Ampeg Svt 2 Pro Troubleshoot

[matzingerz]

Hey TB!

Some days ago at our rehearsal during the loudest part of our set just when I was about to kick out the fat fuzz factory in self oscillating mode for maximum feedback while droning my Amp suddenly got much quieter, i could still hear myself but not nearly as good as before.
We stopped playing and I started testing around. So it sounds like if i overdrive the amp to max if I play louder than a certain volume, and wont get any louder even if i crank up the master or gain further.
if i play at low/moderate volumes it sounds normal more or less I think.
I used the preamp out and fed it into an other power amp section, of an other amp, with also another cab, fully overdriven in gain and drive and the master set to very low (at the svt) and the other amp recieved the singnal without problems or weird noises and I could crank it up as I liked.
The other way round, used the line out of the other amp and fed it to the power amp in section of my svt 2 pro, and again, as soon as i crossed a certain volume threshold it distorted to hell. (I i think a nice fuzz sound, but I cannot enjoy it as I know its the voice of my Amp dying)
So I guess it is a Power Tube problem. I have this amp for about one year now and playing it twice per week at master maxed out and gain to 2 o'clock, ultra low and ultra high knobs, -15db pad active because I use several distorion units and such.
The previous owner says he played the tubes for about 10 reherasals before he sold it to me, so they were quite new.
I guess it almost matches the wear of the tubes that is about 6 to 12 months like I read in other threads, so Im pretty certain its worn out tubes. (Please correct me if I should not do so, because I really have no experience in tube amps before, I only spend hours crawling the forum and gather information every free minute kinda.)
I also asked him (today) how old the preamp tubes were because I did not repace any tubes since this one year, and he replied, about 3 years old.
So Preamp tubes dont wear out that fast, but I read that i should replace the drive tubes with the power amp tubes together, because they wear more than the other preamp tubes do. Is that a fact and I should do it for sure, to prevent damage? (Worn power tubes indicate worn dirver tube?)
And I would like to have your overall opinion to my case.

Im holding my finger over the order button for 6 JJ KT88s.

This is your chance to prevent me from doing a mistake.

Peace

 

[agedhorse]

Tubes should last MUCH longer than that in normal operation.

 

[matzingerz]

Tubes should last MUCH longer than that in normal operation.

What is your defininiton of normal operation? I think I surely do overdrive the Power section immensly for 5-6 hours 2 times a week.
As i said I have the Master Volume maxed out and the Gain to 75% (Drive to 50%).

I would prefer not spending 200€ for tubes also. But I just don't see where it could be another problem, as the weak power section and the fuzzy sound indicate tube failure/ almost failure.
If I take it to a tech, I had to rent a car plus spend an extra 50 bucks and plenty of time for probably somehting I already know. That's my biggest fear.

 

[agedhorse]

I would expect between 5 and 15 years for the preamp tubes and 3-5 years for the power amp tubes based on my knowledge of the circuit's operating conditions.

 

[matzingerz]

Okay, so do you think the problem could be something else and taking it somewhere to get it checked could safe me money?
Also the guy who sold the Amp could have lied about the tubes. I dont think he did but who knows.

 

[agedhorse]

I would suggest using an Ampeg authorized service center, they should have access to all the documentation and also have the experience.

 

[BassmanPaul]

I would me more concerned about your speaker cabinets. Perhaps you lost a driver or three.

 

[The_Janitor]

Before you drop the cash on a sextet of 6550s try replacing the driver tubes in the power section. One of mine failed and it was causing a variety of weird stuff to happen, making it look like I had a bad power tube.

 

[matzingerz]

I would me more concerned about your speaker cabinets. Perhaps you lost a driver or three.

I use my 1000Watts Solid State Amp until the Ampeg is fixed with the same cabs. Already had 2 rehearsals this way, no problems with the cabs.

Before you drop the cash on a sextet of 6550s try replacing the driver tubes in the power section. One of mine failed and it was causing a variety of weird stuff to happen, making it look like I had a bad power tube.

