mherrcat

It appears that the hum balance pot got zapped. I replaced it with the two 100 Ohm resistors on the pilot lamp and that seems to have fixed it.

What did I do to my amp? ('74 SFTR) I had the chassis out and was re-biasing using a Weber Bias Rite. I wanted to check the power tube plate voltage directly with a multimeter. I believe I shorted pin 3 and pin 2 on one of the power tubes; the tabs were VERY close on that tube and either the probe slipped or I flexed the tab and the two tabs contacted. Anyway, sparks at the point of contact, but, here's the weird thing, sparks also on the other end of the chassis at or very near V1. The amp works but there is a constant buzz. Increases and decreases with volume adjustment. Someone suggested that maybe I had put the full plate voltage onto the heater string and damaged some or all of the preamp tubes. I swapped out ALL the preamp tubes with known good tubes and still had the buzz. Any ideas or suggestions appreciated. Also, any recommendations for a good/honest/reliable amp tech in West LA or the greater LA area appreciated. Thanks! Mark (P.S. I hear ya'll snickering out there...)

mherrcat........... In the half power mode (60 watt) you do use the 4 ohm tap as there are half the output tubes on line. With a hot plate, use an 8 ohm hotplate in the 8 ohm jack in 100 watt mode. Using Yellow Jackets, you do want to use these in the 100 watt mode if all four tubes (two of the Mesa tubes and two yellow jackets) are used.OK. So, if I understand this correctly, I can use two Yellow Jackets in the outside tube sockets and leave two 6L6's in the inside tube sockets and run the amp in the 60 watt mode which will only use the Yellow Jackets OR in the 100 watt mode which will use the Yellow Jackets AND the 6L6's. If this is the case, I suppose I would still want the speaker connected to the 4 Ohm jack for the 60 watt mode and the 8 Ohm jack for the 100 watt mode? Thanks for the help!Yes .... and since you seem to be in L.A. from your profile, you may want to come to one of my free amp clinics when I have them, bring your amp, and I can give you a few ideas in person.I've been checking your website daily for the next clinic. Actually, the Mesa/Boogie is currently at my brother's house in Houston. The questions were for his benefit as he was interested in trying to get some different tonality at a lower volume level. I also had him take a look at the Weber MASS and the THD Hotplate.

mherrcat........... In the half power mode (60 watt) you do use the 4 ohm tap as there are half the output tubes on line. With a hot plate, use an 8 ohm hotplate in the 8 ohm jack in 100 watt mode. Using Yellow Jackets, you do want to use these in the 100 watt mode if all four tubes (two of the Mesa tubes and two yellow jackets) are used. OK. So, if I understand this correctly, I can use two Yellow Jackets in the outside tube sockets and leave two 6L6's in the inside tube sockets and run the amp in the 60 watt mode which will only use the Yellow Jackets OR in the 100 watt mode which will use the Yellow Jackets AND the 6L6's. If this is the case, I suppose I would still want the speaker connected to the 4 Ohm jack for the 60 watt mode and the 8 Ohm jack for the 100 watt mode? Thanks for the help!

I have a Mesa/Boogie MKIIb with an 8 Ohm EV12L speaker. Mesa recommends using the 4 Ohm speaker jack when in the 60 watt power mode. If I were to use a THD Hot Plate, would I need to get the 8 Ohm model and still plug it into the 4 Ohm speaker jack? Also, when using Yellow Jackets in the outside tube positions (for 60 watt mode) do I want to avoid switching to the 100 watt mode?