yagoolar's tube amp projects
 

Trying hard to come back to tube amps after some busy time since my last build (RDL by SY). I had collected a lot of tubes and decided to build something easy, small. Must be SE, parafeed and with Po not more than 10W. I have Svetlana 300B, 6C4C, C3g, 6H9C, 6C4Pi, 6C45Pi among others (GM70 as well, but still too little place to build such monster and too scared).
Ok, I am designing C3g+300B SE Parafeed with LEDs in cathodes.

I would like to read your opinions on the posts while making progress.

TIA

Schematic - ver.0.1
 

Schematic - first approach

I think of replacing R3 with CCS in the future.

wouldnt it be more like 5-6watt with parafeed 300B ?

Something around 5,7 W w/C3g. Yes

Operation point
 

Found a reply on another forum regarding C3g and 300B from Thorsten Loesch: Tube DIY Asylum - Re: C3g as driver for 300b

I initially wanted to go 390V/60mA/-61,5V - and there is 430V/80mA/-77V in the post. The only 300B I have are 2 matched (?) SV300B from Svetlana Electron Devices Inc. (Made in Russia) dated Dec 2001. The parameters shown: Ia = 57|58 mA and S = 4,7|5,0 respectively. I wonder what was Ua...

... and convert C3g to triode connected, of course.

Started some output stage simulation with LT Spice. It seems that OP xformer params are critical to bandwitdh. And there goes the question: a transformer is represented by a couple of parameters, one of them is K coefficient - what is a good method to measure one in practice? My transformers were wound a few years ago and there is no chance I can find specs.

A critical factor in a parafeed stage is the type of core you have in the OPT. The best type is the amrphous core because it doesn't store energy at low frequency. Never forget that decoupling cap + primary inductance = resonant circuit! The type of core is just critical as the cap.
45

Be very careful with the SV300B from this era as they tended to develop intermittent shorts between the filament and grid. Hopefully these are the later version (series 3). FWIW these sounded great in PP, and not so good in SE. (SED furnished me with 8 of these for an amplifier design ultimately published in VTV, of these 6 ultimately failed.) I much prefer JJ 300B which have given great service for nearly 10yrs now.

How about fixed bias? Allows for very easy and rather revealing tuning of the operating point. It also eliminates some parts in the signal path at the expense of some additional (limited) supply complexity.

And the Ia listed in the matching data are relevant to the matching only, and with good ones you can run them at any current that stays below 90% of the rated dissipation, and I have found generally the hotter you run them the better they sound. (and the more power you get at a given thd %)

If you want a quiet amplifier heat the 300B with DC, and check out CCS based heating. Rod Coleman here has an excellent design just for this purpose.

How can I tell they are "series 3"?