Need help on oscillation problem, otherwise first test OK...

[Tonegeek]

Put the juice to my HRM today for the first time and got sound, A real ugly, loud, farty, sound that would have toasted my speakers if I had not been quick on the standby switch. Once I removed the feedback resistor, the noise was gone and the amp worked normally. I get a nice clean signal all the way through now.

Got any ideas on what could be the problem? It is a 50 watt build with a Marshall OT, EL-34's biased to 70% plate diss. (37ma). I have the OT secondary wired like Aiken suggests, with isolated speaker jacks, the common going to the jack barrel, then back to the ground point on the buss near the presence pot ground. I am taking the feedback off the 4 ohm terminal of my impedance selector to a 4.7k resistor on to the pi network, 2k pot, etc. I have triple checked the schematic, but don't see any issues. I tried moving some wires around so that the plate and grid wires to the power tubes were not touching. Another weird thing, it only oscillates when connected to the speakers, with my dummy load, it works fine. I don't have a scope, but my voltmeter shows no AC on the dummy load (until I hit a note), and I can listen to the signal via a monitor and it sounds fine.

[Bob-I]

Got any ideas on what could be the problem?

Yes, you have positive feedback. Reverse the OT seconrady leads on the tube sockets. You should have one brown and one blue, reverse them.

I'm convinced that Murphy lives in output xformers, I never get these right the first time.

[Tdale]

Do you sure you don't mean primaries?

Tommy

[pamaz67]

Correct: reverse the primary windings of the transformer.
Small trick: if you don't have a dummy load ( instead of the spkr)and a scope, power up the amp with the st by switch already on. While it is warming (if it's going to oscillate), you'll hear the squealing noise not at full power but raising as the voltages in the amp will raise . This way you'll have the time to switch off the amp and reverse the winding with no fear for your spkrs.
ciao

Paolo

[pamaz67]

sorry, just now i realized you have a dummy load.
Ciao

paolo

[Bob-I]

Do you sure you don't mean primaries?

Tommy

Of course, primaries.

[Tonegeek]

Yes, you have positive feedback. Reverse the OT seconrady leads on the tube sockets. You should have one brown and one blue, reverse them.

I'm convinced that Murphy lives in output xformers, I never get these right the first time.

I realize you mean primary. I had a eureka moment this morning while brushing my teeth and realized the phase of the feedback could be the culprit. At any rate I was going to try swapping the primaries when I got off work today. My brain worked it out while I was sleeping I guess. Too bad it can't do that before the problem happens! At any rate, judging by the response here, I am sure that will fix the problem as it makes perfect sense. Bob, you have been a big help on more than one occasion, so I am saying thanks here for this and several earlier responses to my questions. This was my first ever amp build and I could not have done it without the support of this forum and members like you. At some point I am going to post a "lessons learned" documenting all the mistakes I made and tricks I learned while doing this project to try and give back a little and help some other newbie builder.

Small trick: if you don't have a dummy load ( instead of the spkr)and a scope, power up the amp with the st by switch already on. While it is warming (if it's going to oscillate), you'll hear the squealing noise not at full power but raising as the voltages in the amp will raise . This way you'll have the time to switch off the amp and reverse the winding with no fear for your spkrs.
ciao

Paolo

Thanks for the tip, I will try it tonight (Its good not to wake up the fam at midnight like I did last night when this thing went off!)


BTW the amp does not sound half bad with no feedback at least on the clean channel (OD - not so good), but it is not quite the sound I am looking for. There are plenty of spots to tweak this baby so I am sure I will get it sounding much better soon.

Whit

[Tonegeek]

Yep, that did it - thanks. Pos. feedback is now negative. I am loving this amp aleady and I havent even starting tweaking it beyond the trim pots. I do seem to like it better with about 9k-10k for the FB resistor. I noticed the treble control (and mid control too) seems very subtle even with PAB and mid boost off. I have the Skyline EQ. Is this normal? I have the treble all the way up and the amp is not overly trebly (and I tend to like them dark). My strings (Mex Strat) are dead, my workroom has visible insulation (very dead) and I am using very bass heavy speakers (Hellatones), so maybe that is it. I am not displeased. This amp is already going to be one of my all time favorites, I can tell. The clean channel is absolutely to die for!

I will talk about hum control on another thread. It has a tiny little bit of hum when cranked really high (guitar off). Not any more than some Fenders I have owned, but it could be quieter, I think.

[heisthl]

Make sure you try a different Guitar cable - Just yesterday I had left my normal bench cable (ProCo) in another room and instead used a fat black "Fender" branded cord - the amp I was tweaking lost all its highs and was slightly noisier. Less noise and more highs with good cable.

[Bob-I]

What about shielded cable inside the amp? Where did you use it and what type did you use. Often shielded cable can rob highs.

As for the subtle treble control, yes that's normal. I verified this when I played through a real Dumble a few months ago.

[Tonegeek]

dont think my cable is the issue as it is the same one I use with all my amps and I usually have too much treble. I do agree a cable can make a difference. I have about 14" of shielded cable in the amp just to v1a-v1b grids. Thats it. I do not know specifically the brand or cable designation. It looks kind of cheap actually. I have read posts though where people did notice a difference just removing this one little bit of cheap stuff so I will investigate it at some point. I don't think it is going to be an issue, just verifying the operation of the controls mainly. They seem subtle compared to most amps I have used, but I really like that because the basic tone is already there. This is the first amp where I can actually picture myself using the bright switch. But then again, I am in charge of the cap which in my case is 100pf.


Ok, I am going to post a new thread on hum. I killed almost all my hum but don't know why this works...