bonedog569
399 posts
Oct 19, 2011
7:02 PM
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I finally got the BR9 humming (both ways) again. It did need a new PT - I tried everything else first. It now has a weber WO22913 OT, a hammond 270 DX PT. I also replaced the PI transformer - a Mercury mag GA-PI, and added a choke. (I'm running a standard speaker, not the original field coil)
Only issue is a pretty bad hum (60 cycle I think - not sure) - but only when the volume pot hits certain spots.. The knob goes from 7 o'clock off - to 5 o'clock full on. It's quiet till about 3 when it starts to hum, quiets down around 4, then gets loudest at 5 - full on.
I've tried playing with all the transformer orientations, and unbolted them from the chassi to do so. I took out the choke temporarily too. I can make little differences in the hum sound, but it didn't get to the main problem.
I will trim excess wires next and twist heater leads better - (the original gibson wiring doesn't twist the heater leads at all )
I tried a little experimentation with grounding differently and will do more of that too. Most grounds going to a screw securing the PT - with some near the preamp tube going to the input jack grounds.
Does the heater wiring to the 5y3 tube matter in it's relation to the heater wiring on the other tubes? I could try switching orientation on those.
I couldn't find an exact spec replication of the original PT and had this 270 DX
waitin for the right project.
The sound / tone I'm getting is excellent - as it was before I f-d it up running it with no load. All good except the hum when I'm up near full volume.
Any and all suggestions appreciated. - 5F ??
An asside, I love this little bugger so much , I picked up a twin BR-9 last week for two bills. It was missing the groovy grill but I'm fabricatin that.
I want to have a stereo BR-9 set up going, and will soon.
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bonedog569
401 posts
Oct 19, 2011
9:01 PM
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additonal spec.s
original BR-9 PT put 723 AC onto rect. tube
270DX puts only 598 AC inro rectifier
6V6 plate voltage is close however; 280 VDC original 276-278 VDC with 270-DX
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Last Edited by on Oct 19, 2011 9:02 PM
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5F6H
914 posts
Oct 20, 2011
2:22 AM
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Hi Bonedog,
"I will trim excess wires next and twist heater leads better - (the original gibson wiring doesn't twist the heater leads at all )" Make sure that your new twisted pair wiring doesn't leave any of the wires running close to grid wires (6SN7 pins 1 & 4, 6V6 pin 5) or plate wires (6SN7 pins 2 & 5, 6V6 pin 3).
"...and added a choke." Did the choke replace an original dropping resistor (rather than in addition to) & where did you put the choke (circuit-wise)? If the choke was added in addition to the original dropping resistors then you need to add another filter cap at the junction of the choke & adjacent power supply resistor if there is not one there.
"I tried a little experimentation with grounding differently and will do more of that too. Most grounds going to a screw securing the PT - with some near the preamp tube going to the input jack grounds." The preamp filter cap (8uf on schem) should be grounded to the input jack, the filter caps straight after the rectifier and at the OT centre tap ground to the PT bolt. Does the hum pretty much disappear at "4"? This is often a symptom of grounding all the filter caps at the same point.
"Does the heater wiring to the 5y3 tube matter in it's relation to the heater wiring on the other tubes? I could try switching orientation on those.", No, orientation of the 5Y3 heaters (yel wires to pins 2 & 8) is irrelevant, what did you do with the yel/blk wire on the 270DX?
An old & often used trick with cathode biased amps, to reduce hum, is to connect the PT heater centre tap (grn/yel) to the hot side of the 6V6 cathode resistor, rather than to ground. This biases the AC heater voltage with a dc voltage over the top. Make sure that there is no chance that you are accidentally grounding the hot side of the 6V6 cathode resistor...or you'll need to buy some more tubes...& transformers.
It's often useful to post pics of the amp in question (PT & tube socket wiring?) either here, or mail them to me at markwjburness@btinternet.com
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www.myspace.com/markburness
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bonedog569
402 posts
Oct 20, 2011
10:54 AM
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Thank you so much Mark -
to answer some of your questions :
The choke is added after the dropping resistor. The dropping resistor I had with old PT was a 1K - with no choke, now with the lower voltage B+ it's .5K and then into the choke,
There are 7 wires off the PT. No yell.blk wire - There is a Red/yell - the center tap. It goes to ground.
2 red - secondary stepped up voltage
2 yellow - 5V
2 grn - 6.3 V
Looking at the original schematic I see they drew a 6.V heater circuit center tap to ground - I guess this PT doesn't have one - Perhaps that's a big part of the problem ? ?
I will try some of your tips and tricks my next bench session - though my time is limited till later next week.
