Marshall JCM2000 DSL100 Tube Damage
Cascading Events following an EL34 valve /tube failure
This Marshall JCM2000 DSL100 tube damage results in a number of failures – all caused by a Marshall branded EL34. The valve had short circuited. The EL34 was a TAD EL34 and these were supplied to TAD I believe from Shuguang in China.
Marshall switched to the Tung Sol EL34B a few years ago. the Russian factory supplies them with Marshall branding to Marshall themselves but they ay Tung Sol on the bases of each valve.
So what happened to this Marshall?
When the EL34 went short circuit, the 10K resistor in the power supply was burned out. The screen resistor and the resistor supplying the bias to this EL3434 burned out too. The valve base was also damaged so all destroyed parts were replaced. However the bias could not be adjusted and all the EL34’s were significantly under-biased. The poor EL34’s were being cooked.
Maybe I have been unlucky, because I have seen a number of amps using 6L6 or EL34 from Shuguang that have done similar damage. All due to flashing over in various amps Shuguang have been rebranded as Bugera, TAD, Ruby Tubes etc. At the time of writing the Shuguang factory is not producing valves.
Why the Issue with Under-biasing in this Marshall JCM2000 DSL100?
Mfirst reaction was that this was the well known issue of the printed circuit board leaking current and pulling the bias circuit. TO my surprise, this later JCM2000 had this problem addressed. The El3434’s are now mounted on the chassis and not on the pcb! It was unlikely to be leaking pcb currents.
When the EL34 shorted, the 407 volts obviously burned out the resistors and the 5K6 Bias resistor was connected from the EL34 to the bias circuit albeit via a 220K resistor. this limited the current but the full 407 volts ended up on the bias circuit. This section had 100 Volt and 180 volt operation electrolytic capacitors. They has been hit by 407 volts . I measured the volts and the potential dividers should have been around 36 volts. The reality was 14 volts!
Replace the bias circuit capacitors!
Once this was done the bias could adjusted. Another happy JCM2000.
Our Valve Recommendations
Tung Sol EL34B sound wonderful in these amps. We have them in matched pairs and quads
The Tung Sol 12AX7 really make these amps sing. I always it them to these beautiful British amplifiers !
Go to the bottom of this page to listen to this amp model with the El34Bs fitted. Lovely!
