Using a dummy plug on an early 70s blue line Ampeg SVT

[whitespikebass]

If you are using a dummy plug, I figure that it's safer to plug it into the ext speaker jack.

The dummy plug acts to switch the speaker out jack to the 2 Ω tap on the output transformer. The speaker out jack has a shorting shunt. When nothing is plugged in, the jack is shorted from tip to ring.

If you have the dummy plug in the ext speaker jack and if the amp is mistakenly turned on without a speaker cab attached, the speaker output will be shorted. This will help protect the amp in the short term.

If the dummy plug is in the speaker out jack, a speaker cab is not plugged into the ext speaker jack, and the amp is turned on, the speaker out sees an open circuit. For a tube amp, this is worse than a short across the output.

So plug the shorting plug into the ext speaker jack.

The original SVT had a loop back safety circuit in the four conductor speaker cable. If the amp was turned on without the speaker cable connected to a cabinet, the amp was disabled and it couldn't turn on. A similar idea, employing a different circuit, was used in the B-15. When they removed the speaker cable and substituted a ¼" jack (in 1972?) they need to do something else so they went with a jack with a shorting shunt.

I have the dummy in the external speaker jack.

 

[Bass 45]

Just curious, what do you think if te skunkworks amps? It's been so long since I've had it.
I remember loving it, but not like I love my blue line.
But I also knew less about tone, playing in general, and I had a different cab. Between time and different cabs I can't really give it a fair comparison.

I had number 117 for a while (maybe two years). I had to sell it during a slow gigging/work period. It sounded like a good old SVT to me - but I chose to keep my SVT-2 instead because I had more sentimental value vested in it - and I also preferred the SVT-2's seemingly higher clean headroom and deeper low capability.

On a side note - did run my SVT-2 for a few years with an 8 ohm cab without issues (on the 4 ohm setting).

 

[Bill Hughes]

JimmyM

All of the amps with the three position mid-range use the same values and hence are the same frequencies.

 

[JimmyM]

Thank you, Bill.