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rabski

Time for another preamp

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Bits have been in the parts pile for ages, so it’s about time to glue them all together.

The general idea is to get a basic platform with a really good power supply, to be able to try alternative circuits and valves. Therefore, at least a couple more valve bases in the finished version, and probably a pair of octal sockets. Starter for ten will probably be paralleled 5687s.

Valve-rectified HT, split for left and right channels after the rectifier, CLCRCRC plus a further RC section for filament lift. Sim in PSU designer says HT ripple is under 0.2 microvolts, so it should be quiet. Regulated DC for the heaters with a separate transformer. A two box design would give more space (valve HT regulation, etc.), but rack space is getting a bit tight and the case is going to be quite large as it is. Capacitor coupled initially. Transformer would be nice, but they generally come in two types: costly or poor performing.

This one is going to be a bit foo I’m afraid. I’ve got a Seiden selector switch and a Noble pot that have been looking for a home for ages. Copper chassis, Black Gates for cathode caps, PRP and Mills resistors, cotton-covered copper for all the HT volts, some old Audio Note for the mains and silk-covered silver plus some Kondo for all the music. A selection of capacitors to experiment with, including Ampohm, Obbligato and Miflex.

Busy for the next few days, but this is part of the pile of bits for a rough layout…

layout.jpg

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If anyone is interested here's the basic power supply.

I've used a fairly extensive CLCRCRC chain for the simple reason that I've got all the bits so don't have to fork out for anything :cool: Technically, LCLL would be a bit nicer, but I don't have enough suitable value chokes, whereas I'm drowning in large capacitors. All the caps are 500V rated.

The maximum reservoir value for an EZ81 is 50uF, so the supply will be duplicated for each channel after C1. It's easy to tweak R1 and R2 values to get higher or lower B+ if needed, though that might need the values of C3 and C4 to be changed to match. The last stage (R3 and C5) drops it for the filament lift feed and R4 puts a load on it to keep it stable.

HT.jpg

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Super Wammer

I expect it will be somehow up to snuff

:notworthy:

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We'll see (or rather, hear) :D

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I recently built a very similar power supply for a headphone amp and needed similar values of caps for the power supply. Ale Moglia of Bartola Valves fame told me about some Polypropylenes  which are used for the DC-Link application and are available in sizes like 100uf/500V at reasonable prices. I found some Kemet ones in Farnell which worked very well. I think modern electrolytics are getting quite good but if you are a bit ocd like me I thought you  might like to know  about them if you dont already know of course.

Your build quality looks much better than mine !

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I can be very OCD, believe me! That said, I'm generally quite happy with large electrolytics in power supply applications. Their only purpose here is to reduce ripple and as long as you can take into account ESR values, there isn't much of a downside to using them. I'll stick the output on the 'scope at some point, but as long as you don't make any dumb mistakes (which I have on occasion) the sim in PSU Designer is actually extremely accurate. Kemet and Vishay do 100uF 500V polys and there are others around. I might try some at some stage for 'why not' reasons. I remember that Coco, late of the parish, was quite fond of using banks of motor-run capacitors rather than (very) large electrolytics for much higher voltages.

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Yes I just thought I would mention it as they worked well for me.  In theory they might be better at filtering hf noise on the power rail if that was a problem. In my case I was using triode strapped russian rf pentodes which can oscillate if given half a chance and it was something that Ale recommended. I dont know about 5687,s. Anyway hope your build goes well.  

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Starter for ten will probably be Capacitor coupled initially. Transformer would be nice, but they generally come in two types: costly or poor performing.

If you want to try transformer coupled output I have a pair of Sowter 9560S sitting idle over here, 3.75:1, should be good with parallel 5687. You’d be welcome to give them a try.

https://www.sowter.co.uk/specs/9560.htm

Sent from my iPad using Tapatalk

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Will pm you later Martin. Just seen your post but have to go out for a while. Thx.

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6 hours ago, zeta4 said:

Yes I just thought I would mention it as they worked well for me.  In theory they might be better at filtering hf noise on the power rail if that was a problem. In my case I was using triode strapped russian rf pentodes which can oscillate if given half a chance and it was something that Ale recommended. I dont know about 5687,s. Anyway hope your build goes well.  

Thanks!

The original (well, depending on which one you call original) Kondo M7 circuit uses one 5687 paralleled for each channel and doesn't even seem to bother with a grid stopper so it should be quite robust. Frankly, the biggest pain with using them is that 5687s draw close to half an amp heater current at 12.6V or .9A at 6.3, hence the second transformer just for filaments. Nothing is ever simple...

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