Wireless connections inside tube amps

[Auke Jolman]

Hi All,

I while ago I build my version of the COS: https://el34world.com/Forum/index.php?topic=24788.msg268243#msg268243

It sounds great, but there is always some hum present. Today I had some time to spare, so I wanted to try and figure out what is causing that. After pulling preamp tubes I decided the hum was coming from the FX-loop. A known goot tube as replacement did not change anything so I decided to bypass the signal completely around the fx-loop. The hum persisted. Then I decided to cut the wire going to the PI. After switching on the amp, the guitar signal still was coming thru!

As it can be seen in the pictures of the for mentioned post, I've put the relaisboards on top of the main turret board. The channel switching relaisboard especially sits almost flush on the cap to the PI. When I put it a little bit higher, there was guit some loss of signal.

Putting stuff so close on top of caps is not good practice I guess. I can raise the relais boards to some degree but I'm not sure it will be enough.

Has anybody come across this phenomena of "wireless connections" and if so, what did you do to resolve it.

[kagliostro]

Larry Amps (Boutique amp with similar problem) look to the shield





Franco

[shooter]

love the copper sail! 
couldn't see the silver boxes well enough, relays?, PI filters?, inter_stage tranny?

[kagliostro]

To me relays





Franco

[shooter]

Thanks Franco, looks relay to me

[tubenit]

Then I decided to cut the wire going to the PI. After switching on the amp, the guitar signal still was coming thru!

Auke, can you please clarify this?  Are you saying that the wire between the preamp/fx going to the phase invertor is cut and signal is still passing thru even though there is no wire connection to carry the signal? 

IF there is no wire connection to the phase invertor then there shouldn't be any signal???

Maybe I am misunderstanding what you are saying??

With respect, Tubenit

[PRR]

Wires are a great way to pass signal around.

But Marconi and others proved you can send signal "wirelessly". If you want to send signal a mile you need significant transmitter power. To send signal a few inches is much easier (even 10,000 times easier). It is better to use high frequency but you can throw lows a ways. High impedances catch electrostatic waves best (why we use low impedances on wired microphones).

If you look at an amplifier as a bunch of antennas, there's many signal paths. Normally we connect everything with wires, which forces a semi-low impedance (like 40k) and the through-air signal comes through far less than the through-wire signal. But if you disconnect wires, and have high level stages nearby, you WILL have audio sneakage and at best/worst it may be quite audible.

(In a different field, broadcast center consoles, they have to fight to keep the thru-air leakage VERY low, so different programs coming to the board do not leak into each other, "cross talk".)

[Auke Jolman]

@ Tubenit: You did not misunderstand what I was saying. You should have seen my face the wire being disconnected and still guitar signal is coming thru, as strong as if the wire wasn't cut.

The channel switching relais is almost on top of the cap to the PI. When I changed that to 1 inch the signal was not as strong and distorted, but still it was coming thru. I guess I have to figure out a way to put some shielding in there one way or the other.

[tubenit]

I've never had an amp do that?  OR even heard of that before?  That seems odd but obviously possible.  I'm guessing that may play a factor in the hum issue you have going ?   My 3 relays are between the layout board and the pots.

PLEASE let us know what the resolution is when you get it figured out.

With the volume that I play at normally at home,  you can barely hear any white noise at all with the amp at idle particularly on the clean channel. At idle, it is significantly quiet then the '76 Princeton Reverb I owned at the same volume.  There would be more white noise and maybe some mild hum present IF I were playing at the same level I did playing in a band.

One thing I recently did that helped reduce noise significantly was use 2 watt TKD metal film resistors on all the clean and OD channel triode plates.  However, these are quite large resistors with heavy wire leads and they are expensive ($2/each but I only used 5 of them) .  I experimented with these and found I like them better then the RN65 Vishay-Dale resistors and besides being quieter, they seem to give a smoother tone, IMO.  These resistors are literally about the size of a .0047 Orange Drop capacitor. Not a convenient size.     https://www.partsconnexion.com/tkd-cm2-series-resistors.html

With respect, Tubenit

[Auke Jolman]

I added shielding underneath the relais boards, but that did not kill the hum, but it did end signal passing thru without a wire . When I pull te V1 tube the amp is very quiet, even with the MV and FX-return on full. Still can't figure it out. To me it sounds like 100 Hz hum.

I even changed the filaments from V1 to V4 to DC, with no change. Still the hum is to present. Have to soldier on I guess.

[tubenit]

IF you run the guitar signal straight into V2 and bypass V1 (by pulling V1 and jumpering past it to V2) ………..  does the hum disappear? 

Are you using a 12AY7 in V1?  If so, try a 12AT7 or 5751 or 12AV7 …………. does any of those help?  I've had quite a few 12AY7 tube be microphonic and induce hum.

with respect, Tubenit

[Auke Jolman]

I replaced V1 with a 12AX7 (no hum/buzz in another amp with that tube). The hum persist. When I wire the input jack to V2 the amp is dead quit. With the master en FX-return on full!

