• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

845 Amp

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
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_Hashimoto_Sounds/845_Signal_Pass_Schematic.jpg
845_Signal_Pass_Schematic.jpg
 
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.
 
Administrator
Joined 2004
Paid Member
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.
 
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! :)
 
Administrator
Joined 2004
Paid Member
astouffer said:


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 :2c: YMMV..
 
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.
 
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.
 

Attachments

  • dynamull.jpg
    dynamull.jpg
    72.8 KB · Views: 1,303
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 :)
 

Attachments

  • kt88_3.jpg
    kt88_3.jpg
    67.9 KB · Views: 1,126
astouffer said:
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.


I'll buy that for a dollar.

You payed for it, you built it, and if you don't like the way it sounds when it's done mod it.

:2c:

Out


Nick
 
What I do is build amplifiers with modular top plates. If you want to do this, do as follows.

Think in terms of a standard rackmount size chassis - 30mm deep, 420mm wide and 2U high (=88mm). My top plates are 120mm, 100mm, 70mm and 50mm all out of 4mm aluminium. I can do anything with this method and I recycle constantly. All top plates are 275 deep.

So, think PP amp. Make a power supply of say 450v. Put that on a 120mm plate. Put two PP OPTs of about 6.6K on a 100mm plate next to it. These will be fixed. You can then experiment with the rest of the amp. If you want something big, use PP 845 or 813 low voltage. For those you'll need a pair on two plates of 100mm and the input stages will have to go on a seperate chassis. Or you can use four 300b on one plate of 100mm and you have 100mm left for the input stage. You can experiment knowing that you will always have a working amp in minutes by putting back whatever you started with.

I'm just starting to build exactly this. I will use the same power supply and OPTs and try 845, 813 and 300b and also 2E22 in triode. I will probably put the input stages on a different chassis with a LL1671 PP output.

I went modular about 3 years ago and it was the best thing I ever did. It's kind of like a breadboard but permanent if you want it to be.
 
andyjevans said:
What I do is build amplifiers with modular top plates. If you want to do this, do as follows.

Think in terms of a standard rackmount size chassis - 30mm deep, 420mm wide and 2U high (=88mm). My top plates are 120mm, 100mm, 70mm and 50mm all out of 4mm aluminium. I can do anything with this method and I recycle constantly. All top plates are 275 deep.

I suppose you mean 300mm deep ?
That will match with 275mm top plates

Great idea !
Front and back rack chassis may even receive "permanent" connectors, switches, pots, and so on !

That makes me dreaming ...

Yves.
 
That modular idea is indeed a good one. My other amp projects were all from complete schematics. Its easier to find generally well accepted SE amps, it seems harder to find the same consensus on PSE or PP amps. I guess I should clarify, mainly I'm looking at amps without any sand or outrageous part counts I've also looked at Mikael's KT-88 which fits the bill, I wish it had a few more tubes :)

Yves, I also looked at your 845 amp, was the bias -145? Plus you had two, one with some iron, and one without, which was better?
 
athos56 said:
. . .

Yves, I also looked at your 845 amp, was the bias -145? Plus you had two, one with some iron, and one without, which was better?

Yes, more or less -150V IIRC and 1000V on the anode for some 80 mA current.

The better is ... can't tell !

The simpler (with a single tube and no iron) is really stunning looking to its vy lo complexity.
This was the final choice of the builder.

I must say the the one with two tubes and the iron sounded much more "dynamic" to my ears and had my preference ...

I beleive the builder rejected it when, while explaining him how it works, I pronounced the word "feed back" !
Some audiophiles have real avertion against that.

The only way to know is to check yourself ;)

Yves.
 
Re: hmmm

athos56 said:
I think I'll try and learn how to make a negative bias supply today and put your 845 on the list of possible builds. :)

On a side note why not Self-Bias?

Oh, two at least !

- I dislike to vaste precious and costly volts from the B+ (a bias supply is cheap).

- I dislike to add any impedance in the cathode return, very good bypass caps here is a must, and good caps are not cheap.

My two pence !

Yves.
 
Ok, let me se if I got this straight.

How about something like this. For example one 6D22S damper diode reversed, one leg of the transformer to ground.

Question 1, I assume I can work out the Transformer/resistor/cap values as if it was a positive half-wave rectifier using PSUD II.

Question 2, if the winding had a center tap I would just leave it unconnected.

The other implementations had both legs of the transformer going to the anodes of a full-wave rectifier, the 5v heater going "nowhere" ie not the usual b+ rail. And the Negative supply comming off the center tap. The assumption from question one would still apply. What low current rectifier would you use? I assume that the 5u4 would be an overkill.
 

Attachments

  • example1.jpg
    example1.jpg
    21.9 KB · Views: 716
Administrator
Joined 2004
Paid Member
My thoughts would match Yves pretty much exactly, but I would put even more emphasis on the cathode bypass cap - there is no cap like well, no cap. It is impossible to build completely transparent sounding caps in the value required to do this chore.

I also strongly recommend against using tube rectification for bias supplies - if the rectifier fails for any reason you will loose the output tube and possibly the output transformer and your HV supply as well.

Tube rectifiers are great for their slow warm up (depending on type chosen) and their soft rectification characteristics, however mundane a ss rectifier may appear to be in this application its simplicity and ruggedness make it the best choice. I use tube rectifiers in all of my designs except in the bias supplies..

Not a bad idea to interlock the HV supply to the bias supply - no bias means no HV applied to the output tube. I use a simple circuit to sense bias is present and use this to energize a relay which either switches the primary of the HV transformer or the CT if the transformer is used for other purposes.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.