Dumble SSS/Naylor Superdrive Build

[Travis_HY]

Sounds possible that the regulator may need better heat dissipation. a heatsink etc. Either that or it's not rated to handle the extra current you're giving it with that tube and the relays as well? You'd need to consult the datasheet, there's a max current rating, but I think there's also a max rating with no heat sinking that you can't exceed without using the right kind of heatsink etc. I read up on it in merlin's hifi book but dont' fully understand all the exacting details. You could replace it with one way over amperage to allow it to run cooler, or replace it with the same kind and get some kind of heatsink attached.

I have it mounted to a heatsink bolted to the chassis, which I would figure would be enough to dissipate the heat, but the failure is still perplexing. It's output max is 1A so that's more than enough current available. It should be consuming 400ma max with the relays drawing 50ma a piece and the 12AX7 drawing 300ma. I must be missing something; I know Merlin says on his site that making that 1000uF reservoir filter cap too small can cause hum in the relay/heater supply, which might be the hum I was trying to get rid of, thusly I made the heaters DC. What's so odd is the regulator failure in this application; what was drawing so much current to make it fail? Could it be that the smaller filter cap caused flow-back into the IC? I didn't have a protection diode going from the output to input, anode to cathode, respectively, or the protection diode from the ground pin of the regulator to circuit ground. I dunno. Maybe just cleaning up the relay supply with a larger reservoir cap will negate the need for DC heaters?

https://www.mouser.com/productdetail/?q ... jxLg%3D%3D

I've used these in the past for relay supplies (and for DC heaters on the first 12AX7) and it worked just fine-no problems with hum or reliability. Just weird that a power supply that is supposed to be MORE robust than the one above is having problems. Must be user/designer error on my behalf. Hmm. Damn. And blast.

[martin manning]

12AX7 heaters running on 12V will only need 150 mA, so I don't think you are asking too much of the regulator. I would increase the 1000u to 2200u to reduce the input ripple, and be sure to use a low-dropout regulator.

[bancika]

1000uf is not small. Rule of the thumb is to go 1uf per 1ma of current. In this case you have 250ma, so 1000uf should be plenty. Is it possible that you have a short somewhere aroud the regulator? There shouldn't be almost any heat without the tube plugged. 100ma draw will probably make it dissipate less than half a watt (approximating input dc to 15v after rect) which is fine even without a heat sink. And regulators shouldn't die, they have thermal shutdown and they will wake up once they cool down.

[martin manning]

I agree there is likely a damaged component in there somewhere. Re the reservoir cap size, starting with 12 VAC, two diode drops, ~1.4V p-p ripple voltage for a 1000u reservoir, and 2V headroom for the regulator might be cutting it a bit too close. 2200u would provide some extra margin.

[bancika]

ran the sim and you are right, 1000uF is tight. Good to know.

Cheers

[Travis_HY]

Thanks for chiming in y'all. I really appreciate your help and feedback. Pun absolutely and shamelessly intended. I updated the schematics to reflect the amplifier in it's current form. It's the best this amp has ever sounded. There was a value error on the two coupling caps in the mixer circuit. They are both .022uF not .002uF.

I have since updated and tidied up the 12VDC supply to include an LDO regulator and a larger 2200uF 25v filter cap. No problems thus far and much less heat. That regulator is really a superstar.

I changed the global negative feedback network because I was struggling tonally with the previous arrangement for both for the channels. The clean channel had a lower mid "muck" that the Naylor style NFBL imparted that I didn't dig so I switched over to something that I thought would work better for both channels. I went with a .22uF presence cap to bring out more of the upper midrange response as well a variable feedback control to loosen up the power amp if desired; I might make the 4.7K feedback resistor a 3.3K or 2.7K to get the voltage swing a little wider. Opinions on such a mod would be appreciated as I couldn't really find any info on a variable negative feedback control in a D style amp. I added a small preset Resonance network of 27k/.0047uF to give just a little extra fat in the lower mids like the Naylor NFBL but not nearly to the same extreme degree. It's just enough to give lower volume playing some lower midrange to work with on the dirty channel in the power amp. Without the resonance circuit, the dirty channel sounds too scooped and zingy on the top end for my liking and this gives a happy middle of the road response for both channels to my ears.

