Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Gkworth on December 28, 2015, 11:39:44 am
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Gemtlemen,
I have a Super Reverb head, in which I am ATTEMPTING to install a relay board and supply board (reverb pedal switching) that I got from Doug, at this URL address.
http://hoffmanamps.com/MyStore/perlshop.cgi?action=template&thispage=Lamps&ORDER_ID=372112213 (http://hoffmanamps.com/MyStore/perlshop.cgi?action=template&thispage=Lamps&ORDER_ID=372112213)
It works, but induces 120hz buzz and noise ONLY when the 6.3vac is connected to the supply board. When not powered via the filament winding, no noise is present. The power transformer in this amp has a 6.3v winding with center tap. Of course, I had to lift the center tap and add two 100 ohm resistors (one from each leg to ground), to keep from cooking 2 of the diodes in the full wave bridge. I'm about ready to bail on this project. Any ideas? Thanks for any input.
George :help:
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The power transformer in this amp has a 6.3v winding with center tap. Of course, I had to lift the center tap and add two 100 ohm resistors (one from each leg to ground), to keep from cooking 2 of the diodes in the full wave bridge. I'm about ready to bail on this project. Any ideas? Thanks for any input.
George :help:
The key thing here is that the relay power supply should be floating and therefore not have any reference to chassis ground. (and therefore not be affected by centertap placement or type)
At no place in the circuit should your relay power supply circuit be 'shared' with the amp's circuit/chassis ground.
Put your 6.3VAC legs on the relay ps board and run the 5VDC positive AND negative to your relay circuit without it ever coming in contact with the chassis or amp circuit ground paths. Think of it as a completely separate power supply for the relay coils only. All it powers is relay coil open/close and you don't want it connected to chassis ground at all.
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I have a bunch of hookup info here on that
http://el34world.com/projects/relay_switch.htm (http://el34world.com/projects/relay_switch.htm)
About half way down the page is a diagram that shows two 100 ohm resistors with a bridge rectifier in the relay PS
(http://el34world.com/projects/images/relay3.gif)
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SG answered the way I thought it's supposed to be. Doug's drawing has it differently, that bridge IS connected to amp ground (along with the filter caps, etc.
Which one is right?
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All I can say is I have sold 100's of the relay power supply boards and no one has told me they do not work as I noted :icon_biggrin:
(http://hoffmanamps.com/MyStore/catalog/images/IMG_7176.jpg)
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Thanks for the replies guys. I have it connected as in Doug's diagram, at this time. I put the 100 ohm resistors at the pilot lamp socket, ala Fender. Should these resistors be at the supply board instead, and reconnect the 6.3v CT winding to the chassis? I'll try SG's method and see what the difference is. I was thinking a floating ground RCA jack would be needed, so the foot pedal does not connect anything to the chassis ground. It would use the ground on the supply board only. Does that make sense, so far as keeping the circuit grounds separated?
George
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My diagram up above shows the ground is the same as the chassis ground
Follow my diagram and see what you get
You have to have some sort of center tap on your 6.3v heaters, so leave that in place
If you do not have a center tap on your heaters, you will get a buzz
You use the +5 and Gnd connections on the relay power supply board to your relay coils
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Take a good clear picture of where your relay is located in the amp
You can pick up hum from nearby AC components like fuse holders, switches, transformers, etc
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Here is an image. Having trouble with file size. The boards are mounted over by V1, V2, the first stage preamp tubes. I tried to keep them away from any HV or large fields. I had them un-mounted and hanging, to try some things, but I temporarily re-mounted to the stand offs for the picture. If I connect the center tap of power transformer to the chassis, the diode in the "lower left" of the bridge rectifier diagram cooks, as it's then connected between one winding and ground, so I must use the 100 ohm resistor method on the pilot lamp, etc. I tried a .1uf to ground, from the CT of the winding. No effect on buzz. Position of board does not change buzz levels. Hanging in the breeze, outside of the chassis, or bolted in...same buzz. I'll put a scope on the filament windings and see what happens when the relay supply board is connected. Maybe the 5v regulator is adding garbage somehow. Defective?? 7v in, and 4.99 out, so it's right on the money.
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You say you mounted the relay power supply away from HV energy. Ahh, but by doing so you moved it close to the low-level preamp tubes and thus any hum it induces gets amplified.
It sounds to me like you have 2 separate issues. 1 is the functioning of the relay power supply and the interaction of the transformer CT with your synthetic CT. (the 2 100 ohm R's) 2 is the hum.
My sense is that because you have fed the relay board your 6.3 volts DIRECTLY from the socket for V1 and located the PS board right OVER V1, while the CT, whether real or synthetic, is all the way at the other side of the amp.....you are inducing hum into V1.
The relay power supply need not be located in a "quiet" location. It should not induce noise but it *could* induce noise located right over V1! This is a sensitive spot.
First: You only need 1 center tap, whether real or synthetic. There are several arguments in favor of the synthetic type. However, I think most builders either use the "real" one IF there IS one. Pick one and only one.
The experiment I would do is to remove the syn CT located at the pilot lamp. FOr this experiment you can leave the "real CT if it is installed. Now, hopefully you have 2 extra 100 ohm resistors and you place the dual 100-ohm "Y" structure right at V1, leaving your board where it is. Suspect: will reduce hum, some, not enough.
Next: Move board. I think that is the crux of your hum problem. Really cannot go so close to V1.
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Okay. I took the board out of the chassis and connected the 6.3v to the pilot light terminals, where the 100 ohm resistors to ground are located. With the board hanging in the air, the buzz is identical to when it was mounted in the chassis. Location doesn't seem to bother it. Hmm. (scratching head). I'm gonna try something. If i can find a small 6.3v filament transformer, I'm gonna connect the board to that and see what it does. If it works well, Maybe the easiest thing to do, is use that method. I would go the "isolation" method, as SG recommended... but the foot pedal, on this amp, shares a ground between the reverb and tremolo switches. I would need a proprietary foot pedal, with 2 ground wires and isolated RCA jacks. If someone used a standard one, the buzz would come back.
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Completely floating it should eliminate the problem. (isolation)
If you can convert the jack to a standard 1/4" it will be much easier to isolate the jack from the chassis.
Doug sells these insulating washers that I have used with switchcraft jacks and I imagine that just using a plastic Cliff jack would do the trick as well.
***Nice guitars on your website, by the way***
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Added a small 600mA, 6.3v transformer, just for the relay board. May seem extreme, but it works great. No added noise whatsoever. Picture shows it hanging, but it's mounted now, and all closed up. Thank you all for your ideas!
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Cool, glad you got it worked out