Telefunken ELA V1140 - Knock down gain

[amplexus]

Trying to make this old boy a useful tool. Right now there's so much gain when fitted with a mic input transformer that it just demolishes the thing- heavy distortion even with just an SM57. Noise floor is high as a result, so padding down pre transformer or pre first stage gives useful level but noise floor is basically unusable (Hiss, not hum- psu has been rebuilt and is quiet)
I'm nort sure how much play there is in the feedback network that controls gain- could potentially just altering the value of that 50k be all that's needed?

 

[rock soderstrom]

Trying to make this old boy a useful tool. Right now there's so much gain when fitted with a mic input transformer that it just demolishes the thing- heavy distortion even with just an SM57. Noise floor is high as a result, so padding down pre transformer or pre first stage gives useful level but noise floor is basically unusable (Hiss, not hum- psu has been rebuilt and is quiet)
I'm nort sure how much play there is in the feedback network that controls gain- could potentially just altering the value of that 50k be all that's needed?

View attachment 98146

Bridging the second EF12 by going directly from V1 to V3? (and take the NFB for V1 from V3)

Edit:

A less invasive mod would be to make V1 and V2 also pseudo triodes - I would also turn the NFB potentiometer first...

 

[Bo Deadly]

Where does it start to distort?

If it's not until V3, removing the 100uF on the cathode of V3 should knock it down quite a bit.

Update: That would result in a really wimpy output.

Note that I don't see a massive amount of gain here so you might want to sanity check things a little deeper with a scope if you can.

 

[rock soderstrom]

Note that I don't see a massive amount of gain here

I don't know the OT but with a proper mic to grid IT (1:10?) it adds up to quite a bit of gain.

 

[amplexus]

Where does it start to distort?

If it's not until V3, removing the 100uF on the cathode of V3 should knock it down quite a bit.

Note that I don't see a massive amount of gain here so you might want to sanity check things a little deeper with a scope if you can.

Its the ~15db of gain from the input xformer that’s not doing me a lot of favours.

 

[Bo Deadly]

Actually on second thought, you can't remove the 100uF because you won't get any power into the transformer primary.

I'm now inclined to agree with what rock soderstrom is saying but my concern there would be that the impedance of the OT primary is comparable to the 50K min feedback impedance which could easily cause all sorts of weird stuff to happen.

A simpler route would be to change the 1M grid resistor of V3 to a voltage divider but that would leave V2 just adding noise which would negate the point of using the step up IT.

You need a tube guy which is not me.

 

[abbey road d enfer]

Trying to make this old boy a useful tool. Right now there's so much gain when fitted with a mic input transformer that it just demolishes the thing- heavy distortion even with just an SM57. Noise floor is high as a result, so padding down pre transformer or pre first stage gives useful level but noise floor is basically unusable (Hiss, not hum- psu has been rebuilt and is quiet)

The first question is why the unusually high gain?
I would suggest the reason is that it's intended for very low level sources, such as low-Z mics, in applications where sound quality is not primorial.
I suspect this unit is part of a public address system.
Making it work with an input xfmr requires some reworking of the circuit.

I'm nort sure how much play there is in the feedback network that controls gain- could potentially just altering the value of that 50k be all that's needed?

You may try it, but there's a limit to the amount of NB you can apply this way. You may start with increasing the 100r cathode resistor.
Decreasing the 50k is not right, because it loads the plate too much - the 200k plate resistor won't like that.
Then you'd need to attenuate the gain of the output tube, which is about 26dB, becaue of the local NFB between plate and grid. The simplest solution would be to increase the 0.1M resistor significantly. The noise penalty should not be drastic, since the level there is high enough.
Alternatively, you can by-pass the final EF12 and connect the output xfmr in teh 2nd stage anode.

 

[amplexus]

The first question is why the unusually high gain?
I would suggest the reason is that it's intended for very low level sources, such as low-Z mics, in applications where sound quality is not primorial.
I suspect this unit is part of a public address system.
Making it work with an input xfmr requires some reworking of the circuit.

Yes, I knew I was asking for headaches by adding the input xformer, so I've made this bed for myself unquestionably! It was absolutely meant to feed a PA amplifier, this particular unit had a bunch of inputs on the outer chassis 'summed' with a bunch of pots, one of which was a 'projector' input, so almost definitely for a lecture hall or some similar such thing.

We have no particular need to be precious about keeping this original- just get an interesting sounding, useful tool, so reworking to almost any degree is perfectly ok!

You may try it, but there's a limit to the amount of NB you can apply this way. You may start with increasing the 100r cathode resistor.
Decreasing the 50k is not right, because it loads the plate too much - the 200k plate resistor won't like that.
Then you'd need to attenuate the gain of the output tube, which is about 26dB, becaue of the local NFB between plate and grid. The simplest solution would be to increase the 0.1M resistor significantly. The noise penalty should not be drastic, since the level there is high enough.
Alternatively, you can by-pass the final EF12 and connect the output xfmr in teh 2nd stage anode.

These are all great places to start, thanks! Will experiment this afternoon and report the results.

