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 Post subject: Re: PP 300B questions
PostPosted: August 16th, 2018, 4:39 pm 
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Joined: July 24th, 2015, 4:17 pm
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Location: Parkville, Maryland
Despite the WEB-based recommendations of running the 300B bias at around 60-ma (I even read where 70-ma is desired -- YIKES!) you need to take a page out David's book and set the bias to something a lot more comfortable. My experience has found that, in my amplifiers, 45-ma gets the job done with much improved sound reproduction. Plus you extend tube life as an added bargain. In my SET amps. the 300Bs drive the 845s to good effect using coupling transformers with feed-forward coupling caps.

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 Post subject: Re: PP 300B questions
PostPosted: August 16th, 2018, 5:09 pm 
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Joined: April 22nd, 2013, 12:58 pm
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Sounds feasible. You would have to resistively load the secondary, and could experiment with higher loadings that way.
You would probably want to run the driver in class A, so why not do a constant current source.
Or, as you mentioned before I think, you would do the phase splitting in an earlier stage? In that case you don't need to make the driver into a long-tailed pair.


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 Post subject: Re: PP 300B questions
PostPosted: August 16th, 2018, 5:53 pm 
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Joined: January 14th, 2015, 11:15 pm
Posts: 204
Shashi,

I heard a version of the 300B amps last year in England at the annual get together of a diy group called audio talk.

http://ratbagp.blogspot.com/2017/06/owston-visit-to-diy-audio-land.html

Though obviously limited in power, they were the best of show. You can read more about building them at

https://tubehifi.websitetoolbox.com/post/300b-otl-with-dc-7525758

They have since moved on to another OTL using 6C33C tubes as a cathode follower. About a month ago I did a quick breadboard of this design (with a different operating point) and it sounded promising.

http://www.audio-talk.co.uk/phpBB3/viewtopic.php?f=11&t=7265

Or of course, you could build a MoFo which is getting good reviews. I plan to finish mine when I get back from Australia at the end of September.

http://www.diyaudio.com/forums/pass-labs/313649-build-mofo.html

It's very predictable. I leave the country and Roscoe holds a meet.

ray


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 Post subject: Re: PP 300B questions
PostPosted: August 16th, 2018, 6:04 pm 
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Joined: June 4th, 2013, 2:39 pm
Posts: 142
FerdinandII wrote:
Sounds feasible. You would have to resistively load the secondary, and could experiment with higher loadings that way.
You would probably want to run the driver in class A, so why not do a constant current source.
Or, as you mentioned before I think, you would do the phase splitting in an earlier stage? In that case you don't need to make the driver into a long-tailed pair.


True, I don't need a long-tailed pair, but I'd need to ensure that the current is balanced. That's easy enough. But do I really need to terminate the secondaries? After all, it's really inductance I'm looking for not impedance. I could just as easily use a center-tapped choke. Or am I wrong?

A current source on the cathodes would be a good idea!

...now I've fetched my British GE book and see that for a high-powered AB DA100 amp they use a CT inductor (100-200H) on a pair of common-cathode, triode-wired EL84s to get 200+200V.

ETA: Whoops, those are used in *pentode* mode, and the plate resistors (47K) are shunted by a CT inductor to achieve higher voltage output. Interesting...


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 Post subject: Re: PP 300B questions
PostPosted: August 16th, 2018, 7:09 pm 
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Grover Gardner wrote:
But do I really need to terminate the secondaries?


I think you'd be better off not....

Roscoe


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 Post subject: Re: PP 300B questions
PostPosted: August 16th, 2018, 7:22 pm 
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Joined: June 4th, 2013, 2:39 pm
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dberning wrote:
I make a P-P 300B on special order, and I have a Class A-AB switch that changes the bias current from 19mA per pair to 37 mA per pair. The plate-to-cathode voltage is around 450 V. Output power is 20 watts at 8 ohms and 28 watts at 4 ohms.

