Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: schoolie on June 12, 2012, 12:47:22 pm
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My 5f6a amp was pretty quiet until I decided to make it "better" :l2: I rewired the power tube sockets with the smaller, green 5W screen resistors. I installed ceramic composition 2W 1500R grid stoppers between pin 1 and pin 5 and moved my grid wires to pin 1. I rewired the filament wires from the power section to the lamp. I re-routed my 50V bias secondary to the switch-side of the amp. I didn't change anything in the PI or preamp sections.
Now I have 120hz hum (Right around a low Bb on the guitar). It's not super loud, but it's annoying. It isn't affected by the volume knobs. I've checked my reservoir and filter caps with an ESR meter and they haven't gone bad. I checked the grounding of the reservoir cap and filter caps and they are fine. I've chopsticked the grid wires around with no change. I've returned the bias secondary to the power tube side. I checked the solder joints on the power tubes. moved the wire from the rectifier to the standby switch around, but no difference. I am using Marshall/Hoffman style grounding with center taps, power tube cathodes, bias ground going to a bolt near the PT. Preamp, PI, and presence grounds and input jacks connected to a bus wire soldered to the back of the pots. I'm guessing that it's some sort of grounding problem, but I'm stumped. I'll post some pictures of the chassis tonight, but if anybody has some ideas, I would love to hear them. I just bought a $50 oscilloscope, so that might help when it arrives (if it works :laugh:).
Thanks!
Rob
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I just went through a similar issue - those 1.5Ks need to be between pins 5 & 6. Pins 1 & 8 are tied together and to ground through 1ohm resistors to check bias voltage (current). Those 470ohm resistors should be connected to pin 4. Is your board designed after Doug's layout?
My 5f6a amp was pretty quiet until I decided to make it "better" :l2: I rewired the power tube sockets with the smaller, green 5W screen resistors. I installed ceramic composition 2W 1500R grid stoppers between pin 1 and pin 5 and moved my grid wires to pin 1. I rewired the filament wires from the power section to the lamp. I re-routed my 50V bias secondary to the switch-side of the amp. I didn't change anything in the PI or preamp sections.
Now I have 120hz hum (Right around a low Bb on the guitar). It's not super loud, but it's annoying. It isn't affected by the volume knobs. I've checked my reservoir and filter caps with an ESR meter and they haven't gone bad. I checked the grounding of the reservoir cap and filter caps and they are fine. I've chopsticked the grid wires around with no change. I've returned the bias secondary to the power tube side. I checked the solder joints on the power tubes. moved the wire from the rectifier to the standby switch around, but no difference. I am using Marshall/Hoffman style grounding with center taps, power tube cathodes, bias ground going to a bolt near the PT. Preamp, PI, and presence grounds and input jacks connected to a bus wire soldered to the back of the pots. I'm guessing that it's some sort of grounding problem, but I'm stumped. I'll post some pictures of the chassis tonight, but if anybody has some ideas, I would love to hear them. I just bought a $50 oscilloscope, so that might help when it arrives (if it works :laugh:).
Thanks!
Rob
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I just went through a similar issue - those 1.5Ks need to be between pins 5 & 6. Pins 1 & 8 are tied together and to ground through 1ohm resistors to check bias voltage (current). Those 470ohm resistors should be connected to pin 4. Is your board designed after Doug's layout?
Thanks for the reply fdesalvo! I don't have pins 1 & 8 connected because I'm running 6L6 tubes which don't need this connection. I have my screen resistors running from pin 6 to pin 4--This was how Fender did their sockets, with grid stopper between 1&5 and the screen between 6 & 4. My board is the standard Fender layout on turrets. I do think Hoffman layout is a lot cleaner though.
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I assumed wrong! :icon_biggrin:
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I assumed wrong! :icon_biggrin:
No worries! I'm wondering if connecting the HT centertap directly to the reservoir cap would reduce the hum. AT the moment I have them on separate ring terminals on one chassis bolt. I was dumb and cut the transformer leads short :BangHead:
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Yeah, I just noticed the amp is cathode biased lol. I'd keep that CT to ground at that tranny bolt, opposite of the green wire from the power cord per layout, but I know Aiken recommends grounding in that manner to avoid 120hz nastiness.
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I'm wondering if connecting the HT center tap directly to the reservoir cap would reduce the hum.
