5E3 hums when connected w/ TTE

[pbiagi]

Hello,

This thread is sort of a continuation of this thread -
http://music-electronics-forum.com/s...&highlight=TTE
However the input DC voltage issue was resolved by replacing the preamp tube in the TTE but the basic hum issue remains.

I originally built the 5E3 last May and had some hum.. did a lot of reading..., redid the wiring until I got the hum to an acceptable level...

I had no effect pedals, (and the sound was great).

Then I got the Fulltone Tube Tape Echo. When I try to use it, the hum is unacceptably loud, to the point that I rarely use it..

Here's the deal - with nothing plugged into the amp, there is a bit of hum - noticeable but not too terrible. If I connect the bright channel to the normal, the hum is less. When I plug a guitar in, then the hum is so low that it's barely noticeable - you have to put your ear to the speaker.
But if I put the TTE in the chain, then the hum is like when nothing is plugged into the amp, but much louder.

In the 5E3, if I pull the preamp tube, the hum disappears entirely.
The heater wires are well twisted (I think) and are tucked away..

Today I redid the preamp grounding, connecting to the main input jack. Actually I think that made the hum worse. Now when you touch the chassis, the hum diminishes. I'm pretty sure that wasn't the case before (the latest change).

Pictures are similar to this except for the changes I made today.
http://music-electronics-forum.com/s...ead.php?t=9164
Again this layout works well except for when connecting with the TTE.

Notes:
- A while back I did the two resistors at the cathode to reduce hum on the heater wiring.. (works)
- brass ground plate is still in place - so grounding is kind of dual to the brass plate, and via a copper wire..
- because of my lack of experience soldering, I could have heated resistors(?) too much, however I've measured them all and they seem ok.
- measure resistance from most leading ground wires and the chassis, almost always at .6 ohms.

I purchased a '70s era oscilloscope, French army I think.
I know enough to watch guitar notes make waves on the screen.
If someone can give me a pointer on what to connect to and what the wave form means, that may be helpful in determining the root cause.

I've learned a lot building this amp, and it has been.. well sometimes frustrating, but in the end very satisfying. I'm determined to get the two - TTE and amp - working together even if it means starting over, or doing a lot over.

My next step I was thinking of putting the heater wiring on a separate battery DC voltage. Should that be 6V? What is the best way to do that?
I was figuring at least it would show or eliminate whether the heater wiring is the cause.
But I think the real issue must be "ground potential" or "ground loop".

Since I've tried, done and measure many things, I'm sure there are details I've left out..

Thanks in advance for your help.

Best regards, Paul

[tubeswell]

I bet the hum you were getting with the FTTE connected was a ground loop caused by the different chassis potentials of the echo unit and the amp. Put your 5E3 ground back the way it was when it was quiet, and get a ground lift adapter for the tape echo.

[pbiagi]

Hi Tubeswell!
Yes I had tried that before.. No difference, or worse.
Even tried the ground lift on the amp (wouldn't do that permanently), and on both.. Didn't help.
Paul

[tubeswell]

Have you tried a loop buster in the signal cable between the amp and the TTE?

[pbiagi]

Tubeswell,

I went ahead and tried a ground-lift again to confirm... It had been awhile.
Actually the hum is better with the TTE w/ a ground-lift, however the difference is minimal, and w/ or w/o, the hum is pretty horrible.

Since I setup a line out where the normal extra speaker jack goes, I'll connect up the oscilloscope. I'm afraid I'll be able to visualize the hum, but not sure how to tell if it's 50 or 100hz. (I'm using a 220->110 step-down 750W tranny.)

Paul

[pbiagi]

No I haven't tried a loop buster.. In fact I don't know what it is??
Is it a circuit/box I could build?
What about removing the brass ground plate? Is that worth a try?

[tubeswell]

By loop buster, I mean either a high-quality, high-impedance passive isolation transformer, or an active buffered isolation circuit. (R.G. has some designs on his site.) Or you can use a couple of hi-lo in-line ground-lift adapters back to back between the TTE and your amp. Either way, it isolates the signal between the two units, and only AC signal gets through.

However, if you are still getting hum with the mains ground lift adapter, it is probably not just ground loop hum; it will be likely to be from other sources as well. Did you say you re-did the grounds in the amp making it worse than it was before? (in which case try putting it back the way it was)

[pbiagi]

Hi Tubeswell, everyone,

Please anyone jump in with comments/advice - I've learned from reading others stories and others may learn from this one too..

As stated, after pulling the 1st tube (preamp), the hum went away. So I focused on redoing the preamp and tone stack wiring including grounds.
1) removed the bronze ground plate
2) experimented with floating the input jacks and tone/volume controls (in air), using star grounding near the input jacks, however that made the hum worse, so reverted to grounding jacks, ..., at their base (w/o the brass plate). Since I only had switchcraft jacks, that was easier to do too.
3) replaced a couple ground wires with the green thicker gauge wire.
4) redressed the heater wires at the 12AY7 tube to keep them further way from the sensitive grid wires.
5) redressed a plate wire (from the OT) on one of the 6V6's. It was close to a heater wire - could that be a source of hum??
6) On the input jack wiring, I had missed wiring the Switch (connection) to Ground (connection) on the no. 1 jacks. See this jack wiring diagram.
7) I had used shielded wiring for the grid wires to jacks, but had grounded them near their middle. Now their outer jackets are grounded right next to the connection to the jack.
8) General: shortened all wires.
I probably did more than this..

