Quicksilver full function preamplifier

[Gladiator33]

Hi!

I have a Quicksilver full function preamplifier in daily use. But when power on, the amp swing approx 60V on the outputs and that kill my DC-coupled transistor poweramp - if I don't remember to push in the "mute" switch.

I there any mods to fix that?
A relay with 30 sec delay on and 0 sec off - perhaps?

Any suggestions?

 

[DF96]

As it appears to claim a -3dB LF rolloff at 0.1Hz it probably has output coupling capacitors which are 20-50 times too large in value. This will not help!

 

[nigelwright7557]

Put a resistor across pre amp output to discharge any DC.

 

[Gladiator33]

It is a slow swing ac topping 60v when I turn amp on. Gets into 0v after 30 sek. In that period I must have pushed in "mute". If I forget to do so, my poweramp fry! I think the mute button shorting output to ground.

I need to do some change to prevent amp IF i dont remember to mute.

Resistor will not do the job.
Change components in amp - I dont want to; Mike Sanders at Quicksilver do have a reason for have a big cap - i hope....?

 

[kevinkr]

<snip>

Resistor will not do the job.
Change components in amp - I dont want to; Mike Sanders at Quicksilver do have a reason for have a big cap - i hope....?

Not necessarily a rational one, and it's certainly not a very good idea if the load is a solid state power amp. (It's not really a great idea to do that to the input of a tube amplifier either.)

A -3dB point in the 1 - 5Hz range is certainly a better choice, I can't imagine the size of film cap required to get that LF corner, but it must be huge which likely means plenty of parasitics and less than ideal performance.

You could use a 555 timer IC to drive a relay and unmute the pre-amplifier after 60 seconds or so. Should be designed to mute immediately upon removal of power and fully reset for the full mute time.

 

[Badge]

I put a thermistor in series with the transformer lead to the power supply. This Pre also kills power supply tubes and this helps with that also.

 

[Shoog]

A 555 circuit is over the top for this application. The linked to circuit has never let me down and can be built from easily accessable parts bin finds. Though the circuit is described as a speaker delay circuit, it can easily be applied to the line output of a preamp.

Simple delay speakers – Electronic projects circuits

Shoog

 

[rayma]

You could use a 555 timer IC to drive a relay and unmute the pre-amplifier after 60 seconds or so.
Should be designed to mute immediately upon removal of power and fully reset for the full mute time.

Yes, in a tube preamp, connect a discharge diode between the timing capacitor and the timer supply,
which should be the DC filament supply (it must be ground referenced). This voltage will collapse quickly.
I've been building a similar circuit for many years. Use a low leakage electrolytic for the timing function.

 

[DF96]

Mike Sanders at Quicksilver do have a reason for have a big cap - i hope....?

Yes, I'm sure he had a good reason. Selling product to people who think that a 0.1Hz rolloff is 10 times better than 1Hz may be his reason.

 

[Merlinb]

Put a pair of 16V back-to-back Zener diodes across the output connector.

 

[Burnedfingers]

This is probably a stupid question but isn't there a cap on the output of the preamp that should be blocking DC? I would look ahead of the cap and measure the voltage and then after. Is the cap shorted or leaking?

With respect to mr. Sanders.... a lot of his thinking is questionable. Just ask anyone with one of his mono block amps like the 8417 or the KT-88. Look at the power supply and see a mega cap after the 5AR4 rectifiers which exceeds design rating for the 5AR4.

 

[Burnedfingers]

Why don't you post the schematic for grins so we can see what he has done?

 

[kstagger]

A few years ago I used to own this preamplifier - never had any thump/turn-on issues but I was using it with a pair of old Eico monoblocks using tube rectifiers. The output capacitor - going by memory here - wasn't some massive unit. Maybe 1uF? 2.2uf tops.

Blurry photo of the Quicksilver interior: http://4.bp.blogspot.com/-v--C3wGkq...a8/_NkU9Ei2bd0/s1600/quicksilver_preamp_2.jpg

I've never seen a schematic of this... should have traced one out when I owned it.

 

[DF96]

This is probably a stupid question but isn't there a cap on the output of the preamp that should be blocking DC?

Caps stop steady DC. Varying DC (otherwise known as AC) gets through them. The symptoms he describes are consistent with a large output cap and a large ground leak resistor reacting to a sudden DC shift as the circuit wakes up.

 

[Gladiator33]

Thanks for many suggestions.
I will do it the easy way; an industrial delay item with delay on/fast off simulate my push on the mute button.
Simple and the amp will still be original.