Ampeg svt issue

[jonday30]

No I do get that. It’s just this being 300 watts, it should be blowing the windows out of my house before tube break up even comes close to happening. This is not normal. This also isn’t from the 70’s or 80’s. It’s a svt-av so whenever those came out is what this is.

All I’ve ever owned are tube bass amps. I’ve had Traynor yba300’s for years. I’ve even had a svt-CL with this same Traynor cabinet 810 and then found the matching ampeg cabinet. I moved and sold that rig off. That sound amazing with the exact same settings I’ve always had with the Traynor. There’s no reason why this ampeg 300 watt monster couldn’t be run with the settings I’ve always used.

 

[jastacey]

If it is a bad tube, there could be an issue if one of the resistors being burned out, for that tube, that in it's self could be reason enough to have a service tech, to look inside, do the easy stuff, first, but a properly set up SVT - CL is one loud mother

 

[JimmyM]

No I do get that. It’s just this being 300 watts, it should be blowing the windows out of my house before tube break up even comes close to happening. This is not normal. This also isn’t from the 70’s or 80’s. It’s a svt-av so whenever those came out is what this is.

All I’ve ever owned are tube bass amps. I’ve had Traynor yba300’s for years. I’ve even had a svt-CL with this same Traynor cabinet 810 and then found the matching ampeg cabinet. I moved and sold that rig off. That sound amazing with the exact same settings I’ve always had with the Traynor. There’s no reason why this ampeg 300 watt monster couldn’t be run with the settings I’ve always used.

If it is a bad tube, there could be an issue if one of the resistors being burned out, for that tube, that in it's self could be reason enough to have a service tech, to look inside, do the easy stuff, first, but a properly set up SVT - CL is one loud mother

Agreed but a spare 12ax7 isn't too expensive, so maybe you could swap it around everywhere that uses it (def. want to try V1 pre first), maybe even get a 12au7 while you're at it, and do some troubleshooting on your own.

 

[jonday30]

I didn’t have any spare 12au7’s but I did have a new 12ax7 which I did swap out. Not sure if that’s V1 but the other 2 preamp tubes are 12au7.

I tried a regular speaker cable instead of the speakon cable. I also removed one of the 8 speakers just to see if something was loose. No luck.

At this point its either bias is set to hot or a failing tube. Other than that that’s all I can really check without tearing into it.

I do notice though that every time I turn it on. I’ll have standby on, let it warm up, all volumes down, flip the standby and wait for the green. As soon as I touch the master and start to roll it up the amp pops or distorts the speakers violently.

I have to wait a long time to even touch the volume knobs to avoid this.

 

[beans-on-toast]

The master volume on the CL is an attenuator positioned at the end of the pre-amp signal chain, it is before the power amp. It could be that the master pot needs to be cleaned with a small amount of deoxit D5 to clear up the noise when you rotate it. Till then, rotating the pot back and forth several times may help. There are other possible reasons that can cause this but try cleaning with deoxit D5 first.

The preamp, which is in the top chassis, contains two 12AX7 tubes. The power amp, located in the bottom chassis, has one 12AX7 and two 12AU7 small signal tubes, then there are the six 6550 power tubes.

 

[jonday30]

Oh ok, there are 2 more 12ax7’s then. I have a few more at home I’ll try out then. I did spray some cleaner at the master pot and vigorously turn it.

It was still too late last night to try it out.

 

[beans-on-toast]

Oh ok, there are 2 more 12ax7’s then. I have a few more at home I’ll try out then. I did spray some cleaner at the master pot and vigorously turn it.

It was still too late last night to try it out.

All cleaners are not alike, some can destroy a pot over time.

Removing the top chassis isn’t too difficult but it does take some work. Look for disassembly instructions posted on TB.

 

[jonday30]

I have a contact cleaner for electronics. I’m pretty sure it’s deoxIT. Oh I’ll definitely tear into this tonight.

 

[Old Fart]

The power tube shown in the pic is a Svetlana "Winged C" 6550. Some say these were perhaps the finest ever made. I hope this is not a failing 6550.

 

[JimmyM]

I didn’t have any spare 12au7’s but I did have a new 12ax7 which I did swap out. Not sure if that’s V1 but the other 2 preamp tubes are 12au7.

