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Hello!
This is my first posting. I am new to tubes, and have a lot to learn.
I recently purchased a Don Allen modified Magnavox tube amp. I believe it is of the 9300 series. It has 4 EL84 tubes, 3 12AX7a Tubes (one of those is wired to a dial-- a trim pot maybe?) and a 5U4GB tube rectifier.
I want to use this amp to run a turntable, as well as digital music (from say an iPad). I have no idea what would be a good phono preamp to work with this (DIY or off the shelf). I just don't know enough about how all this works yet.
So to layout some questions clearly:
1) Is Don Allen still making/fixing amps? Supposedly he lives within a mile of me, but the shop I bought the amp from doesn't have any contact info for him...
2) I'm looking for pre-amp (especially phono) suggestions, either off the shelf, or DIY. I have some DIY electronics experience, just not audio.
3) I'm looking for speaker suggestions-- I know the generic rule of finding something sensitive, efficient, and with a relatively flat impedance curve. the shop I bought the amp from auditioned it for me using some relatively large vintage JBL (C35 maybe?) speakers. I am going to be using this amp in a small room, so I'm looking for relatively small speakers. Something no bigger than say 24"x20"x16" at max. I was wondering if perhaps a pair of dynaco speakers would work...
Thanks!
Follow Ups:
Astroman,This has gotten even more interesting. ;> ) It appears that Mr. Allen started with a Magnavox model 9302 and, among other things, added some line stage circuitry that uses an additional tube.
You stated you want 2 sources: phono and digital. It is possible to install a tubed phono preamp on the chassis, without adding to the 3 bottle small signal complement. Please notice that the power transformer has a 2nd, currently unused, filament winding that was used to energize the tuner/preamp. That winding can be repurposed to feed a DC heater supply, for the phono section heaters. The phono preamp uses 2X 12AX7s, which leaves but 1 twin triode for the power section. That is gotten around by using the ZVN0545A MOSFET in a "concertina" phase splitter role.
What I have in mind is to use a tweaked RCA setup for phono and hybrid variation on the Dyna ST35 theme.
You can have exactly what you want. :> D
Eli D.
Edits: 05/29/16
Eli D.
In my reading, I have come across the pre-amp in the RCA tube manual, and wondered if that would work. I'm glad that you think so, and have a much better idea of how to do it that I.
I believe Don Allen is retired and has moved away from PHX, I think FL. I'm surprised we haven't heard from Todd Krieger, a big Don Allen champion for years. He should have Don's contact info if you PM him.
Agree that the Bugle2 is a great value. For a little more, the Lounge Audio LCR phono stage is a better buy, IMHO (MM only).
PM me.
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.
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.
--------------------------
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.
back
overall view
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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