12AT7 in Primaluna

[stoutblock]

I was trying to reduce the gain on my Primaluna Prologue 7 monoblocks. The Primaluna uses 12AX7 for inputs and 12AU7 for the buffers. I had tried some 5751 tubes in place of the 12AX7s but the reduction was minimal. So I picked up some 12AT7 tubes and dropped them in the 12AX7 slots. These are made by Amperex in France and are labeled Conn.

Although I really don't think there is a great loss in gain, I am absolutely amazed at how good my Soliloquy 6.5 speakers sound with these tubes! They seem to have velvety smooth yet still well defined mid-range response and are better balanced across the entire frequency range. The 6.5s might have been a little bright in my listening room before this change. I also notice the upper bass is quite present and seems tighter than before.

Any explanation for this?

Also, do I need to focus on lowering the gain from the 12AU7 slots rather than the 12AX7 slots? My line stage is beyond comfortable listening at 10 o'clock or so.

In any case, these 12AT7 tubes are staying (I assume they can cause no harm?).

 

[Elliot Thompson]

An ECC 83 is more sensitive so it will overload (distort) faster than an ECC 81. The sound difference will be based on the brand labelled on the valves you are comparing.

An ECC 81 falls between the ECC 83 and ECC 82 in terms of sensitivity.

Best Regards,

 

[stoutblock]

Realise that you are dealing with someone that couldn't read a circuit diagram if my life depended on it.

Does not an ECC81 also have less gain then an ECC83? And does this also mean it will send less signal to the power tubes resulting in less volume based on the same signal from the pre-amp?

What I want to understand better is what is going on that makes this 12AT7 sound better than any 12AX7 I have tried in these slots (I've tried most of the older classics). Is it simple synergy with my speakers/room? It can't be just the brand name?

I know the new Premaluna Prologue Premium amplifiers are using 12AU7 tubes in all four locations (I assume with revised circuits). They claim this improved the sound over the 12AU7/12AX7 mix in the older Prologue amplifiers like mine.

I always liked the sound of these amps with my Soliloquys, but using these 12AT7 tubes in place of the standard 12AX7 tubes is pretty dramatic. Anyone with this amp needs to try this.

 

[Primaluna76]

I have a Primaluna Prologue FOUR (tube power amp).

And I have (thanks to the post above) came up (after a lot of testing) with the following setup that works GREAT for me.

2x Philips 12AT7WC JAN - USA
2x Tungsram 12AX7 Hungary -NOS
4x TUNG-SOL 7581A (Russian end tubes)

It is really a budget setup but it doesn't sound like one! I can highly recommend it!

PS: I bought my tubes at www.tube-town.net

 

[Palustris]

Does not an ECC81 also have less gain then an ECC83? And does this also mean it will send less signal to the power tubes resulting in less volume based on the same signal from the pre-amp?

No this is not correct any more than putting larger diameter wheels/tires on your car will make it go faster. The 12AX7 has greater mu than the 12AT7, but it also has higher plate resistance and a different operating point e.g., plate voltage, plate resistance, and bias. Replacing a 12AX7 with a 12AT7 is safe in that they use the same base configuration so the tube will not erupt in a conflagration. However, the engineer who designed the circuit used ancillary components that define the operating point for a 12AX7, so while a 12AT7 might work, it will not be working at the correct operating points and will likely result in considerable 2H distortion. You might like the sound of 2H distortion, but it is not accurate hi-fi, it is a distortion generator.

I suggest you do some reading; the internet is full of technical information on tubes.

 

[Primaluna76]

No this is not correct any more than putting larger diameter wheels/tires on your car will make it go faster. The 12AX7 has greater mu than the 12AT7, but it also has higher plate resistance and a different operating point e.g., plate voltage, plate resistance, and bias. Replacing a 12AX7 with a 12AT7 is safe in that they use the same base configuration so the tube will not erupt in a conflagration. However, the engineer who designed the circuit used ancillary components that define the operating point for a 12AX7, so while a 12AT7 might work, it will not be working at the correct operating points and will likely result in considerable 2H distortion. You might like the sound of 2H distortion, but it is not accurate hi-fi, it is a distortion generator.

I suggest you do some reading; the internet is full of technical information on tubes.

I DON't like distortion and it doesn't! I am a sound engineer so i know how distortion sounds like!