After that I connected the input jack to the V1, second triode and the hum is back. Should I start replacing caps and/or use shielded wires in there?

[tubenit]

Should I start replacing caps and/or use shielded wires in there?

First thing I'd suspect would be silver mica cap IF you have one in the signal chain.  I've had even a new induce significant hum.  Go with a ceramic cap instead.  One forum member had a terrible hum problem that turned out to be a cathode electrolytic cap.  I've replaced a pot before and had that eliminate hum.  So far, I've not ever had a leaky cap cause hum but that's something you can measure for. AND yes, use shielded wiring on long runs and any runs going close to relays or high voltage wires.

Layout and lead dress can make a huge difference.  IF you move the heater wires with a chopstick (non-conductive stick) does that increase hum?   I have a current amp that had a problem with that and I had to silicone a wire in one position. The amp is dead quiet now.

That's all I can think of.  I'd start with silver mica caps, shielding and the layout of the heater wires.

With respect, Tubenit

[Auke Jolman]

Thanks Tubenit.

I've replaced all the silvermica's for ceramics after I first finished the amp. The filamentheaters are DC now. I've moved them around anyways, with no change in the hum.

This evening I will take a closer look at the wiring.

[Auke Jolman]

I wired the inputjack directly to the grid of V1b, bypassing the 200k resistor. In doing so I've got a scratchy volume pot in my guitar. I turns out that upon start up I've got -0.33 V between the tip - ground of the input jack. The voltage tuns more negative. After a minute it's -0.39 V. I've changed the preamp tube with the same result.

Does anybody know the reason for this or how to try and fix this?

[sluckey]

Would you please post the schematic for your amp in this thread?

[Auke Jolman]

I've attached the schematic attached. Sorry for that.

In the mean time I think I've got it sorted out. The cathode bypass cap of V1b is defective. Somehow it gave a short resulting effectively into no resistant at all throwing the bias of that stage completely of which probably caused the dc on the grid. I've got to wire everything back in place. Sadly enough I don't have the time to do that today, so it will have to wait until tomorrow

I will report back tomorrow.

[tubenit]

I look forward to the update!  I'm rooting for you to get this favorably resolved.

With respect, Tubenit

[pdf64]

.sch file?
A more generic file type would be useful.

[shooter]

conversion complete 

[sluckey]

.sch file?
A more generic file type would be useful.

That's a very generic file type on this forum.

[pdf64]

I acknowledge that, perhaps I should have written ‘useful to me’ but I can’t be the only reader who doesn’t have the requisite software on every device they might happen to be browsing the forum with?

[sluckey]

Lot's of people ask how to open those *.sch files. And they usually get a response of where to download the small free program. Heck, there's even a sub forum here dedicated to PC Express. The neat thing about the files is that anyone can edit them. It's a good way to share files . Many more people using that program than Visio. I don't use PC Express but I have it installed on my computer just so I can view the files that people share. I highly recommend getting it. Takes about as much time as it does to read this message. 

[Auke Jolman]

Here's an update.

The defective cap is not the cause of the hum. It did cure the scratchy volume pot though.

Here's a recap of what I've done so far:
- Wiring the input directly to the grid of V2a -> the amps is quiet.
- Wiring the input to V1b -> there's hum present.
- Added an extra cap to the node E -> there's hum present.
- Pulling V1 -> there's hum present.
- Disconnecting wire's to midboost and midshift relaiboards -> there's hum present.

Any advise on to what to do next is extremely welcome.

[shooter]

so it's inside the red box, guessing most of that is wires back n forth to the front panel with wires crossing, lots of parts.  Me I would eliminate it all if it's on the front panel, hardwire out of V1, coupling cap into V2, then add 1 thing n check, then add 2 things n check.......

[Auke Jolman]

@Shooter: thanks for your reply.

I think your red box isn't wide enough, because I had hum hardwiring the input directly to V1b grid. That's how I discovered the faulty cap.

But I followed up your advice anyway. I bypassed everything in the red box. The hum is still there.

[shooter]

The hum is still there.

I' a brute force sorta guy, rebuild V1, 4-6 parts, wires n a socket, hour, use a shielded wire from input jack, ground at input jack (DO NOT ground at tube R).  take way less time than thinking 

[tubenit]

Auke,

I am wondering if all the grounds for the effects loop are tied to the effects loop jacks ground?   In other words, both FX jacks are grounded with each other and everything in the FX loop such as triode cathode and FX pots are grounded to the FX loop?

(vs. the FX grounds being tied to the buss wire ground to the preamp input jack)

Not sure IF that would make a difference in reducing hum, but I thought I'd toss it out there.

Also, you could try isolating the speaker jack from the chassis?  Since it's close to the FX loop jacks, you might be getting a
"ground loop" there causing hum?  IF possible, maybe isolate the speaker jack with electrical tape & add a jumper wire from the output jack ground to the ground (negative side) of the first filter cap.  Does that help reduce hum? 

Not saying any of this will work.  Simply trying to help out with possibilities.

With respect, Tubenit