PP3521 idht

[Paul Barker]

But all throuh this crisis the bigger crisis at home is reminding me Diana isnt getting better. Im on edge waiting for the next twist and turn in her slow torcher like a cat playing with a shrew before it skins it past its sharp teeth on its transit to its gut. Yet another custome has asked for my help, yet again I had to reveal my situation but her response was enormously helpful. Because the majority of humans just dont get what its doing to me. I was in tears more of joy than sadness when this customer wrote back with

[Nick]

I feel your pain Paul

I’ll shed my assumptions about capacitors and deploy them.

Maybe don't shed them, but try and consider that its all a compromise, its very easy to give way to black and white thinking, but reality is more often than not some shade of grey, with hopefully a tinge of colour. I would say though its worth also considering the additional cost (both financially and potentially, audible) of not doing a thing against doing it. I often find myself going down mental rabbit holes looking for a solution to a problem, and have to pull myself up and realize my solution may be worst than the problem (but just different).

A problem may have 10 complex solutions that present themself like excited puppies, the trick is to look past them to see if there is a shy simple solution hiding behind some boxes in the corner.

[Paul Barker]

Thanks Nick for youre heart and youre analagy. Something of the puppy in the corner applied in the amp wherein I had to order another cap, in that the chosen valve uses a low value bias resistor so needs more capacitance. Everything starts with youre early choices. Then there is anothe syndrome the valve you chose to use becomes sacrisanct (however that is spelled) and you do handstands later in the build without ever stopping to think, have I made the right valve choice? Hard to see youre own failing through you're pride.

[Paul Barker]

Ive sat for a coffeee and penned a parallel 6em7 from the ECC88 resistor loaded and ordered the required resistors. So wont be long. Gives me time to remove the bifilar transformers out. Power out will be half probably i.e. probably 4 watts.

[Nick]

Why not use the primary of the interstage as anode chokes and cap couple to the grids? Or are you going to stand the 6em7 on a big cathode resistor and direct couple?

[Paul Barker]

You guessed right. I can bring iit up on variac and stop where 6em7’s are biased safe and sound. With a voltmeter probes between grid and cathode from the beginning.

Same 90uf caps -3db at 6 herz

[Mike H]

Why not use the primary of the interstage as anode chokes and cap couple to the grids?

I was thinking of doing this with the 801A's.

Done interstage, let's try the other.

[andrew Ivimey]

Oh my, I thought the primaries were acting as anode chokes anyway. If you have valves and transformers then bollocks to capacitors eh! Perhaps I should just disappear (?).

[Paul Barker]

Oh booger, the 6em7 is restricted to 1 watt (dissipation is 10 watt but you caant operate it for audio at 10 watts. You cant exceed 150v with 50 mA. Which only gives voltage swing for 1 watt per valve.

EL 84 triode connected is a known glorious sound and it will double the output. You get 1.9 watts single valve so parallel 3.8 watts. Operating point 250v -9v which would drawer 35 mA ish per valve. The 2k opts equivalent to 4k but 6k would be desired. But the 3k R cores would meet that need.

However, more optimal almost 4 watts would sound, Ive ordered the resistors for the 6 em7’s, so lets just go with 2 watts now. At least I should learn something.

Other contenders are kt66 6L6 and EL34 but I barely have any and dont want to buy valves.

Now then I have dht’s. But I was on heater valves not filament valves.

So an inefficient 2 watts is where it lands me or an efficient 4 watts if I buy another set of resistors. There is an indirectly heated version of the 6b4g, somehwere I have a pair but ive forgotten whatthey are called and I only have two so probably a dead end because they are rocking horse pooh no doubt. I have no idea where my two are anyway.

I did think about parallel 6b4g’s similar concept to the parallel 801a’s I last used. 6b4 gs would work on 4k. Future food for thought.

But anyaway I ordered the resistors for the 6em7. So 2 watts limit it is. Its only a stop cap before I can build the GM70 anyway.

[Paul Barker]

In case anyone thonks I missed the 6as7, its so low mu you need a high voltage swing to get the power. Not my bag on this adaption from a fail. And like other better ways such as the EL84 it involves buying different resistors. Too much money to throw about for a pensioner.

[Nick]

Oh my, I thought the primaries were acting as anode chokes anyway

Well, I guess its just semantics. A choke would normally be assumed to only have one winding, a transformer two or more. But yes, the valve sees the same in both cases.

[Paul Barker]

Andrew the bifilar transformers are traumatised they either caused the grid voltage and cathode current excesses or they are casualties of it. I decided not to use as a choke an ex transformer with doubtful insulation integrity. Its not just the bufilar issue in isolation, if the insulation of the winding wire has overheated or stressed it is a danger for earth shorts and a fire hazard. Ive seen chokes arc internally when I played one in the dark. Imagine if I hadnt turned the light off , went out but left the amp on as Russ Ndrews recommended back then?
For sinal use a choke has capasitance if wound for power smoothing where you dont need bandwidth. So you must wind the choke differently to extend hf bandwidth. Just like a transformer you have to reduce capacitance by multi layers which reduces capacitance as capacitors in series reduce by half each doubling. Whereas inductance increases.
Resistors arent always a wrong choice. All choices are thought out especially when considering using iron with a bad experience in its provenance.

[Paul Barker]

Above is why you can only get 1 watt out of a single 6EM7.

You see that 10% dissipation line. You cant get near that. You see your quiescent voltage? Yo cant get higher in voltage than that its sounds terrible. Back in the day I was confused by this and emailed friend Stephie about this.. I dont have my emails from Stephie in those days but what I said about the 6EM 7 before Gary brought it to the community I repeated pretty verbatim on that link I put up here of me on Tubes Asylum.Anyway Gary never discovered anything, Eric Barbour did, I liked the concept from Eric Barbour first used it as a driver and tried it. Found it very distorted because I was trying to use it to full potential. Which you can see on the graph but you cannot go up or right of where Ive put the quiescent point. A waste of potential in a valve. A rare concept and anoying. So just use it within its audio limits and it sounds magical.

The power out is derived from the lowest voltage subtracted from the highest voltage x= divided by 8 x load. = 1 watt! The green line is youre ac voltage maximum on this valve. Thats what you times equal and devide by load times 8. Yet you can see if it were like most other valves we work with you could tap the rest of the grid lines and start further right. Hence why the EL84 triode into 6k gives us almost twice the power 1.9 watts.

On the 6em7 2nd harmonic distortion measuring current at 5 points alongthe green line a left most c mid point e right most point b and d equidistant between a and c and between c and e. I have abcde 70,59,41.5,34 . Formula is 75*(70+34-2*50)/(70+59-41.5-34) = 5.6%

Shall I have a look at the el84 distortion ? I would be using it into 6k with the 3k r core opt’s.

[Paul Barker]

Apologies for wrong image orientation. Mullard and Vade Mecum provided data. The scale of the vade mecum graph is more usual. But I’ll use Mullard as Ive drawn all over my vade mecum.

Well I cant see the optomistic SE power out, I get 1.4 watt per valve so 2.4 watts pse, its not much better than the 6em7. But the distortion is less. I think I,ll be ok sticking with 6em7 pse for 2 watts andI can use the better transformers I wound myself with more inductance and better bandwidth due to 8 layers and they sound beautiful.

[Paul Barker]

Something stands out researching the potential power out is I ought to get winding a phase splitting interstage or phase splitting choke to make realistic power out of these valves. I liked the blend of se up front pp output.