v72a

[rafafredd]

Anyone built it up already?



I´m thinking about this one. Maybe I can substitute this 1:4+4 for a 1:8 or something on the input? Is it really necessary all this components after the input transforme? Can´t I use a simpler yet good input schem for the 6AK5W grid? maybe with a lower ratio, like the OEP?

And about the 25uf eletrolytic at the output? would that be a problem or should I use something else? Maybe Tantalum or high capacitance paper and oil?

Output transformer is not a problem, I think. I´ll just use a 4.8:1 specialy wounded transformer from audiolink.com.br

But the third problem is the chokes. Anyone would guess their values?

And, what the HELL is ERG 110V?

Also the 50p variable capacitor seems really strange to me. What is it for? Should I maintain it as a variable cap?

Thanks for any inputs

[Shalimar]

The components after the trannie is for that specific model. You could probably ditch that, or use whatever the new trannie mfg recommends.

The 25yF should NOT be an electrolythic! Film/oil/paper.. (Well, it should if you're planning on doing a complete original job, which is *impossible*)

Chokes? No idea. Ask Oliver @ TAB-Funk.

ERG 110? Voltage tube or slow turn on device?

The vari cap is for optimizing hf response... I think? It's in the feedbackloop.

I don't think it's worth clonning though. They're not that expensive.

Best pentode baby I've heard though.

[Shalimar]

Btw, there's a lot of good info on Oliver's site (which I'm sure you've read allready). He takes those transformers VERY seriously. He even sells them seperately. Nice guy!

www.tab-funkenwerk.com/#V72history

and

www.tab-funkenwerk.com/blueprints.html

Damn, I'd luv to find a TELEFUNKEN blue book in the thrashcan!

[robflinn]

For the choke check out http://www.sowter.co.uk/
Look at the pro vintage section. He doeas a replacement choke for a V72. Maybe you can work out the spec.

[Shalimar]

Yes, but that's not for the same V72 version. Schematic is on Sowter's site as well. There seem to be as many dif V72 as there is letters.. V72a, V72b, V72c etc..

Maybe that V72 is simpler/easier to build?

[rafafredd]

Is the E180F just acting as a cathode follower output stage or what?

I´ve been thinking about getting the e180F stage out and use just the plate of the 6ak5w. Maybe with an output transformer with a greater ratio and airgapped core (would I be able to pull the 0,1uF capacitor out with a airgapped core?)

Would that be possible? What are the implications in doing this?

Other thing... Would it be possible to substitute the 6ak5w plate choke with a regular plate resistor?

I´m just beginning to understand how this things works, so, I´m sorry if I´m talking about bullshit on this forum... Let me know if this is the case.

[Hollowstateman]

Hello,
It's been a long time since I looked at a V72A and all my schems. are currently in storage but I would make a quick guess that the anode loading choke is 300-400 Henries (it needs to be quite high when loading a pentode) and the cathode one is 50 - 100 Henries. The strange 'ERG 110V' thingy, I believe, is just a light. The networks on the trannie secondary are there to tame THAT PARTICULAR transformers response. If you were to use a Jensen or a Lundahl, a UTC or a whatchamacallit then you should use whatever network (or load) that the manufacturer recommended as a secondary termination. Between 1:7 and 1:10 would be a good turns ratio for this circuit.
Best,
John

[Steffen]

Other thing... Would it be possible to substitute the 6ak5w plate choke with a regular plate resistor?

I don´t think this is a good idea....I strongly believe that DC biased inductors and Xformers are the key to what we like to call vintage sound!

steff

[Mark Burnley]

E180F!!

I've had one of these little beauties stashed away for years! Gold pins too. Now I know what I can do with it!

Ahh, the joys of surplus scavenging and hoarding [img]graemlins/heart.gif[/img]

Mark

--------------------------
"Oscillators don't, amplifiers do....."
Anon.

[cjenrick]

Don't build that V72. Build the one with the EF804S tubes. But thanks for a different schematic!