a new build starts (finally) then stops (again)

[cressy]

I finally got a new build off the ground. Chassis is glued up awaiting a good sanding and prepping and a decision on finish.
The idea is to drive the sv811 with an ecc88 cascode as i have with the el34 amp i am currently using. The psu will have to be in another box due to size constraints.
There will be a square section cut out of the left hand side of the front with the volume and input controls in it. The speaker binding posts will be on the top plate towards the back and the inputs on the left hand side at the front as i have done with the el34.
Op point for the sv811s will be looked at properly when i print out the datasheet, but im thinking around 600v, 10k transformers and about 60ma.
The output transformers are nicks, who gave permission to use them ages ago, hopefully this is still ok nick?
I reckon it could be a goer..........

[IslandPink]

Looks very nice .
I'm a bit concerned about the driver though . That's a lot of Miller capacitance to drive with an ECC88 cascode . Is it the 811-3 or the 811-10 ? Either way you could do with more like 20ma and a decent output impedance below 5k ; otherwise it'll sound pretty 'vintage' . If that's what you need, then so be it .

[pre65]

Fifty shades of grey eh !

You should like the thoriated tungsten sound.

[cressy]

i couldnt be bothered to do anything else with the build, so i decided to do some work on the el34 amp instead. last week i had another look at the curves as i have been thinking about changing the psu as it seemed to be running out of juice. so the amp was rebiased at 250v with 15v at the cathode as according to the mullard datasheet. handily changing the amp to choke input gave 248v at the anode. so close enough. i also swapped out the alps volume pot for a dale stepper. this was much better. however, the amp was still running out of steam, so today the mains transformer was swapped for a 600ma 280 - 0 toroid, the 4h 200ma choke was swapped for a 10h 500ma danbury, a hybrid bridge was put in, and a cap bank was put in consisting of the 220 uf and 150 uf that were in there, plus 2 100uf - 100uf jj caps bringing the cap bank up to 770 uf. still with choke input this psu settled down at 264v so minus the 15 at the cathode there is pretty much exactly what the mullard datasheet ordered. it doesnt run out of steam at all, monitoring the voltage while playing black sabbath and some iron maiden too loud, it didnt change. at all. to me it sounds lovely.
i am now a happy bunny.
i might put some nice components in it now instead of the shite thats currently in there.
sv811 has been put off again.....................

[Cressy Snr]

Yep...
Been talking about this for a few weeks.
Ant's amp just needed a bit of shifting around of voltages to bring it to correct centre-biased operation.
The toroid mains transformer comes courtesy of Antrim Transformers, via Andrew L, who sold it to me a few years ago, when I first got into push-pull pentode amplifiers, in the form of EL84s. It's now in Ant's EL34 jobbie as we can see, and is doing sterling service, providing spot-on voltages for classic Mullard, SE triode-strapped operation of El34s.

Ant and I have both come to the conclusion that there is little to beat the sound of EL34s, properly applied and fed by a power supply with a large reservoir capacity.

Single-ended and strapped as triodes at 250V with -15V on the grid, they sing.
In push-pull pentode at high voltage, (500V) class AB, with fixed bias, and shunt-regulated 250V screens, there is little to touch them. You can stuff your 6L6s KTs and 807s.

EL34s are unfashionable for hi-fi amps so are reasonably priced, readily available in modern production form and sound fantastic, when done right. What's not to like?
Our Ant had a road to Damascus moment on Thursday evening, at my place. We listened to every type of music, but to experience the final movement of Saint-Saens Symphony No3 "Organ" at realistic levels, with the room shaking and the organ putting out controlled thunder, whilst the brass crackled, Timps rumbled and strings swelled, was something else.
That's when he decided to beef up the mains TX , up rate the choke and increase the storage capacity of the power supply on his own ECC88, cascode driven EL34 SE at home.

Seems he made the right decision.

My own amp has killed my constant project building and meddling, stone dead. It is hard to believe, given my previous history that I built my EL34pp in January and nine months later, it is still making music. OK, so it has been developed a bit along the way, but 9 months with essentially the same amp is a record for me

Somehow I think Ant will be just as happy once he has dealt with the rat's nest and tarted up the cosmetics with a different case.

[IslandPink]

Sounds like you guys are really geting the PP stuff nailed. What are you bringing to Owston ?

[cressy]

Next thing to look at is separate driver stage and power stage psu's. I had to leave the original custom danbury mains tx in as it was heating the rectifier and one of the drivers. There is therefore a currently redundant 275 0 275 200ma winding which can be used to power the driver stage via an ez81 rectifier and rc filter. There isnt any more space for another choke in there without a lot of bodgery. Perhaps it could be done with a careful look at the available space inside.
Wether this would be of any benefit i dont know, but it would also allow the voltage on the driver to be put back to the original 300v spec it was designed for

[Cressy Snr]

Sounds like you guys are really geting the PP stuff nailed. What are you bringing to Owston ?

Aye....getting there

I'm bringing the amplifier and maybe a new DAC.

[IslandPink]

Good good !
Re. driver stage supply : can a CCS + VR-tube shunt be used in this case ? Should take up less real-estate than an additional transformer/rectifier. I've always been a fan of this approach since the 'Aurora' days .

[pre65]

I use a CCS on the driver valve on GK-71.

It did need a big heat sink though.

[Cressy Snr]

Re. driver stage supply : can a CCS + VR-tube shunt be used in this case ?

Can't see why not. We'll look into it.
Doesn't really matter that the cascode is down on voltage a bit.
Yes, we'd like it to be perfect, but it's not that big a deal. Still plenty of gain and headroom as it is.

[chris661]

Our Ant had a road to Damascus moment on Thursday evening, at my place. We listened to every type of music, but to experience the final movement of Saint-Saens Symphony No3 "Organ" at realistic levels, with the room shaking and the organ putting out controlled thunder, whilst the brass crackled, Timps rumbled and strings swelled, was something else.

Rather enjoyed that. Now to try it at "realistic" levels...

Cheers
Chris

[cressy]

I can fit an ez 81 in on the top plate easily enough, there is plenty of space between the output transformer covers at the front. I can get another bank of caps on the inside mounted on the left hand side of the case as that side is clear. Looking at the ez81 datasheet it will do what i want easily enough.
I'll have a pike at it this afternoon

[cressy]

ok, so i had a look at the available space and a separate driver supply is no problem to do.
however it means stripping bits out of the nuvistor phono stage i built thats been gathering dust for the last 12 months. out of interest i plugged it in to see if it still worked and it does. the reason it was shelved was that the hum was too much with the cart i was using at the time, the sl15 and step ups. however with the adc qlm 30 mk 3 i am now using it is not a problem and it sounds much nicer than the xlps that mike kindly lent me to use in its place.

so i dont want to pull it apart.

i think what i will do now, is to make a brute force dc heater supply for the phono stage using a sand rectifier and some ruddy great big caps i found while scragging through the spares box for bits to upgrade the el34, see what happens.

this is rapidly becoming a thread about power supplies....................

[Paul Barker]

You are fast becoming a chip off the old block