And yes I wanted to check the preamp tubes aswell. Like getting one that works certainly and replace one after another with it.

I would suggest using an Ampeg authorized service center, they should have access to all the documentation and also have the experience.

Don't want to sound like a douche but i really want to try to go the very low cost way, I really dont have the money and I'm studying electronics and university, so I'm eager to check everything I can myself.
Taking it to an Ampeg service sounds like the most expensive thing I can do.

Getting new Tubes, like one preamp and a set of power tubes won't cost me anything extra, because at some point in my life i will have to change them anyway, and if its not the tubes, I can leave them aside until I need them. But maybe it will solve the problem and I did not need to do any extra work, like bring the amp somewhere or some service costs for just taking a look at it.

But if you have any other tips I can try, like the "check for microphonics" and stuff like that. Maybe some parts in the circuit that are need to be checked regularly?

 

[BassmanPaul]

Im holding my finger over the order button for 6 JJ KT88s.

Your amp uses 6550s. Why would you order KT88s?

 

[agedhorse]

Don't want to sound like a douche but i really want to try to go the very low cost way, I really dont have the money and I'm studying electronics and university, so I'm eager to check everything I can myself.
Taking it to an Ampeg service sounds like the most expensive thing I can do.

Getting new Tubes, like one preamp and a set of power tubes won't cost me anything extra, because at some point in my life i will have to change them anyway, and if its not the tubes, I can leave them aside until I need them. But maybe it will solve the problem and I did not need to do any extra work, like bring the amp somewhere or some service costs for just taking a look at it.

But if you have any other tips I can try, like the "check for microphonics" and stuff like that. Maybe some parts in the circuit that are need to be checked regularly?

Well, that's kind of what it sounds like...

So replacing a quite possibly perfectly good set of tubes, which is not cheap by any means, sounds more expensive to me than correctly identifying the actual problem (may be a tube, it may not) and fixing the problem correctly. If you don't have the money to repair the amp correctly, how do you have the money to replace all the tubes?

All you are doing is guessing, and I have a hunch that you might be wrong. Being a student, one of the most valuable lessons you SHOULD learn is that often guessing is more costly than doing it right. The more you learn, the less you guess. That's the entire basis for troubleshooting, rather than just replacing parts and hoping for the best.

Also, the more you learn, the less likely you are to make mistakes... like replacing 6550 tubes with KT-88's (there are some significant potential problems here, especially with the biasing scheme used), and the assumption that all of the preamp tubes are the same tube (IIRC, and it's been a while since I looked at this, but I do not think so).

A really good, honest, qualified tech in general will be less expensive to get the amp fixed right, with the least amount of additional damage.

 

[Wasnex]

Just because the tubes should last longer does not mean that they will, especially if you are regularly pushing the amp into heavy output drive.

It sounds like you have narrowed the problem down to the power amp. The power amp section has one 12AX7, two 12AU7s, and six 6550s. IMHO, the first think to do is read the manual and check the bias. The status of the bias indicator LEDs will tell you whether the 6550s are biased properly or if a tube is bad.

If it turns out one of the 6550s is bad you may be able to install a new set, adjust the bias and rock on. You may even be able to replace only three of six tubes. However often with these amps, when a tube goes bad it takes out a screen resistor. Also if something else is wrong with the amp, you cold potentially damage a brand new set of tubes. So installing a new set of tubes is not totally without risk.

If you want to swap out the 12AX7 in the power amp, you should be able to do this without a problem. If you want to swap out the 12AU7s, I suggest adjusting the bias for minimal current in the 6550s. Next, turn off the amp and replace both 12AU7s with a fresh tube. Finally, turn the amp on and go through the bias procedure.

The reason I recommend setting the bias for minimum current before replacing the 12AU7s is because they control the bias of the 6550s. If you put a fresh 12AU7 in the amp, it may be significantly stronger than the existing tube. A stronger driver will pull the bias down and cause the three 6550s to conduct too hard.

