It's just regular JFETs (which are an absolute pain to source, but luckily I have a dozen). I really like how it simulates, it has very gradual increase in distortion, and the distortion knee is indeed very gradual (but with the FET clipping a soft clipped signal it becomes even more gradual and dynamic). It won't sound like a tube but should do the trick, the idea is that it should be a Fender-ish kind of amp, crystal clear at low gains, sort of clean but not quite, but not as abrupt and with more of a transition from clean to distorted when pushed. And also capable of heavier distortion when cranked.
If you've heard the recently released Orange Super Crush 100, it has a JFET based preamp. Can definitely get a great overdriven sound. I built a 3 stage (2 x Jfet and 1 Mosfet) which could get into that range when pushed, but stayed clean-ish at low volumes. Ran through a TDA-2030 chip amp at about 14W it was loud enough through a 12 inch guitar speaker.
Seconded. I quite like the Orange Super Crush 100, it's a great amp through the right speaker, and doesn't make you feel wanting for a better amp, tube or solid state. Also very affordable. Do you have any schematics for the amp by the way?
Hi,
I've made some more progress on the cabinet. I've now covered the main part and painted the inside. The next step is the baffle which needs tolex, grille cloth, piping and the fold fascia strip. I found the fascia strip difficult to source and the best I could find was 1/2" gold T moulding from an arcade supplier. I'm slightly worried it'll end up looking wrong but we'll see! I hope to have it finished in the next couple of weeks if all goes to plan.
I've made some more progress on the cabinet. I've now covered the main part and painted the inside. The next step is the baffle which needs tolex, grille cloth, piping and the fold fascia strip. I found the fascia strip difficult to source and the best I could find was 1/2" gold T moulding from an arcade supplier. I'm slightly worried it'll end up looking wrong but we'll see! I hope to have it finished in the next couple of weeks if all goes to plan.
I did the JFET preamp on a perfboard and probably have some schematic and notes written out by hand somewhere in my workshop.. but it's pretty much standard common source stages. On the first stage I had a switchable cap across the source resistor as a Hi-Low gain switch. I had put together a passive TMB stack after the first two JFET stages, before the final Mosfet stage where I put a "master volume" output to the power amp. SInce then I haven't played around with it much..not that I lost interest but just a lot of other projects developed after it. As James UK's project can attenst to, get a final finished amp (or any project) 100% done is a long way from the breadboard stage! Cheers
It certainly is, Shanx! I've spent he best part of two years on this, start to finish. I haven't been working on it the whole time but I have spent more hours than I can count on it. There are just so many different aspects to a complete design and so many minute details that have to be worked out. It's all worth it though, of course!
I think I'm going to have the amp fully completed next week. I've almost finished the cabinet and just need to fit the corners and screw the back panels on. I also need to make a tolex bag to mount the reverb tank to the bottom of the cabinet.
I think I'm going to have the amp fully completed next week. I've almost finished the cabinet and just need to fit the corners and screw the back panels on. I also need to make a tolex bag to mount the reverb tank to the bottom of the cabinet.
Hi,
I completed the amp today, apart from the badge which I'm still making. I'm pretty pleased with the end result. It looks good and sounds good to me and it's very satisfying to have a functioning amp that I made myself.
A huge thanks to everyone for your help in designing and fault finding!
I do have one issue that needs fixing. The reverb feeds back badly to the point that it basically cannot be used. I think to rectify it I need to increase the level of the drive signal and decrease the return level. I'm also considering changing the coupling capacitor to the reverb level control to a smaller value to filter out the low end that's causing the feedback. Any suggestions would be appreciated.
I've attached some photos and a couple of very rough demos of the sound. I'll make some better ones soon.
Thanks,
James
I completed the amp today, apart from the badge which I'm still making. I'm pretty pleased with the end result. It looks good and sounds good to me and it's very satisfying to have a functioning amp that I made myself.
A huge thanks to everyone for your help in designing and fault finding!
