"Superfly" - submini tube version of Doug H's Firefly

[zambo]

Sometimes when amps are not in sheilded boxes they can oscillate pretty bad. If you can find a way to sheild your amp circuit that may help ( i.e. puit it in a metal box ) it gets worse as the gain goes up.

[thomasha]

I had some oscillation problem with the max1771,

the shielded box helped a lot, shielded wire too, but the supply layout is also very important.

Try using a small capacitor between feedback and ground, and between HV and ground, something like 100~200pF, that should remove the high frequency oscillation.

Before the amp B+ add a low value resistor in series and another capacitor, that will form a low pass filter.

Look at the layout, the signal should be far from the inductor, and if possible use a ground plane under the inductor. Depending on the type of inductor you will have more noise (radial) or more voltage drop and heat (toroidal), I don't know if my inductors are the bad type or what, but I experienced this issues.

[duck_arse]

can one of you blokes or blokettes using the circuit above tell me what frequency it runs at? is it high enough that you should be using a low ESR for the filter cap? and are you using a low esr? has anyone (yet) tried the circuit with both types, compared the results?

[MetalUpYerEye]

I used the layout on this site for the power supply. The title says 555 SMPS Layout by frequencycentral. It should be fine... I admit that the whole thing is sitting on my workbench with wires going everywhere but I can't imagine this is a normal level of oscillation. I can turn the volume to max but then the gain only gets to 2 or 3 out of 10 before it starts to oscillate badly. Same thing the other way around, if I max the gain I can only get the volume to 2 or 3 without problems.

I noticed that some of the cap values are different between the 555 SMPS layout i'm using, the Superfly layout and the actual Nixie schematic. It seems like theres a 330uf cap in the Nixie and Superfly layouts but that same cap in the 555 SMPS layout is a 100uf. It appears to be power supply filtering...?  I'll try changing that up to a 330 and hopefully it will help.

[tonyharker]

Hi Duck_arse,  mine clocks at 48kHz approx.  Actual frequency will depend on tolerance of the timer components.

Also frequency will change a little depending also on the voltage selected.

Tony

[Mustachio]

Hey Duck I checked my 4.7uf 350v cap I used on my superfly and it was a general purpose, Not a low ESR. 

[duck_arse]

thanks, tony and mus. I don't know from, but in the app notes for the old 34063, which runs at similar speeds, they recommend low ESR caps. they show how the ESR affect the output ripple, and I quote from a worked example;

A 27 μF tantalum capacitor with an ESR of 0.10 Ω was again chosen. The ripple voltage due to the capacitance value is 28.7 mV and 44.2 mV due to
ESR. This yields a total ripple voltage of:

Eripple(p−p) = (Vout/Vref) 1.5 x 10−3 + (Iout/Co)ton + Ipk ESR
 = 33.6 mV + 28.7 mV + 44.2 mV
= 107 mV

I say quote, I should say paraphrase.

[MetalUpYerEye]

Okay so I have the motor boat problem under control.. I ended up using another inductor which helped as well as a regulated power supply.

Now I have other issues.

I built a baxandall eq into it, between the gain control and the second triode. Its not working as it should. I used a push/pull pot for a tonestack bypass switch, wired as you would wire a dpdt switch to bypass an effects pedal. The input and output go to the middle lugs of the switch, one side is jumpered straight across (for bypass) and the other goes to the input and output of the tonestack. When I have the switch in tonestack mode the treble control works but the bass control works more like a reverse volume control. With it at 1 it gives me full signal and at 10 there is no volume at all. The volume and gain on the amp are what I would expect from a 1/2 watt amp when the bass is at 1. When I bypass the tonestack however the volume and gain drop dramatically. In bypass mode none of the tonestack controls work (as expected) but then the gain knob cuts out the signal after you turn it past 8 or so.

I have double checked layouts, solder points, etc. on the tonestack, preamp/power amp boards many times over and can't figure it out... I am getting 185vdc off the power supply..

Any ideas..?

[zambo]

could you draw a schematic of how you have it wired? sometimes its easier to see that way.

[MetalUpYerEye]

Okay. Got that sorted out. I found an incredibly small (still don't know how I saw it!) solder bridge between pin 1 of the 6112 and lug 3 of the gain pot. Now everything seems to be working fine. It has a fuzzier sound than i'm used to out of tubes but maybe thats got something to do with the pickups in my guitar as they're pretty high output (15-16k ohms) and this circuit seems like its probably suited for something lower output.

The baxandall is interesting.. You really have to pay attention to the sound rather than where the knobs are.

[MetalUpYerEye]

Apologize for the double post, though its probably not a big deal in a pseudo-dead thread anyway..

My superfly has been working for awhile now but was still giving me trouble with the fuzz/fart sound I mentioned in my last post. I swapped out the .02 coupling caps in the preamp section for some .01 and that seems to have fixed it. Now i'm getting a much more modern sound out of the amp and the fuzz is hardly there except at full gain with humbuckers. The overdrive sounds much more refined this way as well.

I noticed there was a bias mod for the second preamp triode.. I think i'll try that next..

[zambo]

on small amps I will use .0022uf and .022uf respectivley. somtimes taming the bass of the first section and letting it bloom on the second one work good. fwiw. Glad its working better 

[MetalUpYerEye]

So you're saying a .0022 on the first stage of the preamp and a .022 on the second?

Originally I had tried something like that, swapping the second stage cap for a .047 so I had .02 and .047 but this seemed to make the flabby farty bass sounds even worse so I started working back in the other direction. I tried reading around on how changing the values of these caps would work towards changing the sound but couldn't find a definitive answer like "change the values up for more bass, down for less"-type of thing. In the end I had to just go for it and start swapping out values. I tried a few combos.. .02/.047, .02/.027, .02/.02, but finally ended up with two .01uf.

