c3g pentode driven 300b set

[Mike H]

question 1, output tx primary is 5k, question 2, dcr of the chokes is 77 ohms, question 3 the winding is rated at 250ma but i dont know what the regulation is on it. its an ae mains tx, i dont know if theres a standard value they use
ta ant

No that'll do, ta

10% is a typical value

[Mike H]

Supposed to be tied up with something else today

Bondage ?

Not that interesting I'm afraid! If only ..

[Mike H]

i thought about that when i started this one following mi old mans experiments with regulated supplies on his pentode amps.

i have considered regulating the g2 supply but again it complicates matters for me

If in the sim I add some series resistance to the B+ supply give us some idea what signal(s) come back to the input stage supply from the o/p stage pulling on the B+. Provided it's decoupled enough (1k in the right place ) should basically work. Well it's usually worked for me so far.

NB: 'motor-boating' oscillation is positive feedback to the previous stage via the supply line.

[Mike H]

psu details are : mains 450 - 0 - 450, 5u4g rectifier, 10h 200ma choke, 80uf 800v motor run cap, 10h 200ma choke, 80uf 800v motor run.

First up, the PSU circuit:

I'm using a load of 2.2k, = 160mA DC

[Mike H]

The plots ~ doesn't quite make 400V DC, but didn't think it would, mine had the same problem!

First up, C1 Volts pink, C2 Volts red ~ turn on at 1 second:

[Mike H]

Second, green: rectifier out / L4 in Volts; pink: rectifier anode #1 current; red: rectifier anode #2 current. So far so good!

[Mike H]

Played with the C3G, had to modify a few resistor values to get it more linear. Basically try to get the anode Volts nearer to half that of its supply line. This is easily tweaked by varying the screen divider values.

Got some gain! The following is its output for input of 400mV (peak). Unless it's telling fibs

[Mike H]

The complete schematic:

I'd think about making C3 or R7 bigger value, as some ripple comes back onto 1st stage supply at low frequencies unless the HT is quite stiff. Unfortunately increasing R7 decreases anode Voltage swing "headroom" for U1, so it's looking more like C3 might be 100uF instead ?

Otherwise seems to work quite well, reaches 5 Watts r.m.s. just on the point of clipping. Non-linearity of U1 is compensated by non-linearity of U2, so it evens out.

Total supply current = 63 mA. X2 for stereo, 126 mA.

[Mike H]

Freq. response plot next, allowing for a bit of transformer coupling loss:

Not slight hump at bottom end, this is cos C5 "slightly too small", but really not worth worrying about and anyway depends if the transformer goes that low TBH.

I think it's Sir Goer....

[pre65]

What about a choke where R7 is ?

I think that was how I did it on my second try at PT15 driven by C3g.

[Mike H]

Could do that, provided it's got enough Henries

[richardcooper2k]

or one of mike's e-chokes

[pre65]

Could do that, provided it's got enough Henries

I think mine was 10H.

[Paul Barker]

R7 is a tenth of value of load resistor a fifth is preferred max for stability.small choke as supplied by you would be icing on cake here

[Nick]

FWIW, I would not build the power supply like that, it has a resonant peak at 4Hz, and the two 80uf caps means its step response is going to be somewhat chaotic. I would use a 47uf cap for the first one, and think about padding the choke DCR out to say 200R with series resistors.

But many seem to like the effect of the power supply getting into the act so YMMV.