Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Mrsedor on September 22, 2022, 02:18:56 pm
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Hi,
I started with a buggy / noisy PCB 65 Deluxe reverb reissue, chassis with all the transformers and choke and wanted to go for a fresh start, converting it to Eyelet, with better components and hopefully a better sound.
Long story short, I have 2 problems, one is the 50hz and 100hz noise (EU power here) that is a bit high, especially when V4 is in, with or without reverb, and not depending that much on the volume of either channel.
Plate voltage at V4a & b are good (170/180v).
The other is, with a probe connecting tool (1ohm in an intermediate socket), I cannot get the bias down to less than 29/30ma, which seems to high for a 6V6, I'd like it at 22ma but cannot reach it...
When powered without 6V6 but rectifier installed, I can reach -42V of negative bias maximum.
When I read the supply values, D+ looks fine at 280v, but C+ is at 340V instead of 325V and B+ is at 440V instead of 415V.
I followed precisely Rob robinette's layout (attached), with 2x100 ohms for Heater center tap, and with the Tick fix capacitor along the photocell.
I used orange Drop coupling capacitors, 5x FT 16uf, sprague atom and RN70 or RN65 dale metal film resistors.
I added a 370K 5w bleeder between first filter cap and ground. It drains all the power in 10sec.
Input jack are isolated from the chassis, with a preamp ground bus running behind them, connecting to chassis via a screw near the normal 1 input. Preamp ground is connected to this.
I run coax with shield grounded at this bus from 68ks inputs to grids and back to volumes for both channels.
RCA jacks for reverb send/return are not isolated and grounded to chassis, with reverb transformer ground and 220k input resistor connected at this ground point.
TRS jack for the footswitch is not isolated. Grounded directly at chassis. Should it be and grounded to the preamp bus ground? This is ground and reverb I/O ground is more distant than in the original, as the RCA plugs are under the chassis and not behind like the footswich plug.
Output jacks are not isolated, grounded directly at the chassis.
Power ground is near power transformer, on a different bolt than the earth, it connects HT centertap (red yellow), 6V6 cathodes, bias board and first stage of filtering.
My wiring does not look as good as certain builds I see on the web, but soldering is good and I tripled check the build with the schematic. Should I shorten the plates and cathodes wires to the minimum?
I had first a big buzz problem, and it got really better when I moved the V4 wiring from the BCD supply lines. It's still there a bit though. I wonder if i should drill another hole in the chassis and run it the way they do in the 64' reissue, meaning on the other side of the eyelet board. They look better twisted also. Should I go for it?
I have tried to swap the OT wires, because it doesn't squeal. These are not twisted. Should I do otherwise?
I will post detailed pictures.
On a side note, one 6V6 is a bit loose between the glass and and the black plastic base. I will order new ones.
Gently tapping with a stick V4 is very microphonic, is it normal?
Sorry, lots of questions, I'm a bit lost after long days of trying to figure it all...
Thanks in advance for your help!
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With more pictures :
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More pictures
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More pictures
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And also, reverb tank is connected, and it is not apparently creating a ground loop (isolated on one side of the tank).
Thanks :)
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R.G. Keen's Troubleshooting page addresses a lot of your symptoms with concrete troubleshooting suggestions.
http://www.geofex.com/ampdbug/ampdebug.htm (http://www.geofex.com/ampdbug/ampdebug.htm)
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R.G. Keen's Troubleshooting page addresses a lot of your symptoms with concrete troubleshooting suggestions.
http://www.geofex.com/ampdbug/ampdebug.htm (http://www.geofex.com/ampdbug/ampdebug.htm)
Thanks a lot, I will look at it thoroughly :)
By the way, do you have any idea about why B+ might be a bit high? Or could it normal to be 440v instead of 420v ?
Thanks 🙏
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First, please post a schematic of what you've actually built. Without one we're all shooting in the dark. Take one clear landscape shot and capture the entire board and chassis in one pic.
If B+ is higher than expected, is is possible you have the voltage selector switch set incorrectly? If not, is it really a problem? Put in 6L6s and call it a Pro Reverb. :laugh:
Okay, what rectifier tube? New or used? What's your screen voltage at "B" node? Record voltages with all tubes in and the amp warm.
Yes, put in new 6V6s--they won 't hurt. I have no experience wrapping control pot wires around others as a bundle, but a lot of builders do it.
Uncle Doug has a YouTube video about 60Hz vs. 120Hz hum (or 50/100Hz in your case) and how to tell the difference. Also ValveWizard.com and Aiken Amps have grounding discussions as well as lots of other info.
http://aikenamps.com/index.php/grounding (http://aikenamps.com/index.php/grounding)
http://www.valvewizard.co.uk/Grounding.html (http://www.valvewizard.co.uk/Grounding.html)
Bias: What's your max and min bias voltage at the 6V6 grids? According to RobRob's calculator, 30mA is 90% of max dissipation. The ValveWizard says 85% is fine for AB push-pull amps with fixed bias. Whereas 22mA would put you at 66%, maybe a tad low? Do you have the 1 ohm 1% resistors between each 6V6 cathode and ground?
