Ampeg J-12T "fuzzy" overtones

[blewgrass]

I picked up this amp DOA as a project, and within one hour had it up and running great. Well almost.

The amp has a distortion type of overtone while playing at low volume. It reminds me of the sound you get when you turn off a tube amp and the last few seconds of "fuzz" you get before the sound fades. (this mostly occurs on amps with no standby switch). It also sounds similar to a stomp box with a dying battery.

Any ideas on where to check? My first theory is when the last person who owned this amp fried the power tubes, they fried some plate resistors or caps along with the tubes.

Thanks for any replies.

Steve

[52 Bill]

The amp has a distortion type of overtone while playing at low volume.

Does the distortion go away at high volumes?

Any ideas on where to check? My first theory is when the last person who owned this amp fried the power tubes, they fried some plate resistors or caps along with the tubes.

Have you checked the resisitors?

The symptoms that you describe could be caused by any number of things. Have you checked the tubes? Have you checked the speaker? Have you checked the power supply voltages?

[blewgrass]

Hi Bill,

Thanks for the reply.
1. The problem exists at all volumes. It doesn't go away and it only begins after the amp "warms up".
2. I have not checked any of the voltages you mention but I intend to this evening and I will report my findings. The schematic is nice becasue it has "test points" with given voltages and resistance to compare with my amp.
3. The speaker works ok until the amp warms up. I will try it through another amp to make sure that is not the problem, but the distortion is different from that of a blown speaker.
4. I will also check the voltages coming out of the transformer.
5. A local amp tech suspects the "phase inverter" and says to start checking resistors too.
6. There seems to be some residue of some kind on the tube sockets. I think I need to check any components near the tube sockets and possibly the sockets themselves.

Thanks again for taking the time to respond. I followed the thread in December where a similar amp was cutting out.

Steve in Nor Cal

[Enzo]

I would be looking for DC voltages that don't belong. The signal would then be having to overcome this DC level to fight its way into the following grids. Most likely reason for DC where it doesn't belong is leaky coupling caps.

[blewgrass]

Thanks for the clue, Enzo. I'll check that when I dig into this puppy in about an hour.
Would I be looking for higher values then? When you say "don't belong" I'm taking that to mean higher than necessary voltages or fluctuatiing readings.

what areas of the circut would you be looking in particular? Tube sockets? Voltages at the speaker... OT?

Thanks

Steve

[Enzo]

No, I am looking for things like a couple volts on a grid where zero DC belongs. I am looking for maybe 6 or 8 volts on a cathode instead of 1 or 2. Looking for plate volts on a preamp tube of maybe 65 volts instead of 165 volts. I am looking on the small tubes in the preamp.

Look at the cathode resistor in the power stage - look OK? Got close to the right voltage across it? Got B+ on both power tube screens? The grids of the power tubes is exactly the sort of place to find some + DC voltage where zero belongs.

There are DC voltages on the schematic at MOST of the tube pins

I am looking for signs of leaky caps, whwich would be the cause of all those things.

I am not worried that I will find 500v where 350v ought to be.

It is hard to "fry" caps.

[blewgrass]

Thanks for all your help, gang. Amp is fixed and running perfectly.
I don't want to admit to you how it was fixed. Ok.. it was the speaker... duh!

I plugged in a mojotone and fuzz went away... The stock speaker for this amp is very wimpy and must have been blown by the last owner. The worst part of it is I bought a pair of JJ EL 84's and paid $36 and realized I could have paid $15 a pair for these... Oh well.

Thanks again. Great board