Corona discharge is a potential issue at 1000v DC + , it is a scientific fact .
Skin contact with 1000V DC B + rail at the current required for a GM70 can cause your heart to defibrillate and or DC burns if the insulation , wire and connectors are not rated for this voltage
Skin contact with 1000V DC B + rail at the current required for a GM70 can cause your heart to defibrillate and or DC burns if the insulation , wire and connectors are not rated for this voltage
I use HV cable plus a teflon sleeve just to be safe!
This circuits works really well and drives a pair of ribbon speakers
This circuits works really well and drives a pair of ribbon speakers
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My PTX is 650-0-650 and after CLCLC I get 889v. I use Belden 8899 00225 wire for the power supply and keep all that wiring away from the signal path.
I've been running a pair of GM70 amps for over 9 years, 1kV plate voltage, fixed bias, 100mA into 7K, interstage transformer driven by triode connected D3A. I get around 20Wrms output.
I do not ever work on the amplifier with the 1kV B+ enabled. Have never had problems with corona discharge, used HV wire and in some places reinforced with teflon tubing.
Around 12,000 hours of service so far without major problems. I change the tubes roughly every year depending on hours of use. I usually like to change them at about 1200 hours. Current ones have about 1600 hours and are still OK, will replace around the end of December.
I do not ever work on the amplifier with the 1kV B+ enabled. Have never had problems with corona discharge, used HV wire and in some places reinforced with teflon tubing.
Around 12,000 hours of service so far without major problems. I change the tubes roughly every year depending on hours of use. I usually like to change them at about 1200 hours. Current ones have about 1600 hours and are still OK, will replace around the end of December.
When I ran them as full range amplifiers I did run copper plates for about 4 years. (3 pairs) These days just graphite since I run them as bass - low midrange amps. (cross at 700Hz)
I got good service out of the copper plates at 90 - 100mA , and ran the graphite at 120 - 125mA, but no longer do (and no surprise they last longer run at lower current)
I got good service out of the copper plates at 90 - 100mA , and ran the graphite at 120 - 125mA, but no longer do (and no surprise they last longer run at lower current)
I have copper GM70 running at 75mA at 1200VDC. Bias is fixed. When I insert graphite ones I'll run them at about 85mA.
@djn
I know it sounds great, but at higher voltage is even better.
@djn
I know it sounds great, but at higher voltage is even better.
What output transformers are you all running these with? I'm planning my build too and I'm thinking about:
Toroidy TTG-GM70SE - Tube output transformer [7kOhm] GM70 SE - Shop Toroidy.pl 7K which is ~200 euro
Hammond1629SEA 6.5K which costs €150
Hammond1638SEA 10K and they go for €185
A friend of mine says go Hammond
I'm also open for suggestions, just don't say Monolith Magnetics (Sorry Ward) or Lundahl, that's way out of my budget 😉
Toroidy TTG-GM70SE - Tube output transformer [7kOhm] GM70 SE - Shop Toroidy.pl 7K which is ~200 euro
Hammond1629SEA 6.5K which costs €150
Hammond1638SEA 10K and they go for €185
A friend of mine says go Hammond
I'm also open for suggestions, just don't say Monolith Magnetics (Sorry Ward) or Lundahl, that's way out of my budget 😉
The pretty looking one is a bit more expensive. I don't want to stick some black wrapped toroid on top 🙂
Before changing to the Monolith Magnetics I used the 1629SEA which actually performed quite well. Square wave looked decent, low end was actually a tad better than the Monolith but the high freq. detail was a little better than the Hammonds. I would of been satisfied with the Hammond but I had the extra cash to layout for the Monoliths.
I would say go with the 6.5K hammonds if you're running these with lower voltage, they sounded great at 1050v, about 95ma (from memory).
Absolutely willing to believe their frequency response is superior! One of the owners is a professor at a university I studied and later worked at for a while. He certainly knows his physics and every now and then the conversation at the lunch table would wander into the audio stuff he was working on which was always highly entertaining. But he operates in a price range that is simply a little bit too much for me so I was looking at the <200 euro range. So hearing that my friend was right about the Hammond being good is nice to hear
I have custom made 7K transformers from FIAT tranformatori (waltube here). It is double C core with 0.11mm laminations. Apsolutely fantastic trafo.
You saw a 10Khz rectangular pulse at the output of the OPT, on a non-inductive (ordinary carbon) resistor?