Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: markmalin on July 07, 2015, 09:02:30 am
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Hi guys,
I have sound :) Fired up the amp last night and it's good so far, but there's a low hum. I don't know if this is characteristic of class A amps, but I'm going to start chasing that down. Here are some pictures of the layout. It's tight. Not much room. Any thoughts on source of hum? One thing, I used a 3H choke (Hammond). Would a larger value help? I known O'Conner talks about adding a second choke between B and C supplies, what are your thoughts on that?
Thanks!
Mark.
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here's a shot of the top side.
Note that I oriented the PT 90 degrees rotated from a typical build, just due to size.
Mark.
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Very nice build Man, congrats !
I don't know if this is characteristic of class A amps, but I'm going to start chasing that down.
As for the hum I wouldn't have placed both PT & OT so close from each other, even in a perpendicular way. Instead I would have put the choke between them.
Any thoughts on source of hum?
BTW, here's an intereresting article about OT placement/orientation just in case of : http://ax84.com/faq.html#ot-place (http://ax84.com/faq.html#ot-place)
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I am not sure what could be causing noise, but the B+ crossing the board could. You have additional wires going back a fourth from your filter caps to your board, but it looks like you have plenty of room to put your filter caps in your main board which would eliminate the long redundant push back wires especially on node A and B. Check it with chopstick and lift the B+ wire off the cathode resistor and see if anything changes.
At least get the B+ from the standby to the A node Filter Cap. You could then connect Node A to B with your choke on you cap board. Just throwing out stuff.
I assume you have a center tap for your heaters and I am not quite sure what the disconnected pot and switch are for. Maybe a Master? Is the switch a triode/pentode switch? Disconnect it until you have a quiet running amp. On a AC 30 I prefer to switch off the 2 outer tubes for half power and just double my speaker Ω. That way it still sounds like a vox.
But then again, what do I know. Heck, I would remove the whole cap board and drill holes and zip tie them to the edge of the board close to where they would be used in the circuit. This would open up the PA end to dressing it nicely. I have seen a lot of great sounding amps that don't look great inside tho, but most of the time when noise is an issue I usually have my inputs and outputs fighting.
Your transformer placement is not any different than a old Marshall or a Dynaco. Put a shield between them and see if it makes a difference.
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I am not sure what could be causing noise, but the B+ crossing the board could. You have additional wires going back a fourth from your filter caps to your board, but it looks like you have plenty of room to put your filter caps in your main board which would eliminate the long redundant push back wires especially on node A and B. Check it with chopstick and lift the B+ wire off the cathode resistor and see if anything changes.
Thanks, it was a debate whether to run the B+ over the board or back past the rectifier across the cap board. I'll get a chopstick in there and test that wire. I like your idea of using the choke to connect the A to B rather than the one pushback wire. I thought maybe since those supplies are all DC, running them long wouldn't be an issue but maybe the DC is picking up AC from something else.
I assume you have a center tap for your heaters and I am not quite sure what the disconnected pot and switch are for. Maybe a Master? Is the switch a triode/pentode switch? Disconnect it until you have a quiet running amp. On a AC 30 I prefer to switch off the 2 outer tubes for half power and just double my speaker Ω. That way it still sounds like a vox.
The heater secondary does have a center tap, yes, so I eliminated the 100 Ohm resistors normally in Doug's schematic.
The big switch is a half power, but it just grounds the control grids of the two inner tubes. I was originally going to switch the two tubes out, but didn't want the end user have to do an impedance change.
Sorry for the confusion on the unconnected pot and switch! Yes, that is a master, and a "master in/out" switch. The micro switch will just short out the master pot, but I wanted to get the amp running before connecting up the master and switch, just to make sure it doesn't induce noise (running wires to the master pot).
But then again, what do I know. Heck, I would remove the whole cap board and drill holes and zip tie them to the edge of the board close to where they would be used in the circuit. This would open up the PA end to dressing it nicely. I have seen a lot of great sounding amps that don't look great inside tho, but most of the time when noise is an issue I usually have my inputs and outputs fighting.
Your transformer placement is not any different than a old Marshall or a Dynaco. Put a shield between them and see if it makes a difference.
I guess I could have put the filter caps on the main board. My next rev I may do that. I hope for now I can keep the cap board as-is, but thanks for the idea!
Mark.
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archaos,
Tanks for the link on transformer placement. The next one I build, I think I'm going to do that. I have one more chassis all cut out but haven't mounted the transformers on that one yet.
I'm not sure what to think about whether to put the choke inbetween the other two transformers. I usually do that just because the other amps I build are similar to standard layouts. Good thoughts, though.
Very nice build Man, congrats !
I don't know if this is characteristic of class A amps, but I'm going to start chasing that down.
As for the hum I wouldn't have placed both PT & OT so close from each other, even in a perpendicular way. Instead I would have put the choke between them.
Any thoughts on source of hum?
BTW, here's an intereresting article about OT placement/orientation just in case of : http://ax84.com/faq.html#ot-place (http://ax84.com/faq.html#ot-place)
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Your OT and PT placement is fine. Obviously this is a huge concern when laying out components but I have a couple of these same builds with my OT even closer than yours. I un-mounted my OT from my chassis and moved it around to as a test to see what other placements would do to my 30 watt build and where I had it was perfectly fine. Hope you find the hum.
maybe swap the tubes as a start? really bad matched tubes can cause you problems. Other than that. I always check grounding and lead dress.
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Your OT and PT placement is fine. Obviously this is a huge concern when laying out components but I have a couple of these same builds with my OT even closer than yours. I un-mounted my OT from my chassis and moved it around to as a test to see what other placements would do to my 30 watt build and where I had it was perfectly fine. Hope you find the hum.
maybe swap the tubes as a start? really bad matched tubes can cause you problems. Other than that. I always check grounding and lead dress.
That's good to know.
It's not a buzz, not like a ground issue, it's more of a low hum. Not terribly strong, but when you stop playing you hear it.
FWIW - I did some lead dress changes, re-routed the B+ as was mentioned above but no change.
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The old amp architecture used poor filtering because of the cost of the e-caps
what you describe seems the normal humm present in an old amp
K
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Any thoughts on source of hum? One thing, I used a 3H choke (Hammond). Would a larger value help?
Your pic shows a 4H choke. The schematics I've looked at show a much bigger 10-20H choke. A larger choke in your amp would probably drop the hum level.
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Any thoughts on source of hum? One thing, I used a 3H choke (Hammond). Would a larger value help?
Your pic shows a 4H choke. The schematics I've looked at show a much bigger 10-20H choke. A larger choke in your amp would probably drop the hum level.
Sluckey,
you're right. I had bought two chokes, a 3H Mercury M and a 4H Hammond and I used the Hammond. But I think I'll pick up a larger choke - the thing was, the big ones are 40 bucks vs 28 or so :P but given your input (and K's) it seems like a worthwhile idea. I'll pick one up today - CE Dist is just down the street from my office.
Mark.
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One update - I pulled the PI and it's quiet. If I install the PI and remove both pre-amp tubes, it's noisy. I'm going to try a different choke, but it makes me wonder.
I did poke around all over the place with a chopstick lifting wires, pushing on stuff, nothing really changes the hum so I don't think it's related to lead dress, though I did shorten several wires while I was in there.
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CE Dist is just down the street from my office
I'm jealous!
I used a Hammond 193B choke, 12H @ 100ma, DCR = 155Ω, in my AC-15. I think it would work well in your amp too. It's big, and pricey. It sits between the PT and OT in this pic...
http://home.comcast.net/~seluckey/amps/VAC15/18.jpg (http://home.comcast.net/~seluckey/amps/VAC15/18.jpg)
There are no guarantees that a larger choke will get all the hum out, but a 12H choke will certainly remove more 120Hz hum than a 4H choke. I'd go for the bigger choke, especially since the schematics call for a bigger one.
That choke smooths the B+ for the screens, PI, and Preamp.
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Thanks for sharing the model and specs! I was just looking on CE's site and noticed the Hammond replacement for the Vintage AC30 is 30 H, while the one for the reissue is only 5, so I was wondering how much is enough? 12H should help :) I think 5 probably isn't that much of a change. Not sure about 30 if that's too much?
Anyhow, if the 193B worked for yours, that should be a good starting place. They're too expensive to just buy a bunch of them and try, so I've got to get close the first time. Thanks!
I used a Hammond 193B choke, 12H @ 100ma, DCR = 155Ω, in my AC-15. I think it would work well in your amp too. It's big, and pricey. It sits between the PT and OT in this pic...
http://home.comcast.net/~seluckey/amps/VAC15/18.jpg (http://home.comcast.net/~seluckey/amps/VAC15/18.jpg)
There are no guarantees that a larger choke will get all the hum out, but a 12H choke will certainly remove more 120Hz hum than a 4H choke. I'd go for the bigger choke, especially since the schematics call for a bigger one.
That choke smooths the B+ for the screens, PI, and Preamp.