Fender biased low (60%) for tube longevity but if you're willing to replace tubes every few years then a hotter bias might be acceptable to you. How does it sound to you? My AB763 rolls around 25mA (75%) on new JJs, and I'm quite happy with it. I don't play out enough to worry about tube life.
The 470 ohm bias resistor controls the range of your bias voltages (assuming the stock Fender circuit.) Has it drifted?
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First, please post a schematic of what you've actually built. Without one we're all shooting in the dark. Take one clear landscape shot and capture the entire board and chassis in one pic.
If B+ is higher than expected, is is possible you have the voltage selector switch set incorrectly? If not, is it really a problem? Put in 6L6s and call it a Pro Reverb. :laugh:
Thanks a lot !
The circuit I built is the one attached, I followed it precisely without any change but the 0.02uf across the optocell, and 0.05uf instead of 0.047uf. I used the layout attached in the first post.
It is possible I made a mistake, but I triple checked all the connexions already... I will check it again.
I used the transformer originally fitted in the 65 deluxe reverb reissue for EU, it was connected to pcb with the violet (live) and white (neutral). But they were other wires to the power transformer that were not connected to the circuit (attached to the PCB still).
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If B+ is higher than expected, is is possible you have the voltage selector switch set incorrectly? If not, is it really a problem? Put in 6L6s and call it a Pro Reverb. :laugh:
There was no voltage selector in the "original" pcb circuit.
Here is the diagram of the transformer fitted in international amplifiers, like mine.
I don't know if someone mess with this connexions before me in this amp.
Violet (live) and white (neutral) from the Mains Switch / fuse CP7 & CP8
Black and White , attached to the PCB but connected to nothing. I believe it was intended for 100V. CP1 & CP2
Black/Red CP3, connected to nothing.
Black/Yellow CP5 & Black/Green CP6, connected together, but to nothing else.
No CP4.
I took a picture of what it was before.
Here power is 230V.
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Okay, what rectifier tube? New or used? What's your screen voltage at "B" node? Record voltages with all tubes in and the amp warm.
The tube is a 5AR4 from electroharmonix, that was before in this amp when it was a Drri.
I will record all the voltage as soon as i can, probably on Tuesday, and report back.
Thanks a lot for your help !
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I wasn't asking for all voltages. My comment was just that once the amp is running, always test with all tubes in, ie, under full load.
A schematic would be more valuable than all voltages at this point.
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I wasn't asking for all voltages. My comment was just that once the amp is running, always test with all tubes in, ie, under full load.
A schematic would be more valuable than all voltages at this point.
The schematic is this one, attached to the post.
Thanks :)
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Take one clear landscape shot and capture the entire board and chassis in one pic.
Is this picture (attached to this post) good enough?
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The picture is fine, the schematic is the standard. All good. Your layout is good and your soldering and lead dress are probably better than mine.
There was no voltage selector in the "original" pcb circuit...
Violet (live) and white (neutral) from the Mains Switch / fuse CP7 & CP8 ... I took a picture of what it was before.
Here power is 230V.
Okay.
1. What is your actual wall voltage at any wall socket, measured by voltmeter?
2. What is the HT voltage going to pins 4 and 6 of the 5AR4? If your DVM doesn't have a range of 0-1000V, then just measure from pin 4 or pin 6 to ground, that'll give half the value, then double it. (Fender's schematic suggests to me that their transformer puts out around 320-0-320V P-P, ca. 650VAC. )
3. What are your 'A', 'B', 'C', and 'D' node voltages.
4. What is the total voltage put out by your bias circuit?
5. What is the maximum and the minimum bias voltages you can get? In other words, the bias voltage at the 6V6 grids with the bias pot turned all the way up and then all the way down.
I've attached an export transformer wiring scheme from a Fender DRRI 65 manual, see below. It doesn't match what you described, but hopefully it'll help to confirm/disprove your conclusions about which windings you're using.
Honestly, although your voltages are slightly higher than nominal it's hard to see them as a 'problem.' The 230V version (of the DRRI transformer) shows four other secondaries from the DRRI that an AB763 circuit doesn't use.
On one hand that would explain why your AB763 version has 'extra' power available to it. It also makes me wonder about how you wired up your bias circuit. In other words, the 'real' AB763 transformer had a dedicated bias tap. Where do you tap power for your bias circuit?
The hum issues are harder to diagnose from a distance. I still recommend Uncle Doug's video; he'll point you in the right direction. Keen's article(s) have a lot of things to try. Report back on what you find.
Good luck!
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1. What is your actual wall voltage at any wall socket, measured by voltmeter?