KOC TUT books and Merlin also say it's best as far as noise.
That is the noisiest ground in the amp. It has the least filtering, because it's 1"st in the B+ rail, has the most current and has the most ripple/filter cap charging currents.
Have you read this on grounding?
http://www.freewebs.com/valvewizard2/Grounding.html (http://www.freewebs.com/valvewizard2/Grounding.html)
Brad :icon_biggrin:
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KOC TUT books and Merlin also say it's best as far as noise.
That is the noisiest ground in the amp. It has the least filtering, because it's 1"st in the B+ rail, has the most current and has the most ripple/filter cap charging currents.
Have you read this on grounding?
http://www.freewebs.com/valvewizard2/Grounding.html (http://www.freewebs.com/valvewizard2/Grounding.html)
Brad :icon_biggrin:
Thanks, Brad! Yes, I've read that excerpt (a great education. I love Merlin Blencoe's writing!). I know the Hoffman grounding scheme was working before I "improved" my amp, but I'll try moving the HT CT to the reservoir cap lead. Eventually I'll have an "aha!" moment...I hope :icon_biggrin:
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You'll find it.
Brad :icon_biggrin:
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My 5f6a amp was pretty quiet until I decided to make it "better" :l2: ... Now I have 120hz hum (Right around a low Bb on the guitar). It's not super loud, but it's annoying. It isn't affected by the volume knobs. ...
Unfortunately, your changes probably introduced the hum.
Since the volume control doesn't affect the hum, the cause is after the volume control. Yank the phase inverter tube. Do you still have the hum with the amp on?
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Unfortunately, your changes probably introduced the hum.
Since the volume control doesn't affect the hum, the cause is after the volume control. Yank the phase inverter tube. Do you still have the hum with the amp on?
Thanks HBP. With the PI tube pulled, there is almost no hum!
(https://lh3.googleusercontent.com/-g1b6YjRbZ8I/T9hHvviMf6I/AAAAAAAAAVk/BIT8-iafp5g/s640/IMG_4381.JPG)
(https://lh4.googleusercontent.com/-ByGp0mF69Lk/T9hKkyIsNsI/AAAAAAAAAWk/SmIbg_lNMvA/s640/IMG_4388.JPG)
(https://lh6.googleusercontent.com/-fmZXyTCmAx8/T9hI9x2asPI/AAAAAAAAAWA/yw89J5N8lkg/s640/IMG_4385.JPG)
(https://lh6.googleusercontent.com/-MoZAFc-TKGU/T9hLvp73fNI/AAAAAAAAAW4/NloeUSFRm8U/s640/IMG_4390.JPG)
(https://lh6.googleusercontent.com/-rXxn1n6d0r8/T9hL2pEvQUI/AAAAAAAAAXA/PjBPPIydCdw/s640/IMG_4391.JPG)
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I need a little more help, please. It appears to be V2 that is causing the hum. If I remove V2 or V3 the hum is gone. Removing V1 does not help. Turning up the Bass increases the hum (I assume because the frequency is in the spectrum that it controls). The treble increases the hiss. The volumes just increase the hiss which is only laud at around 9-10. On V2 pin 6 is 328V and pin 1 is around 184V. Touching the DMM lead to pin 1 or pin 7 produced a loud pop which makes taking a reading difficult. Is that pop expected? I double checked the wiring, and tried swapping in a different tube, but the swap didn't improve the hum. I chopsticked around the tone circuit, and the only interesting thing that I found is that the 0.022uF cap that connects to the wiper of the middle pot is microphonic. Could the hiss and hum be a leaky cap? The filter cap appears to be OK.
Thanks!
Rob
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I'd change that cap. Maybe use an alligator clip and hook up the DMM to the pin while the amp is on standby to avoid the noise. No backwards caps or cold joints in the circuit?
I need a little more help, please. It appears to be V2 that is causing the hum. If I remove V2 or V3 the hum is gone. Removing V1 does not help. Turning up the Bass increases the hum (I assume because the frequency is in the spectrum that it controls). The treble increases the hiss. The volumes just increase the hiss which is only laud at around 9-10. On V2 pin 6 is 328V and pin 1 is around 184V. Touching the DMM lead to pin 1 or pin 7 produced a loud pop which makes taking a reading difficult. Is that pop expected? I double checked the wiring, and tried swapping in a different tube, but the swap didn't improve the hum. I chopsticked around the tone circuit, and the only interesting thing that I found is that the 0.022uF cap that connects to the wiper of the middle pot is microphonic. Could the hiss and hum be a leaky cap? The filter cap appears to be OK.
