owlexifry
Tele-Meister
would adding the 1M, to make the 0.1uF/1M combo on output have any detrimental effect on this? (low impedance / CF output)
MkIIC+ inspired preamp.. DIYLC layout drafting
- Thread starter owlexifry
- Start date
would adding the 1M, to make the 0.1uF/1M combo on output have any detrimental effect on this? (low impedance / CF output)
andrewRneumann
Friend of Leo's
No. A pot would mess things up when the volume level is reduced because it adds resistance in series to the output.
owlexifry
Tele-Meister
ah that’s good to know! thanks
owlexifry
Tele-Meister
just finished it.
put in a 250V 3A fuse.
turned on power (no tubes in)
mains: 238V ac
(primary wired for 240V)
secondary: 318V ac ...supposed to be 300V ac
heaters: 6.9v ac ...
B+1, 2, 3, 4 are all 434V.
dropping resistors don’t seem to be doing anything without the tubes installed...
(because there are no tubes drawing any current?)
plate voltages:
(left to right on board)
P1 427
P2 429
P3 431
P4 424
P5 434
P6 430
with the obvious limited experience i have, i’m hesitant to install the tubes with these values...
will the tubes’ load really bring the voltages down?
or should i put in a larger dropping resistor at b+1?
Edit:
+ i’ve just realized i forgot to drill a hole and hook-up the safety earth for the IEC plug..
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owlexifry
Tele-Meister
i promise i’ll drill a hole and bolt it in with a tag before i do anything else (done for today)
andrewRneumann
Friend of Leo's
View attachment 928186
View attachment 928187
just finished it.
put in a 250V 3A fuse.
turned on power (no tubes in)
mains: 238V ac
(primary wired for 240V)
secondary: 318V ac ...supposed to be 300V ac
heaters: 6.9v ac ...
B+1, 2, 3, 4 are all 434V.
dropping resistors don’t seem to be doing anything without the tubes installed...
(because there are no tubes drawing any current?)
plate voltages:
(left to right on board)
P1 427
P2 429
P3 431
P4 424
P5 434
P6 430
with the obvious limited experience i have, i’m hesitant to install the tubes with these values...
will the tubes’ load really bring the voltages down?
or should i put in a larger dropping resistor at b+1?
Edit:
+ i’ve just realized i forgot to drill a hole and hook-up the safety earth for the IEC plug..
Looks really nice. Good job. Yes, installing the safety ground should have been the 1st thing you did before you ever wired up mains. You will do that on your next build... RIGHT?
The lack of tubes explains the high voltages for the exact reason you suspected. If the HT is 318vac, then the theoretical maximum B+ is 450VDC minus a couple diode drops. Your voltages look right on. I suspect you aren't getting 448VDC because your elevated heater circuit is drawing current even when no tubes are installed, but not sure that explains a 15V sag in the power supply already. Possibly your voltmeter needs some calibration... but all in all I would feel comfortable with what you are seeing.
Start by installing the tube that utilizes B+4. You should see some voltage drops across B+1,2,3,4 when you do that. Voltages will still be a little high, but should come down as you add in more tubes.
Light bulb limiter and variac come in handy at this stage. Do you have either?
andrewRneumann
Friend of Leo's
On that fuse holder, the hot mains from the IEC socket should be on the tail of the holder. The side lug then goes to the power switch. The reasoning is that if you were to replace the fuse with the amp plugged in, you could shock yourself if the end of the fuse made contact with the side lug. Either way--always unplug the amp before replacing a fuse.
owlexifry
Tele-Meister
Looks really nice. Good job. Yes, installing the safety ground should have been the 1st thing you did before you ever wired up mains. You will do that on your next build... RIGHT?
The lack of tubes explains the high voltages for the exact reason you suspected. If the HT is 318vac, then the theoretical maximum B+ is 450VDC minus a couple diode drops. Your voltages look right on. I suspect you aren't getting 448VDC because your elevated heater circuit is drawing current even when no tubes are installed, but not sure that explains a 15V sag in the power supply already. Possibly your voltmeter needs some calibration... but all in all I would feel comfortable with what you are seeing.
Start by installing the tube that utilizes B+4. You should see some voltage drops across B+1,2,3,4 when you do that. Voltages will still be a little high, but should come down as you add in more tubes.
Light bulb limiter and variac come in handy at this stage. Do you have either?
thanks for the tips and feedback!
yeahhhhhh im gonna blame fatigue.
(and the fact the PT didn’t end up having a shield wire - i had planned to ground the safety earth and shield together)
indeed, i was thinking how handy a variac would be at the moment, but nah, don’t have one.
guess i’ll just go for it and install the tubes and see what happens...
i wonder how much plate voltage a 12AX7 can take at idle before something melts...
dan40
Friend of Leo's
They will be fine. If you ever power up the amp with the power tubes removed, the preamp tubes will see very similar voltages. I wouldn't intentionally run them at 400v, but in this instance, they will be fine until you get the power tubes in and biased properly.
owlexifry
Tele-Meister
thanks, i’ll take your vote of confidence
no power tubes here (preamp only),
hence the hesitation, as i wonder how much current/load the x3 12ax7s can really draw to bring down those plate voltages
andrewRneumann
Friend of Leo's
What voltage are you targeting for B+2/3/4?
andrewRneumann
Friend of Leo's
You have many things working in your favor to keep the tube safe even at high voltages. 12AX7 has a relatively high internal anode resistance, plus we put another fairly large anode resistor in series with the valve, plus we use current through a cathode resistor to bias the valve which is self-correcting. This all amounts to one thing… it’s really hard to “melt” a standard 12AX7 gain stage.
