SVT/V4 Preamp/Power Amp Advice Needed

[Old Fart]

Some of you know I have a V4 which is distorting. I want to see whether the distortion is in the preamp section or the power amp section. I can run a cord between the V4 "Preamp Out" and the SVT "Power Amp In" jacks. Would it not be safe and sane to have a cabinet connected to each amp?

With both amps still on, could I then unplug that cord, and then plug it into the "SVT Preamp Out" and then into the V4 "Power Amp In?" Perhaps it would just be smarter to shut both amps off when plugging and unplugging that cord.

Tell me what I should avoid doing wrong.

 

[beans-on-toast]

Yes you always have to have a cab connected to each amp when they are powered on.

Yes you can swap the instrument cords with the amps turned on but at least turn the volumes down to avoid a potential popping sound.

 

[Old Fart]

Thank you, BOT. I have heard that popping sound.

 

[Old Fart]

Something is wrong. Whether the cord goes from V4 Preamp Out to SVT Poweramp In, or the other way around, the SVT makes a robust hum in the speaker to which it is connected. IIRC, when the cord is from SVT Preamp Out to V4 Poweramp In, the instrument comes out through the cab connected to the SVT instead of the one connected to the V4. I think these machines are both going to a tech's bench.

 

[beans-on-toast]

Are both amps connected to the same wall outlet?

Just to confirm, plug instrument into the SVT, connect SVT pre out to V4 power amp in, turn SVT volume up. Sound should come out of both speaker cabs. You have the SVT pre going into both power amps.

What years are the two amps? Modern SVT’s offer more options than the vintage ones.

 

[Old Fart]

The SVT is one of the first MTIs made, and features the "Magnavox Era" circuitry with the 12DW7s. As far as I know, it is a Magnavox SVT in an MTI box.

They were plugged into different outlets, but in the same room. It so happens the V4 has more hum than I would like. And when the two amps are connected, there is an even more substantial hum. I am just a little nervous about the hum, and about the inability to use the V4 Out to the SVT In and get any results. My hazy recollection is that it did not work the other way around either. And I'm pretty sure I had them connected correctly. I'd feel better if I stopped fooling around with these myself, and I know a competent amp tech.

 

[Old Fart]

Oh....the V4 is a late-seventies unit with the Master Volume. Reverb but no distortion knob.

 

[Wasnex]

Something is wrong. Whether the cord goes from V4 Preamp Out to SVT Poweramp In, or the other way around, the SVT makes a robust hum in the speaker to which it is connected. IIRC, when the cord is from SVT Preamp Out to V4 Poweramp In, the instrument comes out through the cab connected to the SVT instead of the one connected to the V4. I think these machines are both going to a tech's bench.

I don't think this is unusual. A ground loop is forming between the amps. I had the same problem when I was experimenting with slaving two Ashdown LB30s. I inserted a 1:1 isolation transformer in the signal path between the two amps, and the problem was gone.

Some amps get along and some do not. Results may be different if you use a different AC outlet.

Unless you intend to actually slave these amps, don't worry about. Focus on troubleshooting the V4.

If you are curious, I used an Ebtech HE2

 

[Old Fart]

Much thanks to you both, but I need to be respectful of my limitations. I sometimes ignore them, but won't in this case. I do not know how to troubleshoot a distortion in a V4, and probably lack the equipment even if I knew what I was doing.

I may just take the V4 to the tech. The SVT was only being used today to see if the distortion is in the preamp or the power amp, and that experiment failed. The SVT works like a champ.

 

[Wasnex]

The SVT is one of the first MTIs made, and features the "Magnavox Era" circuitry with the 12DW7s. As far as I know, it is a Magnavox SVT in an MTI box.

They were plugged into different outlets, but in the same room. It so happens the V4 has more hum than I would like. And when the two amps are connected, there is an even more substantial hum. I am just a little nervous about the hum, and about the inability to use the V4 Out to the SVT In and get any results. My hazy recollection is that it did not work the other way around either. And I'm pretty sure I had them connected correctly. I'd feel better if I stopped fooling around with these myself, and I know a competent amp tech.

That era of SVT should have a switching jack on the PA In. When a cable is inserted, the preamp should be bypassed. This setup can be used as a series effects loop.

The 77 V4 shows this for the Preamp Out PA In:

Two parallel jacks. So they can be used as Slave Out or Slave In, but they cannot be used as a loop.

If you are over your head or tired of screwing with it...nothing wrong with taking it to a tech.

