Hi all,
I bought a the BUD amplifier's PCB and I have noticed that the cathode resistor of the power tube (EL84) is 68Ω. This is resistor R18 in the schematic.
This looks way too low to me. I would use something between 330Ω to 680Ω (470Ωmaybe). This value will pull a current of 45mA through the resistor which is a lot.
It looks that they are applying ≈180V to the anode/plate and ≈160V to the grid2/screen.
What do you guys think, is this a mistake or do they really use this value? Such a large current will shorten the valve's lifetime, I guess. I would use 20mA instead.
Kind regards,
Pedro
This is the link for the build manual.
https://s3.amazonaws.com/tubedepot-...iles/BUD_Manual_-_Working_Copy.pdf?1583168310
I bought a the BUD amplifier's PCB and I have noticed that the cathode resistor of the power tube (EL84) is 68Ω. This is resistor R18 in the schematic.
This looks way too low to me. I would use something between 330Ω to 680Ω (470Ωmaybe). This value will pull a current of 45mA through the resistor which is a lot.
It looks that they are applying ≈180V to the anode/plate and ≈160V to the grid2/screen.
What do you guys think, is this a mistake or do they really use this value? Such a large current will shorten the valve's lifetime, I guess. I would use 20mA instead.
Kind regards,
Pedro
This is the link for the build manual.
https://s3.amazonaws.com/tubedepot-...iles/BUD_Manual_-_Working_Copy.pdf?1583168310
I would worry about the schematic, since it can't work as given. The OT secondary has no ground, so the nfb cannot function.
Thanks for the reply.
If you open the link above and go to page 11, you will find that the value of R18 is 68 ohms.
It's true that they missed the ground on the secondary, but I am just questioning the value of R18.
If you open the link above and go to page 11, you will find that the value of R18 is 68 ohms.
It's true that they missed the ground on the secondary, but I am just questioning the value of R18.
So what's the plate current at 68 ohms?
B+ is pretty low. Typical data sheet idle plate current is 36ma-48ma @250v B+
jeff
Current is around 48mA, which across 68Ω gives 3V at the cathode.
Below is how I thought of biasing the amp.
B+250V, Vg2 = 220V ; Ianode = 25mA instead of 48mA
Vk = 8.3V instead of 3V;Rk=330Ω instead of 68Ω
Tube depot has this video on youtube and you can see the measurements at minute 42:20.
Maybe they use such a low B+, to overdrive at lower voltage/volume? Regarding R18, it the force the signal to be clipped power stage too. Maybe all of this is just a way to achieve the distortion the they want.
Below is how I thought of biasing the amp.
B+250V, Vg2 = 220V ; Ianode = 25mA instead of 48mA
Vk = 8.3V instead of 3V;Rk=330Ω instead of 68Ω
Tube depot has this video on youtube and you can see the measurements at minute 42:20.
Maybe they use such a low B+, to overdrive at lower voltage/volume? Regarding R18, it the force the signal to be clipped power stage too. Maybe all of this is just a way to achieve the distortion the they want.
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25ma would give ~6 watts of dissipation. It's a 12 watt tube. I don't see the point of running it at half it's dissipation. That will give you maybe a watt & a half at the speaker taps, with 5% THD.
jeff
Because I am using a boost converter and can't get 48mA at 250V. Maybe it will be better to use transformers.
To have any effective output power idle will be high.
And absolutely use a real transformer, boost converter wont last long and when they work are poor unless over rated.
More bang for the buck is almost 2x power with same power transformer using push pull.
The poor efficiency of class A is well noted.
People love to tell you romance stories about magic distortion, when its rather farty and spongy with low idle.
They are fun projects to get moving and build. Bud went to a higher range for a reason
And absolutely use a real transformer, boost converter wont last long and when they work are poor unless over rated.
More bang for the buck is almost 2x power with same power transformer using push pull.
The poor efficiency of class A is well noted.
People love to tell you romance stories about magic distortion, when its rather farty and spongy with low idle.
They are fun projects to get moving and build. Bud went to a higher range for a reason
Hi,
I haven't started yet, since I was concerned with the 68ohm resistor.
I will try a boost that I have at home that should be able to deliver more than 50mA.
This is the one:
I have another one that is waayyy better than the above, but it's a development board that I don't want to put inside an amplifier case.
https://www.analog.com/en/resources...aluation-boards-kits/dc2744a.html#eb-overview
This thing claims 700mA at 240V !!! Even if it is just a tenth of that it will do for testing this amp.
I would guess that this one is much better in every single aspect, but it is expensive.
Kind regards,
Pedro
I haven't started yet, since I was concerned with the 68ohm resistor.
I will try a boost that I have at home that should be able to deliver more than 50mA.
This is the one:
I have another one that is waayyy better than the above, but it's a development board that I don't want to put inside an amplifier case.
https://www.analog.com/en/resources...aluation-boards-kits/dc2744a.html#eb-overview
This thing claims 700mA at 240V !!! Even if it is just a tenth of that it will do for testing this amp.
I would guess that this one is much better in every single aspect, but it is expensive.
Kind regards,
Pedro
I have messed around with the top one, sort of like a blind man fumbling around in the dark. Not exactly the most exhaustive look at it, just trying to get the most I can out of it.
https://www.diystompboxes.com/smfforum/index.php?topic=130852.20
I made this little Champ of sorts, The limitations and thoughts in the above link. Will be trying it from a higher supply voltage in order to do a P-P amp. Probably the smallest practical 8" in a cabinet.
https://www.diystompboxes.com/smfforum/index.php?topic=130852.20
I made this little Champ of sorts, The limitations and thoughts in the above link. Will be trying it from a higher supply voltage in order to do a P-P amp. Probably the smallest practical 8" in a cabinet.
OMG you've been through a lot to get this boost converter working as you wanted!
I hope with only 180V I can live with it as it is.
Your champ looks very nice.
By the end, the BUD is some sort of champ with an EL84 instead of a 6V6.
They put a tone stack there too. Maybe I will ad 2 more pots and make it a Marshall tone stack.😃
I hope with only 180V I can live with it as it is.
Your champ looks very nice.
By the end, the BUD is some sort of champ with an EL84 instead of a 6V6.
They put a tone stack there too. Maybe I will ad 2 more pots and make it a Marshall tone stack.😃
Hi,
I built the amp and works well. I cannot tell a lot about the sound quality, at this point, since the only thing I had available was a blackstar Fly 103 tiny external speaker which is super dark. I will connect it later to the 12'' Eminence speaker.
I can tell that the tone control works, the amp doesn't odcillate at max gain and it is silent when I am not playing.
The valves are JJ12AX7 and JJEL84. As I shake really a lot, some solder joints are far from perfect. However I did the best I could.
I built the amp and works well. I cannot tell a lot about the sound quality, at this point, since the only thing I had available was a blackstar Fly 103 tiny external speaker which is super dark. I will connect it later to the 12'' Eminence speaker.
I can tell that the tone control works, the amp doesn't odcillate at max gain and it is silent when I am not playing.
The valves are JJ12AX7 and JJEL84. As I shake really a lot, some solder joints are far from perfect. However I did the best I could.