Blues Jr PT/OT in a Hoffman Stout TMB Solid State Rectified

[jonyoungyi]

Hey all, I picked up some Blues Junior transformers for cheap and instead of building another Hoffman Blues Jr, I thought I'd use them to build a Stout TMB (First time building something other than Fender!).

Anyway, I'm all wired up, but I'm getting an insane amount of buzzing from the build. Besides that, everything works. If I turn up the volume I can hear the guitar signal coming through.

Here's a video of the issue:

&feature=youtu.be


I built this Stout with a solid state rectifier, but other than that, I kept all the component values the same because I wasn't sure what I needed to change.

I know buzzing is usually associated with a ground issue, so I triple checked all my grounds, and all seems well.

For the chassis ground though, does it matter if the star isn't directly on the chassis, but instead, perhaps on the negative of my filter caps?

I'll post some pics soon.

Any and all comments welcome and appreciated, thank you!

[jonyoungyi]

Could it be that I used the smaller 16mm pots vs 24mm? Less power tolerance = dead pot?

I've built a handful of amps just fine, but never had this weird volume issue.

[PRR]

It's not the pot size. Audio pots don't work hard.

You have a bad connection. Keep looking.

[John]

For the chassis ground though, does it matter if the star isn't directly on the chassis, but instead, perhaps on the negative of my filter caps?

If you mean you ran all grounds to the neg side of the filter caps, I think that would introduce a lot of buzz. Or you could simply have a bad (even though new) filter cap.

[jonyoungyi]

For the chassis ground though, does it matter if the star isn't directly on the chassis, but instead, perhaps on the negative of my filter caps?

If you mean you ran all grounds to the neg side of the filter caps, I think that would introduce a lot of buzz. Or you could simply have a bad (even though new) filter cap.

Yeah, at the moment all of the designated chassis grounds are connected to the negative of the filter cap. That includes a ground that is bolted directly to the chassis. The IEC's earth is grounded by it being mounted to the chassis, so I didn't bother running a dedicated wire to the star.

I did this because of bad design haha The chassis bolt is hard to get to, so I thought it easier to ground everything at the filter cap's negative.

[jonyoungyi]

It's not the pot size. Audio pots don't work hard.

You have a bad connection. Keep looking.

Good to know about the pots!

Wouldn't be the first time my eyes missed something, even after a triple check. I'll keep looking after I clear my tunnel vision, thanks!

[sluckey]

Is there a florescent light nearby?

Sounds similar to a guitar cable with a broken shield. Since the volume control seems to kill the buzz look at the circuit between the input jack and the vol ctrl.
 

[dude]

Usually the main power supply star ground is located near the PT or on one of it's legs, all e-caps are grounded there except the first stage filter cap for the pre-amp. This pre-amp ecap filter and the pot grounds, other coupling cap grounds are all grounded to the input jack ground on the opposite end of the chassis. You sometimes see a buss bar ground running along the top of the pots, picking up other grounds ending up at the input ground. But the input pre-amp grounds should "not be connected" to the main star ground near the PT. If you have your input ground to the power filter ground that's a problem, hum. I would think if your power supply star ground is on the first ecap filter, you're ok as long as the ground from there is on the opposite end of the chassis from the input ground. 


Post a gut picture of your built.


al

[jonyoungyi]

Edit: pictures are too big. Need to compress them on pc and repost...

Hey, all, sorry for the late reply and thanks for all your replies!

I'm a little reluctant to send gutshots because of everything going on in there, but I've attached them anyway.

Here are some things I might suspect or things I have tried:

1. I stopped being lazy and ground all appropriate chassis grounds to a chassis star ground (see pic).

2. Regarding the output jack. It's a plastic isolated jack that switches a load resistor if a speaker isn't plugged in, but, check out the ground.

Is it okay to ground the jack via the xlr chassis ground? I'm thinking not since the only thing grounding the xlr in the first place are the screws holding the jack and chassis together. The Jack's sleeve is grounded though.

3. The input jack. So I have a reverb circuit inside this amp and it's wired at the input (compared to the typical fx loop).

1/4" jack -> shield wire -> Reverb -> shield wire + 33k(1m to jack ground) ->  v1 Lug

Hope that makes sense... Is that okay, to move the 33k and 1m to AFTER the reverb circuit, BEFORE v1.

Hope this all makes sense... Tried a lot of things, but that buzzing is consistent.

[jonyoungyi]

Here are the gut shots!

[dude]

Maybe someone else can chime in. That input jack looks close to the power star ground?

[shooter]

power star ground

It's in the lower right, the one in the upper left appears to be the pre ground.

guessing all the orange n blue and green fly-over wires and the IC board all under the pots might be a great source for induced coupling, aka noise/buzz

[dude]

Seems the ground side of the bd is opposite of the usual..? Those fly-over wires would be much shorter if the bd was laid out opposite of what it is? Usually the plate R’s pins, 1 and 6 are on the same side as bd. with short wire length. Be a big job to reverse this. Get he amp to make the noise and chop stick those over the top wires, if you get results, use shield wire or try under the bd.


But hard to see in the pic

[dude]

The bd is not opposite, it’s right, hard to see.

