Bulletproofing an Orange

[drewl]

Hey all, looking for some tips for beefing up an old orange GRO100.

The owner runs it VERY HARD, like flat out into a Hotplate.
I've replaced all the filter caps, rebuilt the bias supply, retubed it and it's running great.
When screaming it into my Hotplate, the mains current is about 3.5 amps, so it popped the 3amp fuse it was rated for back in the early 70's.
I put a 4amp slo blo in, and it's holding up to the abuse.

I'm gonna put some diodes on the plates, to protect from spikes, what are some other good ideas for helping this amp cope with being run flat out?

Thanks.

[Chuck H]

I had to look up the schem... It's a pretty solid design. I can't add much other than maybe an MOV on the AC mains and possibly the diode string protection on the power tubes for voltage spikes.

Reading your post, my first thought was that since the owner uses it through an attenuator perhaps alter the amp so it makes a little less power. You could lower the plate volts with zeners, but on that amp it would require a couple of zeners in series and some heat sinking. Then I thought maybe lower the screen volts a bit. To avoid changing the tone too much you would need to branch the power supply. I don't know how much trouble that would be since I've never been inside one of those amps.

HTH

[pdf64]

http://www.webphix.com/schematic%20h...ge_gro_100.pdf
The significant failure mode is a power tube shorting out somehow, so it would be beneficial to mitigate the collaterol damage when that happens; additionally, as it's there's 4 power tubes, it may be possible to keep it operating with 3.

To those ends, consider-
1/ bringing the EL34 grid leak resistors down some, maybe 100k. The 47nF coupling caps may be ok with that, or may need doubling in value. Alternitively, if feasible, provide each power tube with it's own coupling cap / grid leak / grid stopper. That will help in case a tube developes excessive grid current and starts running hot.
2/ increasing the value of the screen grid resistors, 1k5, 2k2; that should decrease dissipation and wear on the screen grids.
3/ fitting quick blow fuses into the cathode ground returns of the EL34s; may keep the amp operational in the event of tube shorts.
4/ Remove the 100 ohm humdinger pot and fit 2 x 5 watt 100 ohm fixed resistors. Some tube shorts send B+ current into the heater circuit, which can blow insubstantial ground referencing; then all the tube heaters could be pulled up to VB+ by the fault, damaging their heater to cathode insulation.

Pete.