Modulus "Deepfried Marshbar" build debugging!

[JMPGuitars]

You can try swapping pin 7 on the power tubes, but that might make it worse. It doesn't usually matted without NFB, but you have a NFB so it does matter.

Follow the modern ground scheme thread linked in my signature.

Also post your voltages. As I mentioned previously, your power tube cathode bias resistor should probably be 180Ω.

You may also need to do some chopsticking.

Thanks,
Josh

[Iain T]

Hey Josh,

Thanks again for the reply!

That grounding scheme is pretty much what I was considering - in the schematic it's be as simple as cutting the under-board ground wire that goes to the 22uF cap in the pre circuit and moving the ground point from the middle pot over (or routing under-board to the right instead of left.

Voltages for V4/5
2: 9mV / 12mV
3: 11.72V
7: 328.2V / 326.3V
9: 319.2V / 320.2V

If I've got this right the tubes are biased pretty hot at 11.7W dissipation, 97.5%.

Then again, I might be close to 100% wrong as I'm still trying to work out how to calculate tube biasing!

[JMPGuitars]

You calculated correctly. I assume you used this: https://robrobinette.com/Tube_Bias_Calculator.htm

Anyway, yeah, replace the cathode bias resistor with a 5W or 10W 180Ω. To be safe, you might want to get a couple higher values too, like 200 and 220Ω.

You might also want to consider increasing the screen-grid stopper resistors to 1k.

[Iain T]

You calculated correctly. I assume you used this: https://robrobinette.com/Tube_Bias_Calculator.htm

Yeah, that site is an absolute gold mine of info. through most of it before even thinking of building an amp.

I'll have a look at the other bias and grid-stopper changes once I've tried swapping pin 7 over then review the grounding scheme.

Cheers!

[Iain T]

Pin 7's swapped... immediate squeal so I did have that the right way round. My guess is that they updated their output transformers (Modulus wind their own) and colour-coded the two to brown and blue as per the layout. You can buy the kit without the xformers so they are probably catering for ones that are just red-red for the two ends of the winding.

Re-wired the output trans and re-tested. Measures clean outputs at the speaker terminal into my dummy load (measures 4.45 ohms) and got the following peak-peak voltages with a clean sine wave:

Low: 28.8V (p2p)
High: 28.4v (p2p)

This comes out, as I've muddled through the maths at ~23W, a touch high for this alleged 20W amp. Increasing the bias resistor definitely makes sense!

Re-checked the voltages as well and have, V4/V5 and this confirms it's running way too hot... I assume, being able to hear the sine wave in the amp under full load into the dummy isn't ideal and is a sign of the amp running too hot?

2: 3-5mV
3: 11.85 idle, 13.67V under load. That's 99.2% / 113%!!!
7: 314V
9: V4: 322.6V / V5: 324.4V

Other than the above it's sounding great now, I retouched some solder joints and this has improved things a bit. Will adjust the bias and review the grounding scheme but I'm currently very very happy.

[JMPGuitars]

Your target is 85% at idle voltage when biasing the power tubes.

I figured it wasn't the plate connections (pin 7) because you said it didn't squeal all the time. Now you know for sure. Brown/red/blue are common for 18W OT primaries.

What you described sounded like a parasitic oscillation, but there's little point in too much troubleshooting that with your power tubes running as hot as they are, as that could be part of the problem. Your ground scheme can also be a big part of that problem.

Once you have the power tubes tamed, and the ground scheme corrected, if there's any angry noise left, you should hopefully be able to shift some wires with a chopstick to eliminate it.

[Iain T]

Thanks - I'm fairly sure the lead dress is not optimal, it's a fairly tight space to run everything in and I opted to keep the leads as short as possible, a bit more length might actually allow for cleaner routing.

I have some 10W cement resistors on order - 180R, 200R and 220R as well as some ground lugs to work on the grounding scheme.

I'll check back in in a week or so when this is done.

Again, thanks for taking the time to share your experience, it's appreciated.

[Iain T]

Long weekend away so not had any time to work on this until tonight!

Just run through 180R, 200R, 240R, 220R in that order until I landed on a bias that works...

13.65V with the 220r - 87.5% and that seems to have got rid of noise on the power stage completely. Gan on 0 and master up full and it's really quiet, almost silent.

Add any gain and it gets noisy quickly - hiss and interference so grounding scheme and lead dress a must to improve that.

Would adding a 470pf cap to ground off the 10k input resistors help with the hiss? Not quite sure how I'd route it with this layout but must be possible!