2. What is the HT voltage going to pins 4 and 6 of the 5AR4? If your DVM doesn't have a range of 0-1000V, then just measure from pin 4 or pin 6 to ground, that'll give half the value, then double it. (Fender's schematic suggests to me that their transformer puts out around 320-0-320V P-P, ca. 650VAC. )
3. What are your 'A', 'B', 'C', and 'D' node voltages.
4. What is the total voltage put out by your bias circuit?
5. What is the maximum and the minimum bias voltages you can get? In other words, the bias voltage at the 6V6 grids with the bias pot turned all the way up and then all the way down.
Hi,
Thanks for your help!
Here are today's reading:
With all tubes in the sockets.
1. Wall socket : 238V
2. 5AR4 UpP4 Ground : 343V UP6 Ground : 343V
3. A (standby open, off) 461V weird?
A (Standby on) 420V
B 420V in all 3 points in the circuit
C 333V
D 272V
4. With the 2x 6V6 plugged:
at both grids (pin5) -37.1V
at 220k resistors junction -45,2V
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3. A (standby open, off) 461V weird?
A (Standby on) 420V
B 420V in all 3 points in the circuit
C 333V
D 272V
These look very normal. "A" high voltage is normal when STBY is open.
4. With the 2x 6V6 plugged:
at both grids (pin5) -37.1V
at 220k resistors junction -45,2V
The voltage on the grids should be the same as the voltage at the 220K junction. Recheck that.
OK, you have about 420V on the plates of the 6V6s and about -37V on the grids. That sounds normal to me. My DR has 423V and -36V and that gives me 24mA cathode current using 1Ω current sense resistors on the cathodes. I think something is not right with your bias probe. I suggest you get two 1Ω 1W 1% resistors and connect them between cathode (pin 8) and ground and use those to measure your bias current.
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Without the 6V6 in the sockets:
Minimum bias voltage at the grid (p5) : -36,2V
Maximum voltage: -18V
Is it correct?
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With fixed bias amps, the typical 10M input resistance loading effect of most DMMs causes a small (2%) volt drop across the typical 220k grid leaks.
So -50V at the bias supply output will measure -49V at the control grid socket terminal.
Dropping 8V across 220k implies a meter loading resistance of 1M.
I suggest to measure the voltage from one end of a 220k grid leak to the other.
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Without the 6V6 in the sockets:
Minimum bias voltage at the grid (p5) : -36,2V
Maximum voltage: -18V
Is it correct?
I don't like that range. It means that the bias pot needs to be set to one extreme to get a safe bias. But you say you have -45V at the junction of the 220K resistors. You should also have that same -45V at the grids in a properly working amp. Find out why that is not so. Leave the 6V6s unplugged until you resolve this. About the only thing that would cause this difference in a properly wired amp would be leaky coupling caps between the PI and the 6V6 grids. Temporarily disconnect both of those .1µF couplers and see if you now have the same voltage on the grids and the junction of the 220Ks.
BTW, meter are you using?
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At the jonction between the 2x 220k and the wire from the diode I have :
Minimum -45v
Maximum -22V
At the jonction between 220k and .1 coupling cap I have:
Minimum -37V
Maximum -18V
At Pin 1 and at Pin 5, after the 1.5K, I have the same:
Minimum -37V
Maximum -18V
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Right, but that’s negligible.
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Right, but that’s negligible.
At the jonction between the 2x 220k and the wire from the diode I have :
Minimum -45v
Maximum -22V
At the jonction between 220k and .1 coupling cap I have:
Minimum -37V
Maximum -18V
At Pin 1 and at Pin 5, after the 1.5K, I have the same:
Minimum -37V
Maximum -18V
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I suggest to measure the voltage from one end of a 220k grid leak to the other.
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With fixed bias amps, the typical 10M input resistance loading effect of most DMMs causes a small (2%) volt drop across the typical 220k grid leaks.
So -50V at the bias supply output will measure -49V at the control grid socket terminal.
Dropping 8V across 220k implies a meter loading resistance of 1M.
I suggest to measure the voltage from one end of a 220k grid leak to the other.
I mesured both 220k. For each mesurement, one probe at the 2x 220k jonction and the other at the 220k to wire jonction).
0V dc
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I suggest to measure the voltage from one end of a 220k grid leak to the other.
From the 220K to wire jonction of each 220K (not at their jonction) I also mesure 0V dc
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The resistors are within specs when mesured inside the circuit (218k)
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BTW, meter are you using?
I'm using a simple DMM, Mastech MAS830
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One thing I don't understand is:
I have -45v between ground and 220Ks jonction
I have -37v between ground and the grid P5
I have 0v between 220Ks jonction and the grid P5...
:help:
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About the only thing that would cause this difference in a properly wired amp would be leaky coupling caps between the PI and the 6V6 grids. Temporarily disconnect both of those .1µF couplers and see if you now have the same voltage on the grids and the junction of the 220Ks.
I disconnected both .1 couplers, even the wires going to the grid stoppers.
