap is not attached in the same location as the hoffman schematic, but otherwise is the same. I’ll see about drawing it and uploading a pic of how its attached.
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Just put a 47µF 100V cap between the bias pot wiper and ground.
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Just put a 47µF 100V cap between the bias pot wiper and ground.
Right so that is an ADDITIONAL Cap in the bias supply then. The one that is there is like this with the bias connected to the bias pot this way but of course after the bias pot the intensity gets involved which is not the case on the stock twin reverb.
So, do I MOVE the cap in teh bias supply or ADD another cap?
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... The one that is there is like this with the bias connected to the bias pot this way ...
I've encountered the same hum you have one time in a vintage Fender amp. It was a Princeton Reverb, and the hum was present even if I pulled the phase inverter tube from the socket.
The cause was a failed capacitor in the bias supply (the only cap there, same as the one you circled). Capacitor replaced, hum gone.
The Fender bias supply is a half-wave rectifier: the output is 60Hz, same as line frequency.
- The bias supply is the only place besides the heaters (and power cord/PT wiring) where there is strong 60Hz.
- Hum injected from output tube heaters to the audio should be small relative to audio signal.
- Yank the phase inverter, that leaves only under-filtered bias supply to inject 60Hz right to the output tube grids.
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Temporarily add another cap. Connect as I said above. Observe polarity! If no joy, remove it.
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... The one that is there is like this with the bias connected to the bias pot this way ...
I've encountered the same hum you have one time in a vintage Fender amp. It was a Princeton Reverb, and the hum was present even if I pulled the phase inverter tube from the socket.
The cause was a failed capacitor in the bias supply (the only cap there, same as the one you circled). Capacitor replaced, hum gone.
The Fender bias supply is a half-wave rectifier: the output is 60Hz, same as line frequency.
- The bias supply is the only place besides the heaters (and power cord/PT wiring) where there is strong 60Hz.
- Hum injected from output tube heaters to the audio should be small relative to audio signal.
- Yank the phase inverter, that leaves only under-filtered bias supply to inject 60Hz right to the output tube grids.
Did not occur to me that a brand new bias cap was bad but it makes perfect sense! I’ll dig back in hopefully before end of Monday. Need to see if I can find another electrolytic.
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Did not occur to me that a brand new bias cap was bad but it makes perfect sense! I’ll dig back in hopefully before end of Monday. Need to see if I can find another electrolytic.
That's why I said first to measure with a meter, and find out for-sure if the 60Hz is coming from the bias supply.
I supplied a theory, but every theory should be tested. That way you don't blindly replace parts hoping to stumble on a solution.
(This is a huge problem on every forum I've visited; people try "recommended fixes" without actually doing the diagnostic steps to figure out what is causing the problem. Somehow they are surprised when they try the fix, but the problem remains.)
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I do not have a reliable meter but I will attempt. I’m trying to find a Fluke 87V. I’m done with cheap crap, but I
‘m not going to pay $400+ for one. The pawn shops say “people must be working” cause no one is selling their gear. Wish I was one of them!
I wanted to clarify my bias setup as it is and just get a final instruction before I tear into it. See attached. Simply moving the wire from the outside lug to the middle lug with the other wire, would do the trick? I have been wondering about this for a while but it seemed like it would be 6 of 1 half dozen of another. OR....add additional cap to center lug (missing part?)? Or disconnect cap from diode and leave diode on the outside lug and add a wire to put the cap on the center lug?
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I know what your bias circuit looks like. Easy to see from the pic you posted. I can't state it any simpler and clearer than I already did. Just do it!
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I moved the cap to the center lug and left the rest of the bias supply on the outside lug. As far as I can tell without the amp in the cabinet, the hum is gone. Can’t be 100% sure but it appears that way. If I’m wrong, I’ll post back. Here is a video. I wonder if this is NORMAL or not. Its been like this since day one with this circuit. First time I’ve had it plugged into a raw driver so it can be easily seen. There is nothing plugged into the input here. More after the video....
