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Old 22nd August 2008, 12:07 AM   #1
athos56 is offline athos56  United States
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Default 845 Amp

Found this online and was wondering if it was a good design. The B1 is 860v, B2 410v and B3 is 176v. The PSU is really complicated so I used this as an opportunity to practice my PSUD II skills. The amp itself is simple enough so I thought it might be worth looking at.
http://www.tube-amps.net/images/How_..._Schematic.jpg Click the image to open in full size.
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Old 22nd August 2008, 04:05 PM   #2
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I would think that the purpose of using an 845 would be to get an absurdly powerful SE amplifier.

Running B+ at 860V would be throwing away a lot of potential power. Personally, I'd go for more like 1000V if I were to build an 845. You can make a lot of power at that voltage, especially if you can drive the grid positive.

Why use an 845 unless your goal is to shake the walls? If that's not your goal, then a smaller tube would be better/cheaper/safer.
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Old 22nd August 2008, 04:25 PM   #3
kevinkr is offline kevinkr  United States
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I sure wouldn't waste an 845 on a design using the mediocre 6BM8 as a driver, and with global feedback to boot. Properly designed SE amplifiers don't require global feedback at all.

Do you really need the power? Designing and building an 845 amplifier that sounds anywhere as good as many of the smaller dht based amplifiers is a serious enterprise and I doubt this design will get you anywhere close.
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Old 22nd August 2008, 05:06 PM   #4
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Quote:
the mediocre 6BM8 as a driver
Do you mean in terms of driving an 845 or just overall? A 6BM8 is good for a few watts SE so it should have enough 'oopmh'.
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Old 22nd August 2008, 05:35 PM   #5
athos56 is offline athos56  United States
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Thanks for the replies! This is why I ask around before I make any decisions. I also have a 300b I'm looking at from the Tubecad site. I liked the "simpleness" of this amp, which is why I asked, when I see schematics I always look for those might be the elegantly simple types. However I always need it ask if this is the case, there's simple and then there's simple. I don't see many simple 845 schematics around. Anyhow Thanks for the input!
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Old 22nd August 2008, 07:10 PM   #6
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by astouffer


Do you mean in terms of driving an 845 or just overall? A 6BM8 is good for a few watts SE so it should have enough 'oopmh'.

I mean linearity and overall sound quality... The 6BM8 is great for small cheap SE amps, but given the cost of building a good vs. mediocre 845 SE amplifier it's not a good choice. Just my opinion which is worth about YMMV..
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Old 24th August 2008, 05:39 AM   #7
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Don't like it. In the first place, 800Vdc isn't enough for an 845. These puppies like lotsavolts. I'm doing an 845 SET design with the Vpp= 1100Vdc. That'll really make 'em rock.

I also don't like the driver. 845s like to pull grid current even before Vgk actually goes positive. It's long been a problem with this type. To take care of any grid current demand from that, or the high rising transient that'll exceed the grid bias, I like a MOSFET source follower. DC coupled, it can also help set the Q-Point plate current. With a low distortion front end, the 845 will perform a good deal better.
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Old 24th August 2008, 06:05 AM   #8
athos56 is offline athos56  United States
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Default Yeah

So, I've abandoned the 845 idea, I spend most of the day looking for either a PP or PSE amp using EL-34's, KT-88's, or 6L6's to get the power I'm looking for. My home-brew speakers use pro-drivers that are in 90's for sensitivity but also like plenty of power. It seems that the louder I play them the better the response curve looks, by the time my Onkyo HT receiver hits 0 on the volume dial the speakers are rocking but the amp is sweating. I think 20 to 50 tube watts would really rock... The back up plan is the 300b project I have all the parts sourced for, but while I'm still in the planning stages I'm checking out other options.
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Old 24th August 2008, 06:11 AM   #9
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Default Or

This one. The only issue I'm having is that every schematic I find has a thread here where someone I respect tears it down, including this one, and the next one. I kinda wish I would have found a schematic that made everybody say "wow thats great!" But when that happens on Diyaudio is the day I...well its so weird I don't know

I was also looking at the Williamson, but then when I researched it seemed like most felt the available schematics needed modification (read a thread where SY when into this) even though I'm learning, I guess I'm just lazy in wanting a schematic thats ready to go. At least I can cobble together PSU's and source my own parts now and know what most those parts are for, mostly
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Old 24th August 2008, 02:44 PM   #10
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Build something from a stock schematic at first to get experience and then modify if necessary. You're right about never finding a schematic that everyone would agree on. The two you just posted are fine performers. Whats important is how the amp sounds to you.
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