Too humble is what you are too! 🙂 Reading this thread has been a educational pleasure because of you guys!! AND.. when/if I find out where the 'like' button you are using is hiding, I will press it on many of the posts in this thread!
Hello 😀
Well, I don't have a lot of time but I'm working on it, the PCB is here, the drilling is almost done, same for the chassis re-enforcement, i'm gonna make some change all the filaments will be DC powered by a 25A module, the HT bypass will be managed by two big relay, to prevent the 370AC coming to the front of the amp and maybe some other stuff I'm moving slowly
Regards
Alexis
Well, I don't have a lot of time but I'm working on it, the PCB is here, the drilling is almost done, same for the chassis re-enforcement, i'm gonna make some change all the filaments will be DC powered by a 25A module, the HT bypass will be managed by two big relay, to prevent the 370AC coming to the front of the amp and maybe some other stuff I'm moving slowly
Regards
Alexis
As much as I love PCB designing and assembling, I always tried to cheat the mech works. It is because I am not good at it nor that I like doing it.
How did the solder wire stick with the pins of the tube sockets, all good?
Cheers
How did the solder wire stick with the pins of the tube sockets, all good?
Cheers
Hello 😀
Some news, I had to make an another chassis a way bigger one to put all correctly and with some space ! And I started to put together parts
I tweaked the DC module by changing the AC/DC bridge from 25A to 45A and changed the mofset from 20A to 65A (with same specs) and adding a good heat sink under those, now l’m sure it'll be way better cooled compare to the original 0.5mm heat sink 😱 And if needed i'll add a little fan. The PT will be cooler without the 10A to provide the filaments.
More to come 😛
Regards
Alexis
Some news, I had to make an another chassis a way bigger one to put all correctly and with some space ! And I started to put together parts
I tweaked the DC module by changing the AC/DC bridge from 25A to 45A and changed the mofset from 20A to 65A (with same specs) and adding a good heat sink under those, now l’m sure it'll be way better cooled compare to the original 0.5mm heat sink 😱 And if needed i'll add a little fan. The PT will be cooler without the 10A to provide the filaments.
More to come 😛
Regards
Alexis
Hi Alexis,
Just a small comment about the bias potentiometer circuits. Some folks will add a high value resistor across the pot to the grid resistor so that in the event of a pot wiper going open the output tube will get max negative voltage and be spared from a no bias runaway. For example from pin 1 to pin 2 of VR3. A value of say 100k since the pots are 10k.
Great project,
John
Just a small comment about the bias potentiometer circuits. Some folks will add a high value resistor across the pot to the grid resistor so that in the event of a pot wiper going open the output tube will get max negative voltage and be spared from a no bias runaway. For example from pin 1 to pin 2 of VR3. A value of say 100k since the pots are 10k.
Great project,
John
Hello,
Thank you very much for the tips John I'll add two 100K resistor then 😊
A bit more work, PCB almost full soldered need some wire and an another filament PCB stacked on top
Thank you very much for the tips John I'll add two 100K resistor then 😊
A bit more work, PCB almost full soldered need some wire and an another filament PCB stacked on top
Hello,
A bit of soldering time today , the filaments connected and tested, the DC module don't even heat !! That's nice. HT OT also connected, to be honest I did not expected to solder that fast this project. 🤣 It miss just the signals, feedback, vu meters, the OT outputs and the main line caps and self
Cheers
A bit of soldering time today , the filaments connected and tested, the DC module don't even heat !! That's nice. HT OT also connected, to be honest I did not expected to solder that fast this project. 🤣 It miss just the signals, feedback, vu meters, the OT outputs and the main line caps and self
Cheers
Hello 😀
Well it's done !! first some photos
Schematic with voltages measurements and some tweaks to get voltages like in the original schematic but for D+ I still miss 100V ! I'm thinking to tweak R34 to get those 100V
The amp is working very well and sound for me very nicely and dead silent, lot of power even at low volume but without Feedback for now it's completely unstable with !