 

[tinn]

You have to check where and why the distortion happens.
An input Transformer of 1:15 is fine, cause you need that signal for getting
a decent low noise floor on V1. (the V77 has a 1:40 transformer and can take a SM57 vocal mic)
The gain can be high with the 1 Meg pot but should be fine on minimum setting.
To reduce the gain I would change the 100 Ohms (V1) to 1 k and parallel the 2.5k with 2.2 k to
keep the bias on the old level. This will reduce the gain almost to a tenth.
Check the .5 uF in the feedback path as well. Draw a level diagram throughout the amp.
Without scope and generator it will be difficult though.

 

[abbey road d enfer]

You have to check where and why the distortion happens.

There is no doubt that distortion comes from excessive overall gain.

An input Transformer of 1:15 is fine, cause you need that signal for getting
a decent low noise floor on V1. (the V77 has a 1:40 transformer and can take a SM57 vocal mic)

This type of transformer tends to become unobtainium. Most manufacturers tend to restrict to 1:10 for mic inputs.

The gain can be high with the 1 Meg pot but should be fine on minimum setting.
To reduce the gain I would change the 100 Ohms (V1) to 1 k and parallel the 2.5k with 2.2 k to
keep the bias on the old level. This will reduce the gain almost to a tenth.

Even then the gain would be about 60dB, and the output would clip for about 7mV at the input.

Draw a level diagram throughout the amp.
Without scope and generator it will be difficult though.

No need for that. The schemo is enough to do it.
I agree that a level diagram is an essential piece of information, though.

 

[Bo Deadly]

You could make V3 a cathode follower and either put the OT primary in the cathode path or drive the OT primary through a cap (like the 100uF).

 

[abbey road d enfer]

You could make V3 a cathode follower and either put the OT primary in the cathode path or drive the OT primary through a cap (like the 100uF).

Inded it's another possibility.
So many ways to skin the proverbial cat...

 

[Bo Deadly]

Here's one scenario:



Bear in mind, I'm not a tube guy. Be careful. Without the 1K for example, you might burn out your OT.

 

[amplexus]

So actually 90% of the battle was taken care of just by increasing the 100R cathode on V1 and unbypassing the 2k2. I guessed the obvious that the bulk of the issue was in hitting V1 with an additional 15dB of gain from the input transformer. It was partially clipping V1, but also then the output of V1 just demolishing V2 at any appreciable levels. V3 seemed to take it all in stride.

So knocking down the gain in that first stage did wonders for the noise floor and the gain issue. I did also take the suggestion of increasing the 100k grid input resistor on V3 (to 220k), which just helped to tame the output level a touch. Also installed phase and 20db pad switches pre-transformer so theres some extra gain staging options there too.

That said there's still an easy clean ~60db of gain from input to output. -48dbV at the input yields +17dbv at the output unclipped. Quite respectable I think.

Sounds great with an RE20, bass di'd, tried running some mono drums through it to get some 'hair' on em and it does something pleasing to the tracks for sure. Th input transformer is a Urei B11178 which I'm sure helps there.

Thanks for all the advice everyone!

 

[abbey road d enfer]

Here's one scenario:

View attachment 98233

Bear in mind, I'm not a tube guy. Be careful. Without the 1K for example, you might burn out your OT.

Actually the DCR of the primary should be accounted for. It's probably close to 1k.

 

[vinyvamos]

Bo Deadly, I don't think that circuit suggestion will work as the grid1 bias needs to be much higher from ground. Glad you got it sorted though amplexus.

 

[abbey road d enfer]

Bo Deadly, I don't think that circuit suggestion will work as the grid1 bias needs to be much higher from ground. Glad you got it sorted though amplexus.

That would be necessary if the cathode load was a resistor, but when the load is an inductor (in this case the primary of a transformer) it is not necessary. The bias current magnetically loads the core. When the tube is driven negatively, the inductor resistutes energy.
One must be aware that, as with a common-cathode stage, the voltage swing can potentially be twice the B+ voltage., which may present a risk of exceeding the cathode-to-plate voltage.

 

[jokeramik]

It will be a good way to change all old electrolytics. Then connect a generator to the unsymmetrical input and check the signals on the grids. The you can find the stage which doesn´t work. And you know that your amplifier is ok or doesn´t.

Many transformers need a coorection network on their secondary winding. Other not. Transformer from the former RFZ runs under no load conditions. Jensen tranformers need this correction network for best impulse and frequency response.

At first it will be a good way to use a 1:1 transformer.

Best regards!

jokeramik

 

[abbey road d enfer]

It will be a good way to change all old electrolytics.

+1

Then connect a generator to the unsymmetrical input and check the signals on the grids. The you can find the stage which doesn´t work. And you know that your amplifier is ok or doesn´t.

Have you read the other posts? The cause of the issue is clearly identified now. Too much overall gain.

At first it will be a good way to use a 1:1 transformer.

No, because it would not fix the noise issue, which can only be solved by putting a step-up xfmr at the input, but adding about 20dB of transformer gain necessitates a realignment of the gain structure.

 

[jokeramik]

Have you read the other posts? The cause of the issue is clearly identified now. Too much overall gain.

I think that´s not the right way.

Do what you like.
Best regards