REDD.47 a few questions.

Greetings to all.

I have some questions on REED.47

1. In the original scheme, most resistors are designated as - H.S.
Others, like C.
What types of modern resistors can replace H.S. type and type C ?
And how much will the 1/8W old notation be in the modern equivalent?

2. REDD.47 received power from an external source - console REDD.51
If I understand correctly, H.T. volage REDD.51 - it is unstabilized 380 - 400v, and stabilization is built into the amplifiers REDD.47.
Probably in the REDD.51 console it's just a diode bridge or a diode bridge and a capacitor, or something different ?

I would be grateful for the answer.

You can use metal film for both. 1/8W is one eighth watt or 0.125 watts in today's money. I would recommend using MR25 types.

The RED47 HT is internally stabilised. There was big external HT and heater supply box. I have never seen a schematic for this but the HT will certainly be some form of bridge rectifier (tube or SS I don't know) followed by a reservoir capacitorand possibly  one or more stages of RC smoothing. It needed to supply a substantial current  so there is a good chance they use a Metal Oxide rectifier or something similar.

Cheers

Ian

Thank you.

I still have little experience in assembling lamp circuits, so there are a few more clarifications.

- On the forums write that in tube circuits, the more power (larger size) of the resistor - the less noise.
Maybe it's worth using 0.5W instead of 0.25W, or is this information misleading me?


- Can you please tell me if it is possible to make feedback p47, as in this drawing?

vmanj said:

Thank you.

I still have little experience in assembling lamp circuits, so there are a few more clarifications.

- On the forums write that in tube circuits, the more power (larger size) of the resistor - the less noise.
Maybe it's worth using 0.5W instead of 0.25W, or is this information misleading me?

Click to expand...

Excess noise, caused by dc voltages across a resistor, are slightly lower with higher power resistors. The only resistors this really affects are the anode load resistors and the one in the second stage is already specified as high wattage for reasons of power dissipation.

- Can you please tell me if it is possible to make feedback p47, as in this drawing?

Click to expand...

It is possible but why would you want to do it?

Cheers

ian

I am grateful to you for answering my questions.

I bought a DIY REDD.47 preamplifier and there was already such a feedback scheme (and some differences from the original circuit).
So I decided to make it closer to the original.

The preamplifier works, but I do not know, maybe because of this feedback, it does not sound like it should.

vmanj said:

I am grateful to you for answering my questions.

I bought a DIY REDD.47 preamplifier and there was already such a feedback scheme (and some differences from the original circuit).
So I decided to make it closer to the original.

The preamplifier works, but I do not know, maybe because of this feedback, it does not sound like it should.

Click to expand...

It will make a difference to the sound but it may be too small to hear. Hard to say. The capacitor and the output transformer primary inductance form a resonant circuit which can cause a bump in  very low frequency response. With the feedback taken from the junction of the capacitor and the transformer, this effect should be reduced. With the separate feedback capacitor it will not be. Perhaps the previous owner wanted a little extra bass.

Cheers

Ian

Yes, you are right, with a decrease in the gain, the bass becomes very strong.
I still want to change this closer to the original and remove the extra capacitor from the feedback.

In my preamp, there were still significant changes from the original scheme:
- Anode voltage instead of 290 Volts - reduced to 156 Volts.
- The input and output transformer is hand-made.
The input has a ratio of 1: 4 instead of the prescribed 1: 7.

I want to order transformers for РЕДД.47 from a Sowter.
And remake the power supply to get H.T. 290 volts.
There is no oa2 in the scheme.
I think this affects the sound not for the better ?...

Sowter transformers will certainly improve the sound.

The HT voltage is supposed to be 290V. 156V is much too low. I can see no good reason for this.

The most important thing s to have a well smoothed HT supply (the actual value is not critical). Today this is easy because large value high voltage electrolytic capacitors are available cheaply. When the REDD47 was made these were not available so the OA2 and friends were often used. This is a class A circuit so the average current draw is constant (independent of signal level). With sufficient smoothing capacitance you do not need the OA2s. You can use a circuit like this one for example:

https://drive.google.com/file/d/0B_n67A1hN3qtVlAxdXNlWEJwUUU/view?usp=sharing

(Ignore the 75V output, you don't need it)

Cheers

Ian

Hello.
Thanks for the comment.