Still the same, at the jonction of 220k, I have 45v, then 37V after each 220K.
So 8V is dropped across them... not normal, right?
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It is always with 6V6 out of the sockets.
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I disconnected both .1 couplers, even the wires going to the grid stoppers.
Still the same, at the jonction of 220k, I have 45v, then 37V after each 220K.
So 8V is dropped across them... not normal, right?
NOT NORMAL! So, you had the caps and the wires disconnected. That means the only thing in that eyelet is one lead of the 220K. That's an open circuit. Current can't flow and there will be no voltage dropped across the resistor. UNLESS...
There is continuity between that eyelet and chassis. Maybe something under the board is touching the eyelet, such as a screw? Or, the board is conductive, which happens a lot with the type board you are using. Where did you get that board?
Try this. Lift the ends of the 220K resistor from the board, but leave the junction connected. Now measure voltage on each end of the two 220K resistors. I bet they are all the same now.
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There is continuity between that eyelet and chassis. Maybe something under the board is touching the eyelet, such as a screw? Or, the board is conductive, which happens a lot with the type board you are using. Where did you get that board?
Try this. Lift the ends of the 220K resistor from the board, but leave the junction connected. Now measure voltage on each end of the two 220K resistors. I bet they are all the same now.
I got that eyelet board from Banzaii music in Germany :
http://www.banzaimusic.com/eyelet-board-blackface-reverb-ab763.html (http://www.banzaimusic.com/eyelet-board-blackface-reverb-ab763.html)
I will test the continuity with the chassis and disconnect like you suggest.
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By the way I had a look at the DRRI bias schematic. My power transformer comes from it, so maybe it doesn't deliver the right voltage to the bias circuit.
In this bias circuit, the 470Ohm is a 22Ohm, and 100uf replace the 25uf I'm using.
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Try this. Lift the ends of the 220K resistor from the board, but leave the junction connected. Now measure voltage on each end of the two 220K resistors. I bet they are all the same now.
OK, I did it, disconnected the 220k from the circuit. Wire from the bias pot soldered to the 2x 220k, in the air. One leg of each 220k not connected to anything but the meter probe.
Nothing is connected to the eyelet board.
Still the same voltages...
:w2:
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OK I suspected the DMM.
I tried another one and.... NOW I get the same voltages!
I will solder it back to how it was before and mesure with the new DMM at the grid.
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So... everything is soldered back where it was...
With the other DMM I read -45V at Grid...
It was a DMM problem after all :BangHead:
So now at the grids I can read from -45V to -22.
Is that ok?
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So now at the grids I can read from -45V to -22.
Is that ok?
very good.
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very good.
Thanks! I think I can get the right bias now :)
But, regarding my hum problem :
Amp is not humming when only the 6V6s are plugged.
PI plugged, a bit of hum, not much but still noticeable.
V1 plugged, the same.
The hum is not increasing with the volume.
V4 plugged. More hum.
V2 plugged, the same.
Hum still not increasing with the volume.
I suspect something is picked around V4 and V6.
I have a suspect. As I converted this DR from a DRRI, the access from the doghouse to the chassis is a hole located near the tube connexions. In the original and the 64 reissue, it looks like the A/B/C wires are coming from the other side of the circuit, with a more direct access to their eyelet connexions.
In my circuit, now those wire are running across the board as shown in the picture attached. At first they were really close to the tube wires and moving those cables with a stick was changing the noise quite a bit.
Do you think I should drill a hole to route the ABC wires the same way as the original, thus keeping those wire as far from the board as possible? It's a bit of work, but I will do it if you think it might improve this hum.
Thanks a lot for your help!
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I did drill the chassis and re route those wires like they do in the 64 DRRI.
It didn't help with the hum....
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Could this hum be related to a bad Heater wiring?
The heater wires are close to the other wires, like in the picture.
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very good.
As for the bias, I'm scratching my head.
With this bias probe, I'm getting minimum 29mA, which is too much...
I will try to install those 1 ohm resistors.
It's between cathode and power ground, right?
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As for the bias, I'm scratching my head.
With this bias probe, I'm getting minimum 29mA, which is too much...
I will try to install those 1 ohm resistors.
It's between cathode and power ground, right?
If you are getting 29mA when you have -45VDC on pin 5 I would suspect your bias probe is lying to you. Install the 1Ω resistors between pin 8 and ground.
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If you are getting 29mA when you have -45VDC on pin 5 I would suspect your bias probe is lying to you. Install the 1Ω resistors between pin 8 and ground.
I just installed the 2x 1ohm 5w resistors and the reading is : 29mV across the first, and 27.5mV across the second. This is the minimum I can get.
Not good, right?
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Regarding the Hum, it is humming with the PI and 6V6 installed.
It is low 50hz hum, not changing with the volume (when the other tubes are installed).
I have several grounds:
AC outlet ground, alone, bolted to chassis.
