6pi13C known as 6P13S equivalent beam tube to EL36

[Paul Barker]

A quad of these delivered from Eastern Block took about 6 weeks to get here cost me £4.21 each tube cost including delivery.

https://ibb.co/zTcQMVCT
https://ibb.co/NP9SWKQ

So my thought is operate them in the Western monoblocks parallel push pull 2a3 amplifiers.

Supposedly the dc power is 300v but the bias voltage generated by one of the two field coils through the ground of the centre tapped power transformer secondary searching info has yielded a fixed bias of -70v. Until I hook the power supply up I won’t know what actual voltages are. Perhaps anode voltage is 300v and the bias is -70v. Myfield coils are 900 ohms which I power with a 50v telephone power supply to good sound effect. So I first thought is pop a small cathode resistor 400 ohm per valve calculated to make -50 up to 70v total bias, in and bring amp up on variac . The bias will therefore be 5/7ths fixed, 2/7th auto. Ultimately the auto bias resistors to be bypassed.

It is no hardship to use the superb sounding Graetz Field Coils.

I think I’ll order a second batch of these valves. The obstacle for restoring these monoblocks was the cost of 2a3’s . No longer an obstacle.

Obviously I have to fashion a balanced stereo preamp, but actually Anth already built me such a beast. I’ll just use that probably with a further balanced gain stage as the onboard of monoblocks only have a balnced 56 voltage amp. Probably I’ll use ECC88 balanced in the output of the balanced preamp.

Reports from people that have used these amps with the field coils are very positive. Original purpose was Hammond Organ amplifier, but also used as theatre amplifiers, which is where I think mine came from, when our flee pit updated its sound system, probably not for the best. But everyone seems to want modern PA ways today. In my view they over amplify to distortion. My Apple watch warns me the sound level is dangerous.

So my positive findings about the sound from these amps amplifying a Hammond Organ are positive reports, so I suspect the opt’s are good. I’ll operate the Russian tubes at 650 ohm which may require lowish power as to obtain the 650 ohm requires lowish b+. I think I found 2.4 watts se where the loadline for se is, which would translate as 8 watts class A parallel push pull. I think thats enough for me in one room.

[Paul Barker]

I made chipwood Briggs dimension Baffles for the speakers and my sound was heavenly in those days.
https://ibb.co/PGKFrWzT
https://ibb.co/wNWzTJCV
https://ibb.co/hJbLsS10
https://ibb.co/Fq5pcNT8

[Paul Barker]

OK so operating point of 275 ra -42 bias .0625 current the ri is 666 ohm which will be sufficient to equate with transformers made for 2a3’s.

Actually into 2500 se at the above numbers its single valve se 6.2 watts so ppp should come out with 24.8 watts.

No chance of it at this Owston; Next time. One obstacle is the fixed bias. But I can fettle all those matters. I expect the field coil would pull -50v from the centre tap no requirement for auto bias. Which makes 5 watts se ri 800 ohm (which is the published ri of the 2a3) 20watts ppp class A but a tiny bit of AB would increase to 24.5 watts.

So connecting the Graetz engine in the ground leg of the power transformer provides - 50v possibly! So a current meter and volt meter on one of the valves anodes should tell me when to stop raising the b+ voltage. Looking for 310v ra @55mA per valve equates to fixed bias of -50v, after fettling.

Ref the 310v ra. The Late Patrick Turner operated them for audio fixed bias at 380v beyond which arrives runnaway with fixed bias so 310v isn’t an issue. He also said there is no colour change in the anode at 380v. He said conservative limits for the harder work life of tv purposes can be exceedsd for audio.

[Paul Barker]

Edit 275 Ua and 310v Ua not ra.

I’ve ordered another quad of these valves.

[Paul Barker]

Well a fully integrated amp does fit on an ex transformerless amplifier chassis. Mainly used because the input phonos and speaker cable outputs, a power iec input. There is also a stepped attenuator. I’ll have to fit low voltage ac tranformers inside and turret board for resistors coupling caps and d3a load resistors and output stage cathode resistor bypass caps. Its pretty damn heavy right now, so I’ll walk to the station with it on a strong two wheeled sack barrow which will have to come with me. Other passengers may find it amusing.

https://ibb.co/fzM4xmpN

[Paul Barker]

Actually putting the power supply on board was too difficult. So Ive laid out a dedicated power supply. If I put 4 microwave first caps in selecting 3 gives b+ 420 which is perfect for the el36 and the other amp the el38, I would only bring one amp. So that relieves a lot of pressure and anxiety I am suffering with the thought of building at all.

So the least I have to do is bring the power section and the already existing EL38 amp, The most is complete the el36 signal section and the power section.

The el36 produces more power and less distortion compared to the el38. But there are definite different elements in the different signal sections. So its unknown what sound changes occur.

So I guess this means I build the power supply that is more portable than the bench supply. At least that doesn't frighten me off.











The few internal parts for the signal sections if I build it are the last picture plus two 15va toroidal transformers still in the post.

