Advice needed...

Forum to discuss amp circuits besides the 18 Watters and their ilk.

Moderators: JMPGuitars, CurtissRobin, colossal, zaphod_phil, Daviedawg, Graydon

User avatar
TriodeLuvr
Frequent poster
Frequent poster
Posts: 545
Joined: Tue 10/16/18 11:19 am

Re: Advice needed...

Post by TriodeLuvr »

1mS/Div is too fast to see 60 Hz energy. At 1mS/Div, your scope can only display 10 mS of time across the screen (10 divisions). A 60 Hz wave has a period of 16.7mS (1 ÷ 60). 10mS or 5mS is needed to display hum.

That aside, what is the signal on the left side of the display in the first photo? It appears to be something in the neighborhood of 7 kHz.

Jack
1 x

The4thWatcher13
Occasional poster
Occasional poster
Posts: 142
Joined: Sun 03/14/21 9:57 pm

Re: Advice needed...

Post by The4thWatcher13 »

TriodeLuvr wrote:
Mon 05/31/21 6:23 pm
1mS/Div is too fast to see 60 Hz energy. At 1mS/Div, your scope can only display 10 mS of time across the screen (10 divisions). A 60 Hz wave has a period of 16.7mS (1 ÷ 60). 10mS or 5mS is needed to display hum. That aside, what is the signal on the left side of the display in the first photo? It appears to be something in the neighborhood of 7 kHz. Jack
Still images were pretty hard to capture at .5mS/Div. But here is a pic of the repeating waveform. Last night I also moved the heater wiring from underneath the V1 grid. The buzz is still there and doesn't seem to have changed.
The left most aspect of the buzz in the image you mentioned is gone now. I've yet to try to power the V1 heater with DC but maybe I'll try that later today. I definitely have issues with AC here in the garage and I've tried a number of different fixes. I've got several AC filters like B&K TR110 Direct/Iso-pack and OneAC CL1101.5 power conditioner. None affect the current issue as far as I can tell. Setting up AC supply in a shop would be a good instructional for those needing it. It may exist somewhere so I'll look. I'm in a garage with one 15A service to the house. Not optimal I guess. Also new 1M Vol and 2M tone pots, Cliff jacks etc arrived today.
You do not have the required permissions to view the files attached to this post.
1 x
"Things are only impossible until they are not!" - Jean-Luc Picard.

The4thWatcher13
Occasional poster
Occasional poster
Posts: 142
Joined: Sun 03/14/21 9:57 pm

Re: Advice needed...

Post by The4thWatcher13 »

JMPGuitars wrote:
Mon 05/31/21 7:41 am
That buzz is not fantastically significant. It couuuuuuuuuld be the things we're talking about, or it could simply be a need for some chopsticking. Or made worse by the guitar pickups.
I've heard all kinds of amps over the years but only own one other tube amp, '67 Sunn Solarus combo, and it doesn't have anything like this kind of buzz. I think anyone would consider it significant given anything past 50% on the guitar or amp produces a great deal of buzz.
JMPGuitars wrote:
Mon 05/31/21 7:41 am
Regarding the oscillation, you need to determine if it's being caused by the amp, or being picked up by the amp. I spent a year chasing an oscillation once that was caused by a power supply in another room. Chopsticking might help determine both, but if you have anything new plugged in, or power supplies (wall warts, laptop style bricks etc...) that you can unplug and see if the oscillation goes away, that may help determine the source. I use filtered inlets nowadays to reduce this issue. Thanks, Josh
I did Chop-stick the tubes, amp etc. but to no affect except the expected amplification of the tap on the preamp tube. In regard to the oscillation; sometimes I'll hear a little tick, tick...oscillation, then it's gone.
I just started for the first time several days ago as well.
The AC situation here is certainly not optimal. I've turned everything off except the amp and the result is pretty much the same though I can tell that certain things like my laptop do have an a minor effect on the amp. These things just don't affect other audio equipment to the point where I've had to address it to any degree. I went further into my AC situation in my reply to Jack above. Thanks Josh!
0 x
"Things are only impossible until they are not!" - Jean-Luc Picard.