[JMPGuitars]

I don't think you shared any photos of your build. Can you post some? What type of resistors did you use?

Thanks,
Josh

[Iain T]

Hi Josh,

The Bias resistor is a standard 10W cement resistor. The 10ks on the inputs are 3W Metal Oxides.

Will do some more work on this on Sunday and take some detailed pics, here's one from before changing the bias resistor - right-click and open in a new tab for higher res view! Anything you'd want a close-up on?

Thinking of the following ground points:
1: Safety ground lug (as it is)
2: Centre-tap ground lug near the safety.
3: Power-stage ground lug near the left front.
4: Current input/pre ground lug.

2 and 3 could be a single ground point though, not sure if there's much to gain or lose with either.

[JMPGuitars]

I would replace your input resistors with CMF metal film resistors. That includes the 1M, and the grid stopper resistors. Carbon Film isn't as noisy as Carbon Comp, but hiss is usually related to resistor type choices. Metal film being thing least hissy of those choices.

See the thread in my signature for ground scheme improvements.

Thanks,
Josh

[Iain T]

I would replace your input resistors with CMF metal film resistors. That includes the 1M, and the grid stopper resistors. Carbon Film isn't as noisy as Carbon Comp, but hiss is usually related to resistor type choices. Metal film being thing least hissy of those choices.

See the thread in my signature for ground scheme improvements.

Thanks,
Josh

Thanks Josh - already digested the grounding thread. Came across an interesting variant from Lyle (Psionic Audio) where he likes to ground the centre-taps on certain Marshalls separately from the power and filter stage ground. Going with your suggested route is by far the simplest for me as it only requires breaking one link on the existing ground bus and a single hole for the lug!

I'll look at getting some alternate resistors ordered.

It'll be interesting going through each step of changes to see what yields biggest gains!

[Iain T]

Initial attempt to improve the grounding scheme was sub-optimal... much louder earth hum!

Having checked against the "18W Lite 2b" schematic I realise that I had the PI grounded with the power stage via the presence/resonance controls and I hadn't properly traced it through. Swapped that to ground with the pre-stage and it was back to around the same level of hum as per original scheme.

One change - much more noticeable interference that was affected by moving my hand over the input area while the chassis is out of the head cabinet. This wasn't noticeable with the original ground scheme of everything grounding by the input...

Did a full volume test yesterday, pre-changing the grounding, at a local rehearsal space into a 2 x 12 with Celestion V30s... My guitarist friend was seriously impressed by the tonal options and sheer volume.

Amp was much quieter there than testing on my bench at home... realised that the powerline adapters I use for getting internet up to my workspace were adding a huge amount of noise to the mains!!!

Anyway - with master and gain on full using the tone switch in "#34" or "800" positions, there is a lot of hiss. Waiting on the metal film resistors to see how that is impacted.

[Bieworm]

Hi.. I think hiss is inevitable with this circuit. Ever heard a real plexi when dimed?

[JMPGuitars]

Hi.. I think hiss is inevitable with this circuit. Ever heard a real plexi when dimed?

You're correct, however, not using metal film resistors at the input stage will add hiss to the circuit.

CF resistors have less hiss than CC resistors, but more hiss than MF resistors. Any hiss at the input stage gets amplified throughout the circuit.

[Iain T]

Hi.. I think hiss is inevitable with this circuit. Ever heard a real plexi when dimed?

Ha, very very good point, maybe my goal/expectations were too high!!!

I've swapped the 1M and 10k resistors on the inputs for MF. I also took a minute to re-route the low-gain shielded input so it crossed fewer other wires going to V1 - they just cross their respective cathodes with as much vertical separation as possible around 1cm.

I also took the time to completely re-do the input jack wiring. I wasn't happy with the initial job I'd done. It was all sound but messy.

Anyway - master dimed and there is just a little hum.
With the gain and master dimed - low-gain input is just a little hiss, more on the Deepfried/#34 setting, high-gain is more-so but understandable.

I'm not sure that the swap to MF resistors has made much difference but a worthwhile exercise.

On the high-gain channel, with the gain at full and either the DF or #34 circuits engaged it's too much gain for even a good thrash sound but, surprisingly, there's still decent definition of the sound - it doesn't go to flub or fuzz which is a surprise.

Next job it to measure the output on the 'scope and see what clean power we're putting out.

Then to work out what to build next - probably a simpler version of this and on my own, from scratch, rather than a kit-build.

Thanks so much for the input on this guys - it's been invaluable!