Ampeg Svt 2 Pro Troubleshoot

[rickdog]

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).

Okay, you've got me worried now. I put a set of JJ KT-88s in my SVT-CL - the TungSol reissue 6550s I previously had in it had two tubes fail (internal arcing). I wanted something that was rated for higher plate voltage.

So, can you say more about the "significant potential problems"?

 

[bbh]

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


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.



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.

bbh here, you know, I don’t think it was serviced at all. Maybe that was the issue. Thanks belatedly.

 

[agedhorse]

Okay, you've got me worried now. I put a set of JJ KT-88s in my SVT-CL - the TungSol reissue 6550s I previously had in it had two tubes fail (internal arcing). I wanted something that was rated for higher plate voltage.

So, can you say more about the "significant potential problems"?

Plate voltage is only one specification, AND in some cases the maximum allowable plate voltage changes based on the type of amplifier and how the screens are supplied.

Also, the tubes are different electronically, and in some designs, especially those with more complex global feedback (especially around the drivers) the phase margin may be lower. The result can be run-away oscillation which us often catastrophic.

Another difference is the tube's bias curve, and the maximum impedance of the bias supply. Some designs violate the requirements for some tube types and it's not uncommon to see a bias runaway condition (which is also catastrophic)

This is why I recommend following the original designer's intent, they at least knew what they were designing around.

 

[rickdog]

Thanks for the very detailed response. I seem to recall getting an EE degree about 45 years ago , so I actually understood what you said even if I don't have your level of experience designing tube amps.

I'm just trusting that a lot of people have substituted current KT-88s for 6550s in SVTs with no reports of frequent catastrophe. I lost a couple TungSol reissue 6550s (and a screen resistor) due to internal arcing; so far, the KT-88s have been okay. This amp may see more use in the coming year, and I'll be sure to report back on whether it survives.

This is why I recommend following the original designer's intent, they at least knew what they were designing around.

Unfortunately the tubes the original designer intended haven't been available for several decades (unless you want to pay $$$ for dubious "NOS").

 

[Wasnex]

Thanks for the very detailed response. I seem to recall getting an EE degree about 45 years ago , so I actually understood what you said even if I don't have your level of experience designing tube amps.

I'm just trusting that a lot of people have substituted current KT-88s for 6550s in SVTs with no reports of frequent catastrophe. I lost a couple TungSol reissue 6550s (and a screen resistor) due to internal arcing; so far, the KT-88s have been okay. This amp may see more use in the coming year, and I'll be sure to report back on whether it survives.


Unfortunately the tubes the original designer intended haven't been available for several decades (unless you want to pay $$$ for dubious "NOS").

The amp was actually designed to run on 6146s. There was a major reliability update in 1970 that added node H to the power supply to correct a bias problem. Then sometime in 1971 or 1972, they shifted to 6550s. The tube sockets were rewired and a different power transformer was used because 6550s tolerate and require higher screen voltage to make good power.

I believe my 6550 schematic is from 1975. The changes I see from my 1970 schematic is the plate load resistors where changed from 10 ohms to 5 ohms and the preamp and PI voltage was raised a bit. Also some of the pin assignments in the umbilical connecting the preamp to the power amp were shifted around. Other than that, I believe the circuit is pretty much the same.

 

[agedhorse]

Thanks for the very detailed response. I seem to recall getting an EE degree about 45 years ago , so I actually understood what you said even if I don't have your level of experience designing tube amps.

I'm just trusting that a lot of people have substituted current KT-88s for 6550s in SVTs with no reports of frequent catastrophe. I lost a couple TungSol reissue 6550s (and a screen resistor) due to internal arcing; so far, the KT-88s have been okay. This amp may see more use in the coming year, and I'll be sure to report back on whether it survives.


Unfortunately the tubes the original designer intended haven't been available for several decades (unless you want to pay $$$ for dubious "NOS").

There are good choices in 6550's that are quite suitable.

Do as you wish, my comments were for folks interested in why there may be differences and possible issues.

As a designer for many years, I am quite familiar with the reasons why we often choose one part over another.

 

[beans-on-toast]

Check the power tube bias as per the manual. If you can set the bias, the power tubes are probably fine.

Try the amp with an instrument cable patched between the pre-out and the power amp in.

Plug your bass directly into the power amp in. If if you have another pre out, try going through that. This will test the power amp.

If all is well with the power amp, next try the pre-out into another power amp.