Vox AC10 build issues

[Jalmeida]

I have a ton of pictures showing the build here and some recents with circuit mods.

https://drive.google.com/drive/folders/10GvpZVH9rkmf-pj42Q1SvLOgNtprsNCH

[Jalmeida]

https://drive.google.com/file/d/1gYsQiXcAYDc2G_o4whjWgbYpysHUs0KE/view?usp=drivesdk

This shows the most recent of the circuit. Not as tidy given the recent mods and troubleshooting. But will be cleaned up once I get the final circuit figured out.

[Jalmeida]

**Update***

Added a node to the power supply dedicated to the 12DW7. The motorboating went away. But I have a low frequency hum now. For the added node I place another 8uf cap to ground and a 1.5k resistor before the 22k resistor. Will play around a bit with the cap and resistor values to see how the noise changes.

Also, I was advised to remove a few un-need complexity in the circuit by recommendation of a tube amp engineer. Here’s where we are at now.

[Jalmeida]

Another update…

I clipped in a 100uf capacitor to ground in temporarily at the R8 and my low frequency hum went away. I tried this also at the 305V B+ junction at the tagboard and the noise went away the same.

I have a couple things I want to try in this order. Let me know what you all think:

1. replace 8uf reverb power supply node with 16uf, 22uf and 32uf to see where/if  low frequency hum for who drops off.

2. Possibly replace the double reservoir 16uf x 16uf cap for a 32uf x 32uf. Looking over the ac10SRT schematic, they have more power filtering added.

3. Possibly add another RC network between the new node and B+?

[sluckey]

All those ideas are worth experimenting with. As for #3 also consider powering the reverb driver with the higher voltage node but use an additional node with bigger resistor to power the recovery circuit.

[Jalmeida]

All those ideas are worth experimenting with. As for #3 also consider powering the reverb driver with the higher voltage node but use an additional node with bigger resistor to power the recovery circuit.

Thanks for the advise. Curious for my own understanding, what is the concept to separating the driver and recovery sides? And does playing with the value on my 1.5k resistor to a higher value seem worth evaluating? I admittedly don’t fully understand the correlation with the resistive values as far as how they effect power filtering/conditioning.

[Jalmeida]

Okay, so I got the reverb dialed now. Played it for an hour tonight and the sound is great! Lots of reverb.

Along the lines of Sluckey’s suggestion, I added another node and split the drive and recovery power nodes. Ultimately I paralleled(instead using of cascading)the Reverb nodes and broke them into a driver and recovery node. 

[Jalmeida]

After all of the reverb woes, I got the reverb hum resolved but now have a ever present 60hz hum independent I have been chasing for a few weeks now. It was way worse than it is now. But still present.

Since I got the reverb fixed I have been working on the other power supply nodes to filter out the 60hz hum. What I have is a 60hz hum(verified by spectrum analyzed) that does not change with instrument plugged in or not. Does not vary with instrument volume. As soon as the amp comes on with the volumes all the way down it’s present. Here’s what I have done:

1. Changed reservoir capacitor to 32uf+32uf(like AR10SRT)

2. Changed filter caps C13 and C16 from 8uf to 16uf.

3. Added 10K/16uf RC network at V1(similar to AC15)

4. Swapped out V3, V6, V1 and V2 with new.

5. Put reverb level pot ground to be on same groundplane as rest of reverb circuit.

6. Corrected Reverb tank send/receive shielded grounds and isolated connectors(tank has grounded send and receive)

7. Replace signal wire between R45 and reverb pot with shielded wire.

8. Disconnected reverb circuit to rule that out all together.

9. Added ground between OT secondary and preamp ground screw(dedicated ring terminal). Was floating off of connected to C1.

10. With clip terminal lead grounded preamp to numerous chassis points to see if ground point for tagboard better suited for other ground source. No change.


All of the above besides #4, 7 and #9 made improvements. But I have an amp that is too noisy to be useful for anything besides furniture. And I out of ideas at this point.

60hz hum is present with chassis out of cabinet.

I have read quite a bit about grounds on tube amps and know that had I to do all over, i’d stick to the diagram shared here many times. But I stuck with bits I could piece together from the Stephen Grosvener book and what I could find was done on the original AC10. I am fairly certain at this point that it is ground related because touching my instrument strings makes the hum level drop significantly, although not disappear. I am missing something. Looking for last ditch efforts to quiet this thing down before it turn into a boat anchor.
 

[tubenit]

You have a very nice looking amp!   My advice is do NOT give up!   Keep working on it until you have an amp that you can really enjoy.  Yes, you may have to change something to get it where you want it to be.  However, that's way better than giving up on it and not using it at all.  Your build is not boat anchor material, IMO.


The first amp build I ever did literally took my over 2 months of trouble shooting over weekends to get it fixed. It still stands as the most time consuming problem on a build that I resolved.  It was a brand new resistor with a break inside the enamel that would be intermittent when playing. When I would "test" it, it showed good but with bass notes being played it would open and close with vibration. UGH! 


My "best sounding" amp ever took about a year of working on it to get it dead quiet at idle. I probably opened it up maybe 4 or 5 times during that year and experimented with different approaches.


Don't give up!       With respect, Tubenit

[sluckey]

Add two 100Ω resistors to the filament winding as shown...

[Jalmeida]

Add two 100Ω resistors to the filament winding as shown...

I will give that a go. Currently I have 100R going to the EL84 cathodes instead of ground with a adjustable pot(humdinger style). Admittedly the pot does not change the noise level at all when adjusted.

