845 circuit suggestions

Mush · 17th August 2007, 10:16 PM

Hi:
Here's the circuit I have for an 845 based amp. I'd like to hear what critiques or suggestions anyone may have of it.
Thanks -- Minto

HollowState · 18th August 2007, 12:07 AM

Well Minto, I think I'll be the first of many to say that your circuit is not a good one for several reasons. I'll just touch on the obvious weak points.

845's are difficult to drive. They can draw grid current that must be supplied from a good low inpedance drive source. You'll need a high current cathode follower or mosfet source follower like tubelab's to directly drive the grid. (no coupling cap) And the 100K ohm grid resistor is too high in value.

This tube also needs a high amplitude signal to provide any usable power out. I doubt your 12AU7 is going to provide that signal.

DC on the 845 filament is ok (even good), but what is doing with that CRC business with an 8200 (?) ohm resistor?

I assume that 5Z3 is for a slow B+ arrival. I think I'd rather see a delayed relay across a dropping resistor of a suitable value. This is more or less designer's choice.

I also think you'll need some overall NFB.

I see some other minor questionable things futher back, but I think I'll stop here.

Victor

edit for syntax

astouffer · 18th August 2007, 12:49 AM

Agreed on the 12AU7 being too small to push grid current. At the very least I would say a 12B4/6CG7 or a triode strapped pentode like a 6AU5 or 6W6. The 5998 seems ideal for driving an 845 but I dislike the idea of having both channels going into the same tube. My current 845 amp uses a 417A driving a 6AH4 triode. The bottom line is the driver tube must draw a good amount of current, say at least 15mA.

georgehifi · 18th August 2007, 09:19 AM

Here are a couple that I had on file. I also have some with interstage tranies as well if you want.

Cheers George

georgehifi · 18th August 2007, 09:23 AM

and

Yvesm · 18th August 2007, 09:51 AM

Simply :

Yves.

Mush · 18th August 2007, 03:33 PM

Thanks very much for all of your input and the schemaic. This circuit is for the Audioromy 845 stereo integrated amp. I just bought one on Ebay for $320. I look at it as a cheap investment into some parts. I can't afford the good stuff so this will have to suffice.

I am also considering Pete Millett's 12hg7 fron end that he used on his 813 amp. The schematic from Yves is very similar and looks to be be a good one to experiment with. The amp wont be here for at least a month so I am only in the preparatory stages.

Once again thanks and I wish you all well.

Ciao -- Minto

Mush · 18th August 2007, 03:49 PM

Oh yes, George and Yves:
Thanks very much for the schematics. I will have to stick with resistive loads as chokes are added expense. I will go to work today and print out the cicuits you have given me and study them.
Thanks very much -- Minto

tomtt · 19th August 2007, 12:28 AM

...

ak_47_boy · 19th August 2007, 12:47 AM

845's like interstage transformers.

What does the VL208 choke and the two resistors and capacitors before the 6GB8 grid do? Looks to me like a low pass filter?

18th August 2007, 12:07 AM	#2
HollowState diyAudio Member Join Date: Aug 2006 Location: Taxland, New Jersey	Well Minto, I think I'll be the first of many to say that your circuit is not a good one for several reasons. I'll just touch on the obvious weak points. 845's are difficult to drive. They can draw grid current that must be supplied from a good low inpedance drive source. You'll need a high current cathode follower or mosfet source follower like tubelab's to directly drive the grid. (no coupling cap) And the 100K ohm grid resistor is too high in value. This tube also needs a high amplitude signal to provide any usable power out. I doubt your 12AU7 is going to provide that signal. DC on the 845 filament is ok (even good), but what is doing with that CRC business with an 8200 (?) ohm resistor? I assume that 5Z3 is for a slow B+ arrival. I think I'd rather see a delayed relay across a dropping resistor of a suitable value. This is more or less designer's choice. I also think you'll need some overall NFB. I see some other minor questionable things futher back, but I think I'll stop here. Victor edit for syntax __________________ "The supercomputer is technologically impossible. It would take all of the water that flows over Niagara Falls to cool the heat generated by the number of vacuum tubes required." ~ Professor of Electrical Engineering, New York University

18th August 2007, 03:33 PM	#7
Mush diyAudio Member Join Date: Feb 2005 Location: ann arbor, mi	845 Circuit Suggestions Thanks very much for all of your input and the schemaic. This circuit is for the Audioromy 845 stereo integrated amp. I just bought one on Ebay for $320. I look at it as a cheap investment into some parts. I can't afford the good stuff so this will have to suffice. I am also considering Pete Millett's 12hg7 fron end that he used on his 813 amp. The schematic from Yves is very similar and looks to be be a good one to experiment with. The amp wont be here for at least a month so I am only in the preparatory stages. Once again thanks and I wish you all well. Ciao -- Minto

18th August 2007, 03:49 PM	#8
Mush diyAudio Member Join Date: Feb 2005 Location: ann arbor, mi	845 Circuit Suggestions Oh yes, George and Yves: Thanks very much for the schematics. I will have to stick with resistive loads as chokes are added expense. I will go to work today and print out the cicuits you have given me and study them. Thanks very much -- Minto

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18th August 2007, 12:49 AM	#3
astouffer diyAudio Member Join Date: Jun 2004 Location: Pittsburgh, crumbling wasteland	Agreed on the 12AU7 being too small to push grid current. At the very least I would say a 12B4/6CG7 or a triode strapped pentode like a 6AU5 or 6W6. The 5998 seems ideal for driving an 845 but I dislike the idea of having both channels going into the same tube. My current 845 amp uses a 417A driving a 6AH4 triode. The bottom line is the driver tube must draw a good amount of current, say at least 15mA.

18th August 2007, 09:51 AM	#6
Yvesm diyAudio Member Join Date: Jul 2004 Location: Ardeche	Simply : Yves.

19th August 2007, 12:47 AM	#10
ak_47_boy diyAudio Member Join Date: Sep 2006	845's like interstage transformers. What does the VL208 choke and the two resistors and capacitors before the 6GB8 grid do? Looks to me like a low pass filter?

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