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Old 16th April 2010, 03:08 AM   #1
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Default Big push-pull power for dummies

First of all, I know dummies probably shouldn't be looking to build big push-pull amps but lets move beyond that.

I'm trying to wrap my arms/brain around the pros/cons involved in getting big power from PP amps. I've started with some SET amps that I love (Tubelab's SSE is sounding good right now) and am getting ready to start construction on Poinz' Musical Machine. I've been following:

6L6GC AB2 Amp

"Mullard 5-20 KT88 PP blocks!

OPUS 5.0 A Modern Mullard

All three of these threads have given me some great insight but I'm still a beginner. Down the road, maybe next year, I'd like my next amp project to be a pair of PP monoblocks that can do 60-75 WPC (I've got some speakers that I really like that are only 84-85 dB senstive). I'd like to utilize the Russian 6P3S-E that I've recently become enamored with. I'm sure I'll need some specific help down the road but here I'd just like to ask some basic questions.

First of all, I've seen several commercial designs running 4 output tubes per channel (or more) and I'm wondering about the benefits/drawbacks of running more output tubes in parallel. One of the reasons I like these tubes so much is they are very reasonably priced and it wouldn't be difficult to do 4 per channel, at least not nearly as expensive as running 4 of the expensive reissue KT-88's per side. 2 per side... OK, but 4 per side starts to get pricey.

Second question, money aside, is it easier to make a better sounding amp running 2 bigger output tubes per side vs. 4 smaller output tubes per side?

I'm sure I'll come up with more questions down the road but those two should get me started for now.

Thanks in advance!

Last edited by bigjppop; 16th April 2010 at 03:21 AM.
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Old 16th April 2010, 03:45 AM   #2
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Very few have used four per side. After all, it's only a 3dB improvement (or less) over a pair on a side, and the cost in energy, heat, and parts is considerable.

As long as you aren't troubled with parasitic oscillations, you should be okay with it. And of course the transformer must be wound for a correspondingly low plate load impedance. The high end could be a problem, with all that capacitance in parallel. The driver has to be pretty strong.
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Old 16th April 2010, 03:53 AM   #3
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You forgot Pete Milletts current push pull amplifier
Posted new P-P power amp design
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Old 16th April 2010, 04:11 AM   #4
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Quote:
Originally Posted by bob91343 View Post
The driver has to be pretty strong.
Two words: MOS FET
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Old 16th April 2010, 05:24 AM   #5
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By 4 per side, you mean 4 per channel, right? The Chinese tubes you speak of will do 80 wpc with 4 tubes. If you use the KT88, you can probably get 80 wpc with only a pair per channel (higher voltage on the tubes). And both of these amps would be class AB1, so you wouldn't have any driver problems.
Typically, a pair of tubes per channel is easier than a push pull pair (4 tubes) per channel. Easier in that it is easier to match the currents on each side of the output transformer with only 2 tubes. Some people say that more than 2 tubes in a PP amp will muddy the sound, but I don't know if that is to be believed as many tubes on each side will end up averaging out in their characteristics, but you would have to be the judge of that by listening to each example yourself.
I hope this post was understandable....
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Old 16th April 2010, 09:08 AM   #6
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An important benefit of parallel PP is that the OPT primary needs fewer turns and is therfore easier (in theory) to design and build. Another benefit is that the amp can continue in operation if one of the OP tubes should fail - could be useful in a juke box! A disadvantage could be current-hogging by one or more tubes but there are ways to combat this, a simple one being the addition of a resistor of 100 ohms or so in series with each plate; a more complex solution is automated bias.

Last edited by ray_moth; 16th April 2010 at 09:10 AM.
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Old 16th April 2010, 10:28 AM   #7
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Quote:
Originally Posted by danFrank View Post
Some people say that more than 2 tubes in a PP amp will muddy the sound, but I don't know if that is to be believed as many tubes on each side will end up averaging out in their characteristics, but you would have to be the judge of that by listening to each example yourself.
I hope this post was understandable....
Daniel
Bob is correct. One of the reasons is a poorly suited driver stage which slews early and can't handle increased o/p stage Miller effect with parallelled Cin and fixed bias resistors..the demand is high. I've seen an ECC83 driving parallel p-p which is a tribute to an inexperienced designer.
Triode connected pentodes capable of sinking 10-20mA make good Williamson buffer drivers solong they can handle current. I use 12BY7A (now getting rare) will offer 25dB gain in triode mode, and an ECL82 around 15dB.
The mains transformer has to be signifigantly uprated, especially the heaters.

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Old 16th April 2010, 11:24 AM   #8
Loren42 is offline Loren42  United States
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Quote:
Originally Posted by nic6paul View Post
You forgot Pete Milletts current push pull amplifier
Posted new P-P power amp design
Awesome work, but it looks like that is only 17 WPC.

That may not fit the bill if more power is really needed.

You can get a comfortable 60 WPC from a pair of KT88s if you are comfortable with running a plate voltage of about 525 VDC.

As someone else pointed out, more tubes equals more heat, more iron, and more money.
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Old 16th April 2010, 11:47 AM   #9
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If you look toward the end of that thread there are some producing quite a bit more power from that amp. It started out as a 17watt amplifier but it has morphed into something totally different.
Quote:
Originally Posted by Loren42 View Post
Awesome work, but it looks like that is only 17 WPC.

That may not fit the bill if more power is really needed.

You can get a comfortable 60 WPC from a pair of KT88s if you are comfortable with running a plate voltage of about 525 VDC.

As someone else pointed out, more tubes equals more heat, more iron, and more money.
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Old 16th April 2010, 01:01 PM   #10
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Quote:
Awesome work, but it looks like that is only 17 WPC.
Quote:
If you look toward the end of that thread there are some producing quite a bit more power from that amp. It started out as a 17watt amplifier but it has morphed into something totally different.
Is 200 watts per channel enough? I have seen 200 WPC out of Petes board for about 1 hour. This is a bit extreme. However 100 WPC is fairly easy and reliable. Petes 17 WPC rating is rather conservative. Mine made about 35 WPC on first power up with 6JN6's. The only deviation from Petes plan was the OPT, I used what I had which was 6.6K resulting in 35 WPC. More volts, bigger tubes, lower OPT impedance makes more power, lots more power.
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