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I was just looking on CE's site and noticed the Hammond replacement for the Vintage AC30 is 30 H
That's the 194E. I like the sound of that even better than the one I have in my AC-15. It may even drop right in the holes you already drilled for your present choke.
The choke and filter caps work together to form a very smooth B+. Increasing the value of either will result in a smoother (less hum) B+. The reissue AC-30 'may' be using larger filter caps thus allowing the use of a smaller choke. That's just a guess. I haven't looked at a reissue schematic.
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I used the Hammond 159P, 10H 125ma 155Ω. $28.
I did build the same amp as Sluckey. An AC15. The original early 60's was a 30H Choke on the AC30. Their website is down right now, but they have all the specs on the old iron for the 60's Vox amps. They are just adding parts right now. It will be back up soon. The guy that sells the parts suggested the Hammond choke to me since he is in England.
http://livinginthepast-audioweb.co.uk/index.php?p=xfrmrclassic (http://livinginthepast-audioweb.co.uk/index.php?p=xfrmrclassic)
I did overlook your choke comment, Sorry about that. I believe the newer reissues Vox (if you can call them that) have much more filtering using larger caps. I know the ones I have played feel very stiff and completely miss the vox groove. I really do not know anyone who has played an new Vox that thinks they sound anything like the old ones. Not saying they sound bad, just different.
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I just placed an order for both the 194E and 159P. I'm building two of these amps, one for a customer and one as my "demo" unit. I figure both of these will work, so I'll try one in one build and one in the other. Curious to see if I can tell any difference in responsiveness (though amp #2 won't be done for a few weeks).
Incidentally, I had a "new" Chinese made Vox on my bench. I have to say, I wasn't all that impressed with it. On the other hand, I was working on a Bad Cat DC30(?) which I think was based on the AC30. 4 El84's, cut control, etc. Anyhow, THAT amp sounded really sweet. If I ignore the faint hum, this one i'm working on sounds more like the bad cat than the reissue AC30.
:w2: Slucky and Ed, tell me about how your Bass and Treb controls interacted on your builds. I swear the bass control on this build feels backwards. It gets deep and warm when I rotate the pot toward 0, and real thin when I rotated it to 10. The Treb control gets really thin and bright as you dial it toward 10, which seems right. I'm working with a schematic/layout from Doug but I had to do a mirror image of it because of my chassis layout. I've double checked the wiring and it seems right. Is this typical of an AC style build? I have half a mind to wire the Bass backwards to get the desired effect, but doing that without knowing why doesn't sit well with me. (I'm attaching a schematic and layout. The layout is flipped, but obviously not correct because it's just a mirror image and the tube pinouts and pot. orientations are wrong)
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Slucky and Ed, tell me about how your Bass and Treb controls interacted on your builds.
There are no treble or bass controls on the AC-15. The only tone shaping is a simple Brilliance switch and a Top Cut pot after the PI.
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Slucky and Ed, tell me about how your Bass and Treb controls interacted on your builds.
There are no treble or bass controls on the AC-15. The only tone shaping is a simple Brilliance switch and a Top Cut pot after the PI.
Ah. Ok :O)
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the big ones are 40 bucks vs 28
At DIYItalia a friend has tried with success a self made Gyrator
(http://i.imgur.com/nHG7HnY.jpg)
K
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Holy cats! Thats' really something. I've never heard of a gyrator - just looked it up on wickipedia. Fascinating....
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Oh my gosh! I have totally opened a can of worms re the AC30 tone stack. :laugh:
I've been reading up on this and I now understand I haven't wired it wrong, it's the nature of the beast, but what a fascinating story. There is a common misconception that the grounded leg of the Bass control was by accident, but apparently that was on purpose. Anyhow, to circle round, here's a thread on Hoffman about this:
http://el34world.com/Forum/index.php?topic=10166.msg92779#msg92779 (http://el34world.com/Forum/index.php?topic=10166.msg92779#msg92779)
Here's a great article on the real story:
http://www.vintageamps.com/plexiboard/viewtopic.php?f=2&t=94991&start=0 (http://www.vintageamps.com/plexiboard/viewtopic.php?f=2&t=94991&start=0)
I may wire that switch on the front of my chassis to lift the ground on the tone stack rather than use it for cutting the master volume. I really find the action of the standard Vox tone stack bizzare and non-intuitive.
It's funny how you end up learning more about something than you ever wanted to! :P
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I don't know, guys. I swapped in the Hammond 194E and it's still humming about the same.
Also, I took a piece of metal (and end bell from another transformer) and placed it between the OT and PT. I don't hear any difference. I placed it in other spots, like between the tubes and OT, but nothing. Can I assume the placement of the PT and OT being close to one another is not the issue since this extra piece of shielding didn't have any affect on the hum? I didn't actually ground the piece of metal to the chassis (it's painted), but for interrupting any magnetic coupling between the two transformers that shouldn't matter, correct?
Mark.
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The KOC chapter on the AC-30 in TUT3 attributes humminess in the original vox being due to the power supply design and lack of negative feedback in the power amp. He creates another stage in the power supply by adding a cap and a second choke (158T) between the rectifier and the original power supply. The choke has to be able to handle big amperes because the entire amp current flows through it.
Craig
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The KOC chapter on the AC-30 in TUT3 attributes humminess in the original vox being due to the power supply design and lack of negative feedback in the power amp. He creates another stage in the power supply by adding a cap and a second choke (158T) between the rectifier and the original power supply. The choke has to be able to handle big amperes because the entire amp current flows through it.
Craig
Thanks, Craig. I did see that. I wonder if it's worth trying that? It's either that or try to re-mount the PT and OT and orient them differently to see if that helps, but that's a lot of work that may have no results. Adding a cap and a choke is pretty easy...but then again I just re-read that chapter and he doesn't make it real clear...
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If you can post a sample
people who has a Vox can tell you if it is normal humm level or you have more than usual
K
p.s.: You can invest few $ building a gyrator an trying it (with an added e-cap) on your amp
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K,
I'll post a recording (video, likely). I'd like to hear your input on whether it's normal or not. I'll try to come up with something tonight.
Thanks!!
Mark.
If you can post a sample
people who has a Vox can tell you if it is normal humm level or you have more than usual
K
p.s.: You can invest few $ building a gyrator an trying it (with an added e-cap) on your amp
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Here are three videos. Probably have to put on headphones to really hear anything. Two are of the hum, one with nothing plugged in, one with the guitar plugged in. Volume is VERY low. Hum does not change with volume.
What I tried tonight was:
- changed the heater wiring per Gerald Weber's comment to make sure the polarity is right on the power tubes. No difference
- I saw one ground connection was wrapped around the terminal, but not soldered, so I soldered that. No difference.
- I also twisted the tone stack wires per TUT 3 Vox chapter.
https://youtu.be/dBvr-i7klJA (https://youtu.be/dBvr-i7klJA) Hum, nothing plugged in (volume was real quiet on amp)
https://youtu.be/7rwFUfGLCrY (https://youtu.be/7rwFUfGLCrY) Hum with guitar playing a couple notes. (volume was real quiet on amp)
I'm also having a hiss issue that I have not started troubleshooting yet. But preemptively I swapped out a few of the plate resistors for Dale metal film's just in case. Again, I still need to look into this, but it's so bad as you turn the amp up it's very present while you're playing.
https://youtu.be/eYuEBasOgHg (https://youtu.be/eYuEBasOgHg) Hiss.
For the hum, I wonder if the next step is to try moving the transformers around?
Or maybe try to implement TUT 3's second choke idea in the power section?
This is really kicking my butt! It's going to be a great sounding amp, but yeesh...it's getting to be a challenge.
Humbly,
Mark.
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Um, were are the B+ dropping R's?
I don't see them in the filter cap board in your 1st post.
(I listened to Wave, your very good! :icon_biggrin: )
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Um, were are the B+ dropping R's?
I don't see them in the filter cap board in your 1st post.
(I listened to Wave, your very good! :icon_biggrin: )
Hey, Willabe. They're on the main board.
lol....you're not supposed to listen to my other stuff! (just kidding)
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Oh, I see them now.
I liked what you were playin on that song.
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A bit more of what I was aspecting ...
Can you try to feed the filaments with a DC power supply to verify what happens ?
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Put a 100-200n cap directly across the heaters pin on V1, changes anything?
Ciao
Franco
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suggestion:
reroute grounds: segregate power amp ground and LTPI ground. see pic.
--pete
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Thanks, guys! I'll try these things tonight. I was wondering if there was a way to isolate/rule out the heater wires - I need to remember that test. I'll separate the grounds, too.
(Willabe, you're too kind ;)
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Ok, I snipped and re-routed the ground per Dummyload's suggestion, but no difference
I also touched a .1 uf cap across V1's heaters and no difference.
I notice the amp starts out relatively quiet, then as maybe 30 or 40 seconds pass, it grows louder until it levels out.
