Smartest way to 200W?

[Gaz]

I've built a bass amp using the this basic power supply:

DR_200wPS.pdf

My load voltages are 610vdc on the plates and 378vdc for the screens on four KT88s. I'm measuring 170W rms, and was hoping for more like 200W.

Since this is a bass amp, I'd like to bite the bullet and get a custom PT to get me the voltages necessary for 200W (or even more). I don't know how to draw load lines for pull push stages (Merlin had a page, but it seems to be down...), and don't wanna overshoot it and kill the tubes. I already have a custom OT which has a 2K primary.

I'd like to think I can just up the voltages to the values on the schematic, but don't know if that's the best way to do it for my particular OT and tube life...

Thanks for any words of wisdom.

[es345]

some questions and some hints

What is the voltage at HT1 when you measure 170W? - should be not less than 560V
How did you measure the Power - Voltmeter or Scope - ?

- Please share the complete schematic, that helps during the diskussion
- One way to get more power is to go to AB2 mode of operation

[Gaz]

some questions and some hints

What is the voltage at HT1 when you measure 170W? - should be not less than 560V
How did you measure the Power - Voltmeter or Scope - ?

- Please share the complete schematic, that helps during the diskussion
- One way to get more power is to go to AB2 mode of operation

Thanks for the response. The current PT has better than 10% regulation even with max signal, and that's why I think it's just a matter of upping the voltages. the amp does do a nice 270W at full output, but I'm looking for more headroom. The power supply is coupled with this output stage:

DR_200wOutput4.pdf

And this preamp:

DR_Pre4Input_v1a.pdf

So, standard Hiwatt, but less power

[loudthud]

The GEC data sheet shows 4500 ohm OT for 100W from a single pair, class AB1 with B+ 560V. Could your OT be too low? To find out, try fudging the load impedance slightly. Measure power out at 9 ohms (or 4.5 ohms on the 4 ohm tap) load. That would be the same as a 2250 ohm primary.

[Gaz]

The GEC data sheet shows 4500 ohm OT for 100W from a single pair, class AB1 with B+ 560V. Could your OT be too low? To find out, try fudging the load impedance slightly. Measure power out at 9 ohms (or 4.5 ohms on the 4 ohm tap) load. That would be the same as a 2250 ohm primary.

The Datasheet is showing ultra linear operation at 560vdc on plates and screens. I'm running the screen much lower. I know I have to up the voltages on plates and screens, I just don't know how much...

[printer2]

Not to be a spoilsport but how many dB louder will your speakers be driven with 200W rather than 170W?

[Gaz]

Not to be a spoilsport but how many dB louder will your speakers be driven with 200W rather than 170W?

You're right, not much. It's just that the current PT is salvaged, and I don't mind getting a new one for a true 200W, and using the one I have now for something else. I kind of thought it'd be a good excuse for me to understand push load lines as well - I can never learn things unless I have a pressing reason

[loudthud]

Link to data sheet: KT88

At the top of page 3 the data is for tetrode connection with 300V screen supply. Page 4 shows the ultra-linear connection with 553V Vg2.

[Gaz]

KT88

At the top of page 3 the data is for tetrode connection with 300V screen supply. Page 4 shows the ultra-linear connection with 553V Vg2.

Ah, I see it now. Do you really think raising the primary impedance of the OT would raise output power? Although I've already revealed my ignorance on the subject, my practical experience tells me it wouldn't. the only other possible difference I see is that I'm using 1K screen stoppers - not sure what the datasheet assumes, but they could be reducing my clean rms power.

Since I want to keep my current OT, I'm really looking for how to calculate what voltages would be needed to get 200W without killing the tubes.

[loudthud]

It would seem that the 2K OT would work at a lower voltage unless the tubes can't conduct enough current. Is the heater voltage high enough? Measure it right at each tube socket. Does the screen voltage drop too much because of screen stoppers? You can look at it with a scope to see how far it drops at 170W. A DVM doesn't tell the whole story. Something is holding this amp back from making more power. I was just trying to rule out the slight mis-match at the plates.

[J M Fahey]

170W is the same as 200W.
Not distinguishable by ear at all.
I don't understand

the amp does do a nice 270W at full output,

And you are getting 8.5% less voltage than expected , a trivial difference.
I might use a cold rainy weekend to tweak the amp a little, why not?.
Specially if there's nothing good on TV.
But I would not spend $$$ on a new transformer.
Personal opinion, of course.

