Matchless Nighthawk Fizz

[64ES335]

Hi Everyone. Looking for some help with a Matchless Nighthawk: essentially the EF86 channel from a DC-30 mated with their 15 Watt power supply and output section.

When raising the master above 2/3, I get the fizzing/phasing/swirling sound common to low Watt EL84 amps. Looking at the signal at the PI output couplers, I can see some crossover distortion and peaks at the top of the waveform. Output tube cathode voltage at idle is around 11.5V.

I attempted the Paul Ruby diode mod across the dual gang PPIMV pot, on lugs 1 &3 (changed out the crossline for the Larmar MV). Using 1N4007s and 16V zeners. The original 220K output grid leaks are disconnected.

While the diodes definitely help reduce the fizz, it also cuts the low end substantially. Any idea why it affects the low frequencies so much? Expected this mod to have little to no effect on anything but the blocking/crossover distortion.


Appreciate any insights you can share. Thanks!

[Chuck H]

The Paul Ruby mod might affect the low end if signal is predominantly low end through the preamp. This is "typically" less than ideal for most heavily overdriven tones but YMMV and all things are subjective. That said...

If it's a low wattage amp, as in low plate voltage? then examine the bias condition. IMHE Matchless amps are typically biased SHAMEFULLY hot. But I'm not familiar with all of them, what may have been changed on the particular amp you're working with or modern variations. If you're calling this a "low wattage" amp I need to as, what is the plate voltage? Can you provide a schematic" Is this a Matchless or a clone?

There are ways to mitigate fizz with EL84 amps. No one knows this better than I do (he said with no humility whatever ) But there are things to consider...

1) You can't do it if you're interested in tube life.

2) You can't do it if you want to use the cheapest tubes available in a combo type amp.

Power tube life could be limited to a thousand hours (or less). Which goes by pretty quickly if you like an amp and play it for a few hours every day. Once upon a time EL84's were cheap but now they're harder to come by due to ALL tube manufacturers and users being in a combination of political and demographic/market pickle. And I don't expect it to get better. The best tubes are probably NOS or UOS Svetlana. NOS need to be tested and known and UOS need the same. Untested tubes should be avoided.

You're goint to need to push the tubes hard. If the amp in question has been modified this may not be the case. Some Matchless amps that burn up tubes in only a couple of hundred hours get modified for longer tube life too far in that direction and lose their mojo. This needs to be checked for...

Implementing the Paul Ruby mod comes with some measurements of bias conditions as overdrive begins. Was this done or were arbitrary component values from a post used? Awareness!!!

Not enough information about the amp in question so far. Schematic? Bias current in watts?

[64ES335]

Thanks Chuck H! Yes the preamp does seems to have a fair bit of low end. Plate voltage is 320V. It’s a 15Watt 1x12 combo. Yes it’s a real Matchless. No mods to the output cathode resistor value to cool the bias.


As for a schematic, there isn’t one I can find. But reviewing the circuit, the PI, output, and power supply are identical to the Lightning. The pre is the same as the C-30 minus the loop. Attached here.


I can take measurements at onset of distortion later today. Anything in particular I should check aside from cathode voltage (11.5V at ideal) and grid voltage?


Not too concerned with tube life. Just want to get the amp sounding it’s best. Thanks again.

[Chuck H]

Maybe you already know this? What the Paul Ruby mod does is cut off signal that is not part of the audible wave forms in push/pull operation. This reduces charging and allows for for discharge time in the PI coupling capacitors. When those caps charge up too much and can't discharge a negative voltage can build up at the following el84 grid. This negative voltage cools the bias and causes crossover distortion. So the Paul Ruby mod solves for this and if implemented correctly it shouldn't be in the audible part of the waveform.

Since you're hearing a loss of LF with the PR mod in place I think it must be that the cathode voltage is rising above 16V under heavy drive/current conditions so the diode clipping is audible. Since the PR mod acts like a relief drain for the coupling caps you only need enough relief to mitigate crossover distortion. So raise the PR Zeners to something like 20V. Ideally you would just measure the cathode voltage at full clip and then value the Zeners one volt higher than that. But in reality there's enough slop in the gears for this circuit that as long as you're not diode clipping part of the audible waveform the circuit is doing it's job. Or at least most of it.

