• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Load resistor on OPT

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#5
Valve amplifiers like to have some load. If not, clipping can induce big voltages in the output transformer which cause insulation breakdown. Every few weeks, someone pops up on here and says that he thinks he may have blown his amp (often a guitar amp) by accidentally running it with no speaker connected.

The presence of feedback won't help, it could make it worse because when the output stage clips the feedback loop will try to drive it even harder.
 
#6
DF96 said:
Valve amplifiers like to have some load. If not, clipping can induce big voltages in the output transformer which cause insulation breakdown. Every few weeks, someone pops up on here and says that he thinks he may have blown his amp (often a guitar amp) by accidentally running it with no speaker connected.

The presence of feedback won't help, it could make it worse because when the output stage clips the feedback loop will try to drive it even harder.
Click to expand...

DF96,

There have been a lot of discussions on how to protect the Tx. It seems to be easier on a PP amp you can go across the two plate connections with MOV. However on SE the situation is different, a MOV trip would cause B+ short. Was there ever a definitive answer regards SE?

Regards
M. Gregg
 
#7
I used 1/4 inch jacks that have internal switch for when nothing is plugged in. I wired the switch so when no speakers are plugged in it feeds the output into 25 watt 8 ohm(in my case) resistors in my single ended pentode amp. Of course this does nothing if both ends of your wire are disconnectable and you unplug the speaker side
 
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#10
woody said:
Would a 100 ohm resistor be enough load to damp the big voltage spikes that
dammage the transformer insulation?
Click to expand...

What you have to remember is how much of the output you are loosing as wasted heat. OK with this you are only dropping resistance by a small amount. It would probably work however you would not know until you get a speaker disconnected.

Regards
M. Gregg
 
#11
Hi!

M Gregg said:
Was there ever a definitive answer regards SE
Click to expand...

SE amps usually have no feedback and no tendency to break into oscillation. This is much less critical with an unloaded secondary.

I routinely test new amps which I built with unloaded secondary and fully driven input.

In SE amps the voltage across the primary will probably rise no more than 50% with unloaded secondary. The OPT should have more than this reserve in voltage capability

If you are not sure about the reserve in your OPT, a 100 Ohm resistor across the secondary will do

best regards

Thomas
 
#12
Vinylsavor said:
Hi!



SE amps usually have no feedback and no tendency to break into oscillation. This is much less critical with an unloaded secondary.

I routinely test new amps which I built with unloaded secondary and fully driven input.

In SE amps the voltage across the primary will probably rise no more than 50% with unloaded secondary. The OPT should have more than this reserve in voltage capability

If you are not sure about the reserve in your OPT, a 100 Ohm resistor across the secondary will do

best regards

Thomas
Click to expand...

Thomas,

Did this also include cathode feedback SE?

Regards
M. Gregg
 
#15
Hi!
It seems that there are two reason to use load resistors on the OPT's secondary winding.
- for safety reason
- for better (optimal) drive the loudspeaker.

I am interested in the latter. How to calculate the proper value of this resistor. Maybe a kind of Zobel network would be also good?

Tyimo
 
#16
You can also deploy a MOV and series R to progressively load the secondary for excessive voltage swing. The MOV may need to be specially purchased, rather than scavanged mains MOV that are suitable for primary side loading for PP. However given that voltage swing on secondary side is lower, it may be better to implement back-to-back zeners with series R, as they will have better tolerance.

Ciao, Tim
 
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