But if you want some more info on the internet, look on this page a few post above this post...

It works perfectly and sounds better since les amplification on the input.

 

[enginedr]

I have VTL100 mono blocks that are also sensitive to gain . The best solution is to use a TVC or AVC volume control like the ones made by Dave Slagle .This solution would bring your system to another level

 

[Primaluna76]

I have a Primaluna Prologue FOUR (tube power amp).

And I have (thanks to the post above) came up (after a lot of testing) with the following setup that works GREAT for me.

2x Philips 12AT7WC JAN - USA
2x Tungsram 12AX7 Hungary -NOS
4x TUNG-SOL 7581A (Russian end tubes)

It is really a budget setup but it doesn't sound like one! I can highly recommend it!

PS: I bought my tubes at www.tube-town.net

Here you can find some more info regarding replacing 12AU7 to 12AT7: https://forum.audiogon.com/discussions/optimizing-the-prima-luna-premium-integrated-amp

But maybe this works with the Primaluna because of auto bias?

 

[mcgjohn]

I was trying to reduce the gain on my Primaluna Prologue 7 monoblocks. The Primaluna uses 12AX7 for inputs and 12AU7 for the buffers. I had tried some 5751 tubes in place of the 12AX7s but the reduction was minimal. So I picked up some 12AT7 tubes and dropped them in the 12AX7 slots. These are made by Amperex in France and are labeled Conn.

Although I really don't think there is a great loss in gain, I am absolutely amazed at how good my Soliloquy 6.5 speakers sound with these tubes! They seem to have velvety smooth yet still well defined mid-range response and are better balanced across the entire frequency range. The 6.5s might have been a little bright in my listening room before this change. I also notice the upper bass is quite present and seems tighter than before.

Any explanation for this?

Also, do I need to focus on lowering the gain from the 12AU7 slots rather than the 12AX7 slots? My line stage is beyond comfortable listening at 10 o'clock or so.

In any case, these 12AT7 tubes are staying (I assume they can cause no harm?).

I have the stereo version of the same PL amp. Have experimented quite a bit with input stage tubes.

You won't hurt anything subbing in the AT7, but you will drop the plate voltage quite a bit, which if memory serves is tied to the grid on the 12AU7 (next tube in line) which may pull it out of balance. again won't hurt anything, but may not give the best sound.

I have tried everything from an AT7, to an AV7 and an AY7. The guitar guys have driven up the price of the 12AY7s, but it sure is a nice tube.

Its been a few years since I did the test, but think the factory plate resistor is a 100K. you might want to sub a 33K resistor in its place then measure the plate voltage when you have a 12AT7, in the socket powered up. You want around 100VDC on the plate.

You can also cut the the cathode resistor a bit, which will make a slight change in the current draw of that first tube section.
Also keep in mind your amp uses no global neg fdbk. all the feedback is local in that first section, so as you remove a 12AX7 and install a 12AT7, you are dropping the gain, but at the same time reducing the local feedback. Lower feedback usually sounds more open, but not always.

The other significant improvement you can make to your amp, is changing out the coupling caps with Jensen Copper foil caps.

before you do anything you may want to try a set of 12AY7s in that 12AX7 socket and see what you think, but keep in mind the performance of that 12AY7 tube will get much better once the plate load resistor is correctly adjusted.

Best

 

[Primaluna76]

I have the stereo version of the same PL amp. Have experimented quite a bit with input stage tubes.

You won't hurt anything subbing in the AT7, but you will drop the plate voltage quite a bit, which if memory serves is tied to the grid on the 12AU7 (next tube in line) which may pull it out of balance. again won't hurt anything, but may not give the best sound.

I have tried everything from an AT7, to an AV7 and an AY7. The guitar guys have driven up the price of the 12AY7s, but it sure is a nice tube.

Its been a few years since I did the test, but think the factory plate resistor is a 100K. you might want to sub a 33K resistor in its place then measure the plate voltage when you have a 12AT7, in the socket powered up. You want around 100VDC on the plate.

You can also cut the the cathode resistor a bit, which will make a slight change in the current draw of that first tube section.
Also keep in mind your amp uses no global neg fdbk. all the feedback is local in that first section, so as you remove a 12AX7 and install a 12AT7, you are dropping the gain, but at the same time reducing the local feedback. Lower feedback usually sounds more open, but not always.