EDIT: If anything, I'd raise plate voltage by 10% (yes, I already said the differennce is trivial) but keep screens as-is, simply on safety reasons.
And also play with the load impedance.
Of course, since a normal 8 ohm speaker goes from 5 or 6 ohms at mid frequencies to 16 ohms at higher frequencies and up to 40 or 50 ohms at resonance, you can see that small 10% or 20% nominal impedance tweaks are not *that* significative in actual use.

[Gaz]

It would seem that the 2K OT would work at a lower voltage unless the tubes can't conduct enough current. Is the heater voltage high enough? Measure it right at each tube socket. Does the screen voltage drop too much because of screen stoppers? You can look at it with a scope to see how far it drops at 170W. A DVM doesn't tell the whole story. Something is holding this amp back from making more power. I was just trying to rule out the slight mis-match at the plates.

Heater voltages look good at every socket. I just measured voltages at clip, and plates drop to 580vdc from 610vdc. Screens drop to 350vdc. I actually only measured 25.3vac into a into a 4ohm load, so more like 160W. I'm confident everything's working properly, however I would love if there was something I overlooked. Seems like issue with the power supply and output stage just jump out at you though, and everything seems in order. FWIW, the real DR201s had high voltages and put out 200W, so I don't see why I should expect more power at lower voltages.

[Gaz]

170W is the same as 200W.
Not distinguishable by ear at all.
I don't understand


And you are getting 8.5% less voltage than expected , a trivial difference.
I might use a cold rainy weekend to tweak the amp a little, why not?.
Specially if there's nothing good on TV.
But I would not spend $$$ on a new transformer.
Personal opinion, of course.

EDIT: If anything, I'd raise plate voltage by 10% (yes, I already said the differennce is trivial) but keep screens as-is, simply on safety reasons.
And also play with the load impedance.
Of course, since a normal 8 ohm speaker goes from 5 or 6 ohms at mid frequencies to 16 ohms at higher frequencies and up to 40 or 50 ohms at resonance, you can see that small 10% or 20% nominal impedance tweaks are not *that* significative in actual use.

Juan, I agree, but as I said in my last post, the amp's really doing 160W. I'd like to think 40W would give me a little more clean before break up... You may disagree about that though.

I appreciate the comment about not raising the screen voltage, which is what I meant by "smartest" way to 200W. Of course I don't want to burn through KT88s, but it seems like my power level right now is a bit conservative, and I'm no conservative

Can anyone point me to a good digestible source on how to draw push pull load lines?

Thanks again everyone for the comments.

[Steve Conner]

On the contrary I'd say that increasing the screen voltage is the easiest way to increase the power. 358V is awfully low especially with 1k screen resistors dropping the voltage further.

The maximum safe screen voltage depends on the load impedance. The higher the impedance, the more careful you need to be with screen voltage, especially in an amp that gets overdriven a lot.

Simpler than drawing a full load line, you can calculate the instantaneous tube drop and current at the peaks of the waveform with the amp driven to the edge of clipping. Say it turns out (I'm just making these numbers up) that to make 200W, each tube has to have a voltage drop no greater than 50V while passing 400mA. You can look at the tube datasheet to see what screen voltage is needed to make that happen. If it ends up some really high voltage, there may be a risk of burning out the screens and you may need to start again with a higher B+ and a higher OT impedance, so that 200W can be achieved at a lower plate current.

[printer2]

Juan, I agree, but as I said in my last post, the amp's really doing 160W. I'd like to think 40W would give me a little more clean before break up... You may disagree about that though.

I appreciate the comment about not raising the screen voltage, which is what I meant by "smartest" way to 200W. Of course I don't want to burn through KT88s, but it seems like my power level right now is a bit conservative, and I'm no conservative

Can anyone point me to a good digestible source on how to draw push pull load lines?

Thanks again everyone for the comments.

160W with a 98dB speaker is 120dB at 1 meter. 200W is 121dB. Not to say you should not play around and see what you can get and learn, but in terms of practical results you may trade off some tube life for a minor gain in output. Me, I am cheap.