Randall RH150 going into protect

[Anks]

I've got a Randall here that is going into protect every now and again. its a hybrid with a tube driving a mosfet output stage ( I fail to see the point of this).

It seems to work fine most of the time but every now and again it will cut off as the protect circuit kicks in. At first I could cause it to do this with a something around a 70-100Hz signal. This leads me to believe that what is happening is the DC protection is confusing this with DC and maybe a capacitor is failing or a resistor as gone high.

There is a lot about the issues with this amplifier on line but no one seems to have a real fix.

I've disabled the protection circuit and the amp will run fine without getting hot but with it being solid sate I would hate for it to ruin and good 4x12 or the like.

If anyone could have a little look and maybe make a suggestion I would much appreciate it.

[Andy Watson]

It seems to work fine most of the time but every now and again it will cut off as the protect circuit kicks in. At first I could cause it to do this with a something around a 70-100Hz signal. This leads me to believe that what is happening is the DC protection is confusing this with DC and maybe a capacitor is failing or a resistor as gone high.

It's always going to be a compromise between obtaining low-frequency operation and yet protecting against D.C. I would focus on the back to back caps, C2 and C4.  In my opinion, using back to back polarised caps, without a bias, is asking for trouble.

I've disabled the protection circuit and the amp will run fine without getting hot but with it being solid sate I would hate for it to ruin and good 4x12 or the like.

If you're happy to run with less protection, you could add 100uF across each of C2 and C4, and see if the eliminates or reduces the problem.

[Anks]

Thanks for the suggestions. I've just fed the protection circuit DC from my bench power supply and it only cuts in till about 25v so I'm not even more confused.

What I'm thinking now is that as this is a high gain amp used mainly for playing heavy metal (that looks very much like a square wave) it could be seeing anything clipped that is over the 25v p-p and then mistaking this for DC on the output.

Maybe the best way forward with this amp is to just go old school and use a fuse for speaker protection. Do you think any of your suggestions will help in this situation?

Thanks for the help.

[Andy Watson]

If it were mine, I would replace the series connected C2 and C4 with one, non-polarised cap of at least 50uF - probably double-up to be sure.

Musicians ... valves 

[Anks]

Well you maybe onto something here as the caps don't test anywhere near there started value. Ill give it a try and let you know how I get on.

[Anks]

If it were mine, I would replace the series connected C2 and C4 with one, non-polarised cap of at least 50uF - probably double-up to be sure.

Musicians ... valves 

Changing C2 and C4 back to back caps for a 50uf non polarized cap along with changing C1 seems to have fixed it and now seems fine. I've not doubles up the capacitance yet but if the problem comes back ill be doing that.

Thanks for the help it was beginning to drive me mad.