AC30 build lives - a bit of a hummmmm, though

[markmalin]

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.

[markmalin]

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.

[archaos]

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

[Ed_Chambley]

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.

[markmalin]

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.

[markmalin]

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

[hesamadman]

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.

[markmalin]

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. 

[kagliostro]

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

[sluckey]

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.

[markmalin]

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.

[markmalin]

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.

[sluckey]

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

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.

[markmalin]

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

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.

[sluckey]

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.

[Ed_Chambley]

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

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.

[markmalin]

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.

    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)

[sluckey]

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.

[markmalin]

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)

[kagliostro]

the big ones are 40 bucks vs 28

At DIYItalia a friend has tried with success a self made Gyrator



K

[markmalin]

Holy cats!  Thats' really something.  I've never heard of a gyrator - just looked it up on wickipedia.  Fascinating....

[markmalin]

Oh my gosh!  I have totally opened a can of worms re the AC30 tone stack.   


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


Here's a great article on the real story:


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

[markmalin]

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.

[ac427v]

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

[markmalin]

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...

[kagliostro]

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

[markmalin]

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

[markmalin]

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 Hum, nothing plugged in (volume was real quiet on amp)
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  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.

[Willabe]

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!  )

[markmalin]

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!  )

Hey, Willabe.  They're on the main board. 


lol....you're not supposed to listen to my other stuff!  (just kidding)

[Willabe]

Oh, I see them now.

I liked what you were playin on that song.

[kagliostro]

A bit more of what I was aspecting ...

Can you try to feed the filaments with a DC power supply to verify what happens ?

---

Put a 100-200n cap directly across the heaters pin on V1, changes anything?

Ciao

Franco

[DummyLoad]

suggestion:


reroute grounds: segregate power amp ground and LTPI ground. see pic.


--pete

[markmalin]

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 ;)

[markmalin]

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.

[sluckey]

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.

[kagliostro]

Put a 100-200n cap directly across the heaters pin on V1

Have you tried ?

K

[Ritchie200]

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

[markmalin]

Put a 100-200n cap directly across the heaters pin on V1

Have you tried ?

K

Yes.  No difference

[markmalin]

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

[kagliostro]

No difference

No hiss difference ?

K

[markmalin]

No difference

No hiss difference ?

K

No.  No hiss difference, no hum difference.

[markmalin]

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

[shooter]

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. 

[markmalin]

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 ?)

[shooter]

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. 

[markmalin]

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.

[markmalin]

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 :)

[Ritchie200]

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....

Jim

[shooter]

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.......