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PostPosted: 20 Oct 2020, 09:51 
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Hi, They would be OK. Pretty much anything over about 5.5K will be suitable. You will likely get about 1/4 watt more output (big deal) and have about 0.25% more distortion (also big deal). Optimun value for lowest distortion that I could measure in the lab was in the 7.8 to 9K range. Thus the 8K transformers. For best results with the amp use the latest schematics with the small positive voltage boost on the grids and the balance control also in the grid circuit. Use decent caps for coupling the grids (one to ground and one to the driver). I suggest you not use JJ 5751 tubes, they sound kind of harsh to me. YMMV. BTW if you juggle the cathode resistors in the driver stage to keep the same ratio of NFB you can go to 1200 ohms and use 12AX7s and ECC803s. Performance is quite similar overall.

Good listening
Bruce
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PostPosted: 24 Oct 2020, 13:18 
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Location: AB, Canada
Thanks Bruce,
On the distortion side, I looked at the measurements sheet on the site. I have no plans for 25Hz operation, only music, no HT, so would probably be looking at 35 - 50 Hz for low end. With the estimated added 0.25% distortion, one should be at the most around 0.5 % distortion. This should be suitable as a first build. I am not trying to build the "best", just trying to build with what I have (if suitable). I am using this as a great learning experience and introduction into the EL84 world.
Later if I find a good deal on some better OPT's, I will upgrade and see the difference. May go with the monoblocks if I upgrade the OPT's.

I also have already most of the parts for the Baby Huey EL34 except for the Xfrmrs. Looking forward to having these 2 amps in my stable,

Do you see any problems with the 0.5% distortion point.?

Thanks for the help,

Myles


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PostPosted: 24 Oct 2020, 14:27 
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Hi, The typical speaker is way more than that. Tube amps with lower values often have about 20-30 db of negative feedback. I personally believe that it colors the sound (YMMV). So I use very little, about 3 db that is most active in the 35KHZ and up range. It is there as the amps typically have a resonance point due to the output transformer at 60-75 KHZ. Since without the NFB the amps will deliver a significant amount of power there it is at least IAW modeling that it could "ring" or oscillate there with some rather odd speaker loads. Many builder of these series of amps defeat the NFB. The amps will run fine. In the commercial versions I include a defeat switch just for this purpose. 99+% of the time they work just fine. I have not yet (in the 12 years it has been around) ever seen one misbehave. Not even on my electrostatics that drop to about 1.2 ohms at 20KHZ.

Good listening
Bruce

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PostPosted: 25 Oct 2020, 00:04 
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Bruce,

Do you know of any bare pcb's that are available for any of your EL84 projects. If there are not any available, I will have to put my learning skills to work and use the pcb design software that I bought.

Myles


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PostPosted: 27 Oct 2020, 10:48 
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Hi Bruce, have you ever measured/calculated what the output impedance of the amplifier is using the specified EMO 750 transformers? Thanks, Richard.


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PostPosted: 27 Oct 2020, 15:48 
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Hi, I'm not sure what you are asking.....if you wanted the damping factor it is depending on the actual build, in the range of 11 to 15. Pretty typical for such amps.

Good listening
Bruce

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PostPosted: 28 Oct 2020, 09:17 
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Thanks Bruce, so for a typical speaker of say 8 ohms, this would give an output impedance of c. 0.6 ohms, which seems low (in a good way) to me for the size of amp?


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PostPosted: 05 Nov 2020, 13:29 
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Location: Boston, USA
Hi Bruce-

How long do you expect tubes (in this case, JJ EL84 to last)?

I have a Poddwatt which has been on probably 10k hours since I built in in 2017 (it was on 24/7 for a year or so, then I got a timer so it's only on about 8h/day). It was working fine until I moved it. Now one output tube is dead (still lights up but no bias current) and the other pair is way out of balance.

I've got 4 new tubes in there now and it sounds great again! (interestingly the ECC802 preamp tubes are still fine). Is this typical experience? No complaints, just curious.

Best,
Eric

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PostPosted: 05 Nov 2020, 14:11 
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Hi, Not unusual. In many circuits they only go for about 1000 hours. I have some that have gone into the 8-10K region and are still fine. I suspect the tube was damaged by impact during the move. The preamp tubes may not wear out in your lifetime. I have no data on them failing or getting out of tolerance.

Good listening
Bruce

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