Uploaded July 2026 | Updated September 2026, 2 weeks ago
This has to be the most embarrassing failure in Marshall Amplifiers' history. This failure is so well-known, it has its own name - "The Marshall Bias Runaway" or "The Marshall Bias Drift Issue". It affects the JCM-2000 DSL-50, DSL-100, TSL-100, and TSL-122 amplifiers. Fixing the issue requires either a very expensive main board replacement JCM2-60-00 or JCM2-60-02. Marshall no longer supports this model or makes this replacement board, but a good third party substitute can be found here: https://www.bradsguitargarage.com.au/amplifier-parts-accessories/
OR, you can attempt a fix that requires the pads around the output tube grids to be drilled out and some traces on the board severed. The bias voltage then as to be supplied to the EL-34 power tube grids via some other means. In this video, we'll take a look at two main boards - one an Issue 5 and one a later Issue 10, both from DLS-100 amps. Both had bias drift, but one, as we will see, if fixable, while the other still runs away on the bias even after performing the bias fix surgery.
This is a must watch if you're thinking about attempting this bias fix on your Marshall! It might just save you some time and money!
0:00 Marshall JCM2000
1:38 Opening the chassis
3:00 Pulling the Main Board
8:14 Inspecting the Board, Ordering Replacement Caps
9:53 Comparing Issue 5 and Issue 10 Boards
14:50 Explaining the Mods
19:23 Drilling Holes and Channels in the Main Board
23:21 Installing New Grid Stopper Resistors and New Wires
29:10 Installing New Power Caps
30:30 Installing Tubes and Testing the Amp
39:49 Discussing Options After Initial Failure
45:07 Surgery on the Issue 10 Main Board, Testing, Setting Bias to 66mA
51:04 FINAL THOUGHTS
52:30 D-Lab Electronics Shout Out! youtube.com/@d-labelectronics
*SHOW YOUR SUPPORT*
1: Become a Member on Locals: theguitologist.locals.com
2: Donate via Paypal: paypal.me/guitologist?locale.x=en_US
3: Buy Merch: bit.ly/GuitologistMerch
4: Send in Viewer Mail. (Contact Me for details at electrictoothsyndrome@gmail.com)
5: Purchase an ad! (Rate sheets available upon request at email above.)
6: Buy something from a sponsor and tell them I sent you.
7: Stand in the middle of the street naked screaming "LONG LIVE THE GUITOLOGIST!" Then when the cops arrive, tell them about my channel.
8: SUBSCRIBE TO THE FIXOLOGIST CHANNEL: youtube.com/@fixologist1
♠RECOMMENDED BOOKS
-RCA TUBE MANUAL amzn.to/2GOBVqS
-Designing Tube Preamps for Guitar and Bass: amzn.to/2DK5tmt
-DESIGN AND CONSTRUCTION OF TUBE AMPLIFIERS: amzn.to/2HSNMne
This has to be the most embarrassing failure in Marshall Amplifiers' history. This failure is so well-known, it has its own name - "The Marshall Bias Runaway" or "The Marshall Bias Drift Issue". It affects the JCM-2000 DSL-50, DSL-100, TSL-100, and TSL-122 amplifiers. Fixing the issue requires either a very expensive main board replacement JCM2-60-00 or JCM2-60-02. Marshall no longer supports this model or makes this replacement board, but a good third party substitute can be found here: https://www.bradsguitargarage.com.au/amplifier-parts-accessories/
OR, you can attempt a fix that requires the pads around the output tube grids to be drilled out and some traces on the board severed. The bias voltage then as to be supplied to the EL-34 power tube grids via some other means. In this video, we'll take a look at two main boards - one an Issue 5 and one a later Issue 10, both from DLS-100 amps. Both had bias drift, but one, as we will see, if fixable, while the other still runs away on the bias even after performing the bias fix surgery.
This is a must watch if you're thinking about attempting this bias fix on your Marshall! It might just save you some time and money!
0:00 Marshall JCM2000
1:38 Opening the chassis
3:00 Pulling the Main Board
8:14 Inspecting the Board, Ordering Replacement Caps
9:53 Comparing Issue 5 and Issue 10 Boards
14:50 Explaining the Mods
19:23 Drilling Holes and Channels in the Main Board
23:21 Installing New Grid Stopper Resistors and New Wires
29:10 Installing New Power Caps
30:30 Installing Tubes and Testing the Amp
39:49 Discussing Options After Initial Failure
45:07 Surgery on the Issue 10 Main Board, Testing, Setting Bias to 66mA
51:04 FINAL THOUGHTS
52:30 D-Lab Electronics Shout Out! youtube.com/@d-labelectronics
*SHOW YOUR SUPPORT*
1: Become a Member on Locals: theguitologist.locals.com
2: Donate via Paypal: paypal.me/guitologist?locale.x=en_US
3: Buy Merch: bit.ly/GuitologistMerch
4: Send in Viewer Mail. (Contact Me for details at electrictoothsyndrome@gmail.com)
5: Purchase an ad! (Rate sheets available upon request at email above.)
6: Buy something from a sponsor and tell them I sent you.
7: Stand in the middle of the street naked screaming "LONG LIVE THE GUITOLOGIST!" Then when the cops arrive, tell them about my channel.
8: SUBSCRIBE TO THE FIXOLOGIST CHANNEL: youtube.com/@fixologist1
♠RECOMMENDED BOOKS
-RCA TUBE MANUAL amzn.to/2GOBVqS
-Designing Tube Preamps for Guitar and Bass: amzn.to/2DK5tmt
-DESIGN AND CONSTRUCTION OF TUBE AMPLIFIERS: amzn.to/2HSNMne
