Svt cl fault

[sharpiemarker]

I've searched and wasn't able to find an answer to my specific question. Im planning on taking it to a tech but the one I trust isn't taking any more work until he works through his backlog.

So my SVT CL went into fault mode. One set of bias light is out. Which means the at lest one of the power tube is bad. Full disclosure I have been running it at 8ohms which I know can cause issues and conflicting opinions.

The tubes all still glow and I've given them the "tap" test and swapped the two sets of three and the bias lights change side with the tubes. That's as far as my trouble shooting skills go.

Anyone had this issue? Is it a simple case of fresh tubes or should I wait until I can get a tech to check it out?

 

[Wasnex]

I've searched and wasn't able to find an answer to my specific question. Im planning on taking it to a tech but the one I trust isn't taking any more work until he works through his backlog.

So my SVT CL went into fault mode. One set of bias light is out. Which means the at lest one of the power tube is bad. Full disclosure I have been running it at 8ohms which I know can cause issues and conflicting opinions.

The tubes all still glow and I've given them the "tap" test and swapped the two sets of three and the bias lights change side with the tubes. That's as far as my trouble shooting skills go.

Anyone had this issue? Is it a simple case of fresh tubes or should I wait until I can get a tech to check it out?

I believe the fault circuit in the CL kicks in when one of the output tubes exceeds a certain max current threshold.

I believe this is a fairly accurate representation of the bias circuit.

The green LED is essentially an AND logic function. All three tubes in the circuit must conduct at least 15mA to get green.

The red LED is essentially an OR logic function. If one or more tubes conducts more than 30mA you get red.

So if one tube is dead or really poorly matched, you might get red, but no green.

If any tubes pulls drastically more than 30ma the fault circuit puts the amp in Standby.

Glowing tubes just means their heaters are working, it does not mean the tubes are pulling current.

I don't know the threshold, but here is the relevant part of the schematic for the fault detection circuit. The bias circuit is immediately to the right of this on the schematic which can be viewed here: [Invalid or Expired Link Removed]

The red square is the part that generates the fault signal. AFAIK this is a window comparator circuit that monitors the cathode current of all six output tubes. If any reach the overcurrent threshold, a fault signal is sent to the standby relay driver circuit. I believe the fault detection circuit looks at the voltage drop that results from current passing through the resistors in the blue rectangle.

I believe the circuit also monitors the voltage of the DC heater supply at the green square, and a fault can be generated from this part of the circuit as well.

Sometimes the bias indicator circuit goes dead...meaning the LEDs no longer operate. That does not necessarily mean the bias circuit or output tubes have stopped working.

Also it's fairly common for SVTs to blow screen resistors when a tube pulls too much current.

You might get lucky and solve all of your problems with a new set off tubes. It's also possible that the amp has problems that will cause damage to the tubes. The safest thing would be to take the amp to a tech.

 

[Funk With Me]

@Wasnex, where does this come from, that running the amp with an 8 ohm load can damage the amp ? I don't remember anything from the user guide though I could be wrong. Conventional wisdom was always that running an amp with a higher impedance load resulting in reduced power output and cooler operation. It was never considered damaging. This has come up a few times recently. I'm curious what you might know of this. Also, anyone else with knowledge of such is welcome to contribute.

 

[Wasnex]

@Wasnex, where does this come from, that running the amp with an 8 ohm load can damage the amp ? I don't remember anything from the user guide though I could be wrong. Conventional wisdom was always that running an amp with a higher impedance load resulting in reduced power output and cooler operation. It was never considered damaging. This has come up a few times recently. I'm curious what you might know of this. Also, anyone else with knowledge of such is welcome to contribute.

If you are willing to do a little research, the designer of the SVT (Bill Hughes) made posts here on TalkBass specifically stating the design is fine at 8 ohms. The impedance mismatch will reduce power and increase tube wear.

The SVT is a pretty robust design, so don't assume this advice is accurate for every tube amp. Whether a tube amp copes better with a slightly high impedance mismatch or slightly low impedance mismatch varies from design to design. Some designs can handle either, some are better with a slightly high mismatch, some are better with a slightly low mismatch, and some don't cope well with any mismatch. IMHO always best to show the amp the expected load, but I have run an SVT at 8 ohm.

 

[rickdog]

Conventional wisdom was always that running an amp with a higher impedance load resulting in reduced power output and cooler operation.

That's generally true for solid-state power amps - generally not for tube amps.

The problem tube amps have with a higher impedance load is that it can cause the peak voltage on the plates of the tubes to be too high. This can cause the tubes to arc internally (which will probably put it in fault mode), or it may break down the insulation of the output transformer (expensive part), or it may arc from the tube socket to the chassis (to the point that it chars the socket, expensive labor).

As @Wasnex noted, the designer of the SVT is quoted as saying it is okay with 8 ohms. I'd be careful about pushing it hard, though.

 

[sharpiemarker]

Thank you @Wasnex that was explained really well and easy to understand. I will probably just take it to a tech to give it a proper check for peace of mind

 

[micguy]

Conventional wisdom was always that running an amp with a higher impedance load resulting in reduced power output and cooler operation. It was never considered damaging. This has come up a few times recently. I'm curious what you might know of this. Also, anyone else with knowledge of such is welcome to contribute.

Tube amps often come with shorting jacks on their outputs - the designers would rather have you run them into a dead short than an open circuit. Transistor amps are not at all OK with shorts on their outputs, but fine with open circuits - the "conventional wisdom" must be adjusted for the type of amp.