Contex 500 base tube amp

[Staybolt]

Anyone ever heard of it? I can't find anything on it. It has a current issue and eats tubes.

 

[ExitThirteen]

The reason it eats tubes is because it doesn't have any kind of negative bias voltage on the grids. The amp will be fine for AM where the deadkey from the radio "provides" the bias, but in SSB modes, where there is no carrier present, the tubes just run wild and will eventually redplate. I had one of these amps many years ago, and had the same issue, until I discovered that putting about -10V on the grids helps prevent the tubes from redplating.


~Cheers~

 

[Shadetree Mechanic]

I have one, but have never had that problem.

https://www.worldwidedx.com/threads/contex-500-tube-amp.226894/

 

[Staybolt]

Exactly what mine is doing. Do you remember how you did that?

 

[Staybolt]

Any idea how you did that as that's what mine does.

 

[kopcicle]

https://www.worldwidedx.com/threads/hygain-afterburner-plus-482-grid-bias.228873/

 

[Staybolt]

It's fixed. The HV relay in the back was staying keyed in recieve causing the tubes to red plate. Works fine now with good power out. Decent amp but very bare bones.

 

[Shadetree Mechanic]

It's fixed. The HV relay in the back was staying keyed in recieve causing the tubes to red plate. Works fine now with good power out. Decent amp but very bare bones.

Glad you got it going. Yeah staying keyed would give it a workout that's for sure. Yes bare bones, the first time I saw mine I thought it was one step above a box of rocks and sticks. Haha!

 

[Staybolt]

Glad you got it going. Yeah staying keyed would give it a workout that's for sure. Yes bare bones, the first time I saw mine I thought it was one step above a box of rocks and sticks. Haha!

Someone added a hi low and a switch for the fan.

 

[Shadetree Mechanic]

Someone added a hi low and a switch for the fan.

A switch for the fan? That sounds dangerous, might be ok for stand by but I would forget to turn it back on....