Bottlehead Forum

Bottlehead Kits => Crack => Topic started by: Mark Ferring on January 03, 2016, 11:17:29 AM

Title: 90V readings (A1, A6, B1, B4) actually 72-75V [resolved]
Post by: Mark Ferring on January 03, 2016, 11:17:29 AM

Hi guys, first time poster.....

So I just retired and have a pair of Sennheiser 650's. When I was 19 (1976) I built A Dynaco 60W solid state amp and preamp....that still work!

So now that I have time, I thought I would get back into the kit world.

I just finished the wonderful Crack kit. Thankfully, it powered up after the resistance checks with nice glowing tubes and diodes.

However all of my checks for 90v ended up being a little more than 15% low (A1, A6, B1, B4). I assume that these are all related? Anyhoo, based upon Doc's note about tube variability, I assume that this is probably within acceptable range? All other resistance and voltages check good.

I would very much appreciate any of your collective wisdom as to whether this is 1) OK, 2) what could cause it, and finally 3) any degradation to the sound?

Also, any ill effects when I proceed to install the Speedball upgrade at some point (already have it, but want to get familiar with the basic design "sound" first).

Many thanks for anything you can provide!

Mark

Title: Re: 90V readings (A1, A6, B1, B4) actually 72-75V
Post by: Mike B on January 03, 2016, 03:06:50 PM

Does it sound good?

Title: Re: 90V readings (A1, A6, B1, B4) actually 72-75V
Post by: Mark Ferring on January 03, 2016, 03:50:25 PM

Good question.....to me....yes it does! Just wondered whether this type of variance is fairly normal.

Title: Re: 90V readings (A1, A6, B1, B4) actually 72-75V
Post by: Paul Joppa on January 03, 2016, 04:25:09 PM

It is normal. A lower voltage (it can be as low as 60 volts) indicated a 12AU7 with better emission than the specification for 12AU7.

Title: Re: 90V readings (A1, A6, B1, B4) actually 72-75V
Post by: Mark Ferring on January 03, 2016, 04:38:21 PM

Thank you Paul. Pardon my ignorance. Is a lower emission a good thing?

Title: Re: 90V readings (A1, A6, B1, B4) actually 72-75V
Post by: Paul Joppa on January 03, 2016, 06:35:07 PM

High emission is a good thing. Emission slowly fades as the tube ages, so high emission usually presages a long life for the tube.

Title: Re: 90V readings (A1, A6, B1, B4) actually 72-75V
Post by: Mark Ferring on January 04, 2016, 12:16:04 PM

OK, got it. Many thanks.

Approximately how long would I expect tube lifetime to be for the Crack with the original tubes?