Lowering HT voltage on RS124 clone, or not?

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MaxDM

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I've almost got my RS124 together, and I've noticed that the tranformer I'm using, which came out of a V103 summing amp (core type), has a HT voltage which is way too high.. it's rated at 270 VAC at 16 mA.

I was convinced that I could use it with a drop-down resistor, but looking at the circuit, the variable current from the first stage might cause too much of a voltage swing, with an added drop-down resistor

I measured 450V DC with just a cap across the diode bridge and a 100K load. I'm assuming that under 16 mA load, it will drop.

Maybe two 100V and one 60V zener in series and a suitable drop-down resistor?
 

Rob Flinn

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You would be much better off getting a more suitable transformer. The drop down resistor will get very hot dropping that much voltage.
 

MaxDM

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do you know where I can get a suitable transformer?

I figure it should be 6.3 @ 1.5 A and 180 @ 16 mA or so.

They are hard to find.
 

Rob Flinn

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Look at the RS124 thread in the dynamics section & I have linked to the transformer I used. I still had to use a dropper but the transformer was something like 30 euros.
 

Whoops

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Easier way to reduce B+ voltage is using the following simple circuit,
it used a power Mosfet, a simple Zener and 2 resistors:

1622003297638.png

It's used by DIY tube amp builders, I've used it in the past with great results, I highly recommend it.

more info here:


 

abbey road d enfer

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Easier way to reduce B+ voltage is using the following simple circuit,
it used a power Mosfet, a simple Zener and 2 resistors:

View attachment 81255

It's used by DIY tube amp builders, I've used it in the past with great results, I highly recommend it.
Beware that this circuit will dissipate as much heat as a resistor that would produce the same voltage drop! The MOSFET would probably ned some kind of heatsink. The nice thing is the voltage would not sag as much as with a resistor.
 

Whoops

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The MOSFET would probably ned some kind of heatsink. The nice thing is the voltage would not sag as much as with a resistor.

Yes the Mosfet will need an heatsink, I just attached mine to the chassis, works perfectly

This is a much better solution than a resistor because it provides a constant voltage drop not dependent on the consumption.

The links I provided cover the details...
 

MaxDM

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yes, I would rather use it as a series regulator, but for now, I have found that the higher voltage, which turns out to be 320V instead of 275, seems to work fine.

I wonder if there are any noticeable downsides to using the altec/124 circuit with 50 volts higher B+?
 

abbey road d enfer

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yes, I would rather use it as a series regulator, but for now, I have found that the higher voltage, which turns out to be 320V instead of 275, seems to work fine.

I wonder if there are any noticeable downsides to using the altec/124 circuit with 50 volts higher B+?
Maybe the noise performance would suffer a little, but also the dissipation in the output tube would increase, not enough to exceed specs, though. OTOH, the performance under heavy GR would probably improve somewhat.
 

MaxDM

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I've been playing around with the voltages on my RS124-type compressor, and I have two scenarios available,

B+ of 315 V, with 180V on the centre tap of the primary of the output TX, and 4.12V on the cathode end of the 220R resistor

Or

B+ of 258 V, with 148V on the TX tap, and 3.33V on the cathode

SO.. The following files are to show the sonic difference between the two, on a drum loop. The GR meter indicates around 6-7 dB reduction.



I know which one I prefer, and why, but I am curious to know your opinions as well.
 

Winston OBoogie

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My gut tells me this is a 6BC8 tube but I'd be pushing it if I put money on it. An easier tell would be when you have -15dB or so on the meter.

If you go with the higher voltage, then I'd put in an ECC189 instead. I'd put in an ECC189 either way but, each to each. :)
 

MaxDM

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My gut tells me this is a 6BC8 tube but I'd be pushing it if I put money on it. An easier tell would be when you have -15dB or so on the meter.

If you go with the higher voltage, then I'd put in an ECC189 instead. I'd put in an ECC189 either way but, each to each. :)

Yes, it's a 6BC8. I have lots, so that's what I'm using right now.

I am going to get a bigger H.T. transformer, and maybe increase the voltage of the ouput stage further. Either way, at the higher voltages it sucks up more current than my current transformer is designed for.
 

MaxDM

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As usual, I can't tell a difference. :D
I could use either one, as long as it's not the only drum track.
Actually, I think the 1st one is a tad more compressed than the second.
Have you run a comparative DR test?

I've nulled them and the only thing left is a bit of the distortion from the transient 'click'

No, I am not sure how to do a comparative DR test.

I find that the lower voltage is less punchy overall, but that's without compression as well
 
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Winston OBoogie

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Yes the punchy will be there more with an ECC189, it'll also take more -tve control voltage so more compression headroom.

If you're going with higher voltage and current, you could also try a 6SN7 output valve. It'll handle a bit more dissipation.

But it's your unit so, just enjoy it in health and happiness :)
 

MaxDM

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Raising the threshold voltage as the 436c does makes it less soft sounding, you’ve done a bit of this going higher B+.
True,

In this case, though, the cathode voltage, which determines threshold, has changed by less than one volt, which should be relatively inaudible, don't you think?

The amp itself, has a harder sound overall, which some people may like, others would not.

I can say that the harder sound sits more firmly in the mix, in the sense that it stays in the foreground more than the softer sound, so it also depends what you are mixing with it.
 

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