Re: [Harp-L] Fender Vibrosonic

[Special20 <special20harp@xxxxxxxxx>]

>From Mike at KCA NOS tubes
(http://kcanostubes.com/content/newsletter_details.asp?ArticleID=5):

"The phase inverter should always be replaced  when replacing power
tubes. Opinions on how the phase inverter affects tone vary. Some say
it has little or no effect while others claim it's critical to have a
high quality 12AT7 in this slot. ... Some players prefer a 12AX7/A in
this postion as it allows an amp to "break up" or distort at lower
volumes due to being "mis biased" in the 12AT7 circuit. This is a
perfectly safe substitution to make. Try it, you may like it! "

Speaking for myself, I am not sure a lower gain phase inverter tube
has the same effect on the tone as it does in the preamp section, as
you suggest.  They perform two very different functions.

I will be trying Mike's suggestion of using a 12AX7 (a higher gain
tube) in the phase inverter for earlier break-up in a 1972 Fender Twin
Reverb I am rehabing.  I'll report back how it works...

-Rick Davis
the Blues Harp Amps blog:  http://www.bluesharpamps.blogspot.com/



On Wed, Apr 30, 2008 at 9:41 AM, David Harris <snakehead_1@xxxxxxxxx> wrote:
> Jim,
>  I understand about the wife.  My agreement with my wife is that I can use it when there is no one home.  Then I can play it as loud as I want.  Woo hoo.
>
>  Here are tube values according to their gain factor (http://thetubestore.com/gainfactor.html).
>
>  12AX7 = 100 gain
>  12AT7 = 60 gain
>  12AY7 = 45 gain
>  12AU7 = 19 gain
>
>  I wouldn't suggest a tube higher than 12AY7 in the Phase Invertor of your amp.  The 12AT7 and 12AX7 are too bright.
>
>  I also wouldn't suggest a 12AU7 (too saggy) in the pre-amp Normal channel.
>
>  You already have a 12AX7(the original tube from the normal channel), 12AU7 and 12AT7.  You would only need to acquire a 12AY7 and you are ready to start swapping.
>
>  Here are the choices for your test, when you have time:
>
>  Phase Inverter=12AU7.  Pre-amp Normal=12AT7.
>  Phase Inverter=12AU7.  Pre-amp Normal=12AX7.
> Phase Inverter=12AY7.  Pre-amp Normal=12AT7.
>
>  The list has from darkest to brighest sounding settings.  Defintely use the nicest mic you own for this test so you will be able to gauge how it will really sound.
>
>  After several minutes you will be able to decide which config is right for you.
>
>  Have fun,
>
>  Dave
>
>
> "Priest, Jim" <Jim.Priest@xxxxxx> wrote:
>      Thanks for the suggestions Dave. Steve hasn't changed the PI so I'll give it a go. I'd actually forgotten that I already have a 12AT7 there. I'll have a play around and report back on the results. It may take a while... the other drawback to the Vibrosonic that I hadn't mentioned is that my wife has banned me from using it in the house!
>
>  Jim
>
>
> ---------------------------------
>  From: David Harris [mailto:snakehead_1@xxxxxxxxx]
> Sent: 30 April 2008 12:39
> To: Priest, Jim; harp-l@xxxxxxxxxx
> Subject: RE: [Harp-L] Fender Vibrosonic
>
>
>
>  Jim,
>  Great job on your project.
>
>  Your 12AT7 idea in the pre-amp Normal channel makes sense.
>
>  You didn't say if the tech replaced the phase inverter tube.  If he lowered that tube to a 12AU7, you would defintely get more breakup.
>
>  You initially mentioned that you had a 12AT7 in the phase inverter.  Just switch the pre-amp normal and phase inverter tubes.  You would then have a 12AU7 in the phase inverter and a 12AT7 in the pre-amp Normal channel.  That would be a free way to beef up you amp.  :-)
>
>  These changes should make the amp growl a little.  Heh heh.  Of course the right mic would be the finishing touch.  A nice crysal mic would sound badass on your amp.
>
>  All in all, you are very close to harp amp nirvana.
>
>  The best part is you can do the tube swaps yourself now that the amp has been given a clean bill of health.
>
>  Please let me know how the tube changes go.
>
>  Dave
>
>
>
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