I measured voltages and am not familiar with this amp, but does this seem right?
(as you read these values, bear in mind I have swapped the two sides of each pre-amp tube. So V2-A in the schematic is pins 6,7 and 8 and V2-B is pins 1,2 and 3. So in my swapped version, V2 has pins 6 and 2 tied together...pin 7 is the input...pin 1 tied to "D".)
V1 plates both at 153V, which seems reasonable.
V2 plate on pin 1 is at 287V, which seems high. Note that it starts out about 200 when I switch off Standby, then grows gradually to 287 and the hum grows louder as this increases.
V3 plates are both at 168, which again seems reasonable.
Is something wrong with V2 being so high? Why does the voltage increase (and hum increase) over a 30 or 40 second period?
I appreciate the help guys!
Mark.
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Is something wrong with V2 being so high? Why does the voltage increase (and hum increase) over a 30 or 40 second period?
It's normal for V2 voltages to be high. It's a cathode follower and the plate is connected directly to B+ node D. Grid and cathode voltages will be high also.
The 5AR4/GZ34 has a long warm up time. The B+ voltages take a long time to build up to maximum with a 5AR4. That's good.
Wish I had some more ideas about your hum. Was hoping the choke would fix it.
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Put a 100-200n cap directly across the heaters pin on V1
Have you tried ?
K
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The 5AR4/GZ34 has a long warm up time. The B+ voltages take a long time to build up to maximum with a 5AR4.
Do you have another 5AR4 to swap in there just to make sure it's working as it should? If that's already been done, sorry I just scanned through the posts!
Jim
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Put a 100-200n cap directly across the heaters pin on V1
Have you tried ?
K
Yes. No difference
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The 5AR4/GZ34 has a long warm up time. The B+ voltages take a long time to build up to maximum with a 5AR4.
Do you have another 5AR4 to swap in there just to make sure it's working as it should? If that's already been done, sorry I just scanned through the posts!
Jim
I'll try that, Jim. In fact, I have a solid state 5u4 plug in rectifier. Wonder what diff' that will make? I'll play around with tubes next. Then probably remove the OT and mount it further from the PT
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No difference
No hiss difference ?
K
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No difference
No hiss difference ?
K
No. No hiss difference, no hum difference.
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I just swapped a complete new set of tubes in. There's still a hum. Maybe not as bad, but it's subjective (could just be me). Pretty much the same, though, so I think I can rule out tubes. I'm going to try my solid state rectifier.
- edit - I did try the solid state rectifier, but no difference.
Couple observations:
Hum:
- in "half power", less hum (not much to speak of)
- Both volumes off, treble off. Rotate bass all the way off and hum all but gone. Turn bass up and hum increases (could just be the filtering effect of the bass RC circuit I suppose?) The hum definitely increases gradually as you increase the bass control (other controls on 0)
Hiss:
- With treble and bass and Cut off, volume #2 on about 3, major hiss
- Turn treble up, hiss increases
- Turn Cut all the way up and the his is completely gone.
- pulling V2 has no effect on the hiss
- pulling V1 stops the hiss, but that takes out the first gain stage, so no signal is going in to the second gain stage, and hence no hiss gets through to the power amp section.
So it's like the Bass effects the Hum and the Cut removes the Hiss.
I've been playing the amp for a while. W/out these two issues it really sounds pretty sweet. I've got to get the hum under control, but more so get rid of this annoying hiss. I've purchased a dummy load (should be here Wed.) and plan to build up a listening amp to track the hiss down.
Any other thoughts on the Hum?
Is it worth removing the OT and moving it away from the PT?
Should I consider trying to rotate the PT 90 degrees as well the way it typically would be? (it's going to get REALLY close to the rectifier...)
Mark
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This is sorta like "will it stick?" I would break the circuit at the input to C9, and pull all pre-amp tubes. maybe even ground the input to C9 after it's disconnected.
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This is sorta like "will it stick?" I would break the circuit at the input to C9, and pull all pre-amp tubes. maybe even ground the input to C9 after it's disconnected.
So do that and fire it up and see if it hums? (or hisses ?)
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So do that and fire it up
Yes, look at the schematic, to me, it's the most logical place to 1/2 split the amp. If the hum/hiss is gone, or there, you've pretty much narrowed it down to 1/2 an amp.
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So do that and fire it up
Yes, look at the schematic, to me, it's the most logical place to 1/2 split the amp. If the hum/hiss is gone, or there, you've pretty much narrowed it down to 1/2 an amp.
Roger that. Thanks, I'll give that a shot today and report back.
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So do that and fire it up
Yes, look at the schematic, to me, it's the most logical place to 1/2 split the amp. If the hum/hiss is gone, or there, you've pretty much narrowed it down to 1/2 an amp.
Ok, I grounded C9 and the hum was all but gone. Faint, but certainly livable. So it looks like it's in the pre-amp section before the PI, eh? I'm open to suggestions on ways to divide and conquer further.
Again, I sure appreciate everyone's help. I'm learning a great deal :)
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Is your unused input jack grounding as it should? Just trying to think of some of the obvious that has bitten me in the past.... :help:
Jim :icon_biggrin:
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ways to divide and conquer further
re-connect C9, and break the circuit at the treble pot wiper/R20. gator-clip ground to the "bottom" of R20 where it used to connect to the treble wiper. you'll need V1 in, but you can leave V2 out. Should even be able to plug something in, but I'm guessin your problem don't care if something's plugged in.
All this, *might* help, but if your gremlin is living in the B+/ground hole.......
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Thanks, Shooter. I'll give that a try (tonight if I can break away) and report back.
ways to divide and conquer further
re-connect C9, and break the circuit at the treble pot wiper/R20. gator-clip ground to the "bottom" of R20 where it used to connect to the treble wiper. you'll need V1 in, but you can leave V2 out. Should even be able to plug something in, but I'm guessin your problem don't care if something's plugged in.
All this, *might* help, but if your gremlin is living in the B+/ground hole.......
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Ok, I reconnected C9, broke the circuit at treble wiper/R20, grounded the bottom of R20 and powered it up. I had V1 in and left V2 out. Same hum.
I then did the same with the normal channel. I broke the circuit at R19/Volume 1 wiper, grounded R19 (top) and installed both V1 and V2. Same hum.
In both cases, the hiss is still there, though with R20 grounded, the hiss is only present as I turn up volume 1, though not as loud a hiss as in the Top Boost side with the treble up.
Thanks, Shooter. I'll give that a try (tonight if I can break away) and report back.
ways to divide and conquer further
re-connect C9, and break the circuit at the treble pot wiper/R20. gator-clip ground to the "bottom" of R20 where it used to connect to the treble wiper. you'll need V1 in, but you can leave V2 out. Should even be able to plug something in, but I'm guessin your problem don't care if something's plugged in.
All this, *might* help, but if your gremlin is living in the B+/ground hole.......
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K had asked you to try sub-ing DC volts for your filaments, I didn't see a reply on that, I did see you tried the cap he asked about.
I grounded C9 and the hum was all but gone. Faint, but certainly livable
the hiss was gone?
My guess is the hiss is probably bleeding in from V1's associated pieces parts, the hum, I'm still leaning to filaments, unless it's still there using a 6vdc battery to feed the filaments
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K had asked you to try sub-ing DC volts for your filaments, I didn't see a reply on that, I did see you tried the cap he asked about.
No, I don't have a DC supply. I'll try to borrow one today and give that a try.
I grounded C9 and the hum was all but gone. Faint, but certainly livable
the hiss was gone?
Yes, the hiss was gone.
My guess is the hiss is probably bleeding in from V1's associated pieces parts, the hum, I'm still leaning to filaments, unless it's still there using a 6vdc battery to feed the filaments
I ordered an 8 Ohm dummy load, which should be here Wednesday. Once I get that I'm going to build Doug's "listening amp" front end and plug it into a spare solid state head I have and try to determine which part is hissing.
Regarding the filaments, would offsetting them with DC give the same affect (i.e. tying the center tap for the filament supply to a DC source I'd have to create)?
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No, I don't have a DC supply. I'll try to borrow one today and give that a try.
A 6V lantern battery usually works well. You can pick one up at the checkout counter in Lowes.
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No, I don't have a DC supply. I'll try to borrow one today and give that a try.
A 6V lantern battery usually works well. You can pick one up at the checkout counter in Lowes.
Thanks, Sluckey. I'll do that.
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No, I don't have a DC supply. I'll try to borrow one today and give that a try.
A 6V lantern battery usually works well. You can pick one up at the checkout counter in Lowes.
I picked up a lantern battery.
Before I do this test, just so I understand, I'm going to disconnect the 6.3V tap from the PT to the heater, tie them off temporarily (i.e. isolate them), then connect my 6V battery across the heater wires on the amp. Then power it up.
The assumption is that the heaters dont' care whether they are seeing 6.3V AC or 6V DC across them, correct?
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That's all correct.
