Looking at the Pultec MB1 / MEQ5 schematics which have a seemingly unique DC coupled NFB design. I have read some threads on this aspect but I am curious about the output cathode follower. The similar but different designs by Pultec use a paralleled 12AU7 / 6AQ5 for the cathode follower, but I am wondering if a single triode of a 12AU7 is possible as it would be neat to be able to do two channels with only 3 tubes thus requiring less heater current. Does the DC coupling require a beefier setup as seen in these examples? The feedback resistor is 5W so the current is certainly flowing but some other cath follower into OPT preamps I know of are fine with a single triode.
Pultec MB1 Cathode Follower
- Thread starter heatwalk
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ruffrecords
Well-known member
The drive capability of a cathode follower depends largely on the quiescent current. Clearly with two paralleled tubes you can have twice the current and twice the drive capability, The Pultecs were professional products designed to drive 600 ohm loads to high levels so a high drive capability was necessary. If you are just going to be driving a 10K load then you can get away with a single cathode follower.
Cheers
Ian
Cheers
Ian
Thanks Ian. I figured I would use an OPT in the realm of 15k:600. I usually load with a resistor or output level control with a 1k pot which I remember you mentioning in another thread years back which I started doing. Any issues I would come across if really pushing the pre in the modern day?
ruffrecords
Well-known member
Well, 1K is not that far from 600 ohms so it is still quite a heavy load. This means you would not have much difficulty clipping a single cathode follower. As I mentioned, if you load it with 10K it will be OK.
Cheers
Ian
Even with output transformer? One of the other circuits I can remember seeing is the WE 141A. Looks like that can be configured to drive 600 sans output transformer using half of a 6SN7. Is this possible bc less current/ AC coupled?
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It's out of the question it can be connected to a 600ohm load, but what level it could push into 600ohm before clipping.
Of course the cathode follower has a lower impedance but a single small signal triode still has some current limit.
Of course the cathode follower has a lower impedance but a single small signal triode still has some current limit.
ruffrecords
Well-known member
The term 600 ohm output is very vague. That CF has a quiescent current of about 4mA. Ohms law tells us that 4mA into 600 ohms is 2.4V peak or 1.7V rms or +6.8dBu Any level higher than that with a 600 ohm load will certainly clip and in practice it will be well before that. "You cannae change the laws 'o physics".
Cheers
Ian
