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Old 15th June 2019, 11:16 PM   #31
Troy Madden is offline Troy Madden  United States
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Output transformer is a Hammond 1650T, primary impedance is 1.9K. Ultralinear taps to screens through 220 ohm resistors. Biased to 60mA, B+ to output tranny about 485V under load. Im not well versed on calculating load lines and output tube operating parameters as you can tell! I wont be using it for low end, I bi-amp and use a 100+lb sand amp for bottom end, LOL. In my system I will use 4 ohm speakers, on the 4 ohm taps. Although after hearing this thru a 12" full range Eminence in an infinite baffle, I' wondering if I did my home theater all wrong ! The bass was surprising good, and the sound was so much more coherent.
PS, I figured the lower distortion with a lighter load was normal tube behavior, IE, they don't like being loaded down. But then the output tubes are pentodes, and the behavior of pentodes is a bit of a mystery to me. They behave one way connected as triodes, another way with regulated screens, an another way in ultralinear.

Last edited by Troy Madden; 15th June 2019 at 11:30 PM.
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Old 16th June 2019, 12:10 AM   #32
wg_ski is offline wg_ski  United States
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Depends on the pentode. Some really do have less distortion with higher Z loads. Some have a clear optimum. And sometimes you’re interested in maximum power or use of iron, other times it’s secondary.

Coherent sound is a characteristic of good full range drivers. Even if the frequency response isn’t flat. Multi way can be coherent but it NEVER gets that way by accident.
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Old 16th June 2019, 03:55 AM   #33
thoglette is offline thoglette  Australia
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Unless you know what harmonics these are, THD numbers are next to useless.

If it's 2nd and 3rd, those will be almost inaudible. But if they're 7th and up, probably audible.

What's the sensitivity of your speakers? Where's your noise floor? What's your target peak SPL? These set how often you'll get into these levels of distortion.
<rant>
Read your Hamm and Crowhurst. JAES 1973 21(4) and 1957 5(4) respectively.
</rant>b
Quote:
Originally Posted by wg_ski View Post
Wouldn’t be “a little odd” if it effectively becomes class A. Which further pushes down the available current/power because the load becomes Ra-a/2 not /4..
This is the big compromise - pushing it into A1 will drop watts but clean up everything.

Last edited by thoglette; 16th June 2019 at 04:06 AM. Reason: Read through the pages of comments I missed (duh!)
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Old 16th June 2019, 07:26 AM   #34
Diabolical Artificer is online now Diabolical Artificer  United Kingdom
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I got similar THD readings with a 1650TA OPT but with 18dB of NFB. I used six EL34's and a similar driver arrangement as yourself. I used ECC99's/6H6N's paralleled with a MJE340 CCS from a negative supply of -100v (80v loaded). The OP stage is direct biased adjustable. Grid R's are 47k with grid leak R's of 2k2, g2 R's 220r.

A balance pot is an excellent idea, I used a ganged 10k lin pot/s, makes balance adjusting less fiddly. I used two of those little 4mm banana test sockets on the chassis to make nulling easier. You basically connect your DMM from g1 to g1 and adjust/null trying to get as near 0v as poss. I used 1 turn Bourns pot/s, 10 turn would have made adjustment easier as you can't get it bang on but it's not really needed.

The Hammond 1650TA is a good OPT, I got 145w out of my OP stage albeit briefly, the OPT didn't get hot. Bass response is good down to 5hz, flat to 30khz so a decent not expensive OPT.

Re matching you can play with the value of your dummy load to see if it improves things, try lowering it a tad to say 4ohms on the 8 ohm tap or increase it. The old datsheets for the EL34 and KT88 made matching a lot easier as typical anode resistance was given for various conditions, the Tungsol datasheet http://www.tungsol.com/specs/kt120-t...ecs-curves.pdf - I found didn't have these figures, it's a bloody joke as far as datasheet's go. The Valve Wizard site has some good info on working out anode resistance for beginners, more thorough analysis is in Crowhurst and The Radio Designers Handbook.

I'd advise fitting a protection circuit that cuts power to the mains tfmr if Ik on any valve is exceeded, also fit reverse diodes from anode to ground to protect the OPT if no load is attached, a 1k resistor across the OP terminals does the same job to a point.

Lastly make sure to test for stability, this is the trickiest aspect to get right but with a few tricks not too hard.

Good luck, Andy.

Last edited by Diabolical Artificer; 16th June 2019 at 07:30 AM.
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Old 16th June 2019, 12:27 PM   #35
Mark Tillotson is offline Mark Tillotson
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Quote:
Originally Posted by thoglette View Post
Unless you know what harmonics these are, THD numbers are next to useless.

