6DJ8/ECC88-SRPP Tube Preamplifier

[DF96]

SORRY: just read your post properly. You include Ra; I thought you had missed it. I will leave my post here as a tutorial.

If you calculate noise current for a single component then this is understood to be noise current into a short circuit. Similarly, if you calculate noise voltage this is open circuit. In either case you then may have to add whatever load the circuit imposes at that point and that will reduce the noise from the component being considered (while also adding its own noise too).

You can't simply multiply the noise current by the external impedance to get noise voltage as this ignores the internal impedance of the noise source. A 12k resistor has an internal impedance of 12k. You need to treat it as a current noise source (infinite impedance) in parallel with 12k, or as a voltage noise source (zero impedance) in series with 12k. Then do normal circuit theory calculations to get the final noise voltage or current. A valve has an internal impedance too - often the anode impedance.

 

[merlin el mago]

ECC81 exactly as the pic:

An externally hosted image should be here but it was not working when we last tested it.

Could you draw the heater elevated schematic.

Thanks in advance for kind help.

Felipe

Will be better if someone can explain my how calculate the heater elevated.

 

[merlin el mago]

I mind that I know is to calculate a voltage divider to get the needed V from HT

An externally hosted image should be here but it was not working when we last tested it.

Ian example B+ 250V R1 75K R2 33K = 76.389V

 

[merlin el mago]

It's necessary or benefitial add an artifical center tap for heaters? If yes where have I to connect the artifical center tap for heaters, between the tx & the heater regulator?

 

[merlin el mago]

Please help?

 

[ruffrecords]

I mind that I know is to calculate a voltage divider to get the needed V from HT

An externally hosted image should be here but it was not working when we last tested it.

Ian example B+ 250V R1 75K R2 33K = 76.389V

Yes, this is exactly what I meant. You can use an artificial centre tap if you wish but it is not necessary for low gains. For higher gains I would suggest using a dc heater supply.

Cheers

Ian

 

[merlin el mago]

+HT connected in PSU before the HV regulator or after the HV regulator?

 

[merlin el mago]

To HT+ or to HT gnd?

 

[ruffrecords]

To HT+ or to HT gnd?

The HT+ is connected to the same HT supply that feeds the top tube anode. All connections should be after regulators. Attached is a schematic of another project that uses a mu follower with an elevated supply.

 

[merlin el mago]

Thanks for help Ian, so if I use B+ 280V

R1 75K
R2 27K
C1 47uF

I elevate heaters 75V, right?

OK I connect always after regs.

I suppose always connected in the positive rail, right?

Resistors 1/4, 1/2, 1W or more?

Capacitor voltage rated?

 

[ruffrecords]

Thanks for help Ian, so if I use B+ 280V

R1 75K
R2 27K
C1 47uF

I elevate heaters 75V, right?

OK I connect always after regs.

I suppose always connected in the positive rail, right?

The top of the pot divider (R1) goes to the HT+ and the bottom of the pot divider (R2) goes to 0V.

Resistors 1/4, 1/2, 1W or more?

Capacitor voltage rated?

Total dissipation in those two resistors is about 0.77 watts so you should use 1W types.

Since there is about 75V across the capacitor it should be rated at 100V or more.

Cheers

Ian

 

[merlin el mago]

Thank you Ian, sorry I misread the last schematic about cap voltage value, looking the last schematic is very easy to see how can be wired the elevated heaters (one pic best than 1000 words).

Cheers,
Felipe

 

[revintage]

Hey,

Back to square one! You say you are building an preamp for a friend with a X100. This has a sensitivity of something like 2,5V with Zin of just above 10kohm when running it unbalanced.

So what do you need 25-30dB gain for?

 

[merlin el mago]

Thanks Lars, wich gain & voltage RMS swing do you recommend? my friend told me that using X100 the bass is lost....

 

[revintage]

Hey merlin,
Depends on the sources. What output levels do they have?

 

[merlin el mago]

Hi Lars,
At this moment only a DAC, I suppose 2Vpp
Thks
Felipe

 

[revintage]

Hey,
A "normal" DAC or CD has 2rms out. But to get an ample reserve 10dB(ca 3x) should do. So why not try to find a really low mu, low Rp, linear tube and maybe add some local feedback. 12B4, 6H30?

 

[merlin el mago]

Hello,
I really don't know where is the problem with the power amp because make a 3rd person but it's really strange that have problems with his Erno Borbely preamp with 20dBs, I have the same Erno Borbely preamp but balanced so I have two unbalanced preamps inside two chasis (one for PSU + one for preamp and regulators).
The reason to use ECC88, ECC82 & ECC81 is that I have on hand in my stash so I don't have to buy.
Kind rgds
Felipe

 

[revintage]

Fix his power or preamp first, something must be broken. If not, the user don´t know what he is doing!

You can use the 88 like a simple anodefollower with about 15dB feedback go for a sand CCS on top. Dump the 81 and 82.

 

[merlin el mago]

You are right, first of all I need to see the power & preamp, but meanwhile I will try a sweet tube preamp for him without buy nothing only with components that I have on hand. Could you post the schematic for 88 simple anode follower?
Have a nice day,
Felipe