Single-Ended Hum

[Sir Cuitous]

Thanks for all suggestions - good info. I believe this is a "normal" level of hum to expect from this amp. Though I could be wrong, I don't think any kind of instability is involved. It's a very low level hum that increases in the first few seconds as the power tube warms, but it does not amplify as the volume is turned up. I have to listen carefully for it. I'm not trying to waste anyone's time here. I am only seeking to learn and improve what I can. If my efforts do not yield a better result, I will have at least explored many options on an amp that I built and not some vintage masterpiece.

There is no center tap. I have gone from a single wire heaters to artificial center tap with 100ohm grounds and now I have a 200 ohm "balancing" pot to ground. No difference in the hum. I will try more of these suggestions - heater bias then OT placement.

[Sir Cuitous]

OT picking up Hum

That's an interesting theory Nico. I looked at my placement of the trannys and the Champ chassis does not leave a lot of room to move the OT around to check on a better location. If the OT were "picking up hum from the magnetic radiation of the PT through the chassis" then any place near the chassis would be a source of hum for the OT. I am also not sure about your method of checking this. I'm am no expert, but if I disconnect the OT and listen on the primaries, I am just going to hear what the OT would be "hearing". If the OT were picking up hum "magnetically", I should be able to hear it right out of the speaker even with the primaries disconnected, right?

I am now have the artificial center grounded to the cathode of the 6V6. Still humming. I could continue with connecting a positive bias voltage from a B+ node, but I now believe this is something else. I would not have been able to reach this conclusion without everyone's help. Thanks.

[teleamp]

That's an interesting theory Nico. .....

Not a theory. It works

MikeY

[Steve Conner]

I believe that Champ-style circuits hum by default. The single-ended output stage with no negative feedback is lousy at rejecting power supply ripple.

I modified my Champ-style amp to have a CLC filter with two 47uF caps and a choke, which shut the hum up nicely.

[MWJB]

Most champs do have NFB. Interesting to note though that Fender did flirt with choke filters on 5E Champs & Princetons but dropped it again (most likely due to cost). One possible draw back with this method is raised screen voltages, a larger than usual screen grid resistor may be required to drop the screen voltage below plate voltage (if that bothers you, most folks would fit a 470ohm and be done with it - SF champs typically have screens running a little higher than plates).

If filament hum is a problem though, DC biasing is quick, easy & effective.

[Sir Cuitous]

Every Champ I ever heard had some hum. Isn't there about 3dB of negative feedback due to the unbypassed cathode resistor on second triode of the pre-amp?

The filter caps are 16/8/8. I'm sure a 3x16 would have been a better design. I did DC bias the filament to 40v which gave a very slight improvement on the hum. At this point I am enjoying the amp. As my first build, it's decent sounding, but I believe the hum is due to my sloppy wiring technique. At some point I may change the filter caps to higher values along with some other improvements.

[jyoung]

Good Sir, (had to)

Well, as it turns out, I grabbed an old VOM (Voice of Music) phono with a "FREE" sign tapped to it off of a nieghbors lawn. I figured to make a cool little practice amp from the salvage. Upon removing the speaker I was surprised to find it had a trombone bell shaped cone. O.K. I've seen that before. Not often, but I have. So I check the code on the rim...It's a P12R!...With a bell shaped cone!..From 45+ years ago.

+1 Chuck -- I can back you up on this one - 1969 VOM console record player 6V6 PP amp 4 ohm P12R trombone shaped cone - You speak the truth.

[tubeman]

So you are getting hum on your Champ. I built mine using my own layout (Hoffman like). My amp is dead quiet cranked all the way up with only alittle hiss. I suspect you are getting hum from a ground loop. To quiet my amp, I seperated the preamp / power amp caps from sharing a common path to ground. My preamp stage is grounded to the input jack ground lug which is in turn grounded to the volume pot. The power amp caps ground are run to the nearest PT bolt. one last thing, when you are checking your amp for hum make sure nothing is plugged in the input jack. You may be picking up noise from your guitar pickups..

Good luck

[Chuck H]

Thanks. Wow this is an older thread. I never expected anyone would actually READ it.

FWIW I've heard from guys in the know that these old bell cone p12r's are the $h!t. Supposed to sound great for clean and OD but get funny in between (where most gutar players live). Oh well, you can't have it all (unless you pay for vintage Bulldogs). But I'm told that using them is tragedy in the making. The old animal glue will have become so brittle that it will crack up the first time you crank it up. Nothing to lose I guess. Who knows about these speakers anyway...Besides you and I and a select few. Mine is still sitting in a box.

Chuck

[Regis]

Thanks. Wow this is an older thread. I never expected anyone would actually READ it.

FWIW I've heard from guys in the know that these old bell cone p12r's are the $h!t. Supposed to sound great for clean and OD but get funny in between (where most gutar players live). Oh well, you can't have it all (unless you pay for vintage Bulldogs). But I'm told that using them is tragedy in the making. The old animal glue will have become so brittle that it will crack up the first time you crank it up. Nothing to lose I guess. Who knows about these speakers anyway...Besides you and I and a select few. Mine is still sitting in a box.

Chuck

I have an old P12S that was torn and worn, I fixed it with toilet paper and contact cement.

[Sir Cuitous]

After a year...... yes I still get hum, but I have built two more single ended amps since then which are very quiet. My brother is now enjoying that first effort of mine and I may get it back from him some day laugh at my first build. It still sounds good and the hum is still way below guitar threshold so as my boss would say "shut up and play yer guitar".

Funny how this became a P12R thread. I used a new production P12R speaker. Not really expensive, but the speaker works really well in pine/tweed style cabs. I have used them twice now with good results.

BTW Chuck - that repair is a work of art. Nicely done.

[Enzo]

SIngle ended amps don't have the advantage of ripple cancellation like the push pull stages have. So they hum. If your B+ is clean, then they won;t hum because there is nothing to cancel.

[Regis]

"BTW Chuck - that repair is a work of art. Nicely done."

That was my repair, not Chucks...

[Sir Cuitous]

Sorry Regis - kudos on saving that speaker. Cool website too.

I probably could have upped the filter caps and paid closer attention to my grounding. When I get back to Michigan next year to visit my brother I will have another look at that amp.

[lain]

Help me verify please!

On a SE amp wire heaters as a twisted pair, no CT, no 100ohms, no ground reference at all. If you do miss a ground reference you will fry your PT!

From 1st filter cap run a 2W 220K to one side of the heaters. From the same side of the heaters run a 0.5W 27K to ground. You should now have 35-40vdc biasing the heaters. Tweak 27K to make sure that you don't have less than 35v or more than 40v (SF voltage champs might need nearer 22K?).

Shouldn't be necessary on push pull amps.

On my SF Champ, one end of my 6.3V winding is grounded, and so is one of the pilot lamp terminals. So I should unground the 6.3V lead and connect it to one side of the new 2-wire heater circuit, using both 6.3V PT leads? Then unground the pilot lamp terminal and include both terminals parallel to the 2-wire circuit? Then add one side of the new heater circuit to the B+ via 2W 220K, and ground that same side with .5W 27K (maybe near the power cord ground)? Does this sound right?