Thanks!
Rob
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Thanks, fdesalvo! I'll try reflowing the solder joints tonight. I didn't find a backward cap, but I'll probably retrace the circuit until I find something.
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Thanks, fdesalvo! I'll try reflowing the solder joints tonight. I didn't find a backward cap, but I'll probably retrace the circuit until I find something.
Hmm, it's got to be related to something you changed. Hoffman has a great circuit tracing method over in the sticky at the top of this forum. I'll have another look at your first posts on this topic again, too.
Edit - reading your post abut grounding the CT on the (-) of the first filter cap intrigued me, so I moved mine there and noticed a very minor reduction in hum. I'll def continue grounding mine like that in the future.
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Glad that moving the HT CT ground helped. Doug Hoffman's debugging guide is helpful, but I've checked that every capacitor and resistor is good and connections are correct, and voltages are within range. I know it's almost impossible to help much without the amp in front of you. I'm just out of ideas, and was hoping that I could narrow down my search. Thanks again for responding, fdesalvo! I'm going to break out the scope tonight.
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Could my re-routing of the filament wiring have caused it to pick up noise from the rectified HT going to the standby switch? Is it possible that V2 is picking up this noise from the filament wiring?
Also, if pulling out V2 removes the hum, does that mean that there probably isn't an issue with the HT filtering--at least up to V2?
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When I was struggling with my buzzing issue, I read many people attributing 120hz him to filaments. If moving the filament wires away from the signal leads around v2 makes a difference, yes. Did you forget to ground your filaments when revamping the wiring on the power amp side?
Does your amp make this sound on standby or with no instrument cable inserted into the input jack?
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I read many people attributing 120hz him to filaments.
filaments are operating at line frequency - 60Hz. the power supply, if it's a full wave operates at 120Hz.
usually, 120Hz buzz is attributed to poor grounding/lead dress and/or power supply filtering.
--DL
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When I was struggling with my buzzing issue, I read many people attributing 120hz him to filaments. If moving the filament wires away from the signal leads around v2 makes a difference, yes. Did you forget to ground your filaments when revamping the wiring on the power amp side?
Does your amp make this sound on standby or with no instrument cable inserted into the input jack?
My 6.3V CT is grounded. I hear the noise with nothing plugged. I don't hear anything on standby (No HT to tubes).
Thanks!
Rob
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I read many people attributing 120hz him to filaments.
filaments are operating at line frequency - 60Hz. the power supply, if it's a full wave operates at 120Hz.
usually, 120Hz buzz is attributed to poor grounding/lead dress and/or power supply filtering.
--DL
It was farfetched, but I though maybe the wires could pick up 120hz noise from the rectifier and the noise picked up by the tubes. Do you think V2 pull removing the noise means that PS filtering is OK? Maybe the pot wiring is picking up the noise?
Thanks!
Rob
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Thanks!
I read many people attributing 120hz him to filaments.
filaments are operating at line frequency - 60Hz. the power supply, if it's a full wave operates at 120Hz.
usually, 120Hz buzz is attributed to poor grounding/lead dress and/or power supply filtering.
--DL
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Are you using shielded cable from your volume pot(s) to the tube's grid?
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Are you using shielded cable from your volume pot(s) to the tube's grid?
No, I built this amp tweed style, just a bare, under-board wire from the mixer resistors to the the grid. I tried moving the grid wire around but couldn't make an audible difference in the noise. That made me think that the pin could possibly be too close to the filament wires. Definitely a few possibilities to look into in my next debug session.
Thanks!
Rob
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Cool - excluding the mechanical side of building tube amps, I'm a total and utter newb with respect to the actual function of each compnent - well, not utter/total, but I only have so much to offer a person in your shoes haha. I wish the best. Def tracking this.
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Cool - excluding the mechanical side of building tube amps, I'm a total and utter newb with respect to the actual function of each compnent - well, not utter/total, but I only have so much to offer a person in your shoes haha. I wish the best. Def tracking this.
Hey, I really appreciate your help, fdesalvo! It's been really helpful talking through this with you. I'll let you know if I make any progress.
