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Old 12th March 2003, 05:48 AM   #1
amt is offline amt  United States
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Default What to build w/ Akai parts?

Hi,
I just bought an Roberts (Akai) reel/reel which contains a good condition amp. After checking some of the threads here I see that rebuilding an old amp is a good way to start tube diy. I would like to go SE and have a couple questions. The amp came with the following:
2 6BM6
2 12AT7
1 6CA4
The only reference I could find on this amp discribed a rebuild but using 6BQ5s/ 12AD7s. Duncans doesnt show these as equivelent so want show I do? I found a schematic for a 6BM6 amp but Im not sure its really hifi. Any suggestion as to where to start. I would like to stay SE triode if possible (flea power is fine)

Thanks amt
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Old 12th March 2003, 06:33 AM   #2
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Default Did you try them out as is?

Hi

If this is the Akai M-8/Roberts 770X, then the two EL84 amplifiers in there are perfectly servicable as-is. They are actually pretty good monoblock amplifiers (many 'audiophile's have been alarmed to find how much so) - a pretty conservative, safe design ensuring longevity and safety of tubes.
All I would recommend you do is check the PS capacitor's health and add connectors to run them properly as monoblocks, and you'd be surprised how good they sound!

I understand the DIY itch, however, and there are many other designs more worthy of your time I think. For example the JE Labs Simple 45
http://users.starpower.net/je2a3/simple45.htm, is one that I have found is quite suitable for a DIY beginner (like I once was...).

Hope this helps -

Pradeep
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Old 12th March 2003, 02:55 PM   #3
amt is offline amt  United States
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Pradeep,
The model is a 1630 and unfortunately, is part of a preamp and tape recorder "mess" of wiring on the chassis. The direction I wanted to go can be seen at http://rummageads.com/tstereo/SEstereoamp/
Also, I really wanted to use the parts for a cheap first project.

amt
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Old 13th March 2003, 09:48 PM   #4
SHiFTY is offline SHiFTY  New Zealand
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I had one of these with a blown power transformer, used it as the basis for a small SE amp. Mine had the tube lineup of 2x 6BM8 (ECL82) and 2x 12AT7. Output transformers were pretty darn good, actually, as was the ELNA power supply cap (2x 40 uF).

I would suggest completely gutting the unit (with its incrdibly dodgy wiring and leaky ceramic caps) and starting from scratch. You might also want to use a tube rectifier (I used a 5V4G for the hell of it.)

I built the Mullard schematic at Bonavolta

It actually had a really good sound, but I have somehow borked it a little

When I get around to it I will buy some of the Svetlana 6BM8 (they are still being made)

Here is a webcam pic:
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Old 14th March 2003, 05:43 AM   #5
amt is offline amt  United States
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shiFtY,

Thanks for the help and the schematic. That looks to be relatively simple, will require few new parts and will actually sound good too! Perfect for a first tube diy.

Ive already dismantled the amp and am not sure how it was originally assembled. I barely had room to snip the wires much less assemble and solder. The nice Elna cap looks like a beer can that was opened after being shaken. Too bad 'cause it really looked nice.

The ez81 that came w/ it tested fine so I guess I'll use it unless theres a good reason why not to use it.
My next step is to become alittle more adept at reading tube schematics so I dont fry myself or the parts.

amt
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Old 14th March 2003, 06:42 PM   #6
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Default Mullard amp

While I am a big fan of Mullard circuits, the one reservation I have with this ECL82 amp is that the balance control is within the feedback loop, and thus feedback levels will alter with the setting of this control. Even though the feedback level is quite small by modern standards (about 6db) it might be advisable to place the balance at the input of the amp, possibly before the vol/tone control network.
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Old 14th March 2003, 07:00 PM   #7
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Default Another Mullard...

Quote:
While I am a big fan of Mullard circuits
Me to!!!
Quote:
the one reservation I have with this ECL82 amp is that the balance control is within the feedback loop, and thus feedback levels will alter with the setting of this control
Good remark ...i'm with you a second time!

Quote:
it might be advisable to place the balance at the input of the amp, possibly before the vol/tone control network
And why not take out the balance all together...i never use it...and it compromise the sound quality...
also the tone control if was me will be deleted...(i'm a purist)

Last sugestion...this amp has been designed in the good old days of AM radio and ceramics and piezo cristals pic ups ...so the high frequency demands are not so great as today...and the input 1 MOhm volume resistor in conjunction with the Miller capacitance of the first stage will atenuatte to much the high frequencys...so i sugest to change the pot to a 100K log pot...
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Old 14th March 2003, 07:11 PM   #8
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Default Mullard amp

....I also think that if the input impedence is reduced for modern sound sources rather than for medium output ceramic cartridges, the total gain will not be required either, and perhaps the level of feedback could be increased. As it stands, this circuit has quite high levels of distortion at full output (5% at 2W) so any method to help reduce this would be welcome....
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Old 14th March 2003, 07:35 PM   #9
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Default Sentimental reasons...

I think that for us Europeans the Mullard is the some that the Dyna ST 70 from ours friends in the States...the same sentimental reasons...and the some memories of the days when the audio was young...
Quote:
the total gain will not be required either, and perhaps the level of feedback could be increased.
Yes another good idea...now the gain is 34 X ...considering the output power...tha gain can be reduced to half that value...with the consequent halving in distortion....noise...and doubling the damping factor!

Thats the fun of diyaudio...
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Old 14th March 2003, 09:38 PM   #10
amt is offline amt  United States
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Default So you're saying...

to eliminate RV1 (1M log), eliminate RV7 (500k log) and replace RV2 with a 100k log pot. Sorry but Im still in my circuit reading infancy. This is referenced to shiFtYs schematic link. Im also not clear as to where the tone controls are.

BTW, I used to own a ST70 and a PAT3 but sold them to buy "high power" SS stuff. Wish I still had them now since Ive gotten wise to higher efficiency speakers and all my old SS stuff is close to worthless. Oh well.

amt
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