That won't be the queitest, though...Tubetec said:
Because they were intended for DC applications, in analog computers.
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clintrubber
Well-known member
abbey road d enfer said:
That linked pdf above & below again here might help with noise, but if it'll be enough will depend on circumstances
http://w140.com/brophy_122_mod.pdf
This seems to me inescapable for a mic preamp.clintrubber said:
Very interesting; it's the first time I see decreasing heater voltage as a significant means of reducing input noise whilst maintaining transconductance.
clintrubber
Well-known member
abbey road d enfer said:
Same here, not read such a thing before. Makes one wonder why it's not used more often. Clashes with the recommendation to avoid heater voltage deviations of say more than +/-10% though (in view of life time and/or shifting parameters), perhaps that why it's not considered more often. But hey, if it gives lower noise then simply replace the tube more often. We change guitar strings as well.
Step up to grid transformer/resistor Id have assumed would be a first step to modding it for dynamic mics at least ,
That being said would it be possible to match a 48v condenser mic directly to a push pull valve input stage , re-jig the ht+/- to show 48v+ at the grids to energise it ,even if it was only usable on high level sources.
What I find hard to visualise is a resistor with + volts at the top and - volts the other end , so at some point along the resistor is 0 volts ,yet ground and chassis is also 0 volts yet not connected
Many thanks also to Ian and Abbey for the insights on feedback .
He didnt mention AC or DC supply in the modifications , what was the original type 125 I wonder?
That being said would it be possible to match a 48v condenser mic directly to a push pull valve input stage , re-jig the ht+/- to show 48v+ at the grids to energise it ,even if it was only usable on high level sources.
What I find hard to visualise is a resistor with + volts at the top and - volts the other end , so at some point along the resistor is 0 volts ,yet ground and chassis is also 0 volts yet not connected
Many thanks also to Ian and Abbey for the insights on feedback .
He didnt mention AC or DC supply in the modifications , what was the original type 125 I wonder?
That wide body rack mount RM122 would look nice on top of a Tele LA-2a , LF filters are good as they are , HF cut to 50,250,1000hz makes no sense for audio ,repurposed to more usable frequencies in the khz band is easy to do .
122 output stage is only good for hi(ish)-z input though ,
122 output stage is only good for hi(ish)-z input though ,
That is perfectly doable. It is also doable with solid-state circuitry. It would just take lifting the +/-15V rails to +33/+63; not difficult but adds complexity. Then the reason why wanting to do that is getting rid of the input capacitors, but the issue is the output signal is then lifted by 48V, and needs to be shifted down to 0V, which takes at least one of the dreaded capacitors. Or something else (e.g. xfmr-coupled isolator).Tubetec said:
moamps
Well-known member
abbey road d enfer said:
From my experience, 12ax7 can't work at 3V for heater voltage.
Also it is questionable does the heater voltage stay at -6V when the heater current is smaller. It is much higher, IMO, because of installed power resistor across the pass transistor. They stated BTW the minimum heater current of 0.7A for TEK125 power supply. If only one "modified" preamplifier is connected to it, the power supply is out of regulation for filament part, IMO.
Schematic of power supply is here
http://bama.edebris.com/manuals/tek/122_125/
Although I never tried it, I tend to believe that.moamps said:
Not knowing the unregulated voltage value does no help analysis but no doubt the 9 ohm resistor takes over regulation. Then the remaining tube in the third stage, that is not series connected get excessive voltage.
I don't think Tektronix would have approved of the mod.
I have a regulated LT supply on an EL84 based line driver , I did notice hum drops disproportionately as you reduce heater volts , and that even holds true for a relatively quiet dc supply ,everything else apart from gain and anode volts drops too .
I'd be of the mind that if we were to reduce heater volts ,we should also de-rate the anode voltage , in other words if a valve is operating at a fraction of its maximum anode current ,were quite safe to shave a volt or maybe more off the heater supply ,if your running a tube up towards the limits of its disipation dont be mean with heater volts .
3v on an ECC83 heater best of good luck with that unless you want to run it with a 470k load and starvation anode volts well <100.
Its often advisable to check back on the correspondence page letters of advice the following month ,as retractions ,corrections and additions are made quite often . ;D
I'd be of the mind that if we were to reduce heater volts ,we should also de-rate the anode voltage , in other words if a valve is operating at a fraction of its maximum anode current ,were quite safe to shave a volt or maybe more off the heater supply ,if your running a tube up towards the limits of its disipation dont be mean with heater volts .
3v on an ECC83 heater
best of good luck with that unless you want to run it with a 470k load and starvation anode volts well <100. Its often advisable to check back on the correspondence page letters of advice the following month ,as retractions ,corrections and additions are made quite often . ;D
ruffrecords
Well-known member
moamps said:
I can attest from personal experience that it does. A long time ago I made a long series of measurements of double triode tube noise when configured as a mu follower. When I first tested a 12AX7 I accidentally wired the 6V heater supply across the 12V heater pins so each half was working with 3V heaters. It did work but the gain swas eriously reduced. I forget by how much because I soon found the fault and fixed it.
Cheers
Ian
ruffrecords
Well-known member
abbey road d enfer said:
I do not recall the article saying transconductance was maintained. Overall gain increase because the second stage 12AU7 (u=17) was replaced with a 12AX7 (u=100) but I do not see any mention of transconductance.
Cheers
Ian
I should have writ "maintaining some transconductance". Indeed, I would not expect total conservation of electron flow.ruffrecords said:
According to the paper, overall gain increased by a factor 2.8, when the 12AU7 to 12AX7 would have accounted for factor 5. That means that Gm decreased by only about 30-40%. That is very surprizing to me.
moamps
Well-known member
ruffrecords said:
Isn't that just confirmation of what I said? Come on.
12ax7 can work from, let say, 5V because it has small heating element and anode current is low.
However, someone can try cold emission with it and will get some anode current. There are always some desperate free electrons around. But it isn't 12AX7 anymore.
As I said, I believe the heating voltage in question wasn't drop to 3V because of the lousy filament regulator and gain of tubes remain reasonably high.
No need for heated (pun intended) arguments here. The issue there is defining "work". A 12AX7 with 12V plate voltage is not a 12AX7 anymore, or is it? A number of (questionable) products do that, though. I think a more scientific approach is desirable here; anybody with a small jig and a variable PSU could check.
This is the brewery... better to argue about tubes than the typical subjects. :abbey road d enfer said:
JR
ruffrecords
Well-known member
It was simply an anecdote.moamps said:
Cheers
Ian
There have been any number of efx SKUs that ran tubes at low voltage just so they could say the audio path included a tube, and avoid the obvious cost of HV PS and components. How they sounded was not paramount, what the FAB (features,advantages,benefits) card said at POS (point of sale) was.... :
JR
JR
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