If it's 2nd and 3rd, those will be almost inaudible. But if they're 7th and up, probably audible.

That's pushing it a bit - single-tone tests are very limited, in any realistic signal intermodulation distortion is an issue, and 2nd and 3rd order IM products are as audible as 7th or 8th order (for the same amplitude), they all can fall at non-harmonically related frequencies.
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Old 16th June 2019, 12:37 PM   #36
Troy Madden is offline Troy Madden  United States
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Quote:
Originally Posted by thoglette View Post
Unless you know what harmonics these are, THD numbers are next to useless.

If it's 2nd and 3rd, those will be almost inaudible. But if they're 7th and up, probably audible.

What's the sensitivity of your speakers? Where's your noise floor? What's your target peak SPL? These set how often you'll get into these levels of distortion.
<rant>
Read your Hamm and Crowhurst. JAES 1973 21(4) and 1957 5(4) respectively.
</rant>b

This is the big compromise - pushing it into A1 will drop watts but clean up everything.
It's my understanding that triodes produce primarily 2nd order harmonics, with pentodes you start introducing 3rd order, higher harmonics are created when you use feedback. Its also my understanding that our ear/brain doesn't mind the even order harmonics but the odd order harmonics sound harsh. I usually just measure 2nd and 3rd order, because those will be the primary harmonics in a tube amp with no feedback. If I get 2nd and 3rd as low as possible without feedback, it stands to reason the higher harmonics will be as low as I can get them also. I can measure way up the spectrum, and measure individual or defined bands of harmonics, but at this point that wouldn't help me much. If I get 2nd and 3rd as low as I can, the rest will follow. The speakers it will eventually power are about 90 db efficient, my test speaker is around 97. Noise is nearly inaudible with the 97 db efficient speaker, so it will be fine with my home theater mains. I don't listen nearly as loud as I used to , I'm not worried about running out of power. I'm just shooting for low distortion. I'd like to get as much Class A power as possible, but at this point I don't know how to go about that, except to bias the outputs as high as safely feasible.

Quote:
Originally Posted by Diabolical Artificer View Post
I got similar THD readings with a 1650TA OPT but with 18dB of NFB. <snip>
Maybe I should lower my grid resistors, Im at 100K on each tube.
Today I think I'll install a balance pot. It was near perfect with the Sylvania Bad Boys, but with the Russians in there its off a bit. I like the test socket idea, Id hate to have to flip these things over every time I wanted to test that in the future. I would mess around and try and draw a load line for the outputs but with a real life speaker/crossover load the impedance the amp sees is all over the place anyway so....also Im not going to change my output tranny or my B+ at this point. The bias point would be the only thing I can really adjust. I will probably see what effect that has on distortion.

Quote:
Originally Posted by Mark Tillotson View Post
That's pushing it a bit - single-tone tests are very limited, in any realistic signal intermodulation distortion is an issue, and 2nd and 3rd order IM products are as audible as 7th or 8th order (for the same amplitude), they all can fall at non-harmonically related frequencies.
I do have an old Crown IMD tester, practically NOS. But I probably wont go there, LOL

I ordered some 8.2 volt zeners to put under the paralleled 6SN7's at the output of my driver stage, instead of the 830 ohm resistors, per Morgan Jones. Cant wait to pop them in and see what happens with the distortion.
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Old 16th June 2019, 01:21 PM   #37
kodabmx is offline kodabmx  Canada
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Call it good enough?
Why not try a 9V battery for grid bias? :P
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Old 16th June 2019, 01:24 PM   #38
Troy Madden is offline Troy Madden  United States
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Originally Posted by kodabmx View Post
Why not try a 9V battery for grid bias? :P
Good idea! I used battery bias in a phono preamp some years back, but forgot all about it until now, LOL

Heres basically what the amps will look like when done. In this pic, I had Lundahl output transformers, but it turned out the power loss was quite high in my application.

Click the image to open in full size.
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Old 16th June 2019, 04:30 PM   #39
huggygood is offline huggygood  France
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I can not see some photos posted by American members, is this normal?
# click the image to open in full size ... loading ................................... and nothing
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Old 16th June 2019, 05:03 PM   #40
Troy Madden is offline Troy Madden  United States
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Quote:
Originally Posted by huggygood View Post
I can not see some photos posted by American members, is this normal?
# click the image to open in full size ... loading ................................... and nothing
I have to use the "open in a new window" option to see them myself. Not sure why they don't show up in the post.

Last edited by Troy Madden; 16th June 2019 at 05:15 PM.
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