OK…so I’m on wi’ summat

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Cressy Snr
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#166 Re: OK…so I’m on wi’ summat

Post by Cressy Snr »

Power supply now:
Screenshot 2024-03-30 at 18.25.55.png
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Last edited by Cressy Snr on Sat Mar 30, 2024 6:26 pm, edited 1 time in total.
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Nick
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#167 Re: OK…so I’m on wi’ summat

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Cressy Snr wrote: Sat Mar 30, 2024 2:36 pm
Nick wrote: Sat Mar 30, 2024 2:23 pm
A layer of hash has been removed as it was last time.
I have to ask. Did the layer of hash come back when you put the electrolytics back in last time?
Yes, it was just a slight bit of fuzz around the treble. It's not a night and day difference with the motor runs in place but it's there nonetheless.
It was a practical question of how to mount the things in the present chassis in order to use them again, but I overcame that problem by using old L shaped transformer brackets. The caps were strapped to those with cable ties.
Small film bypass caps across the electros might be interesting. Across the motor run as well, for that matter.
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Paul Barker
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#168 Re: OK…so I’m on wi’ summat

Post by Paul Barker »

Is vintage silver mica good?
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
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Cressy Snr
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#169 Re: OK…so I’m on wi’ summat

Post by Cressy Snr »

I've fitted a dual 220uF, BG as the bypass cap(s) for the output stages and removed the bypasses from the input stage, which has reduced the gain, but the amp is so sensitive it's no great loss.

It's the best I can do at the moment. Not tried it on music yet. That'll have to wait until later tonight. Let's see what happens.
Last edited by Cressy Snr on Sat Mar 30, 2024 5:27 pm, edited 1 time in total.
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Cressy Snr
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#170 Re: OK…so I’m on wi’ summat

Post by Cressy Snr »

In the meantime, here is the newest version of the audio circuit:
EL34SEULNobypass.png
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simon
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#171 Re: OK…so I’m on wi’ summat

Post by simon »

Paul Barker wrote: Sat Mar 30, 2024 3:54 pm Is vintage silver mica good?
Good question. I bought some big Russian silver mica caps years ago when Russian caps were still dirt cheap on ebay. The only time I found a use for them was in a high pass filter in front of the 45SET I'm using now and they were pretty horrid, the sound seemed broken somehow. Took them out sharpish. Steve has a pair and might have different experience.

Other caps might be better?
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Cressy Snr
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#172 Re: OK…so I’m on wi’ summat

Post by Cressy Snr »

I tried the silver mica caps from Simon, years ago. Didn’t like them I’m afraid: to call them screechy is being kind. :(
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#173 Re: OK…so I’m on wi’ summat

Post by simon »

Think we can agree those aren't very nice then!
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Cressy Snr
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#174 Re: OK…so I’m on wi’ summat

Post by Cressy Snr »

Was going to go fixed bias (Gawd knows why.) Got all the way to building a pair of bias supplies. Tested them, they worked faultlessly then I wondered what I was playing at and thought, sod it!

Just enjoying the music instead.
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Paul Barker
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#175 Re: OK…so I’m on wi’ summat

Post by Paul Barker »

Shame, it would be worth trying fixed bias having gone so far.
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Cressy Snr
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#176 Re: OK…so I’m on wi’ summat

Post by Cressy Snr »

There is nothing simple, that can’t be made more complex. I needed to take a step back and ask myself if what I wanted to do was actually necessary?

I seemed to be planning to up my solder connections, up the parts count, double my chances to make a mistake. Double my chances for failure, so I thought, bugger it.

Additionally, mounting the bias boards would have been tricky. The EL34 grid resistors would have had to be changed too. In my chassis, fixed bias would have to have been something I would have had to plan for at the design stage. Trying to add it in later was just too much messing about. I may try at a later date, but don’t hold your breath folks. :|
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Ray P
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#177 Re: OK…so I’m on wi’ summat

Post by Ray P »

Enjoy the music, Steve, that sounds like a good plan for staying off the slippery slope.
Sorry, I couldn't resist!
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Cressy Snr
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#178 Re: OK…so I’m on wi’ summat

Post by Cressy Snr »

Ray P wrote: Wed Apr 03, 2024 4:06 pm Enjoy the music, Steve, that sounds like a good plan for staying off the slippery slope.
The combination of this amplifier, the Mac Mini and the RTZ DAC (with the higher quality op-amps) is producing the best sound I’ve had in this room…ever!
I think, trying to wring anything extra out of the amp might be a step too far and risk losing what I have.
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Paul Barker
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#179 Re: OK…so I’m on wi’ summat

Post by Paul Barker »

I get that Stephen its happening to me aswell. I’m real glad I have the 6 watt EL34 amp built, which is simply wound up on the variac.
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Cressy Snr
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#180 Re: OK…so I’m on wi’ summat

Post by Cressy Snr »

Ray P wrote: Wed Apr 03, 2024 4:06 pm Enjoy the music, Steve, that sounds like a good plan for staying off the slippery slope.
Yeah that would have been nice; in fact it was very nice indeed...that is until last Thursday afternoon.

The telly was on, Melanie was watching "Impossible," I was upstairs, not having turned the amp off, no sound through the main speakers, just the telly through its own when of a sudden it was, "STEVEAAAAN! WHAT'S GOIN' OFF WITH THIS BLOODY AMP AGAIN?"

I came downstairs to a loud hummmmmm, which then stopped, then followed by a click through the speakers, it started again, then stopped, then started, and so on. Tapping with a plastic rod on components in the live circuit proved inconclusive. The upside-down amp had gone quiet again and I couldn't provoke it into action...that is until I pushed with the rod on the terminals of one of the motor-run caps. That set it off! To cut a very long story short and using the capacitance setting on the meter, I found that two of the three big 80uF motor-runs had an intermittent internal connection to one of their terminals. Bugger!

The four way terminals look to be spot welded to the internal wires, so how they could have come adrift internally, I don't know. Maybe too many past instances of heat being applied. Maybe push-fit connectors should have been used from day one with them, but whatever; two of the three were well fooked.

The NVA based amp came back.

I was now faced with having to remove the big caps, leaving three ruddy great holes in the top plate. These had to be covered over with a sheet of black gloss acrylic, cut to size and the power supply has to be redesigned. AND it was going to be electrolytics as I was sick to the back teeth of trying to think up ways of mounting oversized caps in an undersized box. I'm well over the nervous breakdown and really enjoy messing about with valve electronics but there are limits to my patience. I mean, the NVA based amp is sooo easy to live with. All you do is just turn it on :lol: :lol:

I've spent three days designing and rebuilding the power supply with the help of PSUD for Mac and have used all electrolytic caps: three 500V, 40uF, blue, Sprague Atom caps, plus one 500V, JJ, 2 x 100uF job. All these are guitar amp favourites, and are the ones I had in my 6B4G mono-blocks from around 2011. The JJ and the Sprague caps combine to produce a nice pleasant tone at the speakers. The background from which the music emerges, is as quiet as the grave. It'll do me.

The only thing different is that the HT is 20V higher than it was because of the 47R series resistor after the rectifier. A 5R4GY brings it all nicely back into line.


The power supply is almost as simple as it gets:
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Internal shot: now that's more like it! Minimalist, uncluttered.
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