B-15 layout

[Tom Phillips]

I found that the layout was missing a plate resistor for the 2nd 6SL7. The one with the red circle around it in the new layout.
And the B+ node for the preamp tubes is now after the 1k dropping resistor.

Matt,
You put a lot of work into that layout drawing and it looks really good. I haven't compared it to an original B-15. How much is based on an original layout and how much of it is improvements to the grounding etc that you have been working on?
I ask because the information will be useful for others to use when working on their amps. I suggest that you add a title block with your name and the date/version and consider submitting it to schematic heaven.
Regards,
Tom

[txstrat]

Tom,

thanks for your kinds words, haven't thought about this before. Great idea. Actually I didn't think my humble work could be of help.
The layout is based on pics I found on the net and got from you. Thanks again. I adapted the layout to my terminal strips as good as I could.
I found the hum was the same like I had in my latest Princeton clone build. It came from the negative bias supply. I had put the electrolytic cap on the wrong side of the dropping resistors. Added a second cap before them and the hum was gone.
BUT working on the amp and trying to reduce the hum improved the circuit hum wise even without a second cap on the bias supply. So I believe it must be VEEERY quiet when I add the second cap.
I still have the amp at my drummers place but will pick it up next week to change the bias circuit.

cheers

Matt

[Tom Phillips]

...I found the hum was the same like I had in my latest Princeton clone build. It came from the negative bias supply. I had put the electrolytic cap on the wrong side of the dropping resistors. Added a second cap before them and the hum was gone....

Matt,
Was that the final fix for the build discussed under the Thread "Transformer hum"? I was wondering if you got the amp working to your satisfaction or if you just accepted it as close enough.
Cheers,
Tom

[txstrat]

Matt,
Was that the final fix for the build discussed under the Thread "Transformer hum"? I was wondering if you got the amp working to your satisfaction or if you just accepted it as close enough.
Cheers,
Tom

Tom,
the final fix for the B15 copy is yet to come.
In the build after that - which is sort of a Princeton - I encountered the same sort of hum. I didn't think of coincidence and since I found the culprit in the "Princeton" I'm sure it's the same in the B15.
My last post in the "Transformer hum" thread contained: "there must be some obvious thing which I haven't been thinking about yet..."
I bet this obvious thing is the bias cap on the wrong side.
But even if it is not, I could live with the hum level, which is not completely satisfying but, as you said, close enough to accept.
I'll report as soon as I picked up the amp at my drummers place.
cheers
Matt

[txstrat]

Here's what I found.
The bias cap (100uF@100V) was actually already on the right side. BUT, as soon as I clipped a second cap in parallel to the first the hum decreased about half so that it's barely audible.

D@mn: if I recall correctly I had already done this to no avail at that time.

The bias circuit as it is now is old fashioned like in tweed or brownface amps.
I'm gonna replace it with an improved circuit as soon as I find the time. Just wanted to keep you up to date.

[Tom Phillips]

That makes me think that the existing 100uF/100V cap was bad. Adding more capacitance as you describe would not normally make that much difference to the bias

[txstrat]

Changed the bias to an improved version yesterday - with two caps before and after the voltage divider. All hum gone.
My bass player checked the amp out at home and told me he's excited and can't wait to check it out in our rehearsal on wednesday.

[drewl]

That makes me think that the existing 100uF/100V cap was bad. Adding more capacitance as you describe would not normally make that much difference to the bias

I have a B15 on the bench with two 100uf in the bias supply, one on either side of the 47K I think.
It's dead quiet.

While looking up the schem, there seems to be alot of cross over between the different varieties of B15's.

This one has 6L6's, fixed bias, with the 6SL7 preamp tubes, kind of a cross between a B15N and B15N-F, no speaker ground connection for point "F" phase inverter cathode ground, plus it has the Ultra High and Low switches in the preamp.

This baby looks brand new inside, I'll try and scan the schem.

[txstrat]

I'll try and scan the schem.

Wow, that'll be great and if it's not too much of a hassle some gut pics of the layout would be great. Unless it's a PCB version which wouldn't be of much help while planning a PTP layout.

[bob p]

The PDF of the layout seems to have been lost. Any chance someone couuld re-upload it?

[bob p]

txstrat, i was looking at some of the other sites, and i saw your posts about noise in your build. did you ever get it sorted out?

[txstrat]

txstrat, i was looking at some of the other sites, and i saw your posts about noise in your build. did you ever get it sorted out?

Yes I did.
I played a little with different layout plans, insulated jacks, different "star groundings" vs. "one wire grounding".
Ended up with the layout posted here. It's not as dead quiet as my other amps, but good enough regarding to my bass player who loves the amp.
(I believe the 6SL7's are a little more prone to noise anyway)
I'll re-upload the layout pdf-file.

[bob p]

glad to hear the news. i'm keeping an eye out for your layout upload.

[txstrat]

Here's my layout plan. It's not the one I've posted before, because I couldn't find it (must be on one of my data sticks - I'll keep looking for it).
I think you can get the picture.
The second triode in a 6SL7 is less prone to hum because of it's inner structure. That's why I would change the input section of stage one to the second troide of the first 6SL7 (other than in the layout).

Here you go...