1625/12SL7 Clementine Completed!

sweet & juicy SE amp for 1626 Darlings and the 6L6 family

1625/12SL7 Clementine Completed!

Postby Thermion » Wed Dec 21, 2011 10:47 pm

I have been working on a 1625 based Clementine amp for a few months now and finally got it assembled and wired up this past weekend. Here is a picture of the amp from the front.

Clementine 02-crop.JPG
Clementine 02-crop.JPG (380.58 KiB) Viewed 1520 times



I used Edcor transformers for this project. This is the first time I've used them in a project, and also first single ended amp I've built so kind of an experiment for me. The outputs are GXSE15-8-5K and the power transformer is XPWR114-120. This power transformer is 660V, 200ma center tapped with a 12.6v at 3A and 5v at 5A filaments. I chose it because of the 12.6v filaments. It's a little overkill, but should make for a conservative design.

I prototyped the circuit in both triode and UL mode with and without feedback. I decided to use UL mode with 6 db of feedback as my targets. I used a 5AR4 as the rectifier and an old Dynaco filter choke in the power supply. With this set up, I have about 406 V B+. For the most part, I used Shannon's recommended values for a "Chubby" 6L6 Clementine, with some variations. I used 1.2k and 1.5k resistors for the cathodes which gives me 667 ohms. I measured around 52ma bias current for a dissipation close to 19 watts. In the power supply, I used a 50 ohm 5 watt resistor for R21 and a 33k resistor for R22. This provides a voltage on the 12SL7 of 317 volts which is fine. I also reduced C7 to 33uF because of the tube rectification. Here is a picture of the underside.

Clementine 14-crop.jpg
Clementine 14-crop.jpg (402.99 KiB) Viewed 1520 times


In this picture everything is wired except the feedback and ground to the speaker binding posts. The binding posts are Cardas and most of the wiring is Kimber TCSS. I used a two conductor shielded Belden cable for the inputs.

The unit powered up cleanly with no problems and I conducted some open loop measurements last night. Open loop gain was about 21.8 db. Tonight I hooked up the feedback using 681 ohm resistors. This gave me around 8.3 db of feedback. This level of feedback is more than my target 6db, but significantly reduces the distortion. Without feedback I reached 5%THD at around 2 watts. With feedback the 5% THD level was about 6.5 watts.

I haven't listened to it yet. That will probably be tomorrow night or Friday.

JT
Last edited by Thermion on Sat Dec 08, 2012 8:32 am, edited 1 time in total.
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Postby SVinzant » Thu Dec 22, 2011 8:50 am

Thermion,
That is a sweet looking amp.
Great work!

Scott
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Postby Shannon Parks » Thu Dec 22, 2011 1:05 pm

Hope I can drop by for a listen tomorrow! Looks great. Love those JAN 1625s.

Shannon
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