Some oddities in the Grosvenor book were the addition of an erroneous jumper and ground right at the ECF82. Those at first power on caused all sorts of issues and were omitted. So right now the only filament ground is to the cathode of the EL84s and are otherwise directly to the transformer.

[sluckey]

Currently I have 100R going to the EL84 cathodes instead of ground with a adjustable pot(humdinger style). Admittedly the pot does not change the noise level at all when adjusted.

Would be nice to show that on the schematic.

[Jalmeida]

All of the grounds on the tagboard are daisy chained and exit the tagboard at C1 that ground to a chassis bolt. There is a ground connection that connects this daisy chained groundplane to the pot/input jack ground bus. I did clip that and clip on to a dedicated control pot ground and the noise level remained unchanged. Part of me suspects the daisy chained groundscheme on the tagboard being suspect. There area some EL84 screen resistors and the tone circuit sharing the same groundplane. But all of the plate and cathode for the EL84 are in the lower chassis power section like the original and have their own ground tagstrips.

[HotBluePlates]

After all of the reverb woes, I got the reverb hum resolved but now have a ever present 60hz hum independent I have been chasing for a few weeks now. It was way worse than it is now. But still present. ...

If it makes you feel better, my 1965 AC10 Twin also hums.

It happens as soon as the power is applied & before any tubes warm up, so cannot be cured without modifying the layout (which won't happen).

[Jalmeida]

Currently I have 100R going to the EL84 cathodes instead of ground with a adjustable pot(humdinger style). Admittedly the pot does not change the noise level at all when adjusted.

Would be nice to show that on the schematic.

Good point. I’m at revision 8 of my schematic haha.

[Jalmeida]

After all of the reverb woes, I got the reverb hum resolved but now have a ever present 60hz hum independent I have been chasing for a few weeks now. It was way worse than it is now. But still present. ...

If it makes you feel better, my 1965 AC10 Twin also hums.

It happens as soon as the power is applied & before any tubes warm up, so cannot be cured without modifying the layout (which won't happen).

Thanks for your encouragement. I was pretty frustrated last night and at my witts end.

I’d love to hear your AC10 if you have a recording.

Here are a few of mine with hum:

No volume idle hum and normal chan

EF86 hum

Both channels bridged

[Jalmeida]

Other lessons learned along the way….


I used 600V rated teflon insulated solid core hookup wire because it was suggested and keeps shape where it is placed. In hopes of keeping a clean build. The wire is great for that one reason alone. But if you chase issues or have to tweak and modify it is horrible. I have had wires break constantly. Lots of fixes with every repair or tweak. Sonif you have a proven amp design and layout proven, the wire is awesome. But I won’t use it again. So chock that one up for lesson learned.

Next is LED power pilot lamp. So I used a 110V LED power lamp right off the mains switch because the indicators were easy to find and I figured why not? Well along the way I have had noises all over and going away and come back. Today playing around with more reverb integration tweaks and playing with mixer resistor values I fins I have a hum back. Or more a buzz… hmm. So the coupling capacitor that connects the normal channel off the volume pot to the PI is where the mixing resistor solders to. And I have been working in this area since I first got the amp powered on. Some while poking with a chopstick I found that if I move C15 one direction the noise gets louder. Towards the pilot lamp. Pop the lamp from the bulb holder and find it is spraying out noise like crazy. Not sure if that is the nature of using a AC mains lamp or LED. But definitely replacing with adifferent lamp and powering off the heaters. How many times I have been down the noise road? It is A problem but I am apprehensive that it is THE problem. Anyways, for anyone wanting to know where this stands… 🤦🏻‍♂️

eature=share

As for the reverb circuit, i have the reverb working and have tried pilling the dry signal the reverb drive from the other side of the reverb so that the drive is goes up and down with volume. I have tried multiple suggestions to try to get the reverb so that it doesn’t start the slow and perpetuating feedback induced with the reverb level above 1/2 up.

Here’s what I have tried and the results:

1. Changed C11 from 500pf to .022uf
2. Changed C24 from 1uf to .22uf.
  - These two cap changes made the howling feedback to a squeeling feedback.

3. Moved C24 dry signal from V2 plate over to between Normal volume wiper and R45.
- The reverb drive was very weak. Feedback squeel starts immediately.

4. Temporarily clipped in resistors to ground at R37 around the existing 1M varying from 750K all the way down to 330K.
-Feedback starts slower each resistor value down I go, but is still present.

5. Replaced R45 with 3.3M(ala Fender).
- extremely weak reverb drive (almost none at all)
 
6. Tried a 750K for R45.
-still weak reverb

7. Moved C24 dry signal feed back to plate.
- feedback seems to be gone with reverb. Need to fix a few other things before I am certain of the reverb being fixed. The dwell also seems better likely due to the capacitor changes not piping so many lows to the tank.

In summary…

I believe that the Swart style reverb circuit is a compromise. If I had a cascaded gain stage I believe I would be able to tie the dry signal drive after the volume and would be good with another gain stage before the PI. But because I go right into a PI from the reverb re-integration, I needing all signal I can to drive this with one tube.
So I will keep the reverb integrated at the plate and it will function like a stang alone reverb where I mix dry and wet signals. The nature of scabbing on a one tube reverb like I am I suppose versus adding another recovery tube. 🤷🏻‍♂️

Attached schematic is prior to temporary mods.