Trainwreck Rocket AC30-ish build NOW WITH REVERB

Keefy

I re-routed the OT primary wires outside the chassis but that made no difference, so I'll probably put them back inside. In the process I have picked up a crackle - bugger!

It belatedly occurred to me to see what happens when I pull the pre-amp valves. V1 and V2 make no difference, but pulling V3 (phase inverter) kills the hum (but not the crackle - that must be down to the re-made OT connections). I already swapped V3, so I don't think it's a bad valve, although I will check again. I have checked all the connections to V3 with no apparent problem.

Puzzled.

EDIT: Comparing the Modulus circuit that I used to the version below, Modulus adds a 220k resistor to ground between R10 and C8. I temporarily removed this from the circuit, and the hum got worse. Something is going on with either that PI circuit or the tone stack (which might be acting a bit odd anyway).

ICBM

Sounds like it's coming from the tone stack.

In what way is it acting odd?

Keefy

ICBM said:

Sounds like it's coming from the tone stack.

In what way is it acting odd?

The ‘flat’ setting is bright and scooped, to the point of being shrill. Treble and bass are hugely interactive, way more so than on my Matchless Lightning. This might be influenced by only hearing it through an unboxed 10in speaker so far, and it might be par for the course - I don’t know. I should perhaps try it through a 12in in a cab!

ICBM

Is the bass control wired the right way round? It's 'upside down', the clockwise terminal should be grounded, which is the opposite of a volume control.

The bass should be very interactive with the mids - that's correct though, with bass up full the mids are zero'd. This was probably a mistake in the original Vox Top Boost circuit, copied from a Gibson schematic which includes the mistake.

Keefy

ICBM said:

Is the bass control wired the right way round? It's 'upside down', the clockwise terminal should be grounded, which is the opposite of a volume control.

The bass should be very interactive with the mids - that's correct though, with bass up full the mids are zero'd. This was probably a mistake in the original Vox Top Boost circuit, copied from a Gibson schematic which includes the mistake.

The bass control is correct as you describe.

I found the crackle - an unmade solder joint on one of the EL84 cathode connections - now sorted.

More chopsticking - if I bring the bundle of OT primary wires close to the PI plate leads, the hum reduces slightly, then gets very loud when they are in contact.

Tried a 12AY7 in V3 - no difference.

Some people recommend elevating the heater voltages, but I don't have that option - they are centre tapped within the transformer.

That's enough frustration for one day. I'm going to play some bass!

Keefy

I'm so close to cracking this!

I temporarily earthed the coupling cap where the signal goes into the phase convertor (on the tone stack side) and that killed the hum - so it's definitely the tone stack, not the PI.

I moved the tone stack earth to a different point - no change.

Then I started adjusting the tone controls, and found that the hum disappears with treble set to 3 o'clock. The bass control doesn't really affect the hum.

Despite, checking all the components before installation, I disconnected all the tone stack parts and checked them - all OK.

So is there a hum going into the tone stack, and if so, where is it coming from?

ICBM

Remove V2 and try shorting the cathode at R7 to ground. Does that stop it?

Keefy

ICBM said:

Remove V2 and try shorting the cathode at R7 to ground. Does that stop it?

I've literally just done that! No it doesn't, the hum is still there. So it's actually being 'generated' by the tone stack. I'm confident of all the connections and component values - I even checked the pots.

PeteC

Clutching at straws a bit but have you resoldered all your earth/ground connections ? They might look ok but might be suspect perhaps ? 

Keefy

PeteC said:

Clutching at straws a bit but have you resoldered all your earth/ground connections ? They might look ok but might be suspect perhaps ? 

Yep, been over those. Also re-sited the tone stack earth but to no avail.

ICBM

Keefy said:

I've literally just done that! No it doesn't, the hum is still there. So it's actually being 'generated' by the tone stack. I'm confident of all the connections and component values - I even checked the pots.

Where the tone stack is grounded to might be important. Try disconnecting the ground end, attaching a long wire, and try out different grounding points.

Keefy

ICBM said:

Keefy said:

I've literally just done that! No it doesn't, the hum is still there. So it's actually being 'generated' by the tone stack. I'm confident of all the connections and component values - I even checked the pots.

Where the tone stack is grounded to might be important. Try disconnecting the ground end, attaching a long wire, and try out different grounding points.

The tone stack was previously grounded via the volume control. I had moved it to the ground side of R7 which seemed logical, but it made no difference. Will try this.

Keefy

Nope, I can ground the tone stack wherever I like, still get the hum. If I break the ground though I get a VERY loud buzz, just as you might expect.

I also tried placing an earthed aluminium plate across the top of the chassis in case it was getting in as RF, but once again no change.

The link between the B+3 and B+4 nodes runs under the board beneath the tone stack components, but by that time it has passed through a choke and a dropping resistor, and been filtered by 3 caps, so I can't see that being the problem. There is no other mains or HT nearby.

It may well be that the hum becomes far less noticeable when the speaker is mounted in a combo on the floor rather than sat on my worktop, but for now it is definitely loud enough to be annoying.

ICBM

Try disconnecting the B3-B4 link, just to eliminate that.

Keefy

ICBM said:

Try disconnecting the B3-B4 link, just to eliminate that.

That'll be fun!

PeteC

How about trying to replicate the same grounding scheme that is on the express?    Basically where each section goes back to the power supply node cap earthing points ?   I did that on my express build and no earth hum. 

PeteC

Dodgy filter cap ? 

Keefy

PeteC said:

How about trying to replicate the same grounding scheme that is on the express?    Basically where each section goes back to the power supply node cap earthing points ?   I did that on my express build and no earth hum. 

I looked at the Ceriatone layout for the Express, and it's pretty much what I'm doing now, ie. one ground point for power amp, one for preamp. Also no changes I have tried on the grounding scheme have made any difference.

PeteC said:

Dodgy filter cap ? 

No the symptoms don't fit. I tested all components before fitting, and use heatsinks when soldering, so unlikely I fried one.

ICBM

If there's a null point on the treble control it does sound like hum being directly introduced into the tone stack, to me.

Keefy

ICBM said:

If there's a null point on the treble control it does sound like hum being directly introduced into the tone stack, to me.

Yes, I found this old discussion where someone describes a similar problem. Gonna disconnect B+4 later.