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SVT/VT-22 Pre-Out/Power-In

Discussion in 'Amps and Cabs [BG]' started by Old Fart, Mar 8, 2013.


  1. Old Fart

    Old Fart

    Mar 11, 2011
    I would like to see how the VT-22 preamp sounds through the SVT power section. And perhaps visa versa. Both amps have "Preamp Out" and "Poweramp In" plugs in the back.

    Is it as simple as that? When I plug a cord into the "Preamp Out" thing in the back of the VT-22, does this disconnect power to the VT-22 output section? When I plug a cord into the "Preamp Out" thing on the back of the SVT, does it shut off the output section of the SVT? I don't want to damage those output transformers.
     
  2. ThisBass

    ThisBass

    Aug 29, 2012
    Germany
    No
     
  3. christw

    christw Get low!

    May 11, 2008
    Dayton OH
    I want to be Tesla (tinkerer at Dayton Amp Co)
    Plug the VT-22 into a cabinet and plug the SVT into a cabinet because both power sections will be active. VT-22 Pre Out -> SVT Pre In and you'll have quite the loud VT-22. I bet that'd sound cool. Get the grit from the 7027A and the depth from the 6550. :bassist:
     
  4. ThisBass

    ThisBass

    Aug 29, 2012
    Germany
    Do a shorting to the PowerAmpIn-plug.

    Power amp out is shortened unless nothing is plugged in, so you are fine.
     
  5. christw

    christw Get low!

    May 11, 2008
    Dayton OH
    I want to be Tesla (tinkerer at Dayton Amp Co)
    If I had a spare amp to use I'd check if the VT-22's preamp is fired up while on standby. I know the B-15 and B-12XTC's can be used as preamps while in standby.

    ...Now I'm getting ideas about running my B-15 into my 300 Pro. :D
     
  6. ThisBass

    ThisBass

    Aug 29, 2012
    Germany
    In general stand by means heating the tubes only, but not powering.
     
  7. Old Fart

    Old Fart

    Mar 11, 2011
    I'm not sure I understand. One comment was to have both amps connected with cabinets, just in case. Another says that I don't have to worry about it since there won't be a signal going into the output section I am not using (VT-22).

    I would think that since these amps have these features and capability, and since there is no warning note printed around those plug connections, that perhaps it is simply okay to just run a cord from the VT-22 Pre Out to the SVT Pre In. But yeah, I suppose it would be safer to have the VT-22 connected to a cabinet. Even though its output section should be inactive.


    Christw, the 7027A's will be taken out of play. Which is part of the goal. The 7027's have (to my ear) a somewhat icky breakup. The SVT has a fantastic growl. the VT-22 has a brighter, clearer, cleaner sound......until the 7027's start to break up. So I wonder how that crisp sound will be shaped by the growly SVT output.
     
  8. Old Fart

    Old Fart

    Mar 11, 2011
    I wonder if maybe the VT-22 should be switched onto Standby, while I am not using its output section.
     
  9. BassmanPaul

    BassmanPaul Gold Supporting Member

    Aug 25, 2007
    Toronto Ontario Canada
    Most standby switches kill the B+ supply to the amp and thus the preamp will be silenced too.
     
  10. The standby switch will cut the high voltage to all tubes in the amp.
     
  11. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    This is your answer. Let me explain in more detail.

    The later VT22 (1976) revision has a power amp in and pre-amp out jacks on the back, so called bridging jacks. If your says this, you can use a shorting jack.

    This is a 1/4" plug that has the tip and ground shorted together. You will need to build one of these by soldering a wire from the center terminal to the outside terminal. Plug it into the power amp in. This will ground the power amp input and turn it off. Then get an instrument cable and connect the VT22's pre-amp out to the SVT's power amp in (ext amp).

    On some VT22's the jacks at the back are labeled ext amp (the earlier models). This revision just has two parallel jacks. They are intended as pre-out or power amp but the design is slightly different than the one above, you can't use a shorting jack. If you do it will short both the pre-amp output and the power amp input. You can remove V4, the 12AU7 (phase inverter) if you want to turn off the power amp in the early model.

