Tube based VU Meter Buffer Schematic?

Has anybody come across a tube based VU meter buffer design or schematic? I've got some tube pre's and want to install a VU meter/buffer, hanging off the tube power supply.
Cheers
Michael

Junction said:

Has anybody come across a tube based VU meter buffer design or schematic? I've got some tube pre's and want to install a VU meter/buffer, hanging off the tube power supply.
Cheers
Michael

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A Vu meter buffer relies on a rather low output impedance since the meter is supposed to see a total source impedance of about 4k. So if you don't need voltage gain you could use a cath follower, but if you need gain, you must have gain from a common cathode stage followed by a cath follower, or a step-down transformer. Most of the times, in vintage (or not so vintage) equipment, VU meters are driven off a transformer - and very often from the unit's output.

You can use the poor man's tube gain make up circuit for this. It is what I used in my most recent all tube mixer. It has quite a bit of gain so you need to use the preset pot at the input to reduce the input level. I also up the resistor in series with the VU meter to 10K further reduce the sensitivity but this can mess with the ballistics.
Cheers

Ian

Can you just tap it onto (one of) the output stage cathode(s)?

MagnetoSound said:

Can you just tap it onto (one of) the output stage cathode(s)?

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Yes I could but a VU meter across the audio path can introduce distortion into the audio, I dont want to mess with the audio, a buffer seems the best way to tackle it.

Why waste a tube for such a mundane task? One or two high voltage MOSFETs will do the job perfectly. Without hurting your glass sound .

In principle, you could power an opamp from the filament power supply.

Best,

Bri

volker said:

Why waste a tube for such a mundane task? One or two high voltage MOSFETs will do the job perfectly. Without hurting your glass sound .

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Yeah good suggestion, however I am going overboard on the tube/iron thing, this sucker has or will have soon, 4 audio trannys, a choke and 2 power trannys (as the original pre was 110V and I am on 240V supply), so only the right think to do it put another tube in there, there is already a spare 7 pin socket on the chassis, so I was thinking of using a 6AU6, not actually that much more work in wiring up a tube that a transistor.

An it has been a cold winter, so any extra bit of tube warmth never goes to waste around here.
Cheers

Brian Roth said:

In principle, you could power an opamp from the filament power supply.

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Though about that Brian, good idea too, but as per my last post, I want to stay inside a glass bottle where I can .... why? ..... coz I can and they look good.

What abbey road said.  Unless your tube pres do not use an output transformer, just connect it from the OT secondary with an appropriate pad/divider network in front and you're done.

lassoharp said:

What abbey road said.  Unless your tube pres do not use an output transformer, just connect it from the OT secondary with an appropriate pad/divider network in front and you're done.

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Yeah fair call lasso, I have been researching several vintage tube mixer schemos and they all run their meter off the secondary of the OT. The vintage VU meters that I have hardly even blink when connected to the output, might be worth tweaking the internal resistor, there is one in there along side the germanium bridge. Definitely worth a try.
Cheers

>just connect it from the OT secondary with an appropriate pad/divider network in front and you're done.

Have a look at the spectra of noise added by connecting a moving coil vu meter across the secondary of OT.

I found surprisingly a *lot* of mid frequency range noise added even with very good quality vu meters.

Enough so that now I only use a buffer or a defeat switch.

Still below the threshold of hearing but surely 'ruins'  an otherwise beautiful unity gain noise plot 

Cheers

Thjanks Alex, yes I have read various comments as such and really thats why I wanted to go the buffer approach in the first place. I'll see if I can get my oscilloscope which hasnt been happy lately, working, so I can have a look at the impact.