Bell & Howell Filmosound 179

[xtian]

My 6SL7 tubes came, so I plugged my new build into a light bulb limiter (with 40watt bulb) and fired it up. All good. In fact, it sounds great. In fact, if I didn't know it was current limited, I'd say, awesome! It's done!

HOWEVER, when I remove the current limiter and plug straight into the 120vac, as it warms up, after about 10 seconds, a loud hum starts and persists. It's unusable in this state. I left it on long enough to take voltages and then shut it down. 6V6 glass was hot, others were fine. No red plating.

Voltages as follows when plugged into 120vac:
B+ = 384, B+1 = 277, B+2 = 261
preamp 6SL7 cathode = 0, plate = 89
PI 6SL7 cathode = 1.9, plate = 167
6V6 cathode = 16.8

What next? Build the current limiter into the chassis and leave it permanently installed?!? ;P

[surfsup]

I heard putting a large bypass cap on the cathode of an octal input pentode solves hum. 50-100u. Easy to try, can just gator clip it in.

Might affect frequency shaping.

Or maybe elevate the heaters....

Can you isolate the hum, where it comes from?

[xtian]

Any other ideas?

[Phil_S]

I think, when you have to bulb limiter in place, your heaters will not get up to the full 6.3vac they should be at and you can't really run the amp with the limiter in place because of that. Once you go directly to the wall supply, your heaters will go to full voltage and whatever noise they might be prone to generate will be present. You don't say whether the noise is 60Hz or 120Hz, but I'm thinking this is 60Hz hum you are hearing. I have no way of knowing. You might have both and may not hear the 120 until you cure the 60 problem.

If you aren't sure, your A string is 110Hz, so 120Hz is almost A#. 60Hz is an octave below that, so you'd need to tune down your E string till it's fairly loose to get that frequency. Try to confirm this, as it sends us in one direction or another. Either way, you can't continue with the bulb limiter, which has served it's intended purpose. Now that you know you don't have a short, put away the bulb limiter.

Your build is jam packed into a small space. (A+ for effort!) There are all sorts of wires that aren't where they are "supposed" to be. I see B+ wires crossing signal wires, and I see untwisted heater wires. Outputs are Switchcraft type and look as if they are not isolated from the chassis. All of these are potential problems, yet, we see examples where amps work fine in spite of the "rules."

So, let's find out what frequency your are hearing for starters. You might also pull tubes to see if that will affect the hum. Pull V1 first, then V1+V2. Report what that does, as well.

[M Fowler]

I have a PTP Fender 5b6 amp I built that is dead quiet using 6SC7 preamp tube and 6SL7 PI tube.

How about getting rid of the ground loop in your amp by taking a look at your photo where I added green ground wires. Actually I would run the put and input jack grounds to the bypass cap/resistor ground tab and one wire to chassis ground such as V2.

Is there more caps under that board for main filter a can cap?

Mark

[xtian]

Thanks for the suggestions. Before I get to soldering, I made these observations. Hum is the same with V1 tube removed. Hum vanishes with PI tube removed.

Hum is primarily 120Hz, with a heavy 60Hz component also. See spectrograph. Recording also attached.

Any further insights?

[Structo]

Hey, tell us about that new Marshall you just scored! (saw it on The Gear Page)

That looks great and you said you got it for a good price.

[M Fowler]

What is your main AC filtering cap value and where is it located?

Needs more AC filtering.

[xtian]

What is your main AC filtering cap value and where is it located? Needs more AC filtering.

Main AC filtering is the three visible 22uF caps.

[xtian]

Hey, tell us about that new Marshall you just scored!

Details here: http://www.marshallforum.com/marshall-a ... 203-a.html

I got to play it for the first time in a live setting with my AC/DC tribute (bonnyscott.com) last weekend. I had been playing my Egnater Tweaker 15watt, and it was fine. But the JCM 800 f***ing rocks. Period.

[Phil_S]

What is your main AC filtering cap value and where is it located?

Needs more AC filtering.

I would have thought 22uF is plenty on this type of amp. I wonder if the cap isn't up to spec? Mark seems to be considerably more experienced than me. I'm not in complete agreement and I'll voice another impression, though I'm speculating quite a bit here.

Pulling the PI kills the hum? Good, as this suggests the problem is in the PI, not V1 or the power tubes.

I am thinking that your 60Hz problem is likely to be heaters over B+ filtering. Since your heaters are on the floor and hard to get to, I'd leave them and try to address the 120Hz first. Maybe it isn't that bad? You've already got a recommendation on ground wires. I'd do that. I can't "draw" quite as well, but I think it might be worth considering re-routing the B+ supply away from any signal wires.

I have a thought that I will try to write here. The cap board is near the power switch. The switch is at the top. Now we have an orientation for top, bottom, left, right. The first filter cap is on the right and the 3rd is on the left.

I'm not fond of the red wire from the first filter cap, going over the two 10K to connect to the tube socket. It think it is very close to the output jack. I might route that wire up towards the switch, under the ground wire, and along/on top of the edge of the red board. Slip it to the left, between the 2 tube sockets, under the brown board, and loop it back around to the place where it needs to connect.

Next is the red wire from f/c #3, the one on the left. Slip that wire between f/c #2 and f/c #3, under the ground wire and straight to its tie point on the brown board. That gets it away from the blue signal wire.

For the wires to the tips of the output jacks, there is a fold along the open side of the chassis. See if you can route those wires into the space between the brown board and the jack to the left, up away from the heater wire, and into that fold by the open side and then on to the tip connections. You may be picking up 60hz heater hum at the jack due to proximity to the output wires.

You have a tight space there. It's challenging, no doubt about it. You have great courage to try to build this circuit in there. Do one thing at a time and check for results. No sense doing more than is needed.

[M Fowler]

That is probably enough filtering.

I have added a schematic showing how I would ground the whole amp.

[tonewood]

Xtian-

What app did you use for the screen shot:
Screen Shot 2012-02-06 at 10.38.54 AM.png

Thanks

[xtian]

Xtian-

What app did you use for the screen shot:
Screen Shot 2012-02-06 at 10.38.54 AM.png

Thanks

Digital Performer.

[surfsup]

Thinking aloud here...since it's in the PI, maybe it is just PI tube heater noise? If so, what about elevating the heaters off B+ to about 70VDC?