Very high B+ (Superlead)

Discuss your builds of MetroAmp Kits.

Moderator: VelvetGeorge

Elad E
Senior Member
Posts: 137
Joined: Sat Jun 29, 2013 9:19 am
Just the numbers in order: 13492

Very high B+ (Superlead)

Post by Elad E » 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.

Roe
Senior Member
Posts: 5054
Joined: Thu Apr 13, 2006 1:36 pm
Just the numbers in order: 7
Location: Drontheim. Norwegen
Contact:

Re: Very high B+ (Superlead)

Post by Roe » Fri May 15, 2015 3:38 am

use the 240v primary!
http://www.myspace.com/20bonesband" onclick="window.open(this.href);return false;
http://www.myspace.com/prostitutes" onclick="window.open(this.href);return false;
Super 100 amps: 1202-119 & 1202-84
JTM45 RS OT JTM50 JMP50 1959/2203/34/39

Elad E
Senior Member
Posts: 137
Joined: Sat Jun 29, 2013 9:19 am
Just the numbers in order: 13492

Re: Very high B+ (Superlead)

Post by Elad E » Fri May 15, 2015 7:12 am

thanks Roe!

last night I rewired the amp for 240V. when I turned it on I measured the B+ and it dropped from 522 to 484.
the heaters dropped from 2.9 to 2.6VAC. not sure if that's good for tube life, would love to hear more opinions about that.

while measuring the B+ the amp started to hum even though all controls, including both volumes were on 0 (PPIMV wide open), and I noticed extreme red plating on V7 and mild red plating on V6 before I put it back on Standby.
I changed the 5.6K swamp resistors on V6 and V7 with a new pair just to make sure and afterwards measured everything again and didn't get any red plating. I calculated maximum cathode current for the "hottest tube" based on the new B+ readings and biased a little a bit colder than maximum.

I'll give it a spin afternoon and report back.

User avatar
neikeel
Senior Member
Posts: 7231
Joined: Tue Dec 06, 2005 8:31 am
Location: Suffolk, England

Re: Very high B+ (Superlead)

Post by neikeel » Fri May 15, 2015 7:44 am

Wonder why the Classic tone PTs all seem to give much higher B+ than the originals. A Superlead by design should have a pin 3 voltage of 480-490vdc. Sure the US early 70s amps were higher but a lot of those ate output tubes (remember the 6550 subs). Same goes for the JTM45, these sound best with an output tube plate voltage of 440-450vdc yet a lot of people seem to quote they are getting 470-480v.

Your heaters will be about ok, I seem to remember 3.15vac +/- 10% (so 2.8vac to 3.5vac being the range) and of course the variac guys go pretty low with their heaters and don't admit to taking out loans for lorry loads of pre amp tubes!
Neil

Elad E
Senior Member
Posts: 137
Joined: Sat Jun 29, 2013 9:19 am
Just the numbers in order: 13492

Re: Very high B+ (Superlead)

Post by Elad E » Fri May 15, 2015 1:24 pm

about a minute playing with ppimv wide open and vol 1 on about 2 with all controls on 0 (mid at about 4) theres an audible thud and noticeable red plating appears on V6 and V7. metallic crackling noises follow (like the sound of the filaments cooling down when turning off the amp after a long session only in an almost constant tempo).

I'm afraid the high voltage toasted the pair of tubes, I'll try switching the V6/V7 tubes with the V4/V5 tubes and see if the red plating moves between the push and pull sides or stays stationary.

Elad E
Senior Member
Posts: 137
Joined: Sat Jun 29, 2013 9:19 am
Just the numbers in order: 13492

Re: Very high B+ (Superlead)

Post by Elad E » Fri May 15, 2015 1:32 pm

btw, I forgot to mention the PPIMV is of the LarMar type with the 2.2 meg resistors installed.
can a bad connection or a bad wiper at the PPIMV pot cause the red plating when bias voltage measures stable?

User avatar
GilmourD
Senior Member
Posts: 144
Joined: Sat Oct 12, 2013 7:47 pm
Just the numbers in order: 13492

Re: Very high B+ (Superlead)

Post by GilmourD » Fri May 15, 2015 2:42 pm

Elad E wrote:thanks Roe!

last night I rewired the amp for 240V. when I turned it on I measured the B+ and it dropped from 522 to 484.
the heaters dropped from 2.9 to 2.6VAC. not sure if that's good for tube life, would love to hear more opinions about that.

while measuring the B+ the amp started to hum even though all controls, including both volumes were on 0 (PPIMV wide open), and I noticed extreme red plating on V7 and mild red plating on V6 before I put it back on Standby.
I changed the 5.6K swamp resistors on V6 and V7 with a new pair just to make sure and afterwards measured everything again and didn't get any red plating. I calculated maximum cathode current for the "hottest tube" based on the new B+ readings and biased a little a bit colder than maximum.

I'll give it a spin afternoon and report back.
It only got up to 2.9VAC and dropped to 2.6VAC? It should be at 6.3VAC. Something soundsd wonky there.

danman
Senior Member
Posts: 1099
Joined: Tue Sep 10, 2013 9:09 pm
Just the numbers in order: 13492

Re: Very high B+ (Superlead)

Post by danman » Fri May 15, 2015 6:28 pm

A bad solder connection on your ppimv pot can cause redplating because the bias voltage rides on those wires. It happened to me last year and reflowing all the solder connections at the pot fixed the problem. I would start with a fresh set of power tubes and monitor the bias voltage closely while playing. Be ready to shut the amp down at the first sign of redplating if it occurs again.

