troubleshooting: preamp tube voltages

[neikeel]

Excellent!
Do not crank it until you have set the bias properly
A few things when biasing:
Make sure you have a load attached
turn your volume pots down (hum or hiss is a signal and you will not be at idle
Clip the earth lead of your meter to the chassis.
Push a piece of wire offcut over the probe tip so that only the pointed bit is exposed.
Make sure you have a consistent wall voltage and consistent plate voltage on your outputs.
Check your output tube grounding bolts have a lock washer adn are tight (ie digging into the metal a little).

Aim for 70% but bear in mind that this is an absolute maximum for new production tubes (+/- 5-10Ma difference is no big deal - in fact adds to some of the harmonics when you push it!
The presence squeal will go when it is installed in the headcab with the aluminium flashing on the bottom to complete the shielding of the board components.
Good luck!

[Elad E]

thanks again Neil!
lots of good advice as always. what's the left off wire insulation on the probe for?

[neikeel]

thanks again Neil!
lots of good advice as always. what's the left off wire insulation on the probe for?

Stop you shorting the bare length of probe against other surrounding terminals or bare wire (avoid expensive sparks!)

[Elad E]

lol, I gathered this might be the case though I got the sparks flying out when I touched pin 8 of the bad Valve Art tube.

anyway, I biased (both tubes' bias resistors read around the 42.8mV mark) and may I say this is one fine sounding amp!!

with the mids rolled back I get a lovely Fenderish sparkle (more into the BF or Brownface territory than Tweed).
I can't really drive it into classic rock territory but it does a wonderful job hitting these tones with my OD pedals.
btw, it takes pedals like a champ (no pun intended) - crazy Cream and Free style wah tones with my Wilson wah.

maybe it's because I'm using a ported bass cab but I don't feel any flabbiness in the lows. tried it with two of my basses - sounds great (it won't blow away any '69 SVT but it's got a thing going for it with bass).

one thing though - I got Groove Tubes' 12AX7s in V1-V3, I put an old Fleetwood 12ax7 (supposed to be a Canadian distributed Mullard - does the famous startup flash) and the HT fuse blew.
I initially had the Fleetwood in V1 when I started testing the amp and was blowing HT fuses by the dozen. I have a Fast-Blo fuse in the internal fuse holder, would switching to Slo-Blo solve the problem? I suspect the tube may be causing it.

[neikeel]

You are probably right.

SLO-BLO T type fuses are recommended in these.

Regarding crunch and break up they usually need breaking in for a while and then reset the bias. Best results for rock are presence 7 or 8, bass 2, mids 10, treble 8 and input upper I on 7 or 8 with unused channel on 2-4 to taste (there is some interaction even with nothing plugged in.
These seem to work best for rock type tones with inefficient G12M greenbacks.
But yes if you dial back volume a little and scoop the mids with a smidge of reverb they are very Fendery.

[Elad E]

thanks, I'll switch to the slow burning fuses as I'm curious to hear that supposedly Mullard-ish tube in V1.

what about the impedance switch? numerous times I encountered the argument that Marshalls sound the best when running on the 16 Ohm tap.
I'm playing it through a 300W 8 Ohm speaker cab so the impedance on the amp is currently set to 8 Ohm.
can I run it on 16 Ohm and mismatch the impedance or would that put too much of a strain on the OT?