Radiotron SE 2A3 Input 6C6 Question

[WntrMute2]

I have built this amp with the changes shown in Red. It sounds very good driven with a Cary 9SLP-98 into a pair of Heathkit AS-21. I do have a couple of questions hopefully someone here can answer.


1) At the input RCA jack i have substituted a 220K ohm resistor for the 100K ohm pot to form a grid leak circuit.

I was hoping to reduce the sensitivity of the amplifier as I can barely move the volume control off idle on the pre-amp. Would changing back to the pot at 100K (or some other value) help in this regard? Adjustment in level would be nice as my left ear is down a few decibels. I'm concerned that running the input signal current through a pot or resistor may degrade the SQ. Is this something to be concerned about?
Suggestions as to how to add adjustable attenuation would be appreciated.

2) The output when measured with a RTA and pink noise shows a significant decrease above 10KHz. I've tried another pair of drivers that are supposed to play a little higher but the measurements are very similar. Any thoughts on changing cap values or something else to boost the top-end a bit?


This is my first foray into building off of a schematic so I appreciate and and all help and information.
Thanks

 

[BinaryMike]

The simplest way to reduce gain would be to disconnect the cathode bypass cap in the first stage. Pentodes can have a lot more gain than desired for this application --- most of us would likely be using a triode here.

 

[WntrMute2]

I understand the triode suggestion but I want to keep the basic design unmolested.
I appreciate the help though.

 

[Palustris]

I was hoping to reduce the sensitivity of the amplifier as I can barely move the volume control off idle on the pre-amp.

This is simply another case of too much gain in the signal chain. There is no need for a preamp when the amplifier can be driven to full output by the source component. Follow the schematic, eliminate the preamp, and you will reduce gain, noise, and distortion.

I am reminded what my Dr. advised me when I told him it was painful to get down on my knees, "Don't do that."

 

[WntrMute2]

If I eliminate the preamp I will lose my ability to choose sources as well as my ability to record, mute, select mono etc. Is there not an attenuation circuit that doesn't force the signal through a potentiometer?

 

[maxhifi]

If I eliminate the preamp I will lose my ability to choose sources as well as my ability to record, mute, select mono etc. Is there not an attenuation circuit that doesn't force the signal through a potentiometer?

I responded to this topic already over at diyaudio, on the same topic. I drew out a voltage divider attentuator for you, and explained how to calculate the resistors. There's no magic to this, just gotta go ahead and do it now. You can also build attentuator into the patch cords you use, to avoid modifying anything as an alternative.

 

[FlaCharlie]

I understand the triode suggestion but I want to keep the basic design unmolested.
I appreciate the help though.

I've never used a 6C6 but . . . maybe try it wired in triode? The data sheet has specs for it wired in triode. https://tubedata.altanatubes.com.br/sheets/127/6/6C6.pdf

I realize everyone sees things differently but in my mind that wouldn't be molesting the design any more than switching from a 2A3 to a 300B.

 

[WntrMute2]

I responded to this topic already over at diyaudio, on the same topic. I drew out a voltage divider attentuator for you, and explained how to calculate the resistors. There's no magic to this, just gotta go ahead and do it now. You can also build attentuator into the patch cords you use, to avoid modifying anything as an alternative.

Thanks for your help. I was trying to find out if there was an attenuation circuit that didn't rely on passing the input signal through a resistor or pot as well as leaving the grid leak resistor as is. D96 on DIY seemed to suggest there was one but cryptically. I will, more than likely, implement the voltage divider you suggested.

 

[WntrMute2]

I've never used a 6C6 but . . . maybe try it wired in triode? The data sheet has specs for it wired in triode. https://tubedata.altanatubes.com.br/sheets/127/6/6C6.pdf

I realize everyone sees things differently but in my mind that wouldn't be molesting the design any more than switching from a 2A3 to a 300B.

I see your point but I feel a pentode driving a DHT remains in character with the design. A triode driving the DHT is different in my mind.