The interesting thing I want to bring up is that actually both operating bias levels operate the tubes in class A until clipping occurs! I actually discovered this with my expensive 845 P-P amps that have a 3-position bias switch. You can see that neither tube actually cuts off until clipping if you look at the plate current with a current probe. As it turns out, on the tube that has its grid being driven more negative, the plate is going positive at a rate that is sufficiently fast so that the negatively-going grid is not changing enough to pinch off the plate current. Once clipping occurs, further increase in plate voltage stops and the negatively-going grid can completely cut off the plate current. The bias switch does increase the linearity in the A position, and for critical listening does sound better in most circumstances. Note that my amplifiers do not use output transformers and are thus not subject to core saturation for the higher bias currents. In principle, you can avoid this saturation if you can maintain balance in the dc for the two haves of the transformer, but it is more difficult to maintain for higher bias currents.

I now use floating 6AU6s operated in a pentode-follower mode and bootstrapped back to a 6SN7 for my driver for the 300B. All of these stages are dc-coupled, including the ZOTL output stage and servo controlled.

David


David, your designs never cease to amaze. ;-) Now, given I'll be using transformer output, is it really practical to bias the 300B's that low? Won't the plate impedance go up considerably? Or does it not make a difference?


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 Post subject: Re: PP 300B questions
PostPosted: August 16th, 2018, 7:26 pm 
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Joined: June 4th, 2013, 2:39 pm
Posts: 142
SoundMods wrote:
Despite the WEB-based recommendations of running the 300B bias at around 60-ma (I even read where 70-ma is desired -- YIKES!) you need to take a page out David's book and set the bias to something a lot more comfortable. My experience has found that, in my amplifiers, 45-ma gets the job done with much improved sound reproduction. Plus you extend tube life as an added bargain. In my SET amps. the 300Bs drive the 845s to good effect using coupling transformers with feed-forward coupling caps.


To clarify, Walt, you're using these as drivers and not output stages, correct? I suppose I could go that low but given the higher transformer impedance required and the lower power output, I'm getting down into 2A3 range, I think. ;-) I try to keep my 300B's at 70-75mA max, they start to sound "tight" at 80. I note that a lot of Japanese designers prefer 60mA and higher loads, but they're also not looking for as much output power.


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 Post subject: Re: PP 300B questions
PostPosted: August 16th, 2018, 7:33 pm 
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Joined: June 4th, 2013, 2:39 pm
Posts: 142
Roscoe Primrose wrote:
Grover Gardner wrote:
But do I really need to terminate the secondaries?


I think you'd be better off not....

Roscoe


That's my thinking, too. Does anyone recall the old Magnequest/Sound Practices designs for choke-loaded drivers? Mike used to make those 70H, gapped plate chokes for 6SN7's, IIRC. I don't recall any plate resistors involved. It's interesting that the GE designs use both a plate resistor and a CT'd inductor. Maybe because they're using pentodes?

I suppose I could use two EL84's in pentode mode, just as they do (gain of 100!) in parallel with an OPT primary, put a 6J5 balanced phase splitter in front and be done...? Kind of like a dual Thorsten Loesch Legacy design...


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 Post subject: Re: PP 300B questions
PostPosted: August 16th, 2018, 11:57 pm 
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Joined: April 22nd, 2013, 12:58 pm
Posts: 177
Hi Grover,

The CT Choke is probably the ideal piece of hardware for the topology you are suggesting.
https://electra-print.com/docs/6bx7pp0001.pdf
But I got the impression you wanted to try and use parts that you already have.....
Leaving the secondaries unterminated means a theoretical infinite resistance and any parasitics will be able to wail away unimpeded. Better to load a typical output transformer with a 1k load and dampen any stray inductance/capacitance that are lurking inside the transformer.

Of course, we are DIY experimenters :confusion-confused: so try it both ways......


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 Post subject: Re: PP 300B questions
PostPosted: August 17th, 2018, 12:58 am 
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Joined: June 4th, 2013, 2:39 pm
Posts: 142
FerdinandII wrote:
Hi Grover,

The CT Choke is probably the ideal piece of hardware for the topology you are suggesting.
https://electra-print.com/docs/6bx7pp0001.pdf
But I got the impression you wanted to try and use parts that you already have.....
Leaving the secondaries unterminated means a theoretical infinite resistance and any parasitics will be able to wail away unimpeded. Better to load a typical output transformer with a 1k load and dampen any stray inductance/capacitance that are lurking inside the transformer.

Of course, we are DIY experimenters :confusion-confused: so try it both ways......


Thanks for the suggestion about loading the secondaries, I'll try it.


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