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Planning a new G*S*G

PostPosted: Mon Mar 01, 2021 11:47 am
by vvelt
Hi All, I just got one of the new GSG boards that jwhitmor made available (http://www.diytube.com/phpBB2/viewtopic.php?f=3&t=7050) and am thinking about building this with 300B tubes. This well made board allows for using the 6BY5 damper rectifier and 2 chokes. I could go with the standard power transformer with diodes for the highest B+, or I could go with a tube rectifier. My previous GSG build (http://www.diytube.com/phpBB2/viewtopic.php?f=14&t=6996) used an Antek transformer with the 6BY5 rectifier.

My question is would I really get that much more out of the amp with the standard power transformer and diodes or would output power and sound be about the same with a tube rectifier and less B+? I've made the following list of data to consider, maybe this will help others decide. Any thoughts would be appreciated, thanks.

6B4G/6A3
Plate 250v @ 60ma
Filament 1 amp

Vintage style has bi-plates, best with lower plate voltage
Modern style has mono-plates, can work with higher plate voltage

Vintage style includes the Russian 6C4C
Modern style available only from Sovtek, these are current production

300B
Plate 300 - 450v @ 60 - 100ma
Filament 1.3 amp

6AX5 rectifier, filament 1.2 amp
6BY5 rectifier, filament 1.6 amp

Edcor XPWR005 power transformer, 330v @ 180ma, 2 x 6.3v @ 2.5ma
Antek AS-1T300 power transformer, 300v @ 170ma, 2 x 6.3v @ 3ma

You can run any 6B4G/6A3 with the standard XPWR005 power transformer and 6AX5 rectifier. Note, using a diode rectifier will leave your voltage too high for these tubes but may be preferred for 300B.

Running 6B4G/6A3 with the standard XPWR005 power transformer and the 6BY5 rectifier will overdraw one of the 6v filament windings by .1 amp.

You can run any 6B4G/6A3 with the Antek AS-1T300 power transformer and 6BY5 by trimming the B+ with a dropping resistor. Could also run the modern mono-plate style tubes without a dropping resistor.

You can run 300B with the standard XPWR005 power transformer and the 6AX5 rectifier for a modest improvement in output power. Can also use diode rectifiers for maximum B+ and possibly more power*. If using diodes you should use a CL-90 or 500v rated filter caps.

You cannot run 300B with the standard XPWR005 power transformer and 6BY5 rectifier, you will overdraw one of the 6v filament windings by .4 amps.

You can run 300B with the Antek AS-1T300 power transformer and 6BY5 rectifier. This will not need a dropping resistor and you could use 2 chokes to boost B+ further.

*Shannon points out that running 300B's at higher voltage won't necessarily give you more output power because the 6SL7 tube can only drive them so much. See:
http://www.diytube.com/phpBB2/viewtopic.php?f=14&t=5176

If wanting to be safe with 500v filter caps they are readily available now at 105C with good ripple and hour life ratings. These were not common when the GSG first came out. See:

Kemet ESG157M500AQ5AA, 150uf
Nichicon UCY2H220MHD3TN, 22uf

Re: Planning a new G*S*G

PostPosted: Wed Mar 03, 2021 4:38 pm
by vvelt
Considering another option now. Use the Edcor XPWR005 I have on hand with an additional 6v filament transformer to supply enough current for a 300b and a 6BY5 rectifier. Verify the phase of the secondaries using the "spark test." Connect the primaries in parallel and 2 secondaries in series. If they measure 12.6 volts then they are in series, if 0 volts then they're in parallel. Keep the 450v rated caps without a CL-90.