 

[beans-on-toast]

The SVT is one of the first MTIs made, and features the "Magnavox Era" circuitry with the 12DW7s. As far as I know, it is a Magnavox SVT in an MTI box.

They were plugged into different outlets, but in the same room. It so happens the V4 has more hum than I would like. And when the two amps are connected, there is an even more substantial hum. I am just a little nervous about the hum, and about the inability to use the V4 Out to the SVT In and get any results. My hazy recollection is that it did not work the other way around either. And I'm pretty sure I had them connected correctly. I'd feel better if I stopped fooling around with these myself, and I know a competent amp tech.

It could be as simple as a bad tube. Worth starting there and swapping one small tube out at a time.

Look for small brown ceramic caps on the main PCB’s with tubes on them. Not on the switch boards. They are suspect when it comes to hum.


Ampeg issued these notes in late 1976:

V4/ VT22 with distortion - Excessive Hum
The 6AN8 Tube should be checked first. We have found that it can cause a noticeable hum problem. The replacement tube should have a spiral filament, as supplied by Ampeg, to minimize the hum level. Four Capacitors were added to the original design to help eliminate hum. These capacitors were 0.47uFD/600V capacitors connected in a series/parallel combination across the blue/yellow and violet wires from the screen winding on the transformer. These should be checked for proper operation.

V4/ VT22 with distortion - Oscillation
The shielding on tube V1 which is a 12DW7 Tube is critical. If the tube shield is not completely extended oscillation may result. When you receive a unit with an oscillation problem check this tube shield first to make sure it is fully extended before proceeding with further repairs.


V4/ VT22 with distortion - Fuse Failure
The first production run of these newer models contained a 6 1/4 amp Slow Blo Fuse in the filament line. We later found this fuse would open prematurely. This fuse should be replaced by a standard 15A fuse.

V4/ VT22 with distortion - Red Plate Condition
It has been found that if the .15 Amp Fuse in the bias line touches the nearby mounting screw the bias will be lost. This will result in a red plate condition on the output tubes. To eliminate this problem the fuse should be moved so that it does not touch the mounting screw.

V4/ VT22 with distortion - Crashing Sound When Hit
On these newer models the input to the reverb is grounded. Due to this, even with the foot switch off, a crashing sound may be heard when the amplifier is hit. The only way to eliminate this is to turn the reverb control down.

Check that these revisions were made to your amp.

 

[Old Fart]

Thanks, BOT. I did swap out the small tubes, one by one, with no change. And I did get the Pre/Out - Power/In test to work, and the distortion is in the preamp section. Somewhat predictably.

I think those service bulletins refer to units with the distortion circuit which this does not have, nor does it have a 6AN8 tube, whatever that is. Bulletin #3 mentions a 15A fuse and this amp certainly does not have any of those!

I'll print and bring this to the amp tech, who may already be familiar with these bulletins but if not, then I wonder if he will know the series/parallel configuration. Hm.

Much appreciate, D.

 

[beans-on-toast]

Ok, then this from 1976 could be your amp’s schematic. The power amp in jack’s shunt (switch) should be gently cleaned with deoxit. Oxidation there can result in a poor signal to the power amp. If a jumper cable from in to out helps, the shunt is dirty. Something easy to check. Also check the input jacks for oxidation against the chassis. Just look for crud there, you don’t normally have to remove the jacks.

 

[JeezyMcNuggles]

Some of you know I have a V4 which is distorting. I want to see whether the distortion is in the preamp section or the power amp section. I can run a cord between the V4 "Preamp Out" and the SVT "Power Amp In" jacks. Would it not be safe and sane to have a cabinet connected to each amp?

With both amps still on, could I then unplug that cord, and then plug it into the "SVT Preamp Out" and then into the V4 "Power Amp In?" Perhaps it would just be smarter to shut both amps off when plugging and unplugging that cord.

Tell me what I should avoid doing wrong.

Should be easy to find out. Line out into a mixer, with headphones. You have to have a cabinet connected when you power up a tube amp. Do not ever have one on without a load attached...ever.

If you have an interface, line out in to that. Easy peasy.

My guess is that it just needs to be biased. Or, retubed...then biased.

 

[crazyBassClown]

They were plugged into different outlets, but in the same room. It so happens the V4 has more hum than I would like. And when the two amps are connected, there is an even more substantial hum.

