Oscillations with my fuchs Lucky Seven...

[Sammas]

Hi there!

I hope you don't mind me asking this question here, but I thought if anyone could help me it would be a wise bunch of TW clone builders.

My lucky 7 is suffering from low end oscillations. Here is what i've noticed/done:

- This only happens very the amp is pushed into breakup so it is volume/ gain related
- The amount of bass effects the amount of oscillation, for example using the neck pickup would bring more oscillation that the bridge... also turning the bass eq higher.
- It affects every note below about the "E" on the "A"string, it sounds fine playing anything higher than that... It sounds even worse when playing chords.
- I've replaced all the valves and biased the single EL34 (this is a single ended amp).
- tried different cables, guitars and speakers.

So I think i've narrowed it down to a problem with the amplifier itself. I was just wondering if you guys could point me in a direction to start looking. My limited knowledge makes me think its a faulty capacitor somewhere in the circuit... could it be the bass cap in the EQ?

Any help would be greatly appreciated. I am in Australia which puts Andy Fuchs a long way away to aid in fixing this so I'd like to try and nail it myself before sending it off somewhere to be repaired.

[Snapcase]

Start checking PSU filter caps and its connetions.

[tubedogsmith]

How is this oscillation manifesting itself?

[loverocker]

I'm with Snapcase - Fuchs' rep pretty much rules out a design SNAFU, which points to a filter cap connection (possibly where it's grounded) being marginal.

[Sammas]

I spose a clip will better show what im talking about...

here is a quite and nasty clip I just recorded... you can hear how the wheels fall off as I move down the next. This is probably the best i've gotten it to sound... It usually has a weird wobbling to it which you can clearly see in the recorded waveform.


anyway... heres the quick and nasty...

http://www.groupdiy.twin-x.com/displayi ... at=0&pos=0

[Omar]

Sammas
Have you tried to contact Andy about this issue? He seems to be a real helpful guy in the few correspondence I've had with him.

Omar

[Snapcase]

Well, listening to the clip It doesn't seems to be an oscillation.

Assumming you tried fresh tubes in the circuit replacing one by one just to be sure there isn't a faulty one causing that farting distortion, first check voltages on every stage. If any of the voltages seems too high or too low there you got it.

With to low voltage present, check the supply filtering cap for this section, the fastest way is replacing it with a fresh one, check voltage again if the voltage drop dissapeared you fixed it.

If a too high voltage is present in any section then check the resistors surounding that stage, if resistors measuring reveals correct values, then check its connections. Next step is checking the bypass and coupling caps, if a cap leaks there will be positive voltage present at the following stage grid causing unatural distortion, once again check cap solderings both at the board and at tube socket pins.

Hope this helps.

[Sammas]

Hi guys, thanks for your help thus far...


here is was my DMM has revealed after a little poking around...

on the first 12ax7 im getting about 184 or so volts on pin 1, on pin 6 however im getting 205volts.

on pin 1 of the second 12ax7 im getting about 195 volts. The se cond half of this valve isn't used.

The EL34 seems to be running comfortably on around 340 volts or so.


Another oddity, when touching the probe to pin 6 of the first 12ax7 i hear almost a mechanical clicking sound coming from somewhere within the amp... Im not sure where. It is almost like the crackling sound when you fiddle with your guitar cable.

At the moment I think the problem lies here with the plate voltage at pin 6 on the first valve...

any thoughts?

[Snapcase]

I don't know the Lucky 7 schem, but maybe V1 pin6 voltage is a bit in the high side, don't know for sure. It seems you only meassured Plate voltages. Did you check cathode and grid voltages?

Anyway check the solderings and wires going from pin 6 to plate resistor and coupling cap just to be sure they're ok.

[Sammas]

I'll give them a go...

In the meantime i have spoken to Andy and he has suggested a few things to try. Fuchs really like to see there customers happy! Its great... if only every amp builder was this friendly and helpful.

[MarkB]

I can't download the clip without a plugin, but I'll give it a shot. Possibly something is messing with the bias of a tube, maybe a bad blocking cap. If the bias is fluctuating you could get a wobble sound, and bass frequencies have the most energy.

[Sammas]

I don't know the Lucky 7 schem, but maybe V1 pin6 voltage is a bit in the high side, don't know for sure. It seems you only meassured Plate voltages. Did you check cathode and grid voltages?

Anyway check the solderings and wires going from pin 6 to plate resistor and coupling cap just to be sure they're ok.

okidoky... I took some measurements.

V1a plate - 195
V1a grid - 0
V1a cath - 1.6
V1b plate - 217
V1b grid - 0
V1b cath - 1.76
V2a plate - 200
V2a grid - 0
V2a cath - 1.527

El34 pin 3 - 347
El34 pin 4 - 334
El34 pin 5 - -24
And its biased to 55ma according to my bias rite. There is a minor flucuation of about .4ma

[Snapcase]

Well, Did you check this in V1b?

If a too high voltage is present in any section then check the resistors surounding that stage, if resistors measuring reveals correct values, then check its connections. Next step is checking the bypass cap... ....once again check cap solderings both at the board and at tube socket pins.

And its biased to 55ma according to my bias rite. There is a minor flucuation of about .4ma

Did you take massurements right after turning standby on? Voltages may need a few minutes to settle down. Perhaps the bias voltage flutuation is only that.