The output tubes are organize in a push pull topology. Three 6550 are on one side of the output transformer and three are on the other side of the output transformer. Each 12AU7 tube controls three of the 6550s so you can consider getting gain matched 12AU7s as well, so both sides of the push pull circuit balance better. The 12AU7s do not need matched triodes.

Good luck.

 

[kosmicwizard]

I've had a SVT ll for a decade, changed power tubes once, put J/J KT88s in it (totally awesome power tubes!), and have never changed the pre-amp tubes. I put a RCA 1958A long black plate (or whatever it's actually called) 12ax7 in V1 because I preferred the tone not because any of the Pre-amp tubes were toast or distorting. Hope that helps. Please report back what you think of the KT88s compared to the 6550s. Cheers!

Hey TB!

Some days ago at our rehearsal during the loudest part of our set just when I was about to kick out the fat fuzz factory in self oscillating mode for maximum feedback while droning my Amp suddenly got much quieter, i could still hear myself but not nearly as good as before.
We stopped playing and I started testing around. So it sounds like if i overdrive the amp to max if I play louder than a certain volume, and wont get any louder even if i crank up the master or gain further.
if i play at low/moderate volumes it sounds normal more or less I think.
I used the preamp out and fed it into an other power amp section, of an other amp, with also another cab, fully overdriven in gain and drive and the master set to very low (at the svt) and the other amp recieved the singnal without problems or weird noises and I could crank it up as I liked.
The other way round, used the line out of the other amp and fed it to the power amp in section of my svt 2 pro, and again, as soon as i crossed a certain volume threshold it distorted to hell. (I i think a nice fuzz sound, but I cannot enjoy it as I know its the voice of my Amp dying)
So I guess it is a Power Tube problem. I have this amp for about one year now and playing it twice per week at master maxed out and gain to 2 o'clock, ultra low and ultra high knobs, -15db pad active because I use several distorion units and such.
The previous owner says he played the tubes for about 10 reherasals before he sold it to me, so they were quite new.
I guess it almost matches the wear of the tubes that is about 6 to 12 months like I read in other threads, so Im pretty certain its worn out tubes. (Please correct me if I should not do so, because I really have no experience in tube amps before, I only spend hours crawling the forum and gather information every free minute kinda.)
I also asked him (today) how old the preamp tubes were because I did not repace any tubes since this one year, and he replied, about 3 years old.
So Preamp tubes dont wear out that fast, but I read that i should replace the drive tubes with the power amp tubes together, because they wear more than the other preamp tubes do. Is that a fact and I should do it for sure, to prevent damage? (Worn power tubes indicate worn dirver tube?)
And I would like to have your overall opinion to my case.

Im holding my finger over the order button for 6 JJ KT88s.

This is your chance to prevent me from doing a mistake.

Peace

 

[matzingerz]

Just because the tubes should last longer does not mean that they will, especially if you are regularly pushing the amp into heavy output drive.

It sounds like you have narrowed the problem down to the power amp. The power amp section has one 12AX7, two 12AU7s, and six 6550s. IMHO, the first think to do is read the manual and check the bias. The status of the bias indicator LEDs will tell you whether the 6550s are biased properly or if a tube is bad.

If it turns out one of the 6550s is bad you may be able to install a new set, adjust the bias and rock on. You may even be able to replace only three of six tubes. However often with these amps, when a tube goes bad it takes out a screen resistor. Also if something else is wrong with the amp, you cold potentially damage a brand new set of tubes. So installing a new set of tubes is not totally without risk.

If you want to swap out the 12AX7 in the power amp, you should be able to do this without a problem. If you want to swap out the 12AU7s, I suggest adjusting the bias for minimal current in the 6550s. Next, turn off the amp and replace both 12AU7s with a fresh tube. Finally, turn the amp on and go through the bias procedure.