I do have one issue that needs fixing. The reverb feeds back badly to the point that it basically cannot be used. I think to rectify it I need to increase the level of the drive signal and decrease the return level. I'm also considering changing the coupling capacitor to the reverb level control to a smaller value to filter out the low end that's causing the feedback. Any suggestions would be appreciated.
I've attached some photos and a couple of very rough demos of the sound. I'll make some better ones soon.
Thanks,
James
Hi,
I'm not having much luck solving the reverb problem, after several hours working on it. The tank feeds back when the reverb is turned up. There isn't much reverb either, it's not like it's drenched in reverb when the tank is turned up, there's very little reverb and just feedback. I've attached the schematic. Any suggestions as to what the problem is would be greatly appreciated!
Thanks,
James
I'm not having much luck solving the reverb problem, after several hours working on it. The tank feeds back when the reverb is turned up. There isn't much reverb either, it's not like it's drenched in reverb when the tank is turned up, there's very little reverb and just feedback. I've attached the schematic. Any suggestions as to what the problem is would be greatly appreciated!
Thanks,
James
Hi,
Thanks for the reply. The Reverb is an 8 ohm tank, code 4AB3C1B.
So far, I've changed C27 from 470n to 47n. My idea was to filter out the lower frequencies that seem to be causing the feedback. It didn't seem to make any difference.
I also swapped R37 for a variable resistor to try and find a maximum allowable gain, at full output volume, before it fed back. It was still just about feeding back when set to 220k. The problem is that it stops he feedback but then there's no reverb.
I think I may need to drive the tank harder but I'm not sure how to increase drive with the arrangement I've got. I tried measuring the voltage across the tank input but connecting the oscilloscope caused oscillation, presumably because it resulted in grounding one side of the reverb tank.
Thanks,
James
Thanks for the reply. The Reverb is an 8 ohm tank, code 4AB3C1B.
So far, I've changed C27 from 470n to 47n. My idea was to filter out the lower frequencies that seem to be causing the feedback. It didn't seem to make any difference.
I also swapped R37 for a variable resistor to try and find a maximum allowable gain, at full output volume, before it fed back. It was still just about feeding back when set to 220k. The problem is that it stops he feedback but then there's no reverb.
I think I may need to drive the tank harder but I'm not sure how to increase drive with the arrangement I've got. I tried measuring the voltage across the tank input but connecting the oscilloscope caused oscillation, presumably because it resulted in grounding one side of the reverb tank.
Thanks,
James
Connecting pin 1 of J1 to ground shorts the feedback loop, so U3.2 then runs open loop. You could use a differential probe if you have one. Alternatively, you can actually easily measure the current delivered to the drive coil with a standard probe by measuring the voltage across R21. 33mV = 1mA with the values shown.
The NE5532 has very high input bias and offset currents, so R19 should probably be reduced significantly. As shown, you could have as much as 950mV offset at pin 5 of U3.2 (worst case at 25C), which translates to 28.8mA DC across the drive coil. 'Typical' values given in the datasheet works out to about 6.4mA DC. I don't know what the saturation point of the drive transducer is at DC, but 29mA is definitely too much DC offset. I'd suggest changing R19 to something around 10k-22k and C15 to 100n-1µ.
Have you checked the DCR of the reverb tank drive coil to make sure it doesn't read open? It should measure around 0.8 ohm from what I can find (the 8 ohm rating is the impedance at 1kHz).
Thanks for the detailed response. I'll measure the current and reduce the value of R19. Does this not risk loading the output of the previous stage and rolling off treble?
If the coil is being saturated by DC offset, would that affect the level of drive to the tank? Is there any adjustment that can be made to increase the level?
I measured the DCR of the input yesterday and it read around 2 ohms from memory.
Thanks again for your response,
James
If the coil is being saturated by DC offset, would that affect the level of drive to the tank? Is there any adjustment that can be made to increase the level?
I measured the DCR of the input yesterday and it read around 2 ohms from memory.
