V 72 Stuff

[CJ]

you do not build v72, you wind v72, and you wind precise choke, as if you cross wires all over the place, you get a piece of random , junk, but no, because nobody is perfect and a lot of tube amps sound differnt.

The way to build V 72 is use Freds in place of poison se.

There is so much less drop across the rectifier that the whole power transformer changes, you ca work a phantom winding into a V 72 if you mod it for fast freddys markham, faster sprinter at san jose hellyer park, winner of the dupont..


goodnight..



http://en.wikipedia.org/wiki/Hour_record

 

[jensenmann]

Any progress on my V72s, CJ?

 

[CJ]

Jenns, use updated shipping address, yes,?
As I recall, youhave moved recently, still at same addy?
Thanks!

cj

 

[helterbelter]

Hi guys !

First of all, CJ, thanks for doing this thing, and Jens too !!


I've got a Siemens V72 that's not working properly.

A collegue (who's experienced in tube electronics) tested the PSU part, and said it was working properly..... The damn thing just wouldn't work. Replaced tubes, replaced all other caps, measured all resistors, and checked the in and outputtransformers, nothing did the trick, hahaha. Then the guy from whom I bought the V72, suggested to replace the choke with a 47k resistor...... I did, and then there was sound !!!! Measured the choke with an Ohmmeter, and it measures 12 Kohm.
However, the sound suffers from noise and rumble with ticks and plops.

Then I moved from Wageningen to Zevenaar, so I put the thing in a box, to leave it for a while.

Until the day before yesterday. I also decided to replace the powercaps. I discovered that the 2nd cap was dead. (Never trust a collegue, hahaha!) I'm waiting for the caps to arrive, so I haven't checked if this will be the cure I'm looking for.

So, and now I'm coming to the point :
The seller of my V72 told me that he once did a (blind) hearing test with a few people with "Golden ears". They listened to a V72 with the choke, and a V72 with the choke replaced with a resistor..... The outcome was that the resistor version sounded best.

It might be clear that I'm an absolute newbie when it comes to intelligent electronics, and tubestuff in particular, so the question to you all is :

What do you think ?!?

I'll be probably wrong here, but I'd suggest that you can't simply replace an inductor by a resistor !?!?

Oh, I have a second question : What should the resistance of the choke be ? I'm not able to measure Henries.....

Thanks in advance !

 

[helterbelter]

[quote author="helterbelter"]
I haven't checked if this will be the cure I'm looking for.
[/quote]


Well, it wasn't the complete cure.... the 50p cap in the feedback circuit is defect too.... Temporary replaced it with a ceramic one, now it works !


:grin:

 

[jensenmann]

:thumb: well done

 

[peters]

I have a question on the V72 choke, as I am building the Drip V72 and using the Lundahl LL1667 5ma choke.

The Lundahl choke is rated for 810H and measures about 2.4K DCR with the coils wired in series per Greg's instructions.

I see that CJ measured 12K DCR on his original rewind and the Sowter replacement choke is rated also at 12K DCR as well (and 750H at 1KHz).

I am wondering how important the DCR value is in the original circuit and whether I should consider putting a 10K or so resistor in series with the Lundahl choke?

Schematic is available here:
http://www.dripelectronics.com/forum/download/file.php?id=36

thanks,
--Peter

 

[CJ]

I tried a resistor, it can help with noise,
but not with sound quality.
It drops the B+ a bit to where the tube makes less noise I believe,
but it was so subtle that i just left it out,
or settled on a half way point, like 4.7 k or something,
watts will be easy to manage with the low current of the ef804s, what is it, 1.3 plate and 0.7 screen?

i am shopping digi cameras right now,
you have to see this new counter i got for my winding machine,
it is Huge! the numbers are about 3 feet high!
did not realize, the evilbay photo was not to scale!
built like a tank,

Veeder Root, built like a frickin tank, riveted together so you can not mess with the data.
my digi counter was dropping 100 turns every 1100!
on ocassion!
not good!

and, it does not count backwards when you unwind, a very important feature to have on a winding machine, believe me.
everybody has overshoots if you wind all day.

so stay tuned, winding stuff for v72 coming up,

input, output, choke...


so this guy is 7" x 3.5" x 4.24" 

the whole bench shakes everytime a digit flips,
but it will loosen up.
carpal tunnel just resetting the dang thing, but it will loosen up,

 

[peters]

Here is a picture of the beastly Lundahl choke:

http://www.jacmusic.com/lundahl/images/products/Plate-choke-detail3.jpg

It has a very unique winding around a circular wound metal core. From the machining it looks like the coil winding was done after putting the core together (toroid like). Apparently this can create high inductance using less wire hence the 2.2K DCR.

I did an experiment and added a 10K resistor in series with the Lundahl choke to bring the apparent DCR up to about 12.2K. The B+ increased from 265 to 266 VDC, voltage across the choke went from 11 to 11.5 VDC, and calculated current flow through the choke went from about 5.0 ma to 5.2 ma.

I am not sure if this is better or not but it is probably closer to the original circuit.

