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Crate GT-50 Standby switch, Howl

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8/12/1997 1:25 PM

Crate GT-50 Standby switch, Howl

I have a Crate GT-50 which is an all tube 50 watt head. This is a great sounding (and cheap) all tube amp. The only problem is that it doesn't have a standby switch. I would like to install one but I'm very unknowledgable as far as tube amps go. My best guess is to install an SPST on the plate supply. Any suggestions?

Also, this amp howls (a sound similar to feedback)when the volume is turned up. It does this when there is a cable plugged in with or without a guitar attached to it. It does not howl when there is no cable plugged in. I suspect that the capacitance of the cable, or the guitar cable combination is causing this, but I do not know why.

8/12/1997 2:26 PM

Install your standby switch in the cathode-to-ground circuit of the output tubes (assuming that this is a fixed bias and not a self-bias output). Parallel the switch with a 10K
resistor to keep the cathodes marginally pulled down when the switch is open.

This is far safer than installing the switch in the high-side B+ line. Use a large body switch rated for at least 240VAC.

As to the howl, clearly the howl occurs when the cable inserted into the jack opens up the connection to ground that is usually wired into the input jacks. It could be a dirty/corroded input jack, or perhaps a preamp tube going south. (It could be some other things, too, so this is going to be one of those "try this" answers.)

Clean the input jack carefully with contact cleaner. Swap the tube for another of the same kind, perhaps the other preamp tube. Does the problem follow the tube or the socket?

If it stays with the socket, touch up all the solder joints on that tube's circuitry with a hot soldering iron, and clean the socket contacts with contact cleaner and inserting/removing the tube.

8/12/1997 3:42 PM

I was looking at a schematic for an old Ampeg SVT and noticed that the standby switch was installed on the AC side of the power transformer and that all that it affected was plate voltage. The way I understand it this is the best setup to prevent stripping.

Regarding the howl, I will replace the preamp tubes and see what happens, in addition to cleaning the input jack and checking the solder joints.

8/13/1997 8:40 AM
Heath	I replaced the Yugoslovian tubes that came with the amp and replaced them with Sovtek 12AX7WXT+'s It solved my howling problem and made the amp sound better.

8/13/1997 10:32 AM
Steve	Just a heads up: Lee Jackson designed this amp for Crate, according to his website. It's at: www.leejackson.com There is also a link to this site at the Ampage's amp manufacturers section. Hope this helps, Steve

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