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Role of paralleled EL34's in AES SixPac monoblocks
Role of paralleled EL34's in AES SixPac monoblocks
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Old 9th September 2018, 10:05 AM   #1
MrJackson is offline MrJackson  Romania
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Default Role of paralleled EL34's in AES SixPac monoblocks

I'm really enjoying my Dennis Had designed SixPac monos, but I do want to roll some other power tubes in it without burning my wallet to a crisp - a full set (12) of Psvane EL34PHs would set me back a cool 900GBP.

So, I was looking on the internet for the possibility of running it with only a single pair of EL34s per channel (the amp has 3 pairs by default). I came across a great video on youtube (on the MrCarlsonsLab channel) where the amp was opened up, analysed, reverse engineered... the works. This confirmed my knowledge that the output tubes are paralleled, but I couldn't understand exactly what's the purpose for this arrangement. In theory, I could easily remove one or two pairs from each channel and run it like that (after adjusting the bias of course).

Some time ago I removed a pair from one channel, as an experiment... and I expected a drop in output power, but I couldn't notice one. Not understanding what happened, I just put the pair back in and used the amps as designed.

However, I'd still like to understand this, as running it with less tubes (cooler, cheaper) would make sense to me, if performance isn't affected (except output power, which I have enough of anyway).

I've attached the reverse engineered schematic for reference.

Cheers!
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Old 9th September 2018, 12:26 PM   #2
Koonw is offline Koonw
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What is the Ra-a (impedance of plate to plate) of the OT? The output impedance of the 1 pairs 6550a in triode mode near your configuration is about 1.3K, so if Z of OT is at least 1.3K or higher, it will work. Else you may increase the reflected impedance by using higher speaker tapping or speaker say 4 to 8 or 16, so it matches Z better for reduced number tubes. Increased gNFB aslo can reduce Z of output tubes.

Last edited by Koonw; 9th September 2018 at 12:35 PM. Reason: add info
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Old 9th September 2018, 01:40 PM   #3
Steve Morley is offline Steve Morley  Canada
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I haven't heard the Psvane tubes but can vouch for the Genalex (Russian reissue) KT77 tubes. They're approximately 2.5X the price of an ElectroHarmonix EL34 but probably a whole lot less than a set of Psvanes. See link.


Cheers, Steve

A Survey of Four Sets of Power Tubes in Two Push-Pull Amps | Wall of Sound | Audio and Music Reviews
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Old 9th September 2018, 02:44 PM   #4
MrJackson is offline MrJackson  Romania
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@Koonw: I have no idea unfortunately and right now I wouldn’t want to open it up and measure it. However, if I understand correctly what you’re saying, reducing the number of tubes would just increase the output impedance as seen by thr OT at its input, right? Would there be any danger to the amp if I go too low? I’m thinking I could just experiment and see if I notice any sound issues with less tubes.

@Steve: Yeah, those are on the list as well, together with some Svetlana Winged Cs (they’re almost half the price of the Psvanes iirc)
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Old 9th September 2018, 03:46 PM   #5
45 is offline 45  Italy
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Quote:
Originally Posted by Steve Morley View Post
I haven't heard the Psvane tubes but can vouch for the Genalex (Russian reissue) KT77 tubes. They're approximately 2.5X the price of an ElectroHarmonix EL34 but probably a whole lot less than a set of Psvanes. See link.


Cheers, Steve
I have used EH EL34 on several occasions and bought them from different sources but always found them under-perfoming respect to other brands. Lower emission and lower gm. Simply couldn't get the same power out of them. So I have come to the conclusion that their lower price is because they are the B tubes coming out of production chain and the A tubes get rebranded under more fancy names...

The JJ EL34's instead are likely the next cheaper option and perform much better. If selected they are excellent performance wise. Only problem with these is that they are delicate. Never touch them when they are still hot or warm as their heater will break. Once cool they are ok to manipulate as any other tube.
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Old 9th September 2018, 04:18 PM   #6
Koonw is offline Koonw
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Mis-match of impedance in tube and OT will cause power loss (tone too). If the tube output impedance is higher than OT, then distortion is higher, the distortion is lower if opposite (the lower Z of the tube the better). If the secondary is not connected there will very high mismatch (reflected impedance is very high). So if the tube is acting on this high impedance (like very high resistor), because it's inductive, it will ring and generate very high spike which maybe more than 2KV, enough to break down the layer insulation. As long as the load is connected, it should be safe. You need to watch the OT, there should be some sort of fusing protect against over load and accidental short circuit.
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Old 9th September 2018, 04:29 PM   #7
45 is offline 45  Italy
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Quote:
Originally Posted by MrJackson View Post
I'm really enjoying my Dennis Had designed SixPac monos, but I do want to roll some other power tubes in it without burning my wallet to a crisp - a full set (12) of Psvane EL34PHs would set me back a cool 900GBP.

So, I was looking on the internet for the possibility of running it with only a single pair of EL34s per channel (the amp has 3 pairs by default). I came across a great video on youtube (on the MrCarlsonsLab channel) where the amp was opened up, analysed, reverse engineered... the works. This confirmed my knowledge that the output tubes are paralleled, but I couldn't understand exactly what's the purpose for this arrangement. In theory, I could easily remove one or two pairs from each channel and run it like that (after adjusting the bias of course).

Some time ago I removed a pair from one channel, as an experiment... and I expected a drop in output power, but I couldn't notice one. Not understanding what happened, I just put the pair back in and used the amps as designed.

However, I'd still like to understand this, as running it with less tubes (cooler, cheaper) would make sense to me, if performance isn't affected (except output power, which I have enough of anyway).

I've attached the reverse engineered schematic for reference.

Cheers!
You can't remove 2 pairs and hope it works the same way with just less power. Need to change the output transformer at least...
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Old 9th September 2018, 04:32 PM   #8
MrJackson is offline MrJackson  Romania
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What I do know (specified in the instructions and confirmed in practice) is that the amps run fine without a load connected. The circuit is constructed such that they do not oscillate. At least with all 3pair or output tubes and no load. They also seem to work just fine with 2 pairs... so I would assume the impedance is still lower than OT.
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Old 9th September 2018, 04:35 PM   #9
MrJackson is offline MrJackson  Romania
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Quote:
Originally Posted by 45 View Post
You can't remove 2 pairs and hope it works the same way with just less power. Need to change the output transformer at least...
I understand now that power’s not relative to the number of push pull pairs. I would be temped to try multiple configurations though as long as I don’t break anything. If it sounds bad, I can always revert.

Also, your experience with JJs vs EH is interesting... I always saw JJs as inferior to EH. From the limited tubes I've tried in my preamp, EH always sounded the best in the budget range. I do need to try some NOS RCAs and some premium new tubes in there.

Last edited by MrJackson; 9th September 2018 at 04:43 PM.
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Old 9th September 2018, 04:45 PM   #10
Koonw is offline Koonw
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Quote:
Originally Posted by MrJackson View Post
What I do know (specified in the instructions and confirmed in practice) is that the amps run fine without a load connected. The circuit is constructed such that they do not oscillate. At least with all 3pair or output tubes and no load. They also seem to work just fine with 2 pairs... so I would assume the impedance is still lower than OT.
It does not oscillate because you have engaged the gNFB switch. Just to be safe, it's an usual safe practice.

You need to use a smaller fuse (not 500mA?) as the current would not be that high with only 1 pairs.
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