I tried a regular speaker cable instead of the speakon cable. I also removed one of the 8 speakers just to see if something was loose. No luck.

At this point its either bias is set to hot or a failing tube. Other than that that’s all I can really check without tearing into it.

I do notice though that every time I turn it on. I’ll have standby on, let it warm up, all volumes down, flip the standby and wait for the green. As soon as I touch the master and start to roll it up the amp pops or distorts the speakers violently.

I have to wait a long time to even touch the volume knobs to avoid this.

Hmm...well David has you covered, I see. Listen to him.

 

[Wasnex]

I know how tubes work and this amp is by no means cranked. This is not my first svt or tube amp for that matter.

With switches out and EQ set flat, no it doesn’t do it. However the tone of after is not in that range of settings. So that’s why I want it sorted out. Tonight I’ll bias the amp again and look into power tubes.

I don't know where a CL maxes out with the Master cranked, but vintage SVTs tend to hit max clean power with the Volume somewhere between about 10:00 and 11:30. The Volume control is not an indicator of how much headroom you have. Most amps run out of headroom long before their controls are maxed.

 

[jonday30]

I don't know where a CL maxes out with the Master cranked, but vintage SVTs tend to hit max clean power with the Volume somewhere between about 10:00 and 11:30. The Volume control is not an indicator of how much headroom you have. Most amps run out of headroom long before their controls are maxed.

I don’t know if I’d consider this one vintage? I actually don’t know anything about this particular svt. I was told to buy this at my local long and Mcquade because I may not find one again. I wanted a USA made svt. This came in used svt-av for $1400CAD so I bought it. These were made in 98? So I wouldn’t consider it vintage.

I also understand that with maxed master volume from 2 o’clock to maxed doesn’t really increase any volume. It’s just with the EQ flat and gain at about 9 o’clock this monster shouldn’t be tapped out for clean volume.

 

[Wasnex]

I don’t know if I’d consider this one vintage? I actually don’t know anything about this particular svt. I was told to buy this at my local long and Mcquade because I may not find one again. I wanted a USA made svt. This came in used svt-av for $1400CAD so I bought it. These were made in 98? So I wouldn’t consider it vintage.

I also understand that with maxed master volume from 2 o’clock to maxed doesn’t really increase any volume. It’s just with the EQ flat and gain at about 9 o’clock this monster shouldn’t be tapped out for clean volume.

Hopefully somebody who owns a classic can tell you the approximate gain settings where the amp will hit it's limit with the master maxed. It's going to vary somewhat depending on how hot your bass is. If your bass has a super hot signal, it will push the amp to max power with less gain.

The first thing IMHO is to check the bias and then determine where the amp is running out of headroom. I.E. is the problem in the preamp or is the power amp hitting the wall.

It could be a bad tube anywhere in the amp. Also I suggest running a jumper between Pre Out and PA In. Sometime the switching contacts in the plugs can become corroded and/or damaged. Inserting jumper bypasses the switching.

Unless you are a tech, about the only other thing you can do is start replacing tubes. There should be two 12AX7s in the preamp and one 12AX7 in the power amp. Try rolling a known good tube through these positions (one at a time).

The power amp also has two 12AU7s. If these are bad, it may not be possible to set the bias. The 12AU7s are drivers. Each 12AU7 controls bias for three of the 6550s. Weak 12AU7s may not be able to pull enough current to put the bias in the proper range, so it will not be possible to get green on the back panel, bias indicator LEDs.


If you want to roll 12AU7s, I suggest that you bias the amp for minimum current, turn the amp off and install the new tubes, then turn the amp on and go through the bias procedure. Installing stronger tubes will the pull bias voltage down and possibly allow too much current to run through the output tubes.

 

[JimmyM]

I don’t know if I’d consider this one vintage? I actually don’t know anything about this particular svt. I was told to buy this at my local long and Mcquade because I may not find one again. I wanted a USA made svt. This came in used svt-av for $1400CAD so I bought it. These were made in 98? So I wouldn’t consider it vintage.

I also understand that with maxed master volume from 2 o’clock to maxed doesn’t really increase any volume. It’s just with the EQ flat and gain at about 9 o’clock this monster shouldn’t be tapped out for clean volume.

