Bottlehead Eros Build Check

[robomaster]

I finally finished building my Bottlehead Eros. I ran all of the checks at the end of the manual and am not sure about some of the values obtained. Particularly on the right channel board the Kreg reading of 0.15 vdc instead of 1.0 vdc. Also the OA should be 170 vdc but is only 121.9 vdc. Also the C4 socket reading is 620 ohms instead of infinity. I will post the results here in hopes someone will take a look at them and advise me as to where any problem areas are. Thanks in advance for any help.

Whoops, just found a typo in my Socket check chart on A5. I will be running all tests again and repost data. Sorry.

Ok. I posted a new 9 pin socket check. Issues seem to be with A4, B4, and C4. Some resistance values are also high.  I have penciled in actual readings for the ones printed on the check sheet from page 76 of the manual. 

Update. Measurements for the socket test were done with tubes in and power off. With tubes out A4, B4, C4 read 620 ohms each. Still not the values they should be (0, * rising, and infinity.

Update: I have run another voltage test on the A,B, and C as described on page 76 of Eros manual. The file is attached. The only issue I see is that the Kreg voltage is 1.13 on the right channel board and 0.62 on the left. I hooked up the Eros to my system and it seems to sound fine. There is noise from the Eros if the volume control on my preamp is turned all the way up with no record playing.

[Paul Birkeland]

Will you swap the EF86s to see if the problematic voltage follows a tube?

Pin 4 on each of those 9 pin sockets is one of the heater pins, if you still have 6.3V at the 6.3V pads on the big power supply board and your tubes are glowing, there likely isn't anything to be worried about here. 

[robomaster]

Thanks for the info. I removed the tubes and checked the resistance on A4, B4, and C4. All now show 620 ohms. Still not spec but different than previous. I also ran another voltage check per page 76 and posted the results as an attachment (Bottlehead final voltage check.txt). It looks good to me except that the two Kreg voltages are different. The right channel is 1.13 vdc, and the left is 0.62vdc. I felt confident so I hooked up the Eros last night and it sounded great. There just seems to be a lot of noise when my Parasound P5 preamp is turned up past 2 o'clock volume position with no record playing. I realize that the Eros is very high in gain @ 50db compared to other units and also being tube based (higher noise levels?) this may be normal.

[Doc B.]

when my Parasound P5 preamp is turned up past 2 o'clock

Is that a listening level, or is that a way too loud to listen level?  If the latter, there is nothing wrong.

[robomaster]

It is too loud to listen at that level. Thanks for your help. I think I'm good on the build. Any thoughts on the Kreg voltages measured? One is 0.62 vdc and the other is 1.13vdc.

[Paul Joppa]

The different kreg voltages are probably because the two EF86's have different transconductance. That's why PB asked that you swap those two tubes and see if that voltage difference follows the tube - this would confirm that theory if it's correct, or if not confirmed, then we'd start looking for other issues.

An ideal EF86 would have a kreg voltage closer to 1.5 volts; some new-production copies will run lower but usually at least 1.0 volts. The lower voltages you measured suggest lower than normal transconductance. This is often due to a cathode that is not yet fully formed, and giving the tube some time in operation (say, 100 hours?) will usually bring the voltages in line. So I would recommend that you listen to it for a while and then re-check those voltages. If they don't improve, post again and we'll take a closer look.

[robomaster]

Thanks for your reply Paul. I have now swapped the EF86 tubes and yes, the problem did reverse. The low reading is increasing though and is now 0.75 vdc. Up from 0.36 vdc on first check. The high side is now 1.21 vdc. I will now break in for a 100 hours or so and check again. I appreciate your help in this matter.

[Paul Birkeland]

If that Kreg voltage is coming up, you may find that it settles to a more reasonable value once the tube has had some run time.

[robomaster]

That's what I am going to do. Break it in for about 100 hours and then run the voltage checks again. It is sounding great as is. There is only some noise when volume is set very high on preamp with no record playing. There is a sound like wind blowing on one channel and a low hum as well in the other. Not noticeable at reasonable listening levels. Thanks for your help.