67-Plexi's "critical observation" thread has me thinking about a systematic method of tuning the Dumble circuit. Any tips on the best way to approach tweaking? For example, starting at V1, adjust plate voltage, adjust bias, find best coupling caps values for tone, move to V2...etc.
Dogears has been more than generous about giving tips for the best values to use in each circuit, but I'd really like to get a deeper understanding of tuning.
Methodically tuning a Dumble
Moderators: pompeiisneaks, Colossal
Re: Methodically tuning a Dumble
Dogears often refers tro a 'system' and I totally agree.
With any amp there is an order and a number of fundamental things that need to be in place and correct before tuning begins. I think the difference beween a 300pF and 470pF snubber is totally moot if your operating voltages are a mile off or your ground scheme is a rats nest.
With any amp there is an order and a number of fundamental things that need to be in place and correct before tuning begins. I think the difference beween a 300pF and 470pF snubber is totally moot if your operating voltages are a mile off or your ground scheme is a rats nest.
Re: Methodically tuning a Dumble
...or you have a treble cap bleeding DC !!!
I always work from the power section forward. the first gain stage would be last.
I always use a known set of 'book value' preamp tubes. There's nothing worse than chasing around a gound loop hum only to find the problem is a crappy 'new' tube.
I always work from the power section forward. the first gain stage would be last.
I always use a known set of 'book value' preamp tubes. There's nothing worse than chasing around a gound loop hum only to find the problem is a crappy 'new' tube.
Re: Methodically tuning a Dumble
Amen to that. In your other thread you mentioned EH...I had a bad batch of them that had me chasing ghosts for a week.'67_Plexi wrote:I always use a known set of 'book value' preamp tubes. There's nothing worse than chasing around a gound loop hum only to find the problem is a crappy 'new' tube.
I gather from your comment that the first thing to dial in is operating voltages. As obvious as that sounds, I've never really gone beyond "close enough" when checking my build. I think I'll do some tinkering this weekend to see how close I can get to known good values. (Funny thing is, my favorite build to date has very low voltage on V1a.)
Re: Methodically tuning a Dumble
I would say that the most critical are V1 and V2, you need to shoot for between 195 and 205 on V1 and 210 to 220 for V2. That's what works best for me anyway. Don't be afraid to change the dropping resistors to attain this. People may get too hung up on getting it 'exactly Dumble' and that means everything like for like, even these resistors. This is all well and good, but trust me from an Engineering standpoint HAD wouldn't care what values he used, he would be shooting for the sweet operating points and that will be different amp to amp.
Also remember that wall voltages vary all the time and place to place. This can have a pretty drastic effect to, especially on the bias voltage. I always use a Variac at gigs. It at least gives me a ball park starting point. My main amp is actually a '67 JTM50 Marshall Plexi and I biased and run it at 110V. Every other amp I have set up to run at 115V. I'm not attempting the 'Eddie' thing, just want a consistent tone gig to gig and it's the only way that works without buying expensive voltage regulators.
I'm sure Dogears could drill this down even more because he has a lot of experience with the Dumble amp design. I'm more the Dogears of the Marshall world
Also remember that wall voltages vary all the time and place to place. This can have a pretty drastic effect to, especially on the bias voltage. I always use a Variac at gigs. It at least gives me a ball park starting point. My main amp is actually a '67 JTM50 Marshall Plexi and I biased and run it at 110V. Every other amp I have set up to run at 115V. I'm not attempting the 'Eddie' thing, just want a consistent tone gig to gig and it's the only way that works without buying expensive voltage regulators.
I'm sure Dogears could drill this down even more because he has a lot of experience with the Dumble amp design. I'm more the Dogears of the Marshall world