#26 pre amp

[coolzero]

Thanks Kevin. What ever the design you have suggested initially, I still use in my pre because it works perfectly except this marginal bass response issue. I have attached the schematic here. Bass issue could be due to my SS amp and its low input impedance. When I auditioned my pre to others who are dealing with high-end systems in daily basis, they were pretty impressed however everyone noticed this lean bass response. Because it is almost near to completion of my pre, I thought of fixing this small issue as well. Below is the detail of my system (amp/speakers) to understand it full. (BTW, my speakers usually have pretty decent bass response when I played the same material without 26 pre.) I use power amp section directly to input the 26 pre.

[WayneK]

175 volts on the plate and -9v on the grid would give very high current for a 26. This may cause the lack of bass you describe?

[kevinkr]

Originally Posted by WayneK

175 volts on the plate and -9v on the grid would give very high current for a 26. This may cause the lack of bass you describe?

I agree, I recommend somewhat lower plate voltage, (+150V) easily accomplished just by adding some resistance in series with the choke. Do not bypass this resistor, I think 3.3K - 3.6K would be optimum - this would also reduce the Q of the choke significantly, and may also slightly increase the effective RL at very low frequencies.

Wondering how the inductance of the choke holds up at high currents. Also if the input impedance of the amplifier is significantly below 50K ohms I would recommend increasing the coupling capacitance to 2.2uF, and below 25K to 4.7uF-5.1uF.. Loads below 20K are not recommended with choke loading - in fact 20K is a difficult load for a 26.. The current -3dB point with this power amplifier is about 6Hz, coupling cap is responsible for about -1dB at 20Hz, and the choke may be responsible for at least another dB or so..

Note that you might also want to try the 112A/12A (note the RCA-12 is a different tube!) which has an rp of around 5.1K with -9V bias and 140Vp and a filament voltage of 5V/250mA.. It would drive the amp better due its lower rp..

[coolzero]

Sorry Kevin, WayneK. I have done this schematic some time back and the voltage mentioned is somewhat different from what I use now. 150V/-10V is the current parameters in place. Hope this is not a problem.

Apart from that, will there be any impact of increasing 220K resistor to higher value like 1M ? (Infact I didn't realy grab the concept out of the readings but keen to know more if you guys could explain).

Kevin, becuase my power amp input is 20K, would it help if I replace the chokes with a similar P2L Trafo of your design explained (UTC HA-133) in Positive Feedback? Or is it really necessory to look for a different tube like 112A in this situation? (I don't want to remove 26 from my setup ;*( )

Thank you guys for the knowldge share as always.

[nicoch58]

Andy any news on grid vs filament bias 26/10Y ?


or

[coolzero]

Kevin, your suggestion of increasing the coupling cap to 4.7u and adding 3.3k resistor in series with the plate choke was spot on. I did those changes and the result is increased bass and overall clarity significantly. you are the man.. thanks again.

[kevinkr]

Originally Posted by coolzero

Kevin, your suggestion of increasing the coupling cap to 4.7u and adding 3.3k resistor in series with the plate choke was spot on. I did those changes and the result is increased bass and overall clarity significantly. you are the man.. thanks again.

Hi Kanishka,
This is good news, glad I was able to help..

[andyjevans]

Just to go back to nicoch46, news on the evolution of my 10Y filament bias preamp is that I've joined the "iron brigade". Chokes, chokes and more chokes. Better people than me worked that out a while ago and I didn't follow this path until I laid my hands on a few 280mH chokes from a local surplus outlet. I was shocked how much better choke input sounded. So out went the voltage regs in the filament supply. Thomas Mayer uses L>C>L and he's right. When I replaced the last choke with a voltage reg the sound took another nose dive. the only thing that preserved all the clarity and tone was, as stated, L>C>L. Chokes are 280mH but Hammond do a 60mH one at 2A which is probably fine. I used 10,000uF reservoir cap. This is a heavy solution, but it does avoid heatsinks and regs. And the Hammond parts are available and not expensive. I'll see if I can post a schematic. Frankly, I think this preamp is stunning.

andy

[coolzero]

Originally Posted by kevinkr

Hi Kanishka,
This is good news, glad I was able to help..

Thanks Kevin for your help as always. One more thing BTW; should I use a larger bleeder resistor for the output coupling cap like 1M instead of mentioned 220K? Will it make the pre little more quarter? I felt it is tad harsh with some records specially with rock.

[coolzero]

Originally Posted by andyjevans

Just to go back to nicoch46, news on the evolution of my 10Y filament bias preamp is that I've joined the "iron brigade". Chokes, chokes and more chokes. Better people than me worked that out a while ago and I didn't follow this path until I laid my hands on a few 280mH chokes from a local surplus outlet. I was shocked how much better choke input sounded. So out went the voltage regs in the filament supply. Thomas Mayer uses L>C>L and he's right. When I replaced the last choke with a voltage reg the sound took another nose dive. the only thing that preserved all the clarity and tone was, as stated, L>C>L. Chokes are 280mH but Hammond do a 60mH one at 2A which is probably fine. I used 10,000uF reservoir cap. This is a heavy solution, but it does avoid heatsinks and regs. And the Hammond parts are available and not expensive. I'll see if I can post a schematic. Frankly, I think this preamp is stunning.

andy

Hey Andy, This seems like a good experiment. Please post the schematic. I'd like to try this out too.