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6DJ8 phono pre build/questions...

Jcricket

Jcricket

not someday, but today
Subscriber
Hey folks,
My next project is going to build myself a 6DJ8 line stage phono preamp. I founs this schematic on line and I have a few questions.

First, whay are tyey using two tubes? Seeing that the 6DJ8 is a dual triode, why not just use 1/2 for each of the stages?

Second, I have heard that if you build the PS on a separate chassis, you will get less noise in your phone pre. True? Why? And how would you connect the amplifier stage to the PS?
 

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Power supply on a separate chassis keeps all the AC, transformer vibrations, etc. away for your gain stage. You should always have the last part of the supply (a cap usually) on the amp chassis to have the power close. You would use an umbilical cord with an appropriate plug to connect the two. Appropriate meaning enough connections, and the high voltage not exposed in a dangerous manner. I don't get your first question; two tubes, and two channels. The reason they don't run a channel per tube is likely for layout purposes. Don't worry about separation issues running both sides through one bottle. The tube sections keep all the electrons where they are supposed to be.
 
You answered my question. Two tubes on one channel - 1/2 of v1 and 1/ of v2. But, channel b us the 2nd half of v1 and v2.

I was looking at the schematic and wasn't sure if it was built like I stated above, or if the circuit needed four 6dj8's - two per channel.

Jay, what have you used for an umbilical/connector in your amps? Got any suggestions?
Mark
 
The use of 2 triodes per channel is to allow buffering the RIAA curve circuit away from the input and the output loads. And also for gain.
 
Figure out how many wires you will have and shoot me a PM.
 

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I was going to build this exact circuit a few years back so I looked at the schematic really carefully and spent quite a bit if time researching phono stage theory. I like the idea of grouping the RIAA compensation networks together as one unit between the tube stages like this design shows. I don't like using feedback to achieve the RIAA curve--just personal preference I guess.

A properly designed RIAA stage takes into account the impedance characteristics of the second tube. The second tube affects the component values in the RIAA network. As I recall, I went through all the calculations and it did appear this design takes that into account, although I don't think the design included a rumble filter--but I think most phono stages don't include them anymore since turntables have gotten so much better over the years.

There are improvements one could consider such as paralleling tubes in the first stage to lower the hiss or "tube rush" noise, or maybe using a white cathode follower in the second stage to give you really low output impedance so it could be used with longer interconnect cables, etc. (however, that would probably change the input impedance of the stage and therefore would change the values needed in the RIAA network).

The overall topology of this design is quite common I think, so I believe you will have good luck with it.

But as tube DIY goes, a good phono preamp might be one of the harder builds to get right. So I think you're wise to build an existing known circuit if this is your first attempt at a phono stage.

Good luck with it...I'll be curious to know how it turns out.
 
kward said:
There are improvements one could consider such as paralleling tubes in the first stage to lower the hiss or "tube rush" noise, ... (however, that would probably change the input impedance of the stage and therefore would change the values needed in the RIAA network).
Click to expand...

The 6DJ8 has a mu of 33, and S=12.5, and if I did the Rp calculation right, the plate resistance is 2.64K. Two of these tubes in parallel would make this Rp be half, And R4 becomes 15K, and the equivalent resistance of that change is 1.21K vs the old 2.64K. so to get the input to the RIAA circuit right, add 2.64K-1.21K = 1.42K between the plates and R5 of the RIAA circuit. Or make R5 84K. As for the grids, each gets its own grid stopper comming off of R1 (which stays the same value), and either make R3 half of what it was, or use separate R3s and bypass caps per cathode.
 
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I have Built that Pre, and have enjoyed it Ever Since...For mine I've added a 3rd tube for Gain, along with Pots on each Ch. for Balance...Also, it helped using a 6v 3A DC Brick for the 6DJ8's heaters...Good Luck with It. It Will be a Challenge...But, worth the Effort...
A Quick Tip...Use Wima MKP10 Caps for C1 thru C4...They might not be 1%, but they Sound better than the Daytons...
 