&feature=youtu.be
So I plugged the PI back in and the quiet storm is of course back. That is the last demon to chase out I think. I don’t know where to start. I could swap the 100K plate resister for a metal film but I do not have a metal film 82K. I don’t have metal film 1Meg Grid stoppers either. Do I need to get this stuff? I mean these amps from the factory don’t have metal film but they don’t have this static storm. I would think the resisters would just cause straight HISS. I pulled the oscillator tube back out so you could hear the storm. Not sure if its loud enough over the cars going down the road but trust me, it is audible in real life.
I let it warm up with just the PI and the power tubes in and flipped it off standby.
&feature=youtu.be
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That sound could definitely be caused by noisy CC resistors. The 100K and 82K are the most likely suspects, but it could also be the two 1Ms. And maybe even the 470Ω and 22K. They're so cheap that I would change all six resistors. Hoffman sell them. Just follow the resistor link below this message.
The node C filter cap is also a suspect if the resistors don't fix it.
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OK thanks, I’ll order all of them. I just find ist crazy that all these resisters are bad. I’m sure it will be a few weeks before I can get back to that due to the ordering and shipping aspect.
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It's probably one or two, the solder iron is hot, you're already there..........
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I just replaced the 100K on the PI Plate and the crackle appears to be GONE!!! The 82K is still carbon comp.
Now I can just hear the RIZZ, now this is with ONLY the PI installed, everything else out.
The rizz (in my line of work rizz is harmoncs of hum, often appears when motors or fluorescent lights (K-mart levels) are on circuit) is VERY low but audible, but only if I stick my head in the speaker. :worthy1:
I would like to get rid of it but not sure what else to do except maybe get better matched tubes? Upping the capacitance on the A supply node won’t help, right?
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As I add more tubes, the rizz gets louder. Quite audible now. I have V3, V4, V6 in and all the controls all the way down and the riz is quite noticeable. Never really heard it this loud before but that “storm” and the hiss was possibly masking it. I still feel like I’ve made progress today
Edit, I think the RIZZ is my soldering iron! I’ll be back in a few and edit again.
Here is a pic before I put it back in the cabinet. If you all have any advice as to any wire routing, I’m open to that. as well. The distortions on the photo are because its a panorama.
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So the HUM is still there when the intensity is rolled all the way down, but there is also a tick from the speakers moving when at full intensity AND the tremolo effect is MUCH weaker than before. I already had the bias really low prior to this to get it strong. I would not think moving the cap would affect the strength of the bias. :BangHead:
I guess I just couldn’t hear the hum in that 10” raw driver. It may well be reduced. The hum is there with or without the oscillator tube installed.
Should I increase the capacitance? I have a 100uF 100V sitting here I could use.
The tick is literally the more responsive of my speakers (JBL I just put in with the EV) and I don’t think I have actually played the tremolo since I put that speaker in there cause that was just last week. The EVs are a lot stiffer. They were also moving (and the one in there still is) but not making the noise. They have a much stiffer spider and a heavier cone.
So is that movement normal or not?
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I do not have a reliable meter ...
... I just find ist crazy that all these resisters are bad. ...
Maybe they're bad, maybe they're not.
There's a way to test resistors for noise in resistors (bottom), but it means a need for more test equipment (and the test exposes you to high voltage, so it's not safe). You also have to remove & test each resistor, then maybe re-install them if they're good. So that's a bit of time.
If your test winds up finding noisy resistors, you'll have to replace them. So you're going to need more resistors on hand anyway.
Where Sluckey is going is resistors are cheaper than test gear. They may be "cheaper than your hourly rate" that would be spent removing, testing, reinstalling even if you had the equipment to do the test.
So it's your call how to proceed:
- "Economize" by saving the cost of more resistors, by buying more test gear and spending more time.
- "Economize" by saving the cost of gear & time, by simply replacing the suspect resistors & see if the noise goes away.
The latter is contrary to my argument earlier that one should use test equipment to find the actual problem. But it still probably better than spending $100+ finding the item to do the test and spending another~$100 of your time, when the resistors could be replaced in 5-6 minutes (if you had resistors on-hand).
(Lately, I've bought some test gear others assumed I always had. I want to spend the time because right now because it suits my interest. Some other time, I may just want to get-it-done.)
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... Now I can just hear the RIZZ, now this is with ONLY the PI installed, everything else out.
