DIY Telefunken V76 Build attempt

[Landins]

@shabtec! Give more info on the primary winding filter. My local radio-repairs man (the only one in my small town) who sits no vast experience says this input filter is placed there to reduce microphone puffs and mic stand rattels ect. And as he sad "If the cleaner turns on the vacuumcleaner in the studio" If you say different i will take this into consideration as I now will leave this out.

@gemini86: Im thinking about your reply and trying to figure out what it really means #wrongeducationalbackgroud

Great forum!

 

[shabtek]

I was only making comments regarding the original schematic; relatively complex...the components that tie the center tap of the input transformer likely tune its response--I've never really noticed such a configuration before

is there a set of iron for this on ebay currently?

 

[My3gger]

It's nice to see more people here, i thought p2p and tubes are not good...
There is OT at eBay, but look at the price, it's incredible. I wouldn't buy input from eBay as it is very fragile.
http://www.ebay.com/itm/ws/eBayISAPI.dll?ViewItem&item=221175868957&item=221175868957&lgeo=1&vectorid=229466
I was looking at Jonte's Therapy Pre yesterday and noticed the same thing. It is easier way to high gain as are some RCAs. But you know, we want Tele.
CJ said OT is nothing special and i will use Cinemag's input and output. I hope Landin's Hammond chokes work (probably not properly), if not i will have to buy expensive Sowters. Can't understand why a simple choke costs more than a good transformer, probably because it says Telefunken. While you are here: for 0,85H, can we buy "normal" choke like we use for Pultec? Farnell often has them for like euro or so...

 

[shabtek]

http://www.ebay.de/itm/Ubertrager-fur-TAB-Telefunken-V76m-Mikrophon-Vorverstarker-Trafos-/181099862720?pt=Filmnachbearbeitung&hash=item2a2a649ac0

 

[Landins]

Lost confidence, backtracked, wonder what I really have gotten in to, studied. Came back. Now I got the understanding of the choke qualities by studying it again. Just for clearence of my bran: the inductor passes dc but opposes ac. As anode choke it passes sufficient dc current to plate, and as anode current starts to swing it builds impedence reative to the swing frequency and amplitude. It essentially becomes a variable plate resistor?

The original part for this unit are so extremely expensive! Hideous prices. And I agree: why are the chokes so expensive? Wire many times around core. Whats the deal. The expensive ones mot likely have low capacitance.

Several schemes for the build are presented. I have noticed a difference. In the original, posted here a few post back, the ground line is connected to negative volt supply. In the filterless and gwagner schemes it is not. I my meaning i should be! Is sounds redicilous to charge R72 with -350V.

 

[My3gger]

Those eBay trafos/chokes aren't expensive at all and look clean. Nice find Shabtek! I can't see on schematic and don't have time to search if "85" is input transformer, but it looks like complete V76 iron. This is cheaper than Sowter even if input needs rewind.
Landis, your mechanical skills are very good, i consider your preamp almost finished even if you don't really understand chokes. At this stage i would contact German seller about the iron, see if it's complete/working and simply use it if it is. Now you probably know what you have gotten into (Un)fortunately i have another preamp to finish but i still enjoy your work as it was mine.
Grounding seems the same to me on all schematics i've seen. One of them is not really clear and R72 looks like it's touching ground directly but it's not. There is no -350V here, only +350 (or whatever HV it is) and 0 volts. It's not dual voltage like typical +-15V we use for op-amps.
Pins 2,4 are electrical grounds, pin 5 looks like chasis ground. It's a bit different from our builds because it is a cassette, on your build it should be simple. Have you already checked Radiotron Designer's Handbook i wrote about? It's really good reading, i'm sure you will enjoy it as much as we all do.

 

[Landins]

Thanks for the support on all respects! "We go to the moon not becauce its easy, but beacause its hard" .

The pricing is good on those trafos! If my prototype works out I have two orders to a friend (gutarist of Swedish punk-icons Refused) so there will be more soldering up ahead. My wallet is somewhat starined right now. Must check finances.

You are correct concerning the groundpath. Circuit (electrical) ground connects to pin 2,4, and so are I/O shield of wires. Thank you for clearing the ground issues out! I got curious because on the v72-schem the minus of psu connects directly to ground path.

When the last tube, e83f, arrives from Tbilisi (!) Im ready for the last of the hookup. Almost there!

 

[Landins]

Just for fun i hooked up an signal generator to the input transformer and looked att the waveform thru tubes 1-3  with oscilloscope(fourth tube havent arrived yet). And voila! Sinewave looks smooth and frequency response is totally, totally flat from sub 20hz and up beyond 20khz! Happy times fo far!

 

[My3gger]

That's nice to hear. Have you added any resistance to the choke? E83F is probably RFT or something from east, can you tell me who is supplier? I would like to find some cheap Tesla EF806s. eBay is full of them at 20 euros, but they show up cheaper from time to time. Some EF86s from RFT (RWN) are also pretty low noise and microphonics when used at the input.