User avatar
TriodeLuvr
Frequent poster
Frequent poster
Posts: 545
Joined: Tue 10/16/18 11:19 am

Re: Advice needed...

Post by TriodeLuvr »

Well, the sweep has gone from being too slow to too fast. :) It should be set to 5 or 10mS/Div, not 0.5mS. The two scope divisions in the last photo aren't displaying 60 Hz. That's the same high frequency signal (6 kHz or 7 kHz) that was evident in the first photo. I don't know if this is being generated by the amp. There are quite a few devices in homes (and garages) now that emit large EM fields. Televisions, computers, sprinkler controllers, even wall-mounted light dimmers. It's possible you're picking that up somehow.

Ignoring the HF stuff for the moment, I think you'll need to gut the filament wiring and redo it from scratch. It's simply not located where it needs to be, and I think running it across the front panel under the pots is really a problem. If it was me, I would start at the transformer wires and go to the output tubes. Then wire from one of the output tubes to the 12AX7, then to the 6AV6. Also, don't push it down onto the chassis. That's where the all the signal circuitry is. Position all the runs one or two inches above the amplifier circuitry and sockets, and drop it down onto the tube pins in each location. It's a misconception that running twisted pair along a metal chassis helps shield it. Not sure what kind of wire you're using; this is obviously easier with solid wire because it holds its shape better.

Jack
1 x

The4thWatcher13
Occasional poster
Occasional poster
Posts: 142
Joined: Sun 03/14/21 9:57 pm

Re: Advice needed...

Post by The4thWatcher13 »

TriodeLuvr wrote:
Tue 06/01/21 5:55 pm
Well, the sweep has gone from being too slow to too fast. :) It should be set to 5 or 10mS/Div, not 0.5mS.
The scope is actually set on 5mS/Div, though it's not shown in the last photo. I think what you're seeing is the styling on the knob. The marker is a raised line pointing to 5mS/Div. You are correct that in the first two photos the scope was set at 1mS/Div tho.
TriodeLuvr wrote:
Tue 06/01/21 5:55 pm
The two scope divisions in the last photo aren't displaying 60 Hz. That's the same high frequency signal (6 kHz or 7 kHz) that was evident in the first photo. I don't know if this is being generated by the amp. There are quite a few devices in homes (and garages) now that emit large EM fields. Televisions, computers, sprinkler controllers, even wall-mounted light dimmers. It's possible you're picking that up somehow.

Ignoring the HF stuff for the moment, I think you'll need to gut the filament wiring and redo it from scratch. It's simply not located where it needs to be, and I think running it across the front panel under the pots is really a problem. If it was me, I would start at the transformer wires and go to the output tubes. Then wire from one of the output tubes to the 12AX7, then to the 6AV6. Also, don't push it down onto the chassis. That's where the all the signal circuitry is. Position all the runs one or two inches above the amplifier circuitry and sockets, and drop it down onto the tube pins in each location. It's a misconception that running twisted pair along a metal chassis helps shield it. Not sure what kind of wire you're using; this is obviously easier with solid wire because it holds its shape better. Jack
You know, I wondered about that earlier when you were talking about "elevating" the heater trans CT ground wiring. At first I thought you actually were talking about physically elevating the heater runs because I wasn't familiar with the function. But when we then started talking about "elevating" the heater trans CT with the two resistors I just assumed that that was the entirety of what you were conveying. I better start asking more questions when in go to wonderin'! Never the less I'll get right on it. Good time, I guess, to install the new jacks and pots as well. Thanks Jack!
0 x
"Things are only impossible until they are not!" - Jean-Luc Picard.

User avatar
TriodeLuvr
Frequent poster
Frequent poster
Posts: 545
Joined: Tue 10/16/18 11:19 am

Re: Advice needed...