The rizz (in my line of work rizz is harmoncs of hum, often appears when motors or fluorescent lights (K-mart levels) are on circuit) is VERY low but audible, but only if I stick my head in the speaker. ...
You've got an open chassis, and hummy, buzzy stuff is in the air all the time. See the video and how much crap the guy picks up with the probes for his signal tracer, unless he shields the probe with his hand, or supplies extra capacitance-to-ground by holding the grounded side of the probe.
So what happens if you put a conductive metal plate over the open side of that chassis? You know, to shield the open side? Which is normally in contact with the metal screen stapled to the inside of the cabinet?
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I swapped out the 50/50 for a 100/100 in the bias supply. I still have it going to the center lug of the bias pot and the voltage going to the outer lug and the other outer lug going to the bias resister to ground. Hum is the same.
Still not sure if I should have 1 cap in the normal supply tied to the diode and going to the outer lug and a second cap on the center lug, or if the idea is just to have the one single cap and just have it moved to a different spot.
Now that I have the 100uF, on the center, I could add another 100uF to the diode where the original bias supply was. That would increase the capacitance very high though right? There would be some resistance between them when the 10KL bias pot was not all the way to one side.
And yes, I tested with the chassis back in the cabinet with the metal shield in place on top.
Edit, clipped 50uf cap on middle lug, move new 100uF cap back to stock locaton, hum gone.
So it seems you need both caps.
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Adding the additional cap to the center post, otherwise leaving the bias stock, solved the hum that comes up when the intensity goes down. This should be added to the PDF when using the BIAS board. One or the other did not work.
Also, putting the bias back to stock (albeit with a 100UF cap) got rid of the tick from the speaker and brought the intensity back UP.
The speakers still MOVE and I still want to know if this is NORMAL or some other issue I have in this amp. I have no point of reference other than all you guys that have built these.
I will order metal film resisters for all the grids, the 82K on the PI and the 22K and the 470 Ohm. Not of this will get rid of the RIZZ though. I could totally live with the steady hiss I have if it wasn’t rizzing. I’ll also order 2 50uF 100V caps for the bias unless you all think the 100/100 on the main supply is OK and the 50/50 on the center lug is also OK. When I bought these parts, I ordered a KIT then substituted a bunch of the parts here and there but the BIAS was as it came in the kit. It was way easier for me to order the parts kid and not use all the parts than to try and sort what resisters needed to be 1W, what needed to be 1/2W, and all the voltage ratings of various caps. This was my first amp. I ordered all the parts back in 2008 or so IIRC. So to be fair the bias cap is 12 years old, but thats brand new for the most part cause its barely been played and I’m sure they haven’t dried out in my home.
I’ll shoot another video from the front of the amp so demonstrate. The amp sounds gorgeous btw. Getting rid of that STORM and the HUM is huge. I could live with the hum keeping the intensity at full but that storm was random, not any sort of pattern. I’ll let the amp sit on for a couple hours and see if anything comes back.
You guys are awesome! Thanks for being patient with me. Interestingly, the Princeton Reverb I built in 2014 had only 1 issue upon first power up which was a reverb send wire was on the wrong eyelet or disconnected and that amp has been perfect since I got it together. This Twin was the one I made all my first timer mistakes on. I cleaned up a lot of solder the last few weeks. Some of the turrets just had TOO MUCH so I sucked it off and reapplied just a tiny bit. Has some PVC that was melted right at the solder connection, stripped those back and cleaned them up. Little things here and there, none of which improved or disturbed the sound, but it looked awful.
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Last update for now.
Everything is back together. The idle rizz noise needs to go but overall its all much quieter.
The speakers move when the tremolo is on and I really would like as many people as possible that have built these 6G16 Tremolos to tell me if that’s normal.
I adjusted the BIAS DOWN (hotter plate disappation) and the ticking from the JBL came back and then I adjusted it further and it went away again, but by that point its too weak. I plan to make tweaks to the oscillator circuit to get stronger tremolo when biased hotter, later on. The point is, there is a spot where the trem is moving the speakers in and out I guess more approaching a square wave perhaps, and the JBL pops. Lower bias and its OK, higher bias and its OK but the trem intensity is too low.
