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FDP Forum / Amp Mods, Repairs, and Projects / Bias again

macguy

USA

Feb 16th, 2018 12:28 PM   Edit   Profile  

Hello...

My 1967 Fender DUAL Showman has 4 Winged C Svetlana tubes...

Tubes read from left to right when installed in the amp viewed from the back...:

#1 27mA, #2 40mA, #3 32mA, #4 32mA.

Should they be put in any particular order until I can afford a matched quad?

Thanks,

MG



Hammond101
Contributing Member
**********
**

So. Cal. USA

Feb 16th, 2018 12:51 PM   Edit   Profile  

They pair up inner two, outer two, so you are about the best you can be until replacements are installed.

You might try moving the left two tubes to other sockets to see if there are any changes for the better. You just never know.

macguy

USA

Feb 16th, 2018 01:26 PM   Edit   Profile  

Thanks Hammond...I have moved the tubes around and the readings remained essentially as I mentioned...I just hope the amp will suffer no damage.

Thanks again,


MacGuy

(This message was last edited by macguy at 04:16 PM, Feb 16th, 2018)

Leftee
Contributing Member
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**********
*

VA

Life makes a man crotchety.
Feb 16th, 2018 04:02 PM   Edit   Profile  

The readings followed the tubes or stayed with the sockets?

macguy

USA

Feb 16th, 2018 06:06 PM   Edit   Profile  

Followed the tubes...that tube that shows 40mA was at 45mA which is what prompted me to check them all. I backed down the bias numbers to get it down to 40mA hoping to keep the others around 32mA...in doing so the other tube dropped to 27mA. I bought those tubes years ago...not sure from whom...and they were supposed to be "matched". They may have been but they sure are not now.

Leftee
Contributing Member
**********
**********
*

VA

Life makes a man crotchety.
Feb 16th, 2018 07:03 PM   Edit   Profile  

They will drift with use. And not evenly.

How does the amp sound?

macguy

USA

Feb 17th, 2018 08:41 AM   Edit   Profile  

The amp sounds great...always gets a compliment at gigs...HOWEVER, after turning the amp on last night the two tubes on the right began to red plate. I switched the tubes around a time or two and now all is well. The hottest tube is at 37mA, the lowest is just under 27mA.
The lower number on bias is the "coldest", correct?

MG

FDP Forum / Amp Mods, Repairs, and Projects / Bias again




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