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Post subject: Re: retube Fender dual showman red knob head
Posted: Tue Jan 22, 2013 5:13 pm
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here some pic of the inside
after looking some video and I think but not sure but I think my cap look ok(no leak or bubble on each side of the cap)and some pic of the tube. those tube I think are original(25 years old) and same for the preamp tube, I decide to change all the preamp tube because the amp make hum noise from the speaker cab when I remove the standby. is not big hum noise but.. and I read some were it was some faulty preamp tube. my tube will arrive in 2 days from tube depot (power tube are JJ tube and for the preamp they are sino tube , I know the sino tube are made in china, but before to spend expensive preamp tube, I will check if those one will stop the hum noise
here some pic

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Post subject: Re: retube Fender dual showman red knob head
Posted: Tue Jan 22, 2013 5:20 pm
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Looks in nice shape. :D

Using cheap tubes to try and diagnose a possible tube problem is not a good idea. If the problem still occurs you don't know it it is due to the cheap tube or something else. JJ 12AX7s and 12AT7s are not that expensive and they can be relied upon to work properly. In any case, all amps will have some level of hum and hiss, particularly high gain amps.

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Post subject: Re: retube Fender dual showman red knob head
Posted: Tue Jan 22, 2013 5:33 pm
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the hum from the cab is noisy, I use the same cab(marshall 4x12 with celestion blackbacks 1976) with my Laney VH100r and no hum noise. what I mean by the hum noise from the cab, all the volume are at zero and the guitar too and even with out the guitar cable, like hummmmmmmmmmm. when I play I don't hear this hum.
update

I know when I try to bias the first time with the original tube I was only able to set at 0.03vdc and the balance was hard to setup too and sometime the noise was more high


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Post subject: Re: retube Fender dual showman red knob head
Posted: Tue Jan 22, 2013 5:51 pm
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Ok. I seriously doubt that this hum is a tube problem. I would first check and make sure all the internal grounds are solid. Hum can also come from the heater wires if they are too close to signal wires. Check the wire dress and make sure everything is neat and tidy.

As far as the low bias goes, you need to check the bias power supply circuit. Make sure all the components are up to spec. Per the schematic you should see around -60 VDC (again, the schematic is hard to read) at the input to the bias and balance pots. If that voltage is good, check the bias circuit from the pots to the tubes themselves.

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Post subject: Re: retube Fender dual showman red knob head
Posted: Tue Jan 22, 2013 10:13 pm
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bluesky636 wrote:
...Check the wire dress and make sure everything is neat and tidy...

+1

Looking at the second picture, notice that the black cable runs right across one of the preamp tube sockets, I would move that cable away from the socket, maybe up under the board. Just to the left of that socket, move the blue wire away from the circuit board. If that doesn't reduce the hum, I would try separating the rats nest of wires above the power supply. I am surprised that the heater wires are not twisted pairs (green wires). :o You might want to try moving the green wires a little also.

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Post subject: Re: retube Fender dual showman red knob head
Posted: Thu Jan 24, 2013 3:17 pm
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Hi ' I just got my new tube. just one more question, the book said .04 VDC and the balance 0 VDC. some said is to cold and said 40 mA at each test point and I need to approx. 80 mA on each teat point. but what is the value in VDC if the bias is .04 VDC and the balance output is 0 VDC.
thanks and I hope you understand what I mean and thanks again for all the info and helping me and sorry for my English because i'm french


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Post subject: Re: retube Fender dual showman red knob head
Posted: Thu Jan 24, 2013 3:33 pm
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simus35 wrote:
Hi ' I just got my new tube. just one more question, the book said .04 VDC and the balance 0 VDC. some said is to cold and said 40 mA at each test point and I need to approx. 80 mA on each teat point. but what is the value in VDC if the bias is .04 VDC and the balance output is 0 VDC.
thanks and I hope you understand what I mean and thanks again for all the info and helping me and sorry for my English because i'm french


You are measureing across a 1 ohm resistor.

I=E/R

1 mA = 1 VDC/1 ohm

You MUST also measure plate voltage and calculate the percentage of maximum plate dissipation as described previously. Measureing 40 mV or 80 mV without knowing the plate voltage at those values is meaningless.

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Post subject: Re: retube Fender dual showman red knob head
Posted: Fri Jan 25, 2013 10:26 am
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thanks again for the info bluesky636
now I order a probe I can measure plate voltage
but another question this amp have hi and low switch. and I realize when I put on hi the volume is lower than when I put on low. and when I bias only with a meter. on low switch I can get 0.04 VDC and the balance 0.00VDC. but when I switch on high the higher I get on the bias is 0.03 VDC.

do you think I have a transformer problem ? or something else and do you think is something I can fix my self or I need to get a tech for that
thank again for helping a newbie


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Post subject: Re: retube Fender dual showman red knob head
Posted: Fri Jan 25, 2013 11:23 am
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simus35 wrote:
thanks again for the info bluesky636
now I order a probe I can measure plate voltage
but another question this amp have hi and low switch. and I realize when I put on hi the volume is lower than when I put on low. and when I bias only with a meter. on low switch I can get 0.04 VDC and the balance 0.00VDC. but when I switch on high the higher I get on the bias is 0.03 VDC.

do you think I have a transformer problem ? or something else and do you think is something I can fix my self or I need to get a tech for that
thank again for helping a newbie


The high and low power switch operates by changing the plate voltage. You are seeing a lower cathode current on the high setting because the plate voltage is considerably higher than on low. You should set bias and balance with the amp set to high power. Then, when you switch to low power, the cathode current and plate voltage will change as will the power output.

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