I intend to get another JBL for this amp cause the JBL just sounds better than the EVs and the amp is insanely heavy with 2 EVs in it. These JBLs are lighter. The EVs tend to be a bit harsh and the JBL mellows that without taking out the sparkle. In fact I can run with the bright switch OFF. With the EVs I had to have the bright switch on to get the sparkle but then turn down the highs to get away from harsh and it was a moving target based on how loud it was. The JBL just seems to work. I guess that’s why so many use the D120F. This is a K120. It works well with the EV but I need to find another one to try it with both. End of the day I will use whichever combination sounds the best. SO, I think I need to figure out why the speakers are moving.
Its as if the oscillator frequency itself is hitting the speakers as AUDIO. Perhaps the frequency is just to low to reproduce audible sound from these speakers. I need to get an O-scope I guess. I actually have one that has never been plugged in since I’ve had it. Its so old, I assume it doesn’t work. Buddy gave it to me. I would like to have a 1MHz compact dual trace unit and I would love it to be analog. I think that might be a unicorn though.
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I'm too slow--you already closed up. But here are ideas for next time...I avoid having signal and high voltage wires run parallel to each other. The four power tubes in your Twin Reverb might benefit from rewiring to reduce possible interaction. You could place each 470 ohm screen resistor on the tube socket and feed them with a single wire. This would daisy chain from one tube to the next and be located between the socket and the back chassis wall. Daisy chain the signal wires to each tube pair but locate them in front of the sockets. The wires are still parallel but far fewer of them and spaced much farther apart. Sluckey shows this very nicely on his layout diagram for a 6V6 Plexi, attached.My other idea is Sluckey-related also. No originality here! I have had great success using his Trem-O-Nator Circuit to wiggle the signal in the preamp stage. The tremolo volume swells are very smooth, like a good bias-vary trem but without affecting the amps bias or fading out at high volume. The circuit only uses one triode so the downside is finding a use for the extra triode :wink:
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I'm too slow--you already closed up. But here are ideas for next time...I avoid having signal and high voltage wires run parallel to each other. The four power tubes in your Twin Reverb might benefit from rewiring to reduce possible interaction. You could place each 470 ohm screen resistor on the tube socket and feed them with a single wire. This would daisy chain from one tube to the next and be located between the socket and the back chassis wall. Daisy chain the signal wires to each tube pair but locate them in front of the sockets. The wires are still parallel but far fewer of them and spaced much farther apart. Sluckey shows this very nicely on his layout diagram for a 6V6 Plexi, attached.My other idea is Sluckey-related also. No originality here! I have had great success using his Trem-O-Nator Circuit to wiggle the signal in the preamp stage. The tremolo volume swells are very smooth, like a good bias-vary trem but without affecting the amps bias or fading out at high volume. The circuit only uses one triode so the downside is finding a use for the extra triode :wink:
RIGHT! I always thought of those high voltage lines as DC but I realize more recently there isn’t enough filtering on them and that could certainly cause some buzz, only thing is when all the little tubes are out, its dead silent. The power tubes aren’t amplifying their own noisy power, but I wonder if some of that noise is getting into prior stages. I still suspect grounding.
Heck, I’m still baffled as to why my TRS foot pedal setup was so problematic. It was exactly the same as the twin reverb reissue, and that works fine. I got fed up trying to figure it out and re-wired it to vintage, which filled the other RCA hole on the chassis, which is good. Magically it fixed a terrible footswitch pop when turning off the tremolo.
My number one question right now is if the 6G16 tremolo is supposed to be MOVING the speakers when nothing is plugged into the amp.
In regards to Slucky’s tremonator. Once all the noisy bits are gone, then I start with mods. The first will be to do all the STRONG tremolo mods to facilitate a stronger bias setting. I will put an LED on the cathode, etc. We’ll see how that works. If all is said and done and I pick up a real set of 6L6 GCs and I can’t get them biased AND have the tremolo I like, I will change to Slucky’s mod. Next will be channel 1 tone stack mod to Marshall or tweed or bassman. I’m leaning toward the marshall stack.
