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Old 18th February 2012, 05:29 PM   #11
diyAudio Member
Join Date: Jun 2011
NFB is debateable. always try to build the circuit perfect. if you start clipping the signal through the path, then apply NFB.
your schematic does not have it. So what. Lets fix whats really wrong instead of lowering the gain characteristics to hide problem.

for a heater transformer I would use a hammond 166ms (5vct 3A) for a each tube. the reason to stray away from 6.3V is less tube noise on the plate and better gain characteristics over the life of the tube.

now a variant of this is to take a transformer that has class F insulation split/dual bobbin type that has two 115V primaries and two 6V secondaries 50VA. Tie the primaries in series (230V operation) but run it at 120V. now we add a layer of rfi by adding a .01UF 1000V cap from the primary's center tap to the FG or earth ground (optional). Tie the two secondaries in series (normally 12V) now you have 6VCT but with core loss, this can be an advantage as any imbalance of heater winding tap will be absorbed in the heater winding core. the reason because of the higher current rating is mainly to lower the DCR of the secondary. Another effect is the extra inductance that bucks against the heater and secondary causes a small amount of heater regulation to be established as the thermionic emission will dynamically change with the signal. This sponginess works really well. And can be increased by putting 5-15mh chokes between the heater pin and the transformer connection.

in a push pull DHT amp I always recommend using separate transformers: one for the inverting and one for the non inverting side of the output section.

Last edited by DavesNotHere; 18th February 2012 at 05:32 PM.
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Old 14th October 2012, 07:32 PM   #12
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Join Date: Mar 2011
Location: Somewhere in Quebec
hi dave.
so I need to get a hammond transformer for each output tube? so four in total?

also, is the 6a5G is a direct replacement in my amp or I need to get new sockets for those tubes?

but im not sure if you stil think I should change my 6b4g?

Last edited by murphythecat8; 14th October 2012 at 07:36 PM.
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Old 16th October 2012, 12:10 AM   #13
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