two channel Gyraf G9 with VU-meters

[johnheath]

I have built a Gyraf G9… two channel preamp and I was thinking about adding two VU-meters, one per channel.

I want to light them up with one light bulb each. The bulbs drawing 100mA each and now I wonder if those two light build would cook the 78S12 regulator?

If so can I just connect another dc feed from the rectified 15vac… a CRC filter bringing the voltage down to 12vdc for two light bulbs. I get it to a 47ohm/ 5W resistor. (rectified 15 vac  x 1,4 = 21 volts dc. 2 x 100mA = 200mA)

Inputs please

Best regards

/John

 

[johnheath]

I guess that two 230 v/ 13mA light bulbs i parallel from transformer 230vac could do just fine as well

 

[PRR]

> 230 v/ 13mA light bulbs

Be wary of bringing high (line) voltage near your audio circuits or front panel.

230VAC throws more electrostatic hum than 12V DC.

If the 230V wiring isn't bomb-proof, there is some chance of dangerous shock.

You can't "cook" these regulators. If they get hot, they shut-down.

 

[johnheath]

Yes... I know that they shutdown ... when "cooked"

So... the question remains if the 12vdc circuit can cope with another two light bulbs?  2 x 100mA?

/John

 

[PRR]

> the question remains if the 12vdc circuit can cope

What, you haven't just-tried-it yet??

The key question is how much heat-sink you have (I can't see it from here). But if it is not enuff, the 7812 will run HOT and then shut-down. So if it runs for a bit, shuts-down, and the heat-sink is HOT!, you have a clue (use a bigger sink), This is quicker than computing approximate thermal resistances.

Next, the transformer could run warm. It won't smoke fast, so you run it for 5, 10, 30 minutes with an occasional feel to see how warm it gets. Modern iron can run HOT without harm, though I prefer very-warm.  And the 30VA part with a 10VA-13VA load should not be strained.

 

[johnheath]

Well, the first transformer is 50VA with two secondaries 15vac and 1,67A each.

I slightly changed the G9 schematic to use it with 5687 tubes for the second stage.

The tubes all together use 1200 mA and with an added led for "power on" it is another 30mA.

And… yet another two light bulbs for 100mA each should be fine from the transformer point of view… all together 1,43A

I asked about the heatsink a couple of weeks ago and followed the advise to go…"totally overboard" and found and tested  a heatsink that works for hours without any shutdowns… but that was before adding the meters and their wanted lights.

I am thinking about connecting another 12vdc circuit from the rectified 15 x 1,4 volts… the transformer can take it and it won't add load the 78S12 regulator… But is there a risk of messing up the circuit noise wise?

Best regards

/John

/John

 

[SSLtech]

I am thinking about connecting another 12vdc circuit from the rectified 15 x 1,4 volts… the transformer can take it and it won't add load the 78S12 regulator… But is there a risk of messing up the circuit noise wise?

You'll be fine.

Although If you really want to have super-long bulb-life, you could always under-run the bulbs slightly... and this could even be done more simply, and cooler.

If you rectified and smoothed the 15VAC, you should end up with something in the region of 20VDC, after 1.2v 'silicon tax'.

Just run the two 12V bulbs in series. This will HALVE the current demand on the secondary, still run the lamps at over 75% of their rated voltage, extend the bulb life, and cost less to build.

-Regulating voltage just for incandescent bulbs is for the birds, and is more evervating for the bulbs when the regulator tries to maintain full voltage across a cold filament, upping the current demand significantly, for brief bursts of extreme 'torture'.

If in doubt, go for the simpler approach.

 

[johnheath]

Thanks… well, I wasn't thinking of a "new" regulated circuit just for the lights though

Series you say? Hmm… why not? I guess it is easy to get "stuck" in the "a channel on its own.. and everything in between".

Thanks

/John

 

[Harpo]

Well, the first transformer is 50VA with two secondaries 15vac and 1,67A each.
..
The tubes all together use 1200 mA

A little less than 1200mA for the tubes, but 1.67A is transformer AC current per secondary winding, leaving ( form factor K=1.8 ) 0.926A DC current after fullwave rectification.
The ECC82 only draws 1/3rd of the 5687 heater current (0.15A vs. 0.45A), so 0.9A instead of 0.3A needed for heaters.

and with an added led for "power on" it is another 30mA.

maybe allow for 10mA, unless you want to build a headlight.

And… yet another two light bulbs for 100mA each should be fine from the transformer point of view

incandescent light bulbs can be supplied with AC voltage as well. Or maybe use the other secondary winding with additional rectifier and bulbs in series, as described by Keef . You are already at the limit of the 1st. secondary winding with your heater current demand.

I asked about the heatsink a couple of weeks ago and followed the advise to go…"totally overboard" and found and tested  a heatsink that works for hours without any shutdowns… but that was before adding the meters and their wanted lights.

Ignoring maybe 1.4V diode drop in rectifier, (15VAC x SQRT(2) - 12VDC) x 0.926A = 8.53W transfered to heat will need a big heatsink.

 

[johnheath]

The 5687 tubes sound a bit better to my ears and I am aware of the vastly greater heater current… thereby my questions about the light bulbs… hmm… Also I have a fairly big heatsink that is mounted to the chassi and it works just fine.

The circuit is using two toroid transformers and there is another secondary wingding feeding the other transformer for B+ and feeding the phantom power. The phantom power and B+ have together less current draw than the secondary feeding the heater supply, but I am not sure if it is bad or not to hook up at this secondary for the lights?

/John

 

[johnheath]

Followed PRR's ...question "What, you haven't tried it yet"?

Bang... in goes the lights on the 12vdc and it works just fine... the amp was left on its own for two hours andeverything is just fine.

/John

 

[pstcho]

Hello, ive always wanted to ad a vu meter on the G9

id love to be able to chose input level, output level, but also " in between " the 2 stages  in order to be able to see the first gain level

any advice ?

thanks