Help! loud hum in onboard loop. THis is serious...

[Tonegeek]

The last part for my HRM just got installed to get my onboard dumbleator working. I am getting seriously loud hum when I switch the loop in making the loop unuseable. Here are some observations:

The amp works normally with no hum when the loop is bypassed. When the loop is in, The hum gets louder as I turn up the send so it would seem it is present right at the start of the loop. The return pot kills the hum when it is down (except for a tiny bit). Of course it kills the signal too.
Hum increases as I turn up the send pot (it sounds like what a loose guitar cable sounds like when you grab the hot end of it - some sort of 60 cycle variant). BTW vol pot is down, and no cable plugged in.
However and this seems really baffling to me, If I turn the send pot all the way up, the hum completely disappears, but only with the pot hard against the stop. I measured the pot thinking the wiper was off the contact, but no it is functioning normally, so right at that 250K to ground position, the hum is gone. ALso, If I grab the .05 input cap, I can make it almost go completely away. I can also touch the .047 pot coming off V2b and it will affect the hum level some but not as dramatically as with touching the grid cap on the cathode follower. BTW the loop works and sounds normal when the send pot is all the way up. One last thing, with the send pot up and no hum, if I turn the mid pot all the way down, the hum returns!

what I have tried so far:

moving wires around with a stick while listening. This has had no affect at all. I replaced the common pot, and jack ground wire with a jumper and tried different ground paths to no affect. (I don't think this is a ground loop.) As touching the cap makes the hum less loud, I am thinking it is a shielding issue, but there is not that much wire and also no gain (at least on the follower side), so that does not fly either. Maybe something is oscillating.

I am including my version of the ODS-101 schematic just in case I missed something. The schematic has been altered by me to show the exact values I used in my build. I used the values NOT in parenthesis. Notice my heater has a center tap which goes to the ground bus.


I really need to figure this one out, because I absolutely cannot use this amp without a loop. I am not happy at the moment!

Whit

[heisthl]

Do you have a layout or picture? I'm just finishing up a non-HRM with FX and I had some similiar wierdness. I got rid of some of the noise by using a sheilded cable from the return pot wiper to my FX in/out switch. On the scope I was still getting the neg peaks of 60 cycle at the speaker out. I grounded the PI with a cliplead where the 8.2k and the 390 (1K in my amp) meet and the noise disappeared. I was using the P/S ground on the board for the 390(1K) and when I moved it off the board and used the bus rail by the presence pot the problem went away. Just for grins put a cliplead to ground after the 8.2K feedback resistor. Oh and one more thing - I don't have panel switches for PAB and OD and even though with no footswitch the relays nc for no PAB or OD I have had the midpot noise you describe - it goes away when I plug in the footswitch. I also had to ground my 5v heater footswitch supply to common for less noise when using the OD.

[heisthl]

Wait - I just noticed your plate voltage on the second FX stage - way too high, should be 200 or less. Try changing your P/S around - heres what I'm using with a 12AT7 tube: 2A goes to the first stage cathode follower and 2B goes to the 100K plate of the second stage.

[Tonegeek]

Wait - I just noticed your plate voltage on the second FX stage - way too high, should be 200 or less. Try changing your P/S around - heres what I'm using with a 12AT7 tube: 2A goes to the first stage cathode follower and 2B goes to the 100K plate of the second stage.

Good catch! Always helps to have extra eyes on the situation. B+6 and B+7 are reversed on my schem. and layout. I will fix that tonight and see how that affects the hum. It should put the voltages more in line with a higher v3a and lower v3b. Also, I am using a Sovtek 12ax7 LPS in V3 which has lots of gain and is prone to hum. I have a 12AT7 and 12AU7 available and will try those instead of the 12ax7. I meant for the LPS to go in the PI socket, but for some reason it got stuck in V3. I am attaching a layout anyway in case someone notices areas for improvement. I hastily drew the ground connections (fat purple lines). Notice I have 3 coming off the main P/S. If you look closely you can see they connect at 3 different spots on the P/S (not counting the relay P/S which was grounded to fix the other hum already discussed eleswhere). They run parallel to the buss so should not be contributing to a loop situation, although some say each B+ section should connect to the local star for the section it powers. My amp is very quiet as is (Except the damn loop!), so I see no need to change it unless of course it is causing my loop hum.

Heres my layout. My schematic is more accurate as to values, especially the bias resistors which I just got working the way I want. Iwill one day synch everything and post to this forum, if I ever finsh this project...
thanks,
Whit

[heisthl]

You also may want to redo the FX In/Out switch and send/return jacks to allow a "hot" send jack at all times for recording. I keep the master hard wired to the loop input, let only the return jack interrupt the pass through signal and then have the switch put ground on the return signal when in "out" mode. See the schematic in this files thread: https://tubeamparchive.com/viewtopic.php?t=1979

[Tonegeek]

You also may want to redo the FX In/Out switch and send/return jacks to allow a "hot" send jack at all times for recording.

