V76 input transformers acting weird

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Bonnie1

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Jan 24, 2016
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Hello Murdock,
If we understand the testing details so far,
-Sec. dcr (not able to measure)
-Sec. L (not provided)

The BV511 ratio is 1:30
If you inject -30dbu (.0245vrms) sine wave into primary, you should have approx. -.5 to 0dbu on Sec. (.735vrms)
(Src. 150- 200 ohm / Load 100k)
If your response signal is super week you have a open winding.
If your response signal is -20dbu then you have a V372 (1:10 ratio) used in V76M.
These levels are in ref. to 1k regardless if lamination has lost some permeability.

If you need further assistance and/or these repaired to original specs. It is most efficient to email AMI/TAB directly [email protected]
 

Murdock

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Jan 28, 2015
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Hello Murdock,
If we understand the testing details so far,
-Sec. dcr (not able to measure)
-Sec. L (not provided)

The BV511 ratio is 1:30
If you inject -30dbu (.0245vrms) sine wave into primary, you should have approx. -.5 to 0dbu on Sec. (.735vrms)
(Src. 150- 200 ohm / Load 100k)
If your response signal is super week you have a open winding.
If your response signal is -20dbu then you have a V372 (1:10 ratio) used in V76M.
These levels are in ref. to 1k regardless if lamination has lost some permeability.

If you need further assistance and/or these repaired to original specs. It is most efficient to email AMI/TAB directly [email protected]
Hey Bonnie1 thanks for the support and offer!
But is 100k not loading it to heavy? With a ratio of 1:30 the reflected impedance is around 111 Ohm which is way to low for a 200 ohm source, right?
I have other good 1:30 transformers and when I use a 100k load I get -10dBu when injecting 24.5mV.
But the two bad transformers have really low level. So another sign for an open winding.
 

Murdock

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Here are some pictures of one of the defective transformers.


Here you can see that they are impregnated with some kind of wax. But they seem pretty original to me.
And the seller had a stock and delivery note from the warehouse management of the "Südfunk" for the five Bv511.
It dates from 1974.
 

Bonnie1

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Jan 24, 2016
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51
Hi Murdock,
I should have specified a min. of 100k load. Your analysis is correct for the "really low level" bv511's.
Appears the 1970's production dipped transformer bobbin with the coils in wax. I have not seen wax in the earlier units.
 

Fairman

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Nov 21, 2020
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If you got the to often wellknow problem with the input transformer I recommend not spending hopeless time on it, replace it with an other about 1:10 to 1:20 tranformer. Ex a Lundahl LL1577 connected 1:14. You'll loose a bit "gain" but get an exellenct frequency respose FAR beonde the "hopeless" original one.
 

Fairman

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After consulting my notes over many years of "V76" expiriences I've got some more info.
The original BV511 has a very outstanding (abnormal) ratio of 1:28, obviously designed to find as much as possibele "noise free" gain.
The many delicate secundary sections windings with extremely tin wire unfortunately have also extremely high internal parasitic capacities.
Mesurred with an 200 ohm generator, and tested with a 5M load, 60Hz is down -6dB, 1Khz - 0 dB, 8 Khz - 4 dB, 10 Khz - 6dB and 20 Khz - 10dB.
Lundahl LL1577 1:14, 60Hz - 0.5dB, 1 Khz - 0 dB, 10 Khz - 0.5 dB, 20 Khz -1dB, 50 Khz -4dB
Further more, very IMORTAND, an original Europian V76 230V wire up and 230V supply will supply the tube heaters with 6.83V. For the correct 6.3V the input voltage should only be about 212V.
Finally i higly recommend bypassing the four 2u capacitors in the middle of the primarys. These are installed to avoide eventually asymetrical 48V phantom to pass the pimarys. If these capacitors by accident are charget, a discharge will induct a very high voltages to the fragilde secondaries, proabebly the reason that so many of these BV511 transformers goes dead.
 

Bonnie1

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Jan 24, 2016
Messages
51
Hi Fairman,
What you may consider deficiencies in the bv511 is very much the sonic character of the V76. Changing to any other transformer will diminish its value.
Not recommended for such an amazing sounding preamp!