You have a lot of tubes on that amp so they may drain the battery pretty quick. Only leave it connected long enough to tell if hum improves/disappears. In, fact, just to lighten the load, I'd pull a pair of the EL84s before even hooking up the battery. Get an idea of the hum level by ear and also measure the AC hum voltage across the speaker for a more accurate reference level.
Then disconnect the two PT filament leads and use some gator clip leads to connect to the battery (polarity doesn't matter). Leave one clip off the battery and turn the amp power switch on to allow the B+ to come up. Then connect the other clip lead to the battery and allow the tubes to warm up. If the filaments were contributing to the hum level then the hum sound level should be less and the hum voltage reading should have decreased.
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That's all correct.
You have a lot of tubes on that amp so they may drain the battery pretty quick. Only leave it connected long enough to tell if hum improves/disappears. In, fact, just to lighten the load, I'd pull a pair of the EL84s before even hooking up the battery. Get an idea of the hum level by ear and also measure the AC hum voltage across the speaker for a more accurate reference level.
Then disconnect the two PT filament leads and use some gator clip leads to connect to the battery (polarity doesn't matter). Leave one clip off the battery and turn the amp power switch on to allow the B+ to come up. Then connect the other clip lead to the battery and allow the tubes to warm up. If the filaments were contributing to the hum level then the hum sound level should be less and the hum voltage reading should have decreased.
Thanks. That's really helpful. The hum is less noticeable with 2 tubes pulled, so I may just leave all 4 in and hope the battery holds out long enough for my test.
I have a Standby switch, so that needs to be set to ON to let the B+ come up. Looking forward to seeing what happens.
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Just to check... The AC wire - switches is far, but can induce some Hmmm? maybe.
And the white wire,comes from D, it passes side the C6 47p Silver Mica? try move with wood stick if, change
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Just to check... The AC wire - switches is far, but can induce some Hmmm? maybe.
And the white wire,comes from D, it passes side the C6 47p Silver Mica? try move with wood stick if, change
Vincentebd, I'll try moving the white wire with a chopstick. I have poked around quite a bit with the chopstick to move wires and haven't had any change, but I can check this specifically.
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I tried out the 6V battery (following your instructions, Sluckey). Before disconnecting the PT heater supply I measured 30 mv AC on the speaker output. So when I desoldered the heater wires and attached the battery, I was able to get the pilot light to light, but I don't think the heaters on the pre-amp or power tubes were lit. They were not glowing, and no sound. I tried removing the pilot light and just running the tube heaters off the battery, but again, it was quiet. I couldn't really see any glow in the tubes. Any ideas?
FWIW - I looked at the wire vincentebd mentioned and shortened that. I also noticed the ground bus on input #1 was not well-soldered (I could move it). I thought that might be the smoking gun, but re-flowed the solder and didn't notice anything major. That said, with that fix and shortening some of the connections to V2, the AC hum on the output is closer to 26 mv Ac now.
Interestingly enough, if I dial the Bass to 10, I see 26 mv AC, when I dial it back to 3, it's more like 7 mv and the hum is negligible, but set the bass to 0 and the hum is gone and I see like 1 mv AC.
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if I dial the Bass to 10, I see 26 mv AC
I'm not qualified here to know for sure, but wouldn't that indicate the bass pot is working, at 0 it cuts the low freq signal - hum, at 10 it lets the hum thru?
Did you monitor your battery volts under load?
Do the tube light re-connecting AC?
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if I dial the Bass to 10, I see 26 mv AC
I'm not qualified here to know for sure, but wouldn't that indicate the bass pot is working, at 0 it cuts the low freq signal - hum, at 10 it lets the hum thru?
Did you monitor your battery volts under load?
Do the tube light re-connecting AC?
Well, the bass control certainly brings out the bass in the signal correctly. When I play a guitar through it, when I increase the bass, the tone darkens and fattens up (typical bass control) and backing it down brightens up the tone (less bass). It's just weird how it lets the hum through - I don't understand that.
With the battery connected, I only saw about 3.5V DC across the heaters.
When I reconnect AC, yes, the tubes light and function just fine.
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You have a lot of tubes on that amp
hence the 3.5vdc, which as you seen, won't cut it. I have a couple rechargeable 6vdc batteries outta emergency lighting signs,(maybe hunt down a maintenance guy at your work and see if they might "loan" you a couple) they're pricey initially, but you can re-charge them and they do come in handy.
the bass pot is a low freq filter, passing or blocking them, hum is low freq, so it makes sense to me, now whether this is a symptom or cause is yet to be determined.
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You have a lot of tubes on that amp
hence the 3.5vdc, which as you seen, won't cut it. I have a couple rechargeable 6vdc batteries outta emergency lighting signs,(maybe hunt down a maintenance guy at your work and see if they might "loan" you a couple) they're pricey initially, but you can re-charge them and they do come in handy.
the bass pot is a low freq filter, passing or blocking them, hum is low freq, so it makes sense to me, now whether this is a symptom or cause is yet to be determined.
I'm going to see if I can scare up a variable DC power supply.
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temporarily split the AC filament rail from the preamp tubes and use the battery for just the preamp tubes or just run 2-3 batteries in parallel.
if you split the filament rail, the hum in the power tubes will cancel. preamp bottles are usually the source of the hum as most are running in class A.
--pete
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temporarily split the AC filament rail from the preamp tubes and use the battery for just the preamp tubes or just run 2-3 batteries in parallel.
if you split the filament rail, the hum in the power tubes will cancel. preamp bottles are usually the source of the hum as most are running in class A.
--pete
Thanks, Pete. That should be pretty straight forward to test. I'll give that a try and report back.
Mark.
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I'm not qualified here to know for sure, but wouldn't that indicate the bass pot is working, at 0 it cuts the low freq signal - hum, at 10 it lets the hum thru?
Well, the bass control certainly brings out the bass in the signal correctly. When I play a guitar through it, when I increase the bass, the tone darkens and fattens up (typical bass control) and backing it down brightens up the tone (less bass). It's just weird how it lets the hum through - I don't understand that.
Hey, shooter. My bad - I originally misread your first reply. For some reason I read the bass pot was not working, which is obviously not what you said! So my response kind of overstated the obvious - which sounded kindof stupid. Sorry! lol
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I originally misread your first reply
I'm dyslexic so it's NBD :icon_biggrin:
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Ok, I split off the 3 pre-amp tubes and hooked their heaters up to my 6V battery. The amp worked, somewhat less hum audibly, but it measured higher (120 mv AC) which is puzzling. I hooked them back up to AC heater tap and now measure more in the 30 mv AC range at the speaker...so that was kind of inconclusive.
Meanwhile, I've dressed the leads a bit (shortened a few) and re-routed the half power switch wires a bit. I tried the following:
- unbolted the power supply filter board and stood it up some to get it further away from the chassis (and heater wires): No difference
- unscrewed the half power switch and stood that way up (way out of the way of everything): No difference
Other observations:
- I notice if the room is quiet, if I turn on power and have the Standby switch in Standby I do still hear hum. Just louder when I turn the Standby to On (to turn on B+) as the tubes warm up. Just interesting it hums faintly when in standby
- Moved a lot of wires with a chopstick, no difference.
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Try the amp in one other room or, better, in one other building
K
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Try the amp in one other room or, better, in one other building
+1
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Thanks - I'll try that. Should be interesting....I'd love it if it was just the room (though that's my workshop!)
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Are you hearing your PT humming when AC power on? Keep the room quite and put you hand on the PT with the AC power only and listen if the hum matched any transformer vibration.
Also, I have had this happen where I did something and messed up the 100K artificial CT. Measure them, but I don't think this is your problem since the battery did not change the hum much, but check them anyway.
And yes, it could be something in the main AC line.
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Are you hearing your PT humming when AC power on? Keep the room quite and put you hand on the PT with the AC power only and listen if the hum matched any transformer vibration.
Also, I have had this happen where I did something and messed up the 100K artificial CT. Measure them, but I don't think this is your problem since the battery did not change the hum much, but check them anyway.
And yes, it could be something in the main AC line.
Ed, the PT has it's own CT for the heater tap, so there's no 100K artificial CT.
I may be wrong in my understanding, but I thought the pair of 100k resistors was only used to provide an artificial CT when the heater secondary doesn't have one, right?
I'll put my hand on the PT and see if I can feel it hum.
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Ed, the PT has it's own CT for the heater tap, so there's no 100K artificial CT.
I may be wrong in my understanding, but I thought the pair of 100k resistors was only used to provide an artificial CT when the heater secondary doesn't have one, right?
Yes, use 1 or the other, not both.
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I'm sure y'all mean 100Ω and not 100K, right? :wink:
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I'm sure y'all mean 100Ω and not 100K, right? :wink:
:BangHead: :cussing: Why do I keep missing little things like that??????
(I'm gonna have to read slower........)
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I'm sure y'all mean 100Ω and not 100K, right? :wink:
:BangHead: :cussing: Why do I keep missing little things like that??????
