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Simple 845 tube monoblock design

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Hello all.
I'm rather passive user of that forum, so this is probably one of very few posts of mine. I'm definitely a humble learner, not a pro :)

I have OB speakers of my own design, that I wanna run in bi amping.
For the bass I have KT88 PP amp.
I would like to have SET monoblocks powering the mid and high frequencies.
I believe that tubes like 845 (and similar) are best when it comes to power.

I have been trying to find some simple monoblock design for 845 tube, but its hard.
I is gonna be build by a pro engineer (I'm not suicidal, to play with such a voltage without proper education:)

My needs:
minimum 10W into 5Ohm.
Simple construction, minimalistic.
CANNOT involve 300B tube in the design (far too expensive)

I just have to find a schematics for him.

I will do the wooden chassis myself.

I'll be happy for any word of advice.

Miku
 
No need for a KV tube for that application. You could do SEUL KT88 for an all KT88 system and get about 8 to 10WPC. There is a build thread in another forum that uses some Schade feedback and is said to sound pretty nice (will know soon as I am building one as soon as the iron arrives).

Could also do triode wired at a loss of a few dB. Have you checked the efficiency of your mids? Without the drag of the crossover you may not even need 10W.
 

45

Member
Joined 2008
I have been trying to find some simple monoblock design for 845 tube, but its hard.

Miku

There is a reason for that. Don't you think so?
I don't think a good 845 amp will be cheaper than a 300B amp. You can save money for buying the power tubes but you will end up spending a lot more money for the supply, the driver and the transformers.
Just use JJ300B or good chinese 300B's like Golden Dragon or Valve Art. They are as good as any other 300B if they don't have defects. Or even cheaper a nice parallel single-ended (or push-pull) with 2A3's or 6C4C's....

Then instead of buying an output transformer with the right load for 8 ohm speaker just get one which gives the desired primary load with 5 ohms speakers!

45
 
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If you are going to the trouble of bi-amping which should give you a lot of refinement and you only need 10W-then I think you would not find the 845 a very refined sounding valve! I do not think a simple schematic is possible because apart from everything else(difficult and permanently dangerous PSU) this valve is very difficult to drive,maybe needing,idealy,an interstage transformer. It is good to hear that someone else apart from me is actually reluctant to build kV amps. I wholeheartedly agree with the advice of mashaffer and 45.
 
According to TubeCAD, you could run an 845 with a B+ of 750 and 90 mA through the tube. Fixed bias, using a hammond 1629SEA output transformer, and get 9.65 Watts at very low distortion(lower than 300B). The bias would be a -86 volts, which is not much more than a typical 300B. So a driver, such as a triode strapped 6688 would work great.
 
Miku, are are welcome to reference my design from 2003. I know of at least seven
people who have built from my design - it was a real crowd pleaser at a couple of audio
get-togethers that I attended where I brought my amp.

Flickr: rockymtnmagic's Photostream
Mine runs at 430V, because I use Klipsch speakers here and dont need boatloads of power. For your application, the 750V / 90mA operating point recommended earlier is ideal... Mainly because 750V is about the limit that you can easily obtain the power supply parts using items such as the Hammond Catalog chokes and oil-filled motor run capacitors. Beyond 800V, suppliers for rated parts become very scarce, and the prices soar astronomically.

Since you are in Finland, you might be able to buy from Lundahl in Sweden at favorable currency rates. On the other hand, with the Euro or the Swedish Krona as a reference, you might be able to buy from the US/Canada from vendors such as Magnequest or Electra-print at exceedingly good currency exchange rates!

Last but not least, if you wanted to built a 2-stage amp, the Lundahl 1660/30mA
part with a high gm, high mu tube such as the 7788/E810F or Soviet 6C45 would work very well!

Have you thought about GM70 ? The Soviet Union is just next door, so buying these tubes, and their sockets, ought to be quite easy.

