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Old 1st July 2011, 04:37 AM   #1
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Question 5E3 Tweed build problems

Hi All

I was having some problems with the 5E3 Deluxe project I am building as
I posted before my vacation.
Getting back to it I discovered that my B+ seems a bit high at 384 vdc
directly at the output of the 5Y3 rectifier.
My book says the 6V6's should have aprox 300vdc on the plates and about 150 vdc on the plates of the 12AX7 & 12AY7 respectively.
Can someone confirm this?
I used a power tranny I salvaged from an old radio which is probably why the
voltage is high.
My book indicates 350 vdc being the maximum for 6V6 tubes.
How can I remedy this over voltage problem?
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Old 1st July 2011, 06:21 AM   #2
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Smile over voltage

It's rather easy if you use a conventional filter following the rectifier. All you have to do is to transform this capacitor input filter into a choke input filter by removing the 1st cap, directly after the recifier. This will decrease the output voltage, and improve regulation. If the voltage has dropped too low, then, simply replace the cap you have just removed by a smaller one, until the output voltage becomes correct. If you use the choke input filter method, you can increase the output capacitor to compensate for the removal of the input one.
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Old 1st July 2011, 07:03 AM   #3
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Actually most guitar amps run 6V6s at 400volts and more. To beat the competition, more bang for the buck rules. Its the only way they get 25W++ out of a pair of 6V6s.

It apears the tubes hold up well to such abuse, so I wouldn't worry. One important thing for tube life (when running such high b+ at least) is to have at least 470ohms in series with the screens (unbypassed). (Pref carbon comps).

As Blaireau mentioned, it's no sweat getting the voltage down if you want. Some resistance in series with the rectifiers is also usually a good thing for tone and response in a guitar amp, and it will drop some voltage when flooring it.

Last edited by SemperFi; 1st July 2011 at 07:05 AM. Reason: spelin'
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Old 1st July 2011, 07:37 AM   #4
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I wouldn't worry about it too much- your 6V6GTs will handle it just fine. If necessary, you can always increase the 250 ohm cathode resistor to keep things a bit cooler with the higher voltage.
As SemperFi says, you can always add some resistance in the power supply to get the voltage down a bit more.
Are you building this P2P or on a turret board? (P2P is a bit easier to add stages..)
Also, not all 5Y3 tubes have the same voltage drop, so you might get a few volts difference by trying a different tube.
This B+ was measured with all the tubes in the amp?
Drop me a msg if you need to borrow a few resistors, if you don't have a well-stocked spares box. It looks like we are local.
John
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Old 1st July 2011, 12:31 PM   #5
Trout is offline Trout  United States
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My 5E3 runs 348-365V dependent on which rectifier I use (NOS 5Y3's)
I used a couple Sovteks once but the voltage was much higher, so needless to say I have been using mostly NOS.

Most 5E3's generally run in that area, some higher, a few lower, but that is ballpark for that amp.

Later amps like the Deluxe Reverb ran higher voltages(in excess of 400V) to get the claimed 22W.
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Old 1st July 2011, 05:10 PM   #6
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Default Tweed build problems

Thanks for the responses.
It seems that the voltage is not much of a problem.
The really big problem is the amp screams and howls in oscillation
even without a speaker plugged in.
I have checked and rechecked the circuit (5E3 fender deluxe) and to my tired eyes all seems OK.
All the tubes test OK.
I have started to get some sparking in one of the 6V6's if I let it oscillate.
If I pull the 12AX7 everything is fine.
I tried a different 12AX7 and it was the same problem.
When I measured voltages during the oscillating condition (all tubes in)
I noted a positive voltage at the grid pin2 and cathode pin3 of the PI
side of the tube about 28vdc on grid and 56vdc on cathode.
I thought maybe a signal cap was shorted but when I pull the tube the positive voltage goes away.
Help!!!!!!!
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Old 1st July 2011, 07:30 PM   #7
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Sounds exciting!
Definitely don't run the amp without either a speaker or dummy load connected to the output.
If the wiring is correct and you are getting oscillation with the input shorted, lead dress (short leads) would be one area to check.
Having the 68k input resistor right at the 12AX7 tube socket is a good idea, if at all possible.
Keep us posted here- all problems have solutions.....believe it or not...
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Old 1st July 2011, 08:12 PM   #8
Trout is offline Trout  United States
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Quote:
Originally Posted by Glowin Plates View Post
Thanks for the responses.
It seems that the voltage is not much of a problem.
The really big problem is the amp screams and howls in oscillation
even without a speaker plugged in.
I have checked and rechecked the circuit (5E3 fender deluxe) and to my tired eyes all seems OK.
All the tubes test OK.
I have started to get some sparking in one of the 6V6's if I let it oscillate.
If I pull the 12AX7 everything is fine.
I tried a different 12AX7 and it was the same problem.
When I measured voltages during the oscillating condition (all tubes in)
I noted a positive voltage at the grid pin2 and cathode pin3 of the PI
side of the tube about 28vdc on grid and 56vdc on cathode.
I thought maybe a signal cap was shorted but when I pull the tube the positive voltage goes away.
Help!!!!!!!
#1, never run the amp without a speaker or load (as mentioned above)

In my book, once a tube arc's, it goes in the bin, they only go downhill once that occurs.

Is the OT secondary properly grounded at the output jack? If that ground to chassis is open, it will indeed squeal like a pig.

I have a 5E3 chassis on my bench right now, so I can give you actual voltages (on mine) but it would be very helpful for you to post a few pictures of the build.
I can generally spot an error from pictures pretty easily. Even better if its a conventional tag/eyelet board build.
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Old 1st July 2011, 09:00 PM   #9
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Perhaps your Push-Pull section is "reversed" and you've got an oscillator now.

I've seen it written elsewhere on the 'net that 6v6's in TV use routinely were hit with near 500v.
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Old 2nd July 2011, 05:21 PM   #10
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Smile Problems

Okay thanks guys.
I have some things to sort out and will get back to you with the results.
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