Noobish quention: How to add a pot in a tube pre amp?

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Tons of gain is what happens with tubes. Get the swing around 100v out of the second stage then step it down with a 5 to 1 output transformer 15k to 600 and you have 20v p to p, you decreased the noise by 5x and you have 5x the drive capability (lower output impedance) that will burn some of that gain off. I personally stay away from negative feedback as a gain reduction technique because I dont like the way circuits clip with NFB. and it messes with the gain control depending where you inject it.
 
What is this modification using a linear pot?
There's a switch not shown that selects either an A100k pot or this 25k linear with the resistors?
It's a Bruce Rozenblit line pre but it had other things that were different like using 5814 with 315v plate supply and a couple of other things.
 

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that's just a poorly drawn 470K/56K (20dB) attenuator - probably making up (or down?) for the un-needed 22dB gain in the SRPP tube stage?

That 470K is asking for noise imo

/Jakob E.
 
Hello Mr Bell,
can you please explain me what means Vk/h+ and Vk/h-
in the data sheet of the 6922 JJ knowing that I 'm building a mufollower stage , that the heater is 6.3V AC,and how to measure Vk/h+ and Vk/h-
Hoping that I'm clear,thank you for your help,
best regards
Tayeb
 
Hello Mr Bell,
can you please explain me what means Vk/h+ and Vk/h-
in the data sheet of the 6922 JJ knowing that I 'm building a mufollower stage , that the heater is 6.3V AC,and how to measure Vk/h+ and Vk/h-
Hoping that I'm clear,thank you for your help,
best regards
Tayeb
for me,h+ and h- does not make sens with AC heater...
 
Hello Mr Bell,
can you please explain me what means Vk/h+ and Vk/h-
in the data sheet of the 6922 JJ knowing that I 'm building a mufollower stage , that the heater is 6.3V AC,and how to measure Vk/h+ and Vk/h-
Hoping that I'm clear,thank you for your help,
best regards
Tayeb
Vhk is the voltage that can exist between the heater and the cathode. For most tubes it has two values Vhk+ being how positive the heater can be with respect to the cathode and Vhk- being how negative it can be. This voltage can have both a dc component and an ac component. The dc component is the dc voltage between the heater and he cathode with no signal. In a mu follower, the top cathode is at a much nigher dc voltage than the lower cathode so if your heaters are referenced to HT 0V the Vhk of the top cathode can be about half the supply voltage

However when the tube passes signal the cathode can move up and down with the signal (especially the top cathode in a mu follower). This is the ac component. The sum of the dc and ac components must not exceed Vhk for the tube.

To measure it simply power up the circuit and connect a voltmeter between one side of the heater and the cathode.

Cheers

Ian
 
Vhk is the voltage that can exist between the heater and the cathode. For most tubes it has two values Vhk+ being how positive the heater can be with respect to the cathode and Vhk- being how negative it can be. This voltage can have both a dc component and an ac component. The dc component is the dc voltage between the heater and he cathode with no signal. In a mu follower, the top cathode is at a much nigher dc voltage than the lower cathode so if your heaters are referenced to HT 0V the Vhk of the top cathode can be about half the supply voltage

However when the tube passes signal the cathode can move up and down with the signal (especially the top cathode in a mu follower). This is the ac component. The sum of the dc and ac components must not exceed Vhk for the tube.

To measure it simply power up the circuit and connect a voltmeter between one side of the heater and the cathode.

Cheers

Ian
thank you very much Maestro:) best regards Tayeb
 
thank you very much Maestro:) best regards Tayeb
Hello Mr Bell,thank you very much for your feedback,
allow me to ask you another question which is a mystery for me:)
in the power supply of the classic solo,we have a voltage divider 66K/22K(2W) to obtain HT/4 to elevate the heater,
my question is: why we have to do such a power divider to obtain ht/4 which is only a polarisation voltage.?
is there to drop down the ht voltage ? in this case is the divider act as a current drain?
If I use a 100K/33K to obtain ht/4 why it's not the same?
thank you very much
best regards
Tayeb
 
Hello Mr Bell,thank you very much for your feedback,
allow me to ask you another question which is a mystery for me:)
in the power supply of the classic solo,we have a voltage divider 66K/22K(2W) to obtain HT/4 to elevate the heater,
my question is: why we have to do such a power divider to obtain ht/4 which is only a polarisation voltage.?
is there to drop down the ht voltage ? in this case is the divider act as a current drain?
If I use a 100K/33K to obtain ht/4 why it's not the same?
thank you very much
best regards
Tayeb
The reason is some tubes specify a maximum allowed resistance between heater and cathode. A lot of tubes do not include this information in their datasheets but the few that do (ECC88) typically mention a value in the region of 20K. Others give much higher values. For instance the ECC83 quotes 150K for phase splitters.

Cheers

Ian
 
The reason is some tubes specify a maximum allowed resistance between heater and cathode. A lot of tubes do not include this information in their datasheets but the few that do (ECC88) typically mention a value in the region of 20K. Others give much higher values. For instance the ECC83 quotes 150K for phase splitters.

Cheers

Ian
thank you very much Mr Bell
regards
Tayeb
 

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