M15MK1 volume level increases after a while

Hi,
I noticed that the overall volume level of my m15mk1 increases after playing a while. Is this normal and if not, what could be the reason?
Speaker cabinet is a new homemade 1x12" with eminence texas heat.

Re: M15MK1 volume level increases after a while

I just played for a while and noticed that the volume increased clearly and I was using the distorting channel but I didn't (damn me) check the clean channel but as far as I remember it effects both channels.
I use the amp a few times per week and I have on like 1 hour or so.
This was the first time I noticed the increase when I was playing.
Any idea what can cause this?
What about the amp heating too much, I built an amphead which
is open in the back but the forward side is closed.

Re: M15MK1 volume level increases after a while

I got this answer at ax84.com:
"If its output stage is cathode biased then maybe as it gets hot it is dropping in value.My Ampeg reverbrocket did the with the stock underrated cathode resistor untill after 40 years of service it gave out. They never new back then that anyone would play the amp flat out cranked up so where you need a 15 watt resistor they put a 12."
Would this be the reason?

Re: M15MK1 volume level increases after a while

I just played for a while and same thing happened and the volume increase happened for both channels. I switched off the standby and after a while I switched the standby on and the sound level had dropped back. Then I played again about 5 minutes and same thing happened (louder voice), I switched standby on and off and and the level had dropped back.
So I could repeat the phenomena.
I'll try and check the same thing tomorrow and look at the tubes all the time and see if there happens anything visually observable.
If I'm going to open the amp what would be the places to measure the voltages for checking things up? (logical next step I'd think)

Re: M15MK1 volume level increases after a while

Ok, I'll measure the DC voltages when I've finished my holidays - I'm not in a hurry... (I have to disassemble the head first).
Otherwise the amp works ok and I very much like the sound and considering buying the G2 too.

regards Tomi

Re: M15MK1 volume level increases after a while

I measured some (not all) voltages with the tubes attached and the speaker connected. Some were ok but the voltage at V3's pin 1 (connected to R18/100k and C16/10nF) was 267 V and voltage at V3's pin 6 (conn. to R17/100k and C15/10nF) was 180 V. They are supposed to be 204 and 206 V. So any ideas what is happening here?
(I did all the measuring without the waiting for the volume increase)

Re: M15MK1 volume level increases after a while

Found the fault! It was the 1M resistor at the V3 which was badly soldered and I think that it contacted other components after the tube warmed up. I checked all the voltages and found that something was wrong but when I started to measure the resitors I found the right place... ;D