Update:
- the buzz does occur in a different building.
- and it does away immediately when I turn off the power (had to listen carefully in a quiet room).
Tomorrow I will replace the last two (preamp) filter caps and dropping resistors.
I assume that my signal-grounding results (buzz goes away when I ground the input to the PI but not the output of the preceding CF) are important clues. Does it tell me that the buzz source is between those two stages?
amp buzz
Moderators: pompeiisneaks, Colossal
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Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: amp buzz
Do not replace your filters!
In looking close at your layout I really think your issue is simply that all of your grounds on the preamp board are bussed together pure and simple!
If you had a screwed up filter or two I think your amp would be more likely to oscillate or motor boat due to the stages not uncoupling and interacting with each other's signal level.
Each gain stage or CF after the PI section sould have all of its grounds for that one section bussed and then have a sperate ground wire run back to a grounding point near the PT ground.
Also just for kicks measure the ac 120 hz ripple voltage at each of your plate resistors and report back as to what that is, and be sure to do this before and after you mod the grounding schem if you chose to do that, which as I said you should as right now tha earlier preamp stages are being modulated by the Higher total level of current draw that takes place down stream!
In looking close at your layout I really think your issue is simply that all of your grounds on the preamp board are bussed together pure and simple!
If you had a screwed up filter or two I think your amp would be more likely to oscillate or motor boat due to the stages not uncoupling and interacting with each other's signal level.
Each gain stage or CF after the PI section sould have all of its grounds for that one section bussed and then have a sperate ground wire run back to a grounding point near the PT ground.
Also just for kicks measure the ac 120 hz ripple voltage at each of your plate resistors and report back as to what that is, and be sure to do this before and after you mod the grounding schem if you chose to do that, which as I said you should as right now tha earlier preamp stages are being modulated by the Higher total level of current draw that takes place down stream!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: amp buzz
I tried to implement the 'galactic ground' scheme in TUT3, which is very similar to the scheme outlined in Merlin's freely available grounding document (see Figs. 13.11 and 13.14; http://www.valvewizard.co.uk/Grounding.html). As everyone here probably knows, a bunch of local stars are daisy chained together in this scheme. I will do a couple minor tweaks before trying your suggested ground scheme.
Unfortunately I don't yet have an o-scope (getting my buddy's Hameg soon), but I will try to measure AC ripple with my meter.
Unfortunately I don't yet have an o-scope (getting my buddy's Hameg soon), but I will try to measure AC ripple with my meter.
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Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: amp buzz
If you lift the AC ground on the primary side of the PT and or lift that ground and reverse the ac input wires on the PT what takes place with that buzz?
Also if you hook up a voltmeter set for AC across the speaker with the amp at idle what MV voltage level do you read?
Also if you hook up a voltmeter set for AC across the speaker with the amp at idle what MV voltage level do you read?
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: amp buzz
Here are some power supply numbers I pulled off a fender vibrolux reverb I recaped last night.
I wil give you the B+ voltages and the 120 hz AC ripple voltage at each power supply node, and note that these ripple voltages where read on a voltmeter, not a scope !
The first filter node is two 20 uf caps in parallel for a total of 40 uf and this feeds the OT.
b+ = 423, ripple = 207 MV.
Screen node.
Another 20 uf filter,and this is after the amps choke.
b+= 421, ripple was 205mv.
Third power supply node is another 20 uf and fed from a 10K resistor.
B+=332, ripple was 165mv.
The fourth and last node is another 20uf, fed by another 10K resistor.
B+=276, ripple was 157mv.
Like most Fenders of this era this one node powers four gain stages, none the less the ripple at the tube side of the 100K plate load resistor at V1 is down to 87mv.
This amp is very noiseless , and with a all the controls turned down and the input jacks closed pumps out only .005 VAC of 120hz ripple at the speaker output at idle, and this is without the metal shielding that takes place once the amp is bolted into its cabinet!
To me any amp idling as I just posted about with .025 of idle ripple at the speaker is a what can be expected of a amp with a good layout and no problems!
I wil give you the B+ voltages and the 120 hz AC ripple voltage at each power supply node, and note that these ripple voltages where read on a voltmeter, not a scope !
The first filter node is two 20 uf caps in parallel for a total of 40 uf and this feeds the OT.
b+ = 423, ripple = 207 MV.
Screen node.
Another 20 uf filter,and this is after the amps choke.
b+= 421, ripple was 205mv.
Third power supply node is another 20 uf and fed from a 10K resistor.
B+=332, ripple was 165mv.
The fourth and last node is another 20uf, fed by another 10K resistor.
B+=276, ripple was 157mv.
Like most Fenders of this era this one node powers four gain stages, none the less the ripple at the tube side of the 100K plate load resistor at V1 is down to 87mv.
This amp is very noiseless , and with a all the controls turned down and the input jacks closed pumps out only .005 VAC of 120hz ripple at the speaker output at idle, and this is without the metal shielding that takes place once the amp is bolted into its cabinet!
To me any amp idling as I just posted about with .025 of idle ripple at the speaker is a what can be expected of a amp with a good layout and no problems!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: amp buzz
Thanks for your input and perseverance, Stevem! I was away for a couple of days, so I didn't see your posts until this evening. Upon my return, I went straight to the amp; so I have yet to measure the residual AC as per your suggestion. But I did learn a few things tonight. Here's what I did:
- wired up heaters to two 6V lantern batteries (wired in series for a 12V source; my heaters are powered by a 12V centre-tapped secondary). Initially the noise was still there, but it disappeared after I was doing some chop-sticking. Moving the shielded wired that connects the MV to the input of PI made the noise go away.
- I replaced the shielded wire; noise still present. But I noticed that turning up the MV to '10' also made the noise go away.
- I replaced the MV pot; the excessive hum/buzz was gone.
- reconnected the heater wires to AC secondaries, and some noise came back. I reconnected the DC heater source: no noise.
So, at present, there is noise with the current AC heater wiring, but it goes away when I wire the heaters to an external DC source. When I have more time, I will play around with the DC elevation...
- wired up heaters to two 6V lantern batteries (wired in series for a 12V source; my heaters are powered by a 12V centre-tapped secondary). Initially the noise was still there, but it disappeared after I was doing some chop-sticking. Moving the shielded wired that connects the MV to the input of PI made the noise go away.
- I replaced the shielded wire; noise still present. But I noticed that turning up the MV to '10' also made the noise go away.
- I replaced the MV pot; the excessive hum/buzz was gone.
- reconnected the heater wires to AC secondaries, and some noise came back. I reconnected the DC heater source: no noise.
So, at present, there is noise with the current AC heater wiring, but it goes away when I wire the heaters to an external DC source. When I have more time, I will play around with the DC elevation...
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Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: amp buzz
The dc bias up should set you full on good, but when done do report back as to what your idle noise level measures at like I posted about !
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!