No. 6 changed the hum behavior. Before there was hum with the amp with nothing plugged in. If I jumpered the channels and plugged a guitar in, the hum was much less. That makes sense since the no. 1 channels "switch" was not grounded.
Now the amp is quiet with nothing plugged in, and with a guitar, the hum is a bit more, but frankly you need to be near the speaker - 2 or 3 ft? - to hear it.
I'm not sure but I believe no. 6 made the most difference for the hum overall.

For the Tube Tape Echo (TTE), I went ahead and used the mains ground lift adapter, and I'd say the hum is an identical type of hum as when the guitar is plugged directly to the amp, but maybe twice as loud, still an acceptable level - not very noticeable for normal playing. And when I use the TTE bypass, the hum level is identical to that of the guitar directly plugged in. That was not the case before - then, the amp would hum all the time when connected to the TTE..
Haven't yet tried w/o the ground lift on the TTE.

The amp sounds great, clean and compressed, and the TTE adds great slapback echo for rockabilly.
I'm sure it can sing but I got to get it out of this small apt.

Questions:
1) Could redressing the plate wire on 6V6 help eliminate hum? (don't think so, but really don't know)
2) Could missing grounding the switch connection on the input jacks have fixed all hum issues?? (seems there was a ground potential or loop issue when connecting the TTE which is pretty much gone)
3) Could using a loop buster eliminate the little amount of hum difference between going direct and using the TTE? (probably not worth the effort)

Even though the hum is at an acceptable level w/ the TTE (it wouldn't be good for recording, although a notch filter could be used..), what else can I do to further eliminate the hum?

Thanks for your alls help, Paul

[tubeswell]

If the ground loop buster didn't eliminate the hum, then the hum is coming from something other than a ground loop between your amp and your tape echo unit.

Connecting the input jack ground tip switch grounds the amps signal path (as intended) so that the amp doesn't pick up any EM interference through that most-sensitive input signal wire whene there is nothing plugged in. Shielding the wires from the input jack to the grid of the V1 should help as well, but you would need to re-do the 68k grid 'stopper' resistors, by putting them at the input sockets, for this to have the best results. And you should ground each cable shield at one end only.

But if the hum you get with your guitar plugged in is not there with no guitar cable plugged in, then it suggest that there isn't any innate hum in your amp itself, and that it could just be that your guitar isn't properly shielded, or that you are using a poor guitar cable. (Does the hum become quieter still with hum-bucking pickups?)

Remember that these old-school basic amps don't have all the hi-freq shunty circuits that modern production amps have, and the grounding is a lot simpler. Therefore anything like a poor guitar cable or a badly-shielded guitar will show up a lot more easily than it would with a modern production amp.

[pbiagi]

Hi Tubeswell,

Sorry I wasn't clear - I never got a loop buster.. So still haven't tried that..
I haven't found much information either.
You mentioned R.G. has some designs for an active buffered isolation circuit.
What site is R.G.'s? (sorry for my ignorance)

Yes the 68k grid resistors are soldered right to the tube pin, shielded wire is direct (hanging) to the input jacks, and ground is at input jack side only.
Did this for the tone cable on V2 as well. Did both a while back.

My guitar cables should be good - George L, and the like - and they work fine on a vintage Fender Super Reverb amp.
The guitars are vintage or reissue too, so yes, they are susceptible to dimmers, etc. I don't have a humbucker guitar - will try to get a friend to bring one over.
But in this case the hum comes up a bit when the TTE is connected, even without a guitar plugged into the TTE.
(However I do agree the vintage type guitars are adding a tiny bit of hum when directly plugged in.)

Sometimes we are victims of wishful thinking...
So later I came back to the amp with TTE (thinking I might have wished too much) ... and really was not annoyed - it works just fine.
The little hum that's there I can live with for now..

Around Aug. I may try isolated jacks, and feeding the jacks ground to the star grounding. I read somewhere someone had a similar experience, he was left with just a little bit of hum and when he got isolated jacks to the star ground, pretty much completely eliminated the hum.

Before then I may try the loop buster if it's convenient enough to try.

Again, the setup is working pretty nicely now - much better! - so now, it's just a matter of getting it perfected a bit more, if that's reasonable.

Thanks, Paul

[tubeswell]

You mentioned R.G. has some designs for an active buffered isolation circuit.
What site is R.G.'s? (sorry for my ignorance)

http://www.geofex.com/

[Groundhangers]

Hi all, im not usually on this site but hey, i am now.

Paul you can try filtering the audio tranny by putting 40-100uf 450v cap from where it attaches at the Standby switch to ground. That will eliminate a fair wack of hum if there is any there.

Does the hum increase even if you run your hand or chopstick if you like, anywhere close to the input section?

Greg

[pbiagi]

Hi Greg,

Do you mean put the cap on the B+ line (at the standby switch) from the rectifier tube, which goes on to the 1st 16uf cap and OT?
Not on the standby/disconnected side?
Would that have any affect on tone?
Why doesn't the 16uf cap take care of the hum?

From memory touching the input wires, or coming close with my hand, does make the hum come up. But I think that would be expected since those wires are so sensitive.??

I'm ordering some isolation washers for the input jacks. They'll take a couple weeks to get here, so I'm planning to work on it again then.
I do have a cap handy to try for the standby switch.

Thanks for the feedback,
Paul