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sled108 said:
I have Built that Pre, and have enjoyed it Ever Since...For mine I've added a 3rd tube for Gain, along with Pots on each Ch. for Balance...Also, it helped using a 6v 3A DC Brick for the 6DJ8's heaters...Good Luck with It. It Will be a Challenge...But, worth the Effort...
Click to expand...

Good to hear, I've been looking at a couple different 6DJ8 phono pre designs and recently bought a half built bottlehead job. I've listened to two 6DJ8 phono pres and they were both fantastic to my ears. If I ever get me a good 6DJ8 phono going, I want to try one or two of the special quality low noise pentodes in a phono pre too.

Did you have any trouble finding quiet 6DJ8s?
 
Did you have any trouble finding quiet 6DJ8s?[/QUOTE]

Oh...That can be Fun...As long as you have a Box O' 6DJ8's and a pair of Gloves to Hot Swap them with...It would be Easier (But, $$$) to just get New, Quiet Ones...
 
Here's what it takes...The Ground Post is for the TT, it goes to Ground at the IEC...
attachment.php

Have Fun..Ha...Those Caps can be Larger than what's shown...
One other thing....With 6DJ8's, the Pin 9 should be to Ground too..
 

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sled108 said:
Here's what it takes...The Ground Post is for the TT, it goes to Ground at the IEC...
attachment.php

Have Fun..Ha...Those Caps can be Larger than what's shown...
One other thing....With 6DJ8's, the Pin 9 should be to Ground too..
Click to expand...

that is just too cool. How did you do that?
 
jaymanaa said:
Did you have any trouble finding quiet 6DJ8s?
Click to expand...

That, IMHO, is the rub, in using these.

It's hard to find one that's TOTALLY without "rush noise" at idle. Whether the noise is loud enough to matter to you, is something you'll have to listen for yourself.

IME, new-production ones just don't "stay" quiet. They may be quiet at the start, but many of them get noisy as they age.

This is why I generally have stayed with 12AX7/7025 tubes on phono stages. If they're quiet to start with, they stay that way. And, if you put a buffer stage on the output (say a 12BH7 or 6CG7 cathode follower), the high internal impedance of the 12AX7 is not an issue...

Regards,
Gordon.
 
GordonW said:
That, IMHO, is the rub, in using these.

It's hard to find one that's TOTALLY without "rush noise" at idle. Whether the noise is loud enough to matter to you, is something you'll have to listen for yourself.

IME, new-production ones just don't "stay" quiet. They may be quiet at the start, but many of them get noisy as they age.

This is why I generally have stayed with 12AX7/7025 tubes on phono stages. If they're quiet to start with, they stay that way. And, if you put a buffer stage on the output (say a 12BH7 or 6CG7 cathode follower), the high internal impedance of the 12AX7 is not an issue...

Regards,
Gordon.
Click to expand...

Hey Gordon,
Do you have a recommendation for a phono pre then?
 
That should work. It's not hard to find perfectly quiet 12AT7s.

And, I liked the way they sounded, when I subbed them into the Heathkit AA100 phono stage...

Regards,
Gordon.
 
It's not impossible to find quiet 6DJ8s or 6922s, and IMHO, they are worth searching out. Of course I have a couple hundred or so, so that may be part of my thinking process.
 
This is getting Interesting...How well would a 12AT7 work in the 6DJ8 Circuit on a 140v B+? By just changing the Heaters to pin 9, you can use Both...Hmmm..
And, you could do it for just the First Tube, or for Both at the Same Time...Why has it not been brought up about using 6.3v & 12.6v Tubes at the same time in a Pre??
attachment.php
 

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140v b+???
Both schematics I see are asking for 250v b+.

Are you refering to another pre circuit? If so, please share.
Mark
 
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