Elad E
Senior Member
Posts: 137
Joined: Sat Jun 29, 2013 9:19 am
Just the numbers in order: 13492

Re: Very high B+ (Superlead)

Post by Elad E » Sat May 16, 2015 3:56 pm

thanks Danman,
I decided to follow your advice prior to changing the V4/V5 tubes with the V6/V7 tubes and after reflowing all PPIMV solder joints I played the amp for an hour with various PPIMV and volume settings with my R8 and my partscaster and my conclusion is that it's just a killer amp!

unlike my JTM45 the controls actually do something (the bass knob actually adds bass, not subharmonic mud) and the breakup tones are out of this world!!
it's really much more versatile than I imagined (rolling some mids and playing on both pups of the R8 with volumes on 2 I get the sweetest 60's R&B rhythm tones).

I'm really looking forward to completing the break in period.
I wonder if I should change the .0022 cap for a .022 though - read some threads and only got more puzzled as to which way would be more to liking, guess I'll have to try.

danman
Senior Member
Posts: 1099
Joined: Tue Sep 10, 2013 9:09 pm
Just the numbers in order: 13492

Re: Very high B+ (Superlead)

Post by danman » Sat May 16, 2015 4:27 pm

Glad to hear that you got it working properly! Changing the .022uf in the bright channel to a .0022uf will cut some of the lows from the signal. If your amp sounds great now, with no flub at higher volumes, you may want to leave it as is.

Roe
Senior Member
Posts: 5054
Joined: Thu Apr 13, 2006 1:36 pm
Just the numbers in order: 7
Location: Drontheim. Norwegen
Contact:

Re: Very high B+ (Superlead)

Post by Roe » Sun May 17, 2015 9:14 am

what's the heater voltage measured on the tube sockets?
http://www.myspace.com/20bonesband" onclick="window.open(this.href);return false;
http://www.myspace.com/prostitutes" onclick="window.open(this.href);return false;
Super 100 amps: 1202-119 & 1202-84
JTM45 RS OT JTM50 JMP50 1959/2203/34/39

Elad E
Senior Member
Posts: 137
Joined: Sat Jun 29, 2013 9:19 am
Just the numbers in order: 13492

Re: Very high B+ (Superlead)

Post by Elad E » Sun May 17, 2015 1:47 pm

Roe,
voltage at the sockets is 5.2VAC with tubes installed (2.6VAC at each heater pin of any given single tube socket).
my JTM 45, which was also built with Classic Tone iron, exhibits the same voltage trends - high voltage on the plates and low heater voltage.

Danman,
I had a .0022 cap from the get go as per the Metro Amps' instructions. I've changed the .0022 for a .022 and the difference is prominent - much more punch and heft in the low end. it does sound more JTM-ish with less fizziness and more of the "barking" quality I associate with the essential Plexi rhythm tone.
I probably should have waited to finish breaking in the Sozo caps but I feel like it's christmas morning whenever I find myself at home during daylight with free time to noodle around...


btw, where is the 100 Watt Superlead/Superbass board? it's like a steakhouse not serving entrecôtes.

Roe
Senior Member
Posts: 5054
Joined: Thu Apr 13, 2006 1:36 pm
Just the numbers in order: 7
Location: Drontheim. Norwegen
Contact:

Re: Very high B+ (Superlead)

Post by Roe » Wed May 20, 2015 4:34 am

strange, my classictone PTs all have correct heater voltage
http://www.myspace.com/20bonesband" onclick="window.open(this.href);return false;
http://www.myspace.com/prostitutes" onclick="window.open(this.href);return false;
Super 100 amps: 1202-119 & 1202-84
JTM45 RS OT JTM50 JMP50 1959/2203/34/39

User avatar
neikeel
Senior Member
Posts: 7231
Joined: Tue Dec 06, 2005 8:31 am
Location: Suffolk, England

Re: Very high B+ (Superlead)

Post by neikeel » Wed May 20, 2015 11:40 am

Roe wrote:strange, my classictone PTs all have correct heater voltage
So what are your rectified secondary voltages (i.e. plates)?
Neil

Roe
Senior Member
Posts: 5054
Joined: Thu Apr 13, 2006 1:36 pm
Just the numbers in order: 7
Location: Drontheim. Norwegen
Contact:

Re: Very high B+ (Superlead)

Post by Roe » Thu May 21, 2015 4:36 am

neikeel wrote:
Roe wrote:strange, my classictone PTs all have correct heater voltage
So what are your rectified secondary voltages (i.e. plates)?
I got 445v on my twin and 440v on my jtm45. I haven't tried the dagnall style PT yet
http://www.myspace.com/20bonesband" onclick="window.open(this.href);return false;
http://www.myspace.com/prostitutes" onclick="window.open(this.href);return false;
Super 100 amps: 1202-119 & 1202-84
JTM45 RS OT JTM50 JMP50 1959/2203/34/39

Post Reply