 

[Landins]

Ef83 is Philips Miniwatts NOS goldpins. Bought the pair for 24$ +8$ shipping from seller bgebus on ebay. Ef804 i bought from a seller i spain for about 20$ each those where.

I tried without additional resistance in series with the chokes. When all wiring is in place i may change my mind .

 

[My3gger]

Good news, thanks for sharing. I went through some reading and it seems like you should expect a little lower gain with lower H chokes, beside other things i'm not so sure about. Choke also isolates psu from tubes, only tests will show if you need regulators because of this. As i understand it adding resistance might not be the best thing, so let's see. Output choke is probably more critical because it makes part of the output with transformer. Edcor could be another cheap source for chokes, i will send them an email with original data and see if they can make at least two chamber bobbin. It shouldn't be expensive.

 

[Landins]

Thanks for reply! Wiring is in process but last tube has not arrived. The low price may explain this *sigh*. I´ve looked att the signal thru the first stage and i can see some noise and quite heavy clipping in the high gain stages. The situation may change when all tubes are in place and output-trx is connected. My psu delivers 275 v which differs from the original 300. I have followed the original resistance in the psu so Im thinking about changing this for higher overall B+ and thereby increase headroom. I only get ca 140 V now. Concerning noise I will add separate wiring for the heater of first tube and install the 250R "noise-bias-pot". Power-trx har double heater wirings. Chokes will be put to test when everything is in place! Ill keep you posted!

 

[My3gger]

A few days ago i checked offer for your E83F, iirc seller looked ok to me. Tube might be cheaper because it is marked differently although it is the same as E83F. I would mail the seller and ask if tube was sent.
B+ is 300V after diode bridge only, can you measure what you get on anodes? 25V difference at HV makes almost no practical difference with tubes, so maybe wait until you wire complete circuit. Isn't first choke already wired? If it's not don't turn preamp on. Well, i would wire complete circuit and then start to check why you have low HV readings, noise, etc. If it was my preamp i would try resistors instead your chokes which are very different from original. I know people tried 47k resistor in V72 instead of choke, difference in sound was not big and circuit worked without oscillations and other problems. I don't know which R values you need here, maybe CJ or others can help with this question.

 

[Landins]

After a couple of tough days buildingwise some progress was made today. Everything is hooked up but the low values were still there. The solution was negative v-return. Ive studied the wrong schematics but the original manual gave answers. R72 is tied to return V.  See attached file. It looks like the chokes will not work in this configuration unfortunatly. So now i have to test correct values. All ideas are welcomed!

http://www.irt.de/IRT/publikationen/braunbuch/V76.PDF

 

[CJ]

wow, did not know about this room,

FYI: you do not need the 0.85 choke BV nr 517a, all it does is wreck freq response,

same with the caps on the front of the input transformer,

they were used to limit the bandwidth of the amp for radio use,

all i have  on mine is a bass boost and low cut, works fine,

 

[Landins]

The man i was waiting for has appeared!

I will ditch the input filter and consider the inbeteween pi-choke. Turretboard makes bypassing and testing easy.

 

[CJ]

if you use EF804s make sure you keep the heater voltage at 6.3 or below,

Ollie says that at 6.6 and above the carbonates burn off the cathode,

 

[My3gger]

Thanks CJ, nice to finally see you here. Landis is probably having problems with his Hammond 156C chokes (150H/8mA/3k7ohms), he uses them as plate loads. I think they won't work properly, what do you think? There is many of us who would like to build something similar to V76 with cheap Hammonds if they work, or resistors. It doesn't really matter if we loose some gain and distortion changes, we can add proper chokes later... What do you think would be starting point for resistor values to replace both plate chokes? I know 47k works for V72, but i have no clue about V76. RDH doesn't have much about it, Google just a little bit...
As you talk about EF86 class tubes; eastern Europe seems full of nice and cheap RFT EF86s, Tesla's EF806 aren't that expensive either and don't need much (if at all when NOS) selection for input stages.

 

[Landins]

Great progress has been made during the last few days. Eventhou the hammond chokes has proven not to work the preamp is alive! Chokes are on the way from across the pond but in the meantime i tried resistors. 100K on V2 and 47K on V4. It passes signal but performance will be better upon arrival of chokes. IMPORTANT: Beware of reproducions of the original schematics, there are faults within these! Go for the original one found in manual attached in thread. I found two faults in the repros. I fixed these and it had dramatic effect on performance.

Problem: Gain works all the way up to the last three gainstages. When step nine kicks in, total whiteout feedback! Cool in som aspects but not so useful  Any suggestions on how to solve this?

 

[PeteC]

great thread - I'm learning from it ....

FWIW,
For anyone in Europe there is a very good supplier of chokes and TXs who I have used in the past for bespoke wound components for Audio.
very reasonably priced and might be a good source for chokes etc
I got some TXs and chokes for a PT15 based monitoring amp custom wound at great prices from him.

AskJanFirst
http://www.die-wuestens.de/eindex.htm

Rgds
PeteC