Somewhere between 102 and 183

[Colossal]

Very nice amp, Fred! I love all the old parts. Good job.

[fred.violleau]

Alright, quick follow up before I jump back to work.
Voltages seems to fall in place, I will post a complete picture of the readings tonight.

So far I have a constant loud hum, not changing when playing with volume or master pot. Will try to debug tonight.

Any advice on where to look first ?

Fred.

[fred.violleau]

Hey Colossal, thanks for the kind words!

I realize that taking the time to build and do it right is part of the fun!

Fred.

[norburybrook]

Alright, quick follow up before I jump back to work.
Voltages seems to fall in place, I will post a complete picture of the readings tonight.

So far I have a constant loud hum, not changing when playing with volume or master pot. Will try to debug tonight.

Any advice on where to look first ?

Fred.

Built in dumbleator. From having done this myself and spoken to people on TAG it seems the on board loop can be an issue. Mine is OK but i could NOT wire in a bypass switch without getting bad hum regardless of what I tried. My loop is always in circuit now because of this.

Just my initial thought from your post.

M

[fred.violleau]

Hey Marcus, thanks for chiming in! On my bluesmaster build, I have wired the d-lator with the bypass switch and it works very well, no hum at all.

This one feels like a bad ground, but I checked each with the meter, but one... May be the culprit... I will check this one tonight as well.

Fred.

[norburybrook]

Hey Marcus, thanks for chiming in! On my bluesmaster build, I have wired the d-lator with the bypass switch and it works very well, no hum at all.

This one feels like a bad ground, but I checked each with the meter, but one... May be the culprit... I will check this one tonight as well.

Fred.

no problem Fred. Tony A also warned me of possible hum issues . I couldn't get a bypass switch to work no matter what I tried hence my thoughts on your question.

If you've done it once then I'm sure you'll be fine.

M

[fred.violleau]

All good Marcus, every advices and potential explanation makes us smarter!

Fred.

[didit]

Alright, quick follow up before I jump back to work.
Voltages seems to fall in place, I will post a complete picture of the readings tonight.

So far I have a constant loud hum, not changing when playing with volume or master pot. Will try to debug tonight.

Any advice on where to look first ?

Pull all the preamp, OD, and loop tubes, leaving just the PI and power tubes. If there is still hum, which is expected, then the poor ground is almost certainly at the PI.

Hum is a witch.

Best .. Ian

[fred.violleau]

Love the way you strategize Ian, sounds like a plan!

Fred.

[fred.violleau]

Pulled the tubes as you said Ian and hum is light at the beginning and then builds up within 15 to 20 sec to become very loud and constant. No reaction to any of the volume pots.

Moved the NFB/PI to V4 to see if it would make a difference, nothing changed.

Checked all my grounds and all of them has a .02 ohm reading on the PS.

I will share an updated version of the layout so you can see all the grounding points.

[didit]

Pulled the tubes as you said Ian and hum is light at the beginning and then builds up within 15 to 20 sec to become very loud and constant. No reaction to any of the volume pots.

Moved the NFB/PI to V4 to see if it would make a difference, nothing changed.

Checked all my grounds and all of them has a .02 ohm reading on the PS.

I will share an updated version of the layout so you can see all the grounding points.

Seems you've got at least one source isolated. Check for loops as well as weak or missing connections. Seeing some points in photo hard to confirm are well & fully soldered. For example, the row along the end of the main board next to the PS board. Hard to trace wires that tuck in under. Keep at it.

Best .. Ian

[fred.violleau]

Ok so here it is

I am grounding the PT HV CT with the heaters CT, the bias supply and the B+ caps.
I am derivating current from one branch of the HV in order to feed the bias supply.
And finally I am using the 5v supply with a voltage doubler to feed the relays.
I don't think I grounded the 5v CT.

Here is a picture of the layout

PSUgnd.jpg

Here is the info of the transformer I am using
https://www.hammfg.com/files/parts/pdf/274BX.pdf

[fred.violleau]

Spent the night reflowing solder joints, making sure every ground is solid.
Found an unmelted piece of solder, but could not test the amp as the kids are now sleeping

Here are the voltages i get :
B+ : 441.4V
B+1: 440.3V
B+2: 419 V
B+3a: 378.7 V
B+3b: 305 V
B+4: 316.8 V
B+5: 309.1V

CL - 12AX7
V1A 1 : 188.5 V
V1A 2 : 0.1mv
V1A 3 : 1.786 V
V1B 1 : 198.7 V
V1B 2 : 0.1mv
V1B 3 : 1.622 V

OD - 12AX7
V2A 1 : 201.4 V
V2A 2 : 0.1mv
V2A 3 : 1.696 V
V2B 1 : 203.2 V
V2B 2 : 0.1mv
V2B 3 : 1.661 V

FX LOOP - 12AX7
V3A 1 : 305 V
V3A 2 : 18.81v
V3A 3 : 37.72 V
V3B 1 : 233.8 V
V3B 2 : 0.6mv
V3B 3 : 1.731 V

PI - 12AX7
V4A 1 : 298.4 V
V4A 2 : 36,09 V
V4A 3 : 57.83 V
V4B 1 : 284 V
V4B 2 : 40.49 V
V4B 3 : 57.83 V

6L6 x 2
V5 1 : 2mv
V5 3 : 442.7 V
V5 4 : 441.51 V
V5 5 : -62.6 V
V5 6 : 441.8 V
V5 8 : 2 mv

V6 1 : 2.1 mv
V6 3 : 442.9 V
V6 4 : 441.3 V
V6 5 : -62.6 V
V6 6 : 441.9 V
V6 8 : 2 mv

I am done for tonight, if you have any theory as to where the hum may come from, please let me know !
I will resume my debug session somewhere tomorrow if the work schedule allows it!

Fred.

[fred.violleau]

Quick update, and the good news is: I was able to rule out the preamp today.

Plugged the send of the FX loop into another amp's loop, and it worked fine.
All the toggle switch are functional, relays as well, volume, drive and level works fine as well.

Then I removed preamp tubes one by one until all four tubes are removed and I keep getting that huge 60 cycle hum.
So maybe it is not located in the PI, but further dowsntream.

I am thinking about the ground of the OT :
It is going from the OT to the speaker jack's ground lugs. These jacks are isolated from the chassis and there is another cable going from the speaker jack gnd lugs, back to a ground lug that located near the PT, which host B+2 filter GND as well as the main filter caps gnd.

I was thinking of moving the OT ground somewhere else and see if it works.

Any opinions?

Fred.

[Guy77]

Hi Fred! Great looking build!
Regarding the OT grounding location, I keep mine close to the PI .
Basically close to the spk jacks and near the PI tube.

I saw you are using the Hammond 274bx PT. I will have to try that one on my next build! I was wondering what your wall voltage was? I assume you used the 125v tap.

Cheers
Guy