LTP tail voltages in 5F6A build

[tubeswell]

Okay now most of the 'issues' with this build are rounded up and dealt with (yay) - thanks to all involved

However the LTP voltages are still intriguing me a little bit

Here is the updated schematic with voltages I took this afternoon. You can see the tail is at 41.2V and the cathode is at 42.6V with teh plates at 246 V (82k) and 232V (100k) and the HT at that point is 371VDC. As you can see the plates are about the same as the schematic values, yet the tail and cathode are about 9 volts higher respectively. The resistor values all check out on my DMM (in fact i replaced all the blinkin' resistors one at a time just to make sure).

So for some reason the stage is not biased as hot as it could be. The amp sounds okay and I wonder if these higher tail voltages are really an issue? (or just in my head). But if I wanted to lower the tail voltages there'd be no harm in using 8k2 with 470R would there? (or am I better to use 8k2 and 560R)?

[txstrat]

Hi TW,

I don't think the higher voltages are an issue.
The values of the cathode and tail resistors are set in a special ratio (might be a run of try and error back in those days when they tried to keep the signal as clean a possible). If I'd go and try different values I'd also try to (sort of) keep that ratio to get both voltages down.
Although if you like the sound as it is now, you might try different bias settings before changing the resistors (I'd leave the intensity of the trem out of the calculations for now).
Your schematic shows 30ma and that would be less than 50% according to Ted Webers bias calculator. (I think this might make the amp sound cold and harsh). You could go up to 45ma and even be short of 70%.
I don't think the tubes will be harmed when you set them to 70% and the trem gets them up to, say, 80%, while going up and down).

HTH
p.s. any soundfiles yet?

[Chuck H]

What is the measured value of your presence pot???

Chuck

[tubeswell]

Originally Posted by Chuck H

What is the measured value of your presence pot???

Chuck

just under 5k

[MWJB]

Your PI tube dissipation is as close as dammit, 3mA vs 3.2mA on the 5F6A schem, not an issue. Variances in tubes could account for more than that. Have you reconnected the NFB loop yet, voltage may be being bled off via the NFB loop? Later 5F6A bassmans used a 6.8K tail resistor (will push up gain slightly), all used 470ohms for the PI cathode.

30mA*438v = 13W, which is fine for this kind of amp, unlikely to make the amp cold & harsh, you'd have to go somewhat colder for that to happen. Higher idle currents may have a detrimental effect on the intensity of the trem effect.

[tubeswell]

Hi MWJB - yes those voltages are with the NFB reconnected. Cheers