I'd like to try some different values for cathode resistors as well. According to the book i've been reading this can have a dramatic effect on the sound. Unfortunately this was built on perf which isn't my cup of tea when it comes to trying out different values/tweaking. Maybe i'll solder a 5k pot in for each of the cathode resistors and see if theres a sweeter spot for each (to my ears anyhow!) and then just measure the pots and install fixed resistors that match.

[thomasha]

hi,
i always use 0.0047uf as the first coupling capacitor and 820 ohms 0.680uf at first cathode.
A bigger cathode bypass cap makes the bass to mushy.

Another thing, after playing for 1 hour the sound changed a lot, and after some measurements I observed a voltage drop at the SMPS. Any ideia which component would be responsible for the SMPS efficiency loss after some heating? It's a 50 V voltage drop, and changes the volume and tone. 

[zambo]

So you're saying a .0022 on the first stage of the preamp and a .022 on the second?

Originally I had tried something like that, swapping the second stage cap for a .047 so I had .02 and .047 but this seemed to make the flabby farty bass sounds even worse so I started working back in the other direction. I tried reading around on how changing the values of these caps would work towards changing the sound but couldn't find a definitive answer like "change the values up for more bass, down for less"-type of thing. In the end I had to just go for it and start swapping out values. I tried a few combos.. .02/.047, .02/.027, .02/.02, but finally ended up with two .01uf.

I'd like to try some different values for cathode resistors as well. According to the book i've been reading this can have a dramatic effect on the sound. Unfortunately this was built on perf which isn't my cup of tea when it comes to trying out different values/tweaking. Maybe i'll solder a 5k pot in for each of the cathode resistors and see if theres a sweeter spot for each (to my ears anyhow!) and then just measure the pots and install fixed resistors that match.

Thats what i am saying . Bigger caps = more bass. Where you place the caps being most important. Amplify to much bass to soon you get farts. To much treble and you get ice pick to ears. we hear most sound in the mids so everyone scoops those. Balance seems tro be that your highs being more distorted ( not amplified ) and your lows being less distorted makes a more palatable tone to most people. So a small cap after first gain stage gets those highs on the way to clipping right away but leaves the bass out. the bigger cap after the second stage lets the bass through but its one gainstage behind the highs and less likley to clip as much in the power section in this scenario.

As far as bias goes, hot bias is ok but can lead to farty boomy bass as well. Judging by your name ( metal) you would probably like cold biasing more. makes more second harmonics pronounced in the sound. Thiink soldano , trainwreck Suhr etc. stages usualy go medium bias ( 1k5 ish ) or cooler biased ( 2k7 ish ) then cold biased ( 10k ish on up to 39k ) and are alternated to maintain balance. This has been my understanding of it anyway and ive read a bunch and built a fair amount. Merlin has a lot of cool stuff on his free web about it.  I read there a lot. Its all personal taste and experimentation anyway but thought i would share for what its worth.

[MetalUpYerEye]

Awesome zambo!

I was wondering about the bias resistors and i've read some of Merlins book. A quick google search didn't seem to turn up any definitive answer (more resistance = colder bias, less resistance, hotter bias) like what you're saying.

I own a Blackstar HT5. Great little amp. I'd really like to get my Superfly somewhere close to that sound. Of course it wont have as much gain without the cascade stuff the HT5 has.

You're right about my name so i'll have to try cold biasing the tubes in my Superfly. Do you think a 25k pot is too much for testing?

[zambo]

no 25 or even 50 k as long as your testing. Probably should calc the voltage drop / ohms law thing and make sure your pot can handle the wattage. since you have to disconnect stuff i would just try a few resistors. try the knowns first and see what happens. 39k etc..  also hit the front of your amp with a clean boost and see what happens. usualy gets them screaming pretty good. beat that first gainstage with a boost and then limit the gain in the amp via the gain knob and magic stuff can happen.

[MetalUpYerEye]

Cool i'll have to give that a try!

I was wondering.. If I were to swap out my 6112 preamp tube for another 6021 could I rehash it to be single ended and use the first triode of the power tube as a preamp section for more gain? I got the idea looking at this schematic...



If so, it seems like I would need a different output xfmr. Or could I use the 125a I have and omit the B+ tap?

[duck_arse]

Another thing, after playing for 1 hour the sound changed a lot, and after some measurements I observed a voltage drop at the SMPS. Any ideia which component would be responsible for the SMPS efficiency loss after some heating? It's a 50 V voltage drop, and changes the volume and tone. 

50V is too much. you'll need to test for temperature, carefully. if your fingerprints stick to any parts, they are too hot. check your IC, inductor and mosfet, which you have, of course, on a generous heatsink. if something is getting hot, it might be affecting componets around it. maybe you got a bad regulator transistor. can we see pics of your board?

[zambo]

Cool i'll have to give that a try!

I was wondering.. If I were to swap out my 6112 preamp tube for another 6021 could I rehash it to be single ended and use the first triode of the power tube as a preamp section for more gain? I got the idea looking at this schematic...



If so, it seems like I would need a different output xfmr. Or could I use the 125a I have and omit the B+ tap?

There is a difference between se and pp transformers and i dont know if it would be fatal to the ot .Air gap and standing dc are terms i see a lot but dont understand so i wont pretend to. However.... if you went se and did the proposed plan you would get about half the volume your getting now as you will be missing half a tube. I would just put a boost in front because they are cheap and fun to build lol. A tube boost is even more fun. just pay attention to cap sizes and low pass/ high pass filters etc. Let your conscience be your guide. if your power supply is heating up and dropping volts i would hesitate to load it up any further.