Hi
I built the RCA 12AU7 cathode follower preamp, and I’m finding it to be a bit bright/Sibilant sounding. It sounds bright with both Mullard, PSvane and Electroharmonics tubes in it. And it’s bright with every amp I put it in front of (DIY store Sony VFet, Hiraga Super 30, Type 45 and a couple of others…)
Been reading about subbing a 12BH7 for the 12AU7’s, and from what I’ve been reading, they could help with the top end.
I’m using a Hammond 369JX transformer and an EZ81 rectifier for the power supply. The transformer has 2.5A available for the heaters, which I believe is enough for the 12BH7s and EZ81 (which add up to 2.2A)
Any thoughts on this swap, and if it will help with the bright top end would be greatly appreciated.
Tyanks
P
I built the RCA 12AU7 cathode follower preamp, and I’m finding it to be a bit bright/Sibilant sounding. It sounds bright with both Mullard, PSvane and Electroharmonics tubes in it. And it’s bright with every amp I put it in front of (DIY store Sony VFet, Hiraga Super 30, Type 45 and a couple of others…)
Been reading about subbing a 12BH7 for the 12AU7’s, and from what I’ve been reading, they could help with the top end.
I’m using a Hammond 369JX transformer and an EZ81 rectifier for the power supply. The transformer has 2.5A available for the heaters, which I believe is enough for the 12BH7s and EZ81 (which add up to 2.2A)
Any thoughts on this swap, and if it will help with the bright top end would be greatly appreciated.
Tyanks
P
Attachments
I can suggest a few things for you to try, without any guarantees. One by one.
1. Add grid stoppers to both grids, as close to the grids as possible. 300ohms to 1kohms,
2. Ground the first grid with 1Mohm to GND (this is mandatory, the potentiometer slider may lose contact),
3. Remove C3 for lower voltage gain,
4. Add a 47ohm resistor in line with C4 (from C4 to output).
5. The operating points can be changed with other resistor values. Let's say the first half of the 12AU7 works with a very low anode current, and the second half is not much better either.
1. Add grid stoppers to both grids, as close to the grids as possible. 300ohms to 1kohms,
2. Ground the first grid with 1Mohm to GND (this is mandatory, the potentiometer slider may lose contact),
3. Remove C3 for lower voltage gain,
4. Add a 47ohm resistor in line with C4 (from C4 to output).
5. The operating points can be changed with other resistor values. Let's say the first half of the 12AU7 works with a very low anode current, and the second half is not much better either.
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Thank you!
I will be placing an order for some parts in the near future, and will add these parts to my order - along with other parts for whatever other adjustments might be suggested.
I will be placing an order for some parts in the near future, and will add these parts to my order - along with other parts for whatever other adjustments might be suggested.
You can remove C1
The 220 kohm seems to be very high; please check the voltage on R3 to know the current in this stage
Also R3 is high
With 82 a good bias current can be 8-10 mA with 300 Vdc
Then use a lower value of pot
Walter
Any chance this is an impedance matching problem? That circuit was designed to drive the amps that they were using back then and you've connected it to several SS amps, yes?
If you look at this 12AU7 Cathode Follower design from CascadeAudio, you'll see that by moving the volume pot to the end of the signal path the output impedance can be lowered, resulting in a more suitable match for SS-based amps.
The word Preamp kinda makes me jittery because it means so many different things.
w
If you look at this 12AU7 Cathode Follower design from CascadeAudio, you'll see that by moving the volume pot to the end of the signal path the output impedance can be lowered, resulting in a more suitable match for SS-based amps.
The word Preamp kinda makes me jittery because it means so many different things.
w
The cathode follower should have a lower output impedance than the 10k potentiometer. And it is not variable. With a potentiometer, the highest output impedance for an AC signal is at half the value of the pot resistance and equals 25% of the total resistance (2k5 for 10k pot). When the slider is at GND the output impedance is 0.
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What tubes are you using? RCA cleartops by any chance? I've read they are very bright up top.
Years ago I bought a basket-case BottleHead Foreplay. After rebuilding it, I discovered that all 12AU7's sound different, and many just sound plain bad. So after a bit of tube rolling, I settled on a pair of Baldwin labeled black plate tubes that came with the Foreplay.
