Old 2 channel Dual, hum, noise

[pgrudge]

Hi. I have a Dual recto with the serial 127. The previous owner got it modded from 110 to 230V (that we have here in sweden) by an old electronics-dude that i think rewound the original transformers. It's been working great.

Anyway i love this amp to death. I've been keeping it several times when i really should've sold it to afford to eat!

Anyways it's been getting noisier and hummier lately. The hum seems to be "in tune" with the slight hum that i can hear the transformer to the right (from the back) make if i put my ear near it. It makes me wonder a little if the previous owner got a good job done, or if it's just amplified by bad power tubes. What do you think?

The noise is kind of a white preamp noise, and i've been switching the preamp tubes around a little without better results. I'm the kind of guy that only get new tubes when something is really wrong. If it aint broke.... Lately my playing has improved a lot and i want to nail a great tone that i can live with and i'm thinking of a full JJ retube hi-gain kit with the kt66's and kt88's. Any users here that have experience from that kit in a recto that old? I play thru a Marshall cab with greenbacks so anything too brittle would be superbrittle in my setup.

Thanks in advance
//Mattias Johansson, Sweden

 

[mikey383]

As far as the hum goes, it may be a preamp tube going microphonic. You could try swapping around the ones close to the power tranny with the ones further away and see if that helps.

For the tubes, some people like the JJ high gain kit, some don't. For me, I think their preamp tubes work great in spots other than V1...I think they're too dark in that position, but I like them in the others. I also have a revision G. Yours is probably a revision C, which is said to be brighter, so maybe the JJs would sound good in V1 of your amp. I don't have any experience with their KT66s or 88s, but I do love their KT77s.

 

[pgrudge]

Thanks. I tried some more swapping around and definatley noticed a change in the noise, suggesting that one or a couple of the preamp tubes are noisy, and when you mention it, a microphonic preamp tube could pobably pick up the hum from the transformer mechanically i guess.

Anyway a change of the noise is reason enough to get some new tubes! Out of curiosity what do you run in V1? I've always used an electro harmonix, but i think it's a little too "knifey" sounding.

 

[mikey383]

If you still have the original Mesa STR7025s in it, they're likely past their prime.

I have an old '65 Mullard 7025 in V1 right now. It's pretty sweet on the top end and has a nice midrange bite to it. You can usually pick them up for fairly cheap on Ebay. I think I paid around $15 USD for it and a Mullard 12AT7. It's followed by an unlabeled tube in V2 (I think it's a JJ), then JJs in 3, 4, and 5. I have a boatload of NOS and new production tubes that I've acquired in the past few years, and that's the combination that I found to work the best for my style.

 

[pgrudge]

Thanks man. I think i'll order a mix of some good brand preamp tubes and experiment 'til i get them in the order that sounds the best for me. I think i'll go for some new TAD 6l6's too.....Only problem i have is that i'll try one combination and then get stuck playing guitar for several hours, but whatthehell you gotta have something to do between gigs, kids, dayjobs and girlfriends right? ;-)

 

[mikey383]

That's what ya gotta do! You'll either find the right combination and want to play for hours, or find that 30 seconds is too long with the wrong combination. Either way, it's fun playing around with tubes.

 

[JonCurcio]

It's an older amp, so it could be bad filter caps. You should have a tech look at them if you're not experiened, as the voltage can easily kill you. Never touch them unless they're been discharged...

 

[pgrudge]

Yeah i've been thinking about that. I have a degree in electronics and won't risk my life poking around with undrained capacitors. If retubing donesn't fix it i'll get the stuff and have an amptech do it for me, cause i haven't been working with high voltages for ages. I'm guessing it's the 4 pretty big ones ? The two HUGE ones out by the rim of the amp even look dangerous! but i have never lookes at their values-