    So it depends on which revision of VT-22 you have.

    Always keep the cabinets connected when doing this. Whenever a tube amp is turned on it need the speakers connected. Even though the power amp is turned off, a spike could get through that could cause damage if the speakers are not connected. So for safety sake, keep the cab plugged in.
     
  12. Old Fart

    Old Fart

    Mar 11, 2011
    Thank you. I wonder if I might simply connect an instrument cable between the "speaker" jack on the VT-22 and a cabinet (like the original combo cabinet which I have, with speakers loaded). Here is more information:

    The VT-22 has two speaker jacks. One say "Speaker" which is normally where one would connect the speakers in the combo cabinet. The other speaker jack says "Ext Speaker" which is used if a second cabinet is added. Between those two jacks it says "Class 2 Wiring Acceptable."

    Near the center of the head there are two jacks. One says "Power Amp In. High Z." Below it is stated the following: ".25 Volts Full Power."

    The other says "Pre Amp Out. 10K Ohm Min."

    Seems like this is getting complicated. I wonder if I could hook a cabinet up to the VT-22 speaker out (to ensure there is a load, in case a signal tries to get through the VT-22 output section). Then simply run an instrument cable from the VT-22 "Pre Amp Out" to the SVT "Power Amp In."

    (By the way, the exact same language is written near the jacks of the SVT. Same .25 V remark.....same 10K Ohm remark.)
     
  13. ThisBass

    ThisBass

    Aug 29, 2012
    Germany
    In general there is no need to plug a load into power amp out jack when power amp in is shortened. Some random power spikes are eliminated at the ot itself. That's like the Stand By "plup"
    But if you have got fear of damage the ot so take a 1/4 " jack and solder a 1 watt or 2 watt resistance (4 Ohm) connected from hot to ground.
    A 1 watt or 2 watt resistance is capable to "heat" 300 watt random short term spikes.
     
  14. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Use a speaker cable not an instrument cable when connecting to the speaker or ext speaker jacks.

    Use an instrument cable when connecting to the pre amp out or power amp in jacks. The reason why is that the pre out and power in signals are low, less that 0.25 V[SUB]RMS[/SUB]. An instrument cable will shield the signal from noise. The output of the power amp to the speakers is in the volts range and has higher currents. You don't need noise shielding but you do need a cable that can handle the elevated voltage and current.

    The .25 volts means that it takes .25 V[SUB]RMS[/SUB] to drive the power amp to full rated power. The 10K means that the output impedance of the preamp is at least a 10K ohms. This tells me that the VT-22 preamp out will drive the SVT power amp without any issues.

    Yes that would be fine but the VT-22 cab will be working as well. It sounds like you have a later revision so the shorted plug in the power amp in will turn off the power amp sound on the VT-22. Remember, just to be safe, always have a cab connected to both amps when running these tests.

    Also, turn the amps off, connect all the cables, then turn the amps on. Plugging a cable into the power amp in can cause a pop which isn't good for the speakers. There is circuit protection in the vintage SVT and STV-VR, but it may or may not be present in an SVT-CL. So follow the precaution just to be safe.
     
  15. ThisBass

    ThisBass

    Aug 29, 2012
    Germany
  16. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    All the revisions of the VT-22 that I have have a shorting jack shown on the schematic. A shorted jack should be in place as indicated in the schematics that you posted. Unfortunately not all amps have this type of jack in place. Some of the old Fender amps are a good example.

    The shorted jack is intended for short term mistakes. You wouldn't want to run the amp very long under those conditions. Some will have more elaborate measures like having a 250 ohm high wattage resistor from an output tap to ground to protect the amp in case it is turned on without a speaker cabinet connected. Ampeg did this on some of their amps.

    The problem is, most people don't know if a tech changed the jack and used the wrong one. Always better to play it safe. Also the impedance selector switch on the VT-22 is not the best thing to run the output through. If it slides over a little you are left with no jacks connected to the output taps.
     
  17. ThisBass

    ThisBass

    Aug 29, 2012
    Germany
    As I told above:
    4 Ohm dummy plug
     

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