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My 5f6a amp was pretty quiet until I decided to make it "better"
- I rewired the power tube sockets with the smaller, green 5W screen resistors.
- I installed ceramic composition 2W 1500R grid stoppers between pin 1 and pin 5 and moved my grid wires to pin 1.
- I rewired the filament wires from the power section to the lamp.
- I re-routed my 50V bias secondary to the switch-side of the amp.
That's only 4 things. Change them back, one at a time, checking to see if the hum is gone. It's possible you no longer have a good connection on one of those changes. The screen resistor change is not likely the cause, unless a poor connection is now a factor. I would definitely use half watt 1500Ω grid stoppers. Mounting between pins 1 and 5 is fine (unless you are running EL34s).
I think reversing those changes will likely reveal the problem, even if the change itself was not the culprit. Sometimes you just have collateral damage.
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That's only 4 things. Change them back, one at a time, checking to see if the hum is gone. It's possible you no longer have a good connection on one of those changes. The screen resistor change is not likely the cause, unless a poor connection is now a factor. I would definitely use half watt 1500Ω grid stoppers. Mounting between pins 1 and 5 is fine (unless you are running EL34s).
I think reversing those changes will likely reveal the problem, even if the change itself was not the culprit. Sometimes you just have collateral damage.
Thanks for you help, Steve! I know you're right. I've already restored the bias wire, so that's only three changes. Is there any issue with using the 2W grid stopper? I thought it would be a good idea since it gets hot from the tubes...but it is *big*.
Thanks!
Rob
Thanks again,
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:l2: I wish I could count the number of posts where Steve suggests 1/2w resistors there. I have 5W monsters on mine - Have no idea where I got that idea from. I think I kept seeing Marshall builds with those giant cinder blocks hanging from the pins!
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:l2: I wish I could count the number of posts where Steve suggests 1/2w resistors there. I have 5W monsters on mine - Have no idea where I got that idea from. I think I kept seeing Marshall builds with those giant cinder blocks hanging from the pins!
5W is 4.5W better than 1/2W, right? :laugh:
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I route all of my leads around the 5w resistors :l2:
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I don't think the wattage is an issue, but mounted as you have it puts it very close to the heater wires. A half watter will lay neatly at the bottom of the socket, away from noisy wires. Another issue may be the solder connections. It looks like you 'may be' simply laying wires or components in the tube socket lugs and soldering without first making a good mechanical connection by wrapping/crimping the lead or wire around the pin lug. If so, that's inviting gremlin type problems, especially as the amp is moved and bumped around.
These may not be issues at all, but you have to suspect the last thing(s) you did as a possible source of the problem.
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I don't think the wattage is an issue, but mounted as you have it puts it very close to the heater wires. A half watter will lay neatly at the bottom of the socket, away from noisy wires. Another issue may be the solder connections. It looks like you 'may be' simply laying wires or components in the tube socket lugs and soldering without first making a good mechanical connection by wrapping/crimping the lead or wire around the pin lug. If so, that's inviting gremlin type problems, especially as the amp is moved and bumped around.
These may not be issues at all, but you have to suspect the last thing(s) you did as a possible source of the problem.
OK, thanks for clarifying that. I didn't crimp some of my tube pin connections, but will make sure to do so in the future.
Thanks again,
Rob
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When I was struggling with my buzzing issue, I read many people attributing 120hz him to filaments. ...
I built an amp, and had a weird, buzzing hum. When I went to figure out the cause, it turns out I'd forgotten to install the 100Ω resistors for an artificial center-tap. Installing the resistors stopped the buzz.
I don't have a good explanation for why (I'd like to see the effect on a scope to confirm what my ears heard), but the sound was a buzz instead of smooth 60Hz hum. The higher harmonics present in such buzzing might lead folks to think the sound wasn't 60Hz because of the higher apparent pitch.
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I built an amp, and had a weird, buzzing hum. When I went to figure out the cause, it turns out I'd forgotten to install the 100Ω resistors for an artificial center-tap. Installing the resistors stopped the buzz.
I don't have a good explanation for why (I'd like to see the effect on a scope to confirm what my ears heard), but the sound was a buzz instead of smooth 60Hz hum. The higher harmonics present in such buzzing might lead folks to think the sound wasn't 60Hz because of the higher apparent pitch.
