RCA BA-2C Buzzing Issue

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mattrad

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May 29, 2021
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Portland, OR
Thanks Tony, finally got some time to work on this thing again. I flipped the power supply area around as you mentioned so that the ground point is as close as possible to the CT. Alas the buzz persists. Read and reread Ruffrecords' grounding document, as far as I can tell everything is shipshape. Strangely throughout troubleshooting this thing the mechanical buzz has become much much less prominent- curious. bolts at the PT are tight, even disconnected it from the chassis to see if that did anything, no change. Maybe the next move is to swap the other one in.

Also, took advantage of the extra tube socket and wired it up for tube rectification, figuring this would solve the problem since it seems to stem from the diodes. Was puzzled to find that the buzz was still there- though its possible it had something to do with the alligator clip leads being about 10x too long. Though, the buzz didn't respond to the leads being moved around and the character of the noise was the same.

Putting a 100nf cap across the PT secondary in addition to the caps across each diode knocks the buzz down further. I also tried a RC snubber across the secondary with a potentiometer and a cap in series, I could tune the pot for minimum buzzing but couldn't get it to vanish completely, and it was hard to tell if it was more or less effective than a cap by itself. Overall it seems like the more capacitance I put around the diodes the less it buzzes, wondering if theres a catch to putting some 1uf caps in there and calling it a day? Still though, one wonders what the underlying issue is thats making all this necessary in the first place!
 

5v333

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Jun 30, 2013
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499
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Gothenburg
Could there be an issue with DC on the mains that makes the PT into a buzzer?

Or Could there be an issue with the PT being loaded too lightly?
 

5v333

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Jun 30, 2013
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499
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Gothenburg
also i would try joining the ground pin of the sec input tx to the cathode resistor and bypass cap instead of a direct path to starground.
 

alphasnk

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Apr 10, 2019
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Location
France
Hi, I'm joining the conversation guys!

I've also built a simili ba-2c, and i was amazed at how quiet it is especially in this state haha :) I got no buzz issues at all

I didn't pay much attention to the grounding, i tried to keep things close together: the input is grounded to the 100k gain potentiometer, the output is grounded to the cathode of V2

I'm currently trying several input transformers: beyer 1:15, beyer 1:10, api 2622. Right now i have a chinese 600:15k which i found amazing for the price, it's very close to the api 2622, same low end response with a lil bit more density, the high frequencies are more dimmed, but it's a beautiful sound!

The output transformer is an edcor 15k/600, 2 156c chokes in humbucker config (it killed any buzz i had with maximum gain), 1uf decoupling capacitor

I'm having fun with the circuit, i replaced V1 with a pf86, the 6j7 works pretty well at 4.5v! The mess on the right is an 6as6 that i'm trying to mate with the circuit, in order to have a clean preamp with the regular version, and some distortion to record drum machines.. Right now it's working more as a compressor than a distortion, but i've got to try more things!

The additional stuff i tried:

triode to pentode mode for v1, works great, triode sounds dryer, more density and headroom, pentode sounds more wrapped around and sustainy.. And an output potentiomer..

It's a great circuit to learn and try things with tubes :)
 

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emrr

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Apr 12, 2006
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7,874
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NC, USA
I would try the big things: different input transformer, different power transformer, try the power transformer external and at distance, etc. Bludgeon the damn thing. If you want a similar trail of pain, see my 1567A thread.
 

mattrad

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May 29, 2021
Messages
27
Location
Portland, OR
Update- settled into a .33uf cap across the secondary as well as a .22uf across each diode. Also came across a sensitive wire while prodding around- from the second stage plate into the cap/choke junction- so I shielded that. All seems to be well in buzz land at this point! Maybe a band-aid fix, but at this point i'll take it.

There IS a slight 60hz hum that doesn't respond to the volume pot. At full gain its barely noticeable against the hiss, more prominent at lower gain levels- definitely workable though! I have a 100ohm humdinger pot on order to replace the two resistors, we'll see if that helps any. Will also try to futs around with the PT and shielding etc to see if I can get any change. Thanks for all the help y'all!
 

mattrad

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May 29, 2021
Messages
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Location
Portland, OR
Hey everybody, figured i'd add another update for those who may be interested-

I ended up swapping the Hammond with a similar value toroid transformer mounted inside the chassis and that essentially solved all my problems. No more buzzing from the rectifier and no capacitors across diodes necessary. Rotated the PT for lowest noise and its very very quiet now.

I tried 2 different Hammonds (bolted and unbolted from the chassis) of the same model so it was not some sort of malfunction. Seems like for whatever reason that particular transformer didn't play well with the particular circuit I had going. From what i've read the steel chassis may have played a part in the mess as well...

Anyways, thanks for the help everybody!
 

Creamman

Member
Joined
Jun 17, 2019
Messages
9
Hey everybody, figured i'd add another update for those who may be interested-

I ended up swapping the Hammond with a similar value toroid transformer mounted inside the chassis and that essentially solved all my problems. No more buzzing from the rectifier and no capacitors across diodes necessary. Rotated the PT for lowest noise and its very very quiet now.

I tried 2 different Hammonds (bolted and unbolted from the chassis) of the same model so it was not some sort of malfunction. Seems like for whatever reason that particular transformer didn't play well with the particular circuit I had going. From what i've read the steel chassis may have played a part in the mess as well...

Anyways, thanks for the help everybody!
Hi ... I have a hum in my circuit, probably from the power supply. Which Hammond Transformer did you use? Mine is currently Hammond 370ex.
 
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