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Old 5th October 2012, 10:30 PM   #2071
jamesdb is offline jamesdb  United States
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Can anyone say what the minimum input DCV is for Rod's filament boards for the 26? I have seen 12V and 6V so assume 6V is acceptable.
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Old 5th October 2012, 10:34 PM   #2072
Magz is offline Magz  United States
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Around 6 is minimum, 6.7 is nominal, 8.2 is listed as max, but I think that depends on heat sinking. I'm running 7.2V.
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Old 6th October 2012, 01:25 AM   #2073
tcqanh is offline tcqanh  Viet Nam
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tube 26 needs an OPT with high impedance of primary, at least 15K. A good quality pair is expensive. If I use 25K-600 ohms OPT, then I have no gain of ouput. Maybe I can add one stage, one tube or step up transformer to get suitable Vout. But the sound now is not only of 26.

How is to get the best sound of 26 without OPT ?

Does anyone build tube 26 input (plate choke maybe) Direct coupling to 27/ 56/ 76/ 6J5 in cathode follower ? I guess the sound is not bad

Last edited by tcqanh; 6th October 2012 at 01:41 AM.
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Old 6th October 2012, 03:03 AM   #2074
Magz is offline Magz  United States
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Quote:
Originally Posted by tcqanh View Post
tube 26 needs an OPT with high impedance of primary, at least 15K. A good quality pair is expensive. If I use 25K-600 ohms OPT, then I have no gain of ouput. Maybe I can add one stage, one tube or step up transformer to get suitable Vout. But the sound now is not only of 26.

How is to get the best sound of 26 without OPT ?

Does anyone build tube 26 input (plate choke maybe) Direct coupling to 27/ 56/ 76/ 6J5 in cathode follower ? I guess the sound is not bad
You could do what I did and load with a CCS then take the mu output. Lower Rp means less stepdown needed. I'm going directly into an autoformer volume control, so only stepdown is in the VC.
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Old 6th October 2012, 06:38 AM   #2075
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Hi!

Quote:
Originally Posted by tcqanh View Post
If I use 25K-600 ohms OPT, then I have no gain of ouput.
Normally not much gain is needed from a linestage. Most linelevel sources have an output level of up to 2V RMS. Typical input sensitivites of power amps is 0.5 - 0,75v RMS for full power output. So most of the time the level will even be reduced in the linestage.

For me the task of a linestage is:

- input selection
- volume control
- drive power amps through potentially long cable

The lienstage should have a high input impedance so it is an easy load for the sources and low output impedance to drive any power amp also through lobger cables.

If you need gain in your system I would rather add it in the sources or power amps

Best regards

Thomas
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Old 6th October 2012, 08:30 AM   #2076
disco is offline disco  Netherlands
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Is that 2.0 volt rms or pp for output voltages? Measurements I took several years ago (with a good dmm and eighties CDs recorded at -20dB level) gave no more than 0,5Vrms on peaks. So I was under the impression that it was 2.0/2.83= 0.7Vrms maximum.

Searching the web results in most curious answers to this simple question (who has a copy of the Red Book? ), among others this wiki on output levels: The impedance is around 100 Ω, the voltage can reach 2 volts peak-to-peak with levels referenced to -10 dBV (300 mV) at 10 kΩ,
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Last edited by disco; 6th October 2012 at 08:34 AM.
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Old 6th October 2012, 08:54 AM   #2077
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Quote:
Originally Posted by tcqanh View Post
tube 26 needs an OPT with high impedance of primary, at least 15K. A good quality pair is expensive. If I use 25K-600 ohms OPT, then I have no gain of ouput. Maybe I can add one stage, one tube or step up transformer to get suitable Vout. But the sound now is not only of 26.

How is to get the best sound of 26 without OPT ?

Does anyone build tube 26 input (plate choke maybe) Direct coupling to 27/ 56/ 76/ 6J5 in cathode follower ? I guess the sound is not bad
In my case the "preamp" is actually part of the amplification chain, and I can't do without the gain. I have the preamp next to the amp and short cables. A more precise way to look at inductance isn't "15K" but the actual value in Henries if it's given. For the 26 I'd be wanting over 100H, and actually well over 100H like 200H or more. The sound really cleans up with the added inductance. It's also a question of the primary and secondary ratio, but when it's 1:1 I'm after around 200H.

I don't use the 26 anymore, but when I did I used two Hammond 156C chokes in series, bolted top to bottom and wired out of phase - all the details are in the thread somewhere. That was a good sound and cheap to achieve. This gives you all the gain of the 26 with a Russian teflon cap in my case. The issue with this setup is hum - the Hammonds are prone to hum pickup so you need to place magnetic components carefully, and well away from the chokes.

This whole question of inductance comes up with the 4P1L preamp I use (it's a lot easier to work with since the plate impedence is much lower at around 2K). Ale is adamant that the 4P1L sounds better at over 20mA, and I'd agree that you get big gains up to 15mA and a little bit more body and bass after that. But here you are in another trade-off with inductance. As the current goes up the inductance comes down, and I like the sound of the higher inductance options so I'm currently at 18mA with the LL1660. It's all swings and roundabouts.

I wouldn't add a stage if you need all the gain - I'd be looking for a 1:1 solution and if necessary put the preamp right next to the amp with short cables. If you don't need the gain Thomas' solution is standard practice. I have a 3 stage setup in total (4P1L>4P1L>300b) and you lose transparency when you add a stage. You can juggle around with transformers and higher mu tubes, and also solid state. We've had a few different solutions on the thread.

Andy

Last edited by andyjevans; 6th October 2012 at 09:03 AM.
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Old 6th October 2012, 09:01 AM   #2078
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@ Thomas

can agree here that's a good point. What gain do you use in your phono stages? I think of increasing it to ~ 60dB.

Advantage of a 2nd stage will be to lower the output impedance, easier way to drive long distance cables. Old German studio gear - Klangfilm etc. - used to have output impedances of 60 .

BR, Joao
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Old 6th October 2012, 10:26 AM   #2079
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Hi!

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Originally Posted by disco View Post
Is that 2.0 volt rms or pp for output voltages?
There is no standardized line level output voltage for consumer electronics.
With the introduction of the CD in the eighties a 'quasi' standard got widely accepted which was 2V RMS. This is the maximum output voltage (at 0dB) most CD players still have.

This is rather high and if you have a linestage with 20dB gain, which is not uncommon, you end up with to much gain and the situation that the usable range on your volume control is very small, maybe up to the 9 o' Clock setting.

But this is not a mandatory standard it was adopted by many. There are exceptions. I have seen digital gear with even higher output level. less is possible too of course.

This is why it is important to understand the gain needs of the system when choosing the right line stage.

Best regards

Thomas
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Old 6th October 2012, 12:39 PM   #2080
tcqanh is offline tcqanh  Viet Nam
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Quote:
Originally Posted by Vinylsavor View Post

If you need gain in your system I would rather add it in the sources or power amps
Hi Thomas,

I want my preampli have 4-6V out, so I can build many tube ampli with only one amplified stage.

many tubes need gain of 50-70V (100-140Vpp) and more, eg. VT52, 300B, also 845, 211, 811A... so most of these ampli have to get 2 stage of amplification.

When you have plan of C3g triode strapped driver for 300B with IT 5k:5k, maybe you need more 2V input to get full fower. If I use C3g doing driver in power, so I don't need pot Volume at the input. Consequently, Miller effect maybe not present.

Last edited by tcqanh; 6th October 2012 at 12:42 PM.
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