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=maximum= 16th July 2007 09:16 PM

845 Low voltage SE AMP

I'm planning to build an amp with tha power of a 300b amp but with DHT 845 tubes.. I hope they are more economical and indistructible at this low voltages! Someone have build this amp?
I have some other low voltage schematics with845 to discuss...

Thanks in advance

Max ;o))

nhuwar 17th July 2007 02:02 PM

I could be wrong but I remimber people on this forum saying 845 dont like to low a plate voltage. But I could be wrong and I'm sure I will be corrected.


Tweeker 17th July 2007 02:08 PM

You wont be getting much power from an A1 SE 845 at low plate voltage.

A 6SL7 isnt going to provide very satisfactory drive to one, either.

MikeR 17th July 2007 03:32 PM

As most things in audio, opinions vary. I like the sound of a low power 845. I have a breadboard mono 845 that I am currently developing. Mine is at a little higher plate voltage, 430 volts and biased to 62 mA with a fixed bias supply similar to yours. I have a 4.8k transformer. Power is about 4 watts.

I do not use any feedback. I have also found that AC on the heaters works pretty well which suprised me since they are 10 volts.

My driver is a parallel 6SL7 also, but running as a mu stage using a constant current source. It seems to work pretty well. Looking at it on a scope, I get about 120 V p-p before distortion shows up.

Play around with the driver stage to find what you like. A simple 6SL7 SRPP also sounded good to me.

Do a search for Jim Doyle and 845. He studied low power 845 designs extensively.

Good luck.

kevinkr 17th July 2007 06:03 PM

Definitely take a look at mu stage and srpp drivers based on the 6SL7 running at around 2mA. Plenty of swing and good linearity, should be enough current available to get you a little beyond 20kHz allowing for C miller in the 845.

Definitely google Jim Doyle 845 project. I have heard a couple of low power 845 amplifiers and they sounded quite good, and know of one currently under construction. Power in the range of 2 - 5W is achievable at voltages below 500V.

I'm surprised to hear that ac heating at 10V worked ok, hasn't been my experience with 5V tubes, but who knows. I would heat these using constant current dc, (LT1084 based or similar) but quick and dirty AC may be ok depending on the particular set of tubes.

Linearity will be good enough that no global feedback should be required.

MikeR 17th July 2007 11:37 PM

I ran across this link to Jim Doyle's web site. Lots of stuff here for low power 845 amps.

Jim Doyle

=maximum= 18th July 2007 06:42 PM

Hello to all and thanks for the interesting website!

what about this interesting link...

the amp shoul delivery about 11w with 750V an 5K load

I also read an interesting article on Italian Magazine CHF Costruire HI-FI that talk about possibility of Use 845 @ low voltages... if someone is interested I can ship some photos of the article....


Gluca 18th July 2007 10:50 PM


I'd suggest to use a 813 (triode strapped). Much easier and cheaper than 845 and probably even better. There are talks here and there on this tube.


quinnling 19th July 2007 11:56 AM

I just recently converted my low voltage 845 amp from Direct Feed to Para-feed output stage.

I started out with 6EM7 driver that was in a Vacuum Tube Valley write up from Eric Barbour. He used it on a SV572 amp project.

I just created a -50v bias and had 490v on the 845 running on 85ma. I used that setup for about 7 years.

I needed the 6EM7 for another project and converted the amp to a SRPP using a 6SL7. This work surprising well considering the 6SL7 is not a hi current tube.

Last week I just converted in to Para-feed and is waiting for the new transformer to break in.

Once you get going, you can modify you amp to taste or if you are bored. If the new change does not work well, you can always change it back.

As they say: "It is the journey that is the reward"

MartinV56 13th January 2013 07:56 PM

?????? DIY Audio Craft • View topic - 8W Low voltage 845 Single Ended Power AMP

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