[Paul Barker]

So different transformers on each amp. The el38 has 5k6 R core, the el36; 4k6 also R core but bigger 10 watt capacity.

The el36 produces 6.5 watts at 7% 2nd h

The el38 produces 3.5 watts at 7% 2nd h

On the bench supply I get more b+ so at home the el38 is 4.5 watts.

I have suffered more symptoms and my gp who is really on the ball, obviously he has studied Cerebral Amyloid Angiopathy has given me changed expectation of driving. “It will be at least a year before you can drive, if you don’t have symptoms for a year.”

Basically that is end of my driving.

Right now I’m in the doldrums and can’t be sure I shall build the el36 and its separate power supply in time. I’m ok about transporting it to the station and on the train with a sack barrow. I can put the valves in my rucksack and mount the signal chassis plus power supply chassis on the back of the sack barrow. The obstacle is actually building things. I’ll have a go, but call time if it doesn't progress enough. Maybe we will do another Owston a year or two later.


On the chart below its the loadline within the max anode disipation curve, restricted by the available b+ from the power supply tailored for the EL36. The loadline I previously drew was including the g2 wattage and well inside that (8 watts is considerable) anyhow EL38 chart only allows anode watts due to limited voltage available. The old loadline I couldnt edit out, just ignore it.

[Paul Barker]

Now that I have taken delivery of a total 8 Russian equivalents the two theatre amps can be built for parallel push pull at a cost of roughly £4 per valve with delivery. At 7% se distortion, looks like pp may shine. Likewise I could populate a pair of quad II’s with same.

[Paul Barker]

On the bright side, unwinding the 6V 15 watt torroid to power the 5u4 just the amount of turns I guestimated was bang on!

[Paul Barker]

I put in a gargantuan effort and butchered the new Hammond chassis with my jigsaw and by miracle the points to solder are clear of metal of the chassis without butchering the chassis top beyond where it is hidden. I did that part of it outside in the sunshine on top of the brown wheely bin which in Scarborough is garden waste, which you have to pay annually. It’s of course nearly doubled this year.

But this is a momentous moment. There is still the valve base the Black Gate the ht out connector the power leed the 5v torroid and the one microwave cap to mount inside.

The huge choke (I couldn’t help myself) measures 10.8 h 95 ohm dcr. Due to the power transformer limited to 230v and my mains 245 I put ht at 458-0-458, which comes up with 367 volts.

So I set the bench supply at that voltage and the el38 amp sounds sweet at that. Its only 3.6 watts though. But a very sweet sound. Even though its 7% 2nd h. The blue loadline.





If I build the 6p13c it will give 6.5 Watts also 7% 2nd h. But thats not built. However the massive challenge of mounting the iron I have won. So there is hope.

[Paul Barker]

I am so full of junk on the top floor I have taken over the kitchen table. We eat at a better table in the living room.

I go outside for drilling and cutting for reasons of swarf. Apparently I spelled Swarf correctly but Apple says that word is wrongly spelled. Yet when I asked Siri it spelled it same way as me.

It never ceases to amaise me how disjoited sisi is when it sn Apple product. Anything you want to ask how to do on youre apple product it says “I don’t know”.

Its worse than cora, the nat west bank ai text helper. I was stood in natwest bank and I asked cora how to get money without me card. It was as much use as a one legged man in an arse kicking context. I asked a member of staff and she knew exactly what I had to do and explained it step by step.

Ai has a long way to go to replace humans..

Also on my gas forum a gas fitter asked ai how to know the date of a worcester boiler from the serial n7mber. Ai came up with the incorrect answer in a nano second. The poster was very oroud of the speed of ai and accepted the answer it gave him without questioning. Not khowing Worcester have a 3 digit factory date a bit like Mullard. Ai aparently hasnt yet learned that. But is quite happy to blag while it is waiting for correct answer.

[Paul Barker]

The wrong answer delivered by ai in a nano second, followed by the correct answer known as the 3 digit factory date.

[Paul Barker]

This is useful feedback on the acuracy of ai from a trade specialist (Yours truly). In particular I am very knowledgeable about Worcesters. Worcester sometimes put the FD chart in the instructions, but if not they are only a phone call away and you talk to a person.

I have the charts going back to the ‘90’s in my photographs and can always find the FD for any Worcester.

The crazy thing is AI made up a reason for their guess, and this non Worcester specialist engineer was convinced by the explanation ai gave.

Hardly any manufacturers write the date in date form, except Vaillant who put the year of manufacture as the second two digits of the serial number.

I worked for Alpha once and was taught the alpha code to release the date but that was 20 years ago and I long ago forgot it. Neither am I bovered. I can always ring alpha for the answer.

Be warned AI are not honest.

[Paul Barker]

I spoke too soon, currently having amyloid spells. Which means right hand numb lips numb mind foggy crashed out with Roxy on the sofa.

Before this came on I managed to get a capacitor out of the latest broken microwave. Before this came on my mind planned mounting the valve base and mount the cap. Later in the week now.

[Paul Barker]

Very pleased to report symptoms died down and achieved mounting Black Gate and 5U4.