s/f Princeton odd bass note distortion

[Alex R]

Here's a silverface Princeton non-reverb, in to get a bit more gain, which I have given it by bypassing the tone stack with tweed-type controls, switchable, and making the neg feedback adjustable. But it also has a strange problem which the owner complained of and which has remained constant through all the work. It sounds like a bad speaker - a bit of fuzz on low bass notes. But it's the same with all speakers, the guy fitted a new one and no change, same on mine.

If I feed a signal through and scope the speaker output at something quite near maximum but not yet near clipping I can see a good sine wave, then as I lower the frequency something that looks like quite marked crossover distortion kicks in below about 90Hz, and gets worse as it goes lower. The amplitude is about the same till it starts to roll off down around 40Hz but by that time the distortion is very marked indeed. So I'm wondering if this is the mystery 'fuzz'. It sounds like it.

I checked the bias voltage and it is pretty steady under these conditions, it rises fom 31v to about 34v as the output level increases, and wobbles a bit, like about half a volt fluctuation with some appearance of regularity even with the vib footswitch shorted out. The vib depth control has an effect of about a volt on the bias voltage as you go from 0 to 9. But nothing you do to the vib controls affects the distortion. Neither does clipping new caps across the old, including the bias filter cap (I was wondering if something was modulating the signal but it doesn't look like that. It looks just like crossover distortion!)

Tried clipping in a new OT, no change.

Wit the scope the problem is not really visible on the 6V6 grids, but it is there on the plates.

This amp had a LOAD of bass which was making the problem worse, so I tightened up some coupling caps to get some sort of rolloff instead of the bass boost it used to have. I thought this would solve it but it hasn't changed the basic issue.

Quite a lot of compression on this beast. The owner wanted less and it is a 5U4 amp so swapped in a GZ34. It gave about 5 more volts but sounds stiffer. Yet if I play a low note quite low there is a kind of bloom on it, the volume grows a bit, fuzzes up a bit, then dies away.

Any ideas?

[Don Moose]

Princetons and Champs have an inherent problem where bass notes 'fart' or 'flub'. There is a fix involving reducing the first couple of cathode bypass caps and maybe the PI coupling caps to cut the bass hitting the power amp.

Try searching for 'Princeton flubbing out' for the details.

Hope this helps!

[Alex R]

Thanks Don, I'll do that.

I was amazed at the amount of bass going through the poor little thing and have already changed one coupling cap to cut it, and things did improve... I'll go further.

I wonder whether the concertina PI is part of the reason, somehow?

[bnwitt]

You could also install a choke in place of the first dropping resistor (1Kohm) and change the first filter cap to a 32uf like the AB763 deluxe reverb. The problem with bass note reproduction in that amp is the power supply doesn't have the cajunas to deliver them. You can filter out the bass like you're doing but that is not fixing the real problem.

[Alex R]

Thanks for that.

Well something strange happened. I dealt with the bass issue, however imperfectly, but back in the box it buzzed again and it was clear that the new weber speaker the guy had fitted had a voice coil rub. So I took it back to the bench and it destroyed my 150w test speaker. I'm going to start a new thread about it as this is a separate problem, clearly.

[Enzo]

WHy should we assume they are separate problems?

[Bruce / Mission Amps]

Wow, you'd think a 12W to 18w Princeton Reverb that destroys a 150w speaker must have dumped in high voltage DC current into the speaker winding or 120v line voltage somehow... or the speaker was about to blow from other high powered amps.

[Alex R]

It also fried its original speaker, and the nice Weber the guy put in to replace it, before it went on to make an even bigger mess of my PA speaker. The owner concluded that the ensuing cone rub noises on his two speakers were coming from the amp, and the huge amounts of rather distorted bass it was putting out did make in sound even worse. But that's dealt with now. And the Weber still has a nasty buzz on bass notes.

I went and started another thread on it, on which Enzo pointed out that a DC leak capable of blowing a speaker would probably also blow a fuse. And it is hard to imagine a HT leak that would blow a speaker but not pop a fuse - 400v into 8 ohms... but if it was the end of one of the primaries touching a secondary now and then you'd have a bit more resistance - 400v through 80 ohms... but the thing has never blown a fuse at all.

Mains voltage (240vAC here)? There would be a memorable hum then, and neither of us remember one.

I ran it into a load with a sweep signal going through it at full output, and got it hot, watching the output on the scope for HF appearing, and for DC making the trace jump, but nothing appeared. Some very transient phenomenon perhaps. So I will order a cheapo speaker for it and watch and wait. Grateful for any further ideas...