-dmax
 

Murdock

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Location
Belgium
After consulting my notes over many years of "V76" expiriences I've got some more info.
The original BV511 has a very outstanding (abnormal) ratio of 1:28, obviously designed to find as much as possibele "noise free" gain.
The many delicate secundary sections windings with extremely tin wire unfortunately have also extremely high internal parasitic capacities.
Mesurred with an 200 ohm generator, and tested with a 5M load, 60Hz is down -6dB, 1Khz - 0 dB, 8 Khz - 4 dB, 10 Khz - 6dB and 20 Khz - 10dB.
Lundahl LL1577 1:14, 60Hz - 0.5dB, 1 Khz - 0 dB, 10 Khz - 0.5 dB, 20 Khz -1dB, 50 Khz -4dB
Further more, very IMORTAND, an original Europian V76 230V wire up and 230V supply will supply the tube heaters with 6.83V. For the correct 6.3V the input voltage should only be about 212V.
Finally i higly recommend bypassing the four 2u capacitors in the middle of the primarys. These are installed to avoide eventually asymetrical 48V phantom to pass the pimarys. If these capacitors by accident are charget, a discharge will induct a very high voltages to the fragilde secondaries, proabebly the reason that so many of these BV511 transformers goes dead.

Than you have another Bv511 than me. Because the good one is essentially flat from 15Hz to 50Khz... And that is with a 150 ohm source and 1M load. Pretty good for a transformer with such a high ratio.
48V phantom didn't exist at the time the v76 were manufactured. The four 2uF caps on the primary were installed for a bass cut to conform to the IRT broadcast norm as far as I know.
But thanks for the tip with the lundahl. But I don't have an v76. I just happen to come across these transformers and maybe wanted to build something around it.
 

rock soderstrom

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Than you have another Bv511 than me. Because the good one is essentially flat from 15Hz to 50Khz... And that is with a 150 ohm source and 1M load. Pretty good for a transformer with such a high ratio.
48V phantom didn't exist at the time the v76 were manufactured. The four 2uF caps on the primary were installed for a bass cut to conform to the IRT broadcast norm as far as I know.
But thanks for the tip with the lundahl. But I don't have an v76. I just happen to come across these transformers and maybe wanted to build something around it.
Yes, the four capacitors form a subsonic (rumble) filter below 40Hz with the inductance of the primary winding. The measured values for the BV511 mentioned by Fairman are not realistic from my point of view, the V76 would not reach its target specifications with that.

The bandwidth of the V76 (in flat mode "Gerade") is limited by the rumble filter and a low pass filter. You can bypass all these filters, then the bandwidth will probably be in the range mentioned by Murdock.

1624251265847.png

1624251371183.png
 
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mjrippe

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....and I just did it again, a new in box, clearly unused RCA input transformer, one winding open.
Ugh, what a bummer! I have had luck once or twice reflowing the terminal solder on used transformers that measured open, not sure if it would help on NOS.
 

Winston OBoogie

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Appears the 1970's production dipped transformer bobbin with the coils in wax. I have not seen wax in the earlier units.

Yes, strange. I remember Oliver telling me that any kind of potting or dipping of the bobbins and coils was/is detrimental.
Is wax more forgiving that shellac?
 

rock soderstrom

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Appears the 1970's production dipped transformer bobbin with the coils in wax. I have not seen wax in the earlier units.
Unfortunately, I can't find the build instruction for the BV511, but there is some interesting information in the one for the V72 input transformer (CT72/1)

Here it is written that when winding the bobbin the lacquer-insulated CU wire should be pulled through hot and liquid wax.

Ok, this is a different transformer, but the manufacturer, style and time period (1952) are similar.
 

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Bonnie1

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Yes, strange. I remember Oliver telling me that any kind of potting or dipping of the bobbins and coils was/is detrimental.
Is wax more forgiving that shellac?
Might you remember any particular type of transformers you were discussing? Oliver used special shellac mixture on many transformer designs, also many of his design's he specifically didn't use shellac.
Wax may be okay for potting compound in canned transformers, but not need with todays bondable magnet wire.
 

Bonnie1

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Unfortunately, I can't find the build instruction for the BV511, but there is some interesting information in the one for the V72 input transformer (CT72/1)

Here it is written that when winding the bobbin the lacquer-insulated CU wire should be pulled through hot and liquid wax.

Ok, this is a different transformer, but the manufacturer, style and time period (1952) are similar.
Yes, that is correct for the time period. The early CTV72/1 and BV511, hot wax was used to bond the individual coils. Bond magnet wire did not exist yet.
In 1996 Oliver designed his 1st winding machine to repair v-series transformers, using parts from pawn shops... sunbeam mixer, analog counter, sowing machine pedal and hollowed out aluminum block filled with wax and attached heater wire. (looks like it came out of a factory from the 50's)
Bond wire is more than double the price of non-Bond.

The pic's Murdock posted, appears the whole bobbin with coils were dipped in wax, and then stuffed with lam's. Or maybe painted wax on the coils after assembly, difficult to determine from the pic's.
 

GeorgeToledo

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I’ve seen pictures of that sunbeam setup and some of the shellac process for that matter…maybe on Oliver’s Facebook page.
 

Tubetec

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I vaguely remember CJ having a view on the wax potting thing in relation to V series input transformers . Probably to be found in one of his teardowns .

Some info here,

 

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