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Old 11th February 2008, 11:25 PM   #1
arnoldc is offline arnoldc  Philippines
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Default 300B - Push Pull or PSE to drive a commercial speaker

After auditioning dozens of speakers, and finally settling on a DIY speaker, for my near-cube room (4m x 4m x 3.5m H), I found a commercial speaker that conquered the room. It is the LSA1 from The LSA Group. It sounds very good, and is currently being driven by my vintage Harman Kardon 430.

However, I wanted a tube amplifier to ultimately drive this, and after selling my Anthem Amp 1, I'm left with flea powered amplifiers that definitely does not have the power.

So I wanted to embark on another project and to be more sensible, I will use a pair of 300B tubes.

The question is, should it be PSE or push pull? I have not heard any of the topology and does not know advantages of one from the other.

Can anybody enlighten me?

I also wanted to salvage the Lundahl LL1660S/10mA that I used in my last 300B to be used as either interstage or phase-splitter. The questions surrounding this would be, should I use it with single ended to push pull, or push pull to push pull (should I opt for push pull finals).

The other goal is to get as much power output as possible, and I've been playing with TubeCad that can do PSE, and I can only get 18W out of the 300B pair with around 200Vpp, with positive grid. Is there a way to do some 20W or more if I use push-pull?

Please bear with me as I'm a total newbie when it comes to push pull or PSE stuff.

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Old 12th February 2008, 12:25 AM   #2
GK is offline GK  Australia
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The other goal is to get as much power output as possible


Well that rules out class A and PSE.
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Old 12th February 2008, 01:54 AM   #3
arnoldc is offline arnoldc  Philippines
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AB1 and Push Pull?
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Old 12th February 2008, 02:25 AM   #4
GK is offline GK  Australia
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Yep. A pair of 300B could make a rather nice AB1 P-P amp with a decent power output, provided that the 300B drive requirements were met.

I really wish these tubes went for the same $ as a 6550 or a KT88......
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Old 12th February 2008, 02:53 AM   #5
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Originally posted by arnoldc
AB1 and Push Pull?

AB2 and push pull. Tubelab's Power Drive takes care of pulling the 300B grids slightly positive. Don't over do it. IRFBC20 FETs will be terrific as the drivers.

A lot of the euphony found in SE 300B amps is going to disappear, due to the inherent cancellation of even order HD products associated with PP "finals". A 6AB4 (1/2 a 'T7) in the voltage gain stage will restore the "moisture". An ECC99 LTP will take good care of phase splitting.

A considerable negative voltage will be needed to bring the 300Bs close to cutoff, for the no signal state. IMO, self biasing will prove very difficult. My hunch is that combination bias is the way to go.

AC heating of the 300Bs should be fine. When the filaments are phased up correctly, hum gets cancelled.
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Old 12th February 2008, 04:19 AM   #6
arnoldc is offline arnoldc  Philippines
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Gents,

Thank you for the replies. I believe I have to go with fixed bias to attain this goal.

Eli, you think the LL1660S has no place for this?
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Old 12th February 2008, 03:28 PM   #7
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Originally posted by arnoldc
Gents,

Thank you for the replies. I believe I have to go with fixed bias to attain this goal.

Eli, you think the LL1660S has no place for this?

Arnold,

You did say this amp will be used with commercial speakers. Adequate control of the woofer voice coils is a concern. A few dB. of loop NFB might be the difference between success and failure in the damping factor dept. Interstage trafos inside a NFB loop are a prescription for trouble. A Mullard style circuit, with the voltage gain stage DC coupled to the LTP has but 1 cap. (at the splitter's anodes) in the signal path. Therefore, phase shifts are well controlled. The use of high gm small signal tubes is protection against slew limiting. Roll infrasonic noise off at the amp's I/Ps with high pass filters that "corner" just below 20 Hz. Killing infrasonic crud off protects the O/P trafo cores from FUGLY saturation.

IMO, you should save the Lundahl "iron" for a different project. How about a fully differential circuit using a 12AT7 and triode wired EL34s? No loop NFB, here.

