SE KT77 (EL34) power questions

[ThaZ]

Hello,

I've some questions about my DOUKaudio EL34 SE amp.

I use now KT77 tubes.

I changed the biasresistor to 150ohm (was 330ohm).
Schematic and measured values are attached.

My problem is that the distortion of the 1Khz sinewave at the output is comming up at 6.2Vrms = 4.8W
I think this must be go twice, around 10W at least ?!?

1. Are the voltages good? Vpin3=333VDC  Vpin4=191.5DC  Vpin1=9.3VDC (gives 62mA)
2. Is the output transformer sufficient/good? (3.5K primary impedance) 
3. Powersupply of 353.5VDC enough ?
4. Is there a method to measure the output transformers rate (watt)?

Thanks !

[shooter]

the distortion of the 1Khz sinewave at the output is comming up at 6.2Vrms = 4.8W

Fwiw;
I have a KT88, about the same #'s, start's to distort ~~ 6.5W, roars like a lion at 16W, but it's meant as a guitar amp

what is the dynamics of your speaker, a really good spl should get you to ~10W of sound

[ThaZ]

what is the dynamics of your speaker, a really good spl should get you to ~10W of sound

I measured it with a 8ohm load. Also at 100Hz or 400Hz etc etc.
I use it for HiFi music listening.

KR, G

[shooter]

measured it with a 8ohm load

scoping the signal at speaker?
A SE amp is only gonna yield at best 45 - 50% of plate dissipation, for clean audio, expect 35-40%.  IF you need loud, push-pull A1 biased gets you about as much as one could hope for.  possibly AB1 but that requires more tinkering.
I typically use a speaker in the 102db spl range, gives a "volume boost" over say a 91db spl one.

[ThaZ]

scoping the signal at speaker?

Scoping at the 8ohm load resistor..... I don't use a speaker for measuring (its no listnening to speakers ;) ).

So I wanna know (startpost):  what could be wrong in de schematic why I dont get more as 4.8Watt without the sinus distortion.

[shooter]

the schematic is probably fine from what I see.  your "limit" is clean swing
the voltage at the cathode of the 77 is essentially a "limit".  same for plate voltage.

I use the Vk to "set my drive" into the PA.  from there the PA tube can only swing so much volts before it hits those limits, once at the limits, the tube compresses, eventually clipping.
so to get more swing raise B+ and re-bias the cathode R.  But there's a limit for clean audio, even with 410vdc on my 88 plate and ~ 40vdc at cathode I only get 6.5W clean audio, 18W full roar.

[jjasilli]

Some ideas:


*  make sure you have proper, clean signal voltage into the power tube grid1.
*  screen voltage seems low and can probably match plate voltage in your low B+, SE operation.  See:  http://tubedata.milbert.com/sheets/086/k/KT77.pdf
*  are you sure the 8 ohm dummyload is connected to the 8 Ohm tap?

[ThaZ]

1.  make sure you have proper, clean signal voltage into the power tube grid1.
2.  screen voltage seems low and can probably match plate voltage in your low B+, SE operation.
3.  are you sure the 8 ohm dummyload is connected to the 8 Ohm tap?

1. There is distortion because of the feedback signal 2kohm from the output of the tranformer. I disconnected the 2k resistor and the distortion at anode of 6n9p was almost gone. (i'm gonna remeasure it for sure)
2. Yeah, you mean the 191.5V isn't it ? I was thinking of that too. Maybe decrease the 10kohm resistor ? (104V at the Anode of the 6n9p is also low (104V) if you compare it to the schematic original 120V.
3. Yes I'm sure about it.

[jjasilli]

Maybe the NFB is positive instead of negative; are the OT primary wires reversed?

[ThaZ]

Maybe the NFB is positive instead of negative; are the OT primary wires reversed?

How can the NFB going positive in a "factory" assembled amp ?
OT wires are original: red on anode, blue on B+

[shooter]

as long as you're measuring;
start at the input with .5VAC rms
measure at the 6n9p plate
increase the input signal till it starts distorting
what is the ac rms?
anything > ~8VAC should start dragging on the 77, which will drag on the 9p
why did you drop Rk from 300 to 160ohms?  more current doesn't equate to more clean

[PRR]

> must be go twice, around 10W at least ?!?

Why?

I estimate 5W-8W.

You could try Rk 100 ohms, 90 ohms, until the power transformer smokes.

But there is not "a lot" more power available here.

The output transformer is not limiting you at 400Hz-1kHz. (It will be a problem for deep bass.)