(I'm gonna have to read slower........)
lol. I knew what you meant ;)
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when the heater secondary doesn't have one, right?
right, like exclusive OR gate, 1 or the other, but not both.
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I tried it in another room and it's the same level of hum. Rats....
I've been reading some things about a star ground. I'm wondering if that might be a good thing to try? I'd have to isolate the input jacks, which is fine, I have some Cliff jacks. Any thoughts?
Actually - can't I assume the hum is being induced before the end of the tone stack since the Bass control will filter it out when rolled all the way off?
Mark.
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Mark, I've been following but chiming in. IF it's not too difficult, and if you have enough loose wire, try unbolting the OT and moving it further away from the PT. I can't really believe that the MM would be that susceptible to hum, but you never know. I only say to try this since I chased hum in my last build for days, and tried the shield between too, until finally I unbolted the OT and moved it away, and presto hum gone. Replaced the OT (it was old and unshielded, mind) with Doug's little SE and it's quiet as a mouse.
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Mark, I've been following but chiming in. IF it's not too difficult, and if you have enough loose wire, try unbolting the OT and moving it further away from the PT. I can't really believe that the MM would be that susceptible to hum, but you never know. I only say to try this since I chased hum in my last build for days, and tried the shield between too, until finally I unbolted the OT and moved it away, and presto hum gone. Replaced the OT (it was old and unshielded, mind) with Doug's little SE and it's quiet as a mouse.
Thanks, John. I've been stewing about whether to try that. I have a spare set of MM trannies for this amp (the plan was to build 2). If I don't have enough room to unbolt this one I can pull this one and wire up the spare and try moving it around.
Along with that, I keep wondering about the orientation of the PT. Unfortunately I don't have enough room front-to-back to mount it with the bells facing front/back. In TUT3 they talk about the bell side causing the most magnetic coupling...but when I think about that I think, the way it's mounted now, really the most magnetic effect is just on the rectifier tube, so maybe that's moot.
Anyhow - I'll try unbolting (or temporarily replacing) the OT and moving it around.
Thanks!
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Since the PT and OT are standup types just unbolt both so you can turn both of them. You probably can't rotate either very much but just moving one or the other or both may tell you if the hum decreases.
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Since the PT and OT are standup types just unbolt both so you can turn both of them. You probably can't rotate either very much but just moving one or the other or both may tell you if the hum decreases.
That's a great idea, sluckey. Should be a quick test to at least see if there is any difference before re-wiring. Thanks!
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I turn on power and have the Standby switch in Standby I do still hear hum
This statement is bugging me, How does the skr get hum with no B+?
Can the tube rec somehow induce "noise" into, what, ground?
Could the rec tube be replaced with 2 diodes that could for testing just be "clipped in"?
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How does the skr get hum with no B+?
It's why I sort of suspect the PT/OT hum thing. On the one I had problems with, it was immediate; flick the power switch onhum, just like that.
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I turn on power and have the Standby switch in Standby I do still hear hum
This statement is bugging me, How does the skr get hum with no B+?
Can the tube rec somehow induce "noise" into, what, ground?
IIRC, when guys do the head phone test to locate where the quietest place is to mount the OT they only hook up the primary on the PT and tape off the PT secondaries.
No load on the PT, it's just 'energized' to get the PT to put out it's magnetic field. Then the head phones are hooked up to the OT secondary to listen for what the OT is picking up from the PT's radiated magnetic field.
Could the rec tube be replaced with 2 diodes that could for testing just be "clipped in"?
I don't think 'clipping' in diodes on the high voltage B+ is safe.
But, you could tack solder them across the rectifier tube socket (leave the rectifier tube out) to do a quick test.
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I turn on power and have the Standby switch in Standby I do still hear hum
This statement is bugging me, How does the skr get hum with no B+?
Can the tube rec somehow induce "noise" into, what, ground?
Could the rec tube be replaced with 2 diodes that could for testing just be "clipped in"?
I actually tried a Yellowjacket solid state rectifier that fits into the tube socket and it didn't make any difference in the hum. Isn't that basically the same thing as soldering in diodes?
(it seemed to run the chassis hotter temperature-wise...which is weird)
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I actually tried a Yellowjacket solid state rectifier that fits into the tube socket and it didn't make any difference in the hum. Isn't that basically the same thing as soldering in diodes?
Yes.
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I also think it's the proximity/orientation of the transformers.
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I turn on power and have the Standby switch in Standby I do still hear hum
This statement is bugging me, How does the skr get hum with no B+?
mutual inductance. think about it...it's just magnetic fields. one inductor excites the other. the second inductor just happens to excite another inductor that's a connected speaker.
--pete
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mutual inductance
I was at work when the light bulb went on :icon_biggrin:
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Just for knowledge
http://home.comcast.net/~machrone/bjr/bjrhum.htm (http://home.comcast.net/~machrone/bjr/bjrhum.htm)
Franco
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Just for knowledge
http://home.comcast.net/~machrone/bjr/bjrhum.htm (http://home.comcast.net/~machrone/bjr/bjrhum.htm)
Franco
Very very interesting...
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this is getting discouraging.... I'm seriously wondering if this is just the nature of an AC 30 style circuit??
- I unbolted the transformers and rotated them, moved them around, but no difference in the hum.
QUESTION: given there is no variation, is it worth removing the OT and wiring in a new one and playing with the location? I mean, it would give me more room to move it AWAY from the PT.
- I took a copper license plate (AZ vintage plates are solid copper) and tried placing it all over, between the transformers, over them, etc. No difference. Makes me wonder if it's not related to magnetic coupling??
- Out of curiosity I tack soldered another 8 uf cap parallel with the cap in the E supply (doubling the capacitance). No difference. I then tack soldered it parallel with the 20 uf "C" cap just for grins. No diff.
- It's funny...yes, you hear it when B+ is off, but it's way fainter than when you turn the standby switch on. Then it becomes more annoying.
This is really got me stumped. Am I making something out of nothing? I've never built an amp that hums like this. Again, it's faint, you only hear it when you stop playing and the room is quiet.
Mark.
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If there was no difference moving around the trannies, then I doubt a different one would make a difference at all. You should have been able to hear *something*.
it's faint, you only hear it when you stop playing and the room is quiet.
Maybe you're just being too picky then. :icon_biggrin: After all, when you stop playing there will be applause to cover up the hum.
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Actually, John, I'm kind of getting to that conclusion. I may be oversensitive and it's just the nature of this design (like O'Conner says in TUT3).
That said, I need to move on to the hiss. I'm working on a listening amp today - I've narrowed it down to something between the first and second pre-amp tube. If I pull tube 1, no hiss. If I re-insert tube 1 and pull tube 2, hiss.
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Did you tied to rotate the PT and OT of 90° eachother (in the other way) ?
K
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I've built a couple amps with the PT and OT oriented just like yours, got it from TUT 3. Their cores are 90 degrees from each other, just like yours. No noise problems.
Merc Mag. makes very good transformers, I doubt the transformers themselves are the problem. AND your PT/OT both have steel end bells that shield them.
I think it's either the Vox power supply or your layout and lead dress or a little of both.
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Did you tied to rotate the PT and OT of 90° eachother (in the other way) ?
K
I didn't. I could get them both about 45 degrees, but they were offset (not lined up). See pic's - that's all the room I had before the wires stopped me
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I built that cable front end for the listening amp that Doug has on his 'Tools' page. I plugged it in to a little solid state marshall practice amp and started poking around searching for the hiss. I guess this doesn't surprise me, but I hear it start at V2 where pins 2 and 6 tie together on my schematic (pins 1 and 7 on Doug's). It's the plate of the A side to the grid of the B side of the tube. Not very conclusive as I've already replaced the 100k (R12) with a Vishay/Dale metal film resistor in case the his was coming from the carbon comp's. I don't hear hiss on the grid or cathode of the A side. Anyhow, I did some reading and someone suggested rarely the 10K metal film resistors in the power supply dropping string can cause hiss, so I swapped out the 10K in the "D" supply. No difference...still have a hiss.
Here's an observation, though, which may or may not mean anything. When I touch the probe of my listening amp to the D supply resistor, there's a definite hum. Poking a listening amp probe at the DC supply may be a no-no, I don't know. Just thought it was interesting re the hum.
I think at this point I can live with the hum, but I really need to do something about this hiss. (...I maybe should start a different thread on the hiss?)
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Put a gator-clip from V2 grid to ground, I know the pot *should* do that but.... re-probe/listen to pin 1/7, hiss?
Haven't looked at the listnin amp but I'm guessing there is a cap between probe tip and amp, that "should" block DC from the power rail, but I wouldn't recommend headphones :icon_biggrin:
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I don't remember, did you tried other preamp tubes ?
A friend, some time ago, solved a hiss problem that way
Franco
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I don't remember, did you tried other preamp tubes ?
A friend, some time ago, solved a hiss problem that way
Franco
I swapped them out when I was working on the hum, but i can swap them out - I have a few sets. I'll try that.