[Justin Thomas]

Hmmmm... Might hafta bookmark this one... I like my thick low end and distortotron...

Jusrin

[64ES335]

Chuck H - Excellent info. Yes, I understood that the PR mod should not affect the baseline tone. However, as you point out, I think the problem is that I was calculating the Zener value incorrectly.

With the LARMAR master cranked, the onset of distortion (using the volume control) at the EL84 grids and at the amp’s output occurs just under 16V at the EL84’s shared cathode resistor. When the amp is dimed, the cathode voltage hits a max of 22.6V.

Happened to have 20V zeners, and the loss of bass was much improved. Well done and thank you. I’ll see about getting 24V zeners and comparing against the 20V.

Grateful for you sharing your knowledge and time. Thank you!

[Chuck H]

Sounds like the 24V Zeners should do it. And...

I think your cathode resistor must be a higher value than what is in the Lightning schematic (120R) to rise up to 22.6V. Not a bad thing for tube life.

EDIT: It could mean the amp is running much cooler than other Matchless designs (for better or worse). If you have a 16V 5W Zener on hand you could try strapping that across the shared cathode circuit with the cathode to the tube cathodes. This would clamp the bias rise to 16V. Which seems like plenty for 320Vp. This will change the feel of the amp a little as it will behave like a fixed bias when clipping. If you do this you won't need to change the 20V Zeners you have in there right now and fizz reduction will be even better.

Alternately you could change the cathode resistor to the more typical "Matchless" value of 120R (I'd go 130R). But this will change the feel of the amp as it is now and across the full tonal range (both clean and dirty). This should also limit voltage rise such that you can leave the 20V Zeners you have now.

Would you measure the cathode resistor? I'm interested to know.

[64ES335]

Sure thing. Will measure later today and let you know.

[64ES335]

Chuck H The cathode resistor is 120 Ohms. Measures 119.9 Ohms. I’m running ANOS GE 6BQ5.

[Chuck H]

Chuck H The cathode resistor is 120 Ohms. Measures 119.9 Ohms. I’m running ANOS GE 6BQ5.

I'm actually surprised the cathode voltage can rise to almost 23V with 320Vp and a 120R cathode resistor. I haven't bench tested this configuration though. My assessment... You could go either way with this. I still believe with all my humble experience that those tubes would be fine with -16V bias at 320V. So you could strap a 5W/16V zener across the cathode resistor and leave the 20V Zeners in place for the Paul Ruby mod. This would mitigate fizz best. Otherwise you could skip the parallel Zener with the cathode circuit and use 24V Zeners in the Paul Ruby circuit and trim most of the fizz. You're choice

[64ES335]

Chuck H - Very cool. Thanks for the recommendations. I’ll double check my cathode voltage measurement. Maybe I’m wrong about the max voltage I’m seeing.

The amp is going on 14 years old. I’m wondering if the 250uf cathode bypass is leaky? Guess it couldn’t hurt to check.

UPDATE: Just ran and rechecked the cathode voltage at full bore. Highest value I got with my Fluke was 23V. Tested with a 1K,150 mV sine wave.

[Chuck H]

With that info post 10 still stands. I ran a CAD for the Vp and cathode circuit values and couldn't get more than about 17V at the cathode. My own amp runs the el84's pretty hot and hard with a 160R cathode resistor and 365Vp and on the bench doesn't go above -22V bias (IIRC). I do have a Zener strapped across the cathode circuit in my amp AND I'm using the PR circuit. But I did test it without the control Zeners when I was determining their values and this is what I remember. All I can guess is that the Matchless circuit is driving the el84 grids even harder than my circuit (hard to imagine?). Regardless, the measurements tell the story and there is a solution

[64ES335]

Chuck H - Thank you again! Much appreciated.

[Mars Amp Repair]

This whole thing very fascinating to me! I have to say I haven't heard of this symptom nor would have ever come up with that explanation & certainly not the solution. This is why I love this forum. So much variety of knowledge & experience.
This forum can never die.
glen