Very high B+ (Superlead)
Posted: Wed May 13, 2015 4:46 pm
since I've finished building my 1969 Superlead using the Metro instructions I barely had any time to play it.
I have Classic Tone iron: 40-18024 PT, 40-18026 OT and 40-18058 choke (3Hy).
I have New Sensor Mullard EL34s and Tung Sol 12AX7s.
the amp is plugged to a 8ohm 1x12 Aguilar DB112 cab which is a ported bass cab rated at 300w RMS. the impedance is set to 8ohm.
I don't know if it makes any change but I have a shared-split cathode SPDT switch wired to V1.
on sunday I had the chance to play it for a little while at various volumes - started out playing it pretty much at bed room levels with the PPIMV and later cranked the volumes to 8-10 with the PPIMV wide open (out of the circuit).
after about 40 minutes I heard a metallic crackling noise coming from the amp (not from the speaker), which sounded as if a power tube filament was plucked violently, and the V7 and V6 tubes started gradually redplating. I immediately put the amp into Standby, the red plating stopped and after about a minute I turned the amp off.
I later checked Bias voltage while on Standby and while off Standby and bias voltage seemed in line around -42VDC if I remember correctly which when considering the plate dissipation was well within the 70% percent envelope of the tube with the highest cathode current (which is V4).
I tried checking the B+ on pin 3 of the power tubes but the Multimeter couldn't get a stable reading (it was set to 1000VDC) and while it stuttered the moment the probe touched pin 3 it eventually settled on ".1" when checking on any of the tubes.
that being said - the tubes didn't redplate.
I started reading every thread I could find which deals with a similar problem and the main culprits seemed to be a shortened OT, the inability of modern tubes to stand up to the rigors of the Marshall power section, bad PI coupling caps, bad PPIMV pots, drifting PI tail resistors' values, bad swamp resistors or screen grid resistors and oscillations at the PI calling for a swamp resistor.
I decided to first look at the PI tube as this was the most straightforward troubleshooting I could do so I swapped the V3 tube (a fresh Tung Sol 12AX7 I bought for this build - has about 1.5 hours of total playing time) for an old GT 12AX7 which came in a Fender PRRI amp and turned the amp on. this time around I could check B+ voltage but I got 522VDC on all 4 tubes!
no tube red plated but I didn't hit the amp with any signal.
I checked the notes I wrote while building and saw that when I calculated the maximum cathode current I got the same B+ readings and thought that was within range because it's not much higher than what's in Metro amps' voltage chart and at the time I wasn't aware this is well beyond what modern tubes can handle as most threads I've read seem to recommend 420-460VDC as the optimal B+ for modern tubes.
is the above mentioned B+ voltage a probable cause for the red plating I witnessed?
what can I do to lower the B+ voltage?
I hardwired the PT to 220VAC which is the standard in my country but the wall voltages here usually run between 228 and up to 232 VAC. could it be that I should have used the 240V tap?
the VAC on the heaters is fine so - that's just a shot in the dark.
any help will be greatly appreciated.
I have Classic Tone iron: 40-18024 PT, 40-18026 OT and 40-18058 choke (3Hy).
I have New Sensor Mullard EL34s and Tung Sol 12AX7s.
the amp is plugged to a 8ohm 1x12 Aguilar DB112 cab which is a ported bass cab rated at 300w RMS. the impedance is set to 8ohm.
I don't know if it makes any change but I have a shared-split cathode SPDT switch wired to V1.
on sunday I had the chance to play it for a little while at various volumes - started out playing it pretty much at bed room levels with the PPIMV and later cranked the volumes to 8-10 with the PPIMV wide open (out of the circuit).
after about 40 minutes I heard a metallic crackling noise coming from the amp (not from the speaker), which sounded as if a power tube filament was plucked violently, and the V7 and V6 tubes started gradually redplating. I immediately put the amp into Standby, the red plating stopped and after about a minute I turned the amp off.
I later checked Bias voltage while on Standby and while off Standby and bias voltage seemed in line around -42VDC if I remember correctly which when considering the plate dissipation was well within the 70% percent envelope of the tube with the highest cathode current (which is V4).
I tried checking the B+ on pin 3 of the power tubes but the Multimeter couldn't get a stable reading (it was set to 1000VDC) and while it stuttered the moment the probe touched pin 3 it eventually settled on ".1" when checking on any of the tubes.
that being said - the tubes didn't redplate.
I started reading every thread I could find which deals with a similar problem and the main culprits seemed to be a shortened OT, the inability of modern tubes to stand up to the rigors of the Marshall power section, bad PI coupling caps, bad PPIMV pots, drifting PI tail resistors' values, bad swamp resistors or screen grid resistors and oscillations at the PI calling for a swamp resistor.
I decided to first look at the PI tube as this was the most straightforward troubleshooting I could do so I swapped the V3 tube (a fresh Tung Sol 12AX7 I bought for this build - has about 1.5 hours of total playing time) for an old GT 12AX7 which came in a Fender PRRI amp and turned the amp on. this time around I could check B+ voltage but I got 522VDC on all 4 tubes!
no tube red plated but I didn't hit the amp with any signal.
I checked the notes I wrote while building and saw that when I calculated the maximum cathode current I got the same B+ readings and thought that was within range because it's not much higher than what's in Metro amps' voltage chart and at the time I wasn't aware this is well beyond what modern tubes can handle as most threads I've read seem to recommend 420-460VDC as the optimal B+ for modern tubes.
is the above mentioned B+ voltage a probable cause for the red plating I witnessed?
what can I do to lower the B+ voltage?
I hardwired the PT to 220VAC which is the standard in my country but the wall voltages here usually run between 228 and up to 232 VAC. could it be that I should have used the 240V tap?
the VAC on the heaters is fine so - that's just a shot in the dark.
any help will be greatly appreciated.