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The U-6V6

Approaching the end : NFB C and R, Pilot Lite, and input shielded cable are still to be installed... Oh, and the fuse in his socket !

l54bXb-P1250795.jpg


T
 
64ncXb-U-6V6-juste-fini-a-tester-03-09-2026-1.jpg

tubelectron said:
Wiring is finished this evening... Awaiting checks, measurements, test and... Possible debugging ! :biggrin:;)

T
Click to expand...

:thumbsup:


The mil spec 6V6GTY is one of my all-time favorite tubes. Run it with the least feedback constraints possible to reveal its full range of sonic glory.

How do you like all those flowery adjectives?

6b770e47-c8c1-4ef9-aab2-b337fdafa89d.gif



6v6gty_Raytheon.jpg
 
Thank you Guys ! :)

Lavane said:
Verey nice. What did you use to punch or cut the cap can hole?
Click to expand...

I drilled three holes at 10mm and used a jigsaw to cut the rest. A bit of filing and voilà... This is masked, so no real importance !

I suspect some bugs : voltages lower than expected values... I will have to clear out these points ! :idea::idea::idea:

T
 
Square Waves at 2WRMS rated output = 4.00VRMS @8R :

F6WcXb-U-6V6-SQ-2W-05-09-2026.jpg


Measured bandwidth at 2WRMS (sinus @400Hz ref 0dB) :
@-1dB = 20Hz - 33kHz
@-3dB = 12Hz - 48kHz

Damping factor : Aw ! Forgot to measure it... Certainly low : pentode operation with only 6dB NFB... Circa 2 maybe ?

THD : no way to measure it for the moment... the usual few percents, I guess...

Well, reasonable performances, given the size and the simplicity of the circuit !

Outside pics :

hLWcXb-U-6V6-finished-out-05-09-2026.jpg


T
 
OK : listening test last evening and this afternoon...

With my large 375L speakers (3 ways / 8R / 12"+8"+comp / 94dB/W/m)

1788803274476.png

The sonic results are bluffing :oops:... Relatively - of course - to the tiny size and the modest resources used in the construction of these amplifiers.

Nothing anemic, nor lack of this or that : way punchy and loud enough. A bit surprising for a 2-watters basic cathode bias / 250V supply / Pentode operation...

Du coup, I re-envision the possible Triode operation switch... So I will make tests and measurements ! Schematic update have to wait... ;)

T
 
DirtyErnie said:
I have dreams of using a multi-way gang rotary switch to handle screen grids, bias voltage, and NFB resistors....
Use it and you owe me a decent meal. ;)
Click to expand...

I suggest La Brasserie d'Alice :

1788849943404.png

This reminds me to check in each mode the damping factor and in Triode mode the level of NFB ! :thumbsup:

In most of the unecessary large SE chassis amps (DIY or not), yes, you could certainly install a big rotary switch as you think about, @DirtyErnie. One delicate point would be to make it silent operation swictching, in order to avoid Peks/Schkrrs/Plops/Oommph in the speaker when you switch the settings, though ! Or you accept to add a SBY switch which cuts the speaker output, that you activate when you want to change the setting of the rotary switch... ;)

T
 
PENTODE vs. TRIODE mode comparison : At LEFT = PENTODE, At RIGHT = TRIODE

1788860692709.jpeg

In PENTODE :
- power output = 2WRMS
- RK = 270R
- NFB level = 6.6dB

In TRIODE :
- power output = 0.9WRMS
- RK = 470R
- NFB level = 5.4dB

Same input signal 600mVRMS sinus 400Hz.
Same output load 8R 200W.

To shift from PENTODE mode to TRIODE mode, it requires :
- switching RK from 270R to 470R.
- switching the screen from 6K8 to direct Plate connection.

OK... Why not ? I'll have to purchase a pair of nice mini-DPDT ON-ON switches... My stock of mini-switches is weak these times ! :confused:

T
 
Can minimize or eliminate the change in resistor value if the screens connect to the same B+ point as the output transformer does. That should have all idle voltage the same in both modes, so same bias needed. 6.8K is an awful large screen resistor too, normally its not more than about 1K specifically to keep it from sagging and causing performance issues.

That will also get more power in pentode mode. I'd expect closer to 4 watts out of a 6v6 at reasonable voltage.
 
gadget73 said:
Can minimize or eliminate the change in resistor value if the screens connect to the same B+ point as the output transformer does. That should have all idle voltage the same in both modes, so same bias needed. 6.8K is an awful large screen resistor too, normally its not more than about 1K specifically to keep it from sagging and causing performance issues.

That will also get more power in pentode mode. I'd expect closer to 4 watts out of a 6v6 at reasonable voltage.
Click to expand...

OK @gadget73 - I'll do the test and see how it behave : maybe a good surprise in sight, even if I'm rather skeptical...

The 6K8 value has been chosen to have a screen and plate voltages close at idle and max output, and yes it's unusually high.

T
 
A 'Standby' switch and the brainpower to remember to use it before moving the rotary switch would suffice.
We would all fail that test at some point.
 
The shorting of the 6K8 screen resistor offered to reach a 2.6WRMS output instead of 2.0WRMS. I opted for 470R, which limited the power to 2.5WRMS for 600mV input. Reaching 4WRMS would have required 300-350V of plate supply, which my little potted power transformers are unable to offer.

Triode-Pentode mode is under work...

T
 
End of the story : updated schematic :
1789144780472.jpeg
  • The triode/Pentode switch can be operated on the run with nothing than a little "click" in the speakers when silent or at low volume.
  • The loudness level shows as expected a circa -3dB drop between Pentode to Triode (roughly half the power output).
  • The damping factor difference between Pentode and Triode is moderated, thanks to the action of the NFB.
  • The power output is largely enough to drive a pair of 94dB/W/m speakers, both in Pentode and Triode mode - at least in my little Auditorium !
Which mode sounds the best : Triode or Pentode ? :idea:

It's not so easy to tell - honestly :oops: - once you have found the correct loudness balance between the two modes for immediate A/B comparison... That said, we all know that - basically - the sound of a Triode is usually better than the sound of a Pentode. Since the Ultimate 6V6 is a basic design, it certainly makes no exception to the rule ! ;)

T
 
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