Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: newguitarsmell on December 20, 2018, 11:39:18 pm
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Here are the pictures I took of the guts and outside of the amp I have that has an obnoxious buzz.
I don't know much about it but it sounds GREAT. OK for gigs in noisy bars, but not when I need things to be quiet.
I've been trying to solve this issue for awhile with no luck.
This is an amp that is voiced for keys. The amp goes from nothing to full blast in about 1/16 of a turn.
I don't really know what pics to shoot - so I am including two links to a photo bucket file. I couldn't get the pics to upload, so this is where I am at with this technology.
This amp has a SS rectifier and I'd like to use a tube rectifier to see if that calms it down some.
Again - I did not build this amp, and don't know much about it. It sounds so good, except for the buzz, I am trying to learn as much as I can about amps so I can fix the darn thing. I have not chop sticked around in there yet.
The buzz is constant - when something is plugged in or not. As long as the amp volume is on, the buzz is there. The volume pot doesn't make any difference to the volume of the buzz... and it buzzes with whatever I use as a sound source.
Thoughts?
AS ALWAYS - a big THANK YOU to you geniuses out there....I appreciate all your help.
<iframe width="480" height="360" src="http://s706.photobucket.com/user/mmayer97801/embed/slideshow/Amp"></iframe>
http://s706.photobucket.com/user/mmayer97801/library/Amp
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Has the amp ever not buzzed?
It's not obvious from the photos whether the amp chassis is properly connected to mains ground? eg use a meter to measure resistance between ground on the IEC mains inlet and the chassis metalwork.
Does the heater circuit have a reference to the amp's chassis / 0V? eg use a meter to measure resistance between any terminal of the green wire heater circuit and the chassis metalwork.
It would be helpful for you to provide a 'key' to what the power transformer wires colour codes indicate, ie what circuit each wire colour is intended for. If you don't have that, how about any sort of part identifier from the transformer end bells?
Are you competent to work (eg take voltage measurements) inside a live amp chassis?
Haha, have you got a meter; from your previous posts I can't find of making an electrical measurement :w2:
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The rectifier socket is wired for a 5 volt rectifier. Put a 5AR4 in the socket. I don't expect it to make a difference.
The filament string is wired backwards, ie, PT to pilot lamp to preamp tubes and finally ends at the power tubes. This means that the high filament current of the power tubes must flow right by the sensitive preamp tubes. I would fix that. Run the wires from the pilot lamp straight to the first power tube, not a preamp tube.
I also can't tell if the filament winding is referenced to ground either with a real center tap or an artificial center tap. Check this as pdf64 stated.
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PDF 64 - I will take some readings tomorrow and see if I can find the schematics for the transformers.
There is neither a green center tap or an artificial center tap from the green wires. I will add that.
I will require the filaments so they go to the power tubes first, then down the line.
The reverb never worked, and I sent the amp back once when I first got I think because it was so noisy. They claimed something happened in shipping, fixed the main problems and sent it back with the buzz.
It has always had it from day 1 though the builder claimed it never buzzed for him.
I took the thing to an amp guy and he did nothing. I think he had no idea what he was doing because when I got the amp back it had different tubes, but everything else was the same. He said the amp "didn't make sense to him"...
As a newbie to amps, I noticed the lack of the green center tap right away - there are some curious things in this amp..l will try those two things first and see if that changes things. Those both make sense to me.
I will report back.
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You mentioned the reverb never worked? Do you have anything plugged into those jacks? When you sent it back were they able to get the reverb to work? BTW are those bias point jacks on the back of the chassis? Gosh I hate photobucket - between the ads and clickbait when you try to go back.... :BangHead:
Jim
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Hi Richie,
Sorry about photobucket. I had to pay the $5 monthly fee to get those stupid ads to go away. No, the reverb never worked though they said it did.
I haven't dug into the amp yet but plan to. It should be easy to sort out those first things - surrounding the filaments. I don't know about bias point jacks - there is a line out, the output for the reverb and a line out - in the event I want to send the bad buzz somewhere else. :think1:
There is a little pot inside that I believe is for bias...
I could try rebiasing the tubes - though you'd THINK the amp tech would have tried that...
It's that bad buzz that brought me into this tube amp building world, so for that I am grateful! This is so much fun and so frustrating and expensive at the same time! And the amps - so delightful. I love them all..
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UPDATE
I changed the filament wiring around and added a center tap. There is some wonky way the reverb is wired up so I can’t remove it without shorting out the amp. I resoldered some stuff on the first preamp 12ax7 and bam. Buzz is gone.
I haven’t tried it with keys yet. Now I wonder if the reverb will work, too? Something was shorted between the reverb pot and the volume pot...
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UPDAtE 2: SUCCeSS
I started learning how to build amps so I could eventually fix this amp I have had since 2007. . The bad buzz one..
I am happy to report it is now fantastic! No noise at all.
Steps - for those who are curious.
I rewird the filament wires so they would go to the power tubes first. I added a center tap ground.
I measured voltages at pin 3 and adjusted the bias. That gave me more headroom, but turning the bias dial also gave me some white noise that didn't go away when I turned the vol to 0.
One by one I started swapping tubes. Preamp 1 - 12ax7 NOS. Tube 2- same thing. Tube 3 12aT 7 replaced with a Mullard NOS I had.. Tube 4 replaced 12ax7 with an amperex 12 au7.
Still a little noise as I adjusted the bias...
Replaced Tube 1 with a new Sovtex 12ax7 - much, much better.
That resolved, I started in on the power tubes.
I had Ruby el34s in there. Swapped for a big bottle Military spec (1944 army navy) 6L6 G - also NOS.. and finally replaced the other El34 with another big bottle Tung Sol NOS tube from the 50s.
Adjusted the bias again and - perfection.
Without all the help from this forum, I would have never been able to achieve my dream of fixing that buzzing amp. I use it with keys, and even going straight in with a line level, it sounds absolutely fantastic.
Thanks everyone!!