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TexasAudio 6th December 2009 09:11 PM

Personal Tube Amp DIY Project
 
Hello Tube Audio Brothers,

I am new to the board. From what I have been reading in the forum I think I found the correct place. I have been thinking about a winter project since the 38 straight days of 100 degree weather we had this summer. I have settled on turning my Emerson model 898 series B Hi-Fi Stereo tube phonograph into a personal amplifier for an ipod or any line input device including acoustic guitar. I chose my Emerson because I already had it and it plays ( I always like starting with something working). It uses 2 50EH5 for audio outputs and one 12AX7 for the preamp. I have electronics experience and have repaired a fair share of tube radios. I have recapped some tube audio gear. I have a good selection of test gear and I know the basics of tubes. I am here for opinions and advice on this project. I have included the best schematic I could find for reference.

My first few questions are

1. Can the Emerson do what I want?
2. Can you do a combo without a lot of work to use with guitar and line
devices?
3. Should I recap and tune-up the amp before I start any mods?
4. What type of capacitor do I use when I recap.
5. Do I just change the connector to connect line in or do I need to modify
the input in some way?
6. Any opinions on mods

That should get me started.

Thanks in advance for all. . . matt

http://i262.photobucket.com/albums/i...bschematic.jpg

http://i262.photobucket.com/albums/i...or/front-2.jpg

http://i262.photobucket.com/albums/i...or/front-2.jpg

Cassiel 6th December 2009 10:27 PM

[QUOTE]From what I have been reading in the forum I think I found the correct place/QUOTE]

Not really. No power transformer equals not welcomed.

kevinkr 6th December 2009 10:51 PM

I have to agree with Stalker on this one, you must absolutely use an isolation transformer with this amplifier. I am very uncomfortable with the idea of relying on the insulation in a 45+ yr old output transformer to keep the mains away from your head when wearing headphones. (Not to mention the hazard to anyone touching that iPod or other source.)

More practically speaking I think you will find the snr and linearity of this amplifier seriously wanting in headphone duty. I have designed more than a few and find this a demanding design area. (Mine are OTL or direct transformer drive - no attenuation.)

You might want to take a look at some small spud amplifiers using tubes like the 5842 or similar and really good opts from Jack Elliano at Electra-Print. A good power transformer with a proper safety ground is also required. The whole thing has to be ridiculously quiet to perform well as a headphone driver.

dsavitsk (Doug) would be the fellow to ping about spud headphone amplifiers, he has designed and built quite a few good ones.

TexasAudio 7th December 2009 01:31 AM

Thanks for the feedback. I don't need headphones, I would rather it just be a small amp to connect a Ipod or CD to listen to. I would like to know how to modify it to connect a line in signal if any one could help? And while asking any opionons on what a good speaker replacment would be?

Eli Duttman 7th December 2009 03:02 AM

TA,

Do you still listen to records? That old Emerson unit employs a piezoelectric cartridge, which is a record destroyer. High freq. info. is literally scrubbed out of the grooves. :(

The previous safety warnings are spot on! Some sort of transformer protection MUST be installed to isolate human external signal source operators from the AC power mains. It is possible to use transformers at the I/Ps, but ease of implementation and cost strongly suggest you do it at the AC mains.

I'm sorry to be a "party pooper", but Emerson's design is very much LOFI junk. Let's start with the 50EH5 O/P pentodes. The 50EH5 data sheet says the type is good for 1.4 W. of O/P. You need big, expensive, 100+ dB. efficient horn loudspeakers in combination with such low power, for serious listening. Do you really want to continue?

m6tt 7th December 2009 03:31 AM

Use the chassis, but expect to redo the power supply some. Also expect to want better output transformers. For instance, you could use a power transformer with a 6.3v winding and use 6AQ5 instead of 50EH5 for output. That would be more than twice the output power. Use a set of 1n4007 to rectify the HT, choose a transformer that will give about 250-300v into the filter cap of choice (rated at least 450v for life & safety). The 12AX7 would be wired for 6.3v operation with 4&5 tied. All of that should be fairly cheap, depending on the caps & transformers. 6005 and some others are milspec 6AQ5, they're about $5-6 each. Tubesandmore.com sells those and a 5k SE output transformer that is something like $8, Fender champ output transformers would work too.

By doing all of that you're not only ensuring you'll get better sound, but also ensuring the safety of you, your loved ones and pets. UL would *not* approve today of some of the things done in cheap equipment from the 60s. They can burn your house down and kill you at the same time. ESPECIALLY when you try to plug a guitar into the circuit. If the outlet is miswired, or the plug is reversed, you might as well stick a fork in the socket.

Jaap 7th December 2009 08:03 AM

Why not buy a pcb from this guy Tubelab Home and build something brand new that also sounds much better ? Lot of documentation here on this designs. Or look here: http://www.diyaudio.com/forums/tubes...mp-design.html

TexasAudio 7th December 2009 01:36 PM

Wow, Ok I now get it. I think what I will do is build a Amp and modify the cabnet the way I intended and install the new amp in it. I will start looking at plans to build a amp. I would like about 15w output and simple tone control circuit. I would like to keep the cost down as much as I can. Any suggestions?

Thanks, Matt

kavermei 7th December 2009 02:55 PM

I guess you could copy the preamp section (first gain stage + tone circuit) from a small guitar amp, like Fender Champ or the AX84.com project. Then use the other half of the 12AX7 as a driver for the output stage. Get some higher power tubes for the output stage, and a better output transformer. Add a switch and a line input before the driver.

That's what I'd do :)

Kashmire 7th December 2009 03:41 PM

Post a picture of the chassis without the lid. From what I understand, some speaker drivers are integrated into the chassis. Do you intend on using the integrated speakers, or external speakers?

Here's what I would do. I'm currently rebuilding a Magnavox with similar problems.

1) Replace the integrated speakers with Tang-Band units from Parts Express. If you don't see a Tang Band you like, try a Galaxy. For high-end stuff, look for Fostex or Jordan. How big are your speakers? I'm suspecting about 5".
2) Redesign the entire circuit, using 12A*7 and EL84 in push-pull. Tubes are cheaper at Parts ConneXion than at Parts Express. Operate the EL84 cool. They are cheap, ubiquitous, and will last a long time.
3) Use Edcor power supply transformer. I like the XPWR057 for EL84 push-pull. Use diodes instead of tube rectifier. Get some 450V electrolytics from Digikey or similar (i.e. Panasonic TSHA series). You can get all sorts of neat chassis-mount capacitors from Parts Connexion.
4) Use Edcor output transformer. You can step up from the XPP to the GXPP to the CXPP (increasing quality and price with each step upwards). The XPP15-8-8K (15 Watts, 8 Ohm output, 8K input) is only $20. That's what I'm using in my Magnavox.

I'm building mine with a $7 Edcor transformer phase splitter. I like input transformers more than coupling capacitors, because they provide phase splitting, true isolation, and protect me and the source component (iPod) from getting zapped. Note For a serious amp, you'll want the $70 Lundahl LL1540 if you use an interstage transformer. For using iPods as the source, the quality of the LL1540 would be wasted for critical listening. Besides, I wouldn't be able to enjoy the LL1540 quality with the integrated speakers in my old Magnavox.

Total BOM of my Magnavox rebuild: matched pair of EL84EH ($20), output transformer ($20), power transformer ($50), other components including interstage transformer ($40) = $130. Add to that the price of the speakers.

Your chassis looks very cool, and with a redesigned circuit and integrated speakers, will be an excellent showpiece to be proud of.


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