Soft start for tubes

[Bob M.]

Hi,

I'm looking for a soft start circuit for the tubes in a homebrew amp. No standby switch is available. I've seen several circuits (mostly Hi-Fi stuff) but all were too large, too involved. I'm looking for something relative simple that can be mounted in an already tightly packed amp, besides using just a simple thermistor. I did see one in the 'Re-Making of a Champ' article. I'm wondering if anyone has built this and how well does it work? Any suggestions would be a big help. My goal, of course, is to get the heaters fully warmed up before applying B+.

Thanks,

Bob M.

[Tom Phillips]

Bob,

This is really a controversial subject. However, I'm going to jump in and suggest that you look into the latest information that indicates that standby switches may not be necessary at all. (This is related to you soft start question) Standby switches originated in circuits such as radio transmitters that ran the tubes at truly high voltages such as 1 to 3 kV or much more. At those voltages, much higher than used in commercial guitar amps, cathode stripping was a concern. Standby switches were included on guitar amps as a convenience that have now taken on a life of their own on the discussion groups. These discussions are sometimes based on little more than repetition of rumors and misapplied "facts."

I have tube type test equipment that gets turned ON and OFF just as frequently as guitar amps and some pieces are still happily operating with the original 45 year old tubes. All without the "benefit" of a standby switch.

I'm away from the shop right now so I don't have my notes to refer you to the latest information. Anyway ... consider this food for thought. I'm sure others will chime in.

Regards,
Tom

[trobbins]

Bob - is there any practical reason why you want this?

I'm just adding another HT delay circuit to the amp I'm presently restoring - but this is because it is choke input HT filter, and the 5R4 lets through sufficient current well before the KT66's warm up enough and so the HT will stress everything for about 5-8 seconds at turn-on.

Ciao, Tim

[soundguruman]

It's a globar resistor, inrush limiter, thermistor...
thermistor R56 Mcintosh MC240 - AudioBanter
It reads 25 ohms cold, and drops voltage, but as it heats up, the resistance drops to near zero.
Cave man technology true, but it works. Used and proven primitive technology. Used by McIntosh and other manufacturers.

[R.G.]

I've always been of the opinion that softer start has to be better, even if only marginally. The items mentioned work, as do some others.

I designed a current limiter which can be put in series with the rectifiers and if you really like belt and suspenders, also between the first filter cap and the rest of the circuit. One resistor sets a current limit, so that the current can be up to that, but no more. If you set the current to just above the full-warp maximum operating current, then anything over that gets clamped.

For instance, when you're in standby and flip to 'on', many amps just switch the rectifiers to the empty first filter cap, and that pulls a monstrous amount of current through the rectifiers and into the cap. In some amps - and there is evidence that the AC30 is one of them - this pulse is so large that it can damage the rectifier tube, and perhaps the cap. Setting a current limit just above full-bore max operating current means that the current limit in effect ramps up the first filter cap to operating condition, but doesn't let huge pulses through.

Practially current limit/delay circuits have pros and cons. Thermistors are GREAT - until some bozo flips the amp into and out of standby repeatedly. The thermistor is already hot and doesn't cool off to limit current on repeated flips. And they get really hot. A time delay relay shorting around a current limit resistor is really good and really controllable - until the relay contacts weld. A MOSFET 'shorting' around a current limit resistor works well, but then you have to concoct some way to drive the gate. My current limit circuit actually developed from that, and consists of a floating gate drive circuit to let the MOSFET control things independently.

[deci belle]

Is it bigger than a couple fuse-blocks, R.G.?

[Austin]

Sorry if this isn't helpful but why can't you add a simple stand-by switch out of curiosity? The chassis is too tight?

[Bob M.]

Oh, but it is helpful. Thanks, R.G.!

Bob