Help/advice with new build poping at hi vol

[frankeg]

Hello all,
I did a scratch build along Fender lines. It is a single chan pp 6L6GC head with reverb. The pre-amp and reverb is Princeton with a TMB tone stack. The PI is a 12at7 LTP (aka. Super Reverb AB 763) and I have a EH 5u4GB rectifier and fixed bias. The output tubes are EH 6L6GC biased at 36 ma/425 vdc. I added a master vol in the form of a 1meg audio pot on the input to the PI. The mid wiper input has a 220k resistor into a .02 mf cap to pin 2. The amp sounds good with low noise (hiss) and no hum up to when the poping starts. I can dime the pre-amp (1st tube is a 12ay7) and run the master up to about 7 (out 0f 10) before it starts to pop. In a dark room, I see no arching during the poping but I do see the output tubes flicker (both tubes). I have turned it up to poping levels several times during troubleshooting but limited it to a few seconds each time. I never dealt with this before and am not sure of my logic but I would'nt think I have two bad output tubes? I thinking a common component? Stand-by switch? Any thoughts? BTW, I chop sticked everything at mid volume and checked solder joints and grounds (using a star gd).

[Steve Conner]

Is it what we would call "motorboating"- a regular sequence of pops, maybe several per second so it's more of a put-put sound? Or does it pop at random?

[frankeg]

Hi Steve,
Once the poping starts, it quickly ramps up in number of pops until I back off the volume which I quickly do. fg

[Steve Conner]

Try disconnecting the feedback from the speaker output, any difference?

[frankeg]

So far I've disconnected the feed-back, pulled the stand-by switch and reverb tube with no change. I've jumpered from the input tube's 2nd stage to the input of the PI at the top of the MV. Same results. Again, the amp sounds great with all tubes installed up to about 8 on the MV, then the poping starts. Tonight, I'm going to start changing out my filter caps because it really appears like it's a power issue and not a signal one. I also need to check if my going to a 5u4GB rectifier instead of the GZ34 may be an issue (I have Merlin's power supply book). Other than a drop in voltage difference, I seem to recall that there is a difference in the amount of capacitance each type will tolerate. BTW, I'm using F&T filter caps which I've used before with great results which is why I've been hesitant in "blaming" them... I'll try to get a pic of my schematic (hand drawn) on here tonight. Perhaps something obvious will jump out. Thanks for helping! fg

[Steve Conner]

In that case the most likely explanation is that you have low-frequency oscillations due to inadequate bypassing in the preamp. Bypass capacitors too small, too many tube stages sharing a supply node, that kind of thing. Does the popping still happen with the first preamp tube removed?

Alternatively it could be parasitic oscillations, but then I'd expect the symptoms to change when you moved the chassis wiring around with a chopstick.

[frankeg]

You called it correctly, Steve! It's motor boating. With the 1st pre-amp tube pulled it's clearly put-put-putting. I guess with the tube in the gain amplified and distorted it to the popping sound. At the lower volume with the pre-amp removed it's not ramping up like before and my power tubes are not flashing. Is it ok to let it motor boat a while as I poke around with my chopstick? I going to re-do my ground scheme which will take a few days as I do this between work and home time. I'll let you know. Thanks again...fg

[frankeg]

Steve,
I have mostly fixed. As you called it, I had too many stages connected to the same node. I split it up and the motor-boating stopped. I still had too much hiss so I kept checking and found 2 bad coupling caps with dc on them. I also increased the resistor from 4.7k to 8.2k on the last stage of the power supply. I need to get a new 500pf coupling cap for the reverb which was one of the bad ones found. It is the third time I've had problems with silver mica caps. I still have a slight buzz that comes and goes on the highest volume settings. I'm still trying to figure that out and I'm driving everyone in my house crazy - it's a 40w amp and I have no attenuator. I'll keep you posted. Really appreciate your accurate advice! fg

[frankeg]

Fixed! I had more than one problem and as I corrected each one, the amp sounded better but the actual positive feedback came from the reverb mixing network. I used a modified Princeton network using a 10pf cap and 2.2 meg resistor (Verses 3.3 meg) which worked well before in a Princeton Reverb clone I built. Apparently, this hybyrd Princeton/Super build did not like that combination. When I went back to the standard Princeton 3.3 meg resistor the feed back stopped. Funny thing is that I had noticed that circuit was particularily sensitive early on when chop sticking but I dismissed it. I got back to it after everything else I tried failed to completely fix it. It took a good deal of time and I was almost to the point of throwing the thing in the neighbors pond to watch it motorboat to the bottom... I must admit (after the fact of course), that it was a great learning experience. Many thanks again to Steve who probably would have found it in a few hours! I'll get there one day...fg