GSG 6A3 Build - Questions.

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GSG 6A3 Build - Questions.

Postby Vapour » Sun Nov 30, 2014 6:31 pm

Hi,
New here, although I've admired these amps for a while. I managed to score a few NOS 6A3 recently & so planning a GSG build.
Received the PCB from Shannon & have been accumulating the other parts & just received a pair of James JS-6113 output transformers.
The last items are the case & power transformer.

I'm in a 230V area & have a very nice power transformer from another project which has dual 6.3V 5A taps. The main HT secondary is adjustable from 250V to 400V in 50V steps. So I guess it should be wired for 350V in the GSG - would I need to adjust any of the PSU component values (R17 & R18 particularly) to achieve the required 300V output, or since it's within 10% would it be ok. I have the specified Triad choke.

Thanks for any advice.
Chris
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Re: GSG 6A3 Build - Questions.

Postby Shannon Parks » Wed Dec 03, 2014 9:37 am

Vapour wrote:I'm in a 230V area & have a very nice power transformer from another project which has dual 6.3V 5A taps. The main HT secondary is adjustable from 250V to 400V in 50V steps. So I guess it should be wired for 350V in the GSG - would I need to adjust any of the PSU component values (R17 & R18 particularly) to achieve the required 300V output, or since it's within 10% would it be ok. I have the specified Triad choke.


Sounds like a great project, Chris! Tell me more about your power transformer - is it a center tapped secondary or will we need a bridge rectifier? Do you have a model number?

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Re: GSG 6A3 Build - Questions.

Postby Vapour » Thu Dec 04, 2014 3:40 pm

Thanks Shannon. Been reading up a bit more, & since this particular transformer does not have a centre tap it could be used with a hybrid silicon/tube rectifier.
These are custom high quality German made (Weiss) for an amp builder.
There is another model which might be better, centre tapped: high voltage secondary 400-350-300-0-300-350-400V and two heater windings, 6.3V winding for signal tubes and a 0-5-6.3V winding for the rectifier. Comes in 2 versions: 300mA or 150mA.
Have to check prices, but would the 150mA version be sufficient in this circuit?
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Re: GSG 6A3 Build - Questions.

Postby Vapour » Sun Mar 08, 2015 4:52 pm

Another question - thinking now that I may build this as a convertible 6A3/300B, so just requires the different filament voltages.
Can't quite think of an elegant way to implement the 2x0R75 resistors in series with a switch. Any suggestions/ideas please?
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Re: GSG 6A3 Build - Questions.

Postby TomMcNally » Mon Mar 09, 2015 11:43 am

Just put two .75 ohm resistors in series in each channel. Then with a DPDT toggle switch, short across one resistor on each side.
Both resistors for 5 volts (300B) and one for 6 volts (6A3) ... simple enough.
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Re: GSG 6A3 Build - Questions.

Postby Vapour » Mon Mar 09, 2015 2:24 pm

Thanks Tom, I realise what's required, it's just the implementation. Would you put both resistors on the PCB & wires to the switch or wires from the board to a switch with the resistors? Or something else...?
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Re: GSG 6A3 Build - Questions.

Postby TomMcNally » Tue Mar 10, 2015 2:07 pm

I'd just put both resistors on the board in an upside down V formation, with the legs twisted together in the center,
and wire the switch across them and locate it in an out of the way spot. I've done it many times, works fine.
Having them up in the air also improves cooling.
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Re: GSG 6A3 Build - Questions.

Postby Vapour » Wed Mar 11, 2015 6:34 am

Thanks again Tom, that was one way I considered.
Another thought, is that the aforementioned transformer has the 0-5V-6.3V winding. As an alternative, could I wire it so that there is a switch in the supply between 5V and 6.3V before the board? Would that work?
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Re: GSG 6A3 Build - Questions.

Postby TomMcNally » Thu Mar 12, 2015 11:24 am

I'd go with the switch. Simpler wiring, and less current through the switch. I've done it with miniature toggles.
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Re: GSG 6A3 Build - Questions.

Postby Vapour » Thu Mar 12, 2015 2:17 pm

Ok, I'll go with that. Been mentioned elsewhere that the resistor value may need to changed if the transformer taps are rated higher than the stock transformers.
Mine are rated at 5A, so would it need higher value resistors?
Another transformer question - go with the 300V or 350V HT tap?
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Re: GSG 6A3 Build - Questions.

Postby Vapour » Tue Mar 17, 2015 6:06 pm

Just a BUMP for my questions above - any ideas please?
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Re: GSG 6A3 Build - Questions.

Postby battradio » Wed Mar 18, 2015 10:57 am

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Re: GSG 6A3 Build - Questions.

Postby Shannon Parks » Wed Mar 18, 2015 1:37 pm

Vapour wrote:Ok, I'll go with that. Been mentioned elsewhere that the resistor value may need to changed if the transformer taps are rated higher than the stock transformers.
Mine are rated at 5A, so would it need higher value resistors?
Another transformer question - go with the 300V or 350V HT tap?


I'd just change the resistors if you measure 10% high. I'd do the lower voltage - especially when using old stock tubes. So 600VCT.

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Re: GSG 6A3 Build - Questions.

Postby Vapour » Wed Mar 18, 2015 10:55 pm

Thanks Shannon.
Will see how it works out with the voltages (yes, NOS 6A3 but of course new 300B).
Chassis ordered, so hopefully done in 2 weeks or so.

Chris.
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Re: GSG 6A3 Build - Questions.

Postby Vapour » Tue Apr 21, 2015 6:05 am

Well I finished my build, did the smoke test. Unfortunately some came out!
The heaters lit up & I left it for a few minutes but not good. The R18 resistor had overheated & burnt the PCB. Seemed to be minimal damage to it but I removed it & measured, still ok at 10k.
Just to be safe I fitted a new 2W resistor & tried again. It started to discolour so I pulled the plug. Then noticed it was loose, so much heat the solder was melting.
Maybe due to the damage to the PCB?

Anyway, I checked a few voltages in short bursts & had 315VAc on J6 so that's ok, 285VDc on J7, 300Vdc/295VDc on J5.
Then checked heater voltages & I'm only getting 3.7VAc on both. Need to sort that, but would that lead to the R18 problem?

I'll need a new board too Shannon. =:o
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