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Old 5th April 2006, 12:19 PM   #21
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Spoken in the best spirit of a DIY'er! Good luck. Amps like this are an opportunity to make a very beautiful sculpture IMO, techno art.
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Old 5th April 2006, 09:17 PM   #22
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Quote:
I am working 9 to 5 with microprocessores and components in size 0402 and 0603.
I know the feeling, maybe thats why I am compelled to build a bigger tube amp. I already have an 845SE and want to build an 833A SE.

We have started using 0201 parts at work. For those who don't know these numbers refer to the size of the parts in mills. 0402 is .020 inch by .040 inch, really hard to solder by hand.

I got 4 813's cheap on Ebay. One turned out to be bad. When I find a transformer I may try UL. Or I may use the 813 for a monster CCS to feed an 845 for a chokeless parafeed design. I built a small one with a 6LW6 CCS feeding a triode wired KT-88 and it sounded great.

What tube would I use for 20 W SE. The 845 is the first obvious choice for me. After that I would try paralleled KT-88's in triode mode. I have not tried the EL509, but some sweep tubes could do 10 watts each. Screen drive requires as much drive voltage as an 845, but less B+ voltage.
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Old 5th April 2006, 09:45 PM   #23
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Hey-Hey!!!,
Look no further than the 813. See the E-Linear article Pete Millett wrote describing his SE KT88 amp.

Expand the scale.

Pete's Page:
http://www.pmillett.com/

and the E-Linear article:
http://www.pmillett.com/elinear.htm

I am building similar with H-K Citation II outputs, and a hybrid cascode( the faux-pentode ), from ART's Merlin v2:
http://www.audioroundtable.com/Group...cts/Merlin.pdf

cheers,
Douglas
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Old 6th April 2006, 04:42 PM   #24
protos is offline protos  Greece
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Hi Rca Victim,
What 805 amp did you get and what mods did you do to it?
I have been eyeing some of those amps and wondering what they sound like (particilarly the output transfrmers)and if they can take mods.
Of course it is not as exciting as building your own though is it?
It is just that it is a big tease to be able to get a big set amp for probably less than what the parts would cost outside China.You could always use them as parts.
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Old 6th April 2006, 11:02 PM   #25
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Quote:
Originally posted by rcavictim



RDF,

You are probably right and I was mistaken apparently about this one.

I had a quad of 813's and have been looking for them now since the last move almost ten years ago without success. I am looking for some good used 813's if anyone has spares they will part with free or cheap please let me know. They won't be re-sold, I just wanna play.

www.tubesandmore.com sells nos 813 for $38.60 and used for $26.30

Regards,
Peter
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Old 7th April 2006, 05:42 AM   #26
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Default My remanufactured MingDa

Quote:
Originally posted by protos
Hi Rca Victim,
What 805 amp did you get and what mods did you do to it?
I have been eyeing some of those amps and wondering what they sound like (particilarly the output transfrmers)and if they can take mods.
Of course it is not as exciting as building your own though is it?
It is just that it is a big tease to be able to get a big set amp for probably less than what the parts would cost outside China.You could always use them as parts.
In my own case it has been as much fun or as much work as building an amp from scratch given the time and effort involved.

I bought one of the early MingDa MC3008AB's. It uses a 300B cathode follower to drive a 805. As supplied, plate supply on the 805 is about 1050 volts, but for 30 seconds allowing heaters to warm before the 805 is biased on the hi B+ soars to very high levels, almost 1600 VDC that endanges the insulation in the output xfmers. I could hear PD (Partial Discharge) as faint static clicking in the loudspeakers as charge carriers re-distributed themselves in the insulation system within the output xfmers. This was a little scary! My re-design delays the hi-B+ until it has a load available.and the PD issue is mute (Ha ha).

My early sn (#10) amp was using a big multi winding power xfmer of apparently good quality and with interwinding shields that provided separate windings for each 300B DC filament supply, each 805 DC filament supply, the master 6.3 volt AC filament supply, the high B+ and also the low B+ and the bias supply for the output stages. It appeared to be operating overloaded and became very hot quickly within the transformer enclosure which had no effective cooling airflow. The power xfmer was potted in epoxy which made heat escape to its own case difficult as well. The searing radiant heat provided by the large output tubes with glowing carbon anodes next to the black outer transformer case was no help either. The voltage doubler making the hi-B+ was employing input electrolytics that could not handle the extremely high ripple current and they got so hot that they failed and the near short circuit, not enough to take out the way oversized mains fuse (oversized by the factory to remedy an occasional surge related fuse failure problem that was not addressed properly), and the by now overheated power xfmer failed in a catastrophic short circuit of the HV winding rendering it as potted jink.

This all occured when I let the amplifier play for about four hours shortly after purchasing it brand new. Ask me if at that time, was I impressed? Not so much.

