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hacknet 7th April 2004 11:21 AM

Detailed and Dry.
 
i have built a couple of preamps to date, all based on either the 6922 or the 5687(6900).

i find that i have only improved the clarity aspect of my preamps. one sounding more clean and defined than the previous one. all still lacking warmth in the mids. i find this is a little odd.

any thing i can take to increase the warmth factor?

SY 7th April 2004 11:39 AM

Usually, that translates to increasing second harmonic distortion. Dropping the value of the plate resistor will accomplish that for most triodes, as will removing cathode degeneration.

hacknet 7th April 2004 02:22 PM

i`ve found removing the cathode capacitor to help. is this in line with the norm...?

fscarpa58 7th April 2004 02:29 PM

from SY:
Quote:

removing cathode degeneration.
from Shacknet:
Quote:

removing the cathode capacitor
these are one the opposite of the other.

Where is the truth?

Federco

fdegrove 7th April 2004 03:18 PM

Hi,

Removing the cathode bypass cap will introduce a local feedback mechanism which would suppress a part of the even order harmonic distortion.

This would give a more clinical sound NOT a warmer one.

Cheers,;)

analog_sa 7th April 2004 04:14 PM

Warmth factor? You may get a lot of mileage out of the passive parts. Try Kiwame or Riken resistors and Jupiter/Audionote caps. For even more warmth (too much?) remove all elcos/films from the PS and bring in the oilers.

thoriated 7th April 2004 08:21 PM

I haven't tried the 5687, but I like the 6bk7b much more than the 6dj8 or 6922, which I think sound rather sterile by comparison. With a little rebiasing, you can usually get a 6bk7b to work where a 6dj8/6922 would. I wonder if the frame grid construction has something to do with this.

Has anybody tried the Svetlana 6N1P which is supposed to be similar but also not have frame grid construction?

EC8010 7th April 2004 10:42 PM

It all depends on what you mean by "warmth". If you decrease the value of the anode load resistor, you reduce the ratio of RL to ra, and 2nd harmonic distortion rises, perhaps sounding warmer. If you remove the cathode bypass capacitor, this increases ra, reducing the RL/ra ratio, but it also introduces local negative feedback, so the effect is not quite so predictable (it also significantly reduces rejection of power supply noise, increasing mud).

Frame grid valves such as the 6922 were designed for use at HF and can easily oscillate at UHF. HF oscillation can often produce a subjective "steely" effect. A 1k carbon film grid-stopper resistor soldered directly in series with the valve grid pin usually stops UHF oscillation, although cathode followers are more intransigent, and may require 10k.

thoriated 7th April 2004 11:23 PM

Hi, EC8010 -

Sounds like a good suggestion. Someone should try that & see if that corrects the sound of those frame grid tubes. I should mention in passing that the 6BK7B's (which were sort of a precursor of the 6DJ8 for RF cascode applications) I'm using in the phono preamp as SRPPs show no signs of RF misbehavior when looked at with a 500 Mhz scope.

Btw, I just ordered some 6N1P's and maybe I'll try 'em in my home brew phono preamp instead of the current 6BK7B's or perhaps my old Carver CD player where I currently have some Amperex 6DJ8 Bugle Boys (the best I've heard of that type). It turns out, though, that the transconductance spec of the 6N1P's is a bit on the lowish side at around 7000 uMho (7mS?) compared to the other two types.

fdegrove 7th April 2004 11:32 PM

Hi,

Quote:

Someone should try that & see if that corrects the sound of those frame grid tubes.
It's easy enough to check and it's well known to work too.

With careful choice of operating points and layout you'll hardly ever have to resort to gridstoppers but, as EC8010 suggests, some tubes can be rather hard to tame.

BTW, I don't think it has anything to do with the tube being of the grid frame type of construction.

Cheers,;)


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