TAB V76 Clone - Another Take

[Falk]

So here comes some relief. At least for me. This marks kind of an end of a journey. UTM -> Great components.
Messured at full gain and @ 12dB(U) output level. Max gain is 68dB due to the lower ratios of the input tx and higher ratio of the output tx.

 

[andre tchmil]

Somehow related to this topic: I have to repair a V76 with a broken input transformer. Customer ordered a Sowter replacement which I installed.
At lower gain settings ( V76 input pad) I have a significant HF roll off starting at 5K. Only with Input pad at 34dB and higher I get the response perfectly flat to 20K.
Now here comes the thing: I did a sweep with the input pad and the Sowter only ( thus taken from he secondary) and I have a flat response at all pad settings.
I did the same test on an original V76 input transformer and there I have a HF roll off starting at a 34dB pad setting. Any setting below that gives a flat response. I'm suspecting the Sowter once build in, but I have no clue.
Anyone ?
And yes the V76 filters are bypassed, actually it doesn't matter, behaviour stays the same.
Thanks

 

[andre tchmil]

been thinking,
I will do a test setup with the original transformer in the "bad" V76 to see if I can rule out any other issues.
I'll report back.

 

[andre tchmil]

so as expected, the odd response is coming from the Sowter input and with the original transformer it gets compensated in the feedback switching.
The 2uf cap in the first 4 positions or do I see it wrong ?

 

[Ilya]

Do you mean the cap in the anode of the second tube?
I believe it’s there to remove DC from the feedback network.

 

[andre tchmil]

oh yes , now I see it's in the circuit all the time.
I wonder what's going on with he Sowter.

 

[Ilya]

I think CJ was right when he said that he doesn’t know any good 1:30 transformers of current production.
Cinemag openly tells that they don’t make mic trafos with such a high ratio.
I’m very disappointed with Sowter, frankly. This is not how a professional manufacturer should behave. It clearly shows that they haven’t tested their replacement transformer in real circuit.

 

[andre tchmil]

agree. But I wonder how the clone builders deal with the HF roll off, yet keeping the character intact.

 

[gyraf]

..probably this why it was never cloned in any real sense..

 

[Ilya]

agree. But I wonder how the clone builders deal with the HF roll off, yet keeping the character intact.

The only V76ish type "clone" I'm aware of is made by Mercury. They use 1:10 input trafos.

 

[rock soderstrom]

They use 1:10 input trafos.

I can understand that, because above that the air becomes very thin. I'm not a transformer mystic, but certain transformers really do require rare components and, above all, knowledge. It's not easy to build such gems, it requires a lot of experience and careful consideration of all the important parameters. The whole thing is a compromise at the highest level, it's a great pity that a lot of expertise and experience has been lost in this area in recent years.

In my opinion, some classics (not only germans) can only be replicated to a limited extent for precisely this reason. The iron used (including the chokes) is something like a copy protection dongle, it is really hard to get them sound like the originals...

 

[andre tchmil]

It's a pity Sowter advertise their replacement transformer as a drop in without mentioning the artefacts.

 

[CJ]

Original 76 has that 1:30 input wrapped in a ton of mu metal,

A Jensen 1:10 sounds better than the stock 1:30 to me at least. And there is plenty of gain to make up for the ratio.

I told Ollie this and he started to offer his 76 with a 1:10.

 

[kidb]

Original 76 has that 1:30 input wrapped in a ton of mu metal,

A Jensen 1:10 sounds better than the stock 1:30 to me at least. And there is plenty of gain to make up for the ratio.

I told Ollie this and he started to offer his 76 with a 1:10.

Stop giving away all the good secrets CJ!!!!

 

[kidb]

I think CJ was right when he said that he doesn’t know any good 1:30 transformers of current production.
Cinemag openly tells that they don’t make mic trafos with such a high ratio.
I’m very disappointed with Sowter, frankly. This is not how a professional manufacturer should behave. It clearly shows that they haven’t tested their replacement transformer in real circuit.

It’s difficult to make a good transformer with that high of a ratio, I’m surprised when it works, I believe 1:10 is about all that should be made, not sure why you’d need a bigger ratio outside of phono cartridges or something, but I agree would have expected better too.

 

[CJ]

You can use taps to increase the ratio, that's what the MC cartridge crowd does with the likes of the Peerless K-241-D etc.

The 1:30 uses the disc method of winding to keep the HF response decent, It also uses a huge core which is why people like the 76 for the kick drum.
Put a 4 inch wolfer over the sound hole with a step up transformer into the 76 and you can get sub wolfer sounds to mix with the internal mic and the 57 near the beater.

 

[Wilhelm]

Good evening gentlemen,

about a year ago a had a job in Berlin and for a week I was commuting every morning and evening for a bit more than 30 minutes. I cant recall why I started to draw the V76 schematic in spice but a some point I had a well working model of the preamp. I played around with that model to get to know the circuit better. Also I read through the V76 threads here in GDIY and gathered quite a bit knowledge about the preamp. During the past year I could gather two Sowter V76 input transformers and four Lundahl anode chokes on Ebay. Output will be some Edcor iron I have here. From the U23 build I have power transformers and a PSU PCB what could do the job.

So I plan to build a stereo unit with PSU included. PSU is regulated with DC filaments. I needed to extend the PSU and designed a voltage quadrupler 12 -> 48V. I am quite curious if this will work. I designed pcbs along the tips and tricks I found in the other V76 threads. I integrated a DI input, skipped all the filter stages. Added in and output pad.

I will use EF86 or PF86 instead of the EF804. All capacitors in the signal path are film. Front panel comes from Frank. He did an awesome job again. I will drive the VUs from a chines VU buffer. The PCBs offers two positions to hook it up. I ll see what works best.

Today I assembled the front panel. Thanks again @gyraf for the knobs. I still live off of them. I will stuff the PSU PCB the next days and make a documentation of the build here. I hope you people enjoy the thread.

Best,
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