Single-Ended Pentode Redux

[Cressy Snr]

The tiny voltage drop given by the solid state bridge rectifier combined with the overvoltage due to the oversize mains transformer has allowed me to try a choke input supply, providing I revert to 6V6GT and stay with them
The output of the supply is at 260V.

The 6V6 run at 34mA anode + screen current each and the 12AT7 unit runs at 4mA per side. We are looking at 75mA or so of total draw, so the 4H main choke is adequate in terms of the minimum current requirements, using the V/L rule of thumb. There is a 1K/10n Zobel across the mains TX secondary to keep flyback spikes out of it, which from a Linear Audio article by MJ is supposed to work a treat.
This Zobel is built onto the rectifier board anyway, so the whole rectifier/snubber is its own self-contained module. The channels still split off into their own LC filters after the first LC.
Everything is dead quiet and nothing in the PSU chain gets even slightly warm.

I’ve had mixed results with LC supplies in the past, probably caused by trying to draw too much current through chokes not up to the job, but with this smaller, relatively unstressed supply I’m bowled over with the results. Treble speed, shimmer, decay and overall solidity elsewhere in the frequency range are all quite a way better compared to the previous arrangement. I’m sold on LC.

The amp now puts out about 3W.

[Paul Barker]

Thank god someone else gets it at last!

[Nick]

I just have to ask, you have changed the rectifier to a solid state one, removed a cap before the choke, changed the anode voltage, added a zobel, even it sounds like changed the valve used. So you have created a lower voltage power supply with a entirely different response to changes in load. And having heard a difference decided the cause of the difference is because of some generic description you apply to the supply?

[Cressy Snr]

Yes.
I had the SS rectifier right from the start with this build, the Zobel was there anyway, I simply highlighted it because I believe LC supplies need these things more than a cap input would, I know the sound of the 6V6, EL34 etc as I’ve built around twenty previous amps with them and I know what I hear and have to go by that as there is no other way of saying it.

[Nick]

Thank god someone else gets it at last!

Hang on. just how many years now have myself and Andrew L said that power supply's matter? To the point where I think both of us gave up saying it.

[Paul Barker]

There is a 1K/10n Zobel across the mains TX secondary to keep flyback spikes out of it, which from a Linear Audio article by MJ is supposed to work a treat.

Ive ordered the book because Morgan deserves payment. But was this recommended for capacitor input filters? Because the Choke input snubber in all his editions of Valve amplifiers serves to “a snubber across the choke to protect the inter-winding insulation of the mains transformer from the spikes”. Its also different values for choke input snubbing. I havent got the article but it appears its focused on the leakage inductance of the power transformer oscillating with the diode stray capacitance. Whereas the choke snubber is designed to select a value of subber Capacitance so that it resonates with the mains transformer leakage inductance.

Until Ive read the article I wont know, but I anticipate Morgan was dealing with the spikes caused by the diodes and transformer leakage inductance. whereas the choke input snubber he recommended from first edition Valve Amplifiers onwards is designed for the specific case of choke input. I imagine applying both might alter values of each or one. Until reading the article or talking to Morgan I would chose the choke snubber first. At least experiment.

You cant measure transformer leakage inductance without expensive equipment. Morgan has designed something that seems to work but it’s probably better to adjust values accordingly. Now his choke input snubber is usually ok with his values for any choke. But a look at the wave form while trying different values would reveal the ideal values. Certainly introducing two zobels serving similar purposes within extended range but focused on transformer leakage inductance might introduce variables Morgan didnt write his one network fits all solution mindful of.

Until checking with Morgan Id go choke input snubber.

[Nick]

I may still have some of these boards from when I got some and offered them.

https://www.diyaudio.com/community/thre ... ig.243100/

[Paul Barker]

ooh, I was unaware of this, yes please if you find the boards? I presume I have to get the instructions and parts list from the diy audio thread?

[Nick]

Will take a look. Yep, the part list is at the head of that diyaudio thread.

[Ray P]

The gerber files are attached to the DIY Audio thread so easy to order a small batch if needed Paul.

[Nick]

Found one. Do you have a working scope Paul?

[Paul Barker]

I had to giggle at that question. i have a load of old sh1t aswell, but I use a digital dual trace 150mhz 25Gs/s scope, never let me down. And a huge brick Tectronics with about a 100 E88CC’s gold pin in, which is why I bought it, to sell the valves, but it works and seems a shame to…. I need to let things go before I go! Cant remember the tec no. but very early days, probably 50mhz but its down at lockup. I dont use it in anger, I just use the digital, mainly because of the digital calculations, saves measuring widths myself to find frequency. Digital is so much easier. Ive had a ton of analog tec scopes, used to buy them, fix the power supplies and sell them on. Their tantalum caps fail short and sometimes take something with them.

[Nick]

So a yes then. I will put the board in the post.

[Paul Barker]

Thank you

[Paul Barker]

I’ve been thinking on the choke input snubber of Morgan Jones in combination with the transformer snubber and think it’s worth trying it. It’s capacitors 220 nf from each end of choke to ground using dc resistance of choke as the resistor.

Since Nick is sending me Quasimodo boards, I could implement his choke snubber then use Quasimodo for rectifier capacitance snubber to null that. Probably work. But I still feel the choke input choke is the most important to support.

My digital scope is very portable so we could give you exact value for youre transformer ringing snubber. When I next call in Steve. it’s very quick process. Might not impinge since reading the diy thread the capacitor of the snubber isn’t the key, the resistor is. As after choke is a big capacitance, might have to put a resistor after 2nd 220nf AOT ideal no more resistance than necessary, due to the down side of resistor’s affecting b+.