Output jacks ground, bolted to chassis
Pedal trs jack ground, bolted to chassis
RCA send/return reverb ground, bolted to chassis.
Power ground, with HTCT (red/yellow, bias ground, 6V6 cathodes ground, first filtering stage ground). Bolted near PT.
Preamp ground, on a bus wire running from PI ground (at 47 ohm) to jack, getting all the other small signal grounds, bolted near the jack input. All 4 jack inputs are isolated.
Do you think it could be a ground loop? I though I respected the layout, but I could have made a mistake...
What do you think?
Thanks a lot,
:worthy1:
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By the way I had a look at the DRRI bias schematic. My power transformer comes from it, so maybe it doesn't deliver the right voltage to the bias circuit.
In this bias circuit, the 470Ohm is a 22Ohm, and 100uf replace the 25uf I'm using.
A dedicated bias tap should be in the 50-60V range. Why don't you measure the (AC) voltage coming into your bias dropping circuit? What is that voltage? Shouldn't be too hard to measure if you have clip probes. If it's much higher maybe that's one problem.
Also, that 220 ohm resistor looks low to me but I defer to sluckey and pdf64.
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A dedicated bias tap should be in the 50-60V range. Why don't you measure the (AC) voltage coming into your bias dropping circuit? What is that voltage? Shouldn't be too hard to measure if you have clip probes. If it's much higher maybe that's one problem.
Also, that 220 ohm resistor looks low to me but I defer to sluckey and pdf64.
At the PT tap red/blue and 470 Ohm resistor jonction I read 37 VAC to Power Ground.
This could be the problem?
Thanks a lot
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I read on the normal Deluxe Reverb PT specs that this is supposed to be 50VAC.
So either mine has a problem or the DRRI has a different PT, which would kinda explain the different bias circuit?
Do anyone has ever encountered this issue?
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The DRRI uses a different PT than used by an original AB763 DR. So, replace your 470Ω with a 220Ω like the DRRI uses. You'll get a little more -VDC.
But, you already said, "So now at the grids I can read from -45V to -22". That's a perfectly good bias voltage range for 6V6s that will likely be set to about -36V when biased properly. There is no issue here.
BTW, my DRRI PT puts out 38VAC on the bias tap and I used a 470Ω with no issues. And my bias pot is set for -36V on the grids and that gives 24mV on the cathode 1Ω resistor.
You need 1Ω 1% 1W cathode resistors for accurate mV readings on the cathode. Your 1Ω 5W resistors are probably only 5% tolerance. Which meter are you using now? Maybe it doesn't read dc mV accurately.
Try this... adjust the bias pot to provide -37V at the junction of the 220K grid resistors. This is according to the DRRI schematic. Do the 6V6s redplate? Ignoring the hum, how does the amp sound?
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Try this... adjust the bias pot to provide -37V at the junction of the 220K grid resistors. This is according to the DRRI schematic. Do the 6V6s redplate? Ignoring the hum, how does the amp sound?
Hello,
Sorry about the delayed reply, I had to manage health issues and just got home now.
So, I progressively provided -37V at the junction of the 220k. The 6V6 both redplated, so I switched the standby off and reverted to -45V.
This is not normal, right?
Thanks a lot.
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Could it be as simple as bad tubes? I don't have 6V6 spares yet to try it...
What do you think?
Thanks
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Yes. Tubes always need to be eliminated as suspects. Try a new set of 6V6s.
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Yes. Tubes always need to be eliminated as suspects. Try a new set of 6V6s.
Thank you,
I have ordered a new pair of 6V6gt TAD.
I will be able to try them at the end of the week I guess.
In the meantime, in your opinion is there any test I can conduct?
Thanks!
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Ear test.
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Ear test.
Just so I don't do any harmful mistake :
You mean, playing and hearing with the tubes set at a bias corresponding to -37V at the 220k junction, even if they are redplating?
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No!
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No!
I didn't.
I Just didn't understand what you wanted me to test by hearing.
Sorry if I don't get everything, please forgive me, English is not my mother language.
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The DRRI uses a different PT than used by an original AB763 DR. So, replace your 470Ω with a 220Ω like the DRRI uses. You'll get a little more -VDC.
...
Just to note that R69 is 22Ω, rather than 220Ω.
https://el34world.com/charts/Schematics/files/Fender/Fender_65_deluxe_reverb_manual.pdf
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The DRRI uses a different PT than used by an original AB763 DR. So, replace your 470Ω with a 220Ω like the DRRI uses. You'll get a little more -VDC.
...
Just to note that R69 is 22Ω, rather than 220Ω.
https://el34world.com/charts/Schematics/files/Fender/Fender_65_deluxe_reverb_manual.pdf
Thanks!
Yes I will change for a 22Ohms.
I notice 2 other changes in this circuit:
A 100uf100V instead of the 25uf50V
A 22uf63V is added at 220K Junction.