I always had some stability problems with extreme EQ settings on the clean channel in every spec I tired (Treble and Volume turned all the way up) when the 3-way LNBF was engaged and the simple fix I found was to add a 560k series resistor out of the Treble wiper into the Volume control a la Hiwatt/Soldano/Naylor/Friedman. I'm sure this instability is a lead dress issue with the 2nd stange's LNFB, but the series resistor tames the oscillation, the tone was improved and the noise floor was reduced, so it was a win-win-win change to me.

I updated the Filter controls to Martin's specs. Between those changes and the power amp changes, this amp is definitely closer to what I imagined it to be for the big-loud-clean HAD Dumbleland kind of tones. The steps are much more uniform across the guitar range and definitely more useful to me. The previous Filters were too bright and extreme for my tastes. The Audio taper Lo-Filter potentiometer will be swapped to linear taper in the future; I think the mucky power amp made the taper hard for me to hear, but I hear that it wants to be linear or a less extreme taper anyway with this spec. Thanks again for your wonderful work on that circuit, Martin. Really great sounding changes.

The last change was swapping the plate and cathode resistors on the clean channel to High Plate specs. From what understand, the High Plate specs weren't used with the Filter circuits? I had the amp open and tried it for the hell of it and I loved the change. Louder and cleaner with more squish than the previous Low Plate specs. Won't be going back to Low Plate specs any time soon.

Now time to go play it!!! Thanks again!!!

[martin manning]

You're welcome and it's good to hear the filters are working better. I'd definitely swap the "Low" pot out for a linear taper. It will be easier to dial in the bass to your liking.

[roberto]

Hi travis, I'd increase the 10k going to the 2N3904s to 100k. It clips better IMHO.

[Travis_HY]

Hi travis, I'd increase the 10k going to the 2N3904s to 100k. It clips better IMHO.

Thanks Roberto! I'll have to try it out-right now it cuts volume much more than I wish it would and it's a hilarious amount of gain; just unholy. It still sounds great in some contexts, and what I may do is add another relay that engages the clipping circuitry and switches to an additional Master volume pot so I'd basically have a 3 channel amp: Clean, Naylor Superdrive, Naylor Superdrive with clipping circuit but with a second Master to be set louder. I'm still getting used to how this amp works and what it does with my pedals and other gear. It's taking a moment!

I know some say other clippers sound better, but those were just what I had lying around. I think zeners and Mosfets are fairly popular? I don't understand which would be favorable in this application because they are just diodes signal clipping, so I'm not really sure what the differences would be? What clippers do you like in this context yourself?

[roberto]

I have to admit I'm not a huge fan of diode clipping, but it has "that" sound that many are looking for.
I say that I'm not a huge fan because I've not deeply investigated on it, but I've read alot and done some tests.
My preference is to use a 100k resistor (you can use a pot to find the best spot) in series with two mirrored zeners around 30-40V each. Less gain than your solution, less volume drop, less "boolean distortion" as well, as I call it.

[Travis_HY]

I have to admit I'm not a huge fan of diode clipping, but it has "that" sound that many are looking for.
I say that I'm not a huge fan because I've not deeply investigated on it, but I've read alot and done some tests.
My preference is to use a 100k resistor (you can use a pot to find the best spot) in series with two mirrored zeners around 30-40V each. Less gain than your solution, less volume drop, less "boolean distortion" as well, as I call it.

Thanks for that tip Roberto; making that 10k a 100k is a cool change, but it almost was too subtle to notice it with the SD preamp being pretty rowdy already. 10k was far too clipped in this amp for me, so I settled on 68k. Cut the volume just a tiny bit, but still gave that clipped hashy thing thats fun when you're doing Eddie squeels! Panama!

[roberto]

Zeners have been invented for big hair squeels.