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Post subject: Fender Twin 65 RI - bias and tube question
Posted: Tue Oct 11, 2011 10:19 pm
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Hi,

I have had a fender twin 65 RI for around 10 years and it's a great amp although I have had a few problems with it recently. I use the bias probe from the link below to adjust the bias when I change the tubes. I cannot however read the bias current or the plate on the right most power tube (when looking at the back of the amp). Also after having the amp on for around 5 mins this tube stays substantially cooler than the other tubes ie. too hot to handle the other tubes while I can still touch this one. All tubes light up to a similar level when on standby and when the amp is on. I have tried swapping the tubes around and failing to read the bias is always associated with the slot and not the specific tube.

I had a problem a while ago where a tube drew too much current and blew out some components, although I can't remember which slot the tube was in.

I was wondering if anyone had any ideas what this could be? I won't attempt to fix it myself, but I would like to know out of interest.

http://www.amp-head.com/product_info.ph ... ucts_id=30

Thanks for any suggestions
Cheers
Chris


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Post subject: Re: Fender Twin 65 RI - bias and tube question
Posted: Tue Oct 11, 2011 10:26 pm
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Is your V7 6L6GC receiving any plate voltage at all? You should be measuring 440vdc on pin 3.

Arjay

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Post subject: Re: Fender Twin 65 RI - bias and tube question
Posted: Wed Oct 12, 2011 2:11 am
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Try to tightened socket contacts. And contact cleaner on tube pins and you put and remove tube few time on his socket this will clean contact. But thightened contacts first.

Look like you have wrong contact or connexion on this socket;


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Post subject: Re: Fender Twin 65 RI - bias and tube question
Posted: Sun Oct 16, 2011 1:22 am
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cheers for the suggestions.
I measured ~440V on pin3 of V7 from within the amp although not sure about the continuity from the pin to the board.
I haven't tried cleaning/tightening the pins will try that next, I guess it has to be something like that right?


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Post subject: Re: Fender Twin 65 RI - bias and tube question
Posted: Sun Oct 16, 2011 1:34 am
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V7's pin three is tied in parallel with V8's pin three -- both are wired directly to the output tranny's primary. You should see the same B+ voltage at TP21......

http://web.archive.org/web/201001051826 ... manual.pdf

If both tubes are receiving the same (correct) plate voltage, you may have a bad OT.

Arjay

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Post subject: Re: Fender Twin 65 RI - bias and tube question
Posted: Sun Oct 16, 2011 1:45 am
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Ok your tube have power on socket

This does't mean voltage go to the tube if contact between tube an socket are defective.
_____________________________________________________________________________________________
Are you sure you know how to bias correctly ? But now this is not the problem

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Retroverbial,
IMO there nothing wrong with output transformer. Problem with tubes or /and bad contacts


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Post subject: Re: Fender Twin 65 RI - bias and tube question
Posted: Sun Oct 16, 2011 1:46 am
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Is it also possible that you have a dead tube? You are getting anode plate voltage. But, there is no current flowing through the tube---because of a defective grid or cathode plate. It happens.

Try swapping a good testing 6L6GC in that position and see if you get a bias current.


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Post subject: Re: Fender Twin 65 RI - bias and tube question
Posted: Sun Oct 16, 2011 1:52 am
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stratele52 wrote:
Retroverbial,
IMO there nothing wrong with output transformer. Problem with tubes or /and bad contacts


This may be true, and I'm hoping it is -- a tube is less costly to replace than a transformer.

Also, it might be wise to re-flow all the solder connections on V7's socket as well as cleaning and re-tensioning the contacts.

Arjay

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Post subject: Re: Fender Twin 65 RI - bias and tube question
Posted: Sun Oct 16, 2011 2:12 am
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I'm sure about that; OT is alright with the information I read from sorge


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Post subject: Re: Fender Twin 65 RI - bias and tube question
Posted: Sun Oct 16, 2011 2:38 am
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I have been through about 3 sets of tubes and never been able to read a bias current or voltage on V7. I checked the voltage on V8 pin 3 and get the same ~440V.
Tried tightening the sockets and now getting a plate voltage but no bias current, will try cleaning pins again. Is it normal for them to be kind of loose in the socket?

I have been able to bias the other tubes fine (~60% dissipation) and amp sounds really good.


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Post subject: Re: Fender Twin 65 RI - bias and tube question
Posted: Sun Oct 16, 2011 3:28 am
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Ok you have the plate voltage now but not able to read bias . Other socket pins are defective ; bad contacts;

Tube should not bee loose on socket. But sometimes the tubes is tight on pins but the pins move on te socket ; it is normal.

If you plug your tube in the socket it should be tight when you push it , Is it ?


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Post subject: Re: Fender Twin 65 RI - bias and tube question
Posted: Sun Oct 16, 2011 5:49 am
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The tube fits tightly into this socket.
I checked the continuity between solder joints on the pins inside the amp and the pins of the tube socket, they were all good. Checked the connections between the solder joints inside and the bias probe leads and they were all good as well, ie. checked between pin 8 and the leads on the end of the probe and have a connection.
Tried again with bias probe but still can't read bias current, so I am a bit confused..

cheers for all the suggestions


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Post subject: Re: Fender Twin 65 RI - bias and tube question
Posted: Sun Oct 16, 2011 6:26 am
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Checking continuity between solder joint does'nt mean they are good ; Better to reheat them with new solder.

There is something you did not see around socket . Are you shure pin 8 are grounded ?

Reheat solders first.

And if does'nt work check carefuly around socket. Tube could be thiht with few socket connector but loose with one ?


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Post subject: Re: Fender Twin 65 RI - bias and tube question
Posted: Sun Oct 16, 2011 8:59 am
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Hi sorge,

Lots of good suggestions here, I'll throw one more into the mix.

Have you checked the screen grid resistors R50, R51, R52, and R53? Find the one that corresponds to the tube that you have no current for. An open screen grid resistor could cause the problem that you've described. With your negative meter lead connected to the chassis, put the positive meter lead to each side of the screen grid resistor that you want to test, and the voltage should be very close on each side, with only a few volts difference. The voltage should be approx 440 VDC on one side of all screen grid resistors, and maybe around 437 VDC on the other side. If you read a high voltage on one side of the resistor, and very low or no voltage on the other side, then that resistor is open. :idea:

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Post subject: Re: Fender Twin 65 RI - bias and tube question
Posted: Sun Oct 16, 2011 6:58 pm
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Thanks again for the suggestions.

I will check the screen grid resistors when I get home from work.
I am almost convinced that pin 8 is making good contact with the tube pin as the bias tester sits between the tube and the socket and I have tested that the bias tester is making good contact with the wire to pin 8.

Is it possible that this socket is receiving no bias current and therefore sort of behaving like class B, with no cross over?


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