Don't Know What I'm Hearing

[boots]

Against my better judgment, I will post a couple pics. I am particularly bummed about the OT secondary routing under the board. This was something that made sense as I was mounting the transformers, but once I got the rest of the stuff wired up underneath, I realized I should have done that differently. I thought I'd see how it works first, in order to avoid drilling any more holes unless I really have to.

TT, I hear what you're sayin'. And maybe you're right that it really doesn't matter. But I guess I am also in the "shouldn't motorboat at all" camp. Mostly because this is a hobby to me, and I am interested to know what the heck is going on. I believe it can be cured. But hell, I'll still play the amp on 8 anyway until I get it figured out!

[boots]

Ugh - couldn't get the photo to attach.

[boots]

I figure that posting a gut shot here on TAG is like "Go cut yourself a switch" and I think I am man enough to take all the constructive criticism you guys can dish out. Maybe even a little bit of ridicule. This is how we learn.

I was getting pretty good at making Champs look tidy, but this one kept getting worse as I went along!

[tictac]

Oh so you were really playing us.... that's an excellent layout, very tidy, I would certainly not be ashamed with that! nice color coding too....

you must be a perfectionist of sorts, is that why you can't let go of this motorboat thing? You're like, "It sounds so good... but I know if I crank it, it will make that sound nope, I just can't have that!"

Building amps from scratch like we do makes you respect the the guys like Leo Fender, Dan Reeves, etc. The hundreds of hours they put in to get it right... If you don't build an exact copy of a vintage amp; when you decide to tweak the layout, the way things are wired, mounted, soldered... then you're building a prototype. Once you do that be prepared for some suffering unless you just plain luck out, (or are an experienced builder with outstanding construction and layout skills and an understanding of why things go where they do...

One thing I noticed is your grounding of the pots is unconventional. IT looks nice but alot of builders perfer a nice big bare copper ground buss wire with an individual ground for each pot soldered to that...

If you really want to troubleshoot the motor you should plug into a resistive load hooked up to a scope then do some probing with a chopstick, moving wires around and see if you can change how the amp behaves... you're in for alot of hours going that route. You could also increase the values of the filter caps but what happens it you lose some of the bassman vibe and tone when you do this. (some student in Germany actually wrote a paper about this with the 5F6-A circuit, really interesting)

TT

[tictac]

Looking at your layout more, maybe a few things to check and correct layout wise...

1) Grounding scheme, looks all over the place...see vintage layout

2) The one area of the amp which actually is sloppy compared to the rest of the amp is the area around the power tubes... better would be plate leads close to the chassis away from screen and control grids...

3) and is that a presence control next to the power tubes? I'd take that out wires and all and move it to the front hanging out of the amp and do your cranking thing afterwards...

4) you got no grid stoppers on your power tubes. 1K5 to 5K, the connection to pin 5 should be as close as physically possible

That's where I'd start....

TT

[boots]

Thanks TT, those are good comments. I actually followed (somewhat) Martin Manning's "Modernized 5F6A Bassman Build Guide" (THANKS Martin!). Which is actually pretty close to the original Bassman layout, I think.

The grounding scheme seems different than the star grounding I've used in other builds, but seems to make sense.

Yes, the presence control got mounted on the rear panel because I screwed up and didn't drill enough holes in the front panel when I layed it out. Would've been a lot cleaner with the pot on the front panel.

I just put in 5.6k grid stoppers today at each EL34 socket. Didn't seem to affect the problem, but I think they need to be in there anyway.

I agree the lead dress around the power tube sockets isn't too pretty. Maybe part of my problem?

[Leo_Gnardo]

In all the comments, I have yet to see "I looked at the output on an oscilloscope." So... are we flying blind? You would see ultrasonic noises on a scope in an instant, then maybe develop some ideas what to do about them.

If it's really motorboating, that's the sign of insufficient power supply filtering. Clip in extra filter caps here & there, if it stops, there's your culprit.