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Put a gator-clip from V2 grid to ground, I know the pot *should* do that but.... re-probe/listen to pin 1/7, hiss?
Haven't looked at the listnin amp but I'm guessing there is a cap between probe tip and amp, that "should" block DC from the power rail, but I wouldn't recommend headphones :icon_biggrin:
Shooter - last night I swapped out some of the carbon comp resistors around V2 and actually this morning poked around with the listening amp and no longer hear the hiss prevalent in V2. I grounded the grid of V2 with a clip lead, with or without the grid grounded now I don't hear anything worth mentioning. Now I hear it just at the plate side of the PI, which seems peculiar. Not on the grids of the PI, but on the plates. I may try some different PI tubes to see if it changes anything.
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I think you're in the home streach now, pretty soon you'll be building the 2nd one, bug free :icon_biggrin:
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I think you're in the home streach now, pretty soon you'll be building the 2nd one, bug free :icon_biggrin:
Thanks, shooter. I sure hope so! but I'm not out of the woods yet, unfortunately.
I thought I had narrowed it down to the PI, but I started tracing the hiss again with the gain way up on my listening amp. I've traced it back quite a bit, actually. Seems like the 500pf cap, C3, has his on the output side of the signal (toward the volume) but not so much on pin 1 of V1-B (or pin 6 if you use Doug's original schematic). So it's like the plate is somewhat quiet, but after that 500pf cap there is more hiss. The difference is faint (and there's enough 60 hz hum in my listening amp from stuff in my shop) that i wonder if that could be the issue, or perhaps V1 or the 33k resistor on the input...but then again, why fainter on the input side of the cap if it's not the cap??
So I ended up finding a 500pf silver mica cap, swapped it in and still have the hiss. I swapped in a different 12AX7 for V1, granted just one to see, but still same hiss.
Which makes me wonder...the 33k (R4) is a carbon film, but I do have a Vishay/Dale metal film I could put in there so I'll try that.
Is it possible the tube socket could cause noise? I'm using these porcelain style...bad idea??
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Is it possible the tube socket could cause noise? I'm using these porcelain style...bad idea
I have had issues with the ceramics, you should be able to "roll the tube around" and hear something. I "rock" the tube in a circular motion while monitoring, be carful!! the tubes are hot and ceramic isn't very flexable.
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I actually swapped out ALL the remaining carbon comp resistors, and the 33k carbon film on the input of V1 pin 2 with Dale metal films. Still hissing. I experimented with more pre-amp tubes, still have the hiss.
I've ordered some non ceramic tube sockets and plan to swap those in tonight. I hope that does it because I"m running out of ideas....(and have pulled most of my remaining hair out!) Wonder if it's worth posting this on Ampgarage too, just for the heck of it?
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Wonder if it's worth posting this on Ampgarage too
As long as you warn them ahead of time, your "problems" have infected my build :sad2:
I might have to report you to the CDC :l2:
Although to be fair , I only caught the hummmm
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Wonder if it's worth posting this on Ampgarage too
As long as you warn them ahead of time, your "problems" have infected my build :sad2:
I might have to report you to the CDC :l2:
Although to be fair , I only caught the hummmm
No way! Your build is humming? :BangHead:
I'll gladly give you some hiss if you like :P I have little hope at this point that new sockets are going to do the trick...
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Ya know, I was looking at your pictures again. The only thing I really see is the B+ wires running pretty close to your pots. Maybe if you routed them up along the top side of the board instead?
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It's worth a try, I guess. Would that induce hiss?
I guess this begs the bigger question. What exactly is hiss and what can cause it? From what I've been seeing on the interwebs (as Strongbad would say), it looks like biggest causes are (from this link http://www.geofex.com/ampdbug/hiss.htm (http://www.geofex.com/ampdbug/hiss.htm) ):
1) tubes (I've tried enough pre-amp tubes to satisfy me that it's not a tube, unless they ALL hiss)
2) carbon comp plate resistors (I've replace literally all the carbon comp's with metal film (but left 2 carbon film cap in the bass and on intput 1 which is not hissing))
3) noisy resistors in the "B+ decoupling string, often around 10k in value" (as a shotgun approach, I swapped out the 10K metal film with another)
Those are some "common causes", but what exactly is it? Why would it happen when there is no input signal (i.e. the input jack is grounded and nothing is plugged in)? Some phenomena is "generating" it? Is it that whatever "noise" is being picked up in the circuit, this "signal" passes through a component that somehow induces the hiss?
I really thought a systematic approach with a listening amplifier would allow me to pinpoint it. But it's basically very faint but present at the plate of V1 (the half that has the tone stack) and gets amplified greatly at the cathode follower. But where is it COMING from?! :BangHead:
But alas...I digress....
John, I'll try moving those lines when I swap the sockets. I stuck them down there because the wire I had has such silly big insulation. I'll see if I can scare up some other wire. On the D'clone builds I do I run them along the top of the main board...but on AB763 style builds I run them under the main board and along the front edge...but not near the pots...
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Mark, *as far as I know* those wires wouldn't induce hiss, just hum. I've never had to chase a hiss, myself.... yet! I'm sure I will though. I only mentioned those wires because I had a similar hum I was trying to track down on my push pull amp, and it turned out I had run the B+ wire too close to a grid resistor in the preamp. What makes trouble shooting so tough (especially for me) is it can be so many things.
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What exactly is hiss
I think it's something like describing color to a blind man :dontknow:
When I look at "hiss" on a scope I see a complex set of frequencies like looking at white noise. I'm not gonna re-read everything, sooo, have you tried clipping caps from plate to ground, or plate to cathode?
If not start around 500pF and move up to maybe .01uf. These WILL have effect on amp tone.
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What exactly is hiss
I think it's something like describing color to a blind man :dontknow:
When I look at "hiss" on a scope I see a complex set of frequencies like looking at white noise. I'm not gonna re-read everything, sooo, have you tried clipping caps from plate to ground, or plate to cathode?
If not start around 500pF and move up to maybe .01uf. These WILL have effect on amp tone.
I haven't tried the cap idea, but I want to do that here soon. I know in the D-clone style amps, there is often a 330 pf to 220pf cap across the second gain stage plate and cathode. Might be worth playing around with that.
I've replaced 2 of the 3 ceramic sockets with plastic style (yeesh...these things are getting more cheaply made as the years go on!) and have re-routed the B+ supply wires to the top of the board, but it still hisses like a wild banshee...so tomorrow it's 500pf caps from plate to cathode!
Questions: if I want to temporarily bypass parts of the tone stack, would I ground the input to the cathode follower and jumper the wiper on the Volume pot to R20?
Or could I just jumper C6 to R20 and leave the cathode follower in, but bypass the tone stack?
Mark.
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I would try it this way and see
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I would try it this way and see
Ok - so actually desolder where the X's are, or wouldn't a jumper from X to X cause the signal to take the path of "least resistance" being the jumper? I'm just curious - desoldering is no problem, just trying to learn here ;)
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I'm just curious
You're right in a shorted wire "should be" fine, I personally would un-solder, just so when I fall asleep I can be certain the TS was physically removed. AC doesn't play as well as DC, I have found it wandering on grounds that measure fine in a DC world.
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Well, not much luck. Here's what I've tried.
- I bypassed the tone stack (I left it in place, I just wanted to try it quick) with a clip lead. Hiss worse, I suspect because of the additional gain due to no tone controls
- experimented with a 500pf cap across V1 cathode and plate (the section feeding the channel with the tone stack). Still hisses the same though I didn't measure it, just listening
- same cap across V3 ( the PI ) no difference
(I need to mess around with more caps, just haven't had a lot of time)
- I replaced the 4 1.5K carbon comp's on the EL84's with metal film. No difference in the hiss
- I replaced all 3 pre-amp tube sockets with non ceramic. No difference in the hiss
- I re-routed some wires and did some lead dress improvements.... No difference
- I noticed V1 was pretty microphonic, so I grabbed a JJ Tesla and put that in. No difference in hiss, but less microphoncs
So I'm stumpped guys. This is getting discouraging.
I was wondering about the El84's bias setting, just wondering if they're running too hot. I split the pairs out and am using 120 Ohm resistors, one on each pair. I measure ~11 VDC across the resistor. Is that about right? That's like 95 ma/pair. This is idling.
Plates on the EL84's are at 355 VDC.
FWIW - The Plate voltages on the other tubes are sitting at 155 V for V1, 192 on pins 2&6 of V2 (the cathode follower), 302V for V3 the PI.
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That's like 95 ma/pair
I'd call that good, I've built 4 amps with 84's, self biased and I shoot for 45-50mA/tube, with plate V's around 250.
Did you try and set the amp on a steel plate, surface? When I did that, my hiss went from bad to completely acceptable. I tried an aluminum plate and that worked also, so I cut one out, hope to install it soon.
