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Old 28th October 2008, 12:17 PM   #1
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Default Tweaks for 845 Power amp

Hi Folks,

After months of indecision, I have finally committed to this amp design -

http://www.audiodesignguide.com/New845/New845v2.html

I have done a fair bit of reading, including the critical comments but at the end of the day I have settled for this one. In fact, the tubes (845's) and caps are on the way and I am in in the process of getting the power transformer custom made locally.

I have decided on a few simple tweaks as follows -

1. The soft start circuit of course
2. Damper diodes on the B+
3. A voltage regulator on each of the 10v heater windings
4. Probably the ECC803 instead of the 12ax7 (I know, not much of a change).

Now here's the question, any useful tweaks/changes without altering the capacitors (I have paid enough for them), the 845 tubes or the transformers? I have noticed that the 12ax7 as a driver has caused some distress but I appreciate the rationale behind it's use.

Debate away whilst I benefit from a hopefully robust discussion.

Cheers,

Rob
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Old 28th October 2008, 03:53 PM   #2
Gluca is offline Gluca  Italy
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If I were in your boots I'd go for the GM70, cheaper and better IMHO than the 845. IT can work at almost the same operating range of 845 and you can use the same OPT (i'd suggest 8K as a minumum load). You need to supply 3A@20V for the filaments.

I do prefer GM70.

Gianluca
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Old 13th November 2008, 02:33 PM   #3
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Default Ciuffoli's 845

Bob, I have built this amp. Simply superb. Seriously you will be amazed. (and I am NOT a relative of Ciuffoli )

If you have any queries or doubts as you go, don't hesitate to drop me a line. And if you take my advice don't bother with voltage regulators.
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Old 13th November 2008, 11:15 PM   #4
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Hi Federico,

Well I am committed to this amp now having just received the Jensen caps (those guys are great to deal with) and the ordered all of the Lundhal transformers (also great to deal with).

I am glad to hear the vote of confidence - it is doing to take me a while to get it up and running as I am going to make this the 'perfect' amp. I will take your advice and drop the regulators.

If nothing else, the Italian heritage was enough to convince me to build this one - you guys have taste and style

Cheers,

Rob
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Old 15th November 2008, 12:48 AM   #5
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Looks sweet Rob..............please tell me you are going to redo the chassis? The one example noted is really esthetically challenged.
No really.... it IS a very crude chassis, quite unworthy of the builder...considering the time, expense, mental effort...........please...some more time & brainstorm a REALLY nice chassis.
_______________________________________________Ric k.......
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Old 15th November 2008, 08:31 AM   #6
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Hey Richard,

Don't worry..... chassis are my specialty - take a look at my previous ones. I have attached a photo of my last one with the 18ct gold top!

I am going to go all out on the 845 and spare no expense. I also happen to know a sheet metal worker who owes me a favour so I am looking at something special.

I am thinking black nickel top, copper sides and transformer covers with brushed chrome air vents and a couple of old analogue bias meters at the front but if you have any suggestions, I will certainly consider.

Cheers and thanks for the interest.

Rob
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Old 15th November 2008, 11:51 AM   #7
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Hey Rob

your gold top amp is really quite stunning. I've used black-nickel on a couple of amps too.
Its not really my habit to intrude on other people's builds with a bunch of unasked for opinions, HOWEVER my experience with this amp is that it simply gets too hot for any shiny metal on the top pannel reflecting the heat back onto the valves. To the extent that I'm considering some sort of forced ventilation.

My only other advice is don't hold back on the plate voltage. I (like most people) was scared of the voltage and gradually went from 800 to 1000 to 1200 volts, the sonic leap is outstanding. Oh.... and make the bias adjustment truly easy to access, the amp is simply too hot and too heavy to warrant fiddling around inside while its under steam.

attached a pic of my cherrywood offering.
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Old 15th November 2008, 02:21 PM   #8
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Rob....Sweet previous build, I like the slender/micro toggles as it brings about a precision aura....& the blue displays brings an eighties Pioneer flouroscan air.......Great integration of styles.
Ever thought of using a pair of EM-87s for VU indicators????
Check out www.youtube.com for a look in case you haven't seen them in person.
I would mount them inside the chassis, horizontally, although I don't know if horizontal mounting of the tube is allowed.
Tubes inside of course would make for forced air cooling.
Just throwing out ideas.........................
____________________________________________Rick.. ......
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Old 15th November 2008, 10:57 PM   #9
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Hey guys,

Thanks for the comments. The amp is not finished in that picture - it is without the base but it was the only one that I could put my hands on quickly.

Federico, regarding the bias, I have always made my potentiometers accessible with the chassis closed and this time around I am going to have a toggle switch and on the back so I can adjust all bias with just two meters on the front. I use voltage drop across a small cathode resistor so I am only measuring 1-2 volts instead of high voltage/current with the potentiometers accessible through a small hole in the chassis. I don't know if you have sen my other thread but I am also replacing the 12AYX with ECC82 to reduce the gain a little bit which seemed to be a small problem with the original Ciuffoli design - although clearly only a small one. I will have a serious think about the heat issue with the metal top. The plate voltage did concern me a bit, I have already ordered the transformers and they are as Ciuffoli specified so I hope 900v is enough.

Hi again Richard, I thought about something like this but I wanted to keep the top of the amp very minimalist and I also wanted to avoid forced air cooling just with regards to noise - but I must admit, they are pretty cool.

I will keep you guys posted - it will be a long build esp as nothing gets done over Christmas/New year. I do love those 845 tubes - I really did not realize just how big they are in real life!

Rob
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