EQP 1-A tube program (Analogvibes project) Help needed

[Takroc]

OK, I listened to your second video/sound cllip. As you say, the high pitched oscillation has gone. What you have now sound to me like "motor-boating". This occurs when the negative feedback stability breaks down at low frequencies. It is often caused by too little or poorly implemented HT decoupling. So you need to make sure all the electrolytics are the right values, fitted the right way round and securely connect to 0V.

The other thing that concerns me is that swapping the leads changes the voltages on the two anodes of the 12AX7 are not the same. Can you post exactly what these voltages are and also post the voltages on pins 3 and 8. It is possible the motorboating is affecting the dc readings so it would be an idea to measure the 12AX7 voltages with the 12AU7 tube removed.

Cheers

Ian

I am pleased you got it going. That capacitor is not a polarised type so it should not matter which way round it is fitted. Perhaps one of the joints was not good. That capacitor is part of the circuit that tames the high frequency response so it will make a difference to stability. On the other hand, even some unpoloarised capacitors do have a 'preferred' orientation which is a function of the way they are constructed.

I should point out that your current construction is not ideal from the point of view of ensuring stability. Particularly the cables from the output transformer - you should aim for it to look more like the picture yo posted with leads cut to length and dressed appropriately. I made exactly the same mistake on the first tube audio amplifier I built back in the 60s. It squealed like a pig.

Cheers

Ian

Hi Ian, I think it was a bad connection. If you watch the first video you can hear the noise change as I push the one capacitor. This is the one with the lower voltage. I also was thinking if by some weird chance the cap works better in one direction. I am planning on cleaning everything up, It was neat and tidy until I started troubleshooting. I’m just happy it’s working.

 

[RoadrunnerOZ]

I think those caps are non polar - the sound of HF squeal varying as you moved the capacitor(s) and wires tend towards sounding like bad solder joints - one end of one cap not terminated with a properly tinned joint will possibly have it acting as an “antenna” - or an internal failure of the cap leg junction to the foil - this can happen if you twist/bend them around a lot and crack the casing at the entry point of the leg. When you tidy up I’d suggest going over every joint with fresh solder.

 

[Whoops]

I also think your problem was a Bad Solder joint.

But in the Topic of correct orientation of Non-Polarized capacitors, although they're not polarized there's still a preferred orientation in order to achieve the lowest noise possible and that has to do to which pin the outer foil is connected to, here is a great video explaining all that and testing it:

 

[RoadrunnerOZ]

We used to do that check on unmarked caps when we were building guitar amps back in the ‘70s - made a big difference in the noise level of the whole circuit. Even though caps are non polarised they do have a preferred direction for placement - schematics would use a symbol like a saucepan with a lid (now obsolete) - the pan was the outer layer and the internal lid was the inner layer - for electrolytics they added a + sign, there is also another with outer layer identified as a winged or curved line:

 

[RoadrunnerOZ]

The 20pF capacitor between pins 1 and 6 is to prevent parasitic oscillation - the solder joint at pin 1 is your likely culprit but I’d be resoldering all your connections at the tube bases.