6V6 line preamp

[merlin el mago]

Your welcome, is very very good. Do you use the gyrator?

 

[Salas]

More or less the same gain that I have with the 12B4 x6,5 that I'm just listening now with the bias you adviced B+ 160V 15mA CCS, sounds wonderful.

Since you have a good alternative for the weak NoDAC as output stage, I suggest you don't go modify the 6V6 to noNFB max gain before you install in it a good quality volume pot so you also test it as a line preamp for all your sources first. Has proper gain for line preamp use as it is with NFB resistors.

 

[Salas]

is very very good

@Nikos made a very good 6V6 preamp construction indeed.

 

[indaco]

Hi, I've just finished making the preamp with AZ1 as rectifier tube and final stage on the power supply as suggested by Elvee. The sound that comes out is wonderful but from the beginning I came across a perpetual hiss that grows when turned on and then tends to stabilize.
I tried various tricks but they didn't work: I didn't take measurements but it doesn't seem like ground loop noise or insufficient PSU filtering.
The noise seems to increase with the potentiometer (power amp) and with the shorted inputs it doesn't change, any suggestions about it? I thought it might be D.H. rectifier (used in hybrid mode with pair of diodes).

 

[6A3sUMMER]

Trouble shooting is one of two things:

1. Troubleshooting with a correct schematic in hand.
Please post your schematic, Some of us are not going to read through 4,604 posts in order to find what version of that preamp you are using.
Thanks!

2. A Blindfolded Archer shooting arrows and trying to hit the target.

 

[kodabmx]

My experience says most hiss like you describe is caused by oscillations. See if the hiss goes away if you short the input grid to ground? The other time I've encountered that type of noise, it was caused by heater cathode leakage.

 

[indaco]

Yes, sorry of course there are miriad of post here.... These are the schematics I followed:

 

[Salas]

See if and what kind of noise is in the B+ line before and after that series reg. A 20MHz bandwidth limited scope will help you do that.

 

[indaco]

I carry some measurements after the regulator, and at the output channel. I hope I took them right.
Particularly in the former there seems to be a periodic frequency alternation (as with wave beats?). I don't know how to interpret this though...

 

[Salas]

Looks like the regulator oscillates. Stable is a continuous featureless line as a goal. The channel output looks noisy for 50mV vertical. Random noise, that's the hiss.

 

[indaco]

Hi Salas, you are right. Beyond other hypothesis I had this suspect me too, and you confirmed.
In fact just replacing the regulator with a plain R-C filter (1k-220uF) the preamp is silent. What was wrong with the SSR suggested by Elvee? I followed his schematic, I changed only the Zd (I had that value) and the lower resistor network...

 

[Salas]

I don't know the inner workings of his circuit. I could try guess but better not since the author is a reachable member. You let him know and he probably has a solution.

 

[Salas]

In fact just replacing the regulator with a plain R-C filter (1k-220uF) the preamp is silent.

At least you got a simple working solution until sorting out the Elvee reg.

 

[indaco]

Yes, it's just as you say. Being not me the creator of that circuit I don't know actually what's wrong.
Waiting for Elvee's reply meantime, since I have Maida populated PCB set in a drawer could be fine to re-use it in the preamp?

 

[Salas]

Maida has been applied before in the 6V6 preamp with success. Mosfet Maida version it was. Try it since you have it, why not. Compare the sound and background noise to your silent enough simple RC filter solution too.

 

[indaco]

Thank you Salas, meantime I can manage like this.

 

[Salas]

By the way how much headroom you have for regs? Vin-Vout I mean. Also, got a picture of your build? Lit up AZ1 rectifier is nice to look at especially if mesh plate.

 

[indaco]

The ΔV between the regulator and the previous LC filter is about 70V (from 375 to 305-310).
Ok gladly, as soon as I post the picture although is not fully completed (it lacks the 3P4T rotary switch to be wired and assembled).

 

[Elvee]

Hi Salas, you are right. Beyond other hypothesis I had this suspect me too, and you confirmed.
In fact just replacing the regulator with a plain R-C filter (1k-220uF) the preamp is silent. What was wrong with the SSR suggested by Elvee? I followed his schematic, I changed only the Zd (I had that value) and the lower resistor network...

Can you show the modified schematic, and post a picture of the physical build?
This regulator has been tried by a number of members, and is normally not very demanding

 

[indaco]

Ok. Here's the attachments.
I changed both resistors in the R divider for a better matching in the sim, and the lower zener...let me know, thanks.