Hi,
I am re-designing a mic pre I have due to the fact that I wanted to eliminate the totem pole follower output. The rest of the mic pre is single ended, no negative feedback and I wanted to keep that topology through out as a I prefer that sound. The mic pre has a 225vdc B+ and all tubes are heated in series using a 50.4 vdc H+. That gives 12.6 vdc to the heaters of each of the 4 tubes and allows a phantom power supply without need a separate supply.
The way I have it set up right now is 1:10 input tranny > 7062 (e180cc) common cathode no negative feedback stage > coupling cap > stepped attenuator > 7062 (e180cc) common cathode no negative feedback stage > coupling cap > 5:1 (15k:600) output tranny. The former design uses 2 6072 stages and a third totem pole follower stage using a 12au7.
Although the 7062 is very close in mu to the 6072 and more current drive ability I cannot quite get the same output and voltage swing due to the beefy totem pole follower the original had. I want to add a third stage to better drive the output transformer. I plan to try a 12ab5, which is a lower voltage 6v6 with 12.6 volt heaters in a miniature Noval footprint. I plan to wire it as a triode in common cathode no feedback which should give 10 mu, 5000 gm, and 1965 ohms internal resistance. I believe this will be a nice fit but I only know enough to be dangerous. will it be too much for a mic pre output? Am I gonna blow up my converters with this thing?
I am re-designing a mic pre I have due to the fact that I wanted to eliminate the totem pole follower output. The rest of the mic pre is single ended, no negative feedback and I wanted to keep that topology through out as a I prefer that sound. The mic pre has a 225vdc B+ and all tubes are heated in series using a 50.4 vdc H+. That gives 12.6 vdc to the heaters of each of the 4 tubes and allows a phantom power supply without need a separate supply.
The way I have it set up right now is 1:10 input tranny > 7062 (e180cc) common cathode no negative feedback stage > coupling cap > stepped attenuator > 7062 (e180cc) common cathode no negative feedback stage > coupling cap > 5:1 (15k:600) output tranny. The former design uses 2 6072 stages and a third totem pole follower stage using a 12au7.
Although the 7062 is very close in mu to the 6072 and more current drive ability I cannot quite get the same output and voltage swing due to the beefy totem pole follower the original had. I want to add a third stage to better drive the output transformer. I plan to try a 12ab5, which is a lower voltage 6v6 with 12.6 volt heaters in a miniature Noval footprint. I plan to wire it as a triode in common cathode no feedback which should give 10 mu, 5000 gm, and 1965 ohms internal resistance. I believe this will be a nice fit but I only know enough to be dangerous. will it be too much for a mic pre output? Am I gonna blow up my converters with this thing?
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Nice idea in principle, but I believe this only works reliably if you use heaters that are all designed to heat up at the same rate, usually indicated on the data-sheet by something like "controlled heater warm-up time of 11 seconds".
The trouble is that cold heaters have much less resistance than hot ones, so if some heaters take longer to heat up than others in a series string, they will see less than 12.6 volts, while the heaters that were first to heat up will see more than 12.6 volts, overheating them and shortening tube life.
You could probably get away with four identical tubes from the same manufacturer - even if not controlled to 11 seconds, all heaters will still match. But four more or less random tubes? I'd be concerned about that.
Before doing that, I'd like to better understand why the transformer is so difficult to drive in the first place.
So: how much signal voltage do you need from the secondary of the output transformer? Are you shooting for instrument level (say 100 mV)? Or line level (closer to 1V)?
If you only need a fairly small voltage, you may be able to "pad" the primary of the transformer with a series resistor, which will simultaneously drop the signal level, and make it easier for your preamp to drive the transformer.
As an example: suppose you want a 100 mV nominal output signal.
Your 15000:600 transformer has a voltage step-down ratio of 5:1. (This is the square root of 15000 divided by 600).
Therefore, if you want 100 mV output at the 600 ohm secondary, you need 500 mV, or half a volt (AC), at the 15k primary winding.
If the last preamp valve in your circuit generates, say, a 5 volt AC signal, you need to pad that down to only 0.5 volt before feeding it to the transformer.
This can be done with a series resistor that has 9 times the impedance of the transformer primary: 9x15k, or 135 kilo ohms.
The load seen by the preamp will then be (135k + 15k), or 150k.
This is quite a light load for a 12AU7 or similar fairly low rp triode. I'm sure your 12AU7 or similar preamp tube could happily drive a 150 kilo ohm load all day long without breaking a sweat. 🙂
It is possible that it will take more than just a 135k series resistor: the transformer may want to see a load of 15k across its primary in order to have a proper flat frequency response. If so, and using the signal levels mentioned above, wire 15k straight across the transformer primary, then use something like a 68k series resistor (rather than 135k).
One last thing: I looked up the 7062 datasheet, and found the words "The 7062 is not intended to be used in circuits critical as to hum, microphony, and noise".
I would say a mic pre is a circuit that is quite critical of hum, microphony, and noise. Maybe this is not a great application for a 7062 tube? 😱
-Gnobuddy
Worrying about whether the tubes are rated for controlled warmup is like worrying about wrong lug nuts on a flat tire. At 12V, a 7062 heater is 200mA, a 6072 heater is 175mA, a 12AU7 heater is 150mA, and 12AB5 heater 200mA. Ain't gonna work.
Agree that finding a sensible heater-string is essential.
You got 12AU7 totem-pole. While a TP can be a super-clean design, you say you don't like it. Then _I_ would strap both sections 12AU7 parallel as a "single" triode, resistance-coupled. With 225V supply I would aim near 10K RP (equiv 20K per section) 8mA (4mA/section). 4V bias, so 500r RK. rp will be about 10k per section, 5K/tube. 5K rp ||10K RP makes a 3.3k source with bypassed unloaded gain around 13. Peak swing about 60V. Driving un-loaded 10K:600 OT you get 15V peak swing which ought to be ample. THD at this point will be several percent; but if you wanted 0.000%THD you would have used a 990.
You got 12AU7 totem-pole. While a TP can be a super-clean design, you say you don't like it. Then _I_ would strap both sections 12AU7 parallel as a "single" triode, resistance-coupled. With 225V supply I would aim near 10K RP (equiv 20K per section) 8mA (4mA/section). 4V bias, so 500r RK. rp will be about 10k per section, 5K/tube. 5K rp ||10K RP makes a 3.3k source with bypassed unloaded gain around 13. Peak swing about 60V. Driving un-loaded 10K:600 OT you get 15V peak swing which ought to be ample. THD at this point will be several percent; but if you wanted 0.000%THD you would have used a 990.
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