Thanks again for your response,
James
No. The NE5532 can do at least 9V RMS into 600Ω on ±15V supplies.
I've not tested it, but I'd expect that the drive transducer wouldn't work well at all if it was saturated with DC. A changing magnetic field is required to move the magnets attached to the springs. If the core is saturated, the field does not change unless it is driven out of saturation.
You can reduce the value of R21 to increase the drive current. However, I don't think that this is the problem (unless the wrong value was installed) as 33Ω should give plenty of drive current.
I just realized that I made a serious error in my analysis above—I forgot about C18. The DC current is zero regardless of the input voltage offset (assuming C18's cathode doesn't go significantly positive, anyway). Oops. Still probably not a bad idea to reduce the voltage offset though.
In any case, measuring the drive current across R21 as I suggested before might reveal something useful.
In any case, measuring the drive current across R21 as I suggested before might reveal something useful.
Hi,
I've measured the current to the tank and I'm getting:
1kHz sine input:
Clean: 8.6mA RMS
Max overdrive: 21.4mA RMS
400Hz sine input:
Clean: 5.6mA RMS
Max Overdrive: 8.6mA RMS
Does that sound okay or too low?
I measured DCR to be 1.2 Ohm.
I'm not sure how much sound I should be able to hear from the tank transducer itself but it was extremely faint. What I also found strange was the fact that the current measures exactly the same with the tank connected or disconnected. I expected it to change or am I just wrong here?
I have managed to reduce the feedback by resting the tank on some cushioning. I had it mounted inside a two-layer tolex bag and loosely screwed to the bottom of the amp, like a Fender, but it seems this might not be enough isolation. Doing this seems to have eliminated the low feedback but now I have a high-pitched squealing feedback, but not at such a low gain. Clearly there is also some work to be done in this respect.
Thanks,
James
I've measured the current to the tank and I'm getting:
1kHz sine input:
Clean: 8.6mA RMS
Max overdrive: 21.4mA RMS
400Hz sine input:
Clean: 5.6mA RMS
Max Overdrive: 8.6mA RMS
Does that sound okay or too low?
I measured DCR to be 1.2 Ohm.
I'm not sure how much sound I should be able to hear from the tank transducer itself but it was extremely faint. What I also found strange was the fact that the current measures exactly the same with the tank connected or disconnected. I expected it to change or am I just wrong here?
I have managed to reduce the feedback by resting the tank on some cushioning. I had it mounted inside a two-layer tolex bag and loosely screwed to the bottom of the amp, like a Fender, but it seems this might not be enough isolation. Doing this seems to have eliminated the low feedback but now I have a high-pitched squealing feedback, but not at such a low gain. Clearly there is also some work to be done in this respect.
Thanks,
James
1.For several reasons the reverb return amp should incorporate a highpass with a cutoff frequency in the region of 500Hz. With R35=220R und C25=10u you get fc about 70Hz. Reduce C25 to 1.5uF.
2.Next step: Your reverb spring with 8Ohms input impedance requires high current that the emitter followers should be capable to deliver. With a good driving level you can hear the springs "sing" when playing the guitar.
3. To avoid a muddy sound the primary coil should be current driven - not voltage driven. This can be achieved by a series resistor or a current feedback inside the driver circuit closing its feedback loop at a shunt resistor in the GND-path of the primary coil.
Probably a little low, but still high enough that it should work OK. Lower drive levels have been used successfully in commercial amps. Rod Elliot suggests around 30-60mA RMS for 8Ω tanks.
There's still a load (R32) with the tank disconnected, so the current will stay the same (unless the driver circuit clips, which doesn't happen in this case).
I noticed that you're using an NE5534 for the recovery stage. That chip isn't unity gain stable without an external frequency compensation capacitor between pins 5 and 8. I don't see any external compensation, so that stage may actually be oscillating because of C26.
It already is current driven in this design.
Definitely not. To achieve this, the inputs of the opamp must be swapped at least.