Theoretically true, but in reality, running the bias of a 12AX7/ECC83 below 1V is an invitation to grid current distortion. Quirk of that particular tube.
Below 1V you could get grid current with other valves too, although I would expect it to be worse with high gain valves as the grid is very close to the cathode. It is probably more noticeable with the ECC83 because that needs to be biased near that region because of its high mu.
ok, great success!
rebuilt the circuit using new parts and it fired straight up, with loads of bass, but lacking in treble and mids, so i swapped out the jj long platers for some cheap chinese units and the mids appeared almost perfect, bass still ther, but little treble, well not nearly as much as there should be with the tone on the power amp all zeroed, but i think this is down to the cheap capacitors i have used (high quality caps rent available off the shelf in new zealand) so i will order in some good ones from the usa.
however, (i just have to complain about something right) there is the same drive issue as im having with the stock yamaha internal preamp.. the tube preamp also is not driving my power amp hard enough.... i have set the voltage as high as i can, but it starts to distort due to over voltage on the tubes i think... so ive set the power supply series resistor to 33k, and it sounds ok at that but lacks drive still.... do any of you think i should look into adding a third tube to the system- a triode each on the output stage and, then control the amount of output by lowering the supply voltage as needed?
if any of you know a good "boost" circuit, using a 12au or 12ax please let me know!
also im unsure of the input impedance of my amplifier, but its a yamaha avx 20 from the mid 1980s..... the yamaha cd player and akai gxc 39d, akai gx210 d decks drive it fine, it just seems this issue is with the phono alone....
thanks for your continued help and education, it honestly is true that different makes of 12ax7 have different sound charactaristics and tha one needs to invest in good quality caps in the audio path!!!
rebuilt the circuit using new parts and it fired straight up, with loads of bass, but lacking in treble and mids, so i swapped out the jj long platers for some cheap chinese units and the mids appeared almost perfect, bass still ther, but little treble, well not nearly as much as there should be with the tone on the power amp all zeroed, but i think this is down to the cheap capacitors i have used (high quality caps rent available off the shelf in new zealand) so i will order in some good ones from the usa.
however, (i just have to complain about something right) there is the same drive issue as im having with the stock yamaha internal preamp.. the tube preamp also is not driving my power amp hard enough.... i have set the voltage as high as i can, but it starts to distort due to over voltage on the tubes i think... so ive set the power supply series resistor to 33k, and it sounds ok at that but lacks drive still.... do any of you think i should look into adding a third tube to the system- a triode each on the output stage and, then control the amount of output by lowering the supply voltage as needed?
if any of you know a good "boost" circuit, using a 12au or 12ax please let me know!
also im unsure of the input impedance of my amplifier, but its a yamaha avx 20 from the mid 1980s..... the yamaha cd player and akai gxc 39d, akai gx210 d decks drive it fine, it just seems this issue is with the phono alone....
thanks for your continued help and education, it honestly is true that different makes of 12ax7 have different sound charactaristics and tha one needs to invest in good quality caps in the audio path!!!
oh and i do have loads of hum, i think caused by the heater ac supply, all it has is two 100 ohm resistors connected to negative.... hat size filter cap would be recommended for dc heater supply to three 12ax7s?
thanks again!
mike
thanks again!
mike
Post the circuit that you actually built.
As you've been told already, a 12ax7 common cathode stage will work very poorly into a low impedance, such as a SS amp.
"Loads of bass" and "lacking treble" are meaningless. Measure frequency response and post it, then we can comment.
The fact that you are not "driving the amp hard enough' may be simply that you do not have enough gain for the cartridge used. Changing the B+ voltage is hardly the way to change gain.
Look at the cartridge output, phono stage gain, your amp sensitivity for full power output and you will be able to see how much gain you need vs how much you have.
As you've been told already, a 12ax7 common cathode stage will work very poorly into a low impedance, such as a SS amp.
"Loads of bass" and "lacking treble" are meaningless. Measure frequency response and post it, then we can comment.
The fact that you are not "driving the amp hard enough' may be simply that you do not have enough gain for the cartridge used. Changing the B+ voltage is hardly the way to change gain.
Look at the cartridge output, phono stage gain, your amp sensitivity for full power output and you will be able to see how much gain you need vs how much you have.
ok, i have managed to get ahold of a signal generator app for my phone, but i need to get ahold of a true rms multimeter, which i might buy this weekend.
just out of interest, are those soundcard scope softwares any good for measuring this?
im also looking for more info on my cartridge, the stylus says "stanton a600"....
will let you know once i have a curve printed....
just out of interest, are those soundcard scope softwares any good for measuring this?
im also looking for more info on my cartridge, the stylus says "stanton a600"....
will let you know once i have a curve printed....
ok i have brought a true rms meter and am using a signal generator app on my smartphone to get the desired signal, however i cannot adjust the volume below an output of 1.6v
someone told me that i should be measuring with about 40mv to the input and go from about 10 hz to 20 000 and i should see the freq fall in certain areas... am i on the right track?
thanks
someone told me that i should be measuring with about 40mv to the input and go from about 10 hz to 20 000 and i should see the freq fall in certain areas... am i on the right track?
thanks
.. am i on the right track?
with all the love in the world, probably not. Post a schematic
Please!
with all the love in the world, probably not. Post a schematic
Please!
True, placing two in series or using an IR led (better IMO) would address this concern. Still the fact remains that many vintage commercial designs ran these with bias voltages well below 1V - and yes there is usually some measurable grid current. I'd agree that this is not a good excuse to do the same in a new design however.. 😀
Yeah, the input impedance of the Yamaha is about an order of magnitude too low for good performance with this circuit.
I looked at the link, but no matter what I did I was unable to get a good look at that schematic, although I can tell it was a pretty standard implementation for a simple passively eq'd phono pre-amp - indeed one of my very early designs (1988) is quite similar in most respects.
I looked at the link, but no matter what I did I was unable to get a good look at that schematic, although I can tell it was a pretty standard implementation for a simple passively eq'd phono pre-amp - indeed one of my very early designs (1988) is quite similar in most respects.
Battery bias also works great for both stages. The first stage is really easy - lift both channels' grounds and connect the battery between both channels' (common'd) grounds and chassis. Bolt the valves' cathodes to chassis. Done deal.
The second stage's battery connects between the (common'd) grid leak resistors' (220K) bottoms and ground. Valves' cathodes to ground and done.
All good fortune,
Chris
The second stage's battery connects between the (common'd) grid leak resistors' (220K) bottoms and ground. Valves' cathodes to ground and done.
All good fortune,
Chris
Wow. That makes no sense, does it? Let me try again:
Lift the input jacks' ground connections and connect the battery between them and ground. The 47 K load resistors stay across the input jacks. Cathodes connect directly to circuit ground (the spot where the battery attaches is perfect).
Sorry,
Chris
Following up on the general discussion of PN junction and LED bias, etc. Battery bias gives the highest possible gain and lets the cathode connect as directly as possible to ground, with a small advantage in noise and linearity (compared to various diode bias schemes).
Thanks,
Chris
thanks, ive never seen this done before...everyone who seems to have tried it seems to like it, so it might be on he cards to give it a try once i get my impedance issues sorted....which might mean i need to look for a low impedance tube to go with instead of the 12ax7lps🙁
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