Wouldn't it benefit the bias circuit if I change them as well?
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But, you already said, "So now at the grids I can read from -45V to -22". That's a perfectly good bias voltage range for 6V6s that will likely be set to about -36V when biased properly. There is no issue here.
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But, you already said, "So now at the grids I can read from -45V to -22". That's a perfectly good bias voltage range for 6V6s that will likely be set to about -36V when biased properly. There is no issue here.
Ok I understand, there's no issue here. So the issue should be either the tubes (I shall have a new pair at the end of the week) or somewhere around theses tubes. I will double check the sockets, resistors and connections.
Thanks
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When the bias is set for -45v on pins 5 are the tubes redplating? How does the amp sound?
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When the bias is set for -45v on pins 5 are the tubes redplating? How does the amp sound?
When it is set at -45v, it is not redplating.
The amplifier sounds about ok, nothing obvious but the hum I described previously.
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Yes. Tubes always need to be eliminated as suspects. Try a new set of 6V6s.
So I finally received a new pair of 6V6 and part of the problem is solved:
With bias to the minimum I have 16,9mV across the 1 Ohm resistors. I can set the bias now around 23/24 mA for both tubes, safely and without redplating.
Thanks a lot for helping me finding this, I learned a lot!
Now I still have the hum.
I can hear it with V1, V6 and power tubes installed. it is not related with volume. It is low hum, not a lot of harmonics, so I think it is more 50hz than 100hz.
Is there any test I could do to troubleshot it?
Thanks a lot for your advices :worthy1:
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I suspect a grounding issue. Most likely a ground loop or possibly a shunt/breaker tip on the input jacks not making proper contact when unused.
/Max
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I suspect a grounding issue. Most likely a ground loop or possibly a shunt/breaker tip on the input jacks not making proper contact when unused.
/Max
Thank you.
I still have the hum with V1 pulled out. Only pi, 6v6 and rectifier installed.
Does that rule out a failed input jack tip grounding?
Is there a good method to test ground loops?
Thanks a lot
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Today I moved the center tap for the 100ohm heater resistors from bias ground to 6v6 cathode.
I felt it helped a bit with the hum, but it might be just an impression, as there is still to much 50hz.
Would an humdinger be of any help here in your opinion?
One thing I never noticed before is, with nothing plugged, and input gain to maximum I can hear a slight buzz added. It is higher pitch than the other hum and much much lower in level. This buzz disappear whenever I touch the chassis or jack inputs. I must say it is almost not noticeable, and I could definitely live with it, but I mention it because it could maybe be a symptom of something wrong.
Would it imply bad grounding? The amp is plugged in another outlet in the basement, earth may not be perfect...
Thanks for your help ☺️
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Well, I'm making progress here !
I changed the bias capacitor, 25uf /50v Atom for another Vishay 100uf/100V.
No more hum !!!
I guess somehow either the original capacitor was shot, or it was not happy with the DRRI transformer, or both. But now this annoying hum is totally gone. Very happy !
Ok it is not the end of the story, now I've got to figure something else:
With the amp open, and only V1, V6 preamp tubes I get a bit of buzz (100hz and harmonics), that goes away when I touch the chassis and put my hand 10cm above the circuit (not touching it). So I guess it's picking up RFI.
With all tubes inside it is way more, but same behaviour. With me touching the chassis and my hand above the circuit, there is no more hum.
It is the most with reverb, as could be expected.
As I said previously, earth in this basement is suspect. I will try it tomorrow in a good known electrical environment.
Any idea if this could indicate a ground fault? Or is it just RFI because the chassis is still open and not properly shielded / earth grounded?
Thank you, I'm so happy I solved this annoying hum!!! :worthy1:
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Ok so, now the amp is back in its casing, no more EMI affected, that's a good thing.
But everything is not solved.
Normal channel is good :
With V1, V6 and power tubes, no problem, just a tiny bit of 50hz hum, really low, that I probably can tame with a humdinger, I will try it soon.
Vibrato channel is more problematic:
With all the tubes installed, I get buzz even with everything at zero volume.
I get another buzz that is superposing to the first one with the reverb pot:
Reverb pot at 0, buzz added, at 1 or 2 less buzz, and it is coming back after 3. This is when reverb is out from the footswitch. When it is on, same behaviour, with a bit more buzz.
Looks like a faulty ground around here, or maybe a ground loop? should i try to isolate the plugs? not really easy...
Of course when I remove V4, everything is back to Normal channel only, with no noise.
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Reverb pot at 0, buzz added,
This is what's troubles me the most
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Looks like I was a little bit overenthusiastic yesterday... :BangHead:
Back with only V1, V6 and power tubes and ... hum . I think less than before but still problematic.
Installed a humdinger. Could reduce the hum a tiny bit, but nothing signifiant.
I went back to all the grounds, nothing apparently wrong there.
