"Jonokuchi" - new desktop amp, or "Little red board"?

[frozenoak]

One last quick question. In the beginning I will be running this amp into a pair of 8 ohm 91db bookshelf speakers. After I build a phono pre-amp I plan to upgrade, possibly build, a better set of speakers. Is the specked GXSE10-8-8k (which I can't find on their website) still the desirable OPT or should I look for another one?

Regards,
Dale

 

[mike567]

The Edcors will be hard to beat but you could consider using transcendar TT-235-OT or TT-236-OT 's. More $$ but multi output impedence taps and are faster to get than Edcor.

edcor listing: https://www.edcorusa.com/gxse10-8-8k

 

[Burnedfingers]

Question if I may..

Wasn't the 13EM7 amp first designed by Fred Nachbaur? If memory is correct he designed it using common parts and used several 12v transformers to obtain both the heater voltage and a doubler on the second transformer for his B+?

Just wondering.

 

[mik000000]

any idea what the current draw is? i am casing mine and am going to use an E24 power switch. i have some relays on hand rated for 1A and not sure if this will be enough

 

[lordearl]

Mine seems to use about 40W, if that helps...

 

[mik000000]

i have finished (almost) the casing and remounted everything. i powered it up and have power to the tubes, but not the bias led's and no sound.

any suggestions?

 

[mik000000]

found it.... loose contact from xformer

 

[Bradrock]

Is anyone using this amp with a turntable? Would you need a seperate phono preamp?

Thank you, Brad

 

[audiowize]

The Jono needs a phono preamp to work with a turntable.

 

[Bradrock]

Thank you. Any budget friendly reccomendations?

Edit; I have an Onkyo P-306RS preamp sitting here that should work.

 

[Pegasus123]

I purchased this board to use with my Plitron 3050-SE output transformers. Can this board be modified to run 6DN7's with a few resistor changes?

 

[audiowize]

Changing resistors and output transformers then sure, you could make the board work with 6DN7s.

 

[Pegasus123]

Run at the same voltages as the 6EM7 does anyone know what resistors in the BOM to change and to what values for the 6DN7?

I'll be using the 4-ohm tap to present a 10K load which will satisfy both tubes.
I would really like to try both to see which I prefer.

 

[audiowize]

You are making a lot of assumptions that running an 8 ohm load on the 4 ohm tap will actually present 2x the load impedance. I would absolutely assure you that this will not happen at all at low frequencies.

 

[Pegasus123]

Would you mind teaching me why this is? The Jeff Bagby Continuum speakers have a solid 8 ohm impedance so on a 4-ohm tap why would this not be a 10K transformer? And for 300 ohm headphones on a 4-ohm tap why would this not double the impedance of the transformer compared to an 8-ohm tap?

 

[audiowize]

Here's an image of an output transformer's impedance with the secondaries shorted, open, and appropriately loaded.

You can see that in the open loading scenario, the impedance has a huge hump in the middle of the pass band and dips down to barely more than the loaded line at either frequency extreme. If you have a triode operating with an Rp of 2K and you select an output transformer that is 6K or more, then you are not going to see much of a gain variation across that giant hump in the unloaded scenario. If you take the 2K Rp of the beefier half of the 6DN7, you'd want each end of the hump to be at about 6K. If you have a 5K transformer, and worse maybe a 5K transformer that is already much less than 5K at 20Hz, when you run it unloaded with too much Rp driving it, you will hear that uneven response far more.