Headfirst MCII build issues

[beachbuminthesun]

Hello,

New to the forum and not at all a technical person. I recently acquired a Headfirst MCII 100w clone. It came on consignment from a store, so I don't have a direct line to the original builder. It has some issues and I'm planning on taking it to a tech, but in the interest of giving them all the info up front, I'm looking for some information which may come in handy when dropping off the amp. If anyone knows what could be going on here, any insight would be really appreciated. I received the build docs from Jason, my PCB says Headfirst 2019.

Current issues:

• 1. RF crosstalk - amp picks up cell phone noises and is overall pretty noisy
  2. Crossbleed - there's a low level fizzy sound on the rhythm channel which disappears when the lead channel is turned all the way down
  3. Master volume jump in level - I thought "hmm this is pretty quiet for a 100w amp and the master is at 1o'clock!" and then fiddle between the channels a little and suddenly the volume jumps up and my insides were exploding

I'm also looking to install a depth, contour, send level and footswitch which the amp is missing and wondering if that is worth bringing up with the tech or if it will be too difficult to install on a fully built amp.

The amp sounds great right now, went to a friends house who had a superplex and a BE-100 and the amp was doing its own thing and sounding just as good as either of those.

I am worried about blasting it with my 212 G12565s. I'm looking to get a 412 just to be able to play with peace of mind and not worrying about blowing a speaker in case the amp is putting out more than 130w at full volume.

Lastly if anyone know what changes were made from this PCB vs the later revisions (2.3)? From having a glance at Jason's build docs, it seems the newer PCB's are more similar to real wizards than previous versions. But not clear if there would be any benefit in changing some components to 'update' the amp.

Thanks in advance,

[xtian]

To address question 1, make sure there are grid-stop resistors directly connected to V1 pins 2 and 7 (you'll have to remove the FX loop PCB to see V1).

[dcribbs1412]

Good looking build, congrats its a nice circuit. I've built a couple.
Main diff with newer PCB is on board tube loop.
Check V1 what xtian said for RF crosstalk.
As far as mods footswitch is nice to have, contour and depth also.
If it sounds tho good run with it.
If you are playing at full volume and worried about speakers...you are rockin my friend.

[beachbuminthesun]

haha thanks for the info! I don't get a lot of time to blast it, only when the kids are out of the house. I have a pretty dense basement, big carpets, lots of bass traps cushions and couches. So it never sounds as loud down there as it does elsewhere. Lent it to a friend who does demos, so maybe will post a sound sample soon. I play mostly clean, I have a jtm45 and bass spec 1986 and 1992 plexis (everything else is 60s american style combos) so this is my one 'dirty amp' that I got on a whim because of the price, but I REALLY have fun playing it. MV works exceptionally well.

[danman]

I built one earlier this year and it really is a great sounding amp. I used Jason's revised board and went with the tube buffered effects loop with global master volume. Mine has a nice, gradual volume ramp up and the global MV is very useful when playing late at night. As for the crosstalk in your amp, it may be the lead dress between the board and pots. There is a mess of wires there that could be routed a little cleaner.