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12.10.199.11
In Reply to: RE: Don Allen modified magnavox tube amp help posted by AstromanoftheFuture on May 23, 2016 at 12:16:05
Pictures please! If you have a schematic, upload it.
A nice, good performing, and modestly priced phono preamp is Jim Hagerman's "Bugle2". Yes, it is opamp based, but it uses passive EQ, just like quite a few "in vogue" tubed units.
Eli D.
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underside
If you touch that chassis at the same time as one that's grounded, you can get a nasty shock. Maybe that's why the seller doesn't want to be contacted. You should have a qualified technician install a three-wire mains cord before further use.
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Buy Chinese. Bury freedom.
Reading other postings/other forums, supposedly the reason for not using a three prong plug on the magnavox amps is because not using the chassis as a ground will create a signal loop/60hz hum. Some post argue for or against star grounding. Some posts argue that there are ways of fixing the hum, but in any case it seems to be a issue that's debated. I do want it to be safe. I'm new to this, more research is necessary-- but I very much appreciate everyones efforts and inputs!
Astroman,
Safety is dealt with by "bonding" the chassis to the 3rd wire of the AC mains cable.
Hum is dealt with by employing a reasonable facsimile of a single point ground (SPG). Star and bus are 2 good ways of SPG implementation. A capable technician will work out which of the 2 methods fits the project well. Repeat, repeat, the chassis is a shield, not a signal carrier.
Eli D.
T/K is 100% correct about the need to install a proper, 3 wire, safety grounded, power cable. A bottom plate to keep inquisitive, small, fingers out of harms way is also indicated. Sheet metal would be best, but 1/4 inch plywood will do.
Another obvious deficiency is the use of the chassis to carry signal. The way the speaker connections are installed is a clear indication of the issue. The chassis should provide shielding and nothing else. The signal ground should connect to the chassis at only 1 point.
The photos sadly suggest that the minimum amount of work needed to push the thing out the door was done. For instance, the OEM RCA jacks were not replaced. I also have my doubts about the twistlok multi-section capacitor.
Now for some good news. As far as console amps go, that "maggotbox" model is top shelf. The power transformer and rectifier also energized a tuner/preamp, which makes them competent. The filter choke is an almost unheard of item in console stuff. That too is a sign of some quality.
Push/pull 6BQ5/EL84 O/P tubes are good for about 15 WPC. I think you will find that 91 dB. sensitive speakers, with a reasonably flat impedance curve, will prove quite satisfactory. The DECWARE 94 dB. sensitive speakers I've linked below will get you to "bleeding ear territory".
Contact "inmate" Mike Samra or another capable technician about getting the unit whipped into shape, along with adding switching for a pair of sources. As long as the unit is bandwith limited, in the deep bass, to protect the smallish O/P "iron" against core saturation, things should go well. You will be able to play a "standard" double bass, whose lowest note is 41 Hz. Playing 32 foot organ stops is out.
Eli D.
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I'll post pictures as soon as I can, thanks!
Please provide a pic of the area around the RCA females, with a ruler of some kind laid on the top surface.
I'm thinking Mouser part # 612-100-M1111 could be suited to switching between a pair of I/Ps. I want to get a "feel" for the space available.
Eli D.
sorry for the delay in the reply, I've been out of town for a few days.
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