| Author |
Topic: Preamp tube question |
Gary Lynch
From:
Creston, California, USA
|
Posted 16 Jan 2008 5:06 pm |
|
Do you know if all four of the 12AX7 pre-amp tubes on a Fender Twin Reverb power both channels together? Or does 2 of the 12AX7 pre-amp tubes power the Normal channel and the other two for the Vibrato channel?
1. In-other-words, if one preamp tube was semi bad, would only one channel be effected?
2. If a 12AT7 was semi bad, would that effect the sound of the Vibrato or Normal channel if the reverb and Tremolo was off?
Thanks in advance. |
|
|
 |
Ken Fox
From:
Nashville GA USA
|
Posted 16 Jan 2008 5:16 pm
|
|
V1 12AX7/7025 channel 1, two gain stages
V2 same channel 2, two gain stages
V3 12AT7 both halve drive the reverb transformer
V4 12AX7 1/2 is reverb recovery from the tank, the other 1/2 is the mix of channel 2 dry and the reverb return signal
V5 12AX7 tremolo
V6 12AT7 final mix of channel 1 and channel 2 |
|
|
|
Gary Lynch
From:
Creston, California, USA
|
Posted 16 Jan 2008 5:30 pm
|
|
Ken,
Great information, thank you What I was wondering is that my Twin started making a buzz/hum (loud) above volume 3. I changed some tubes around (12AX7's) not knowing zip about it, and now the Normal channel works great but the Vibrato channel is really awful above volume 3, with;loud hissing, etc.
I just order a full set of JJ 12AX7 and two matched JJ 12AT7. I am hoping this will fix it. It's brand new!
The Normal channel sound real clean and has super volume if I want to turn it up.
I am assuming the power tubes are ok since the clean channel works well? Could the power tubes make the Vibrato channel hiss loud? I figured since the Normal channel works the power tubes work?
Any info is good info. Thanks. |
|
|
|
Ken Fox
From:
Nashville GA USA
|
Posted 16 Jan 2008 6:22 pm
|
|
Power tubes would affect both channels.
Tube V6 is common to both so I would rule it out. Tube #2 and #5 would be suspect. You can swap V1 and V2. If the problem goes to channel 1 it is the tube. If it stays in channel 2 then try swapping V1 with V 4. If the problem goes to channel 1, the tube is a problem. If no tube swaps move it to channel 1 then the tubes are not the problem. The it is time for a tech to look at it! |
|
|
|
Gary Lynch
From:
Creston, California, USA
|
Posted 16 Jan 2008 6:27 pm
|
|
Ken,
Thank you. This will really help me out. It's a 3 plus hour drive to the Fender service Center one way! |
|
|
|
Gary Lynch
From:
Creston, California, USA
|
Posted 16 Jan 2008 7:22 pm
|
|
Ken,
I followed your instructions and found I had a bad V2 tube thank goodness. I was about to lose it seeing that my amp is new. I had a few other odds and ends of old 12AX7 Fender tubes and a ECC-83. I have the worst of those in V4 now and it limits most the noise.
The bottom line it that one or more of the new tubes failed partially. I think they are probably cheap junk. I'll see how the JJ's work. If you (or anyone out there) know of a preamp tube brand that actually is really quiet or close to it please let me know.
I want a real quiet amp if possible. I know they make some noise being as powerful as it is.
All suggestions welcome.
Thank you. |
|
|
|
Ken Fox
From:
Nashville GA USA
|
Posted 16 Jan 2008 8:52 pm
|
|
| JJ is the only tubes I sell or stock in my shop for preamp tubes! |
|
|
|
Gary Lynch
From:
Creston, California, USA
|
Posted 16 Jan 2008 9:10 pm
|
|
| Do you have a web site for internet sales? |
|
|
|
Ken Fox
From:
Nashville GA USA
|
Posted 18 Jan 2008 5:46 am
|
|
I do not have a website. Another project I must look into this year.
You can get a great deal on JJ tubes at:
http://tubesandmore.com/
for JJ:
12AX7 is an ECC83 series of tubes (they have several to choose from)
12AT7 is an ECC81 |
|
|
|
Gary Lynch
From:
Creston, California, USA
|
Posted 18 Jan 2008 6:59 am
|
|
| Thank you |
|
|
|
