Blackstar Artist 10 high voltage collapsing

[Stratfordade]

Got a Backstar Artist (single 6L6GC) on bench with no output (apart from usual very quiet hum) from either the normal input or via FX return. With the master vol all the way up you can just hear the input signal if the room is quiet!

It's tricky as there's no schematic available, and a lot of surface mount stuff!

But I have probed voltages and usefully there are test points marked on the main board with expected voltages. What I've found is that all the voltages are as marked or very close, but the high voltages collapse as soon as a jack plug is inserted into the input (which you have to do or the amp is muted).

For example TP 10 (anode of one of the triodes in ECC83/12AX7) is marked 200V and is 203V with nothing in input socket, but as soon as I insert a jack into input socket the voltage drops to 89V. Similar results for other high voltage points e.g. TP5 marked 390V (expect this is anode for the 6LG) drops to 295V when a jack inserted, TP6 (290V so probably a screen voltage for 6L6) drops to 124V.

Both the 6L6 and the ECC83 test fine in my tester, and just in case the 6L6 was doing something weird when in the amp (my tester has max 250V anode voltage) I tried a known good 6L6GC with same result of collapsing high tension supply.

Lots of chopsticking failed to find any cause, though owner who sold me the amp said it was intermittent no sound problem so I'm optimistic!

This would be relatively simple with a schematic so all I can do is ask for tips for finding where in chain the voltage is collapsing as soon as the power valve draws a current. Maybe someone has seen this behaviour on the same amp or one of its cousins (the Backstair Studio 10 series)? Absent a schematic best I can do is post a good photo of the board.

The only clue I've found so far is that this behaviour at every test point happens whether the standby switch is set to have the amp off standby or on standby, and I can't measure any volts either side of the standby switch!

Thanks in advance

[x-pro]

Insert one jack into the input jack, not the one in the photo.
Take a closer look. Maybe the schematic will help in understanding the wiring of the connector-tube

[Stratfordade]

Insert one jack into the input jack, not the one in the photo.
Take a closer look:

Thanks but the one in photo is a mono jack but just has adapter for two phono plugs.

I have used other jack plugs in the input but always same results.

[x-pro]

What is the voltage at the grid of the first half of the lamp when the jack is connected?
It is possible that the lamp grid does not have a 1 megohm ground resistor, or it is open circuit.

[Helmholtz]

Schematic is for a 12BH7 PP amp?

[Stratfordade]

I was suspicious of the fact that the standby switch had no effect on the high voltages I was measuring. The switch itself is OK, but I disconnected the two wires being switched. With amp powered up one of terminal measures 0V and the other 0.63V. I'm used to amps with very high voltage on one side of standby switch but I don't know how the Backstar Artist design engineers the standby switching but it does seem odd to only have 0.63V.

The actual switch terminals look blackened at their base but I don't know if this is charring or not.

[Stratfordade]

What is the voltage at the grid of the first half of the lamp when the jack is connected?
It is possible that the lamp grid does not have a 1 megohm ground resistor, or it is open circuit.

It looks wrong. The grid of one triode is -0.09V, and the other is -0.03V.

I can't access the connections to the 6L6 as it's on a daughter board under the main board, but of course I can check voltages at the socket but with the 6L6 removed.

[x-pro]

For example TP 10 (anode of one of the triodes in ECC83/12AX7) is marked 200V and is 203V

and "With amp powered up one of terminal measures 0V and the other 0.63V."
Disagreements

[x-pro]

Schematic is for a 12BH7 PP amp?

I see your point. The pattern might just be similar. So far, the author of the thread hasn't made the comparison.

[Stratfordade]

There are two 1M resistors near the ECC83, R49 and R50 which could be the grid resistors.

The grid of first section (pin 7) is connected to one end of R49, but the grid of second section (pin 2) is not connected to either end of R50. I will need to pull the board to examine traces underneath but want to leave it in for now so I can continue to look for any obvious faults.

[Stratfordade]

I see your point. The pattern might just be similar. So far, the author of the thread hasn't made the comparison.

It's a very different amp so it's hard to make comparison. I can't even find the standby switch and connections on the 12BH7 amp which is only similar in having an ECC83/12AX7.

[mikeydee77]

Quite a lot of the Blackstar range use the input jack as a control of the amps bias. So when there is no jack inserted the power amp is biassed way into cut off, something like -70v and when you insert the jack it comes up to normal e.g. -42v. Just example voltages. Maybe this amp has similar mechanism but is not functioning properly.

[Stratfordade]

Thank you I’ll check what happens to the bias volts, though it does look like a problem with all the anode voltages. I think I’ll remove board and check for cold joints after checking bias behaviour you suggest.

Happily I also have a Studio 10 KT88 that’s working great and near identical so in absence of schematic can compare measurements too.

[g1]

Standby switch does not always carry high voltage. Sometimes it makes a ground connection (at CT of HV winding, or grounding the (-) terminal of a bridge rectifier).

[Stratfordade]

Thanks that would make the standby readings less mysterious and mean issue may still be simple. I think I’ll start looking for dodgy solder joints, but after some comparisons with the near identical Studio KT88 I have (the PCB is even marked with links to select Studio or Artist version). Nothing happens chopsticking board but it’s very firmly held down with screws so doesn’t flex. I know from my Studio KT88 that it’s a lot of work getting it out (you need to remove jack panel board too and carefully disconnect lots of spade terminals) but not too bad compared with some.