The other significant improvement you can make to your amp, is changing out the coupling caps with Jensen Copper foil caps.

before you do anything you may want to try a set of 12AY7s in that 12AX7 socket and see what you think, but keep in mind the performance of that 12AY7 tube will get much better once the plate load resistor is correctly adjusted.

Best

Thank you for your insight and additional information!

I'll save this feedback since it could help me do some upgrades. And I'll try the 12AY7, since you are a big fan of it. Replacing the 100K resistor is no problem, but I am really happy as it is right now. The 2x Philips 12AT7WC JAN - USA & 2x Tungsram 12AX7 Hungary -NOS work really well together.

PS: You are right, the sound space did gain "airiness" and sounds more open with the Lower feedback, using the 12AT7WC witch I love. (-;

With kind regards.

 

[stoutblock]

I have the stereo version of the same PL amp. Have experimented quite a bit with input stage tubes.

You won't hurt anything subbing in the AT7, but you will drop the plate voltage quite a bit, which if memory serves is tied to the grid on the 12AU7 (next tube in line) which may pull it out of balance. again won't hurt anything, but may not give the best sound.

I have tried everything from an AT7, to an AV7 and an AY7. The guitar guys have driven up the price of the 12AY7s, but it sure is a nice tube.

Its been a few years since I did the test, but think the factory plate resistor is a 100K. you might want to sub a 33K resistor in its place then measure the plate voltage when you have a 12AT7, in the socket powered up. You want around 100VDC on the plate.

You can also cut the the cathode resistor a bit, which will make a slight change in the current draw of that first tube section.
Also keep in mind your amp uses no global neg fdbk. all the feedback is local in that first section, so as you remove a 12AX7 and install a 12AT7, you are dropping the gain, but at the same time reducing the local feedback. Lower feedback usually sounds more open, but not always.

The other significant improvement you can make to your amp, is changing out the coupling caps with Jensen Copper foil caps.

before you do anything you may want to try a set of 12AY7s in that 12AX7 socket and see what you think, but keep in mind the performance of that 12AY7 tube will get much better once the plate load resistor is correctly adjusted.

Best

Man, this is an old thread! I sold that amp years ago. In the end I found the big 6.5 Soliloquys sounded best with a more powerful SS amp and tube preamplifier.

 

[mcgjohn]

Thank you for your insight and additional information!

I'll save this feedback since it could help me do some upgrades. And I'll try the 12AY7, since you are a big fan of it. Replacing the 100K resistor is no problem, but I am really happy as it is right now. The 2x Philips 12AT7WC JAN - USA & 2x Tungsram 12AX7 Hungary -NOS work really well together.

PS: You are right, the sound space did gain "airiness" and sounds more open with the Lower feedback, using the 12AT7WC witch I love. (-;

With kind regards.

NP.

If you want to check the voltages, you should have around 95-100VDC on pin 1 and pin 6 of that 12AX7 socket. When you insert a 12AT7, you are likely to see that plate voltage drop to 50VDC or so. Follow the path from pin 1 and you'll see it goes to the grid on the 12AU7, pin 2. This should also be at 95-100VDC, but with AT7, it will drop. The plate voltage is set by R3, a 150K resistor. With a 12AT7, or AV7, you can cut that value roughly by a third to get the plate voltage back into spec with an AT7. With the proper plate load resistor the amp will sound much better than just inserting a 12AT7 into the socket. But keep in mind, if you deicde to switch back to the 12AX7, you'll need to return the plate load resistor back to 150K.

The other neat trick you can do that will clean up the sonics on the amp, is get rid of that 62.5K power resistor feeding the plates on the two 12AX7s, and replace with a Mills Wirewound, may have put two MILLS in series to achieve the same resistance value. Just be sure to discharge that 22uf 450V cap before sticking your soldering iron in there. There is also a zener diode in that power supply circuit to help maintain the voltage.

in the attached image, see the MILLS that is located under the 22uf 450v cap.

 

[PooPoo]

I DON't like distortion and it doesn't! I am a sound engineer so i know how distortion sounds like! How the hell is he gonna tell you your shit sucks and your listening to distortion? I’m sure your ears ain’t playing possum with you eh?

But if you want some more info on the internet, look on this page a few post above this post...

It works perfectly and sounds better since les amplification on the input.