V241 Restoration

[smilan]

Hi,

I have a couple of Lorenz V241 preamps for service.

I measured all the resistors / capacitors and many need to be replaced.

In general, does the big oil capacitors need to be replaced over a 60 years period (like electrolytics) or can stay if measured good?

One of the 1uF oil caps (C4) needs to be replaced, which type of modern capacitor would be the right replacement for it?

Also I would like to replace all the 20uF/450V filter caps. Is it possible to find new capacitors with this type of screw / nut mounting arrangement (like the one in the picture) or should I remove the old ones and install terminal strips and new axial F&T's instead?

 

[beatnik]

For the electrolytics look for F&T SZ series, available in EU from Reichelt and Distrelec

Also this store has a very good selection of capacitors for vintage restorations and might stock the value you need

https://www.die-wuestens.de/eindex.htm

 

[rock soderstrom]

Ask Jan First ® ; electron tubes and more

+1

If you have some time to spare, could you please take some nice photos of the V241s for us?

 

[smilan]

Here are some photos, the components that needs to be replaced are debouched at one point from the circuit.

 

[smilan]

I got the new resistors and capacitors for the Lorenz V241 / 300 preamp.
The voltage readings are way too high comparing the the schematic that I have.
With the Variac set to 215V I get a filament voltage of 6.4V, 275V (instead of 250V) at the plates of the ECC865 and plate voltage of 143V (instead of 90V) at the EF86.
The mains power here is 230V so it won't be a good idea to run it on such high voltages.
This preamp had similar symptoms with the old components as well, so I don't think I made a mistake while replacing the old components.

Should I play with the resistors values to get closer to the voltages of the schematics?
Any other ideas how to make those preamps to work in the correct voltages?

 

[rock soderstrom]

Have you already changed all the tubes, including the EZ80? Otherwise, of course, you could increase W17 to bring the voltages down. There is still the high heating voltage...

 

[emrr]

Old weak tubes will raise voltage. Sometimes an old weak rectifier tube is a nice cheat, though it may not last long.

Oil caps may last forever, what you want to know is the ESR. If they’ve been well used it may be very high, then you should replace. If they are high ESR but correct capacitance, you might risk a parallel capacitor to save the trouble of sorting out a new mounting method. Not necessarily advised….

 

[tinn]

I would check the old capacitors and it is possible that most of them still work fine
for the next 40 years. My experience.
I would put a small resistor (around 220 Ohms) in series with the mains Fuse (Si1) to
get the heating voltage down to 6.3 Volts. As well the mains transformer does
not get in saturation with higer mains voltage then.
I would not be worried too much about the Hi Plate voltages.
as mentioned by emrr old tubes can be the reason.
Check the current draw according to the voltages at C14, C15, C16.

 

[smilan]

Update:

I've installed few different tubes (rectifier and amplification tubes) and still the voltages are too high comparing to the schematic, with feed of 220V from the variac to the power transformer, the 6.3 filament voltage was 6.58V, the plate voltage of the ECC865 was 264V (250V at at the schematic) and the plate of the EF86 was 123V (90V at the schematic), the screen of the EF86 was 61V (90V at the schematics).

With 213V coming from the variac the 6.3 filament voltage was 6.3V, the plate voltage of the ECC865 was 268V (250V at atthe schematic)and the plate of the EF86 was 120V (90V at the schematic), the screen of the EF86 was 60V (90V at the schematics).

You can see all the other voltages on the picture:



I have no idea if the voltages of the schematic are wrong / there's a problem with the tubes / other components.

The biggest difference between the schematic and the measurements I took was at the EF86.

Do you think this difference is critical / it will be safe to work with it with a variac at 213V?

 

[volker]

I've installed few different tubes (rectifier and amplification tubes) and still the voltages are too high comparing to the schematic, with feed of 220V from the variac to the power transformer, the 6.3 filament voltage was 6.58V, the plate voltage of the ECC865 was 264V (250V at at the schematic) and the plate of the EF86 was 123V (90V at the schematic), the screen of the EF86 was 61V (90V at the schematics).

All of that is perfectly fine and within usual tolerances, except for the EF86. I would try a couple different ones and see if it changes.

 

[smilan]

All of that is perfectly fine and within usual tolerances, except for the EF86. I would try a couple different ones and see if it changes.

Thanks, all the four vintage EF86 seem to be faulty, with a new 6*32n tube the plate voltage of the EF86 is 85V and the screen is a little bit too low at 65V.
Do you think it will be helpful to reduce the value of W5 and raise the screen voltage or will it be OK to run it at 65V?

 

[jokeramik]

Where do you come from?

Do you measuring the signal with a oscilloscope? Sometimes these preamps are oscillating. Also look on the anode and the grid of Rö 1.

C 8 (cathode bypass capacitor) must be changed for right function. Also all other electrolytics. They are very old and they are out of capacity and function.

After these long times also foilcapacitors also like to fail.

I think up to 5% difference of the voltage measurements are ok. At these times the tolerances of the components between 5% and 10%. The measurement instruments at these times w

Don´t change the resistors in the RC-filtering. This is problematic for the operating points of the the tubes.

The measuring instruments at that time were analogue and not digital, so the readings were never exactly accurate to 3 or 4 digits.

Best regards!

 

[smilan]

Where do you come from?

