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Topic: Twin Reverb Noise Question |
Ian Sutton
From:
San Francisco, CA
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Posted 16 Jan 2013 11:13 am |
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I recently purchased a '71 Twin Reverb (pre-master volume) and it has this annoying hiss that I can't seem to get rid of. I've had a tech do new power tubes, and a few new preamp tubes, as well as replace the carbon comp resistors, and plate resistors, and it had a recent cap job before I bought it. The tech says it's about as quiet as it's going to get, but i'm dubious of that statement.
At this point, it's gig-able, but I was previously playing a borrowed BF Pro Reverb that was church-quiet.
Do any of you amp gurus have any thoughts/suggestions on what I can do remove this hiss?
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Tim Marcus
From:
San Francisco, CA
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Posted 16 Jan 2013 12:09 pm
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You're in SF? I have some ideas - quieting amps down is very important to me. I've got a few tricks 
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Ian Sutton
From:
San Francisco, CA
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Posted 16 Jan 2013 12:17 pm
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Hi Tim,
I am in SF indeed, and am a bit green with the voodoo they call vintage tube amps. I've been neck deep in Gerald Weber books, and have been hoping it's something reasonable like a funky pre-amp tube or something, but it's the dreaded 'seashell to your ear' kind of hiss.
Any insight would be much appreciated. Feel free to email, or PM -- whatever is convenient for you.
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Stephen Cowell
From:
Round Rock, Texas, USA
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Posted 16 Jan 2013 12:20 pm
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Start by locating the noise... turn down all controls... still noisy? Then pull the first tube for each channel... my guess is that this will get you quiet. Divide and conquer! Go one channel at a time... you can start pulling plate resistors to isolate stages if you have to, after you put one channel's tube back in. Good luck, it's normally the first tube in one of the channels, or one of their plate resistors. Could be a cathode resistor as well, you're suspecting a component passing DC current.
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Too much junk to list... always getting more. |
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Ian Sutton
From:
San Francisco, CA
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Posted 16 Jan 2013 12:21 pm
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Hi Stephen,
Thank you, I should have mentioned that in my first post. I've isolated it to V4. When I pull V4, the hiss goes away. Different tubes in this position produce varying levels of the aforementioned hiss.
Thanks!
Ian
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Ian Sutton
From:
San Francisco, CA
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Posted 16 Jan 2013 12:30 pm
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In addition, the noise is present regardless of volume level once the amp is taken out of standby. And again, goes away if I pull V4 (reverb recovery/ 1/2 of gain stage for vibrato channel).
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Tim Marcus
From:
San Francisco, CA
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Posted 16 Jan 2013 12:32 pm
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Probably the 3M3 resistor in the reverb mix circuit or lead dress to the plate of V4b
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Tim Marcus
From:
San Francisco, CA
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Posted 16 Jan 2013 12:34 pm
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Also note that pulling V4 takes out V1 and 2 as well. It takes out the entire preamp circuit
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Ian Sutton
From:
San Francisco, CA
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Posted 16 Jan 2013 12:38 pm
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Thanks Tim! Regarding the other tube positions.
V1 removed:hiss
V2 removed: hiss
V3 removed: hiss
V4 removed: no hiss
From what I understand, V5, and V6 don't affect tone in a way that would contribute or resolve my issue.
Regarding the 3M3 resistor in the reverb mix circuit, wouldn't that mea that the hiss would be isolated to the vibrato channel and not affect the normal channel?
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Tim Marcus
From:
San Francisco, CA
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Posted 16 Jan 2013 3:19 pm
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No - because the output of V4b mixes in to the phase inverter with the output of channel 1 (V1)
The problem is Sutro tower - it's the bane of my amp building existence. Those old carbon comp resistors sound great, but are very susceptible to RF noise in a high gain circuit. I use a carbon metal film for the 3M3 mix resistor. If you can get an amp quiet in SF it will be quiet anywhere. It's a worst case scenario.
V4b (the second triode of V4) is always on no matter how you have the amp set, unless there's a master volume. It must be quieted down or you can clip out the bypass cap to reduce gain of that stage (not a terrible idea for pedal steel anyway - keeps the output clean)
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Ian Sutton
From:
San Francisco, CA
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Posted 16 Jan 2013 3:29 pm
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DUDE!
Sutro friggen tower. I live off 9th ave in the Inner Sunset and it has been the bane of my DAW for the past two years. I have a drawer full of ferrite chokes from when I was chasing my tail with RF computer noise. We are line of sight to the tower.
