My beloved XTC sounds really bad now

[prsplayer86]

You mentioned that the amp was set at a 8 ohm load when you got it back. That tells me that the shop most likely had a 8 ohm cab to test it with. You never mentioned setting the ohms back to a 4 ohm load when you got it back. Might be a no brainer dumb question.. but are you still running your head at 8 Ohms on a 4 Ohm cabinet?

No i run it at 4 OHM using a 8 OHM cab. As written above and according the manual if you use the half power switch (amp always was in half power mode- they didn`t change my settings) you have to reset the the ohms switch using half of the actual value of the cabinet (A 16 ohm cab should be set to 8 ohms, an 8 ohm cab to 4 and don`t use a 4 ohm cab in half power .
So if they did it right they used a 16ohm cabinet since they used the 8 ohm setting in the half power mode. I highly doubt they used a 16 ohm cabinet!
Could they have ruined my amp using the wrong resistance?

I doubt it that your amp is ruined. However it might be biased too cold, or just not biased at all. So this is what I would try in your shoes, bare with me if you've already attempted these troubleshooting steps:

1. All new cables ( speaker, and new guitar cable)
*nothing in the loop either
2. Run the amp at 8 Ohms to match your cab
3. Go back to the shop with your amp. Unannounced - Ask to play the head through the same cab they used
* try to hash it out face to face to get them to fix it, even good technicians make mistakes
4. Make the call/email to Bogner
* There are a handful of reputable amp guys that they'll be able to point you too
5. If all else fails, buy a bias probe, read up and watch tutorial on youtube, and do it yourself.. really isn't that hard, once you learn how

 

[FourT6and2]

When you bias an amp you have to bias it at FULL POWER and the amp and cab impedance needs to be MATCHED.

Maybe the tech didn't bias the amp at full power.

 

[Everyonesdarling]

When you bias an amp you have to bias it at FULL POWER and the amp and cab impedance needs to be MATCHED.

Maybe the tech didn't bias the amp at full power.

That is good to know. Will call them on wednesday and ask them which cab they used. If they used a 8 OHM cabinet amp and cab impedance were not matched. They did not use full power but half power if that`s what you meant?

 

[Everyonesdarling]

You mentioned that the amp was set at a 8 ohm load when you got it back. That tells me that the shop most likely had a 8 ohm cab to test it with. You never mentioned setting the ohms back to a 4 ohm load when you got it back. Might be a no brainer dumb question.. but are you still running your head at 8 Ohms on a 4 Ohm cabinet?

No i run it at 4 OHM using a 8 OHM cab. As written above and according the manual if you use the half power switch (amp always was in half power mode- they didn`t change my settings) you have to reset the the ohms switch using half of the actual value of the cabinet (A 16 ohm cab should be set to 8 ohms, an 8 ohm cab to 4 and don`t use a 4 ohm cab in half power .
So if they did it right they used a 16ohm cabinet since they used the 8 ohm setting in the half power mode. I highly doubt they used a 16 ohm cabinet!
Could they have ruined my amp using the wrong resistance?

I doubt it that your amp is ruined. However it might be biased too cold, or just not biased at all. So this is what I would try in your shoes, bare with me if you've already attempted these troubleshooting steps:

1. All new cables ( speaker, and new guitar cable)
*nothing in the loop either
2. Run the amp at 8 Ohms to match your cab
3. Go back to the shop with your amp. Unannounced - Ask to play the head through the same cab they used
* try to hash it out face to face to get them to fix it, even good technicians make mistakes
4. Make the call/email to Bogner
* There are a handful of reputable amp guys that they'll be able to point you too
5. If all else fails, buy a bias probe, read up and watch tutorial on youtube, and do it yourself.. really isn't that hard, once you learn how

Thanks for the tips i think i have a good plan now.

 

[Alex_S]

Understood. Do you know which tubes they put in?

Opened up to check out: Tube amp doctor EL 34 SVT and TAD ECC83 selected.

Then the price they asked for is ok. TAD tested tubes are pricy (also for your repair shop) but they are generally tested quiet well. The biasing was expensive imho though.

Also these tubes are the ones recommended by Bogner, so they did there homework there as well. So all I hear from you sounds like they did at least care.

For the preamp tubes: Bogner uses in most amps (relatively cheap) Chinese tubes (don't have an XTC so I don't know about that specific) that are a very open, slightly harsh sounding but fit very well with the overall design. These make a huge difference. Might be these tubes (why did they change the preamp tubes at all? These usually are lasting MUCH longer than power amp tubes).

