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

 

[CJ]

I have seen a version that has a phase inverter driving a WCF or SRPP, I forget which,

If the reissue is just using one 6V6, that says that the original did not use a white cathode follower, if they are using the same circuit.

 

[DaveP]

This is how far I have got in making a V72 type circuit with a 6BX7 cathode follower.
Please note that it has both ac and dc feedback.

I will gradually fill in the circuit values and update as I go.

C4 may not be needed if DC connection is possible.
C6 would probably be around 2~5 uF and should be chosen to give a flat response with the resonance of the chosen OPT.
best
DaveP

 

[MidnightArrakis]

This is how far I have got in making a V72 type circuit with a 6BX7 cathode follower.
Please note that it has both ac and dc feedback.

I will gradually fill in the circuit values and update as I go.

[I will gradually fill in the circuit values and update as I go] -- Good Deal!!! Looking forward to a completed schematic!!!

[ac and dc feedback] -- AC/DC were known for their feedback!!!

/

 

[CJ]

UTC HA 113 would work well for this project with the 200 mW level,

This dealer says there is a Cinemag output xfmr in there, probably the 2840 which would suggest a follower circuit,

12AT7 tubes are listed on another site, I bet the original used 12AX7's, maybe they wanted lower gain to compensate for the 6V6 being used in place of the 6BX7, strap the 6V6 for triode operation and you could probably use 12AX7's,

https://ardasuppliers.com/en-lv/pro...hSIj6XxkwOpBQVBjRk0q0PSi0W_fLhzTYNcfORXW9zE2d

 

[CJ]

Found a gut shot, non invasive,

Cool on the tube socket shock mounts !

The 6V6 being that close to the open frame xfmr worries me,

 

[PermO]

This topic sort of reminds me of an interview with Pino Palladino.
They are sitting in a studio and Pino has this beautifull fretless bass.
At some point the guy asks.. "can I play it ?"
"Yes sure" Pino says...
The guy plays this wonderfull bass and says .. "it sounds like me"

Pino replies "What did you expect ?"