OK, so with a bass with typical vintage style output like a Precision or Jazz, you turn the master all the way up and use gain for volume, and they usually start breaking up around the 11:00 mark on gain. Just before that, even though it may be painfully loud, it should be pretty clean. Of course, a hotter bass might cause breakup at a lower gain setting, but generally that's the spot.

 

[jonday30]

Hopefully somebody who owns a classic can tell you the approximate gain settings where the amp will hit it's limit with the master maxed. It's going to vary somewhat depending on how hot your bass is. If your bass has a super hot signal, it will push the amp to max power with less gain.

The first thing IMHO is to check the bias and then determine where the amp is running out of headroom. I.E. is the problem in the preamp or is the power amp hitting the wall.

It could be a bad tube anywhere in the amp. Also I suggest running a jumper between Pre Out and PA In. Sometime the switching contacts in the plugs can become corroded and/or damaged. Inserting jumper bypasses the switching.

Unless you are a tech, about the only other thing you can do is start replacing tubes. There should be two 12AX7s in the preamp and one 12AX7 in the power amp. Try rolling a known good tube through these positions (one at a time).

The power amp also has two 12AU7s. If these are bad, it may not be possible to set the bias. The 12AU7s are drivers. Each 12AU7 controls bias for three of the 6550s. Weak 12AU7s may not be able to pull enough current to put the bias in the proper range, so it will not be possible to get green on the back panel, bias indicator LEDs.


If you want to roll 12AU7s, I suggest that you bias the amp for minimum current, turn the amp off and install the new tubes, then turn the amp on and go through the bias procedure. Installing stronger tubes will the pull bias voltage down and possibly allow too much current to run through the output tubes.

Hey thanks! I’ll be rolling some 12ax7’s tonight to rule those out. First thing I’m checking here in about a hour is the bias. Then I’ll go from there.

OK, so with a bass with typical vintage style output like a Precision or Jazz, you turn the master all the way up and use gain for volume, and they usually start breaking up around the 11:00 mark on gain. Just before that, even though it may be painfully loud, it should be pretty clean. Of course, a hotter bass might cause breakup at a lower gain setting, but generally that's the spot.

I’m using a 1960 custom shop pbass. So definitely not on the hot side. That’s also what I figured if it was starting to break up, 11 o’clock gain setting not 9 o’clock.

 

[JimmyM]

I’m using a 1960 custom shop pbass. So definitely not on the hot side. That’s also what I figured if it was starting to break up, 11 o’clock gain setting not 9 o’clock.

Ya, I'm smelling a bad tube...just not sure where. V1 pre is a distinct possibility. Not the only one, but I've had that happen before with V1 pre in an amp more than once.

BTW, here's a little tip for when you take the preamp out to check the two tubes up there...take the back screen off so you can counter-balance the pre with your other hand when you go to put it back in for good. Just don't touch anything you shouldn't and remember to double check that it's unplugged when you go in there.

 

[Wasnex]

Here is the signal path for the CL preamp:

You may be able to figure out where the distortion is occurring by fidling with the controls while reviewing this doc.

Signal Path: Input Jack/Pad, V1a, Ultra Low Network, Gain, V1b, Bass/Treble, (V2a Midrange), Master, V2B

For example if the signal is getting distorted with the Gain at 9:00 and Master wide open. Turn down the Master and see if the signal cleans up. If NO the problem is most likely in the preamp; if YES the problem is most likely in the power amp or possibly V2b. Turn the Master down more and crank the Gain to see exactly where the preamp starts to develop distortion.

If you narrow the problem down the preamp. We know the problem occurs after the Gain control if you get a clean signal at any setting, because the V1a has fixed gain.

So with the Gain control at the point where distortion starts, turn down the Bass and Treble controls. If this clean up the signal, the problem is most likely after these controls.

I grouped V2a and the Mid control together. This is because the mid control is located in a feedback loop between the plate and cathode of V2a. So V2a is actually part of the mid range circuit.

The mid range circuit feeds the Master, which sets the level hitting V2b. V2b is feeds the Pre Out.

 

[jonday30]

Here is the signal path for the CL preamp:
View attachment 4458450

You may be able to figure out where the distortion is occurring by fidling with the controls while reviewing this doc.

