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6GF7 and 300B amp

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crazyfrog

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Joined 2009
#1
Hi. I had in mind to build a 300B amp some day and saw something interesting with a 6EM7 as front tube. (Coincident Frankenstein and audiodesignguide comes in mind).
I chose 6GF7 ( novar equivalent) because it's now cheaper than 6EM7.
Interstage transformer (from private winder) on the diagram has been designed for low rp tubes (600-900 ohm) and current 40-80mA. It can also give 85Vrms (specs).
6GF7 plate current, triode 1: around 1mA (the plate resistor could change a bit depending of the power supply)
...triode 2: 46mA
300B plate current: 60mA (output of 8.3W @ 350V according to WE table)
LL1664 as OPT (probably). Guess primary inductance will be around 30H at that current (heard good things about this transformer for its price).
Rod Coleman regulators for 300B filaments
Suggestions? Comments?
Thanks!

note1: 6GF7A is smaller than 6GF7 (but specs seem the same).
note2: 6SL7 could be used as input tube instead of 6GF7, triode 1 ( I don't remember who tried this on Audio Asyl.)

https://frank.pocnet.net/sheets/049/6/6GF7.pdf
Western Electric
http://www.lundahl.se/wp-content/uploads/datasheets/1664.pdf
 

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#2
If you are looking at an interstage trafo, why not consider having it at the input instead..... it would reduce the ultrasonic and general broadband noise getting into the amp, and would eliminate ground loops around the DAC, which can be a real problem. And it would allow a differential link back to the DAC, which is more in keeping with the differential output stage of a high-performance DAC.

I think the trafo would make more difference in this position. 300B grids can be driven very well using a variety of power tubes - including current-production 6V6s. I don't find blocking-distortion to be a real problem in practice, but if you do, you can always add a FET follower.
 
#3
Actually I have a 01A line stage (low gain) with AVC at output. Do I need that 3 stages amp? Probably not. However, the triode 1 of 6GF7 is not pushed very hard and triode 2 is a low rp triode (good match with my IT) having a good drive (can swing 300Vpk if I'm right). But of course the input tube could be something else with enough headroom to kick the 300B.
Thanks
 
#5
LL1664 versions for a 300B

Hi. I'd like to try LL1664 OPTs in a 300B tube amp. Since I don't intend to run it hot (around 60mA), is there a reason other than inductance for choosing a 60-70mA version? Will a 80mA will "fit" well?
Thanks
Pierre
 
#9
cathode cap vs interstage: distorsion?

Hi. I intend to drive a 300B tube with 6EM7 (triode 2) via an interstage transformer and would put a bypassed cathode resistor for biasing. But it seems the capacitor can resonate with the primary inductance of IT inducing distorsion. Since I use Solen caps which are metallized polyprop. (ref. cathode resistor of 6EM7, triode 2), is there a data I should check?
BTW someone has his own idea of 6EM7 operating point in that situation?
Thanks
 
#10
There are many unknown values of the components, so there cannot be a conclusive answer. For example - cathode bypass capacitor value, interstage transformer inductances, secondary type of load.
Remember the triode's internal resistance dampens this potential LC circuit. Also it matters how the secondary is loaded. The primary's DC resistance also matters.

I wouldn't worry much about it at all, but you may give some details if you feel very worried.
 
#12
The other factor is that the interstage will drive grid current, putting the amp into A2 or AB2 and so produce more power and have a different clipping characteristic.

Perhaps the "resonance" is actually the effect of two cascaded filters, potentially producing a "bump" in the LF response at the corner frequency?? In which case change the value of the bypass cap.

_-_-
 
#13
Should not behave any differently than a cathode biased SE output stage. LF roll off behavior is determined by both the increasing capacitance reactance of the bypass cap and the decreasing inductance reactance across the transformer primary.

I'm not aware of any significant LF "resonance" issue with this topology. I would probably choose a bypass cap that results in the transformer primary inductance determining the LF cuttoff point.
 
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