Fun with Feedback

[Nick]

Hmm

2 x Pi x 4200 x 50e-6

I make that 1.3 sec

[ed]

my bad!. I think I was over simplifying and also cheating using the digikey thickos calculator..

https://www.digikey.co.uk/en/resources/ ... e-constant

[Nick]

Fair enough if you want the answer in radians per second

[Thermionic Idler]

Merlin Blencowe (I think it's him - the Valve Wizard) has a good article on the Valve Wizard website about fusing HT secondaries - I've had fusing on both of the 300B amp's HT secondaries from the get-go based on the advice given therein - which probably saved my mains transformers when I miscalculated the power supplies initially (remember when I had those arcing rectifiers?).

[IslandPink]

The fuse is there to avoid fusing transformer secondaries !

[Cressy Snr]

Fixed.

Amp is now stable by the simple action of reinstating the original first stage step network of 4K7/200pF in parallel with the 100K anode resistor and 100pF across the feedback resistor.

Power supply had been completely rebuilt, reconfigured, time constants fecked about with, all to no avail. “Putt putt putt putt putt.” Reinstate original step networks: problem gone.

[Cressy Snr]

Even replaced the VR tube shunt, screen regulator with a simple dropper resistor because I had thought I’d got a localised relaxation oscillator.
Anyway, it keeps me out of mischief and puts the grey cells through their paces. Will restore the cascode CCS fed, glow-tube screen reg tomorrow.

[ed]

Hmm

2 x Pi x 4200 x 50e-6

I make that 1.3 sec

Any chance you have any refs for this so I can read up on it. My understanding is vague to non-existant and I've failed to find anything online.

all the stuff I have found says 5 x Tau where Tau = R X C
On reflection I haven't tried RDH or MJ yet...I will have a look in there first.

[Nick]

I just used the -3dB point of a RC filter

fc = 1/2𝜋RC

Though I may be answering a question you haven't asked.

[Cressy Snr]

I've carried out extensive work on this piece of gear:
Power supply is split to each stage after the output stage caps, which have been changed to motor runs.
Cascode CCS feeding the glow tube regs can be seen next to the left hand motor run cap. Glow tubes themselves have been repositioned between the two EL34s in order to give the shortest possible path to their screen grids.

IMG_0107.jpeg (135.15 KiB) Viewed 1494 times

Top of amp:
I put the cheap Hammond output transformers in the conventional position, on the top, behind the valves as opposed to having them inside and tarted them up with some spare end bells I had lying around. I've oriented them vertically.
I covered the holes left by the old VR tube positions with the badge from the front. I'll eventually get something bigger made, that will cover the holes properly, but it'll do for now.

IMG_0108.jpeg (91.24 KiB) Viewed 1494 times

[Cressy Snr]

Audio:

Screenshot 2023-01-13 at 16.51.05.png (59.23 KiB) Viewed 1482 times

[Cressy Snr]

Power Supply:

Screenshot 2023-01-13 at 16.50.14.png (81.53 KiB) Viewed 1482 times

[Cressy Snr]

There’s a typo on the audio circuit diagram: feedback resistor should be 10K, not 15K.

[Cressy Snr]

Anyway here is the corrected diagram:

Screenshot 2023-01-14 at 14.38.22.png (62.1 KiB) Viewed 1414 times

[Cressy Snr]

Deleted.
Waffle