Next after that, I might try something else like using the polarity switch on the back panel to switch NFB or replacing it with a knob for the same purpose. I really don’t want to make the amp TOO versatile though. I am OK with a 1 trick pony here. I think I will ultimately end up with a dumble clone, or I might start from scratch with another fender and impliment all the early dumble mods to tighten up the bass and the changes to the tone controls, and cascading the channels, which is something I’ve wanted to try for 25 years on this amp actually, which was why I originally bought it. I’m not sure I’ll be adding any pedal PAB or GAIN mods to this one though cause it sounds SO SO good as is and just needs this noise gone and to be able to be biased hot with the correct power tubes. It will be in my rental stock. I don’t really rent backline but I might do it. I have a dream of having a rehearsal studio too. No one really does that right where I want to be living.
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Hey guys, I made a video so you could hear the noise in reference to the actual sound of the amp. I suck at playing but I know you guys are tone fanatics. I actually changed the setup of the guitar the other day which killed a lot of my tone but its a LOT easier to play now. I’ll deal with it. I still have another one with fat strings and high setup.
&feature=youtu.be
Let me know what you think of the amp. I just gotta finish getting rid of that rizz/buzz and maybe I can get the hiss down a tiny bit more with some more metal film resisters.
The knob settings in the photo are the way it was set at after I turned the speed down on the tremolo.
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Guys, I think I found a small error. Some years ago I had one of the 470Ohm resisters smoke when I first got it going. I think it shorted on a chassis screw. I replaced with a carbon comp in the parts kit I bought (which had a lot of errors, one of which I missed apparently). Looks like I used a 47k. I found it looking at the gut shot I posted earlier in this thread. I’ll measure it when I open the amp.
What is the effect of the second grid one one of the power tubes being 47k instead of 470? Would that cause enough of an imbalance to make the rizz I’m chasing audible, if just barely?
I’m taking your advice and re-wiring the octals in the more vintage way like the sluckey attachment.
So does that look like an Orange ring or a brown ring on that carbon comp? I’ll measure it. That’s my SOP before installing so its hard to believe I missed it.
EDIT: its brown, just looks orange int eh drawing. Ordering all the parts to swap now.
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Better?
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I tried wiring the bias like the 6G16 and it brought a little bit of the hum back so I left it as I left it with the 2 caps cause its nice and quiet with intensity up or down, other than that rizz I am chasing.
I have a different problem.
This was there before I just did these changes today.
If I crank up the amp LOUD it can jump to a BUZZY distortion sound. I can play a note and keep turning it up while I keep playing the note over and over and if I play the note hard enough with the volume up enough the tone, which is awesome converts to this nasty buzz. I raised the bias up a bunch to see if that was related but its not. I didn’t mention it because I thought it might be a speaker issue, but its not.
The buzzy distortion is the worst thing I’ve ever heard. If I let the note ring when it drops enough it turns back into normal tone.
This distortion is far worse than the idle noise issue. I’ll try swapping some tubes with another amp just to make sure its not V4B or V6.
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Removing V3 (reverb driver) eliminated this buzzing distortion issue. I’m thinking blocking distortion but I’ve never heard it before so I can’t say for sure that’s what it is and I don’t have a working scope to track it down. I did move the lead dress around a bit a couple weeks go after some comments so maybe I created the problem. I swapped in another 12AT7 and that is a known excellent tube out of another amp and the problem persists.
&feature=youtu.be
I am turning it up gradually while hitting the A string. The difference in volume from the first not to the later notes is substantial but the phone compresses it.
Can anyone confirm this is blocking distortion? Is this a thing that happens on the reverb driver? The amp has to be way loud before it happens and I know even if the reverb return is down all the way, the driver is getting nailed by V2. I’m not sure how it ends up in the signal path though. Does it go back up from the grid and then get amplified by V4B along with the guitar signal? Should I add a resister between the 500pF and the 1Meg at the grid? If so, what size? I think I’ll go dig up a chop stick and move the wire around while hitting the offensive note and see if that makes any improvement.
