audio-talk

Paul Barker wrote: ↑Thu Mar 10, 2022 8:04 pm

Bases still at Russian Port, who knows if they’ll leave port for UK?

I'm surprised at you Paul.

What I have bought too many. 4 coppers bought that long ago. Two came in the big stash I got my graphites in. It was before the world wanted copper and the military surplus sellers separated coppers and charged more. Then again the new pair I forgot about would have been bought 20 years ago, probably £60 each. Graphites probably 25 each with two cheap coppers in that bundle. I was buying valves then I valued considerably more than their cost. GM 70 is up there with the 212. Forget the 845!

Oh boy now I’ve found that I had bought 6 pcs Mazda PP3521 from Langrex in 2013

http://www.r-type.org/exhib/aag0163.htm

http://www.r-type.org/articles/art-194.htm

A sort of indirectly heated PX4 1936. 600ohm mu of 6. Guess what’s driving the GM70’s?
Brand spanking new and a pair of bases.






I didn’t find my rubber British Gas drain down hose for a job tomorrow: but I found these looking for it!

Looks like 3% 2nd harmonic distortion where Id operate them with a 3k load. To get max power from the GM70 at 1200v I nead 120v peak output from one of these, they will provide that with plenty of headroom even at 200v anode. Nice. As used in 801a, ive got a d3a formula thats mint sounding. So d3a triode IT hammond 10k to pp3521 IT AE Europe 3k IT to GM70. fixed bias probably to maximise gm70 b+ from the pair of 2920 vct 250mA transformers perfect for GM70 choke input. Should net 1200 anode volts. The switch mode power supplies can remain on the shelf as a workshop power supply. Im uncomfortable about using switched mode as it may modulate away some of the midrange beauty. Ive been trying to study filtering the switch mode noise, but my research has lead me to learn its not simple and you can more easily spoil than you can fix! Ill stick to my usual old fashioned ways! Simon will be pleased its at least bi-amped!

I just need two big matched 250mA choke input chokes. That will be the main delay of the build. I lied, my condition mental and physical will be the main delay. But these projects are therapy!

Nice items !

Thanks Mark, what I like is there are probably fewer than and handful of indirectly heated true triodes available. I do have a pair of indirectly heated 6a3’s that would also do, but they remain lost! These however might sound better having the single anode and being from 1936, because some say they sounded better then.

An alternative would be the 6CB5A ( is it ? ) that Thomas Mayer found and uses for some of his (cheaper) amps. I have a few upstairs.

IslandPink wrote: ↑Sun Mar 13, 2022 4:08 pmNice items !

Back in the day when I bought these a project like this would have been on my mind. Its really odd that I dont recall at all ordering them. I think its just the stresses ive suffered since then taking their toll, and Ill recover. Otherwise its something worse. I am regaining my knowledge, so I dont think its anything major, and Im definately becoming functional enough to start work again. With background neuralgia, but its much lower pain now, and will probably go away soon.

Managed to see my own dentist at last and he also thinks it neuralgia but we’ll monitor progress.

I remember you ordering them Paul, I got at least 4 of them.
I think I eventually found another in my stash.
I built a breadboard se amp about 5 years ago.
As on output valve they had a sound of the cosser 2p, and the anode size is similar. ( bit smaller than px4)
Very nice valves all the same, but subtle differences between the top valves, I'm convinced anode size and material is the reason. But what a driver they would make, I have those lined up when I eventually build an amp for the 4300a

Thanks, and thanks for the memory jog. I remember now.

Ooh, I was so excited when a Russian parcel came.

Oh hang on a minute, look at the included valves!


Something lost in translation! But when you analyse the situation, destroying the base of the valve gives you less losses/degradation. But

Small mercies; no import duty!

I think it would be better to drill out my 813 bases.



OTOH something about them allures me….. electrically shorter path less solder, silver collets…. Maybe experiment with removing base of one of my graphite’s.

“Nobody said it would be easy” my mentor’s favourite phrase when I was an apprentice.

One valve has no flash either. But can’t say that matters, proof of pudding awaited!

Erm Mia culpa, clear from pictures what to expect; oops!


Text says This bases are for military high reliability!

On balance this might not be such as bad a situation as my first impression! Which was

OTOH insert Waily Waily emoticon!

Yes, that looks a bit inconvenient, but I'm sure you'll find a way .

Those bases are excellent quality Paul, I use them for my 6C33C amplifier. 6C33Cs are notorious for burning through valve bases due to their filament current, but I've had no issues with this type of base.

Thanks for the inspiration Ray.

Decided to simplify and drill out the ceramic of 813 bass for the fat pin; piece of cake with 5mm tile drill. Also less valves wasted as some breakages expected. Also ties in better with the expectation to pop out graphite and pop in copper as desired.