I have a 274BX which more closely matches the original PT specs, but it too larege to fit in this amp. I could temporarily wire it in to see if it makes a difference. It's got 13 leads off it, - including a yel/blk
Thanks again - I'll email you for some flollow up - unless this thread generates more interest from others.
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Last Edited by on Oct 20, 2011 10:57 AM
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chromaticblues
1036 posts
Oct 20, 2011
11:41 AM
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Bonedog if there are only 7 wires than the 6.3 volt heater circuit isn't centertapped and that is your problem. You can rectify this problem by running a 100 ohm 1 watt resister from each terminal the green wires connects to on the first socket they are wired to. Then ground the other end.
If I was doing it I would install a stand up terminal close to the transformer and ground it at the PT bolt and then run the wires to the socket.
2 wires for power
2 wires for rectifier
2 wires for 6.3 volt heater tubes
and the power centre tap.
Thats 7
Hope that helps
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5F6H
916 posts
Oct 20, 2011
11:49 AM
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"Looking at the original schematic I see they drew a 6.V heater circuit center tap to ground - I guess this PT doesn't have one - Perhaps that's a big part of the problem ? ?"
Bingo! You bet it is, you currently have no ground reference for the heaters, run a 100ohm 1/2W resistor from each side of the heaters (say from pin 2 & pin 7 from the same 6V6) to ground, or to the 6V6 cathode resistor at pin 8 (whichever way gives least hum).
Don't worry about the 274BX, a PT doesn't have to have the blk/yel or grn/yel to run quiet, I just didn't realise your PT didn't have filament centre taps & most folks don't know what to do with a yel/blk when they see it.
So add the 2x100ohms & the filter cap between choke & 0.5K and I'm pretty sure you'll be rockin' again.
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www.myspace.com/markburness
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5F6H
917 posts
Oct 20, 2011
12:00 PM
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@Chromaticblues - Simulpost! :-)
@ Bonedog - Looking at the BR6 schem, you don't need the 0.5K in the power supply at all with the choke (either way the choke is in the B+ chain before the OT centre tap, so needs to be rated for the full current draw of the amp, say 70-90mA?), I'd add a 0.5K/470ohm 1W (or better on the W) to each 6V6 socket between pin 4 & the B+ supply.
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www.myspace.com/markburness
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bonedog569
403 posts
Oct 20, 2011
12:23 PM
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Thanks much guys -- I'll get on this as soon as I get some bench time.
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bonedog569
404 posts
Oct 20, 2011
6:11 PM
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Bingo indeed - that does it - Yea Team ! I'll post some pic.s of my
two amps - and maybe some sound clips , when I get the chance.
I am learning from my mistakes, that's a good thing - I'm just not confident I'll remember what I learnt when the next similar scenario comes along - We'll see.
Thanks again guys.
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Last Edited by on Oct 20, 2011 7:20 PM
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chromaticblues
1037 posts
Oct 21, 2011
6:09 AM
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Glad to help. That is to cool of an amp to not use!
I think those are great blues harp amps. I'm not a fan of real loud amps.
I was going to comment about the dropping resister and the choke, but I thought I may have misunderstood what you did. So I did comment.
You can use both if the dropping resister is wired one end to the pin 8 (I think It's pin 8 I haven't done this in awhile) of the rectifier and a say 20uf 450 volt cap to pin 8 and the other end grounded. Then connect the other end of the dropping resister to the section of the power supply that has the OP transformer wired to it. Then put the choke in between the Plate and power tube grid section instead of a resister. You have to test the voltages after to see if you need to add a resister at the socket to drop the grid voltage a little. You should be able to find a schematic that gives the voltages and just adjust resisters untill you either get the original voltages or something that sounds cool to you.
The only important thing is the the running grid voltage has to be lower than the plate.
Useing the choke will get you closer to the original sound because the fieldcoil speaker had the same thing as a choke.
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bonedog569
407 posts
Oct 21, 2011
10:00 AM
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This is the schematic I used as a reference - along with the old original Gibson hand drawn one. Originally I only used a 1K resistor, now I have a .5K and the choke in series. I'm on my way out of town to New Orleans but will look into this more when I get back.
http://www.gregcutshaw.com/Gibson%20BR-9/Gibson%20BR-9%20Amp%20Version%202.pdf
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chromaticblues
1039 posts
Oct 21, 2011
11:37 AM
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@Bonedog Yeah I would just remove the .5K power resister altogether. It's a low voltage amp as it is. They are very thick sounding amps. Removing the resister should give it a little more punch. I thinkit would sound better without it!
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