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I moved the leads around and separated them on V3, but of course the 2 loops that cross the top of the tube can’t really be moved. It made no difference. I played for a few seconds without the tank plugged in and that also got rid of this distortion. That confuses me even more. If it is oscillating, I would think it would be more likely to do it without the tank plugged in. I didn’t want to burn up the transformer so I kept that to about 5-10 seconds. FWIW, the reverb cables are braided shield type. I can’t see them being an issue. I checked at lower volume that the reverb works good. I then used the foot switch to turn it off which shorts the return. Cranked it up and the issue is still there.
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That’s not blocking distortion.
It’s either a bad connection (may be internal to a component) or a parasitic oscillation.
Is it vibration related, eg does moving the amp chassis away from the speaker cab, or plugging into a different, physically separated cab, affect it?
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https://el34world.com/Forum/index.php?topic=23989.msg281848#msg281848
I’ll plug into another cabinet first but someone else on FB said to put in a 10K grid resister between the 1meg and the grid on V3 (which is V2 on that Princetion). Of course the sound clip is gone so I can’t hear if its the same.
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So this is strange. I clipped in a 10K, then 5K, then 2.8K, then for the heck of it cause someone said 200Ohms might work, I tried 470. They all worked. So I soldered in 470, doesn’t work. So I soldered in 5600, doesn’t work. So I soldered in 10K, doesn’t work. Clipping in the reverb works, and there is no distortion. Soldering in, I get the distortion. I have to completely remove the amp each time and put it back in the cabinet because if the reverb isn’t plugged in, there is no issue so testing outside of the box is of no use.
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> if the reverb isn’t plugged in, there is no issue so testing outside of the box is of no use.
Why? Is the tank glued to the box with too-short cables?
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Yea, I’m not pulling the reverb tank or the cables out. Its the original 1968/ 69 reverb tank from this cabinet and everything is nice and tidy. Every time I pull a screw out it goes in looser than before.
I pulled the cathode bypass cap on V3 (as Sluckey suggested in that other thread) and the problem is gone (the 10K grid stopper is still there) but the reverb does NOT sound as good as it did before.
Perhaps Sluckey will explain what is going on here?
I cannot understand why I could clip the resister in with alligator clips and the reverb would work and sound great, then when I soldered it in, it would go back to this mess.
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I’ve encountered that same issue previously, and ascribed it to ‘parasitic motorboating’ (my conjoined terminology to describe an unusual merging of instability subsets), via the 0V return rather than the HT supply.
Due to the V3 reverb driver being supplied via the screen grid HT node, which makes the 0V return path for its HT current rather convoluted and intermingled with preamp 0V returns.
Note that the fine engineers of the SF era got rid of V3 cathode bypass.
My solution was to add a small resistor (470ohm) in series with the cathode bypass cap, so as to limit peak currents.
I used to have a sketch of that arrangement on photobucket, but they’re charging even to access the old stuff now. Let me know if you’d like it uploading somewhere else, and I’ll get around to it :icon_biggrin:
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I’ve encountered that same issue previously, and ascribed it to ‘parasitic motorboating’ (my conjoined terminology to describe an unusual merging of instability subsets), via the 0V return rather than the HT supply.
Due to the V3 reverb driver being supplied via the screen grid HT node, which makes the 0V return path for its HT current rather convoluted and intermingled with preamp 0V returns.
Note that the fine engineers of the SF era got rid of V3 cathode bypass.
My solution was to add a small resistor (470ohm) in series with the cathode bypass cap, so as to limit peak currents.
I used to have a sketch of that arrangement on photobucket, but they’re charging even to access the old stuff now. Let me know if you’d like it uploading somewhere else, and I’ll get around to it :icon_biggrin:
Right so its as if the V3 is actually interacting with the V2 and that is what is making it audible?
OK so assuming a stock AB763 Reverb circuit, are you lifting the cap at the ground connection and inserting the resister, or at the cathode end? I considered doing this but I didn’t have a value for the resister. Is a 1W resister OK? I happen to have several 470Ohm 1 Watt carbon comp so I could try that easily.
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The positive feedback loop somehow involves the power amp, hence it only occurring at higher power outputs. And I’ve only encountered it on TRs, so higher power amp currents than other reverb amps. But not (yet) the 135W version, which has a different power amp configuration / 0V plan.
The 470ohm resistor should only dissipate low power, even 1/2W is probably far more than necessary.
