REDD.47 a few questions.

[vmanj]

Thank you.

If I understand you correctly, then it can look like on the drawing and the number of  RC filters  only affects the quality smoothed HT ?

 

[ruffrecords]

Thank you.

If I understand you correctly, then it can look like on the drawing and the number of  RC filters  only affects the quality smoothed HT ?

That is exactly right. The dc voltage drop is the same in both cases. Even if you made the total capacitance the same in both cases, the version with more RC stages achieves much lower ripple. This was well known to the ancients. A guy called Scroggie wrote an article on it in Wireless World in 1949:

https://drive.google.com/file/d/0B_n67A1hN3qtUWJDRFlnenlZODg/view?usp=sharing

Cheers

Ian

 

[vmanj]

I think I'll follow your advice and change my power supply to solid state.
But I still want to remove the inductor L1 from the circuit, I hope this does not worsen the sound.

 

[ruffrecords]

I think I'll follow your advice and change my power supply to solid state.
But I still want to remove the inductor L1 from the circuit, I hope this does not worsen the sound.

L1 will make no contribution to the sound.

Cheers

Ian

 

[vmanj]

Greetings to all.

There is one more question.
What is the power of the anode resistor ECC88 - 8K2?
In the schemes that I found on the network, this information could not be read (poor photo quality).

 

[ruffrecords]

Greetings to all.

There is one more question.
What is the power of the anode resistor ECC88 - 8K2?
In the schemes that I found on the network, this information could not be read (poor photo quality).

As I recall it is several watts. Not hard to work out the dissipation (volts across resistor times current through it) and add a safety factor of say 100%.

Cheers

Ian

 

[vmanj]

Thank you.

It turned out slightly more than 5W.
I think 8-10W will be enough.

It is also not clear from the voltage of the capacitors 0.47u, 0.022u and 1u.
Is it possible to supply a not very high voltage of capacitors, for example 200V, or is it necessary to 350-400V ?

 

[ruffrecords]

It is also not clear from the voltage of the capacitors 0.47u, 0.022u and 1u.
Is it possible to supply a not very high voltage of capacitors, for example 200V, or is it necessary to 350-400V ?

Worst case is at switch on when the tubes are cold and do not conduct. You should also consider the case where a tube is faulty and does not conduct. In this case, the 1uF can certainly reach the full HT voltage. To be safe I would recommend using 400V types for all three.

Cheers

Ian

 

[vmanj]

Thanks I got it.

 

[vmanj]

Hello.

I have a question about Rumble Filter in REDD47.

Scheme (1) - original connection Rumble filter.

Can the sound deteriorate, if the shunt resistor P24 is made non-disconnectable from the primary winding of the input transformer?

 

[ruffrecords]

In some cases yes. Not disconnecting it means the input impedance is lowered and this will affect the frequency response of some microphones.

Cheers

Ian

 

[vmanj]

Thank you, in that case I will follow the scheme (1).

In my REDD47 there is one more inaccuracy, which I probably have to change.
In the figure, I indicated this.

I can not understand for what purpose the author of my preamplifier changed the input circuit so much ?

One clarification: - input transformer of its own production.
Measuring its exact characteristics I can not, but judging by the DC Resistance, it's 1: 4 instead of 1: 7.

 

[ruffrecords]

You cannot really tell tell the turns ratio by measuring dc resistance of the windings because usually the secondary is made of thinner wire if the transformer is a step up type. You could try applying a small signal to the primary and measuring the primary and secondary voltages in order to work out the actual turns ratio.

Tube mic pres really need a good step up at the input for good noise performance. 4:1 is not really enough. 10:1 is the normal value.

It is difficult to say why the circuit has changed the way it has. The 200 ohm series resistor is not needed and will only increase the noise sightly. The other components depend on the catual transformer.

Cheers

Ian

 

[vmanj]

I thought that it can be calculated (approximately) by formula:
√ RDC(sec)/RDC(pri) - ?

 

[ruffrecords]

I thought that it can be calculated (approximately) by formula:
√ RDC(sec)/RDC(pri) - ?

I do not know where you picked that up but it is not true.

Cheers

Ian

 

[vmanj]

I found this formula on the net.

This does not give an exact answer, but one can only have an approximate idea of the transformation ratio.

I will give some examples:

Jensen Transformers
---------------------
JT-11K8-APC
Microphone Input Transformer
1:3.5 STEP-UP FOR LOW-NOISE FET AMPLIFIERS

DCR Pri - 26.2 Ohm
DCR Sec - 257.5 Ohm

√257.5/26.2 - 3.1 ≈ 1:3.1
-------------------------------

JT-13K7-A
Microphone Input Transformer
1:5 STEP-UP FOR MEDIUM IMPEDANCE AMPLIFIERS

DCR Pri - 17.4 Ohm
DCR Sec - 471 Ohm

√471/17.4 - 5.2 ≈ 1:5.2
-----------------------------

JT-115K-E
Microphone Input Transformer
1:10 STEP-UP FOR HIGH IMPEDANCE AMPLIFIERS

DCR Pri - 19.7 Ohm
DCR Sec - 2465 Ohm

√2465/19.7 - 11.1 ≈ 1:11.1
----------------------------------

And my transformer:

DCR Pri - 31 Ohm
DCR Sec - 477 Ohm

√477/31 - 3.9 ≈ 1:3.9
----------------------------

In my opinion it works, what's wrong here?

 

[ruffrecords]

Several of things wrong . It only works if the wire gauge used on the primary is the same as the gauge used on the secondary. Even then it is not likely to be very accurate because the length per turn of the larger secondary varies as the the winding gets bigger so the resistance is larger than you would expect. It might work in a few lucky cases but it cannot be relied upon as a general rule.

Cheers

Ian

 

[vmanj]

And I did not say that this is an exact calculation.
On the contrary, this is a conditional approximate representation of the transformation ratio.
In all three of the examples I cited, it works (of course, approximately), or all three examples are a coincidence?

Maybe I'm wrong and it's really not right, then please show your examples where this formula will not work.

 

[radardoug]

As usual, Ian has given you good advice, and explained the reasons. Please re-read his answers and understand them fully.
The only way to get an accurate figure for the ratio is to either have full winding details or do a voltage measurement.
To do this, apply an a.c. voltage to one winding, measure the voltage applied, and then measure the voltage across the second winding. 
Please bear in mind that people on this forum are giving you the benefit of their wisdom, which does take some time. Do not ask them to give you examples when they have already given you all the information you need. Rather learn from their wisdom, and find your own examples. Or actually measure some transformers.

 

[bluebird]

UTC A-24 is a 5:1 but your formula produces 4:1

UTC A-26 is a 7.5:1 and your formula produces 5.4:1

So the formula can be off especially when dealing with higher ratio transformers.  Jensen mostly makes lower ratio transformers where the wire gauge for the primary and secondary will be more similar than higher ratio transformers.


* Transformer data courtesy of master butcher, CJ...