Post by TriodeLuvr »

The4thWatcher13 wrote:
Tue 06/01/21 6:22 pm
The scope is actually set on 5mS/Div, though it's not shown in the last photo. I think what you're seeing is the styling on the knob. The marker is a raised line pointing to 5mS/Div.
When my cursor hovers over the image, a flag pops up that says, ".5mS-Div 100mV-Div.jpg." That's what my comment was based on. In any event, those waveforms are much higher than 60 Hz, even if the sweep is 5mS. At that setting, a single 60 Hz sine wave would occupy more than three divisions.
You know, I wondered about that earlier when you were talking about "elevating" the heater trans CT ground wiring. At first I thought you actually were talking about physically elevating the heater runs because I wasn't familiar with the function. But when we then started talking about "elevating" the heater trans CT with the two resistors I just assumed that that was the entirety of what you were conveying.
When I originally said to elevate the wires, I was referring to the application of a positive DC offset. Now I'm saying to also physically distance them from the signal path by locating them up above the other circuitry. This isn't the first time I've suggested this, but I guess it got lost in the mix.

Jack
0 x

The4thWatcher13
Occasional poster
Occasional poster
Posts: 142
Joined: Sun 03/14/21 9:57 pm

Re: Advice needed...

Post by The4thWatcher13 »

TriodeLuvr wrote:
Tue 06/01/21 7:55 pm
When my cursor hovers over the image, a flag pops up that says, ".5mS-Div 100mV-Div.jpg." That's what my comment was based on. In any event, those waveforms are much higher than 60 Hz, even if the sweep is 5mS. At that setting, a single 60 Hz sine wave would occupy more than three divisions.
Yup, that was my mistype of the file name. Guilty as charged...and an extra month for confusing the Judge! :)
TriodeLuvr wrote:
Tue 06/01/21 7:55 pm
When I originally said to elevate the wires, I was referring to the application of a positive DC offset. Now I'm saying to also physically distance them from the signal path by locating them up above the other circuitry. This isn't the first time I've suggested this, but I guess it got lost in the mix. Jack
Indeed. It all makes sense now. Never the less I'm all over it. I've already identified some challenges but nothing too steep as of yet. Back soon..
0 x
"Things are only impossible until they are not!" - Jean-Luc Picard.

The4thWatcher13
Occasional poster
Occasional poster
Posts: 142
Joined: Sun 03/14/21 9:57 pm

Re: Advice needed...

Post by The4thWatcher13 »

Alrighty then, first the changes, then the results:
As can be seen in the pics the heater wiring was completely redone and rerouted per instruction from the power tubes to the preamp tube. The wiring has also been physically elevated to a point as high as possible but where it won't touch the bottom plate when installed. I'm hoping I didn't over do it. The electrically elevated CT lead of the filament transformer was also physically elevated away from the 3 B+ lines. The Vol & Tone pots were replaced with Alps 1M & 2M pots, respectively. The V1 socket was rotated 180 deg so the grid isn't as near the heater runs, which are also no longer runing under the two pots. The preamp supply cap ground was also rerouted around the preamp to the preamp ground instead of keeping the previously elevated run. The input jack was replaced with a Cliff jack and the grounding scheme was done exactly as Josh recommended, as a duplication of the 18 watt lite 2b layout. The Result: Excellent! In regard to the "buzz" it's all but gone. There remains pretty much what I'd expect from any tube amp. No sign of oscillation at all but the "proximity effect", that is, getting the guitar near the amp (1.5' ft.) results in an increase in noise. More a hum than a buzz I'd say. I can't help thinking that it might be the un-shielded transformers. I'll try tube shields on the pre and PI tomorrow. That's it for now Gents, more to follow tomorrow, pics below. I'm a happy camper again. Thanks much!
You do not have the required permissions to view the files attached to this post.
2 x
"Things are only impossible until they are not!" - Jean-Luc Picard.

User avatar
TriodeLuvr
Frequent poster
Frequent poster
Posts: 545
Joined: Tue 10/16/18 11:19 am

Re: Advice needed...

Post by TriodeLuvr »

Yay! Looks like you're almost done. Personally, I wouldn't care much what happens when the guitar is less than two or three feet away. However, if it's too bothersome, 3M makes a shielding material (sheet) called Ultraperm 80. I've used it to help quiet phono stages where the voltage gain was 1,000 or more. It could be wrapped around one or both transformers to reduce the field. A simple copper strip an inch or two wide could also be wrapped around and soldered closed. Most power transformers intended for audio use have a copper flux band built in. Isolation transformers seldom have it, so that could be where the guitar is picking up the noise.