It’s perhaps better have the cap’s negative terminal (ie its outer foil) connected to 0V.
As it seems unlikely that these amps behaved like this when new, it may be worth considering what might have changed over time. One possibility is that one or more 0V to chassis connections has developed excessive resistance, due to eg loosened fasteners, oxide build up, failed solder joints.
Problem being that a Fender relies on the numerous such connections all being good.
Every jack and pot and chassis tab fastener has a star washer that needs to bite into clean shiny metal for its connection to be properly established. And then there’s the brass plate and the corrosion that can build up between it and the chassis, due to dissimilar metallurgy.
Carbon type resistors should be avoided for use as screen grid resistors, now that the far more appropriate MO are available. As the carbon is a fuel load and catches fire in the case of a severe screen grid overload / short. Whereas MO are extremely unlikely to, and have a flame retardant coating just in case (pretty much anything can burn if it gets hot enough).
Plenty of old amps show the signs of the above happening.
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I just pulled the input jacks out a few days ago and re-wired them so they are very solidly to ground. Several small changes have been made to attempt to clean up the noise I was getting in this amp and in the process, I may have created this problem.
The CAP is an electrolytic. I’ll connect the - to the ground bus and the + to the 470Ohm and the other side of the 470Ohm to the cathode with the 2.2K.
BTW, this is the layout....
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I used Rob’s amp picture and dropped in my turret board and did my best to show how its wired. I have no idea what might have changed, as you put it. I can tell you other than doing a scratch build, if I do any more Princeton kits, I’ll be buying an eyelet board rather than using the cardboard crap that comes in the kits. I picked up the PR I built in 2014 cause it has a scratchy pot and the thing has been in air conditioning and shade the whole time and the board is actually crinkling up a bit like the old ones did. Its not been to a gig and since his arthritis has taken over big time, its barely played. I digress.
Here is the layout of my amp. The actual lead dress is different but you have seen that in the photos.
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I currently have everything put back to normal. The buzzy distortion persists.
I have the amp out of the cabinet now, but still attached to everything. Makes no difference to the distortion. I still need to try adding the resister in series with the cathode bypass cap but I still want to know why I need to reduce the gain of the tube to fix this when there are thousands of these amps out there working just find without needing that. I would really like to find the source of the problem rather than put a bandaid on it.
Here are my voltages. EDITED with fluke!
All controls DOWN
All controls DOWN
V1
1 248
2 0
3 1.72
6 250.9
7 0
8 1.81
V2
1 250.1
2 0
3 1.75
6 246.9
7 0
8 1.787
V3
1/6 410
2/7 0.027
3/8 7.97
V4
1 244
2 0
3 179.4
6 237
7 0 hum
8 1.89
V5 Trem footswitch off controls down
1/7 169
2 0
3 1
6 415 small pop
8 1
V6
1 203.7
2 54.7
3/8 82.9
6 207.9
7 54
V7
3 414
4 414
5 -43.6
8 0
V8
3 414
4 414
5 -43.6
8 0
V9
3 414
4 414
5 -43.6
8 0
V10
3 414
4 414
5 -43.6
8 0
I moved the ground of the middle cap on the board from the ground of the series caps to the ground of the other 2 preamp filters. That made no difference to the operation. It may or may not have affected the noise level, but I’ll determine that after the distortion thing is fixed. Its easy to move back. Open to comments on that.
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I'm confused by the P1, P2.. on robrob's referencing for connections off to the doghouse cap board; sorry if I've overlooked it but are those defined anywhere?
...I moved the ground of the middle cap on the board from the ground of the series caps to the ground of the other 2 preamp filters...
And then there's your referencing system, presumably for the doghouse ecaps; some specific, common terminology is required for effective communication. eg the AB763 schematic refers to the HT cap nodes as B, C, D, and we can assume 'A' as the reservoir cap.
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I’m sorry about the confusion. Also all those voltages i posted earlier are WRONG. My fluke JUST arrived and I already confirmed the plates are 414 on the 5881s and the bias is -43.
I had posted a layout of MY amp earlier and I marked P1 P2 P3.
Basically P1 is the MIDDLE cap, P3 is the next one of the same size, P3 is the same sized one on the end. The other 2 caps on the other side are in series to double the voltage handling. I think they are 70µF each so 35µF total.