If someone has a good explanation for what the choke is doing in this type of circuit I'd certainly like to hear it. I have pretty much bottomed out on my tube circuit knowledge here.

thanks,
--Peter

 

[CJ]

plate chokes are exactly like plate resistors, only different.  ;D



amplification- you stick a small signal in, you get a bigger signal out

with plate resistors, you are wasting 1/2 the power supplied to the tube in heat.
kind of like an internal combustion engine.
it is freezing in Germany, they hate to waste heat.
look at the volkswagon, they forgot to install a radiator

the vacuum tube is just a variable resistor
so the plate resistor stays the same, but the vacuum tube resistor moves with the signal.
so you have a voltage divider, and one of the resistors changes with the music
so you can tap off that divider with a cap, and steal the ac signal

then transfer it to the next stage, and do it all over again.

well, the choke is like putting a turbo charger on a car, you have no dcr (on fantasy island at least)

so you get close to 100 percent B+ on your tube
this means, as the tube resistance swings up and down, you get the full swing, from 0 to B+
much more amplification

but the choke is frequency sensitive, so it will start to chop bottom end
harmonic distortion, not good...

so you need max henries to keep the bass safe,  this is why you see crazy values like 400, 600, 800 henries on some plate chokes.

there are other things going on with the choke, but they are minor

with the choke, you now have added a reactive element to the circuit, which can affect phase, which in tune ties in with the neg feedback

the plate choke cap and output xfmr combine to form a pi filter, so you have a lot of math

with the resistor, everything is linear, no phase issues, no bass rolloff, so they both have their benefits

the choke was probably used to squeeze enuff gain out of 2 ea. EF804s in order to get 36 db or whatever it is.

lower dcr in the choke is actually better, but it will not be exactly like the original choke

also, capacitance changes as you add chambers, LL has two coils, Telefunken has three, which is a pain because nobody makes a three chamber bobbin anymore, so ollie had to roll his own.

you divide capacitance by number of chambers squared, so

LL = 2^2 = 1/4 C
Telefunken = 3^2 = 1/9 C.

so you have about half the high end roll off with the Tele choke.
but that should be well after your ears quit at 15, 750hz.


the lundahl will be less noisy as it is wound in a humbucking fashion.
but the tele choke is surrounded my tons of mu metal, so...

that is a regular c core, the coils are wound first then slipped over the core which is then gapped and banded with strap and varnish dipped.

there do exist small Beyer transformers that are indeed wound in the fashion you state

you de reel and wind the xfmr iron strip into a round core, but as you do, the strip goes through a bobbin.
the bobbin has a breakoff flange that rides on a rubber wheel as wire is spooled.
very efficient, no radiator like the vw

 

[MrZpliff]

Thank's for the explanation CJ. Now I've learned something today 
Been wondering too.... what the choke really does.

 

[peters]

CJ,

Thanks a lot for putting that together; That help out a lot!

--Peter

 

[emrr]

Soon I hope to play with some various vintage american plate chokes I've collected, and run some response charts to see what's up, when tried in the same circuit.  Should add some dimension to the v72 discussion, in a divorced sort or way.  The quality of the choke in question is a bit beyond the casual viewers grasp.  Many will work, but to what quality?  It's a tough thing to get your head around, without seeing a real world example.  At least for a gutter theorist like myself. 

 

[CJ]

ok, more cool stuff on the choke from Bottlehead...

"Parallel Feed

In parallel feed, the usual air-gapped output transformer is divided into an air-gapped plate choke and an output transformer with interleaved core laminations. They are coupled together with a high-quality capacitor. The choke does not need to have interleaved windings, so it can be made to have a large inductance (good bass) and still retain a small capacitance (good treble). The transformer can be much smaller because it does not have to handle the DC current, and the smaller size permits it to have good high frequency response without excess interleaving and the capacitance that results. Sonically, this means deeper, cleaner bass without sacrificing the highs. 

The plate load choke also isolates the output stage from the power supply. This suppresses the power supply hum and noise. Sonically, this means less noise and power line interference, and blacker backgrounds. And because it isolates the audio current loop from the power supply, the power supply capacitor does not affect the sound. More and more, we are also returning the loop current to the cathode in order to get the cathode bypass capacitor out of the loop. By getting these large capacitors out of the loop, their distortions are removed without the bulk and expense of large film capacitors. 

If the parafeed coupling capacitor is correctly chosen, and if the speaker impedance is nearly resistive near the low frequency cutoff, the tube can be made to see a nearly resistive load to a lower frequency that one would calculate from the inductances alone. This smaller phase shift reduces tube-generated low frequency distortion., giving a clearer, more articulate bass.

 

[glbaudio]

Any advice on replacing original bridge rectifier on a Telefunken V72? The model is an AEG 330 B 60 M

tnx in advance

 

[jensenmann]

These are selenium rectifiers. Check german ebay, occasionally some original parts pop up over there.
Replacing these with  silicon rectifiers is possible, but you´ll get a different B+ then.  But that can be adjusted by changing the series resistors in the PSU.