• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

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!
 

Attachments

  • Screenshot 2018-09-09 at 11.43.08.jpg
    Screenshot 2018-09-09 at 11.43.08.jpg
    235.1 KB · Views: 411
Disabled Account
Joined 2013
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:
@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)
 

45

Member
Joined 2008
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.
 
Disabled Account
Joined 2013
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.
 

45

Member
Joined 2008
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...
 
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.
 
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:
Disabled Account
Joined 2013
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.
 
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.

I haven't looked at the schematics in detail but in general if you remove 2 pairs the remaining pair will work into 1/3 of the usual load. If one pair of EL34 works in class A into, say, 10K then for 3 pairs that load would be 3.3K. If you remove 2 pairs now the remaining pair will work into 3.3K. Heavy class AB operation. That will work but only in certain conditions which will be different. I see the quiescent current is 220 mA and appears to be the total for 6 tubes which is less than 40 mA per tube. With one pair the quiescent current should be in the region of 55-60 mA I GUESS otherwise distortion, especially a low and medium levels, will not be low.
Anyway if you objective is to roll tubes for making a final choice I don't think it is a good idea because the sound might be different between one pair and three pairs whiteout changing anything else.
 
If possible, I'd always run it with a single pair... these amps get REALLY hot and it's not quite a pleasure in a hot summer. But I take it it's a bad idea.

As a side note, from Mr Carlson's findings in the video (when he adjusted the bias using a signal generator and a scope on the output), the bias can be set as low as 170mA on a full complement of tubes, so I guess 220 is a guideline for being in the safe zone to avoid crossover distortion.
 
If possible, I'd always run it with a single pair... these amps get REALLY hot and it's not quite a pleasure in a hot summer. But I take it it's a bad idea.

As a side note, from Mr Carlson's findings in the video (when he adjusted the bias using a signal generator and a scope on the output), the bias can be set as low as 170mA on a full complement of tubes, so I guess 220 is a guideline for being in the safe zone to avoid crossover distortion.

I have being playing with a simple PP amp with EL34's in triode for the entire period of August holidays. It works fine with 3.3K but quiescent current is 60 mA per tube with 385V anode voltage I get very clean 17-18W. The sound with lower quiescent current is thin and I measure higher distortion. With 10K and 400V plate voltage the output power is less around 14W but it is better from every point of view. Actually with 3.3K it definitely needs some feedback to work best while with 10K it doesn't need any feedback.
So if you want to use one pair just change the output transformer using 3X primary load. That's just my advice.
 

PRR

Member
Joined 2003
Paid Member
If you take out four tubes, it won't be a "Six Pack".

Yes, you can probably cut from six to two *if* you change the load fro 8 ohms to 24 Ohms (not easy) and accept that you have 1/3rd maximum power.

If you do not change the load, the power is even less. 1/9th in simple theory. Since EL34 is a soft-curve pentode, maybe like 1/7th.

Since a "six pack" of EL34 is WAY more than many home hi-fi systems ever need, such reduced powers may still be ample.

If money is a concern, you maybe should get a smaller amplifier.
 
Wow!

I would not run that amp as it is designed, unless I did at least one of these things:

1. Purchase an IR thermometer (even an inexpensive one from Harbor Freight Tools).
Then carefully check the temperature of each one of the EL34 tubes.
There is No provision to balance the current of the 6 EL34 tubes.
One or more may be running very hot, and others very cool.
Just purchasing a "Matched set of 6", might not have current balance at the particular plate voltage and grid bias of your amp.
Just a 1V change in bias results in 11mA change in each tube's current; exact matching is paramount. Such high transconductance tubes are hard to really match with fixed bias.

2. Purchase a "Matched set of 6" that are Custom-Matched at the exact plate voltage, and at 220mA/6 (36.7mA), or a lower current such as 170mA/6 (29.3mA) that you will use in Your Amp.

And do not forget, if one or more tubes ages more quickly than the other tubes, they will not be balanced any more.

3. You could convert to individual self bias for each tube, but that requires 6 power resistors and 6 bypass caps for each channel. Probably no room for all of that.
The advantages would be:
A. Better matched currents, and better match as the tubes age.
B. A little less power dissipated in each tube, running cooler, and longer tube life.
The slight reduction in output power would not be noticed, you have plenty of power.

Note: The schematic shows these tubes to be wired in Triode Mode. Seems like a good thing to me.
 
Last edited:
If you do not change the load, the power is even less. 1/9th in simple theory. Since EL34 is a soft-curve pentode, maybe like 1/7th.

I disagree. It can be a bit more than 1/3 if you remove two pairs and adjust a little bit the bias without changing the load. The EL34 is triode connected here and I have played with a similar power stage for 2-3 week in August.
Using one pair biased at 400V/60mA per tube the only things that change going from 10K to 3.3K plate-to-plate are output power (class A and class AB, respectively) and distortion but not so much in terms of power. With 10K P=14W with less than 1% THD, with 3.3K P=18W at about 2% THD. The big difference is at low and medium level where the class A amplifier has got really low distortion in the region of 0.1-0.2% @1W while the class AB amp has already got 0.5% THD @1W. The latter needs some feedback to work best.
 
Since a "six pack" of EL34 is WAY more than many home hi-fi systems ever need, such reduced powers may still be ample.

If money is a concern, you maybe should get a smaller amplifier.

It's only 50W per channel... depending on your speakers, it might be barely enough. As for the money, yeah, you have a point... but only if we're speaking about hi-end tubes (you can't tell me $1000 is a small amount for a full tube completement). If we're talking about EH tubes, it's not a problem.

@6A3sUMMER: I haven't found any glaring issues with the matched tubes I've got so far. Sound is really good overall... but it does go to about 55ºC.

@45: I'll probably give it a shot at running with only 2 pairs per channel this weekend and see how it goes. I might be able to live with a higher THD (it's about .2% with no feedback)