Motown Direct Amplifier-inspired Preamp?

[DaveP]

Continuing my calculations:-
To find the operating points for the tubes we need to work backwards from the output. We are given that clipping started at +27dBm (600 ohms).
This is 17.35Vrms on the secondary, therefore there must be 5x 17.35V on the primary which is 86.75V . As the CF has a gain of 0.9, the drive voltage must be 86.75/0.9 = 96.4V. I have previously calculated the gain of the two tubes with NFB to be 30dB, therefore the input voltage to the first tube must be 30dB less or 96.4/31.62 = 3.05V at clipping. From this we can say that the first tube will clip at 3.05 x 2.828 =8.62Vp-p, dividing this by two gives us the cathode voltage =4.31V. If the IPT is 1 : 9, then the input to the amp would be 4.31/9 = 480mV which sounds like the right ballpark to me.

A cathode voltage of 4.3V puts us in ECC82/ 6SN7/ 12AU7 territory.

I am drawing up a schematic based on the V72 but with triodes instead of pentodes.
best
DaveP

 

[DaveP]

I have tried to chart various tubes with a Vk of 4.2V and a gain un-by-passed of 17-18 and I cannot find one.
The gain is from 8 to 13 only, so I guess that DC feedback is needed to increase the current in order to get 4V on Vk
best
DaveP

 

[CJ]

Possibly a cathode follower driving the WCF?

here is the scratch pad with an LA2a type circuit on the top, looks like too much gain unless you knock it down with dividers and NFB,

the bottom drawing is a Marshall type cathode follower feeding the WCF, and some NFB. this would cut down on the gain as the second part of the follower has close to unity gain,

i have an RCA 6BX7 on the way from evilbay,

there is also that Jensen line driver with the 6AU6 driver which might be interesting

 

[ruffrecords]

I have never understood the fascination with the White follower. Yes, it has an incredibly low small signal output impedance but its main advantage is that it is push pull so we save a lot of quiescent current. Its biggest problem though is it has unity gain. This is why I prefer to use an SRPP output stage. It has the advantage of being push pull and it has a decent amount of gain. Its downside is higher distortion but the NFB takes care of that.

Cheers

Ian