FWIW I just completed modding a Ceria "Champ" to a "Prinzeton". All good except a blipping buzz noise when volume turned all the way up. If I turned it down, even a tiny fraction, no more noise. So I measured, how many ohms off the top of the 1 Meg control stops the noise? About 4K to 5K. So I soldered in a 6.2K resistor between treble wiper & top 'o the volume pot. I know it has no audible effect on the audio - so it's a "gimmick" that stops the noise. I could have analyzed, hypothesized, and rationalized until the cows came home... instead slapped in a cheap & handy "fixit." Job. Done. Who knows, might work for you.

[boots]

I do have a scope and that's the next thing I will look at. However, it will be a few days before I get back to this, since we are going on a quick family vacation for a few days. There will be no tinkering till we get back.

Thanks for all the help guys! I'll either figure this out or end up living with it. The amp sounds great either way.

[M Fowler]

The amp looks great to me Boots

Either bad solder joint at or near the filter caps or bad cap itself. I usually find a bad solder joint when I get motor boating on a new build.

Mark

[boots]

Thanks Mark. This is the chassis and transformers I got from you a couple years ago. Took me this long to get around to building it. I'll dive back into this thing when I get back from vacation.

[Reeltarded]

you totally lost all your credibility with me.. it looks great..

(you liar)

Sometimes lil things like 10k grid stops on the PI can do wonderful things for stupid problems right there at the end.

[M Fowler]

Thanks Mark. This is the chassis and transformers I got from you a couple years ago. Took me this long to get around to building it. I'll dive back into this thing when I get back from vacation.

Oh that was a JTM45 tranny set I believe, don't remember the chassis

[frankdrebin]

I'm not sure if I'd know a parasitic oscillation if I heard it. Maybe one of you folks can recognize this problem. I've done some searching on this site but not sure how to describe the problem.

I'm still working the bugs out of this 5F6A/JTM thing I just finished. I pretty much followed the Bassman layout, but I'm using EL34's.

The amp sounds great! Until I crank the volume pot past about 8 or so and drive it hard. Then I get a "Thup Thup Thup Thup" sound that continues on its own until I back the volume pot down. The thupping is rhythmic, about 2 times per second. The sound is accompanied by flashing inside both EL34's - I can see it through the holes in the plate. Could it be the screen?

Does this sound like "motorboating"? Could it be blocking distortion? I don't know what that sounds like. The flashing in the tubes is disturbing. I have tried a different set of tubes, and the problem persists. The problem seems to be independent of bias - it still happens even when biased very conservatively.

Any ideas?

i don't see any grid stopper on pin 5,i'd start with a 4,7k or more there,and 1k on screen grids.
What's the value of the feedback resistor and on which tap is it connected?

[TUBEDUDE]

Yeah, I wish my layouts looked that crappy. Very clean layout. If I posted one of mine I'd be banned for life.

[boots]

Well, I got back into this thing and I'm still stumped.

I re-flowed the solder joints for the filter caps. No difference. Then I tacked on an extra filter cap per Leo's suggestion. No difference on B+1 or B+2, but it did reduce the severity of the problem with the extra cap paralleled with B+3. So I replaced the 20 uF cap with a 47 uF for the B+3 leg, which is the PI supply.

However, the problem still exists and after experimenting a bit, I find that the motorboating can be set into action by simply turning both channel volume controls up past 6 or so, without any guitar or other signal plugged into the amp. And it continues indefinitely until I either turn down the bass control or one of the volumes.

On the scope, I'm not seeing anything that looks like HF, but there is a nasty, random-looking waveform that corresponds to the motorboating. I'm still not sure if what I'm hearing is classic motorboating, or something else.

Could this possibly be a symptom of a bad coupling cap? I used some salvaged Micamolds, but I tested them for leakage and they seemed OK. Also, I used a salvaged choke, which came from an organ that used a pair of 6L6's. I believe it is an 8 Hy choke, based on seeing one just like it for sale on eBay. The coupling caps and choke are the only questionable components I used in this build.

The amp certainly is not un-useable, but this bothers me and I'd like to find the cure. I'm considering adding some grid stoppers to the PI stage as suggested by Reeltarded.