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Old 2nd February 2011, 01:57 AM   #1
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Join Date: Feb 2011
Default first 5f1 build is acting funny :/

this is my first amp build and everything went smooth.

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i have triple checked all connections. it works and sounds great, almost no hum.

after playing it for about an hour i notice a small red spot on the 6v6 plate so i went through and checked the voltage levels. the first day on pin 3 of the 6v6 and i get 357 volts. on pin 8 i get 23.35vdc. pin 4 at minimum volume i get 358.9vdc and at full volume i get 365.8vdc. pin 7 and 2 both are 3.484vac and and pin 1 of the 12ax7 was 191. that day i didn't check the high side of the 10k resistor.

the second day i play it for another hour and turn it off.

today i decide to warm it up and check the voltage levels again and they are allot different.

high side is 383.2

6v6gta
pin3 is 353.7 low volume 361.8 high volume
pin4 is 343.8
pin8 is 23.00
pins 2 and 7 are 3.v7 "AC"

12xx7a
pin 6 is 187.6
pin 1 is 189.4

5y3gt

pin 4 is 333.4 "AC"
pin 6 is 332.3 "AC"
pin 2 is 383.7
pin 8 is 383.2

and the small red spot is still there.

should i be worried? the voltage levels seemed to have dropped with use, do i just need to break it in more?

the actual resistor values are
10K = 9.96
22k = 21.74
cathode resistor is 471 ohm
220k = 216.1k

should i be worried? the voltage levels seemed to have dropped with use, do i just need to break it in more? i just don't want it to start smoking
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Old 2nd February 2011, 04:37 AM   #2
RTF671 is offline RTF671  United States
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You built it to spec? What transformers did you use? I can see you used carbon comp resistors. Just it be safe you could swap out the cathode resistor on the 6V6. If it's being problematic you might be better off using a metal film just for that one (if this keeps going on).

As for the glow, some is ok although generally near the top. A good safety measure for the power section is to place a fuse in the cathode between the ground and the cathode. In which if the resistor fails and sorts, the valve will draw heavy currents and blow the fuse which saves the valve and your OT.

Another thing, what is your heater voltage? It looks like you said 7vAC? It would be best if they were within 6.3 to maximize valve life. Also is your heater winding center tapped? Do you have it grounded and elevated?
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Old 2nd February 2011, 05:16 AM   #3
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oh i meant 3.7vac sorry about that and i have no idea about the transformer. i used a kit from triode electronics 5F1 Tweed Champ Tube DIY Amp Kit

and as far as i can tell it is identical to the fender 5f1 schematic, well again no idea on the transformer. and can you explain grounded and elevated?
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Old 3rd February 2011, 01:23 PM   #4
RTF671 is offline RTF671  United States
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The heater voltage is quite high or low if it's 3.7. You should get it toward the appropiate voltage.

Heater elevation is good practice which reduces noise. Sometimes its necessary if you had a cathode follower circuit but you don't on this design. If your noise level is ok there is no need to complicate it further

Would it be possible to get a picture of the glow? Tubes (especially power and rectos) glow a little normally.
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Old 6th February 2011, 12:33 AM   #5
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Join Date: Feb 2011
i finally got a chance to snap a photo of the spot on the tube. that is quite difficult :/ but after 30 or so tries i got one to show up. http://img687.imageshack.us/img687/6625/sam0042u.jpg
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Old 6th February 2011, 05:44 AM   #6
RTF671 is offline RTF671  United States
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It's looks normal or so, at least from the shot. How are the voltages and how is the sound?
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Old 6th February 2011, 02:59 PM   #7
Trout is offline Trout  United States
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Location: Midwest Madman
I would just raise the cathode resistor to around 560R or 600R. Those triode kits are pretty nice, but the transformer is just a tad hot. This is a very common occurrence and the topic pops up a lot on guitar amp forums regularly.

Weber Bias Calculator

The above link might be useful. Get it into or closer to the 60% dissipation range and it will cure the redplate/spot.
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Old 6th February 2011, 04:07 PM   #8
jjman is offline jjman  United States
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You idle is about 16watts. You may need to go to 700R or more to get down to something kosher. Different tubes will idle differently. 13watts is a good target but trial and error is needed since it's not adjustable. I have a 100 ohm pot in line to fine tune my cathode resistor on my 5F1 build, but I'm a little obsessive.
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