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Personal Tube Amp DIY Project

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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

898bschematic.jpg


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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.
 
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?
 
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?
 
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.
 
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
 
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 :)
 
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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Matt,

The Emerson chassis has a 9 pin socket and a pair of 7 pin sockets. If you recycle the sheet metal, 5 WPC is what you'll get, but mid 90s efficient speakers are OK with 5 W. The 6BQ5/EL84 suggested previously is a fine tube, but it needs a 9 pin socket. OTOH, the 6AQ5 (data sheet here) is fine too and it takes a 7 pin socket. :D

There all sorts of single ended circuits you can use. I suggest you use a 12AX7 voltage amplifier to drive an ultralinear (UL) mode 6AQ5. You will get the gain you need to drive the mandatory NFB loop. The most cost effective way to get a quality B+ supply is via a well executed Greinacher ("full wave") voltage doubler. A Triad N-68X, which costs $11.41 is perfect for the needs of this project. You will need heater power. The $8.39 "Rat Shack" catalog # 273-1352 disposes of that item.

Forget tone controls. Tone controls done "right" require fairly sophisticated circuitry, including gain, and (IMO) are not in character with this project. Just use separate left and right channel volume controls. Alpha Taiwan makes OK controls that cost little money.

Edcor's model GXSE15-8-5K is a good fit for the 6AQ5. The 15 W. power rating is not overkill, as magnetic headroom is needed to avoid core saturation from a NFB error correction signal.
 
Matt,



There all sorts of single ended circuits you can use. I suggest you use a 12AX7 voltage amplifier to drive an ultralinear (UL) mode 6AQ5. You will get the gain you need to drive the mandatory NFB loop.

Do a search on the RH-84 amp. This is simple (low parts count), should sound excellent and doesnot have the NFB loop.
 

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After listening to everyones comments and looking at my tube stash I am going to start looking for a EL34 12AU7 Amp to build and put it in the Emerson cabinet. I want tone control and volume only. I think I want to build a Single Ended amp not sure yet. Any suggestions welcome

Kashmire thanks for the enthusiasm. Here is a photo of the chassis, Not pretty but functional.

chassistop.jpg
 
¿ the 100K between anodes isn't NFB?
nice design by the way...

Of course it's NFB. The loop is short, not global. That will work with high quality O/P "iron". OTOH, GNFB can help when the O/P trafos to be used are not quite full range. It's quite possible to combine both approaches in an RH84 variant. Not much in the way of GNFB will be necessary, as it's primary role is to linearize the O/P trafo. The short loop takes care of O/P tube linearity and damping factor.

Yet another possible tweak is g2 B+ regulation via LR8 ICs (1/"final").
 
After listening to everyones comments and looking at my tube stash I am going to start looking for a EL34 12AU7 Amp to build and put it in the Emerson cabinet. I want tone control and volume only. I think I want to build a Single Ended amp not sure yet. Any suggestions welcome

Kashmire thanks for the enthusiasm. Here is a photo of the chassis, Not pretty but functional.

chassistop.jpg

Dude,

That little chassis will not acomodate EL34s and appropriate O/P trafos. The space is insufficient. Also, to use 12AU7s requires cascaded gain stages. The μ of the 'U7 is too low, at 20.

I've attached the schematic for a single channel of "proper" tone controls. Even if the cathode followers are replaced by ZVN0545A MOSFET source followers, you need a pair of 12AX7s, just for tone controls. A quad of tubes is needed if you forego "sand". Again, I ask, where's the room?

Build a RH84 variant that uses 6AQ5 O/P tubes. Look here, where a RH84 variant with 6V6 "finals" was worked out. For all practical purposes, the previously mentioned 6AQ5 is a 6V6 in a 7 pin mini package. ;)
 

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Eli, Thanks for all the info, I have decided to scrap the ideal of using the old chassis at all. I will build a amp from scratch and use the cabinet only to get the old school look. Later today I will have to gather parts once I decide which way to go. I think I will try to build it to be a nice amp if I have to start from scratch. Until I ge tthe parts I have decide to use the amp below to play with. Any suggestions on it would be appriciated.....thanks, matt

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Any suggestions on it would be appreciated/QUOTE]

Here's a nice suggestion: redo all the first triode's circuit. Follow the attached
schematic. ECL86 is a very good sounding tube, but doesn't last that long so maybe you will need a new one for best sonics.


R2 sets the amount of feedback, here personal taste dictates but a value between 1.2 K -3.9 K. should be OK.
 
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