6l6 in parallel with el84 with non lethal voltage

[Manowar1985]

Hi everyone. I am new here. I would like to share my idea about building a tube amp with non lethal voltages.
So here's the thing. I have many spare power tubes.
I have 16 el84's and 8 6l6's.
To make 2 channels I will use half of them for each channel. This will be 4 pairs of el84's and 2 pairs of 6l6's all in parallel at push pull mode. Total 6 pairs in push pull mode. if I was to use only one pair of them with a 5 k p-p load that would give for 6 pairs 5 over 6 equals 0.833 k p-p. Would this thing work. Because it seems to me a very unusual ops

[Dave H]

EL84 and 6L6 are too dissimilar to run in parallel. It would be better to make two amplifiers, one with the EL84s and a second with the 6L6s.

[DrGonz78]

So what is the desired b+ plate voltage for this project? Have to ask why non-lethal voltage is a factor of the design idea? I guess you want to see if it can be done and if it might sound unique.

[mozz]

I don't see why it can't be done, b+ around 300vdc.

[DrGonz78]

I wouldn't consider 300vdc to be a non-lethal voltage. I was wondering what voltages the OP had in mind.

[Leo_Gnardo]

Very unusual ops indeed. A figure around 48 volts is generally taken as non lethal. By coincidence that's the voltage used on phantom powered microphones & DI boxes. Y'know, so nobody gets shocked to death onstage or in the studio. So ... do your maths with a B+ of 48V.

Also, consider powering all those filaments. No worries, big filament transformers are available. Check Antique & some other parts suppliers for prices.

Practical amps that run on fairly low power/voltage can be derived from old portable radio designs. Tube radios run from batteries, like the one Grandma made Grandpa haul to the beach when they were teenagers back in the 1940's. Might get half a watt at full crank, just right for a dirty sounding but non-offensively loud guitar amp.

FWIW this reminds me just a little of our - endless - discussions with a certain "genius" named Gerry from out Bowling Green way. His quest was to issue a modular tube amp kit set that was "kid friendly," meaning nothing shocking please. And have US lot design it for him. Fund it too! Ahh, Gerry.... He went over like a skunk at the sorority party. A very very persistent skunk. So, if it looks like a bust, let's keep it short. And friendly. Now I'm verklemt, let the rest of yez take over, discuss...

[Manowar1985]

OK. First of all I know that all this sounds pretty unusual but I planned to operate it at about 100 v DC. My primary concern is not to get electrocuted. I totally agree with Dave H that these two types of tubes are very dissimilar ones. For this I have been thinking over and over again so I finally see that I have to deal with 3 things(correct me if I am wrong please) :

Different effective load resistance
Different gain
Different vbias for the same operating point.

For the first we see that 6l6 run at 5k p-p
And the El 84 at 8k p-p. If we force the El 84 to work a 5 k (and hope that everything will go well) we divide 5k over 6 pairs to find the new impedance of the primary of the Output transformer

For the second if we think the tube as a current source controlled by grid voltage then we see that if 2 different current sources but same direction are connected in parallel they are added so if the first tube is Ia=gm1*Vgk and the second tube is Ia=gm2*Vgk then the equivalent tube will be Ia=(gm1+gm2) *Vgk. This is for two different tubes in parallel. It should have no problem amplifying.

And for the last I thought about using different grid leaks for the different tubes to apply dirrent bias voltages to make them all operate at the same point.

Sorry for the huge text and thank you for participation and opinions

[J M Fahey]

FWIW even 100V DC can be deadly.
It depends **a lot** on sweat, which being salty, is conductive, so at way lss than 100V you can pass too much current anyway.
FWIW , once I was working in a VERY hot and humid cellar, sweating profusely, repairing a 24V battery powered system.
I was grabbing the chassis (0V) and my elbows just brushed against the +24V terminal ...I felt distinct tingling and arm muscles contracted a little.

Scary to say the least, because I had felt very safe up to that point.

I conswider 100V "safe" only if applied to dry *unbroken* skin.

And we all know that in our job itīs *easy* to get scratches or minor cuts.

Personally Iīd build regular amplifiers, with proper voltages,and enclose danger areas inside a grounded metallic chassis, and attach a label stating: "Dangerous voltages inside" , "No user serviceable parts" and "Refer servicing to qualified personnel".

As of:

if I was to use only one pair of them with a 5 k p-p load that would give for 6 pairs 5 over 6 equals 0.833 k p-p

That applies *only* at original voltages, think 300V to 450V area, whatever datasheet examples ask for; if you modify voltages you must redo calcuations with new voltages.

[Helmholtz]

...and even 100V plate supply can produce up to 250V peak plate voltages.