Also, the CCS that came with this preamp made a huge difference on the voltage amp section, compared to just resistor loading.
jeff
Thank you all for that! At work, so going to be a quick response.
Will check and post voltages when I get home. Have a scope, but it’s been sitting in the box for over a year - need to find time to figure out how to use it. Last time I used one was in electronics shop in highschool…
I chose this circuit just to try a simple build. That, and it supposed to have a low output impedance which should, play well with the SS amps I have. As I said above, it does the same with my Type 45 amp.
And I’m happy to implement a different circuit. The box is built and it should easily me changed up.
Any suggestions on what to possibly look at?
P
Will check and post voltages when I get home. Have a scope, but it’s been sitting in the box for over a year - need to find time to figure out how to use it. Last time I used one was in electronics shop in highschool…
I chose this circuit just to try a simple build. That, and it supposed to have a low output impedance which should, play well with the SS amps I have. As I said above, it does the same with my Type 45 amp.
And I’m happy to implement a different circuit. The box is built and it should easily me changed up.
Any suggestions on what to possibly look at?
P
I have Mullard tubes, rebranded as Rogers. I have some Psvane Tii tubes and some electro harmonics. All have the same issue on the top end.
All very different tubes , with the same issue.
All very different tubes , with the same issue.
Mullard(NOS) and PSvane 12AU7 are not particularly "bright" tubes. NOS RCA 12AU7 heavily carbonized plate (1940's) will sound even less bright than those tubes and very rich and dense.
Many 12BH7 will sound less bright than the average 12AU7, so to satisfy curiosity, various Tung-Sol, Raytheon 12BH7 can be tried. Various Sylvania 12BH7, while my personal favorites, will have sparklier, brighter top end than other 12BH7's..
check what I wrote
The we can discuss with a better working point
In case this works perfectly; it is with ECC99 similar to ECC82
Walter
Attachments
I’ll take a photo of the interior, get measurements as I don’t remember off the top of my head, and send a list of what caps I’ve tried, and what is currently in the amp when I get home.
Here is the inside of the preamp
I’ve tried Solen caps, Clarity caps, Russian Paper in Oil Teflon caps and I’m using Mundorf now, solver gold evo oil coupling and aluminum evo on the output.
And here are the voltages I’m seeing. Surprised at the 324v on the power supply. Thought I adjusted that down? And I don’t understand hey there is a discrepancy between the left and right channels. Something is off….
Top number is left, bottom is right.
I built the unit as per the schematic, with the exception of removing the 0.1uF on the input, and I put a 0.22uF on the coupling cap.
I’ve tried Solen caps, Clarity caps, Russian Paper in Oil Teflon caps and I’m using Mundorf now, solver gold evo oil coupling and aluminum evo on the output.
And here are the voltages I’m seeing. Surprised at the 324v on the power supply. Thought I adjusted that down? And I don’t understand hey there is a discrepancy between the left and right channels. Something is off….
Top number is left, bottom is right.
I built the unit as per the schematic, with the exception of removing the 0.1uF on the input, and I put a 0.22uF on the coupling cap.
Perceived sibilance or brightness may be as much an absence of sufficient midbass or bass as it is to trill at the high end. The DIY Sony V-Fet in particular may be a lost cause for being light on driver control and for having a trafo in the signal path, so I wouldn't make decisions on the sound of your preamp based on that amp alone. For example, I have an ancient Sony CD player that is narrow and trill at the best of times that is nevertheless cleaned up by a tube amp with sufficient bottom end. Few other amps in my quiver make that player sound good.
The VFet is indeed light on bass. But I’m getting the same issue with my Hiraga, which is far from light on bass, Aldo with my Adcom GFA545 and the Type 45. It’s consistent across all my speakers also. Lowthers, Fostex FE168Ez in double horns, Cornwalls, Seas Coax etc. My Tubes4HiFi SP14 does not have he same sibilant presentation. Nor does the super cheap Accuphase 3850 ChiFi clone preamp driving these amps.