-- Jim
 
Hello all.
I'm rather passive user of that forum, so this is probably one of very few posts of mine. I'm definitely a humble learner, not a pro :)

I have OB speakers of my own design, that I wanna run in bi amping.
For the bass I have KT88 PP amp.
I would like to have SET monoblocks powering the mid and high frequencies.
I believe that tubes like 845 (and similar) are best when it comes to power.

I have been trying to find some simple monoblock design for 845 tube, but its hard.
I is gonna be build by a pro engineer (I'm not suicidal, to play with such a voltage without proper education:)

My needs:
minimum 10W into 5Ohm.
Simple construction, minimalistic.
CANNOT involve 300B tube in the design (far too expensive)

I just have to find a schematics for him.

I will do the wooden chassis myself.

I'll be happy for any word of advice.

Miku

Instead of discouraging you from building an 845 SE amp, I am going to encourage you to do it. What you want to do is entirely possible and a lot easier than most people think.

Chinese 845 tubes are not that expensive and sound OK for everyday listening. NOS 845's obviously will sound better, but they are stupid money.

I am traveling, at the moment, on a business trip, but will post a schematic and some other advice when I return home around October 25th.

You can easily drive an 845 tube with a 6SL7 (paralleled) or SRPP as the main gain stage direct coupled to a 6SN7 (paralleled) as a cathode follower. With about 900VDC on the 845 plate (-104VDC bias) and a 5k/10k output transformer, you can get @ 13~14 watts RMS @ 5 THD. I would not recommend the Hammond 1629SE transformer. I tried one a few years ago and it had a terrible dip in the frequency response around 13k, if I remember correctly.

This information comes from practical experience, not simulation (no offense to anyone) or conjecture.

The sound? Well I think it sounds good, but, that of course, is subjective.

Do something different and build an 845 amp! I don't think you will be disappointed.
 
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Mainly because 750V is about the limit that you can easily obtain the power supply parts using items such as the Hammond Catalog chokes and oil-filled motor run capacitors. Beyond 800V, suppliers for rated parts become very scarce, and the prices soar astronomically.
-- Jim
When building high voltage power supplies, you can easily use Hammond chokes and other similar type products by inserting the choke on the "negative" side of the power supply, not the positive side as usual.

In this manner, the voltage drop across the choke is only a few volts and it is close to ground potential.

This is an old trick the amateur radio guys used to use when building transmitter equipment. I read about it in a volume of the amateur radio handbook.
 
thanks for all the advices.

Could you then suggest any well described designs for 300b or paralel 2a3 (and similar tubes)?
I dont want PP.

Thanks!

Audio Note Neiro. It is an excellent starting point. It could also be definitive....
If you use the 6C4C's you better think of a DC filament supply.
Other than that you don't need any modifications.

As it is the amp will give at least 8W with 3% THD typically.
To get 1K25 primary load for 5 ohm speakers you can look for 2K/8R output transformers. Also 2K5/8R, which is more common, will do.

45
 

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Someone I know recently bought an original Japanese Neiro; it is a lovely amp. I am not sure how much of the performance is due to the fact that Kondo,as far as I know,made many of the parts by hand.

It is excellent also with normal components. This is the first kind of "high" power SE I built and it works (sounds) very well. In place of the 5687 you can also use 6SN7's at higher anode current than the original (8 mA for each section). Of course you can also change the input stage and you can get up to 11W with 5% THD and so on...
In other words you have a lot of solutions to tweak your sound and power without changing the output stage!

The big advantage of the 2A3/6C4C PSE is that you have approx. 10mA/V gm and 400 ohm plate resistance and so you are fine with 250V supply which makes the power supply cheap in comparison to other solutions for the same Pout. If you want the same performance from a 400-450V supply you have to spend a lot more.....
With further higher voltages it doesn't come that good. Independently of the cost!

45
 
Hello THD+N,I have heard about this trick with chokes as well.Have you any further details?
Please see the schematic for details.

Note the 2H choke in the plate power supply negative (ground) circuit.
 

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Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.