GK-71 - economical build.

What people are working on at the moment
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pre65
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#391

Post by pre65 »

JamesD wrote:I had a crazy idea...

Filament bias!!!

72V bias at 3A filament current only 24 ohms!!!


and 216Watts - Ooooooopppppppsssssssssssssssssssssssss!

Oh well I said it was a crazy idea...
I like crazy ideas. :wink: :lol:
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pre65
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#392

Post by pre65 »

I'm not sure how to calculate filament bias, I've searched Google but there seem no easy explanations. :? :(
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pre65
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#393

Post by pre65 »

The HT is a bit low (500v - 70v) so I'm changing the PSU a bit. Rather than go cap input, I'm going to retire Mikes E-choke/regulator and replace it with another 10H choke and 500uf of capacitance.

The caps are 2 X BHC 1000uf 450v low ESR in series.

This is similar to what was on the "old" GM70 amp.

Is 500uf too much for this amp ?
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Paul Barker
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#394

Post by Paul Barker »

Difficult. You raise the filament supply voltage by the bias voltage. Don't expext you can with Andrew's module. Then filament ground to one end of your bias resistor other end to one filament pin, other filament pin to supply plus out.

Other end of bias resistor to circuit ground and filament minus

Bias resistor ohms law incorporating bias voltage devised by filament current. Wattage is bias volts x filament current. Cathode current insignificant in this instance but you could add it to be pedantic.
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pre65
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#395

Post by pre65 »

Thanks Paul, I'll do my sums later. :wink:

I'm head nurse today as Jean has "woman flu". :lol:

The mods are coming along. As the HT has (or should have) increased I'm in the process of adding a 4K7 50W resistor in the 12HG7 CCS feed. The CCS heatsink gets quite hot so I don't want to make it worse.

The drill I was using to make the resistor mounting bracket broke, and attacked my thumb, so now it's bleeding a bit and throbbing.

Ivimeys law strikes again. :lol: :lol:
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#396

Post by pre65 »

One amp is done now, and when plugged in has lost most of the hum it had before. :D

Just as I was re-assembling the amp I noticed I'd wired up the two caps after the first choke incorrectly. :(

Instead of the balancing resistor being between plus and minus on each cap I'd put a resistor between each cap. DOH. :?

Perhaps that was causing the hum ? Anyway, I tidied up a few wires, and re-routed the 12HG7 heater wires as well for good measure.

Just need to mod the other amp now. :)
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#397

Post by pre65 »

2nd amp is done, and just played a couple of Eva tracks to see if everything is OK.

Almost no hum is good. :D

The anode HT is now 603v and the cathode is 83.3v (820R) so current is 102ma.

Everything sounds just right.

Will have a good session in the morning to see how it is on a selection of tunes. :wink:
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#398

Post by Dave the bass »

I've got a Derek Bailey album you can borrow if you want to run it in Phil :)
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#399

Post by pre65 »

Thanks Dave.

I hope you won't be offended if I decline your kind offer. :lol:

Yesterday I put a 4K7 resistor in the line feed to the CCS to drop the volts a bit and keep the CCS heatsink from overheating.

Should I decouple after the resistor with a cap ?
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#400

Post by Paul Barker »

No need.
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#401

Post by IslandPink »

I'm not sure the CCS should be fed from a high impedance ( 4.7k ) .... perhaps Nick can comment ?
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Nick
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#402

Post by Nick »

I can't see how it would make any difference either way, as long as the CCS has enough voltage behind it to stay positively biased it should be fine. Of course there is equally no reason why you would not put the resistor after the CCS either.
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#403

Post by pre65 »

I'm doing the cascode CCS driver mod this morning.

I needed to reduce the voltage to the CCS a bit more, but could not find a suitable resistor. In the end I paralleled two 50W resistors, 10K and 22K, which was near enough.

I've got to put things back together now, and then test. If the first one is OK I'll do no 2 later. :)

EDIT. No 1 works OK. :D
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#404

Post by pre65 »

I've never been unhappy with the GK-71 sound, and it seemed to be well received at Owston.

But now, with the cascade CCS driver,Andrews filament supply and all the little details sorted out, it sounds REALLY good.

Last tweek will be to replace the 80uf Sprague Atom electrolytic cathode bypass cap on the GK-71 with a polywhosit version. :)
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pre65
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#405

Post by pre65 »

I managed to find a pair of Solen MKP capacitors on Ebay at reasonable cost, so that should be it upgrade wise for a while.

May not be here for several days. :(
The only thing necessary for the triumph of evil is for good men to do nothing.

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