Guitar Tube Amplifier Volume Intermittent Volume drop

[SkidFx]

Hi Experts,

Looking for some guidance, My amp (BlackStar HT Club 40 MKII) began having very loud humming, my actions as they occurred :

1. Install new 12Ax7 (EC83) pre-amp tubes - JJ tubes
2. Started up the amp, played with same hum no change
3. After i flipped the amp over and began taking readings with MM, i hit a connection with positive lead and the Output fuse 500Ma250v blew.
4. I replaced the fuse, same hum issue
5. I then decided to check the Power tubes, i removed the first Ruby EL34 Power Tube in line and swapped it with a Winged C el34, the mains fuse blew.
6. I stopped- ordered matched set of JJ EL34 power tubes, replaced the fuse
7. Cranked the amp up- first thing i notice is the hum was very low now on hi gain stages, feeling great i biased tubes
   1. This model has two green LED and Pots you turn when green glows bright they are biased correctly per design) I did not use MM for this btw
8. After playing for about 6-10 minutes i experienced a volume loss then immediate return of volume-
   1. Swapped cables, guitars, cabs (16ohm Cabs btw)
   2. Problem is still there.

I do not have the schematic, Blackstar will not release it to a non approved service center- I am not looking for a definitive answer here- just a idea of where to start- What to look for, possible troubleshooting techniques.

Note i have a chain of pedals as well- not cheap pedals name brand i run through the loop. I have not began to eliminate these yet but will after this post.

Please note also i am a hobbyist, yes i understand tube amp voltage and how to bleed the voltage so i can poke around and stay alive thanks for concerns, i am not a expert in electronics but have been hobbling with circuits for a little over a year now. By no means expert--- if you respond please dumb it down a bit for my sake and so i do not aggravate you with unintelligent questions lol.

Thanks for the consideration to help out. I have some video if that helps but cannot seem to be able to post here.

 

[SkidFx]

Hey Experts sorry to not post this -

Power Tubes EL34 - Two
Pre-amp tubes 12AX7 (EC83_ = Two
40 Watts
Outputs - 16ohm, 8 ohm, 4 ohm, using 16ohm

 

[Nigel Goodwin]

Well valve amps are crude and simple, so you don't really need a circuit.

If you've got pedals connected, then disconnect them - it's just another potential source of a fault, so isn't helping. If they loop in and out of the amp, the contacts and switches on the sockets could be an issue?.

You've got multiple controls, see if they all work, and what they do - particularly the volume controls - does the hum vanish when the master volume is turned down - if so does it vanish when either, or both, of the preamp volumes are turned down.

 

[gophert]

Hey Experts sorry to not post this -

Power Tubes EL34 - Two
Pre-amp tubes 12AX7 (EC83_ = Two
40 Watts
Outputs - 16ohm, 8 ohm, 4 ohm, using 16ohm

Sometimes the power transformer magnetic field overlaps with the output transformer and can send hum right to the speaker if the box is too small. The power transformer may be on a 1/2" NPT mount. Try rotating the transformer 90°. It may have flopped over when you moved it recently.

 

[BobW]

Have you checked the electrolytic capacitors in the power supply? One of them may have failed or has an intermittent connection. Electrolytics are the most failure-prone components (on par with power output tubes).

 

[Nigel Goodwin]

Have you checked the electrolytic capacitors in the power supply? One of them may have failed or has an intermittent connection. Electrolytics are the most failure-prone components (on par with power output tubes).

Not usually a problem with valve amps?, as far as I know, and I've repaired a great many over the decades. By FAR the biggest problem with valve amps, and particularly guitar amps, are the anode load resistors for the double triode ECC83's. These VERY, VERY commonly go high in value and O/C, and are so common I can even remember the pin connections for an ECC83 (from 1 to 9 - anode, grid, cathode, heater, heater, anode, grid, cathode, heater CT). So checking the voltages on pins 1 and 6 soon shows if there's a problem. If any of them are faulty, best to replace them all with higher wattage resistors - 1 watt resistors should last for ever (due to their higher wattage, AND their higher voltage rating).

Also in valve amps, historically the next most common fault was leaky coupling capacitors - anode of one valve to grid of the next - these VERY commonly used to go leaky, giving a DC grid bias on the following stage. But I'd like to think modern capacitors have long since stopped doing that?.

As fas as electrolytics go?, there aren't that many in valve amps - and I'm not even sure if I remember ever changing any?. The main problem with modern electrolytic failure is their use in high frequency switch-mode power supplies, and the fitting of cheap sub-standard ones.

 

[SkidFx]

HI Guys,
Update, i removed each pedal one by one, then all together took the loop out of the picture plugging straight in the input. Same issue. This amp has a watt selector, its a push button - pushed in and you get 12 watts, compress another push and out = 40 watts. When i push it in to the 12 watt option all is well but when i push again to get 40 watts the amp gets loud then like a pop noise then reverts back to 12 watts. Can the switch be bad, how do i determine a push button is faulty. I do not want to jump to conclusion, as all of you say can be cap, or resistor as well, causing the switch to malfunction.
If there was a bad circuit int eh power tube, could this affect the switch performance?

 

[Nigel Goodwin]

HI Guys,
Update, i removed each pedal one by one, then all together took the loop out of the picture plugging straight in the input. Same issue. This amp has a watt selector, its a push button - pushed in and you get 12 watts, compress another push and out = 40 watts. When i push it in to the 12 watt option all is well but when i push again to get 40 watts the amp gets loud then like a pop noise then reverts back to 12 watts. Can the switch be bad, how do i determine a push button is faulty. I do not want to jump to conclusion, as all of you say can be cap, or resistor as well, causing the switch to malfunction.
If there was a bad circuit int eh power tube, could this affect the switch performance?

Well, that's a VERY different story. The fact it works fine in 12W mode eliminates any problem in most of the circuitry.

However, as BlackStar refuse to issue circuits I've no idea how the switching is been done - I did manage to find a circuit for the original club 40, but not the mk2 - and I couldn't see any obvious power switching on that?.

It's also not really a valve amp, it's essentially a solid state amp with a handful of valves chucked in, and a fairly weird overall design.

 

[SkidFx]

Nigel,

Completely true, when i pulled the amp out the housing this became clear. Can you share the link for the club 40? Many thanks

 

[Nigel Goodwin]

Here's the circuit.

**broken link removed**