Jack
2 x

User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 3107
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

Re: Advice needed...

Post by JMPGuitars »

Looks great! I'm looking forward to hearing your demo.

I'm assuming this is going into a cabinet of some form? If it's going into a head shell, you can shield the inside of the shell if it's really a problem. I'm guessing it's a guitar with single coils?

Thanks,
Josh
1 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright

Modern Ground Schemes
Soldering Technique
B+ Voltage Reduction


Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale

The4thWatcher13
Occasional poster
Occasional poster
Posts: 142
Joined: Sun 03/14/21 9:57 pm

Re: Advice needed...

Post by The4thWatcher13 »

JMPGuitars wrote:
Sat 06/05/21 7:38 am
Looks great! I'm looking forward to hearing your demo.
I'm assuming this is going into a cabinet of some form? If it's going into a head shell, you can shield the inside of the shell if it's really a problem. I'm guessing it's a guitar with single coils? Thanks, Josh
Thank you, Josh. At first I figured it'd go into a cab but when I switched to a different chassis I thought it'd end up kind of as it is. I am working on a cage of some sort tho. Yes, all I have for electrics are single coils. Putting this into a small head type enclosure is still possible I think. My response to Jack, below, covers current shielding efforts.
Thanks again, Josh.
1 x
"Things are only impossible until they are not!" - Jean-Luc Picard.

The4thWatcher13
Occasional poster
Occasional poster
Posts: 142
Joined: Sun 03/14/21 9:57 pm

Re: Advice needed...

Post by The4thWatcher13 »

TriodeLuvr wrote:
Sat 06/05/21 12:00 am
Yay! Looks like you're almost done. Personally, I wouldn't care much what happens when the guitar is less than two or three feet away. However, if it's too bothersome, 3M makes a shielding material (sheet) called Ultraperm 80. I've used it to help quiet phono stages where the voltage gain was 1,000 or more. It could be wrapped around one or both transformers to reduce the field. A simple copper strip an inch or two wide could also be wrapped around and soldered closed. Most power transformers intended for audio use have a copper flux band built in. Isolation transformers seldom have it, so that could be where the guitar is picking up the noise. Jack
I do have some 1"in. copper tape used to shield guitars. And after one "belly band" around both AC trans the result is better I believe.I'm wondering if more or differently oriented wraps are OK. I keep flashing back to what you said about the grid picking up EI.."I'm thinking the grid circuit is picking it up, and it's riding into the guitar - and the wiper of the guitar's volume control - through the input jack." I've got two types of shielded wire so I may try a shielded lead from the jack to the grid of V1. I may be able to pull off example "B" in the Valve Wizard image below.
Well, here's where I am now.
The tone is pretty nice, gain is respectable, but I've been looking into cathode bypass caps and their effect on amp voicing, and using some of the online BPC calculators. I kinda feel like there may be just a little too much low Hz with the 10uF/25V BPC on V1. Just a thought.
You do not have the required permissions to view the files attached to this post.
0 x
"Things are only impossible until they are not!" - Jean-Luc Picard.

User avatar
TriodeLuvr
Frequent poster
Frequent poster
Posts: 545
Joined: Tue 10/16/18 11:19 am

Re: Advice needed...

Post by TriodeLuvr »

Those diagrams all show the input jack with its sleeve grounded to the chassis or earth. That's OK for a schematic, but these are clearly intended to be pictorial representations of the actual mechanical wiring. The proper way to do this is to connect the coax shield at the grid end of the cable to the same ground point being used for the grid resistor. The other end of the coax should connect to a floating input jack. The ground (sleeve) tab of the jack should not be connected to anything except the coax shield. This creates a single, continuous ground connection all the way from the grid resistor to the guitar. Alternately, if the connection between the jack and the grid is so short that coaxial cable isn't warranted, connect the ground tab of the jack with a wire to the ground at the grid resistor. Spiral it loosely around the "hot" wire if you're concerned about shielding. And I'll say again - none of this should be grounded to the chassis. Sometimes you can get away with that, but oftentimes not, and that makes it bad practice.