[st]

Layout of the amplifier checks out. You have added two solid state circuits, however (one reverb; what's the other?). This messes with the layout. Your input, e.g., goes across v1 and v2 to the reverb board, then back to v1. Secondly, it might mess with power: how do you provide power to these two boards and are you sure it's clean power? Can you bypass them? If yes, does that make a difference? Would it be possible to completely remove them from circuit (power included) for now, so we can focus on the amplifier first?

[jonyoungyi]

Layout of the amplifier checks out. You have added two solid state circuits, however (one reverb; what's the other?). This messes with the layout. Your input, e.g., goes across v1 and v2 to the reverb board, then back to v1. Secondly, it might mess with power: how do you provide power to these two boards and are you sure it's clean power? Can you bypass them? If yes, does that make a difference? Would it be possible to completely remove them from circuit (power included) for now, so we can focus on the amplifier first?

Hey St, thanks for the reply!

The other circuit, currently disconnected, is a H&K Red Box Clone that'll be phantom powered. I've added this to one other amp build and it worked great, so I figured I'd add it to this one, which is the reason for the load resistor.

You are right that the input flies over to the reverb and back to V1 and I have tried bypassing completely and wiring like the layout/schem shows, but that didn't change anything. It wouldn't hurt for me to try to disconnect the 9v step down completely!

One thing I'm not really sure about is what resistor values to change since I'm using solid state and a different PT. I think when I calculated, the PT/solid state is pushing less voltage than what the original schem is calling for.

I'm wondering, maybe that's my issue?

[jonyoungyi]

Here are the PT specs:

CURRENT PT (Stock Blues Junior PT)
Secondary: 256 VAC @ 68ma (based on the http://el34world.com/Hoffman/files/Hoffman_BluesJunior.pdf)
Filament: 6.6 VAC / 2.6A

RECTIFICATION CALCULATOR
Selected PT: 260-0-260
B+: 356.2v (Subtrac 6v because of Choke)
Fialment Current: 2.18A
Current at Load: 88.23mA at 8000R Load


ORIGINAL PT (Heyboer 18 Watt PT)
Secondary: 290 VAC @ 120ma
Filament: 6.3 volt CT

RECTIFICATION CALCULATOR
Selected: 300-0-300
B+: 375v (Subtract 6v because of choke)
Fialment Current: 2.18A
Current at Load: 93.3mA at 8000R Load


Amplified parts show the stock Blues Jr as having 68ma after rectification. Could that be my issue?

Also, should I just adapt the Hoffman Blues Jr's power section to the Stout's, including that bias circuit?

One thing I will change, for sure, is the output jack grounding. I asked about it earlier, but the answer is obvious. Bad practice to have a ground that's only mechanically held in place haha

[jonyoungyi]

Man, I'm so close to finishing this! The amp works and the guitar sounds great, but there's that annoying, loud buzz in the background.

I printed the layout on two separate occasions and went through each component, link, wire, connection, etc. Everything seems fine. I have NOT physically measured the actual resistance of each resistor yet. I've just visually confirmed the colored stripes, etc.

I also tried removing the 9v step down and reverb circuit completely, so the input is tied directly to V1, and there's no AC going to the step down, but that didn't change anything.

Buzzing is usually associated with a grounding issues, especially if the amp is working, but all my ground connections look good according to the layout. I also "chopsticked" on more than one occasion and none of the wires being jostled change the buzzing sound. So it really does sound like some kind of ground issue..

I measured my voltages and everything seems fine, but I'm not 100% sure what's fine any more. I'm basing this off of the Hoffman Blues Jr Conversion's voltages:

A: 321v
B: 319v
C: 282v
D: 272v

Anyone have any other ideas?

[ncusack]

I only took a quick skim of the previous posts but did you provide an artificial center tap for the heater supply? If not something like 100R from each side of the 6.3V referenced to ground could help with the noise.

[jonyoungyi]

I only took a quick skim of the previous posts but did you provide an artificial center tap for the heater supply? If not something like 100R from each side of the 6.3V referenced to ground could help with the noise.

Thanks for the suggestion! I was convinced it's not heater hum, but at this point I'll take anything hahaha

[shooter]

I'll take anything

measure the hum freq, if it's 120hz it's ps, 60hz it's fil or induced(coupled) through wires.  If it's neither, it's FUN 

[sluckey]

What I heard is not PS hum or filament hum. It's more like a guitar cable with a broken shield (ground).

[jonyoungyi]

What I heard is not PS hum or filament hum. It's more like a guitar cable with a broken shield (ground).

The Artificial ground fixed it, thanks ncusack!

But yes, in the video, very audibly, the hum/buzz is definitely not heater, or AC for that matter.

The PT had none of it's secondaries going to ground. There is no wire for center tap. In the original blues JR, there's a bias wire that goes to ground, but I omitted those two wires and shrinked/taped them off.

So, I'm assuming it's because the PT wasn't grounded? Is that a thing?

[jonyoungyi]

The amp is working great! Thank you everyone who commented and got my brain rolling!

There's still a bit of hum, but it's a perfectly normal amount. It could even be my old apt's bad electrical (there's no earth in one of my rooms!! dangerous!)