• 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.

anyone ever use EL86/6CW5?

PP integrated amp with EL86/6CW5.
 

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PP integrated amp with EL86/6CW5.

Hello Davorin,
I was searching for good designs of amps with EL86 power tubes and discovered your post. Very nice work! I especially like the chassis, wood and cover you made.

Looking at the schematic I wondered what tubes you used in the position of E1 and E2. I noticed you had an earlier design using EL86 and 6J6 front tubes. Is the later amp better, and do you perhaps a more recent update?
 
Simply and cheaply stereo integrated amplifier with EL86/6CW5 and ECC83/12AX7 (or 2xEBC91/6AV6):
 

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I have just finished a clone of the Tubelab SPP, using PL84, which is a EL86 variant. I have one on the PCB, and this was a way to get some practice creating a point to point wired amp, with the PCB as a reference.

After one teething problem - due to the difficulty reading the colours on those tiny 1/4 watt resistors (I am going to start to use 0.6 watt to make it easier) - I had a living, breathing amp, producing sweet music. The sound was comparable to my 6P14P (Russian EL84) one.

However after 20 minutes or so I noticed the tubes were starting to take on a orange-ish hue, the dreaded red plate.

Checking voltages, I see (with some variation between the tubes)
V(k) - 17v
V(a) - 210v
V(g1) - 0.02v
V(g2) - 211v

Voltage across the screen resistor (100 ohms) is 0.3v, so I(g2) = 3ma
Voltage across the cathode bias resistor (270 ohms) is 17v, so I(k) = 63ma

Plate voltage is (210 - 17) = 193v. Therefore dissipation is 193 * (63 - 3), which is 11.6W.

Assuming my maths is OK, and my rookie knowledge is not too lacking, then I suppose that dissipation figure is not so high, but I have read that cathode current should be restricted to 50ma.

Should I contemplate a 390 ohm bias resistor?

Out of interest - why is the bias resistor 5W? Is there a spike in voltage or current at some point?

And also, what is the benefit of having seperate resistors? Lots of PP designs using a common resistor, and a lot of the tube data assumes a single resistor in a PP setup.


I have two pairs of tubes, one is Philips NOS and the other is HQ, which looks like an old brand but I had not heard of before I bought them.

The HQ tubes are not as well balanced as the Philips ones (19.6 and 15.6 V(k)), and I suppose that is down to them not being a quality brand.

Does anyone ever fit a balancing trimmer from each of the bias resistors with the wiper going to ground?
 
Thanks Kay!

I have substituted pl84 for el84 in the SPP with ultra linear toroidy OPTs. B+ has been adapted for an EL86, i.e. max 210 B+ and greater VA figure.

I am comfortable the power supply is ok, so it is pointing to the topology. Is UL possible with a restriction on max screen voltage? Do I need to look at a pentode output stage?
 
Yes, it is a special transformer for EL86:
3.3k ohms Raa, UL 43%, 200ma, nominal power 40W.

It is Toroidy - and I have noticed that they are quite big on EL86 solutions in Poland, and EL36 come to that - maybe more of a culture of exploring cost effective solutions? They also sell Polam EL86's on their site n their quite small selection of tubes.

It has proved to be quite difficult to find anything definitive in the tube data sheets. It should be that 200V is the upper limit of screen, according to some data sheets.

The most comprehensive data sheet is for the UL84:
http://www.r-type.org/pdfs/ul84.pdf

There is a curve for Vg2 = Va = 200V, but I do understand that the SPP would be right on the limit.

But it does look like Rk is different in this documentation when comparing with EL84. Looking at that curve I mentioned, there is a point for a grid voltage of -17v, but it is on the bottom of the curve, which is non-linear, and looks like I(k) should be much higher - something like 150mA.

This has been a good exercise in dscovering how much I have to learn. From the planning of the point to point layout, the execution, and the fault finding!
 
Thanks for that. More grist to the mill ;-)

It puts 200v on the screen as the design limit.

AIUI I can increase the cathode bias resistor somewhat so that V(g1) becomes more negative relative to V(a), so V(k) becomes 20v and V(g2) ad V(a) are around 190v. That would put V(g2) into safer territory.

