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 Post subject: Re: Output tube biasing
PostPosted: October 27th, 2020, 3:43 pm 
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Joined: February 28th, 2013, 1:19 pm
Posts: 491
There are advantages/disadvantages to SE as well as PP. It is not clear that one is intrinsically better than the other, it is a factor what is important to you in the sound, the music, and what your speakers need.

A PP Class A no feedback triode amp using DHTs can be a very fine amplifier, offering much of the sonics we like in a SET amp, but with better control, quiet background, and more power. A PP 2A3 or 300B amp could be the ticket given higher efficiency box speakers. You can get up to 20 watts or so with PP 300Bs. This is probably the best compromise amp and is relatively rare, and don't understand why it does not get more respect.

A Class AB PP UL amp with feedback gets alot of bad rap, but does provide power, control, dynamics and is better suited to large orchestral music than low power SETs. With alot of care and attention, you can get a pretty good sound. Maybe not that touchable magical sound, but sometimes you want to listen to more than a single vocalist or jazz combo.

IF you want power with SETs, you are talking about pretty big amps (like my 120 lb each 20W GM70 monoblocks). So there is a practical limitation, and cost goes through the roof.

I like the sound of all three approaches. Since I tend to listen primarily to classical music, full orchestral, PP UL works well to convey the power, size and control the music demands. But my GM 70 amps are close at hand.

David


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 Post subject: Re: Output tube biasing
PostPosted: October 27th, 2020, 4:02 pm 
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Joined: July 24th, 2015, 4:17 pm
Posts: 1302
Location: Parkville, Maryland
David McGown wrote:

IF you want power with SETs, you are talking about pretty big amps (like my 120 lb each 20W GM70 monoblocks). So there is a practical limitation, and cost goes through the roof.

Then there are the Cary 805s and 211s. My 805s (845 triode) are rated at 50-watts output and the 211s (parallel 211 triodes) can push 150-watts.

And yes, they serve the music and not some "Hi-Fi" expectation.

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 Post subject: Re: Output tube biasing
PostPosted: October 27th, 2020, 4:48 pm 
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Joined: February 28th, 2013, 1:19 pm
Posts: 491
Must be some pretty hefty iron in the parallel 211 amp. That is alot of SE power. My Tamuras (in the GM70 amps) are 50 W rated, as I recall. Also getting 50W out of a 845 SET takes a bit of drive.

David


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 Post subject: Re: Output tube biasing
PostPosted: October 27th, 2020, 5:59 pm 
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Joined: January 15th, 2015, 7:19 am
Posts: 1317
Location: Baltimore MD
Walt’s Cary’s are amazing


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 Post subject: Re: Output tube biasing
PostPosted: October 27th, 2020, 6:14 pm 
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Joined: July 24th, 2015, 4:17 pm
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Location: Parkville, Maryland
David McGown wrote:
Must be some pretty hefty iron in the parallel 211 amp. That is a lot of SE power. My Tamuras (in the GM70 amps) are 50 W rated, as I recall. Also getting 50W out of a 845 SET takes a bit of drive.

David

You are correct. David Had, the designer, did his homework. He uses a 300B as a driver through a coupling transformer with a feed-forward capacitor. When the 845's grid goes positive the transformer scheme pushes the 845 to a design limit of 50 watts. 845s have a plate rating of 75-watts. That is the only 845 amplifier that I am aware of that gets more than 20 watts of audio power to the load. Straight capacitor coupling "hits the positive-grid brick wall."

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 Post subject: Re: Output tube biasing
PostPosted: October 27th, 2020, 6:15 pm 
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Pelliott321 wrote:
Walt’s Cary’s are amazing

Thank you - Thank you - Thank you - Thank you!!! :D

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 Post subject: Re: Output tube biasing
PostPosted: October 27th, 2020, 7:57 pm 
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Joined: February 28th, 2013, 1:19 pm
Posts: 491
Walt,

I hope that when the Covid-19 pandemic is over, I get a chance to hear your Cary amps. But have to stay safe and get through this healthy!

My GM70 amps use a single stage driver (D3A transformer coupled), but definitely does not have the drive for A2 operation. Over the past couple of weeks I have been considering reworking the amps and use a power tube driver in addition to the voltage amp at the front. The amps just do not compare in dynamics and clarity with my PP KT88 amps. Of course, I have alot of hours on the GM70s in the amps. so it could be that as well. They are short lived tubes.

David


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 Post subject: Re: Output tube biasing
PostPosted: October 27th, 2020, 11:09 pm 
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Location: Parkville, Maryland
David McGown wrote:
Walt,

I hope that when the Covid-19 pandemic is over, I get a chance to hear your Cary amps. But have to stay safe and get through this healthy!

My GM70 amps use a single stage driver (D3A transformer coupled), but definitely does not have the drive for A2 operation. Over the past couple of weeks I have been considering reworking the amps and use a power tube driver in addition to the voltage amp at the front. The amps just do not compare in dynamics and clarity with my PP KT88 amps. Of course, I have alot of hours on the GM70s in the amps. so it could be that as well. They are short lived tubes.

David

I was planning on a brisket-less group meet then the "kooties" arrived. :obscene-drinkingcheers:

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 Post subject: Re: Output tube biasing
PostPosted: October 27th, 2020, 11:31 pm 
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Joined: January 14th, 2015, 11:15 pm
Posts: 389
Just a reminder about the Tubelab Powerdrive circuit. The article refers to getting > 30 watts out of an 845.

http://tubelab.com/articles/circuits/power-drive/

I've used this circuit for years and I like it. It's very simple and cheap to build though the 2SK2700 is now unobtanium and I use a 2SK3564 instead.

My stuff is still on its way on the boat to Australia. However, just before it reached the Panama Canal it turned around and returned to New Jersey. It is no trying again and is now in Charleston. I am hoping to get my stuff by Xmas.

ray


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 Post subject: Re: Output tube biasing
PostPosted: October 28th, 2020, 10:14 am 
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Joined: July 24th, 2015, 4:17 pm
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Location: Parkville, Maryland
ratbagp wrote:
Just a reminder about the Tubelab Powerdrive circuit. The article refers to getting > 30 watts out of an 845.

http://tubelab.com/articles/circuits/power-drive/

I've used this circuit for years and I like it. It's very simple and cheap to build though the 2SK2700 is now unobtanium and I use a 2SK3564 instead.

My stuff is still on its way on the boat to Australia. However, just before it reached the Panama Canal it turned around and returned to New Jersey. It is no trying again and is now in Charleston. I am hoping to get my stuff by Xmas.

ray

Was air freight too expensive? Did you find out why the ship turned around? Moving problems? You've had the absolute maximum of moving problems. YIKES!!

I've only moved twice in my lifetime and those moves were a royal pain-in-the-ass, but nothing compared to your experience.

Although I have great respect for MOSFets it somehow seems sacrilegious to feed a gallant power-triode that way. :twisted:

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