[ThaZ]

> must be go twice, around 10W at least ?!?

1. Why?
I estimate 5W-8W.

2. The output transformer is not limiting you at 400Hz-1kHz. (It will be a problem for deep bass.)

1. KT77 must be up to 10W in penthode (thats what i presume) ? But yeah 8W is pretty close to 10W.

2. That's what I think indeed, its a cheap chinees OT (see attachement). Is there a method to test his power ?


 

[ThaZ]

as long as you're measuring;
start at the input with .5VAC rms
measure at the 6n9p plate
increase the input signal till it starts distorting
what is the ac rms?
anything > ~8VAC should start dragging on the 77, which will drag on the 9p

1 .why did you drop Rk from 300 to 160ohms?  more current doesn't equate to more clean

Nice, I go testing soon ! Thanks !

1. My thoughts: Running the KT77 a bit hotter (more power) ? But this isn't true ?  Better go back to 330ohm or .....?

[ThaZ]

start at the input with .5VAC rms
measure at the 6n9p plate
increase the input signal till it starts distorting
what is the ac rms?
anything > ~8VAC should start dragging on the 77, which will drag on the 9p

What I did (without KT77 tube in the socket, no feedback):

0.5VAC rms 400Hz sinewave at the RCA input (volumeknob to 0)
start turning the volumeknob rotating to the right, measering Vrms~ at the plate (p5) of the 6n9p.

When I had the volume at 3/5 of max volume, the measurement is about 8Vac rms (and this is indeed the position of the knob when I normally measure distortion at the output).
Then turning the knob at max -> 22Vac rms is what I measured.

If I do something wrong, please tell me ;)

[shooter]

guessing the 22vac was clean.

OT is about laminations, wire, things beyond me 
In my audio stuff, I try for OT 2X power and UL

what's your objective?
we've proved enough, you have what you can get +/- 1W or so

so, short of re-design, moo money, moo time.  hook it up to a good 100+ spl audio speaker and enjoy :)

[ThaZ]

1. guessing the 22vac was clean.

OT is about laminations, wire, things beyond me 
In my audio stuff, I try for OT 2X power and UL

2. what's your objective?
3. we've proved enough, you have what you can get +/- 1W
so, short of re-design, moo money, moo time.  hook it up to a good 100+ spl audio speaker and enjoy :)

1. I measured it with a digital Philips true Vrms meter.
2. I will make sure that there are no "faults" in this design. And i like measuring etc etc ;)
3. Yeah maybe true, AMP sounds good by the way ;)

Shall I put the original Rk (330ohm) back in your opinion ?

[tubeswell]

Try doubling the reflected load. For SE, I get a better load line with 6k, where Va=333, Ia=62mA, Vg2=191.


https://www.vtadiy.com/loadline-calculators/loadline-calculator/

[ThaZ]

Try doubling the reflected load. For SE, I get a better load line with 6k, where Va=333, Ia=62mA, Vg2=191.
\

Wow, OK thanks! The reflected load = primaire impedance of the output transformator (now it is 3.5k) ?

[ThaZ]

Measured/calculated the output transformer's primary impedance: 4.7kohm

(20.9VAC rms 50Hz input -> 0.874VAC rms output)

It's been spec. at 3.5kohm .......mmmmmm

[tubeswell]

4k7 will be great for how you’re running it.

[shooter]

fwiw;
once I'm up and working I use a "muti-tap" dummy load from ~4Ω to 20Ω and "walk" it around my original design load.  My last build tube datasheet wanted 10k:8Ω
it peaked at ~14Ω  (80hz to 4K guitar range) and did "increase loud" enough to notice using a 16Ω speaker.  audio spectrum is more a trade off, that's where quality speakers become way more important.

[ThaZ]

start at the input with .5VAC rms
measure at the 6n9p plate

I remeasured this with the scoop:

Vinput 400Hz 183mV rms -> 788mV rms at anode 6N9P
Vinput 10Khz 2.2V rms -> 6.4V rms at anode 6N9P

without any clipping of the signals !

FYI

[shooter]

Vinput 10Khz 2.2V rms -> 6.4V rms at anode 6N9P

I could be all wet so..;
inputs from most "AUX" audio sources are ~~ 1V rms, Ideally you want your 6.4v with 1V rms input  (think it's called input sensitivity  ) 

[ThaZ]

inputs from most "AUX" audio sources are ~~ 1V rms

Spec from amp: input sens: max 2.0V (and the potentiometer also in the path)

so....... no problemo, the fact was that I measured it with a digital RMS meter.

So the result is quite diff.......;)