If your six valves draw 5mA total, then you are looking at 50V of drop across that first 10K resistor. Does 385V at B+2 sound good?
If your six valves draw 5mA total, then you are looking at 50V of drop across that first 10K resistor. Does 385V at B+2 sound good?
owlexifry
Tele-Meister
B+2, B+3 target = 397V
B+4 target = 392V
yes! thank you. this is the gold i come here for. saved me a lot of searching.
assume you calculated for *six triodes*? so each triode draws 5ma?
385V B+2 will be very close... 10V more and just about there ...just as well i've got a spare 8.2K 5W if i need it
andrewRneumann
Friend of Leo's
I just assumed about 0.8mA per triode on the average, but to be honest I didn’t go back to look at your design. It is 3 12AX7s right?
owlexifry
Tele-Meister
ahhh right, 5ma total, of course - should have read that properly...
(got me scratchin the noggin there, all the reference material i see mentions 1.2ma.... but they never seem to specify if that's 1.2ma per triode, or per pair)
correct, x3 12AX7s
owlexifry
Tele-Meister
safety ground should have been the 1st thing you did before you ever wired up mains. You will do that on your next build... RIGHT?
can i be taken off the watchlist now?
ok.
tubes in.
B+2 was 356V.
immediately turned on the iron.
tried 4.7K - B+2 390V
then 2.2K - B+2 408V
given the 3.3K dropping resistors between each filter cap (values kindly recommended by @Lowerleftcoast ), i suspected the voltages further down at B+3, B+4 were going to drop short of target no matter what i do, so i decided to overshoot further up and left the 2.2K dropping resistor in.
so all up:
(B+1 425)
B+2 408
B+3 393
B+4 385
not too bad.
plate voltages (L to R on board)
1) 211v
2) 257v
3) 270v
4) 208v
5 408v (CF -> 153.7V)
6) 260v
end result:
it sounds fantastic.
and i can’t believe how much gain / overdrive there is.
here’s a quick demo:
https://youtube.com/shorts/q5Z1gVQF7Vg?feature=share
but there are a few problems:
- master volume control is backwards.
- i tried to use an overdrive pedal in front of it. no sound.
tried another. no sound.
tried and checked everything.
no sound.
tried petersen strobe tuner in between - no sound in either true bypass or buffered bypass modes.
this is where it gets even more confusing:
it was completely fine to run my ISP decimator noise gate (rack version) between guitar and preamp input, whether bypassed or not.
but as soon as i put a pedal (overdrive, tuner) between the guitar and the preamp input, it doesn’t make any sound, whether bypassed or not.
i’m completely confounded.
would have thought it’d make sound in true bypass surely?
something to do with the input?
current design is input tip goes direct to grid.
please help.
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andrewRneumann
Friend of Leo's
hmmm… can you verify that with an ohmmeter? Plug a cable in and measure resistance from tip of cable to chassis. Should be 1M.
Unrelated but related: did we just miss the lack of shunt tip jack for the input or was that on purpose?
owlexifry
Tele-Meister
well i’m glad you asked me to check, cos i found the problem.
checked the resistance from tip to ground bus bar + IEC ground pin.
initially this was close to zero..
looked closer at the tip / shunt/switch arrangement. it was bent out of correct alignment.
turns out with jack plugged in, the shunt was touching the tip, and not touching when unplugged..
bent it back in shape, checked resistance again, and yep good, 998K
plugged it all in.
pedals work
i’m surprised it was making any sound at all like that..
just gotta sort out the backwards master volume control now
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owlexifry
Tele-Meister
i changed the jack from regular (type 11) to switching (12A) during a revision earlier on
andrewRneumann
Friend of Leo's
well i’m glad you asked me to check, cos i found the problem.
checked the resistance from tip to ground bus bar + IEC ground pin.
initially this was close to zero..
looked closer at the tip / shunt/switch arrangement. it was bent out of correct alignment.
turns out with jack plugged in, the shunt was touching the tip, and not touching when unplugged..
bent it back in shape, checked resistance again, and yep good, 998K
plugged it all in.
pedals work
i’m surprised it was making any sound at all like that..
just gotta sort out the backwards master volume control now
I was wondering if you had the Tip and Sleeve swapped. A guitar or transformer coupled device might still work, but most things won’t. Glad you got it sorted. Thanks for the sound clip BTW—very cool. Nice to see it all come together. I’m happy for you. I’m pretty sure you’ll figure out what to do on the reversed MV.
