Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Ronno25 on February 28, 2022, 12:45:46 pm

Title: ATTEMPT #2 - Hammond AO-39 conversion to Sluckey ac-15 lite
Post by: Ronno25 on February 28, 2022, 12:45:46 pm: Back in April of 2021 I attempted to copy Sluckey's AC15 lite point to point build. With all of your help I got it up and running. But it had bad parasitic oscillation that I couldn't get rid of. I decided to rebuild it.

For the rebuild I used a more traditional turret board layout instead of point to point. It works and sounds good. But it does have hum that I have thus far been unable to get rid of. The hum increases with the volume pot and goes away completely with volume on 0. Sometimes after I've played through the amp for a few minutes the hum almost compltely goes away on it's own. I've reflowed every solder connection in the amp with no change. My voltages also check out:

I've scoped the amp and located that the hum first appears in the circuit at the plate (p6) of the ef86 (v1). So basically the signal going into the tube is clean but it comes out with hum. I've tried swapping the tube with no success.

Any ideas what the hum problem may be and what I can try to fix this?

On a side note: the ef86 squeals when I crank the cut and volumes controls while the amp is in a combo cabinet. I've mounted the socket on rubber grommets and put tube rings on the tube without success. I'm considering giving up and just putting it in a head cab. Are there other solutions I could try to let me keep it in the combo?

Thank you in advance.
Title: Re: ATTEMPT #2 - Hammond AO-39 conversion to Sluckey ac-15 lite
Post by: sluckey on February 28, 2022, 02:36:37 pm: I have my Dual Lite in a combo cab. No problems.

Use a gator clip test lead to put a ground on pin 9 (grid). Does this kill the hum? The turret for the cathode R/C ground does not look good. Make it pretty.
Title: Re: ATTEMPT #2 - Hammond AO-39 conversion to Sluckey ac-15 lite
Post by: Ronno25 on February 28, 2022, 06:57:21 pm: Sluckey, grounding v1p9 doesn't kill the hum. I'll make sure to fix up that turret.
Title: Re: ATTEMPT #2 - Hammond AO-39 conversion to Sluckey ac-15 lite
Post by: Ronno25 on March 01, 2022, 05:03:40 pm: Sluckey, what would be the next step to trouble shooting this?

Thanks again!
Title: Re: ATTEMPT #2 - Hammond AO-39 conversion to Sluckey ac-15 lite
Post by: sluckey on March 01, 2022, 06:30:33 pm: I think your layout is a major factor. You have long wires for the EF86 passing right over the OT plate leads. The entire chassis looks much busier than need be for this simple amp.
Title: Re: ATTEMPT #2 - Hammond AO-39 conversion to Sluckey ac-15 lite
Post by: bmccowan on March 01, 2022, 06:43:16 pm: Following Sluckey's lead I have built several different amps from AO-39s - including an AC-15 Lite. Along the lines of his advice, I found the Hammond wiring scheme to cause problems - signal wires located too close to B+ feeds. BTW you have tried swapping the EF-86? Some are just plain noisy even in you isolate them.
Title: Re: ATTEMPT #2 - Hammond AO-39 conversion to Sluckey ac-15 lite
Post by: Ronno25 on March 01, 2022, 07:36:06 pm: Yikes, am I in for attempt #3?!? :BangHead:

I did already try swapping the EF86.

Would it be a possible solution to wire the EF86 point to point as much as possible and ground components right next to the tube?
Title: Re: ATTEMPT #2 - Hammond AO-39 conversion to Sluckey ac-15 lite
Post by: bmccowan on March 01, 2022, 11:28:19 pm: I doubt you need to do that. I think sometimes folks follow an edict of making leads as short as possible, which leaves out the possibility of moving them around to see if they can change the noise floor. Sometimes moving wires around makes a huge difference. I know you scoped it, but how about a listening amp? That is typically my saving grace.