I agree that "fixed" bias is needed, but I would "stand" the 300B cathodes on unbypassed 470 Ohm resistors to provide protection against run away. It's combination bias.
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Old 12th February 2008, 04:58 PM   #8
kevinkr is offline kevinkr  United States
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Hi Arnold,
Long ago (like 10 yrs now) I designed a 30W PP 300B amplifier with regulated supplies which was published in VTV and became part of the SED technical library, (long gone) and while I am not recommending you build this particular design I do recommend you at least read the article which is on my site.

I don't recommend AB2 operation for any 300B, as the grids are rather easily over-dissipated, plus at 400V B+ and 120mA quiescent current you can get 30Wrms out with a primary PP of 3.8K and at less than 2% thd with no feedback. Any transformer in the range of 3K - 5K will provide good results.

I used fixed bias in all my designs and recommend that.

This design depends heavily on the quality of the regulated power supplies and was very carefully designed to provide the required drive on a +400V supply, unfortunately the LTP based driver stage has limited bandwidth when driving the miller capacitance of the 300B.

Lots of people have built these amplifiers, and in addition I sold a fair number of a simpler version of the 300B PP amplifier commercially as well.

Note that I have not found ac heating to be very acceptable long term in my commercial 300B based PP amplifiers, and ended up retrofitting at least 5 of these amplifiers (KTA3030) for dc heating at great expense. All subsequent units had dc heating. All would be well initially and then after a few weeks or months the tubes originally well matched would age and drift out of balance - hum would become intrusive. The procedure of setting the bias and then nulling the hum was tedious for the average user, and not as effective as expected. The JJ 300B is the only tube I have used that did not have this issue over the long term.

Here's the link: http://kta-hifi.net/projects/amp_pag...mp/300bpp.html
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Old 12th February 2008, 07:41 PM   #9
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Kevin, thanks for the link! Nice schematic.
I'd like one day to build a "universal" P-P amp and try different output tubes. Both triodes and pentodes. And as I read in your article the input stage can provide 170Vpp per plate. I think it's a nice choice. What do you say?
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Old 12th February 2008, 10:01 PM   #10
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Quote:
Is there a way to do some 20W or more if I use push-pull?
I built a P-P 300B amp several years ago. It started out as a variation on Kevins design but morphed into something different to work with the parts that I had. I have used this amp on and off for 6 or 7 years. I usually grow tired of my amps after a few years and sell them, trade them away, or rob them for parts. I still have this one. There is something about the sound that I have not yet been able to duplicate. Last year the enevitable happened, I flipped the power switch, and zap... poof....twang...it was dead. I am sure that one of the electrolytics shorted leading to some toasted diodes.

I never drew a proper schematic, but am now attempting to reverse engineer one of my own amps to figure out what I did. I know this:

The input stage is an LTP phase splitter using a 5751. It feeds two seperate SRPP driver stages using 6CG7's. The SRPP stages are capacitor coupled into the 300B grids. Fixed bias is used. AC heating is used on all tubes. The power supply is a SS bridge on a 350 VAC large toroid feeding a choke input filter. The B+ is about 375 volts. - 200 volts is generated using some more diodes and capacitors somehow.

I measured the amp when I built it and it made about 25 watts. The output tubes are biased so that they operate in class A for power levels up to about 10 watts. I am using a 6600 ohm OPT.

I made this amp with some cheap parts with the idea of upgrading it later. I used some guitar amp OPT's that I got for $15 each. The power supply components came from computer power supplies, the coupling caps came from ceiling fan controls. There is some synergy happening between all of the junk because every time I have attempted to upgrade it the amp sounds worse. I learned to leave it alone.

Now that it is dead, what do I do? My current plan is to change the dead parts and do nothing else. I want to keep it as a standard by which to measure my next attempt. I plan to do another 300B P-P amp, but with more updated circuitry. PowerDrive, Augmented Cathode Follower, I don't know yet, but it better sound as good as this one, or it gets dissected and I start over! I want the driver board to have "universal" drive capability, but if it can drive 300B's it can drive most anything else except for maybe 845's.
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