Tel Aviv, Israel

Do you measuring the signal with a oscilloscope? Sometimes these preamps are oscillating. Also look on the anode and the grid of Rö 1.

I didn't connected the audio jacks yet, I would like to make sure that the voltages are right before testing the audio.
The plate resistor of Rö 1. tested good as well as W1 (the grid leak resistor).

C 8 (cathode bypass capacitor) must be changed for right function. Also all other electrolytics. They are very old and they are out of capacity and function.

All the electrolytics has already replaced, as well al all the resistors that measured the wrong values (+/- 10% or more)

Don´t change the resistors in the RC-filtering. This is problematic for the operating points of the the tubes.

I replaced one of those resistors since it was out of spec.

 

[jokeramik]

Tel Aviv, Israel

I didn't connected the audio jacks yet, I would like to make sure that the voltages are right before testing the audio.
The plate resistor of Rö 1. tested good as well as W1 (the grid leak resistor).


All the electrolytics has already replaced, as well al all the resistors that measured the wrong values (+/- 10% or more)

I replaced one of those resistors since it was out of spec.

At these times 10% tolerance of the resisitors where ok and this was the specs.. We talk at 1950 or 1960. Electrolytics at the present time have minimum +/- 10 % tolerance.

Do you make a measurement with an scope? Sometimes the amp can oscillate. You can see oscillating only with a scope. Also a problem can be the rotary switch. This part is very important for the stability of the amp.

At the pic the C4 is an old one.
Best regards!

 

[smilan]

Now with the new 6*32n tubes and new caps in both preamps both of them works at the right voltages and sound great.
This is no doubt one of the best sounding mic preamps I've ever heard.
There's one issue with one of the units, when the +48V phantom power is engaged there are some "frying" sounds coming from this channel.
First I was thinking that something with the phantom switch went wrong, so I replaced the location of the two channels and the noise went with the preamp, so it's not the phantom switching network, it's something inside the preamp.
I've also tried to swap the 6*32n tubes between the two preamps with no change with the noise.
What else should I check to try to eliminate this issue?

 

[smilan]

Update:
I was able to cancel the noise by adding two .47 coupling caps between pins 2 and 3 of the input XLR to the primary side of the input transformer.
Do you think it's a good idea to add those caps?
Also it's interesting why this noise accusers only in one unit? Why the other one does not react the same to the DC power on the primary side of the input transformer?

 

[jokeramik]

Now with the new 6*32n tubes and new caps in both preamps both of them works at the right voltages and sound great.
This is no doubt one of the best sounding mic preamps I've ever heard.
There's one issue with one of the units, when the +48V phantom power is engaged there are some "frying" sounds coming from this channel.
First I was thinking that something with the phantom switch went wrong, so I replaced the location of the two channels and the noise went with the preamp, so it's not the phantom switching network, it's something inside the preamp.
I've also tried to swap the 6*32n tubes between the two preamps with no change with the noise.
What else should I check to try to eliminate this issue?

Be carefully! Due to the high ratio of the input transformer (1:18), it is necessary to slowly ramp up the phantom power. This should be very well stabilized and clean. The phantom power resistors (6,81kOhm) should also have very close tolerances (0.01% or better to each other). I recommend the modules and resistors from Funk-Tonstudiotechnik Berlin. These are the best power supplies I've ever used. Starting up too quickly can destroy the input-transformer very quickly (law of induction).
Best regards!

 

[jokeramik]

Update:
I was able to cancel the noise by adding two .47 coupling caps between pins 2 and 3 of the input XLR to the primary side of the input transformer.
Do you think it's a good idea to add those caps?
Also it's interesting why this noise accusers only in one unit? Why the other one does not react the same to the DC power on the primary side of the input transformer?

You are at low impedances. I recommend you to use coupling capacitances around 100µF/63V. Coupling of the inputs via the phantom power or grounding problems?
Do you have an oscillogram?
Best regards!

 

[smilan]

Be carefully! Due to the high ratio of the input transformer (1:18), it is necessary to slowly ramp up the phantom power. This should be very well stabilized and clean. The phantom power resistors (6,81kOhm) should also have very close tolerances (0.01% or better to each other). I recommend the modules and resistors from Funk-Tonstudiotechnik Berlin. These are the best power supplies I've ever used. Starting up too quickly can destroy the input-transformer very quickly (law of induction).
Best regards!

Good point, I found this circuit online, do you think it will be good for this application?

I'm pretty sure it's not the resistors that making on preamp to be noisier then the other one, I tested both preamps with the two inputs / outputs sets and in both positions the same preamp was noisier comparing to the the other one.

You are at low impedances. I recommend you to use coupling capacitances around 100µF/63V. Coupling of the inputs via the phantom power or grounding problems?

I replaced the coupling capacitors to 220uF/100V.

Do you have an oscillogram?

I have a Tektronix scope, I can take a screen shot with my phone camera, what should I look for?

 

[jokeramik]

Good point, I found this circuit online, do you think it will be good for this application?

I'm pretty sure it's not the resistors that making on preamp to be noisier then the other one, I tested both preamps with the two inputs / outputs sets and in both positions the same preamp was noisier comparing to the the other one.


I replaced the coupling capacitors to 220uF/100V.

I have a Tektronix scope, I can take a screen shot with my phone camera, what should I look for?

Look at the waveform of the noise.
Best regards!