ultra low switch on vintage ampeg V4B

[echoplant]

I just picked up an ampeg V4B with a 1970 straight back 8x10. It sounds really great, but I'm wondering if there is something wrong with the ultra low switch. on my svt classic, the ultra low switch sounds like it cuts the upper bass, and boosts the sub bass, but on the 4vb, it sounds like it just cuts all the top end, and doesn't shape the low end at all.

Is this how it's suppose to sound?

 

[JimmyM]

Yeah, pretty much.

 

[coreyfyfe]

>the ultra low switch sounds like it cuts the upper bass, and boosts the sub bass

The SVT-CL manual lists the ultra low boost as +2db @40Hz/-10db @ 500Hz

The vintage V4B manuals are a little harder to find, but the product literature I can find only lists the mid-cut aspect. It's possible that's the difference you're hearing, or that the mid-frequency cut is different/not as drastic.

 

[beans-on-toast]

The SVT CL and early V4B ultra low circuits are both located close to the input between the first and second stage 12AX7 tube. The ultra low circuits, which are a passive resistor capacitor networks, are not quite the same, they are voiced differently. Of course, component drift in an old amp might be adding to what you like in the V4B.

If you feel that your CL is not performing up to spec, any chance that your SVT CL preamp tubes are due for a change? There are two tubes in the preamp, not the ones that you see in the power amp from the back. If the first tube is weak, it will lack articulation. If the amp is being serviced, the ultra-low switch should be cleaned with some deoxit.

 

[echoplant]

The ultra low switch on the svt CL sounds really good. The ultra low switch on the v4b is really crackley, so it may need some cleaning. the same switch on the v4b does not sound good. It sounds dull and muddy. I will try cleaning it and see if that does anything.

 

[beans-on-toast]

The ultra low switch on the svt CL sounds really good. The ultra low switch on the v4b is really crackley, so it may need some cleaning. the same switch on the v4b does not sound good. It sounds dull and muddy. I will try cleaning it and see if that does anything.

Crackling can be a dirty switch or it can be a leaking capacitor that needs to be changed. Dull and muddy often means that caps need to be changed in the tone circuit because they are out of spec. It isn't unusual to have to do this on any old amp. I've attached the schematic for the ultra low circuit for a 71 V4B. The full circuit is available at http://www.ampegv4.com. The circuits for channel 1 and 2 are in the red box.

[Invalid or Expired Link Removed]
​

 

[echoplant]

Thanks beans-on-toast. I will check into that.

 

[SirMjac28]

The SVT CL and early V4B ultra low circuits are both located close to the input between the first and second stage 12AX7 tube. The ultra low circuits, which are a passive resistor capacitor networks, are not quite the same, they are voiced differently. Of course, component drift in an old amp might be adding to what you like in the V4B.

If you feel that your CL is not performing up to spec, any chance that your SVT CL preamp tubes are due for a change? There are two tubes in the preamp, not the ones that you see in the power amp from the back. If the first tube is weak, it will lack articulation. If the amp is being serviced, the ultra-low switch should be cleaned with some deoxit.

Beans is this true for the new V4B reissues?

 

[beans-on-toast]

Beans is this true for the new V4B reissues?

Yes. Lack of articulation or clarity is a classic sign of tube on the way down hill. There are more details on when to change tubes here: http://wiki.talkbass.com/index.php?title=Tubes#How_do_I_know_when_I_need_to_change_my_tubes.3F

 

[SirMjac28]

Yes. Lack of articulation or clarity is a classic sign of tube on the way down hill. There are more details on when to change tubes here: http://wiki.talkbass.com/index.php?title=Tubes#How_do_I_know_when_I_need_to_change_my_tubes.3F

Beans when you say first and second stage 12ax7 tubes is that between the two preamp 12ax7's or is the second stage the driver 12ax7 and the 12au7 tubes? I'm guessing the preamp tubes would have more to do with the amps tone over the two driver tubes or do they work together and I would have to change all four to notice any difference?

 

[beans-on-toast]

Beans when you say first and second stage 12ax7 tubes is that between the two preamp 12ax7's or is the second stage the driver 12ax7 and the 12au7 tubes? I'm guessing the preamp tubes would have more to do with the amps tone over the two driver tubes or do they work together and I would have to change all four to notice any difference?

I see where you are going. The JJ ECC83S aka 12AX7, JJ ECC82 aka 12AU7, are both dual triodes (two tubes in one, A and B),

In the V4BR the tube lineup is as follows:
V1 ECC83S, input stage
V2 ECC83S, tone stage
V3 ECC83S, gain makeup, cathode follower
V4 ECC82, phase inverter
V5, V6 6L6GC, power tube pair
V7, V8 6L6GC, power tube pair

V1B: ultra low, gain, ultra high -> V1A -> treble/bass -> V2B -> midrange -> master -> V2A -> pre out -> power amp in -> V3A -> V3B -> V4A and V4B -> V5, V6, V7, V8.

For the most part, change V1 and V2 for tone. Don't underestimate the grind contribution of the phase inverter when the amp is pushed, it isn't just in the power tubes.

 

[SirMjac28]

I see where you are going. The JJ ECC83S aka 12AX7, JJ ECC82 aka 12AU7, are both dual triodes (two tubes in one, A and B),

In the V4BR the tube lineup is as follows:
V1 ECC83S, input stage
V2 ECC83S, tone stage
V3 ECC83S, gain makeup, cathode follower
V4 ECC82, phase inverter
V5, V6 6L6GC, power tube pair
V7, V8 6L6GC, power tube pair

V1B: ultra low, gain, ultra high -> V1A -> treble/bass -> V2B -> midrange -> master -> V2A -> pre out -> power amp in -> V3A -> V3B -> V4A and V4B -> V5, V6, V7, V8.

For the most part, change V1 and V2 for tone. Don't underestimate the grind contribution of the phase inverter when the amp is pushed, it isn't just in the power tubes.

Wow Beans this is perfect thanks so much!!!