I am going to revise my voltage numbers that I already posted so once I edit that, those should be right.
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Here is where I am with it......
I should probably start a new thread since the tremolo hum was solved several days ago.....
I have a great sounding Twin Reverb with the 6G16 BIAS VARI TREMOLO. Only problem is when I dime it, something goes wrong with the reverb driver tube and it causes a horrible distortion you can hear in this video. The volume was being turned up gradually in the video. The distortion gets a lot worse with more signal. https://youtu.be/fUi-ziwMOkI (https://youtu.be/fUi-ziwMOkI)I built the amp several years ago and lately I have been chasing noise and I’m fairly sure this was not a problem before that. I cleaned up the ground bus that was soldered to the back of the pots just to make sure I didn’t have multiple paths to the chassis through the pots. This did not affect the noise. I have swapped out carbon comp grid and plate resisters with metal film. This made a huge difference in the noise. The Tremolo intensity pot and bias are intermingled in this circuit. When the intensity was turned all the way down, I had a hum. Putting the bias on the center of the bias pot as in the 6G16 didn’t help but putting it on the outside lug, adding a cap to the middle lug and also attaching the middle lug to the intensity made that hum go away. I had the 470ohm resisters on the board with 1ohm resisters for the cathode to check bias. Someone thought that mess of wires might contribute to noise so I changed that whole area to the more traditional style with the resisters on the tube sockets. It cleaned up the look but had no affect on the noise. During all of this the amp sounds fantastic, but I have a small rizz left. A buddy that used to have a Twin, asked me to dime it and record some noodling for him and thats when I discovered this nasty distortion. So far I have done the following.Pulled out V3 reverb driver tube - problem solved, new problem created, no reverbPulled out the reverb send cable - problem solved, new problem created, no reverbDisconnected the cathode bypass capacitor - problem solved, new problem created, reverb sounds ringy and is weaker at normal decibel levelsPlaced a 500pF cap between the plate and ground. This did not work but it moved the distortion to a higher gain setting, it also killed the reverb.Placed a 500pF cap between the plate and the cathode - problem almost solved but new problem created, VERY weak reverb, mostly lower mids, required the reverb return to be up around 8 or 9 where 3-4 is usually plenty, consider there is a 500pF cap between the plate of V2 and the grid of V3 already so the other 500pF is sending everything above the cutoff to ground and the 500 in series with the grid is filtering out everything below the cut off so there isn’t much signal left. The resistance in the circuit will change the cut off frequencies, and I don’t have the math to predict that but its not a solution anyway.I placed a .1µF orange drop between the B+ going to the reverb transformer and ground, made no difference, but may have affected the reverb sound, but I didn’t dig into that.Tried 5 values of grid resisters from 470-10k.I have moved the leads around but that doesn’t make any difference.I have swapped the tube with a known good tube. The tube is a JAN Phillips 12AT7 and was new going in. I roll tubes periodically when chasing noise on this amp so that tube may well be brand new. I have a stack of these NOS.What I have not yet tried is adding a 470Ohm resister between the cathode and the cathode bypass cap. That was suggested on a forum. It reduces the gain of the tube and may be a compromise, but I would like to find the root of the problem rather than bandaid it.A few people have indicated that a bad filter cap could be the cause. I will order some filter caps. If I don’t need them for this, I can use them for something else later. The filter caps were all new F&T when I installed them.The file size limit is too small so I can’t add any pictures. Here are links but they ill eventually expire unfortunately.
https://www.dropbox.com/s/6rey6utl1k1dn ... s.pdf?dl=0 (https://www.dropbox.com/s/6rey6utl1k1dncs/My%20Twin%20Reverb%20Docs.pdf?dl=0)
https://www.dropbox.com/s/bf7sih2yjeeb5 ... 3.png?dl=0 (https://www.dropbox.com/s/bf7sih2yjeeb5yf/TR6G16%3AAB763.png?dl=0)
https://www.dropbox.com/s/fo26h6ts7vo9v ... .heic?dl=0 (https://www.dropbox.com/s/fo26h6ts7vo9vok/IMG_0659.heic?dl=0)