Roadking pop goes the preamp!

konstantine RK · Oct 31, 2010

ROADKING HEAD... REPLACING V4 AND V5 (MESA 12AX7'S) WITH MESA SPAX7'S

I was putting in new SPAX 7's to v4 and V5 each.... Then I put in matched sets of JJ's EL34's and 6L6's also a set of 5U4GB JJ rectifiers...basic tube swap out.

then all hell breaks loose after I turn it on... I hear POP POP POP POP POP POP over and over so I turn the Roadking head on standby and turn it back onto power... pop pop pop pop I turn it off. I then swap out the new spax7's and now I notice the FX loop is distorted and weak sounding no matter what I put into it. I swapped the old output tubes back but the loop is gone.

So Im taking it in tomorrow first thing. any comments would be welcome...

these were new SPax 7's from Mesa btw. :| :| :|

espboogie123 · Oct 31, 2010

Does it still pop when the new power tubes are in and old preamp tubes are in or does it only pop when spax7s are in? Popping is a sign of a bad power tube or two. It happens...even with new tubes. Some tubes don't survive the shipping process.

konstantine RK · Oct 31, 2010

tried both.... new and old. No more popping as I took out both SPAX7's... replaced both preamps with the old 12AX7's (mesa) and put the old output tube set back in. The popping lasted for about 10 seconds until I turned it off, then turned back on (popping came back) turned it off and replaced the output set.

did I fry anything in the circuitry????

When the loop is switched off everything seems to be fine but when it is engaged the loop sounds weak tired and distorted.

Im taking it in tomorrow and I hope to have it back in a week.

94Tremoverb · Nov 1, 2010

It sounds like you did fry something in the FX loop circuit, yes. The most likely cause was using a Russian preamp tube - the FX loop position is a cathode follower and these tubes often won't take the cathode voltage in these. It's very rare for a faulty preamp tube to damage an amp but this is one of the very few cases where it can, because in a cathode follower there is no plate resistor to limit the current if the tube shorts cathode to filament. Do not use Russian tubes in any cathode-follower position (V3 or V4 on this amp I think).

It's not a good idea to change all the tubes at the same time by the way - because if you do get a problem like this, you have no idea which one(s) caused it. Change power tubes and rectifier tubes as sets (bearing in mind that the 6L6s and EL34s are two separate sets in this amp), and preamp tubes one at once, testing the amp each time. That way you will hear a bad tone change immediately as well as any more serious problem.

konstantine RK · Nov 1, 2010

thank you for the advice! Had no idea what I was getting into there. My music store owning pal just got done yelling at me for tinkering around and being counter productive.

Its the reason why Im offering to buy a Roadking II in the classifieds

94Tremoverb · Nov 1, 2010

There's nothing wrong with changing your own tubes, but it's best to be really methodical and analytical about it, so you know exactly what messed up when something did. It's natural for me to be like this, but I know it's not normal for musicians. After a couple of decades I finally realised that's why I'm a tech and not a rock star.