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 Post subject: 6HS5 amplifier
PostPosted: Feb Fri 09, 2018 7:56 pm 
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Saw an ad in the classifieds and was looking at the tubes for sale. Saw some compactrons and got to looking them up.

According to this.

http://vinylsavor.blogspot.com/2011/07/ ... -6hs5.html

it can be used for an audio amp.

Would it be a tube that I would want to mess with given the high voltage required?


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 Post subject: Re: 6HS5 amplifier
PostPosted: Feb Fri 09, 2018 9:30 pm 
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Just about any tube can be used as an audio amp. Doesn't require that high of voltage either unless you need maximum power.

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 Post subject: Re: 6HS5 amplifier
PostPosted: Feb Fri 09, 2018 9:57 pm 
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Location: Saskatoon
If you operate those beam triodes with a positive grid bias instead of the usual negative bias, then you can operate them at much lower plate voltage (around 250 volts), and still get their rated plate current. I have an audio project on the go, using a similar tube with positive grid bias.

Edit:
I dug up the schematic of my test/prototype using a 6JD5 beam triode similar to the 6HS5:
Attachment:
6JD5 Amplifier.png
6JD5 Amplifier.png [ 27.1 KiB | Viewed 385 times ]


In order to work with positive grid bias on the output tube, you need lots of current drive at the grid, hence, the Darlington arrangement. The interesting characteristic of this arrangement is that the non-linearity of the two tubes in the output stage cancel each other out, giving very good linearity.

Bear in mind that the schematic represents a work in progress. It's really a test setup. The potentiometer adjustment for the 6KV8 pentode screen was included to set the screen voltage for optimum linearity. Once the best voltage is found, this should be replaced with a fixed voltage divider.

You should also make note of the fact that these beam triodes draw a lot of heater current. The 6HS5 heater current is double that of a 6L6, and the 6JD5 that I'm using is even worse.

I put it on the back burner when I got to the point of trying to decide whether I wanted to go single ended or push-pull.


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 Post subject: Re: 6HS5 amplifier
PostPosted: Feb Sat 10, 2018 4:08 pm 
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Very cool.

With me I'd probably go push pull.

Now would it be possible to use a triode in place of the pentode?


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 Post subject: Re: 6HS5 amplifier
PostPosted: Feb Sun 11, 2018 12:10 am 
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I would prefer a push pull version too. Supplying all that heater current for a stereo version is a bit scary though. On the other hand, the output iron would be a lot cheaper for push pull.

I see no benefit at all for using a triode as the driver. It would be hard (but maybe not impossible) to find a triode with the very high transconductance and plate current ratings needed in for the driver. In this application, a pentode is ideal for several reasons. The non-linearities between the two tubes can be made to cancel by adjusting the pentode screen voltage with the divider network. You wouldn't have that option with a triode, and you'd have to live with the non-linearity. Also, because the plate current of the pentode is largely independent of plate voltage, it's practical to tie the pentode plate to the triode plate, giving a bit more output drive to the load, and simplifying the wiring.

The 6KV8 is a cheap TV tube, and its triode section is similar to 1/2 of a 12AT7, which makes a good preamp section.

BTW, the original idea for this circuit came from a post by Joe Sousa on RadioMuseum.
https://www.radiomuseum.org/tubes/tube_6jd5.html
One of the advantages of this type of tube running with positive bias is that its plate curves have a fairly sharp knee at a relatively low plate voltage that allows for a much larger than normal linear plate voltage swing.

As for the earlier comment about cancelling out non-linearities, I was able to demonstrate this with my test circuit. By setting up a scope in X-Y mode, and connecting the input grid signal to one channel and the plate signal to the other channel, you get a diagonal line on the screen. If the output signal is free of distortion, then the diagonal line will be straight. If there's distortion, then the line will be curved. By adjusting the screen voltage pot, I was able to change the curvature of the line, from being curved in one direction, to being completely straight, and then to being curved in the opposite direction. So, this circuit is definitely capable of very linear operation. For the final circuit, once the approximate optimum screen voltage is known, I would change the screen divider network to use a much smaller value pot for a finer range of adjustment.

For a push pull version, we still need to address crossover distortion, but that should not be any different than any other tube P-P amp.


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 Post subject: Re: 6HS5 amplifier
PostPosted: Feb Sun 11, 2018 5:38 am 
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I would only build a mono version.

I do have a pretty good self split phase inverter that would drive both pentodes.

Used it in two different amps and it works great. Got the circuit from the schematic to a GEC 912 Plus amp.


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