GM70 PP

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izzy wizzy
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#151 Re: GM70 PP

Post by izzy wizzy »

The rectifiers arrived yesterday and as always, had the day off work on my birthday so perfect timing. As the MVs ran of 4V and have the same sockets as the DW4, it meant only moving 1 wire and alter the tapping on the HT transformer to be in business. I very much like the result and this whole seperate power supply for the front end is proving a winner all round.

So having a bunch of others to choose from, I popped them into PSUD to see what's going on and whether anything is predictable. When messing around with step changes, the response of the different rectifiers to the step change was interesting. They all did different things; either speed to stabilise or overshoot or oscilations getting there or in trying to settle.

There always seems to be discussion around rectifiers making or not making an audible or measureable difference. It seems to me they do audibly so it'd be nice to know why. HT variation is not what was happening here within reason. This points to an indicator that their dynamic behaviour in response to varying current draw will modulate the power supply as will the way they turn off and on and I guess their impedance will alter the PSU characteristics as well.

There's a whole lot of variables here and I'm not sure I'm willing to mess about with all of them. My aim was to try a few and see what works best for the setup I have and then secure a supply going forward. Popped another DW4 in the phono instead of the MVs and got a similar result. I'd not tried anything different in there for about 16 years.

Overall at the end of the day, I noticed that the volume control was a click or two lower to be giving me the same satisfaction and the whole thing sounded a bit more relaxed. So all round, heading in the right direction.
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#152 Re: GM70 PP

Post by RhythMick »

Nice work and nice result.
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#153 Re: GM70 PP

Post by izzy wizzy »

I realise this next question is like "how long is a piece of string" but ...

How long lasting are GM70s? Do they have decent life, long life or short life relative to expectations. Not sure how to phrase this properly. All valves will have a shorter life if thrashed; obvs. I'm running them about 80%; around 100W plate. Most other folk will be running them SE so proabably similar dissapation. Trying to get an idea as to how many spares I'll need for the next 20 years or so.

I'll try again. Are GM70 folk thinking, "gee, I replaced that one sooner than I thought" or is it, "blimey that's lasted longer than I expected"
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IslandPink
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#154 Re: GM70 PP

Post by IslandPink »

Simon's probably your best bet on that advice.
ps. a couple of us might be interested in piggy-backing on your order to help you save postage - if you are able to meet up again further down the line.
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#155 Re: GM70 PP

Post by simon »

Not sure I can really, mine only get occasional use really and I'm fairly gentle with them - 550V 90mA.
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#156 Re: GM70 PP

Post by simon »

I found a post I recall on DIYAudio - kevinkr (who knows what he's doing from all I've read) talks about 1500 hours for coppers and them soon being ready for replacement.

https://www.diyaudio.com/forums/tubes-v ... -gm70.html
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#157 Re: GM70 PP

Post by IslandPink »

That's disappointing, I've been running my KR300B's for nearly 10 years. I think 211's last longer than 1200 hours.
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#158 Re: GM70 PP

Post by izzy wizzy »

Spose there's a few ways of looking at the life. The DIYAudio post does look short life (it's in the data sheet) but there were some things to take from it. He is running them quite hard and being conservative on replacement not wanting to take a risk. But yes compared to others, it's relatively short but then again, they're relatively cheap. There's no free lunch. Looks like I should buy a few and not leave it on willy nilly. I was considering a throttle back switch to lower the bias for general background use.
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#159 Re: GM70 PP

Post by IslandPink »

Yes, seems like a good plan. Ought to be possible to push the hours up a bit with less dissipation, and Andrew's supplies.
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#160 Re: GM70 PP

Post by Paul Barker »

The link is part of the miscelaneus section of Stephie’ tube pages. It’s a useful comparrison of this transmitting valve life expectancy, and the small signal valves of the report. up to 85 years life expectancy. https://jacmusic.com/techcorner/SBENCH ... belife.txt
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izzy wizzy
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#161 Re: GM70 PP

Post by izzy wizzy »

Looking at my plans to house these amplifiers. I like a copper top plate with valves poking through similar to Audio Note and others without any fixing on the top plate. Board holding it all underneath so no weight carried by the copper as any top mounted stuff will have ply underneath. So looking for 2 to 3mm copper sheet about 400mm x 200mm. Does anyone have sources of this they could recommend?
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#162 Re: GM70 PP

Post by pre65 »

I had a quick look on Ebay, searched for 2mm copper sheet.

Quite a few suppliers.

One was for 2mm (14 swg) 400 x 200 @ £23.10 post free.

Or, for 3mm (10 swg) same size @ £36.50 post free.

I had a quick look at Chronos (engineers supplies) but what I saw looked more expensive.
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pre65
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#163 Re: GM70 PP

Post by pre65 »

I was going to suggest copper clad FR4 circuit board, but you might have trouble with the size.

I made a pre amp years ago and I hid everything ( that needed to be) under a cover made from blue acrylic sheet, I thought it looked good.
Life is what happens to you while you're busy making other plans. John Lennon

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#164 Re: GM70 PP

Post by izzy wizzy »

The valve sockets/audio circuit will be point to point on PCB underneath the copper. I have SWMBO approval for the cosmetics so far so sticking with the plan for now. The wooden casework plan is for Iroko as I have a kitchen worktop Iroko offcut for the "stereogram" shelf part for the rest of the system and iroko veneer for the speakers. But plans sometimes change.
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#165 Re: GM70 PP

Post by izzy wizzy »

Did some very crude frequency response and power tests on the amps. As they're only two stage with direct heated valves, gain is low and tested as modelled at about x2.8, 9dB voltage gain but that's all I need.

Power out was about 50 to 55W before the tops of the sine started to round off. I could push it to 60W and it still looked smooth but more rounded off. 55W requires about 7.5V in. Working back, 21V out though that OPT is about 880V peak so with HT about 920V, the limit is the ouput stage not the driver but these are guesses.

Frequency response is limited to what I can generate so at 1W at 20Hz saw no drop off. At 10k to 20k a slight rise. This might be due to unterminated input transformer at a guess but I didn't look further. At full power, didn't see any tail off at 20Hz and the slight rise at the top was brought back to level.

Through all this, I realised to go further, I need a much better generator. Used Audacity generated tones and had to feed the RPi into a SS spare power amp to get enough volts so not very practical. I tried a 1k square wave but the generated signal didn't look pretty however the amps output looked the same without any additional overshoot or droop but a 1k square wave should be easy.
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