What ever the resistance I tried (R4) 1K to 20K but nothing work and for now I don't have an oscilloscope so it's gonna be hard to tune, if I understand well to have stability I'll have to tune C1/R5 and R4/C2 but How to tune it ?
For R4 that's the formula but I don't see "the Speech coil impedance" on the OT datasheet:
Regards
Alexis
Well it's done !! first some photos
Schematic with voltages measurements and some tweaks to get voltages like in the original schematic but for D+ I still miss 100V ! I'm thinking to tweak R34 to get those 100V
The amp is working very well and sound for me very nicely and dead silent, lot of power even at low volume but without Feedback for now it's completely unstable with !
What ever the resistance I tried (R4) 1K to 20K but nothing work and for now I don't have an oscilloscope so it's gonna be hard to tune, if I understand well to have stability I'll have to tune C1/R5 and R4/C2 but How to tune it ?
For R4 that's the formula but I don't see "the Speech coil impedance" on the OT datasheet:
Regards
Alexis
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10K would be the appropriate feedback resistor from the 16 ohm tap, for 20dB feedback. Are you saying it's unstable with feedback applied? What are the symptoms?
How did you arrive at the step filter of 1200pF + 1800? Is that something I suggested, or from my Williamson schematic? It might not be appropriate for the Hammond. You could try the standard step filter of 200pF and 4.7K, and see if it settles down.
How did you arrive at the step filter of 1200pF + 1800? Is that something I suggested, or from my Williamson schematic? It might not be appropriate for the Hammond. You could try the standard step filter of 200pF and 4.7K, and see if it settles down.
Right now you have two capacitors in the gain stages captured by the feedback loop. That will decrease the phase margin at low frequencies.
It will likely work as you have it presented but the phase margin at lower frequency will be more limited.
Have a look at the dynaco example, there is only one capacitor inside the feedback loop.
Harder to do a design with only one capacitor however it will improve the amplifier's stability at low frequency if global feedback is used.
If you have spice run a frequency sweep starting at about 0.01Hz with the load impedance increased by 1,000 times (so from 8 to 8,000) as a test.
If you get a big bump at some low frequency then your phase margin is limited. Lots of designs will fail this test, just an improvement if your design passes.
Of course how much of a issue this is in your design will depend strongly on how much feedback is applied. Have fun!
It will likely work as you have it presented but the phase margin at lower frequency will be more limited.
Have a look at the dynaco example, there is only one capacitor inside the feedback loop.
Harder to do a design with only one capacitor however it will improve the amplifier's stability at low frequency if global feedback is used.
If you have spice run a frequency sweep starting at about 0.01Hz with the load impedance increased by 1,000 times (so from 8 to 8,000) as a test.
If you get a big bump at some low frequency then your phase margin is limited. Lots of designs will fail this test, just an improvement if your design passes.
Of course how much of a issue this is in your design will depend strongly on how much feedback is applied. Have fun!
If the feedback is reversed it can howl or go thud, thud, thud at a very low frequency. Depends on where the feedback is greatest in the particular design.
Both can pretty quickly damage the amplifier or speaker.
By the way you have done a very nice looking circuit board for this design. Top class looking construction as well.
I noticed the slits for the tube sockets in place of holes. Nice touch. What software did you use for the pcb layout and who made your boards?
I just noticed the polarity dots on the Hammond transformer data sheet does it show the transformer is revered polarity from primary to secondary?
The blue wire on the primary has a black dot and the black wire on the secondary has a black dot.
Both can pretty quickly damage the amplifier or speaker.
By the way you have done a very nice looking circuit board for this design. Top class looking construction as well.
I noticed the slits for the tube sockets in place of holes. Nice touch. What software did you use for the pcb layout and who made your boards?
I just noticed the polarity dots on the Hammond transformer data sheet does it show the transformer is revered polarity from primary to secondary?
The blue wire on the primary has a black dot and the black wire on the secondary has a black dot.
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