I isolated the footswitch pedal and connected its ground to V4cathode ground on the preamp ground bus. No change. Next step is to isolate rca grounds and connect them to the same spot.
I'm starting to loose hope to save this amp... :sad2:
Any idea about how tracking this hum?
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I isolated RCA plugs and footswitch pedal. Connecting their ground to V4 cathode ground (on the buss bar). No change.
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Soundclip of the buzz?
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Soundclip of the buzz?
Here it is, with all the tubes installed in. I switch on and of the reverb that has a lot of noise in this position. But there is idle buzz anyway.
Hope the level is high enough.
With only PI and Power tubes there is the same buzz, but less pronounced. I can also make a video of that if you want.
Thanks a lot for your advices
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Soundclip of the buzz?
Here it is, with all the tubes installed in. I switch on and of the reverb that has a lot of noise in this position. But there is idle buzz anyway.
Hope the level is high enough.
So the reverb buzz is much lounder than anything else?
How about if you unplug the footswitch?
If that doesn't help, check that the reverb tank input end isn't closer to the PT than the output end.
I'm not sure that the layout noted in the OP is ideal, particularly that the negative terminal of the HT C node cap seems to be directly connected to the A node negative.
Martin Manning has done a nice redesign of the 0V common layout of black panel Fenders, which might be of interest https://ampgarage.com/forum/viewtopic.php?p=428877#p428877
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If that doesn't help, check that the reverb tank input end isn't closer to the PT than the output end.
I'm not sure that the layout noted in the OP is ideal, particularly that the negative terminal of the HT C node cap seems to be directly connected to the A node negative.
Yes on both, check how you have the verb tank oriented and try moving the C node ground over to the D node ground. Won't hurt anything to try moving that C node ground, you can always put it back where it was if it doesn't help. It's only 1 wire.
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So the reverb buzz is much lounder than anything else?
How about if you unplug the footswitch?
With the footswitch unplugged it is the same :
Please note that I push the reverb to 10, which is a lot.
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If that doesn't help, check that the reverb tank input end isn't closer to the PT than the output end.
The reverb tank is not in its right position, it is now outside of the cab, as you can see on the right of the video.
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Martin Manning has done a nice redesign of the 0V common layout of black panel Fenders, which might be of interest https://ampgarage.com/forum/viewtopic.php?p=428877#p428877
I just registered to ampgarage to be able to see this document, but my account needs to be reviewed by the administrators, so I cannot see it at the moment.
For now, the reverb recovery noise is high, but the idle noise is also not quiet, that what is bothering me the much.
With only PI and Power tubes I have low 50hz hum, not that buzz, but still, it should be quiet:
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try moving the C node ground over to the D node ground. Won't hurt anything to try moving that C node ground, you can always put it back where it was if it doesn't help. It's only 1 wire.
You mean the preamp ground? (At the left of the picture)
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I'm hearing also a little bit of oscillation from the vibrato (not active at the footswitch) since I isolated the footswitch jack (I think, didnt hear it before).
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try moving the C node ground over to the D node ground. Won't hurt anything to try moving that C node ground, you can always put it back where it was if it doesn't help. It's only 1 wire.
You mean the preamp ground? (At the left of the picture)
Now that I see the dog house wiring, just disconnect the B node (B+2) screens cap ground for the C node (B+3) cap ground and now move that B node (B+2) screens cap ground to the A node OT CT (B+1) cap ground.
See attached pic below.
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Now that I see the dog house wiring, just disconnect the B node (B+2) screens cap ground for the C node (B+3) cap ground and now move that B node (B+2) screens cap ground to the A node OT CT (B+1) cap ground.
See attached pic below.
Thanks a lot,
will do in a minute and report.
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If that doesn't help try this;
The K R and bypass cap on the reverb recovery tubes pin 3 and 8, move those 2 parts off the board and onto pin 3 off the verb recovery tube socket and ground them with the recovery tube 220K grid leak R.
Parts shown and where to ground them in pic below. But I didn't show them moved. But you know where to move them.
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Sorry, I have to go do some things. Be back later. :icon_biggrin:
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Now that I see the dog house wiring, just disconnect the B node (B+2) screens cap ground for the C node (B+3) cap ground and now move that B node (B+2) screens cap ground to the A node OT CT (B+1) cap ground.
I just did that. (see pictures)
It doesn't seem to change much, still buzzing idle noise, and lots of noise in the reverb when pushed (but this could be the tank position).
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I looked at how it was on the DRRI PCB.
It seems that they separated D, and kept A, B and C negative nodes together.
Should I try this?
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If that doesn't help try this;
The K R and bypass cap on the reverb recovery tubes pin 3 and 8, move those 2 parts off the board and onto pin 3 off the verb recovery tube socket and ground them with the recovery tube 220K grid leak R.
Parts shown and where to ground them in pic below. But I didn't show them moved. But you know where to move them.
As for now, it is like this schematic (attached), almost because footswitch is a TRS jack.
