Loading the outputs of a tube amp

So I've got an Altec 356A on the bench, it is a little PA head with mic and line in and a pair of EL84 (it claims 18 watts) on the output.  There are output taps at 8, 16, and 250 ohms. The 250 is for 70v lines.  Of course the owner wants to use it as a mic pre with minimal modification.  Would it be possible to load the 16 ohm tap with a 25w 15 ohm resistor and then use the 250 ohm tap as an unbalanced out?

I am not sure about using the 250 ohm tap. 18W into 250 ohms is 67V rms which is what you expect because this is a 70V line output and it will be this level even if you connect an 18W load across the 16 ohm tap. For 18 watts, the 16 ohm tap will still give nearly 17V rms which is which is nearly +27dBu which is more like the max level you would expect from a regular line output.

So load the 16 ohm output by all means but take the output from there as well.

Cheers

Ian

Hi Rippe ,
Something like that might be possible ,but you stand a good chance of redlining or blowing up anything you feed it into from the 70 volt line output . Id say use the 16 ohm output with the load as you said but build an attenuator with a couple of more resistors to knock back the levels a bit ,probably no harm to do a bit of work on reducing hum in the psu as well ,big new electrolytics will help here. Its going to be one power hungry pre-amp either way ,but you know what, it could end up sounding nice too .

All good points, I'd compare response from the 16 versus the 250, it may be that the 250 is better and would be ideal with a U pad buildout. 

Thanks all for your replies and advice.  I will load the 16 ohm tap and try it into 600 and 10k inputs.  I will also check the 250 ohm tap.  It makes sense that the smaller step down would result in higher levels as Ian said, but this thing will never be run flat out as there is plenty of gain even before the EL84 stage.  I would opt for picking the signal off earlier in the circuit, but the owner wants minimal modifications.

Also, I drew out the preamp section of this amp if anyone ever needs it.  Could not find a schematic online.

mjrippe said:

Also, I drew out the preamp section of this amp if anyone ever needs it.  Could not find a schematic online.

Click to expand...

I'd like to glance at  it if you are offering...

Thanks!

mjrippe said:

Thanks all for your replies and advice.  I will load the 16 ohm tap and try it into 600 and 10k inputs.  I will also check the 250 ohm tap.  It makes sense that the smaller step down would result in higher levels as Ian said, but this thing will never be run flat out as there is plenty of gain even before the EL84 stage.  I would opt for picking the signal off earlier in the circuit, but the owner wants minimal modifications.

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Tube power amps are not very quiet as a rule. You need to run it at a decent level in order to get  a reasonable signal to noise ratio. If you run it from the 16 ohm tap at a nominal +4dBu then you have plenty of headroom (over 20dB) but the noise will be 20dB worse. If the noise is80dB below 18W (not a bad spec for a tube power amp) then at +4dBu from the 16 ohm output the noise will be -60dBu - not so good. You may need to consider reducing the gain.

Cheers

Ian

If you could share your final noise results with the pad in place while driving a mic level sine wave in I would like to see where you end up. I was thinking about doing something very similar recently, using a power amp style push pull section and then knocking it back at the transformer or with a wire wound set of R's forming a U or H pad

Good point, Ian.  Spec sheet attached shows the figures are much worse.  Also 113dB gain available on mic input is ridiculous.

And the preamp schematic...not my best drawing but it should be accurate.

You could porbably go in straight into the 0.1uF just before  the 200K pot.

Cheers

Ian

113db of gain on a microphone preamp? That has to be a new record!

Is there any feedback around the el84s?

iampoor1 said:

113db of gain on a microphone preamp? That has to be a new record!

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Standard for tube consoles.

emrr said:

Standard for tube consoles.

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Interesting. Is that an old broadcasting standard?

And I thought Ampex 351 preamps with 90db of gain were hot!

iampoor1 said:

113db of gain on a microphone preamp? That has to be a new record!

Is there any feedback around the el84s?

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Yes, there is feedback to the second 12AX7, I did not include it in the drawing.

I did some testing with the 15 ohm 25 watt resistor across the 16 ohm tap, it did not even get warm.  I could easily get +4 (and much more) from either the 16 or the 250 ohm taps, but I could not get a THD+N of better than -35dBU or about 1.8%.  This is a 1kHz signal from a 150 ohm source into a 600 ohm load with the 400Hz high pass enabled to rule out hum components.  Not really surprised.

Then I talked to the owner and he agreed to let me ditch the power tubes and pick off the signal from the preamp.  Of course I'll have to adjust the power supply a bit as the voltages rise without the load of the EL84s.  I had considered picking the signal off just before the 200k pot, as Ian suggested.  However the owner wants this to be a "color" preamp so I was thinking of changing the second 12AX7 to a 12AU7 and taking the output from the first plate, or even making it into a cathode follower.  That way one could open up the first pot and turn down the second to get some (hopefully pleasant) distortion.

So the new questions: Pros and cons of single triode output vs cathode follower?  Good inexpensive transformer options for either?

Whether you can use a plate output or need a cathode follower or something similar depends on what you are going to feed it into and how far away it is.

Cheers

Ian

Would replacing the output transformer with a "proper" 6.6k (IIRC correctly this is the ballpark figure for most el84 Class AB pushpull amps) to 600 ohm output transformer help? I know Edcor makes one.

A 6dj8 SRPP stage or white cathode follower is always a nice option too.

try the 250 ohm tap and measure  thd,  would be interested in seeing if using the whole winding improves distortion and upper freq response,

mjrippe said:

I could not get a THD+N of better than -35dBU or about 1.8%. 

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Sad.. thanks for following up with that.

Are you able to come out of the preamp section unbalanced with a nice DC blocking cap?

I'm also curious about this 6.6k to 600 ohm idea. Maybe you could find the impedence of the primary you're using