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 Post subject: PP 300B questions
PostPosted: August 16th, 2018, 1:41 pm 
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Joined: June 4th, 2013, 2:39 pm
Posts: 137
This is a bit rambly so bear with me. :-)

I had to put my ongoing PP 300B project on hold because I couldn't afford to keep replacing Electro-Harmonix 300Bs. I've had a really high failure rate with these things, and I'm not running them particularly hard. So back to SE for the moment, and thought I'd try the Psvane Hi-Fi 300B for $120 a pair after reading some good reviews. I think these are an exceptional bargain, very sweet and musical, with a nice midrange and a clean top end. It's been a long time since I've been able to afford a pair of high-end 300Bs, but I prefer these to my remaining pair of EH Gold Grids. If you're on a budget I think the Psvane's are a great deal. We'll see how long they hold up. :-)

In the meantime I came across this MJ Magazine archive mentioned at the DIY Audio board. Some of you might be interested in browsing for ideas. Too bad the text is in Japanese but a lot of pretty schematics!

http://www003.upp.so-net.ne.jp/kirifuri ... 17-18.html

I was particularly intrigued by a 1999 article on PP AB 300B curves, since I've never come across any 300B AB designs before. I've included the article below for interest. So now I'm inspired again but I have a question about drivers. I'm not sure I want to get into interstage trannies, but this PP AB design would need a lot of voltage swing. I'm thinking of two options:

1) A standard front end with a separate B+ supply to afford a lot of headroom for the driver stage.

2) A transformer-loaded driver. That is to say, using a spare output transformer as a load for a pair of driver tubes. I could put the phase splitter up front, then use, for instance, a pair of triode-wired EL84s or something along those lines.

So my question is, can I use a PP output tranny to load the drivers, then cap-couple to the 300B grids? If so I could easily use a single PS. Would I need to terminate the OPT secondaries? I feel I use to know all this but I've forgotten it. ;-) I'd appreciate any thoughts.


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 Post subject: Re: PP 300B questions
PostPosted: August 16th, 2018, 2:02 pm 
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Joined: February 28th, 2013, 10:38 am
Posts: 1027
The best bargain in 300Bs is the 6CB5A ;)

Roscoe


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 Post subject: Re: PP 300B questions
PostPosted: August 16th, 2018, 2:11 pm 
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Joined: June 4th, 2013, 2:39 pm
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Roscoe Primrose wrote:
The best bargain in 300Bs is the 6CB5A ;)

Roscoe


Yes, I've tried those. :-) Pretty much limited to 6-7 watts, though. They're also getting scarcer. A PP pair needs 5A of heater current, which means a fairly hefty piece of extra iron.


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 Post subject: Re: PP 300B questions
PostPosted: August 16th, 2018, 2:27 pm 
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Joined: July 17th, 2016, 6:24 am
Posts: 570
You might want to look at PP 300B OTL amps. Schematic is published in Bruce Rozenblit's book. I will be building this as a stereo amp in the winter....

https://www.transcendentsound.com/pinnacle.html

_________________
Shashi


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 Post subject: Re: PP 300B questions
PostPosted: August 16th, 2018, 2:29 pm 
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A lot of "all-in-one" power transformers have 5A 6.3v windings, use that for the 6CB5s, and a smaller one for the drivers.

Of course, there's always 813s ;)

Roscoe


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 Post subject: Re: PP 300B questions
PostPosted: August 16th, 2018, 2:57 pm 
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Joined: April 22nd, 2013, 12:58 pm
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Hi Grover,

Sounds like you want to use an Output Transformer as a choke load for the driver stage. The transformer would have to be optimized for SE operation in that case, unless you went with a parafeed arrangement where there is no DC flowing through the choke. Google "parafeed driver circuit" and you'll see some examples....

But....what to do with the secondary winding???? You could run it in series with the primary, or terminate it.


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 Post subject: Re: PP 300B questions
PostPosted: August 16th, 2018, 3:04 pm 
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Joined: March 12th, 2013, 11:12 am
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No idea about the PP OT as driver load but I'll throw out my 2 cents worth on a couple of 300Bs. I am also using currently using PSVane 300B (the budget, ceramic base $130 for a matched pair ones) and they are pretty nice sounding. Well balanced and a very traditional 300B sound. Longevity is an open question as I only have 6 months of use on my pair. PSVanes Treasure (?) sound similar but better, somehow being both more lush and more detailed at the same time. Prior to the PSVanes, I was using JJ 300Bs (the ones with the heavy glass and the little lever heater positioner thingie). The JJs sound more muscular, better lows, but a bit harder overall, a wee bit more of a modern sound. I get at ~7000+ hours out of a set of JJs. Sovtecs and EL 300B eehhh. For a much different sound, the FM meshplates are open, airy and fast but don't like to be pushed hard and only last ~3000-4000 hours before they start acting up.

BTW, NOS rectifiers are becoming harder to get and I am running low on my precious Mullard GZ37s, I found the Svetlana 5C3S (black plate) to be a great, low cost, well performing substitute.


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 Post subject: Re: PP 300B questions
PostPosted: August 16th, 2018, 3:53 pm 
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Joined: June 4th, 2013, 2:39 pm
Posts: 137
TubeDriver wrote:
No idea about the PP OT as driver load but I'll throw out my 2 cents worth on a couple of 300Bs. I am also using currently using PSVane 300B (the budget, ceramic base $130 for a matched pair ones) and they are pretty nice sounding. Well balanced and a very traditional 300B sound. Longevity is an open question as I only have 6 months of use on my pair. PSVanes Treasure (?) sound similar but better, somehow being both more lush and more detailed at the same time. Prior to the PSVanes, I was using JJ 300Bs (the ones with the heavy glass and the little lever heater positioner thingie). The JJs sound more muscular, better lows, but a bit harder overall, a wee bit more of a modern sound. I get at ~7000+ hours out of a set of JJs. Sovtecs and EL 300B eehhh. For a much different sound, the FM meshplates are open, airy and fast but don't like to be pushed hard and only last ~3000-4000 hours before they start acting up.

BTW, NOS rectifiers are becoming harder to get and I am running low on my precious Mullard GZ37s, I found the Svetlana 5C3S (black plate) to be a great, low cost, well performing substitute.


Yes, those are the ones. I'm glad you agree, I thought maybe I was kidding myself. ;-) A very nice sound for the money. I too just discovered the 5C3s--well, I had some in my stash, can't remember when I ordered them, ages ago, pulled them out for the SE amps and they really are very nice.


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 Post subject: Re: PP 300B questions
PostPosted: August 16th, 2018, 4:08 pm 
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Joined: June 4th, 2013, 2:39 pm
Posts: 137
FerdinandII wrote:
Hi Grover,

Sounds like you want to use an Output Transformer as a choke load for the driver stage. The transformer would have to be optimized for SE operation in that case, unless you went with a parafeed arrangement where there is no DC flowing through the choke. Google "parafeed driver circuit" and you'll see some examples....

But....what to do with the secondary winding???? You could run it in series with the primary, or terminate it.


Well I actually want to do a common-cathode PP driver. Assuming the inductance would be sufficient (say a small 8K p-p tranny with a pair of triode-wired 6V6's) and I include a balancing pot on the cathodes to ensure equal current, then I could cap-couple the plates to the output tubes, yes?


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 Post subject: Re: PP 300B questions
PostPosted: August 16th, 2018, 4:26 pm 
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Joined: March 2nd, 2013, 2:43 pm
Posts: 147
Location: Potomac, MD
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


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