No ground around the connectors, I have isolated them. Ground connection is at cathode ground.
When I move the capacitor and the resistor, should I restore the ground of the RCA and Footswitch jack, or could I just restore the ground of footswitch jack with still a ground wire to the isolated RCAs?
Thanks a lot for your help :)
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I'm not sure that the layout noted in the OP is ideal, particularly that the negative terminal of the HT C node cap seems to be directly connected to the A node negative.
A negative is connected to the power ground, near PT, along with HT centertap, Biasground, 6V6s cathodes 1Ohm resistors and 10k from Bias pot.
Now B negative is also connected to this point.
C node cap is connected, with D node cap at preamp ground buss, between Mid resistor and V2 V3 cathode.
As the jacks are isolated, the buss bar is passing by them to ground them. So this connection of C and D node is between the vibrato jacks ground and the normal jacks ground. Is it a bad thing?
Thanks !
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I'd re-do that filter cap board just to rule out soldering related issues. The thick jumper wires between the caps grounds are overkill and hard to work with. Google pics on how the original looked or how other builders have done it. The boards have eyelets for a reason.
/Max
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I'd re-do that filter cap board just to rule out soldering related issues. The thick jumper wires between the caps grounds are overkill and hard to work with. Google pics on how the original looked or how other builders have done it. The boards have eyelets for a reason.
/Max
Thanks,
They show good continuity, but maybe there is more to it?
I will redo it as well. Do you think it could also be a bad filter cap? They are new though...
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I suggest to wait and see Martin Manning’s design.
Due to the NFB, reverb driver and trem oscillator, the 0V common return paths in these amps are something of a hodge podge.
Lumping all the 0V commons of the board to a bus, as per the OP layout, may not result in a reasonable hum level.
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I suggest to wait and see Martin Manning’s design.
Due to the NFB, reverb driver and trem oscillator, the 0V common return paths in these amps are something of a hodge podge.
Lumping all the 0V commons of the board to a bus, as per the OP layout, may not result in a reasonable hum level.
Still now allowed to see Martin's design on ampgarage, but it looks like it's been reposted on Groupdiy.
Hope it's ok I repost it here, If not I'll remove it.
Is it the grounding layout you were referring to?
Thanks a lot.
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Yes.
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You should read this on grounding if you haven't read it yet. It will help you understand Martin's grounding. Read it and re-read it until you understand. I read it several times before it sunk in and so have many here.
http://www.valvewizard.co.uk/Grounding.html (http://www.valvewizard.co.uk/Grounding.html)
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As for now, it is like this schematic (attached), almost because footswitch is a TRS jack.
No ground around the connectors, I have isolated them. Ground connection is at cathode ground.
When I move the capacitor and the resistor, should I restore the ground of the RCA and Footswitch jack, or could I just restore the ground of footswitch jack with still a ground wire to the isolated RCAs?
No, leave it as you have it now.
I didn't know you added isolated RCA jacks and ran a ground wire back to the K R and bypass cap ground.
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Thanks a lot !
I will study both and probably come back with a few questions :laugh:
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Yes.
Thanks !
I have a few questions regarding this layout:
BC- ground is going to the right corner. Is it connected to chassis near power tubes sockets via a screw? It passes over Speaker tip but I assume it doesn't connect there. Is it the same point as power tube cathodes?
My input jacks are isolated. Is that a problem if they are grounded at preamp ground with the violet wires? or Should I revert to non isolated jacks?
Is the new tremolo resistor 180K? Were does the red wire connect? Is it ok if I leave this part aside for the moment?
Thanks a lot :worthy1:
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Any grounding scheme can be done with those RCA jacks isolated. Leave them and just run a ground wire over to where ever it needs to go.
The less chassis grounds connections the better. They sometimes can be a problem.
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BC- ground is going to the right corner. Is it connected to chassis near power tubes sockets via a screw? … Is it the same point as power tube cathodes?
See Martin’s layout (https://i.ibb.co/3RPRCyS/6-CBDF12-A-671-F-4-BA9-A868-845-B027-EFA12.png)
The BC- chassis connection point looks to be near V7.
The output valve cathodes look to have their chassis common connection by ghe rectifier socket.
Dunno what the rationale for not all using the same star point is :dontknow:
It passes over Speaker tip but I assume it doesn't connect there.
Definitely not :icon_biggrin:
My input jacks are isolated. Is that a problem if they are grounded at preamp ground with the violet wires? or Should I revert to non isolated jacks?
That should be fine. Maybe use the isolated socketed for the speaker, if they’re not already isolated.
Is the new tremolo resistor 180K? Were does the red wire connect? Is it ok if I leave this part aside for the moment?
The new trem resistor on the board has just been relocated from the speed pot, it’s still 100k.
There’s an additional 180k in the footswitch, across the trem switch, to help reduce any pop.
If you’re not running the trem just yet, then no need to bother with them.