The reason I recommend setting the bias for minimum current before replacing the 12AU7s is because they control the bias of the 6550s. If you put a fresh 12AU7 in the amp, it may be significantly stronger than the existing tube. A stronger driver will pull the bias down and cause the three 6550s to conduct too hard.

The output tubes are organize in a push pull topology. Three 6550 are on one side of the output transformer and three are on the other side of the output transformer. Each 12AU7 tube controls three of the 6550s so you can consider getting gain matched 12AU7s as well, so both sides of the push pull circuit balance better. The 12AU7s do not need matched triodes.

Good luck.

I read that part about the resistors aswell, and okay thats good to know. i thought only the 6550 are in the power amp. Thus only these need to be matched. I also thought since everyone says preamp tubes need no matching and i read that you can swap 12au7 and 12ax7 like you want, to experiment with the gain, I did not read anything about it affecting the power amps bias.
Since I'm not sure anymore I called a tech and he said he actually has an svt 2 there right now and it also has broken screen resisitors and he could explain me in detail what could happen if thats the case in my amp. So ill rent a car and bring it to him i guess.

Thanks for your answers

 

[ezra1]

If you use stomp boxes into the front end of your amp you might be over working the first preamp tube anf frying it.
Try replacing that one and see if it improves.

 

[kosmicwizard]

You don't need a matching sextet either. 3 pairs is how the amp is designed. I took my amp to a music store near me and they had a tube tester, they ran my tubes for free and it saved me a lot of money because I only had to buy the tubes that had failed and not all of the tubes in the pre-amp and power amp sections. Going to a tech is a good idea, it could be caps or a resistor, screens and so forth. Hopefully it is just tubes, and your tech will make sure you are good to go for a long time on a dime.

 

[bbh]

I had a Sunn Model T way back. I was gigging every weekend and it just ate pwr tubes. I changed them with new ones every 3 weeks. The biasing was never far off the mark.
You used to be able to go to an electronics store and use their tube tester. Very hard to find a good tester these days...

 

[agedhorse]

I had a Sunn Model T way back. I was gigging every weekend and it just ate pwr tubes. I changed them with new ones every 3 weeks. The biasing was never far off the mark.
You used to be able to go to an electronics store and use their tube tester. Very hard to find a good tester these days...

There was something very wrong with your amp that was never correctly diagnosed and corrected.

If you use stomp boxes into the front end of your amp you might be over working the first preamp tube anf frying it.
Try replacing that one and see if it improves.

Preamp tubes are generally quite forgiving of damage, even when overdriven, which is not any harder on them than normal operation. This is due to the load line operation of a class A common cathode circuit. The dissipation curve is quite flat compared with other configurations.

You don't need a matching sextet either. 3 pairs is how the amp is designed. I took my amp to a music store near me and they had a tube tester, they ran my tubes for free and it saved me a lot of money because I only had to buy the tubes that had failed and not all of the tubes in the pre-amp and power amp sections. Going to a tech is a good idea, it could be caps or a resistor, screens and so forth. Hopefully it is just tubes, and your tech will make sure you are good to go for a long time on a dime.

Actually, this is not correct. What's more important is the balance between halves of the circuit, there are 3 tubes per half and equal current sharing between tubes in each half is important.

The bias circuit and fault monitoring is a little different in these amps.

 

[Wasnex]

Actually, this is not correct. What's more important is the balance between halves of the circuit, there are 3 tubes per half and equal current sharing between tubes in each half is important.

The bias circuit and fault monitoring is a little different in these amps.

Each trio of tubes in the output section has it's own bias and bias sense circuit. The three tubes in each trio need to be fairly closely matched or the bias sense circuit will indicate a fault.

Matching between the trios is less critical. Since there are separate bias controls for each trio, it's possible to balance the current of one trio against the other even if the two trios are a bit different.

I do agree that it's better to have a matched sextet, but IMHO the amp should work fine with two trios of matched tubes.

 

[agedhorse]

Right, the balancing of the trio is more important (within reason).