AC30 build lives - a bit of a hummmmm, though

[markmalin]

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.......

[markmalin]

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.......

[shooter]

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

[markmalin]

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)?

[sluckey]

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.

[markmalin]

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.

[markmalin]

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?

[sluckey]

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.

[markmalin]

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.

[vicentebd]

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

[markmalin]

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.

[markmalin]

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.

[shooter]

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?

[markmalin]

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.

[shooter]

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.

[markmalin]

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. 

[DummyLoad]

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

[markmalin]

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.

[markmalin]

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

[shooter]

I originally misread your first reply

I'm dyslexic so it's NBD

[markmalin]

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.

[kagliostro]

Try the amp in one other room or, better, in one other building

K

[Willabe]

Try the amp in one other room or, better, in one other building

+1

[markmalin]

Thanks - I'll try that.  Should be interesting....I'd love it if it was just the room (though that's my workshop!)

[Ed_Chambley]

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.

[markmalin]

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.

[Willabe]

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.

[sluckey]

I'm sure y'all mean 100Ω and not 100K, right? 

[Willabe]

I'm sure y'all mean 100Ω and not 100K, right? 

            Why do I keep missing little things like that??????

(I'm gonna have to read slower........)

[markmalin]

I'm sure y'all mean 100Ω and not 100K, right? 

            Why do I keep missing little things like that??????

(I'm gonna have to read slower........)

lol.  I knew what you meant ;)

[shooter]

when the heater  secondary doesn't have one, right?

right, like exclusive OR gate, 1 or the other, but not both.

[markmalin]

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.

[John]

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.

[markmalin]

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!

[sluckey]

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.

[markmalin]

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!

[shooter]

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"?

[John]

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.

[Willabe]

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.

[markmalin]

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)

[Willabe]

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.

[clyde]

I also think it's the proximity/orientation of the transformers. 

[DummyLoad]

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

[shooter]

mutual inductance

I was at work when the light bulb went on

[kagliostro]

Just for knowledge


http://home.comcast.net/~machrone/bjr/bjrhum.htm


Franco

[markmalin]

Just for knowledge

http://home.comcast.net/~machrone/bjr/bjrhum.htm

Franco

Very very interesting...

[markmalin]

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.

[John]

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.    After all, when you stop playing there will be applause to cover up the hum.

[markmalin]

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.

[kagliostro]

Did you tied to rotate the PT and OT of 90° eachother (in the other way) ?

K