Well, since my tech confirmed we replaced the resistors in the reverb circuit, that would take the 3.3m resistor out of the equation.
I gig in North Beach tomorrow, I wonder if it will be any quieter there.
In addition, I wonder if the Pro Reverb was quieter due to the smaller OT etc. being a 40 watt amp?
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Tim Marcus
From:
San Francisco, CA
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Posted 16 Jan 2013 4:39 pm
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yeah - less fizz overall with less power
Sutro tower has been a curse and a blessing for me. Curse because it ruins any chance of recording at home I attempt (I live in the lower haight - also line of sight to the tower) Blessing because if I can get my Milkman amps to be dead quiet in sight of a radio tower, and with 100+ year old terrible wiring in my house (which has no grounds on the outlets) then I know they will be dead quiet anywhere.
My guess is that the 3M3 was not replaced - or if it was, maybe he put another carbon comp in there.
If you want an easy solution that will help some I would try pulling out that bypass cap from the suspicious gain stage. Its very easy to solder back in if it does not work.
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Ian Sutton
From:
San Francisco, CA
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Posted 16 Jan 2013 5:22 pm
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Hi Tim, i'll check with my tech and let you know how I make out. I'm still a little timid to tear it open and work inside on my own. My only other amp work was doing some BillM mods on my BluesJr.
BTW, I've been hearing a lot of great stuff about your Milkman amps, who else in town is playing them?
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Tim Marcus
From:
San Francisco, CA
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Posted 16 Jan 2013 5:29 pm
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Josh Yenne has one, and I have one, and occasionally I lend one to Tom Heyman and Dave Zirbel- but SF needs to step it up! NYC is starting to make us look bad - most Milkman amps go out of state.
You are welcome to come by and check it out any time that I am here - I also have the new Sideman 50W amp prototype here that you can be one of the first to try out.
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Ian Sutton
From:
San Francisco, CA
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Posted 16 Jan 2013 5:37 pm
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Thanks again Tim - I may take you up on that sometime. The sideman would be perfect amp for me - one side for telecaster, one side for steel.
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Stephen Cowell
From:
Round Rock, Texas, USA
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Posted 17 Jan 2013 2:52 pm
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| Tim Marcus wrote: |
| Also note that pulling V4 takes out V1 and 2 as well. It takes out the entire preamp circuit |
Oops! Look again... ch1 is still hot.
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Too much junk to list... always getting more. |
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Stephen Cowell
From:
Round Rock, Texas, USA
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Posted 17 Jan 2013 2:58 pm
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| Ian Sutton wrote: |
| In addition, the noise is present regardless of volume level once the amp is taken out of standby. And again, goes away if I pull V4 (reverb recovery/ 1/2 of gain stage for vibrato channel). |
... and you've tried winding the Reverb control up and down, no difference? Since different tubes hiss differently, I'd try a *real* quiet tube here, like the Sovtek 12AX7WB... although this is a low-gain position in the amp, so I'd normally suspect plate resistors.
Good luck!
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Too much junk to list... always getting more. |
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Donny Hinson
From:
Glen Burnie, Md. U.S.A.
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Posted 17 Jan 2013 3:41 pm
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If it's not a tube or plate resistor, lift one end of the negative feedback resistor and check the value. This resistor is really critical to the amps high-freq gain and hiss. If you can snag a 2k trimpot and mount it somewhere accessible and put in place of the resistor, you can adjust the N/F on the fly.
(The N/F resistor is hooked to the O/P tranny secondary.) |
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Tim Marcus
From:
San Francisco, CA
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Posted 17 Jan 2013 6:00 pm
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| Stephen Cowell wrote: |
| Tim Marcus wrote: |
| Also note that pulling V4 takes out V1 and 2 as well. It takes out the entire preamp circuit |
Oops! Look again... ch1 is still hot. |
so much for attempting to do this by memory - yeah, they join at the PI via 2 220k resistors from what I remember. I said that once, and then contradicted myself. Too much time playing with single channel amps!
I usually experience a bad plate resistor as crackles, not so much hiss. However, there is a lot that could have happened between 1971 and now. A Twin should be pretty damn quiet even in sight of Sutro tower.
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Tom Tobey
From:
Florida, USA
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Posted 24 Jan 2013 4:21 pm
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| I would have the amp tech check the bias on the power amp tubes. there will be a noticeable noise difference between properly biased and improperly balanced tubes. If you have done everything else, give this a shot. cant hurt and always helps to have it done every once in a while. it only takes about 15 min to do. |
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