That applies *only* at original voltages, think 300V to 450V area, whatever datasheet examples ask for; if you modify voltages you must redo calcuations with new voltages.

Typically power tube datasheets don't even provide information on characteristics with a screen voltage of only 100V.

The least I would expect from a plate and screen supply of only 100V would be very low power of only a few watts per tube pair. As well as strongly reduced gm/gain.

For the second if we think the tube as a current source controlled by grid voltage then we see that if 2 different current sources but same direction are connected in parallel they are added so if the first tube is Ia=gm1*Vgk and the second tube is Ia=gm2*Vgk then the equivalent tube will be Ia=(gm1+gm2) *Vgk. This is for two different tubes in parallel. It should have no problem amplifying.

Tubes are not ideal current sources as they have finite internal plate impedance (e.g. 5K at low plate voltage), meaning that they can also drain current. If you parallel power tubes having different internal plate impedances, the lower impedance tube will load the higher impedance tube resulting in low efficiency.

[Manowar1985]

Thank you all for sharing such valuable information. I am really alone in this thing. I have been trying to gather information from books and experts but found nothing that has to do with low voltages. What I want to do is design and built an amplifier and understand as much as I can.
So I would like to ask 2 last but not least important questions :

First

When the tube is operated at triode mode we want the plate resistance to be as close as possible to the load resistance for maximum power.
But what happens in pentode mode. Why the plate resistance is much larger than the load resistance and we still get higher power than triode mode.
And if plate resistance doesn't matter in pentode mode why dont we place directly an 8 ohm load (with isolation transformer)

Second

Is it true that if we have given grid curves for say Vg2=300 V then to find the curves for say Vg2=100 V lower the y axis (anode current) by 3

This is just an approximation



Note

I read something on my book of my school(electrical engineering). The book is called electrical installations.
I read that at up to 120 volts dc and up to 50 volts ac at infinite contact there have not been serious accidents

Thank you all again

[J M Fahey]

Your book is fine, just letīs put it in context.
As Helmholtz reminded us, if your +B is 100V DC, if driven to clipping when one tube saturates, you have a see saw in the OT; center point stays at 100V, one plate swings down 100V to "almost zero", the other plate swings UP 100V to almost 200V.
And that if load is resistive and sinewave *just* reached clipping ... in practice given the inductive nature of speakers and overdriving the amplifier you will have 250V peaks or even more.
There goes your "safe" voltage

As of:

When the tube is operated at triode mode we want the plate resistance to be as close as possible to the load resistance for maximum power.

Actually no.

But what happens in pentode mode. Why the plate resistance is much larger than the load resistance and we still get higher power than triode mode.

Proof that what I just wrote above still applies.
Those are very rough approximations; the real "optimum load impedance" is found , in principle graphically, trying to maximize and optimize voltage and current swing.

And if plate resistance doesn't matter in pentode mode why dont we place directly an 8 ohm load (with isolation transformer)

Oh, it does matter, a lot, just that the "ready made " phrases about impedance do not apply, truth is found using the datasheet curves.

As a side note, these do not show much detail with 100V supplies because simply tube performance at such voltages is appalling, so who would want to use them that way?

The problem behind all this is that it is VERY difficult to have electrons jump through vacuum, period, so you need hundreds of Volts for that.

Yes, a little current will pass even at lower voltages, but not enough for practical uses.

A couple years ago a German student came here (or was it at DIY Audio?) with a similar project: his graduation homework was to build a working amplifier.
He chose to make it tubed, so far so good, but by School regulations, which in due time followed German safety rules, also shared across the EU, maximum allowed voltage was 40/45/40V , not sure which now but in any case "all about the same" and insufficient for Tube use.

He specifically wanted to build a "45V JCM800"

Closest he could get to that goal was building a preamp using 12AU7 which are the ones which can work with lowest voltage and drive a chipamp.
He could do nothing useful with power tubes under those constraints.

[Manowar1985]

OK. I think that I will try to do my best getting out of my amp as much power as I can. I won't be using different tubes in parallel. Final decision.
I will look for high sensitivity speakers too and if this isn't enough I convert it to a headphone amp. But I think that with so many tubes in parallel I will have more current at the very small voltage and I hope that I will get some power out. Of course I will have to deal with the very high input capacitance.

Again thank you all for your advices and If you like I will keep you informed about the results

[Helmholtz]

Of course I will have to deal with the very high input capacitance.

No, wrong information.
Power tetrode/pentode input capacitance (including Miller effect) is generally very small.

[Manowar1985]

Helmholtz even with 4 tubes in parallel?. Or 8?

[Helmholtz]

Helmholtz even with 4 tubes in parallel?. Or 8?

Expect around 20pF per EL84.