The good news was before it melted, it sounded FABULOUS !!! When i bought this amp I had a chance to ABC it with several other MingDa amps, one the MC34B which I also bought, has been trouble free as manufactured and which enjoy very much. From this MC3008AB amp I loved the tone, the SET presence and soundstage, and the command of the bass that you do not get with most SET amps (because they are wimpy) This one makes 40 watts/ch. I think the output transformers are sonically very good. They are physically quite large and appear well made. I finally had an amplifier in my home that I had wanted the likes of for years and I was not going to let this little meltdown spoil my fun. .

Not having a proper replacement power xfmer, and not wanting to pay someone in North America $600 or more for a custom replacement with 12 windings, I decided to manufacture my own multi winding filament xfmer to take care of the specialized filament needs (these DH cathodes all have audio and bias on them and need to be separately shielded and isolated), and some high quality transformers I had in stock. I did not want to go with voltage doubler on the Hi-B+ so I used a 300 VA CCS rated Hammond plate xfmer (about 1300 volts CT IIRC) and full wave bridge into a beefy choke input filter. A large blue computer grade cap bank now delivers the Hi B+ with 0.1% regulation from no output to full output with both channels driven.

Having to cram about 12 transformers into a chassis that originally held only three required some work to enlarge the case. I wanted this remanufactured amp to look better than the original so it became the object of several month's work. A hard maple case was made and finished in 18 coats of hand rubbed lacquer for a rich piano black finish. I employed a very unique control stage idea of my own conception which helps this amplifier stand out.

I have upgraded the power supplies, enhanced user safety and improved cooling. I did not modify the original audio circuitry other than to roll some different tubes into the position of 12AU7 SRPP driver to the 300B which made a really noticeable improvement in available output power and sonics. I found the E180CC does a fine job in this hole.

With 1050 volts on the plate caps of the 805's I decided that it would be a good idea to rig SS, zero crtossing relays with low voltage DC control looped through the protective cage over the tubes. With the cover removed the amp will not come on.

The amplifier has also received proper attention to component cooling both under the chassis and in the upper transformer enclosure. A quiet squirrel cage blower is installed in the bottom plate that pressurizes the chassis, cooling the bridge rectifiers underneath which were not cooled properly from the factory, then up to the transformer enclosure which is really effective in keeping the plate and output xfmers cool, then finally forward to the tops of the 805's under the plastic top plate which keeps that cool and from deforming.

This new amp has been my pride and joy now for about two years or more and has been in daily use with no problems. I like it so much I have plans to purchase, and similarly upgrade another when I can afford to. If you can end up with a $20,000 amplifier through DIY it doesn't hurt to acquire a spare.

I am using the originally supplied ShuGuang carbon plate 300B's without any problem but never gave the Chinese 805's much time, replacing them early on with some nice vintage Westinghouse tubes that I had in stock. They are run at their full rated CCS plate dissipation of 120 watts each.

The new amplifier weighs about 130 lbs and I need help to move it very far. It is bIG, as large as a big microwave oven. The spun aircraft aluminum footers contain full sized hockey pucks as the soft interface to the setting surface.

I enclose a before picture of this model amplifier with silver trim, the picture of mine with gold trim was lost in a HD crash. In the next post I show the finished results of my handiwork.

From a point of view of buying one of these Chinese SE 845 or 805 SET amps as a basis for upgrades, if you get one like I did with good output iron I think some of them I see on ebay are priced low enough that they represent a bargain just as a source of parts.
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On April 01,2012, my radioastronomy Project TARGET SETI search succeeded. The signal came via extradimensional subspace, not radio waves. I now get a lot of 'visitors'. I took my avatar pic of one friend watching me in my work yard from just 20 ft. away.
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Old 7th April 2006, 05:45 AM   #27
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Well, ta da. Here is my pride and joy. I was considering actually manufacturing these and offering them to the marketplace, hence this artsy "glamor" photo.

Am I going to do this? NO. To darned much work!

How does it feel to have the only model of this amp in the world? Priceless!
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File Type: jpg 805dun.jpg (18.5 KB, 735 views)
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On April 01,2012, my radioastronomy Project TARGET SETI search succeeded. The signal came via extradimensional subspace, not radio waves. I now get a lot of 'visitors'. I took my avatar pic of one friend watching me in my work yard from just 20 ft. away.
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Old 7th April 2006, 05:15 PM   #28
protos is offline protos  Greece
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WOW !

Now that surely must make it in the record book as the most complex P.O.O.G.E. (progressive optimisation of generic equipment) one has ever attempted.

I am now more seriously looking into those Meixing amps.At least after I finish my series connected (two output txfmr per channel) 6c33 design.
The high voltage mods though would make me a bit stressed.
What did you compare it to ?
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Old 8th April 2006, 08:30 AM   #29
tubalu is offline tubalu  China
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805
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