Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jeph8871 on April 25, 2012, 12:07:44 pm
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I have a Bedrock Royale that came in with buzz and hum. There were two types the high pitched oscillation from bad grounds and poor lead dress and an underlying hum that sounds like a poor ground.
Here's the kicker. The preamp has a boost function that takes the bypass cap in and out on the second triode of the preamp. When it is pulled out the noise goes away.
electrolytics and filters have been replaced. I have ruled out an aux 5v transformer for the recto filaments (disconnected and went with SS). The tone and volume pots have no effect on the noise. I have redressed the first stage and replaced the shielded cable (I got new stuff from you) as well as the jack.
My theory is that the lower gain signal (with bypass cap removed) is just low enough to pick something up in the PI stage. When the bypass cap is engaged the much higher gain signal is just strong enough to "get around" it.
There are no schematics around for this. But it is much like an Ampeg / Orange running cathode biased.
Just looking for general thoughts not a solution. I know you don't know me from Floyd but I figured I've been getting some parts off you and maybe you might what to help a guy out :)
Another thing I did notice is the speaker jack reference to ground had been left off. However, restoring this has no effect.
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This may not apply here because I can not see a schematic?
IF the boost feature is relay operated, it may be that the "open" connection is with the boost engaged and the "closed" connection has a magnet that shifts the DPDT (or whatever switch). Sometimes those relays when closed, will have some noise. My understanding is a magnet switches those relays and can induce noise.
Take a Dumblish type amp. It is possible for the OD (if relay is open) to be quieter (in noise and hum) than the clean (less boosted & overdriven section) when the relay is closed.
IF this is not relay switched, just ignore what I have said.
Hopefully Sluckey or Geezer or someone more knowledgeable will chime in.
With respect, Tubenit
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Never heard of this amp. Is it boutique or slap-dash?
Unless it is somehow precious, I would consider re-working the ENTIRE ground system per any known-good philosophy. That might cure many "unrelated" troubles. Like lower noise with higher gain. Of course ponder this against the significant labor and the value of the amp.
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Early 90's boo'tique. Google it. Great sounding amps
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Sounds like some ground issues, bad solder flow on some grounds.... or maybe a leaky cap.
Tubes? any IWT ? (intermittent when tapped)
Found these schemos.
Hope they help.
http://bedrock27.tripod.com/bedrockamp/id4.html (http://bedrock27.tripod.com/bedrockamp/id4.html)
Regards, Mike
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I agree. I was thinking just this morning that they used a copper clad board and it has one common ground.
Going to try moving some.
Checked for leaky caps and everything was in low mV
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One main ground can do it. bzzzzzzz
I do separate grounds for PS, PA, and preamp.
Interesting that Bedrock didn't ptp the components and have a better ground scheme.
Then they did a copper clad instead.... :dontknow:
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If adding the bypass cap eliminates the noise, it sounds like either a bad/noisy preamp tube, or noise is being induced into the cathode of that stage (either from heater to the cathode, or across the 5.2k cathode resistor).
I'd be looking at the wiring from that cathode/bypass cap to the switch, and see if it could be picking up noise. Swap that tube to see if a known good one eliminates the noise. See if the wiring to/from the switch is shielded, and where the shield is grounded.
My theory is that the lower gain signal (with bypass cap removed) is just low enough to pick something up in the PI stage. When the bypass cap is engaged the much higher gain signal is just strong enough to "get around" it.
So the "new theory" is that the 5.2k cathode resistor is a big enough impedance to cause noise to be picked up by the wiring to/from the switch. When the bypass cap is switched in, it effectively shorts those noise voltages to ground, which kills the noise. Switch open, now the cap doesn't bypass the noise to ground, and the cathode is an additional input terminal.
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That makes sense
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I'll try placing a shielded wire there and see if it stops it. It does make sense. I'll let you know.
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Is that wiring not already shielded?
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Well shielding made no difference. I doped out on this one. If I haven't learned the first time I may not by the 1000th. Never, assume just because a customer installed new tubes everything is fine.
Turns out the "new" Tung sol preamp tube he put in was highly microphonic, it was picking up vibration from the OT. Strange design having it right under the preamp input. Thinking of ways to minimize the transference to the chassis. Maybe grommets on the bolt holes.
Replaced it with a good tube and the noise was cured. I also, seperated the grounding to a star grounding scheme. They have a pc/clad board with one tie to ground for everything right at the input. Once I changed this some other oscillation became much easier to tame.
Cleaned up the tone control wiring a bit.
It had really great tone the whole time..but now it's quiet and more useful.
Would love to get my hands on one of these for..myself .
Thanks for the help
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Cool, you got her up and running silent.
Always do the IWT test on all the tubes before you do anything.
Just tap with a pencil (eraser end) and listen for whistling, popping, microphonics, etc.
I've had bad tubes right out of the box.....
I'm sure re-arranging some of the wiring helped too.
Good job.