• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

First HiFi Tube amp

When I grow up, I want to know half as much as @trobbins about valves, but that would probably be twice as much as my brain can hold.

Thanks, Tim.
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

420694018_915517893348921_3332980133502504738_n.jpg
420532824_1125029312264406_8981607949210399383_n.jpg
420671848_402163335507137_4075357513842099664_n.jpg


Regards
Alexis
 
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.

428084140_3798311603730703_4826201717657316192_n.jpg
428219186_430042149375215_346769728874313649_n.jpg
428136018_955979546176919_7827247870023247384_n.jpg
428084132_946307037058258_8325394444660083581_n.jpg


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
 
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
428378960_840054577878695_3930203723736747527_n.jpg
428496328_952337636308668_61140404131735647_n.jpg


Cheers
 
  • Like
Reactions: decramer
Hello 😀

Well it's done !! first some photos

423455052_1445901902671692_2175155854236022845_n.jpg
423221595_735111558710537_5328375279948578075_n.jpg
423455081_1085333406023302_6221868208756984359_n.jpg


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

PREAMP - Copie.png


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:

Capture d'écran 2024-03-03 174944.png

Capture d'écran 2024-03-03 174958.png


Regards
Alexis
 
Last edited:
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.
 
  • Like
Reactions: Orion24
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!
 
  • Like
Reactions: Orion24
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.
 
Last edited:
  • Like
Reactions: Orion24