I wish that were the cause, but my filament CT is grounded. I've reversed my changes, but still get the hum. I think it's somewhere between V2 and V3. I don't see any noise on V2 grid, but I do see noise on the negative side of the 250pF cap. I think it's somewhere in the tone stack. Maybe I'll try reflowing the solder around the caps and pot leads. If connect my DMM between the neg side of the 250pF cap and the ground bus, the hum gets much louder(same for the bass and middle caps) . The wire from the mixer resistors to pin 2 of V2 is microphonic and pin 2 is somewhat microphonic too.
Thanks!
Rob
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I wish that were the cause, but my filament CT is grounded. I've reversed my changes, but still get the hum. I think it's somewhere between V2 and V3. ...
My comment was only about how heater-related hum could be confused with 120Hz hum. No, you're right that given your test results, the source of your hum is probably related to V2.
To check the ground for that tube, I'd suggest clipping your black meter lead over at the far left side of your chassis, where you have the power cord ground bolted, them measure resistance from the V2 cathode pins to verify the path for ground and rough resistance of the associated parts.
And push the filament wiring towards the opening in the chassis (which moves it further away from the circuit board).
What is the pot by the speaker jacks?
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My comment was only about how heater-related hum could be confused with 120Hz hum. No, you're right that given your test results, the source of your hum is probably related to V2.
To check the ground for that tube, I'd suggest clipping your black meter lead over at the far left side of your chassis, where you have the power cord ground bolted, them measure resistance from the V2 cathode pins to verify the path for ground and rough resistance of the associated parts.
And push the filament wiring towards the opening in the chassis (which moves it further away from the circuit board).
What is the pot by the speaker jacks?
Thanks, HBP! I will double check the resistances, as you suggested. The pot by the jacks is bias adjustment.
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Probably beating a dead horse, but how many different tubes did you try in V2? I once had two bad ones out of the box.
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Probably beating a dead horse, but how many different tubes did you try in V2? I once had two bad ones out of the box.
Funny you should mention that. Fedex just delivered a Tung-Sol gold pin 12ax7. I'll give it a shot tonight.
Thanks!
Rob
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Yuu've been very methodical in your approach to troubleshooting - I wouldn't be surprised if it's a faulty component at this point in the cycle. You've basically traced the problem down to the very tube socket V2 occupies.
Probably beating a dead horse, but how many different tubes did you try in V2? I once had two bad ones out of the box.
Funny you should mention that. Fedex just delivered a Tung-Sol gold pin 12ax7. I'll give it a shot tonight.
Thanks!
Rob
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Well, it's a new build, so faulty components are very rare. Most often, there's a wiring error or a 470k resistor where a 470Ω should be.
I made exactly those kind of errors on my last build.
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To check the ground for that tube, I'd suggest clipping your black meter lead over at the far left side of your chassis, where you have the power cord ground bolted, them measure resistance from the V2 cathode pins to verify the path for ground and rough resistance of the associated parts.
And push the filament wiring towards the opening in the chassis (which moves it further away from the circuit board).
Hi HBP,
With one lead connected to the power ground, the V2 cathodes measured 888 Ohms on pin 3 and 103 KOhms on pin 8, which agrees with the schematic. All of the preamp ground measure 0.1-0.2 Ohms from the turret to the power ground point (Which is about zero Ohms, since I see 0.1Ohms with the DMM leads connected). The ground buss and backs of the pots also show 0.1-0.2R to the power ground.
Thanks!
Rob
Yuu've been very methodical in your approach to troubleshooting - I wouldn't be surprised if it's a faulty component at this point in the cycle. You've basically traced the problem down to the very tube socket V2 occupies.
Thanks, bud! Hope I can make some progress tonight. That Tung-Sol reissue 12ax7 was noisier than the ElectroHarmonix tube. I did find a bad solder joint on the 250pF cap, so I reflowed the solder on all tone caps, but it didn't help. I verified that there is no DC voltage on the pots. Better go to bed now...
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I think the hum is quieter, after fixing the bad solder joint. HotBluePlates, you're absolutely right. I checked out the noise at the output transformer secondary and see 60hz noise, around 10mV PtP (that seems low for an audible hum, maybe my scope is off). The good news is that I don't see any noise anymore from V2...so it's on to V3.