Sluckey AC-15R Build Reverb Issue

[shaun]

Heyo, and Happy New Year!

About a year ago, I built - with a lot of help from this forum - Sluckey's AC-15R. It's a great sounding amp, and I matched it with an aging Jensen 12" ceramic, probably from the 60s. Loud as all hell.

Anywho, I've never been able to trouble-shoot the loud hum that occurs when the reverb is turned up. I've tried a variety of grounding systems, and I've replaced the V3 tube several times. I have a 12AT7 in V3 right now - I forget why I used that instead of the 12AX7 suggested in the schematic - probably got less hum that way :). The hum continues when the tank is disconnected - I jumped the RCA return plug to check this. I've checked the wiring about a million times.

The only thing I can say is that ALL the components attached to the V3 pins are very microphonic when I chopstick the amp, especially the .0022 cap running from Pin 1 to Pin 7. I checked the cap and it reads okay when disconnected from the circuit, so I'm assuming it's okay - I haven't had an orange drop go bad before, but I suppose it happens. The other components seem fine. Voltage on Pin 1 = 201vdc, and on Pin 6 = 104 vdc.

Anybody else had anything like this occur? I should point out that the Reverb is from a Matchless Chieftain.

Many thanks as always, and may you all have a great 2021.

[Williamblake]

Maybe the 220k grid leak is bad? Maybe take a picture?

[Latole]

Look you may have some bad solders.

[sluckey]

You probably have the one and only AC-15R. I put that together just for you. Use a gator clip test lead to connect V3 pin 7 to chassis ground. Does this kill the hum?

Try a lower gain tube for V3. I suggest 5751 or 12AY7.

PS... Lot's more info on this amp here...   https://el34world.com/Forum/index.php?topic=25468.msg276088#msg276088

[tubenit]

I think your best bet would be to keep at it until you resolve the hum problem.  You like the tone and I agree the video you made of this sounded lovely.

Back up option ………………………   rewire V2 and V3    I am not familiar with the approach Sluckey took on adding reverb into the LTPI.  I know that's been done successfully.  I simply have never tried it. 

You could rewire V2 and V3 to have a one tube reverb OR use a mosfet CF and have something closer to a Fender strength reverb?  see schematics below

With respect, Tubenit

[sluckey]

Just my opinion... I think adding an extra gain and tonestack after the EF86 will work but will also kill the AC-15 vibe. That EF86 with no tone control is capable of driving the power amp to full glory.

The reverb circuit is not my design. I simply copied the Matchless Chieftain circuit.

[tubenit]

Just my opinion... I think adding an extra gain and tonestack after the EF86 will work but will also kill the AC-15 vibe.

I agree with you.  It would change and probably kill the AC-15 vibe.  I do think getting it to work without hum (as is) would be a better option. 

The "back up option" is essentially a HoSo56 type approach  (ala DaGeezer)

With respect, Tubenit

[tubenit]

Things to try in trouble shooting:

1)  Sluckey's idea of a 12AY7 in V3

2)  jumper over V3a which has a 22k plate resistor and the microphonic .002 cap and adds gain.  IF this works and you like
     the reverb you can leave out V3a ……….. OR  V3a with 100k plate resistor and V3b as a cathode follower for no gain
     increase and eliminating the .002 coupling cap

3)  replace the suspected microphonic .002 cap

With respect, Tubenit

[tubenit]

Another option to consider.  One of the merits of this one is that it helps preserve some of the AC-15 vibe with the EF86 driving the LTPI.  IF the 220k mixing resistor lessens the gain too much, then try 100k.  Because the reverb pot is 1MA and you have a 330k resistor connected to the reverb pot, you can probably use a smaller mixing resistor  (220k to 100k)? (think of that 1MA pot and the 330k resistor as a "mixing resistor" since you won't be turning that 1MA pot all the way up).

This eliminates some things that may be contributing to the hum problem:

- 22k plate resistor
-.002 microphonic (?) cap
- going into the other side of the LTPI
- minimal rewiring which allows you to use the layout board you already have with few mods to it

Just trying to help.  Not saying this would necessarily be "better" but perhaps it would?

Sluckey,  what do you think about this as an idea?   

With respect, Tubenit

[sluckey]

The Matchless circuit is fine just as it is. No need to change it. I believe the problem lies with his layout and workmanship. Did you look at the other thread?

[shaun]

Wowza! There's a lot to consider here. I'll study all this closely, first eliminating all possible suspects before modifying. I'll post my results, although it may take a day or two.

Thank you!

[shaun]

As usual, Sluckey was right. I checked all components once again, and all looked fine. I looked through tubenit's great mods and was about to launch into them when I decided that, before I did, I'd try lifting the reverb footswitch off the chassis with a fiber washer. Well, that meant routing the jack's ground to the center of the star ground, and it made a very big difference. So I'm one happy camper. I've looked at jack isolation washers and wondered what sort of effect they would have, and now I know.

I'd also like to say thanks, tubenit, for those schematics; I learned a lot from them. Even though I didn't get that far in applying them, when I analyzed them, they helped me realize how straight forward circuits sometimes can be. I'm still quite intimidated by what I don't know, and your mods turned the light bulb on for me, I think primarily due to their clear practicality. Much appreciated. I'll keep them handy for the next time I run in to reverb difficulties.
 

[sluckey]

Well, that meant routing the jack's ground to the center of the star ground, and it made a very big difference.

What do you mean by "the center of the star ground"? There was no star ground on my layout. Could you post a hi-rez pic that shows what you did?

[shaun]

Well, I mean to the primary grounding point, which I usually keep near the PT. I've found that when I use it as the ground point for difficult wiring jobs - which this was for me - it can solve ground loops etc.

I can post a pic, but I'm sure you'll remember what a mess it was. Do I have to embarrass my self again? 

A few minutes after I posted the above, I realized the amp was already back in the cabinet, which was no small feat - my cabinets are as crazy as my chassis, so it would take a lot of work to get it out. Sorry.

What's the plural of chassis?  Chassises? Chasses?

[sluckey]

The plural form of chassis is also chassis.