If the problem only exists while the 2 amps are connected it is almost certainly a ground loop hum, so don't worry about that particular problem. Problems beyond that there is certainly no shame in giving to a qualified tech to look at and fix.

 

[anderbass]

Do both input channels of your V4 amp have the same distortion issue, or only one of them?

PS: You might already know all this, but just in case:
It's totally normal for an Ampeg V4 amp to start overdriving with it's volume knob at the 10 o'clock position. And even at lower volume knob positions if you have the amps Bass knob boosted and/or are using a bass guitar with a high output level; and/or if your using an active bass with it's bass EQ frequency's boosted.

 

[Old Fart]

This amp is way too old to have a line out.

I will check Channel 2.

Both input jack nuts and washers are visibly corroded. Yeah, it would probably be good to open her up again to clean stuff. The Master Volume knob is scratchy.

This amp has a schematic pasted to the access panel at the top of the chassis.

I did get the amps to work together, with their Pre Out and Power In jacks. The V4, by itself, hums somewhat. When both amps are connected, a significant hum is heard, certainly in the cab connected to the SVT.

The crusty old amp tech has not called me back yet.

This is not that big of a deal, or at least it wouldn't be to many. The distortion basically cannot be heard in a cabinet that lacks a tweeter. To me, it is quite audible in cabinets with tweeters that are cranked. The V4 hum and distortion might not bother everyone, but I don't like it.

 

[numberfourthree]

Sounds like it could use some general maintenance on an amp this old.
Clean pots with Deoxit
Clean jacks with contact cleaner, or isopropyl.
Visually inspect solder joints and circuit board traces. If you see areas that need cleaning, do so carefully with a soft toothbrush and isopropyl or contact cleaner.
Check caps (for leakage, bubbles as suggested above).
If you have a means of testing tubes do so, or swap positions to see if signal if affected.
Test pots if you have a multimeter. Hum Balance pots in old Ampegs often fail.
If all is good, re-bias.

or to the Tech....

 

[agedhorse]

It could be as simple as a bad tube. Worth starting there and swapping one small tube out at a time.

Look for small brown ceramic caps on the main PCB’s with tubes on them. Not on the switch boards. They are suspect when it comes to hum.


Ampeg issued these notes in late 1976:

V4/ VT22 with distortion - Excessive Hum
The 6AN8 Tube should be checked first. We have found that it can cause a noticeable hum problem. The replacement tube should have a spiral filament, as supplied by Ampeg, to minimize the hum level. Four Capacitors were added to the original design to help eliminate hum. These capacitors were 0.47uFD/600V capacitors connected in a series/parallel combination across the blue/yellow and violet wires from the screen winding on the transformer. These should be checked for proper operation.

V4/ VT22 with distortion - Oscillation
The shielding on tube V1 which is a 12DW7 Tube is critical. If the tube shield is not completely extended oscillation may result. When you receive a unit with an oscillation problem check this tube shield first to make sure it is fully extended before proceeding with further repairs.


V4/ VT22 with distortion - Fuse Failure
The first production run of these newer models contained a 6 1/4 amp Slow Blo Fuse in the filament line. We later found this fuse would open prematurely. This fuse should be replaced by a standard 15A fuse.

V4/ VT22 with distortion - Red Plate Condition
It has been found that if the .15 Amp Fuse in the bias line touches the nearby mounting screw the bias will be lost. This will result in a red plate condition on the output tubes. To eliminate this problem the fuse should be moved so that it does not touch the mounting screw.

V4/ VT22 with distortion - Crashing Sound When Hit
On these newer models the input to the reverb is grounded. Due to this, even with the foot switch off, a crashing sound may be heard when the amplifier is hit. The only way to eliminate this is to turn the reverb control down.

Check that these revisions were made to your amp.

View attachment 4834863

If the OP doesn't know what he is doing and understands proper safety practices, there is no reason to get anywhere near this deep.

Should be easy to find out. Line out into a mixer, with headphones. You have to have a cabinet connected when you power up a tube amp. Do not ever have one on without a load attached...ever.

If you have an interface, line out in to that. Easy peasy.

My guess is that it just needs to be biased. Or, retubed...then biased.

If you read through the symptoms, you will have seen that the distortion has been identified as coming from the preamp. He has also replaced preamp tubes one at a time with no change in the symptoms. This rules out the amp needing to be rebiased (power amp is fine) and retubed (power amp is fine, changing tunes in the preamp made no improvement).

Your guesses are not helpful as they are incorrect information based on what the OP has already posted.