5e3 motorboating?

[chuckb]

Hey guys, I just finished wiring up my 5e3 build.I have alot of hum and when I turn up either volume pot past 4 I get that motorboating sound. I have guitar sound coming through and it sounds ok other than the hum and motor boating. I'm suspecting it's probably a filtering problem on the B+ ? Any suggestions on what approach I can take to address this problem. I don't have any shielded cable on my inputs so that may also be a reason for the hum. I have a chrome plated 5e3 chassis, so when hooking up any of the grounding points do I need to grind off the plating to get a better grounf? Any help would be greatly appreciated.

Chuck.

[MWJB]

Sounds like a grounding issue, check pots & jacks are tight in the chassis, check that grunded eyelets on the board actually are grounded, measure resistance to groundfrom these eyelets.

Yes, it is a good idea to grind away the chrome, especially if soldering grounds direct to the chassis...but lots of guys bolt down ground points, direct to the plated chassis, with no issues.

Why do you suspect the filtering? change any cap that you suspect, but I'm not sure that this is your primary issue.

[chuckb]

thanks for the reply. I will check all my grounding points and see if I can track in down. I suspected a filtering issue because I had similar issue in another amp and it was a cap.I know that with grounding issues you can get hum,So poor grounding can cause this motorboating as well ?

Chuck.

[MWJB]

Poor grounding & layout, loose grounded components can all cause motorboating.

What type of grounding scheme have you used?

[chuckb]

All my board grounds all go to one solder lug which is bolted to the chassis. My pots and input jacks are also on on solder lug along with the ground for the output jacks. I have both CT soldered to a single lug which is attached to a tranny bolt. My power cord ground is soldered to a lug on a seperate tranny bolt. I was going to use star grounding but I ran out of solder lugs, so this is why I used the single lug terminal strips.

Chuck.

[chuckb]

Hey MJWB, I think I may have to take out the board and check the connections underneath. I really am reluctant to do this but my couriosity is getting the best of me. I really need to git rid of the hum and motorboating. This is the first thime I've used a eyelet board. I have built three 5e3's before but I used the old way of PTP wiring with very few problems.

chuck.

[chuckb]

I've removed the board and discovered that on of the leads were long and protrouding throug the board on the underside. This may have been coming into contact with the chassis. I trimed the lead and hopefully it was the problem.I'm usuing 1/4" standoffs on the board. are these high enough as to no cause any problems with contact to the chassis? I'm using all cloth covered solid core wire for the board connections.I have 18Ga teflon coated wire for the tube sockets. Any suggestion on how to install this board where I won't have to de-solder all the leads just so I can remove the board,or is that common practice when wanting to view the underside of the board?

Chuck.

[chuckb]

Anyone?

[MWJB]

Why only one ground point? Use one for preamp circuit & onefor power amp (main & screen filters, 6V6 cathode resistor, 220K grid loads, B+ CT) at the PT end of the amp.

Disconnect the speaker output ground wire from the same ground point as the other grounds, in fact use grounded jacks for speaker & input jacks...like a 5E3.

[chuckb]

Hi MWJB, Thanks for your reply.I'm sorry you misunderstood but I have two ground points,one for the preamp and one for the power section. Upon removing the board again, I noticed that some of the leads were a little long going through a couple of the eyelets and they were probably grounding on the chassis.I trimmed those and I have the amp all put back together again. Everything works great with no hum whatsoever. I have one input that isn't working but I suspect it's a bad solder joint. The remaining inputs work fine. The amp sounds beautiful,I'm going to enjoy it.I have to build a cab and do the tolex etc. I'll post some pics when it's finished. Thanks for all you help I appreciate it.

Chuck.

[chuckb]

Hey MWJB,
I still can't isolate the problem with that one input jack. Could it be a faulty jack? I get some sound when the vol is turned all the way up,it's very faint though. All the other jacks are working fine. I replaced the 1M resistor and re-soldered the wirecoming off the input jack going to the 68k resistor, still nothing. I may try another input jack and see if that's the problem?

Chuck.

[chuckb]

I Replaced the input jack,nothing. I replaced the 68k resistor,nothing. I replaced the 1M resistor on the input jack ,still nothing. I can't understand why three out of four input jacks are working and the forth one isn't? It's the #1 input jack on the bright ch that's not working, only very low sound when vol is maxed out. Any ideas what else I can try?

Chuck.

[chuckb]

I found the problem,I had the input jack wired wrong! I guess we do learn from our mistakes. Working fine now,thanks for your replies,I appreciate it.

Chuck.