Load line vs safe operating limits

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pcardoso73

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Joined 2021
#1
Hi all,
I am using a calculator on a website (https://www.vtadiy.com/) which is amazing. The figure attached comes from this website.
I know this is a silly question, but I am having a brain freeze now !
I would like to plot a load line for a given transformer with a primary impedance of 25000 Ω and sec of 8Ω (Hammond 1750A)
In this case, the load line is almost flat and I am concerned about reliability.

Since the biasing point is below the max Power curve, should I be concerned with such a slope? In principle this just means that AC can swing quite a lot.
Maybe the best would be just to choose another valve ....

Best regards,
Pedro
 

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#3
What if the loudspeaker cable disconnects from the amplifier output.
That is probabley worst case for Pentode and Beam Power output stages, whether Push Pull or Single Ended.
There may be Arcing.

That is a bad thing, generally whether the load line is flat, or the load line is steep.
And, sometimes no load will make an amplifier that employs global negative feedback, and is normally stable, but with no load, it goes into full bore oscillation.
 
#4
6A3sUMMER said:
What if the loudspeaker cable disconnects from the amplifier output.
That is probabley worst case for Pentode and Beam Power output stages, whether Push Pull or Single Ended.
There may be Arcing.

That is a bad thing, generally whether the load line is flat, or the load line is steep.
And, sometimes no load will make an amplifier that employs global negative feedback, and is normally stable, but with no load, it goes into full bore oscillation.
Click to expand...
Good to know.
And yes, the worst case scenario for stability happens for low loads. That is, when the current being pulled is low.
 
#5
I've compared the results given by that calculator to some amps I've built & their test results.
The calculator is in very good agreement with these measurements.

What are you building? Which tube(s) will drive the OPT? Sounds like a 6SN7.
 
#6
jhstewart9 said:
I've compared the results given by that calculator to some amps I've built & their test results.
The calculator is in very good agreement with these measurements.

What are you building? Which tube(s) will drive the OPT? Sounds like a 6SN7.
Click to expand...
Hi
Thanks for taking the time to verify the calculator.
I am just trying to do a single tube guitar amp (low power) and thought to use either an ECL84 or an ECF80 (since I have them). The triode would be the preamp and the pentode the power tube.
Cheers
Pedro
 
#8
With that high of a primary impedance, you could cut the bias in half and not lose output power. Which will only be about half a watt. Dissipation comes down.

Usually lowering the screen to reduce bias rather than dropping vg1 more negative will preserve gm (gain) to the extent possible.
 
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#10
If you are using pentodes without NFB and there is the risk of load being removed, use a shorting plug when the load isn't present. For triodes or pentode with NFB open load generally doesn't care. An alternate solution for the first case is to use a MOV in parallel to OT and with a rating about 2Ebb for the tube.

I wonder wh such a high load value. This tubes were used in phonographs, TV's and radio usualy used 5000R. Pentodes optimal load is taken as Ebb/Ib, say, the same as the DC RESISTANCE of the tube, NOT ITS INTERNAL SLOPE RESISTANCE.
 
#11
The high load R was likely an attempt to keep the output power low. It’s not a bad match for a video output tube or even a small signal/driver tube.

The problem however, is that even a 1 watt tube guitar amp is an eviction notice waiting to happen.
 
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#13
Osvaldo de Banfield said:
If you are using pentodes without NFB and there is the risk of load being removed, use a shorting plug when the load isn't present. For triodes or pentode with NFB open load generally doesn't care. An alternate solution for the first case is to use a MOV in parallel to OT and with a rating about 2Ebb for the tube.

I wonder wh such a high load value. This tubes were used in phonographs, TV's and radio usualy used 5000R. Pentodes optimal load is taken as Ebb/Ib, say, the same as the DC RESISTANCE of the tube, NOT ITS INTERNAL SLOPE RESISTANCE.
Click to expand...
Hi,
Thanks for your reply. I am using a 25000 transformer, because that's what I have available. Maybe that would be better suited to other tube ...
Cheers,
Pedro
 
#15
wg_ski said:
The problem however, is that even a 1 watt tube guitar amp is an eviction notice waiting to happen.
Click to expand...
You are so right. One watt into a typical guitar speaker is deafeningly loud. Loud enough to cause hearing damage.

For example, one single watt into an Eminence Rajin' Cajun 10 at 3 KHz will produce about 108 dB SPL. That's about as loud as a nearby jackhammer (!!), and 23 dB louder than the 85-dB threshold above which you start to suffer permanent hearing damage.

A few years ago, I breadboarded a roughly 200 - 250 mW tube amp. Plugged into the stock speaker (and cab) in my '65 Princeton Reverb reissue, even 200 mW was much too loud to overdrive in my apartment at night. Maybe on a weekend, if the neighbours were feeling charitable.

But for apartment-friendly clean tones, the 1/5th watt amp was about right.

I used the pentode section of a 6JW8 as the (single-ended) output tube, and the triode section of the same tube to drive it. There still wasn't enough gain for a tone-stack or overdriven sounds from both stages of the 6JW8, so I added a 6AQ6 triode input stage before the 6JW8.

The 6JW8 is a small-signal tube, and the pentode section has a maximum dissipation rating of only 1.2 watts. That's far more than I needed for this application, however.

The schematic is in post # 67 of this thread: https://www.diyaudio.com/community/threads/mini-amp-for-output-tube-distortion.326999/page-4

There was a final tweak to the schematic in post #73 on the same page.

I've finally managed to find a way to get clean and overdriven guitar tone at apartment friendly levels, but not by using tubes. Instead, I'm using a little Flama FS06 Preamp pedal. It sounds more "tubey" than many tube amps I've heard, and you can set clean or overdriven loudness exactly where you want it with no compromise in tone. (The output of the pedal has to be fed into a suitable flat-response full range loudspeaker.)

-Gnobuddy
 
#17
Who is worried about distortion from a guitar amplifier?

Only about 1/2 of electric guitar players. The other 1/2 of electric guitar players seem to want distortion.

I have had some guitar players tell me they want a low power guitar practice amp that is distorted, just like a high power model that is driven so hard (like when there is either red-plating or melted G2 screens).

This is an Instruments and Amps Thread (Some Clipping, Hard Rock, etc.; versus Some cool Jazz, etc.).
. . . It is Not a Tubes / Valves Thread (Hi Fi).
 
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