So my guesses would be either
- a bad tube (can always happen)
- you don't like these preamp tubes (my guess)
- wrong bias

Btw, you are right about the half impedance if you use the half power switch. Have you tried it full power already, if it sounds the same way? Not sure how the half power switch us done with the xtc but most of the time it's done by just using two of the four power tubes. So if you use full power you should hear all four tubes instead of two. If you have a bad power tube its impact should diminish (1/2 vs 1/4). Maybe worth a try.

Another experiment would be exchanging the preamp tubes. If you don't have others at hand, just switch positions. Each has a different significance on the sound. You don't need to rebias for preamp tubes, so you can exchange them safely. That way you could find if you got a bad preamp tube...

Hope that helps.

 

[Everyonesdarling]

Understood. Do you know which tubes they put in?

Opened up to check out: Tube amp doctor EL 34 SVT and TAD ECC83 selected.

Then the price they asked for is ok. TAD tested tubes are pricy (also for your repair shop) but they are generally tested quiet well. The biasing was expensive imho though.

Also these tubes are the ones recommended by Bogner, so they did there homework there as well. So all I hear from you sounds like they did at least care.

For the preamp tubes: Bogner uses in most amps (relatively cheap) Chinese tubes (don't have an XTC so I don't know about that specific) that are a very open, slightly harsh sounding but fit very well with the overall design. These make a huge difference. Might be these tubes (why did they change the preamp tubes at all? These usually are lasting MUCH longer than power amp tubes).

So my guesses would be either
- a bad tube (can always happen)
- you don't like these preamp tubes (my guess)
- wrong bias

Btw, you are right about the half impedance if you use the half power switch. Have you tried it full power already, if it sounds the same way? Not sure how the half power switch us done with the xtc but most of the time it's done by just using two of the four power tubes. So if you use full power you should hear all four tubes instead of two. If you have a bad power tube its impact should diminish (1/2 vs 1/4). Maybe worth a try.

Another experiment would be exchanging the preamp tubes. If you don't have others at hand, just switch positions. Each has a different significance on the sound. You don't need to rebias for preamp tubes, so you can exchange them safely. That way you could find if you got a bad preamp tube...

Hope that helps.

Thanks a lot Alex for your help. It actually sounds a bit better in the full power mode but not nearly as good as before.
Seems like i have to get my own amp tech. What power/ preamp tubes do you guys recommend for a 20 th anniversary EL34 XTC?

 

[Alex_S]

If you were happy with the sound before, use the same again (if you know what was in). As I don't own an XTC I can't really recommend anything,sorry. As said Bogner uses in most amps winged c svetlanas and Chinese preamp tubes. I guess it is the same with the xtc, but there should be a hint in the manual. But best would be if you stay what was in if you liked it...

 

[Alex_S]

Ah, and if sound improved by switching to full power it really might be a faulty power tube amp. Just a hint, but a first indication...

 

[Loudness250]

The tech probably didn't notice that you were running it at half power. He might have biased it without noticing that. In that case it would be a low bias and sound shitty and cold. Might be nothing wrong with the tubes.

 

[FourT6and2]

They did not use full power but half power if that`s what you meant?

The amp must be biased in full-power mode. Not half power.

 

[Everyonesdarling]

The tech probably didn't notice that you were running it at half power. He might have biased it without noticing that. In that case it would be a low bias and sound shitty and cold. Might be nothing wrong with the tubes.

Thanks. It was biased in half power mode. So maybe that is the explanation. Recognized another thing when playing in channel III and that is a crackling sound. Maybe i manage to get that on "tape" with my i phone so i can post it here.

 

[FourT6and2]

Yeah, if tech biased the amp in half-power mode, your bias is going to be too cold.

 

[halebox]

Also excursion needs to be on tight and presence on zero when biasing also full power.

 

[FourT6and2]

Also excursion needs to be on tight and presence on zero when biasing also full power.

Why does presence need to be on zero?

 

[halebox]

Don't know just read it online

 

[FourT6and2]

Don't know just read it online

Well don't repeat some BS nonsense you read somewhere unless you know it's true

Because it isn't. True, that is.

 

[halebox]

http://hugeracksinc.com/forum/viewtopic ... 91&start=0

 

[FourT6and2]

Yeah I see it where someone wrote that (in 2007). I don't profess to know all there is to know, and maybe the XTC is a special case, but the presence circuit has nothing to do with bias. You usually turn the volumes down all the way when you bias for a few reasons, but the EQ and presence circuit shouldn't affect the process. Anybody else wanna enlighten me?

 

[Loudness250]

I usually turn everything down but I have no info that would indicate the presence would affect bias.