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bikerhifinut · 23rd Dec 2015, 9:13 pm

Cheers Ken,
I had an idea that may be the case.
Anyway the problem is mostly solved

I used 4 of "Andy's patent isolation washers" under the "simon frame" between it and the steel chassis. They are actually a hard rubber tap Washer from the plumbing spares shelf at the DIY shop................
Result! There's a residual "tizz" amplified a bit by the top cap and It sounds to me like the windings rattling. The transformers had an extra vacuum impregnation and bake so I reckon there's no improvement to be had now.
I will continue to fine tune the remaining buzz, I think I could damp the top cap a bit and wonder if a pad of some sort under the top cap would be possible.

Now to the other bit.
I am a bit miffed that the transformer was wound to a 230V primary. Our mains was 246V last night and it's caused issues on the secondaries. I had a word with majestics and they say they wind to the standard for the mains which is 230V, apparently in Poole thats as high as it gets. No good to me though. In future if I get a transformer wound to my spec i shall ask for a tap on the primary so it can be used 230 or 240 V.
The issue is that the EL34 heaters are running at 6.8V measured on the new DMM and they are visibly brighter at a yellow shade of orange which looks a bit hot to me. The other valves are no problem as I have them on a different secondary and regulated at 6.3V DC.
The Ht needs to be managed down, with a 220R in each anode of the GZ34 I am getting 483V off the cathode and after the choke its still up at 463V. I dont want more than 450V on the CT of the output transformer.
I gave them a quick run up after checking the rest ov the voltages and i thought the amp wasnt working, total silence at the speaker with ear pressed against it. But a quick unplug and re insertion of the input RCA plug proved otherwise. So the REGulated DC on the EF40 and ECC81 has been a definite advantage, although the amp was already pretty darn quiet.
BUT............. I put a signal through it and as the music started i saw the dreaded "Red Plate" effect begin.
The only parameter I have changed is the mains transformer so I am assuming that this particular circuit configuration of EL34 with cathode bias of 470 ohms in UL with 43% taps was already "On the limit" at the design voltage of 450V at the CT. I reckon the valves were passing between 76 and 78mA which is about 10mA more than specification.
So its bung another 100 ohms on each anode, i rearranged the tagboards so that i could use different resistors either singly or series and parallel to get the correct voltage on test. That will make the tweaking easier.

Anyway fingers crossed that i have got over the hump, and thanks to all for their very useful input as many of the hints and tips have been put into use.

Happy Xmas everyone!

Andy.

23rd Dec 2015, 9:13 pm	#129
bikerhifinut Octode Join Date: Nov 2013 Location: Penrith, Cumbria, UK Posts: 1,993	Re: EL34 power amp project Cheers Ken, I had an idea that may be the case. Anyway the problem is mostly solved I used 4 of "Andy's patent isolation washers" under the "simon frame" between it and the steel chassis. They are actually a hard rubber tap Washer from the plumbing spares shelf at the DIY shop................ Result! There's a residual "tizz" amplified a bit by the top cap and It sounds to me like the windings rattling. The transformers had an extra vacuum impregnation and bake so I reckon there's no improvement to be had now. I will continue to fine tune the remaining buzz, I think I could damp the top cap a bit and wonder if a pad of some sort under the top cap would be possible. Now to the other bit. I am a bit miffed that the transformer was wound to a 230V primary. Our mains was 246V last night and it's caused issues on the secondaries. I had a word with majestics and they say they wind to the standard for the mains which is 230V, apparently in Poole thats as high as it gets. No good to me though. In future if I get a transformer wound to my spec i shall ask for a tap on the primary so it can be used 230 or 240 V. The issue is that the EL34 heaters are running at 6.8V measured on the new DMM and they are visibly brighter at a yellow shade of orange which looks a bit hot to me. The other valves are no problem as I have them on a different secondary and regulated at 6.3V DC. The Ht needs to be managed down, with a 220R in each anode of the GZ34 I am getting 483V off the cathode and after the choke its still up at 463V. I dont want more than 450V on the CT of the output transformer. I gave them a quick run up after checking the rest ov the voltages and i thought the amp wasnt working, total silence at the speaker with ear pressed against it. But a quick unplug and re insertion of the input RCA plug proved otherwise. So the REGulated DC on the EF40 and ECC81 has been a definite advantage, although the amp was already pretty darn quiet. BUT............. I put a signal through it and as the music started i saw the dreaded "Red Plate" effect begin. The only parameter I have changed is the mains transformer so I am assuming that this particular circuit configuration of EL34 with cathode bias of 470 ohms in UL with 43% taps was already "On the limit" at the design voltage of 450V at the CT. I reckon the valves were passing between 76 and 78mA which is about 10mA more than specification. So its bung another 100 ohms on each anode, i rearranged the tagboards so that i could use different resistors either singly or series and parallel to get the correct voltage on test. That will make the tweaking easier. Anyway fingers crossed that i have got over the hump, and thanks to all for their very useful input as many of the hints and tips have been put into use. Happy Xmas everyone! Andy.