I agree and this is exactly how mine is wired (only return breaks the path). It just never got updated on the schematic which shows both jacks as switching. I never record off the preamp out, but I do use power amp in sometimes, and of course FX.

I keep the master hard wired to the loop input, let only the return jack interrupt the pass through signal and then have the switch put ground on the return signal when in "out" mode. See the schematic in this files thread: https://tubeamparchive.com/viewtopic.php?t=1979

I saw your schematic and that makes sense for your application. Have you done an A/B listen with the send always on vs completely out? It seems like it would not be an issue like you have it. I wanted to completely bypass the loop in case it messed with the tone too much only because I got the impression from some other forum members that it would alter the sound, especially on the clean channel. After I get it all working, I may try some alternate wirings. I need to change the bypass switch any way as It appears to be going bad (it was a parts box switch but never had solder on it. I guess age corroded the contacts even tho it never saw service).

How do you like the Master vol as shown in the schematic you referenced? I made mine a clean master on the theory that I don't need super clean but like to push even my clean sounds a bit and this seemed like a good way to do it and still get the OD to function about the same. I am however curious as to what the global MV sounds like in comparison.

Whit

[Tonegeek]

I swapped my plate supplies on the loop tube and put in a 12AT7. My voltages are now Send > 277, Return > 84. The 84 seems a bit low as I thought the consensus is 200 is about right on the return. I can certainly fix that with one resistor, but I will wait and see if anyone chimes in before I change it. BTW when I tried my 12AU7, the voltage went down to 47 on the return. Anyway the PS changes did nothing to fix the hum, but I think I got that figured out although i can't explain why what I did works.

I noticed I Could move the plate supply on the Send half of the tube around and the hum would completely go away at some spots. however, if I turned down either the loop receive, OD level, or clean master all the way, it came back right as the pot hit the stop. Crazy huh!! I looked at the schematic and thought well maybe the .05 input cap to the send grid needs some resistance ahead of it since it is humming when it gets sent to ground by the level pots, so I put a 100K resistor there and guess what, no more hum! I don't know why this works. Maybe someone with an EE background can explain it. I sure would like to hear from other people who have experience with the on board loop.

I played some with the loop engaged and bypassed and could hardly tell the difference in the sound (at least at a low - wife asleep level). The loop seems to fatten it up a tad. It also adds a tad more noise but only if you crank the return up higher than the level you have when it is bypassed. I don't see that being an issue.

Tomorrow it is off to the cabinet maker for a fitting. I am lucky that there is a guy 10 miles from me who does cabs for Fuchs, Germino and others. I was not looking forward to building my own although it will be 2 months till mine is ready...
Whit

[heisthl]

Glad you got your noises fixed, what are your p/s rail (B+) voltages now for the 2nd half of the tube? The 277 is good.

Answers to your previous post: No the always on send does not affect the amp tone/volume whatsoever. The non-HRM uses the single Master as standard fare, it works great because the OD level control allows the OD and clean volume ratios. Your HRM should have 2 masters for level matching.

I found some more wierdness with the internal loop on my current project tonight - loud hum when NOT using the OD. I was using an unshielded wire from the .05uF loop input cap to the first loop grid - once I shielded it no more noise, not sure why this occured with no OD only. I don't feel bad about using shielded internally for the loop - You have to use patch cords to the FX anyway.

[Tonegeek]

Glad you got your noises fixed, what are your p/s rail (B+) voltages now for the 2nd half of the tube? The 277 is good.

I will check tonight. I suspect I will need to change the resistor feeding that cap section to a lower value to fix.

The non-HRM uses the single Master as standard fare, it works great because the OD level control allows the OD and clean volume ratios. Your HRM should have 2 masters for level matching.

After reading a bunch of posts about the Master vols. I decided against a global master. When I go to OD it takes the clean master out of the circuit (except the 1meg to ground) so changes to the clean master would not affect the OD level (although of course clean vol changes still do affect OD). Between the OD trim, Drive and Level controls, I should still have plenty of independent control over the OD vs. the clean channel. I guess it could be argued that the gain structure is now changed, but that is what appeals to me, so I am experimenting a bit with this. If it does not work, I will simply rewire it to be a global master, but so far in my limited low volume testing, I am not displeased. I really need to be able to crank this amp and should get the opportunity this weekend. Then I will have more opinions about the masters.
[/quote]

I found some more wierdness with the internal loop on my current project tonight - loud hum when NOT using the OD. I was using an unshielded wire from the .05uF loop input cap to the first loop grid - once I shielded it no more noise, not sure why this occured with no OD only. I don't feel bad about using shielded internally for the loop - You have to use patch cords to the FX anyway.

Yea, this sounds like what I was going through with my loop, i would fix it for one channel, then the other channel hummed. I almost put shielding on it because when I touched certain components, the hum receded indicating it might be a shielding issue. The resistor mentioned earlier did the trick. If you think something is shorting the loop's .05 grid cap to ground (like the MV or some other control), you might try the 100k resistor. I noticed on the Hybrid A schem. there is an optional 100k resistor feeding the send output after the master vol. It looks like this may not be optional after all.
w