Mesa Stiletto problem

[drewl]

Anyone work on these?
I know the schems aren't available.
This one loses output when switched from 150watt to 50watt mode in both channels.
In 50watt mode it lowers the high voltage by about a couple hundred volts and the bias stays the same so the output tubes are no longer conducting.
Built in power scaling maybe?
I don't know if this is normal or not.
It still has signal up to the output tube grids, but the output tubes no longer conduct, I'll see if the bias is supposed to change, maybe it's supposed to throw a relay.

[drewl]

Okay, update.
The voltage drop was being caused by a bad rectifier tube, it doesn't drop in diode mode, replaced the bad 5U4 and it doesn't drop in tube mode either.

Switching to 50w mode engages a relay on the output board which is doing something to the output tube cathodes, because cathode voltage changes from 20mv in 150watt mode to 40volts in 50w mode.
Maybe changes it to cathode bias or switches in a resistor between the cathodes and ground, which may be bad.
Wish there was a removeable plate to get to the output board like the marshall TSL's etc.

[drewl]

Okay, got it figured out.
In 50w mode the relay on the output board switches out the cathode resistor except for the far two output tubes at each end, which just happened to be dead, and were not in there when I swapped in 4 new tubes instead of all six.
Popped the extra two new tubes in the end sockets and it works in 50w mode.
Now I just have to increase the cold ass bias and it's good to go.
Thanks for all the help!

[Pedro Vecino]

If you use it with diode rectification there can be a problem in a power tube. In the Trident exteriors tubes (1 and 6) are the active ones in 50 watts. You can withdraw and put there the second and fourth to verify if voltages are kept in them. The amplifier does not have another system to activate the tubes that of assigning the cathodes to earth.
HT voltage is the same for both modes (50/150 watts). In 50 watts, since it has less consumption, it grows lightly.
If you use it with tube rectification, remember that only one 5u4 remains active in 50 watts. A bad functioning of this tube also might generate the fall

Regards

[drewl]

Well that explains the scenario alright, bad rectifier in 50w mode and bad 1 & 6 tubes in 50w mode.