RFT/VEB MV4056

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valco

Well-known member
Joined
Sep 14, 2010
Messages
48
Location
Norway
I've been pulling my hair out on this one for some time now. So I really need some good advice.

I have installed an input transformer and a 100k pot between C1 and the grid on EC92, but there's still insufficient gain to get a decent signal into the daw.

What is it that I don't understand here? Is it the original design that's not powerful enough? Or is it a impedance problem?
 

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Yes, because the 100k Ohm is a too heavy load in here...

What is the input impedance of the PC/soundcard ? The PC92 can't handle it probably.

A buffer stage or matching transformer would be needed.

Also the value of the coupling capacitors (10nF and 50nF) is too small for that.
 
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Is the internal resistance :11K ohm or the grid resistance :1 M ohm that is the input impedance of the EC92?

Do you suggest to replace the coupling caps with higher values ?
 

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If you replace the 1M resistor with a 100k resistor then the capacitor needs to be bigger. But that would only change the frequency response not the issue of the mismatch of the impedance. Do you need to reduce the gain here ?

First try the amp with the original values. As mentioned before it could be that your soundcard or input device for your DAW has a too low impedance for this amp anyway. In this case a buffer or matching transformer is needed. Look at schematics of the V72/V74 etc.
 
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Here is a brief translation of the original description:

Due to its input circuit, the compact, small preamplifier is particularly suitable for high-impedance AF sources, e.g. crystal microphones (piezo technology) or pick-ups of electric guitars.
By using the triode wired EF 86 pentode with low microphonics, hum and noise in the input section, a good signal-to-noise ratio is achieved.
The output stage (EC 92) has an output impedance of approx. 16 kΩ, which means that 'longer' connecting cables can be used without losses in the high frequencies.
(..but only in the old days when the following transformerless tube input was very high impedance!) Three-pin diode sockets are provided as input and output.
The potentiometer on the rear panel, colloquially referred to as the 'humdinger', is used to balance the 'virtual center tab' of the tube heater, thereby reducing possible hum pickup.
These models were later manufactured at VEB Funkwerk Kölleda
.

Thus, this circuit is to be described in the original as a high-impedance instrument amplifier.
If you want to use this amp as a microphone amplifier, you need an input transformer with a 1:7 or higher transfer ratio.

Which transformer do you have at the moment?

As it was said before, the 100k pot is a very big load for the EF86, 500k or 1M would be better. Basically, a different gain control might be useful. A variable output pad could be more useful.

The second tube is not very current potent, but could work with a proper transformer (and a $$$ anode choke). You could replace the EC92 (half an ECC81) with an EC90 (something like half an ECC82). You would swap gain for current.

An output transformer would be helpful to drive low impedance inputs. What interface or mixer are you feeding? The EC90 would be better suited to drive cheaper transformers. In that case W5 needs adjustment.

C2 definitely needs to be increased, depending on the load.

Have you done a general health check of the amp, checked all voltages, are the capacitors still OK? A leaky coupling capacitor can shift the bias of the following tube, so that not much signal arrives at the output. Check C1!
 

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So according to that description, maybe its more suitable as a tube DI.

I tried with a Beyer 1:5 first, but that didn't give much signal on the test bench. So I tried an unmarked old mic transformer I had lying around. Its probably 1:10

Im using a Orion antelope which has 4 x D-SUB 25 (32 channels total), +20 dBu max, 11.2 kOhms input impedance. So that should work

What ratio would be suitable as an output transformed? I see that the REDD 47 has a EF86 and uses 1:7 on the input and 7:1 on the output.
 
The REDD has a beefier buffer for the output transformer. So you will need a transformer like 20:1 or so on the EC92. Why not add a buffer (can be a seperate unit not to modify this one) with a high impedant input ( can be solid state, so is your PC anyway).
 
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