This is the high-voltage part of my preamplifier.
Can you please tell us what can be changed in this scheme (without changing the 6C4P to a solid-state diode bridge), what would get 290 volts ?

The only real option is to change the HT transformer secondary to a higher voltage.

Cheers

Ian

I'm understood, thank you.

I decided to replace H.T. transformer secondary to a 270-300v, think this will be enough.

Tell me please, what power (mA) will be enough for the secondary ?

vmanj said:

I decided to replace H.T. transformer secondary to a 270-300v, think this will be enough.

Tell me please, what power (mA) will be enough for the secondary ?

Click to expand...

Hi, one channel of REDD47 will draw around 30mA (the original 2xOA2 Version), with safety margin I guess 45-50mA would be on the right side.

I have thought about you current PSU an I think something is odd here. Your transformer secondary provides 245VAC and your HT is only 156V? 

You say: "Anode voltage instead of 290 Volts - reduced to 156 Volts."

Anode voltage means HT= voltage after the OA2 "stabiliser" in the original schematics 290V or the voltage at the plates of your tubes?

How big in H is your inductor? How big is the resistance in ohm?

Do you really need that 3k resistor in front of the inductor?

I'm not very experienced in electronics, so I can not exactly answer all the questions.
The preamplifier I bought - it turned out with changes in the circuit, so I want to bring its scheme closer to the original one.

I measured the voltage under load, after the inductor and it was 156 volts.
The inductor has a resistance of about 600 ohms.
For what the resistor R3 is installed I do not know.
But can reduce its value (or completely remove it), and the anode voltage will rise? But I'm not sure of the safety of this.

It's also unclear why the R1 and R2 resistors are needed?
And why C2 is not electrolytic ...

I think R3 is much too high. If the unit draws 30mA then there will be 90V dropped across it. It was possibly done because C2 is higher than the recommended 10uF maximum. I recommend you change C2 to a 10uF 450V electrolytic and reduce R3 to 300ohms. This will increase the HT voltage but you may also get more hum.

If it were up to me I would do away with the  tube rectifier altogether. I do not believe it makes any contribution to the sound.

Cheers

Ian

I think that this was done for a smooth supply of anode voltage.
And about the sound, I do not know how much better a tube or solid state?
In some forums they write that even oa2 make a sound ...
I do not really trust this, because Chandler does it on a solid-state circuit.

If I replace C2 by 10uF 450V and R3 by 300ohms, how did you write, then the voltage can rise to 290v ?


But it's still easier for me to change the circuit to solid state.
If I do this scheme https://drive.google.com/file/d/0B_n67A1hN3qtVlAxdXNlWEJwUUU/view?usp=sharingng
My transformer will be enough to get 290v ?

Yes, I think it is the best way to do it with solid state diodes. You will come close to the 290V, you can define that with your resistors. Even if you are 20 Volts below or above, it will work without any audible difference. The most important factor is how clean is your HT.

"I'm not very experienced in electronics"

Be careful, you are working with lethal voltages! You caps will store this voltage for long time after you have switched off your amp if you work without bleeder resistors! Check everytime after you have switched off your amp how much tension is stil there.

Work clean and safe, keep one hand in your pocket. Don´t kill yourself!

Thank you, of course, I follow the safety rules.
I meant that my knowledge of electrical circuit calculations is not very strong.

With silicon diodes, there will be very little voltage drop and the voltage across the reservoir  capacitor will reach close to the peak of the secondary waveform i.e. 245 x 1.414 =  346 volts To drop this to about 295 you need to set the series resistors in the following RC filters to drop about 45V at the normal load current. If this is 30mA then the total value of the resistors should be 45/30 = 1.5K. Divide it equally between the resistors.

Cheers

Ian