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That's like 95 ma/pair
I'd call that good, I've built 4 amps with 84's, self biased and I shoot for 45-50mA/tube, with plate V's around 250.
Did you try and set the amp on a steel plate, surface? When I did that, my hiss went from bad to completely acceptable. I tried an aluminum plate and that worked also, so I cut one out, hope to install it soon.
So you set the amp on a steel plate?? Interesting. I'll have to try that, but why would that affect hiss I wonder?
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So you set the amp on a steel plate?? Interesting. I'll have to try that, but why would that affect hiss I wonder?
It's shielding. It helps greatly to reduce any air born junk from getting into the circuitry.
All the old Fender BF amps had a piece of steel window screen stapled onto the cabinet above the chassis opening. (I don't know if the brown face and tweed had this also.) Later Fenders had a solid piece of thin metal stapled in the same place.
Same exact concept as shielding a guitars control cavity.
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I see. I've done that on other builds to cut down on buzz, but I thought hiss was more thermal reactions in the components. I'm going to get some sheet metal and see if I can make any difference in this.
Anyone messed around with testing the outside foil on coupling caps? Probably grasping at straws, but I can't get it out of my head that something inside the amp is literally generating this noise. I'm going to put a master volume in which will help keep the noise floor down, but it's just so bad I wanted to reduce it before adding the mv.
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Anyone messed around with testing the outside foil on coupling caps?
Yes, look at this thread link started by Tubenit;
http://el34world.com/Forum/index.php?topic=11427.0 (http://el34world.com/Forum/index.php?topic=11427.0)
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Anyone messed around with testing the outside foil on coupling caps?
Yes, look at this thread link started by Tubenit;
http://el34world.com/Forum/index.php?topic=11427.0 (http://el34world.com/Forum/index.php?topic=11427.0)
Good stuff. I read the article on Aiken the other day and printed it out. As my last ditch attempt (...now second to last ditch since I'm gonna get a big sheet of steel) I'm going to go through, desolder all the caps, test/mark outside foil, and re-install them. I'll work on that tomorrow...I need beer...
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Have a look at this, if you haven't yet, it's on grounding schems;
http://el34world.com/Forum/index.php?topic=18828.msg193010#msg193010 (http://el34world.com/Forum/index.php?topic=18828.msg193010#msg193010)
And, I only see 1 shielded piece of wire, might be more?
Grid wires need to be as short as possible or they need to be shielded to keep out noise.
Any signal wire AFTER the plate coupling cap is now a grid wire. So wires after the plate coupling cap going to a volume/MV control then going to the next tubes grid might need to be shielded, also the TS wires after the TS caps as these caps take the place of the plate caps and after the PI coupling caps. These are all common places you'll find shielded wire runs.
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Have a look at this, if you haven't yet, it's on grounding schems;
http://el34world.com/Forum/index.php?topic=18828.msg193010#msg193010 (http://el34world.com/Forum/index.php?topic=18828.msg193010#msg193010)
And, I only see 1 shielded piece of wire, might be more?
Grid wires need to be as short as possible or they need to be shielded to keep out noise.
Any signal wire AFTER the plate coupling cap is now a grid wire. So wires after the plate coupling cap going to a volume/MV control then going to the next tubes grid might need to be shielded, also the TS wires after the TS caps as these caps take the place of the plate caps and after the PI coupling caps. These are all common places you'll find shielded wire runs.
Thanks, Willabe. This is good stuff. I've read GW's stuff about grid wires being short. I think he said plate wires should be on the long side (?). I added a shield on the volume to V2 wire. I may add others. I appreciate the explanation "any signal wire after the plate coupling cap is now a grid wire". Good point. I'm going to keep hacking ;)
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I think he said plate wires should be on the long side (?).
I think your right on that, if he did, he gave an explanation as to why.
Think about it this way.... Grid wires act like an antenna and pick up air born hash/noise/radio signals, etc. Plate wires tend to reject them, it's an impedance thing.
So, we want to go from point A (plate) to point B (grid). And the tube sockets are on the back side of the chassis. But we have a volume control in front of the 2nd/next tubes grid and it's on the face of the chassis. So we have to go from the 1st triodes plate/back of chassis, to the volume control/front of chassis then back to the grid of the 2nd triode. Depending on the chassis and where the tube socket(s) and volume control are located AND where we put the coupling cap, now we have 6", 8", maybe 10" of grid wire. Could be a problem.
If we mount 1 end/far end/AC side of the coupling cap directly to the pot then we don't need shielded wire going to the pot, we only need it going from the pots wiper to the 2nd triodes grid.
So it's that were trying to put the coupling cap closer to point B and further away from point A in our layout. But you can't always do it like that if you have a pre made eyelet/turret board. So just use shielded wire if you need to.
Some guys hang/mount the TS (tone stack) caps on the tone pots for the same reason.
Remember back in the 50's/60's, when most of the classic amps were designed, they didn't have anywhere near the air born 'hash' we have now. There we no light switch dimmers, florescent lights were much less prevalent, if at all yet, PC's/lap tops, cell/smart phones, things like that. (Also AC power grid line noise was very low compared to now. They just didn't have the amount of traffic and devices plugged into the AC grid.)
Plus they weren't building hi gain amps either, so they didn't have to deal with shielded wire for the most part. A good steel chassis with some type of shielding over the back/open side of the chassis was enough. :icon_biggrin:
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I don't know about that! All the neon beer signs in the bar would make quite a racket when the back of the stage was next to the front window! :laugh:
Jim
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:laugh: That's true, I forgot about those. :icon_biggrin:
But at least we'd have that beautiful neon signage to look at!
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great write up, Willabe! Thanks :)
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Yeah Willabe, thanks! I read your post and changed the layout for my re-wire job I'm doing a little bit to get the coupling caps close to the pots.
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I'm checking the outside foil on all the caps now. Wondering, in the tone stack how do you orient the outside foil? Most descriptions talk about coupling caps (outside foil toward the plate) and caps to ground (outside foil toward ground), but what about in the tone stack??
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Ok, I've done all the following and it still hisses like a mad man:
- I went through all the caps, desoldered them, checked for outside foil and re-installed them. (only the orange drops, that is).
- I tried replaced the input jacks with isolated jacks (and rerouted the grounds to accommodate)
- I actually went through and did more of a star ground while I was at it.
- I changed out the 47pf cap for a larger 120pf cap to see if it toned down the hiss. nope
- for grins I attached that 120pf treble cap at the pot. nope
- I took the 500pf cap right after the plate of V1 and soldered on another 500pf to double it just to experiment. same thing
Nothing seems to touch this hiss.
I was reading something Tubenit wrote about hiss years ago - he mentioned increasing the treble cap, and some other ideas like shunting high freq to ground. I need to go back and re-read that to try some things 'cause noffin is working yet.
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other ideas like shunting high freq to ground
That's what you did by running caps from plate to ground or cathode. They can also be paralleled with the plate R.
Did you try hanging a BIG cap - 100-250uF - cathode to ground on both pin 3 and 8?
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Does this amp follow Hoffman's schematic? IE, it has a normal and a top boost channel? Is the normal channel hissy?
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Does this amp follow Hoffman's schematic? IE, it has a normal and a top boost channel? Is the normal channel hissy?
Sluckey - Yes, this is his schematic except for the half power switch.
The top boost channel is hissy. The normal channel is real quiet!
That being said, I'm wondering what happens if I play around with the first coupling caps. The normal channel has like a .047uf I think, while the top boost has a 500p followed by what is effectively a "bright switch" cap of 100p across the volume. I mean this amp is SILLY bright, I wonder about modifying these caps.
:think1: This is off topic - but what's with the 0.1uf cathode bypass cap in the top boost?
1) Normal cathode bypass caps in the first gain stage are like 25uf, 50uf, 10uf. If I understand correctly, the smaller the cap value the more high frequency will be allowed to pass through the signal path. Maybe 0.1uf is just too small? What if I put something more fenderish in there, like 25uf or 5uf cap? Maybe it would tame it down??
2) Why is this cap non polarized, is it just that it's such a small value? I've got a big honkin' 0.1 600v polyester in there!
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other ideas like shunting high freq to ground
That's what you did by running caps from plate to ground or cathode. They can also be paralleled with the plate R.
Did you try hanging a BIG cap - 100-250uF - cathode to ground on both pin 3 and 8?
Shooter - I didn't try a big cap on both 3 and 8 to ground. I'll give that a try. I'm not sure if i have anything that big, I juse used like 220 pf. I may have some 100uf's left over from bias circuits. Did you mean pins 3 and 8 of V1 or V2?? And from 3 and 8 to ground? So I'm basically increasing the size of the already present cathode bypass cap as a test, yes?
Thanks!
Mark.
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Remove the .1 cap from V1B pin 8. Also remove the 22µF from V2A pin 3. Replace the 220K resistor on V1B pin 6 with a 100K.
All this will reduce the gain of the top boost channel which will also reduce the hiss. Does this take you in the right direction?