I've tried to find some other examples of EL86 (or similar) in a UL PP setup, and I can't find any schematics, which is unusual.

Is it something to do with the conditions changing when the tube is under load? The screen voltage will have the signal from the OPT in UL? Is UL a realistic topology with an EL86, due to te screen voltage limitation?
 

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Toroidy OT for EL86 power tubes?

Yes, it is a special transformer for EL86:
3.3k ohms Raa, UL 43%, 200ma, nominal power 40W.
I’m interested in a pair of transformers like this, but did not see it on the Toroidy catalog. Could you provide the model number and link to the specifications, please. Or was it a custom wound transformer for you?
 
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DiyAudio thread on 6CW5 with power toroid as output transformer

OldHector, in case you did not see it, check out: 6N2P/6CW5 design with toroidal outputs

This thread is about an interesting “economical design” for 6CW5/EL86 using low-cost toroidal power transformers for outputs. While it is not UL it might be informative for you to read through the relatively short thread.

The Blumlein Garter bias scheme is quite interesting in this design.
 
Last edited:
Hello Davorin,
I was searching for good designs of amps with EL86 power tubes and discovered your post. Very nice work! I especially like the chassis, wood and cover you made.

Looking at the schematic I wondered what tubes you used in the position of E1 and E2. I noticed you had an earlier design using EL86 and 6J6 front tubes. Is the later amp better, and do you perhaps a more recent update?

E1 is 5963 and E2 is 5751.
Both amplifiers work well and it would be difficult to decide which one would be best if tested just by listening music.
Amplifiers work at my friends homes without any problems and failures for years, and on the second amplifier I have built an octal socket for powering a gramophone preamplifier (MM + RIAA Correction with E80CF).
 
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E1 is 5963 and E2 is 5751.
Both amplifiers work well and it would be difficult to decide which one would be best if tested just by listening music.
Amplifiers work at my friends homes without any problems and failures for years, and on the second amplifier I have built an octal socket for powering a gramophone preamplifier (MM + RIAA Correction with E80CF).

Thank you that you spotted my previous question. The reply was worth the wait :D
 
OldHector, in case you did not see it, check out: 6N2P/6CW5 design with toroidal outputs

This thread is about an interesting “economical design” for 6CW5/EL86 using low-cost toroidal power transformers for outputs. While it is not UL it might be informative for you to read through the relatively short thread.

The Blumlein Garter bias scheme is quite interesting in this design.


Hey, my daily-driver build! :)

Still running all day, every day as my main rig. If someone is home, the amplifier is on. Still delivering great sound, and haven't had a single issue with it. Highly recommend folks try these tubes! A really lovely tube when run with a regulated screen supply and global negative feedback. I would say they sound more like the 6V6 than the EL84 just off of what I hear and instinctual reaction, but with greater power capability.

Screen voltage is about ~225, plate voltage about ~310, cathode voltage ~32 (across both garter bias resistors, so 16 volts x2) so actual operating parameters are close to 190 screen to cathode, 280 plate to cathode, -16 g1. Nice operating point with a 3~4k (I'm close to 4k) anode to anode load. Cathode voltage is nearly identical (within a few mV) on each tube, so far as I can measure with my DMM, which is good enough for my purposes. :)
 
Thanks for the tips. It is quite a different topology to the UL PP setup I have it built. I’d like to iron out the issue with the red-plating. At least be sure I understand the cause, since I can’t see that I am being particularly abusive to the tubes.
I have bought some prototype boards, so I hope to try out your design on those.
 
Due to their very efficient cathode and their low µg2g1, one could extract more than 40 watts from a pair with a plate voltage of 350 V or somewhat more, half this value as screen voltage (that makes the PSU simpler :D) and a Rpp of about 3k5 to 4k. Yes, this exceeds the plate voltage rating, but only this one, and would do no harm. Remember that the European PL84 member of that family also was used as a TV frame deflection amplifier, with plate voltage peaks of more than 1 kV.
Best regards!