Hello!
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))
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))
An externally hosted image should be here but it was not working when we last tested it.
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.
Nick
Nick
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.
A 6SL7 isnt going to provide very satisfactory drive to one, either.
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.
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.
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.
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.
Hello to all and thanks for the interesting website!
what about this interesting link...
http://www.vt4c.com/bb/viewtopic.php?t=17
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....
M@X
what about this interesting link...
http://www.vt4c.com/bb/viewtopic.php?t=17
the amp shoul delivery about 11w with 750V an 5K load
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
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....
M@X
Ciao,
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.
Gianluca
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.
Gianluca
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"
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"
I can't think of any technical reason why low voltage (ie. 700V) would be bad for the 845. Go ahead and try it. The GM70 would be an economical alternative to the 845.
I would bias the crap out of those things if using lower voltage. Looking at the graphs, if anything the tube would run more linear at such operating points.
I would bias the crap out of those things if using lower voltage. Looking at the graphs, if anything the tube would run more linear at such operating points.
I have the same idea with you.
With Vak ~ 750V, 70-50mA and opt 5-6K, we will have near 14-15W which is enough to dirve almost speaker in market. In this operation point, 845 has very very longlife.
I've heard some try AC heater for 845 with small hum noise. But I never try it.
🙂
With Vak ~ 750V, 70-50mA and opt 5-6K, we will have near 14-15W which is enough to dirve almost speaker in market. In this operation point, 845 has very very longlife.
I've heard some try AC heater for 845 with small hum noise. But I never try it.
🙂
I didnt't check it myself, but a very experienced guy said that the plates are covered with titanium dioxide which has a gettering effect only if it is heated at more than 450°C, that corresponds to 90W dissipation.
So the 845s may live longer if they are run hotter!
So the 845s may live longer if they are run hotter!
I have built several SE 845 amps running at 750 volts 3.5k out put transformer {Sowter}.
I drive mine with a Screen driven 5881 and{ peter millets} HI mu Pentode.
advantage of the screen drive 5881 it can handle 750 volts; I use a tube voltage regulator for the Pentode. Amp is very dynamic.
Phil
I drive mine with a Screen driven 5881 and{ peter millets} HI mu Pentode.
advantage of the screen drive 5881 it can handle 750 volts; I use a tube voltage regulator for the Pentode. Amp is very dynamic.
Phil
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Joined 2009
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845 starts to become more linear with 800V on the plate (if correctly biased).
6SN7 doesn’t have the stones to drive it well. Maybe 6BL7 or better yet us a mosfet follower.
Ian
6SN7 doesn’t have the stones to drive it well. Maybe 6BL7 or better yet us a mosfet follower.
Ian
Depends a lot on the 845. I used ones with a metal plate that did not have any Titanium dioxide. I also had graphite plate which I didn’t like as much. I don’t recall them looking white.
Titanium dioxide is quite white.