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Did you mean
I didn't realize one channel was good. Follow Sluckey's lead, I was just looking for ANY change, so on the bad channel, v1 Big cap across the cathode R, and it can be in || with whatever other cap you have there. It should be a quick, clip n test.
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1. If I understand correctly, the smaller the cap value the more high frequency will be allowed to pass through the signal path.
2. what's with the 0.1uf cathode bypass cap in the top boost?
3. Why is this cap non polarized, is it just that it's such a small value?
1. Close, the smaller a caps value the less mid range/bass it will let pass through, so it sounds brighter by subtracting mids/bass end. Caps and resistors are passive, they can only subtract, they can't add anything.
2. The K bypass cap value is used to 'tune' the amps signal. The smaller the K bypass cap the less bass/mids that gain stage will amplify.
3. Large value caps are usually made of electrolytic construction to keep their size and price down. Large value film caps are made but their huge and way more $$. E-caps are polarized, film caps are not.
used like 220 pf.
I think you mean you used 22uF?
220pF = 0.00022uF.
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used like 220 pf.
I think you mean you used 22uF?
220pF = 0.00022uF.
No, I only used 220pf. I misunderstood shooter's original test suggestion and was thinking 100-200 picofarad from plate to grid. I may have tried 500pf as well, but nothing that big. My bad.
Thanks guys. Sluckey, Shooter, Willabe, I'll try these things tonight. I'd like to hear some difference with these big change tests so I understand how to head in the right direction.
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Hm... nothing!
I took put a 25 uf cap in parallel with the cathode resistor of V1 (the Top Boost Channel) in place of the 0.1 uf cap. Still hissed.
I soldered a 100uf cap in parallel with that, so 125uf and still the hiss was the same.
Sluckey, I removed the 0.1 uf cap, replaced the 220k cathode resistor with 100k, and pulled up one side of the 22uf on the cathode of the other half of V1 (the "normal" channel) to remove that from the circuit. Still hisses! :BangHead: It may be a touch less hiss, but it's still enough to where you could never record with this amp. If I turn the treble down to 2, it's livable, but as soon as you turn the volume up to maybe 3 or 4 it's there again. Or if the volume is down (livingroom volume, maybe 1 or 2) and the treble is up to 5 or more, it's just too much hiss.
My test case (listening) has been to run the amp with the volume on 3 and the treble on 8 (treble and volume knobs are pointing in opposite directions). This has been my test each time I make a change. If I turn the Cut control up, the last 15% of rotation of the cut silences the hiss. This has always been the case.
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and pulled up one side of the 22uf on the cathode of the other half of V1 (the "normal" channel)
Wrong cap. I said "Also remove the 22µF from V2A pin 3". Pulling the correct cap should reduce the gain of the TB channel. Hopefully hiss will go down too.
You can also try rolling a bunch of 12AX7s thru V1 and V2. If you have some 12AY7s and/or 5751s try those too.
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and pulled up one side of the 22uf on the cathode of the other half of V1 (the "normal" channel)
Wrong cap. I said "Also remove the 22µF from V2A pin 3". Pulling the correct cap should reduce the gain of the TB channel. Hopefully hiss will go down too.
You can also try rolling a bunch of 12AX7s thru V1 and V2. If you have some 12AY7s and/or 5751s try those too.
Right, that makes more sense. I did a quick check before calling it a day, lifted that cap and powered it up. The hiss level is less with that cap out. Still there but reduced due to the gain change.
I've tried a few sets of pre-amp tubes, but this weekend I thought I'd run a bunch of them in V1 and V2. I have Mullards, JJ's and EH's, as well as some old stock.
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I played the amp some last night and the hiss is under control. The amp is less gainy, obviously, I'm tempted to add gain back in in small steps until just before the hiss starts getting annoying.
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FWIW, most, if not all of the schematics with the top boost channel show a 25µF cathode cap on the first stage and no cathode cap on the second stage. The only place I've seen that 22µF cap on the second stage was on the original 'top boost' mod that was a separate module to be added to an amp. Looks like by the time the top boost channel was incorporated into the AC30, that second stage cap had been eliminated.
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FWIW, most, if not all of the schematics with the top boost channel show a 25µF cathode cap on the first stage and no cathode cap on the second stage. The only place I've seen that 22µF cap on the second stage was on the original 'top boost' mod that was a separate module to be added to an amp. Looks like by the time the top boost channel was incorporated into the AC30, that second stage cap had been eliminated.
Interesting. Just so I'm clear, you're saying you typically see 25uF in place of the 0.1 uf for C1b in the picture, and no C5? If so, that's about where I'm at.
I've never built anything based on the AC30 circuit until now, so it's all very different from the Fender/D'clone style amps I've built in the past.
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Just so I'm clear, you're saying you typically see 25uF in place of the 0.1 uf for C1b in the picture, and no C5?
Yes. Take a look at the AC30 schematics in the schematics library at the bottom of this page.
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Just so I'm clear, you're saying you typically see 25uF in place of the 0.1 uf for C1b in the picture, and no C5?
Yes. Take a look at the AC30 schematics in the schematics library at the bottom of this page.
Ah, I see.
So help point me in the right direction. What's troubling me is, I've sort of put a band-aide on the hiss by reducing the gain, haven't I? The amp sounds ok now, but I haven't played it long enough to know whether it's lost any of that sweet EL84 "ping".
If I understand correctly, taking the bypass caps out allows audio negative feedback and effectively "reduces" the gain. Please correct me if I'm wrong. If it's not an issue having these out, and if I feel the tone of the amp is good, I guess I'm done -- but if I want to add more of that bite back into the tone, wouldn't I want to have some value bypass cap in V1 and V2 (the cathode follower), but maybe change the cathode resistor to lower the gain instead?
I don't mean to confuse the issue. At the end of the day, I want a nice "EL34" tone amp. I need to grasp what these changes have done, whether it's OK to leave them out, and what I would do if I wanted to introduce a little more gain.
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What's troubling me is, I've sort of put a band-aide on the hiss by reducing the gain, haven't I? The amp sounds ok now, but I haven't played it long enough to know whether it's lost any of that sweet EL34 "ping".
Gain is directly proportional to noise (hiss). Higher gain = higher noise. No way around that. This is certainly not what I'd call a 'high gain' amp and from just looking at the schematic, I'm surprised that the hiss is too high for you. But it is a bright preamp and hiss is heard better in a bright amp. I kinda suspect the reason Vox eliminated the second cathode cap was to intentionally lower the gain, maybe to reduce hiss. Just a guess.
I don't consider removing that cap as a 'band aide'. I'd like to think you are now copying the real Vox circuit. I'd also increase the value of that first cathode cap to .68, or 1, or 5, or even 22. The real Vox uses 25. Some say a 25 makes for a muddy sound but with a 500pF plate coupling cap the mud should be eliminated. Heck, I'd probably replace that 500pf with a .0022 or even a .022 and even remove that 100pF bright cap from the volume control. If you do all this you should have a quieter amp and I bet you will like the tone. At least give it a try so you'll know.
You're not really gonna get an EL34 tone from an EL84 amp.
If I understand correctly, taking the bypass caps out allows audio negative feedback and effectively "reduces" the gain. Please correct me if I'm wrong.
That's correct.
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Thanks, Sluckey. That was super helpful.
I too was thinking of either removing or increasing the size of the bright cap (I put 120pf in there, but i may just take it out all together). Was also wondering about increasing that 500pf. I'll play around with these things.
Again, many many thanks
You're not really gonna get an EL34 tone from an EL84 amp.
D'oh! fat fingers. I meant EL84. (corrected above)
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For a pretty good read on Hiss reduction go over to the AX84 site and do a search for a thread titled "Experiments in Hiss Reduction". Makes for a pretty good read. Will be on about the second page when you do a search for "hiss"...Thread is about three years old.
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For a pretty good read on Hiss reduction go over to the AX84 site and do a search for a thread titled "Experiments in Hiss Reduction". Makes for a pretty good read. Will be on about the second page when you do a search for "hiss"...Thread is about three years old.
Thanks labb. I will.
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To close the loop here, in the end I used a 25uf cap on the first gain stage cathode (C1b), and a 5uf on the second stage (C5). C5 was a tossup, but it seemed to sound better with the master volume in and the input volume set high (i.e. to get overdrive). The gain is less and the hiss is minimal. So:
C1b is 25uf
Plate resistor on V1 changed to 100k
C5 changed to 5uf
47pf treble cap value the same, but changed to silver mica and hung off the treble pot
Here is a picture of the build. I added the MV, which works, but I need to modify that a tad. Thanks for all the help guys, the amp is sounding great :) FWIW - Here's a link to an iPhone video for the guy I'm building the amp for to update him on the progress. (it's just an iPhone, so quality is not good, but at least you can hear the amp). https://youtu.be/Iqss6zkv52E (https://youtu.be/Iqss6zkv52E)
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see good bye happy with amp :m5 :m17