Hopefully, the copper tape will be enough. I can remember seeing old vacuum tube tape decks and small amps where the power transformer was enclosed by an external circular band of metal that was glued to the transformer or bolted to the chassis. Looking back, I suspect it was an alloy similar to mu-metal. Whatever it was, there's no question that - in addition to a flux band - an iron-based shield the same height as the transformer can be beneficial. I think you have everything reasonably well managed at this point though, and nothing of that sort is likely to be necessary.

About the frequency response, reducing the size of V1's cathode bypass can significantly degrade stage gain even at midrange. Maybe it's worth a try, but I would recommend changing coupling caps C0, C6 and C7 first. And of course, before you do anything, be sure it's not a problem with the speaker. If you have an audio generator, it should b e possible to use the scope to create a table comparing the gain at, say, 50 Hz, 100 Hz, 1kHz, 5kHz and 10kHz.

Congrats! You have an amp!

Jack
1 x

User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 3107
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

Re: Advice needed...

Post by JMPGuitars »

The4thWatcher13 wrote:
Sat 06/05/21 8:14 pm
I do have some 1"in. copper tape used to shield guitars. And after one "belly band" around both AC trans the result is better I believe.I'm wondering if more or differently oriented wraps are OK. I keep flashing back to what you said about the grid picking up EI.."I'm thinking the grid circuit is picking it up, and it's riding into the guitar - and the wiper of the guitar's volume control - through the input jack." I've got two types of shielded wire so I may try a shielded lead from the jack to the grid of V1. I may be able to pull off example "B" in the Valve Wizard image below.
Well, here's where I am now.
The tone is pretty nice, gain is respectable, but I've been looking into cathode bypass caps and their effect on amp voicing, and using some of the online BPC calculators. I kinda feel like there may be just a little too much low Hz with the 10uF/25V BPC on V1. Just a thought.
Option A is bad. Option B isn't a shield anymore, it's a ground wire. Option C is grounded and shielded.

If you feel there's too much bottom end, then try a much lower value, 1µF or a little lower.

Thanks,
Josh
1 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright

Modern Ground Schemes
Soldering Technique
B+ Voltage Reduction


Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale

The4thWatcher13
Occasional poster
Occasional poster
Posts: 142
Joined: Sun 03/14/21 9:57 pm

Re: Advice needed...

Post by The4thWatcher13 »

OK, I hate to do this but I'm going to revisit the "buzz" issue for a second because of a somewhat obvious observation. Today I noticed something that I'm attributing to my experience working on mostly solid state equipment and "comfort" in my work space arrangement. Firstly my work space is severely limited. So the current tube amp project has been on the bench in front of me, belly high. The cabinet I use for testing guitar amps is behind me, a little to my left, belly high! Which means that while I've been testing this amp I've been standing 2'ft from the amp and 2'ft from the cab with guitar, amp and cab all at the same height. I've only been worried about the kind of feedback we know and love, of which there has been none. So I put the cab on the floor 180deg. behind me 5"ft. away and started moving around the room with the guitar. The buzz disappears completely when I'm standing at about a 45deg. angle to the cab regardless of distance from it or the amp. It's present at every other orientation. Near the amp I think the buzz is reinforcing itself through the guitar at most angles due to proximity alone. I think the only effect the shielding has had is changing the shape of the magnetic field around the amp. Maybe even amplifying it or causing more interaction with the chassis or circuitry. Apologies, I generally don't move around much when I'm testing stuff on the bench. (force of habit)
Question: Does it make sense to wrap the living crap out of both the iso trans and filament trans? I hate to waste shielding tape but at this point things are getting a bit weird. With the amp and the cab separated this thing is interacting with the guitar much more obviously. :cry: And the suggestion to put it in a shielded enclosure has not fallen upon deaf ears. I can make a cab if I can get to a wood shop. Apologies if this sounds goofy, late or both! But I'm determined to defeat the buzz, one way or another!
You do not have the required permissions to view the files attached to this post.
0 x
"Things are only impossible until they are not!" - Jean-Luc Picard.

Post Reply