Shared/Split cathode question ?

[JB007]

Its been a while since i've been on here so here it goes with a dumb question. I like the split cathode of a SLP but i'm curious as to what would happen if i put a 1k/.68 Shared cathode in my JTM45 RI ? Would it be simular to the superlead or would it sound awefull because it's shared cathode and when doubled up it would mush out ? I never use the normal channel of a SLP and the JTM45 is to dull/dark for what i'm looking for. But one thing i like about the JTM45 is the recto tube feel you get over solid state when you open the amp up.

[neikeel]

Only one way to find out!

Your RI board has a space for the necessary cap and resistor, you will need a short piec of yellow wire and to snip the jumper on the valve base.

Will be a hybrid between an 1987 and JTM45

[PCollen]

Its been a while since i've been on here so here it goes with a dumb question. I like the split cathode of a SLP but i'm curious as to what would happen if i put a 1k/.68 Shared cathode in my JTM45 RI ? Would it be simular to the superlead or would it sound awefull because it's shared cathode and when doubled up it would mush out ? I never use the normal channel of a SLP and the JTM45 is to dull/dark for what i'm looking for. But one thing i like about the JTM45 is the recto tube feel you get over solid state when you open the amp up.

I used 1.5K/22uF and 2.7K/0.68uF when I split the cathodes of my JTM45. I also removed the bright cap off the volume control AND the bright channel mixer bypass cap which is the source of the NORM channel
dullness in these amps. That mixer bypass cap shunts NORM channel highs to ground .

1K/0.68uF combo will really make the channels brighter..the -3dB point will be about 380Hz meaning the roll-off starts an octave higher at 760Hz. 820/10uF is a popular set-up .

[JB007]

Thanks so much Pcollen! I did the 1k/10uf change on the ViA/B and it rocks.. No more loose farty bass and the amp seems to be alot smoother too. I can't believe the difference a simple mod like that can make. Thanks a million,it really makes the JTM45 bass knob usable again.

[AJW]

I've read that the shared cathode resistor value is in effect doubled (e.g. 820 measures as 1640).

Does this have any effect on a shared bypass capacitor value? That's something anyone needs to know to work out the roll off frequency of the shared cathode bypass Rk, and something I'm confused about.

[JamesHealey]

thats correct it's doubled in value so 1k would be 2k, in effect you want really to have a 1.35k resistor with a 0.68uF to achieve that Super Lead split cathode sound on the bass and bright channels.

but surely splitting the cathode with two 2.7k and two 0.68uF would be easier?

[PCollen]

I've read that the shared cathode resistor value is in effect doubled (e.g. 820 measures as 1640).

Does this have any effect on a shared bypass capacitor value? That's something anyone needs to know to work out the roll off frequency of the shared cathode bypass Rk, and something I'm confused about.

Since the current through the shared V1 Rk is (theoretically) 2x of that through each individual V1 segment , a shared 820 Rk gives the same Vk (bias) as would a separate 1620 Rk on each V1 segment..a 1500 seperate Rk is close and a standard value.

If Rk = 820, 1500, or 2700, using 330uF or 250uF isn't going to make an audible difference ..the -3dB rolloff is going to be less than 1 Hz . You need to play with Ck values around 10-22uF +/- 10uF to hear an audible difference and then yes you need to halve the cap value (ex: 22u > 10u) if you double the resistance (ex: 820 > 1500) to keep the same approx -3dB roll off , and v.v.