Signal Path: Input Jack/Pad, V1a, Ultra Low Network, Gain, V1b, Bass/Treble, (V2a Midrange), Master, V2B

For example if the signal is getting distorted with the Gain at 9:00 and Master wide open. Turn down the Master and see if the signal cleans up. If NO the problem is most likely in the preamp; if YES the problem is most likely in the power amp or possibly V2b. Turn the Master down more and crank the Gain to see exactly where the preamp starts to develop distortion.

If you narrow the problem down the preamp. We know the problem occurs after the Gain control if you get a clean signal at any setting, because the V1a has fixed gain.

So with the Gain control at the point where distortion starts, turn down the Bass and Treble controls. If this clean up the signal, the problem is most likely after these controls.

I grouped V2a and the Mid control together. This is because the mid control is located in a feedback loop between the plate and cathode of V2a. So V2a is actually part of the mid range circuit.

The mid range circuit feeds the Master, which sets the level hitting V2b. V2b is feeds the Pre Out.

Ok well I’ll be saving this info right here, thank you!!

So I think I did find and fix the problem!! I removed the 4 screws on the top of the amp and slide the control portion out. Found the 2 12ax7 tubes. The one closest to the gain knob was barely seated!!!

I removed it and it looks brand new. All the preamp tubes do beside the power tubes. Anyway I put that tube back in and proper seated the tube. They have little stickers with numbers, not sure what that means. I’m pretty sure someone was in here before.

Anyway fired it up with my preferred settings and it just pounds. No speaker crackle or distorted farty noises. Just a huge difference. Even with the switches both in, EQ flat, master cranked and gain at 9 o’clock it pounds. I did turn the mids down about 9:30 and the frequency to 1 setting.

Got the amp hot and checked the bias. Control #2 was just slightly flickering the red LED. Turned it back so both green and good to go. So it was probably just a poorly seated tube. Maybe that’s V1?

 

[beans-on-toast]

Ok well I’ll be saving this info right here, thank you!!

So I think I did find and fix the problem!! I removed the 4 screws on the top of the amp and slide the control portion out. Found the 2 12ax7 tubes. The one closest to the gain knob was barely seated!!!

I removed it and it looks brand new. All the preamp tubes do beside the power tubes. Anyway I put that tube back in and proper seated the tube. They have little stickers with numbers, not sure what that means. I’m pretty sure someone was in here before.

Anyway fired it up with my preferred settings and it just pounds. No speaker crackle or distorted farty noises. Just a huge difference. Even with the switches both in, EQ flat, master cranked and gain at 9 o’clock it pounds. I did turn the mids down about 9:30 and the frequency to 1 setting.

Got the amp hot and checked the bias. Control #2 was just slightly flickering the red LED. Turned it back so both green and good to go. So it was probably just a poorly seated tube. Maybe that’s V1? View attachment 4458549

Yes, that is V1. The numbers relate to matching each triode within the tube. They are considered matched with those numbers. Sounds like you got it all figured out. Good job.

BTW, don’t touch the tube with greasy fingers. Residue can cause a hot spot on the glass envelope.

 

[Wasnex]

Ok well I’ll be saving this info right here, thank you!!

So I think I did find and fix the problem!! I removed the 4 screws on the top of the amp and slide the control portion out. Found the 2 12ax7 tubes. The one closest to the gain knob was barely seated!!!

I removed it and it looks brand new. All the preamp tubes do beside the power tubes. Anyway I put that tube back in and proper seated the tube. They have little stickers with numbers, not sure what that means. I’m pretty sure someone was in here before.

Anyway fired it up with my preferred settings and it just pounds. No speaker crackle or distorted farty noises. Just a huge difference. Even with the switches both in, EQ flat, master cranked and gain at 9 o’clock it pounds. I did turn the mids down about 9:30 and the frequency to 1 setting.

Got the amp hot and checked the bias. Control #2 was just slightly flickering the red LED. Turned it back so both green and good to go. So it was probably just a poorly seated tube. Maybe that’s V1? View attachment 4458549

Awesome!

AFAIK, the stickers are transconductance ratings. 1200/1250 tell you the two triodes are fairly close to balanced. If you knew which tester was used (and it was properly calibrated) you could tell a bit more about the health of the tube. Usually there is a Bogey which is the minimum rating for a new tube, and the tube is considered good until the number drops to 65% of the Bogey.