Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mosrite1 on January 09, 2011, 02:03:10 am

Title: High voltages in 66 Ampeg B15nf
Post by: mosrite1 on January 09, 2011, 02:03:10 am

So I have a 66 Ampeg B15nf, I just replaced all of the electrolytics and tubes ( except the rectifier), I am getting 516vdc on the plates ! 510vdc on pin 4 and -57vdc on pin 5. all of the voltages before the output section is fine, a little higher than the schematic due to the 123.5ac from the wall 
The bias seems fine, and it sounds good, I am just worried about that 516vdc plate voltage eating up tubes

here is the schematic
http://www.kbapps.com/audio/schematics/tubeamps/ampeg/b12nfb15nf.html

any help would be much appreciated

thanks

Title: Re: High voltages in 66 Ampeg B15nf
Post by: Maschinenmann on January 09, 2011, 06:51:48 am

I've seen this on a few vintage amps using new production glass; the voltage drops are not the same as the originals due to material and coating differences. As long as you aren't exceeding the cap ratings and you bias it properly, you should be ok. A better solution would be to build a voltage "browner" to get the input voltage down to '60's era 115-117 VAC.  :wink:


http://geofex.com/Article_Folders/vintvolt/vintac.gif

Title: Re: High voltages in 66 Ampeg B15nf
Post by: Geezer on January 09, 2011, 07:21:19 am

Yep, you need to build a "Vintage Voltage Adaptor" (MM's link above). I built one & they work great!

I worked on a friends B15 a few years ago & the voltages were almost exactly what you got, maybe even higher.

G

Title: Re: High voltages in 66 Ampeg B15nf
Post by: PRR on January 09, 2011, 09:31:51 pm

> 516vdc on the plates !

On 6L6GC? Or any modern "6L6"?

Forget about it. That won't hurt these tubes. The "450V" rating was ultra-conservative for marketing reasons.