Blown screen resistors and more

[titser_marco]

Go ahead! I am learning from these discussions, and this is a good way to wait out the results as I wait for the PT to be done as well as cleaning/replacing the sockets.

[davidjon_99]

A carbon track along the wiring side of the socket or the tube side? Is there any way I can still use the sockets after cleaning them?

This is an image from a Mesa Boogie I bought that had several issues. One of which was very loose output tube sockets. This thing had been arcing for some time. IMHO, it's best to replace the sockets.

[tictac]

Wouldn't some pictures of the inside of this amp be in order?

especially the area around the power tubes, power supply, and bias supply?

TT

[xtian]

Hey, Phil: When you talk about DC voltage being proportional to the number of winds, this is a relationship between primary and secondary windings. Contrast this to the UL discussion, where the center tap is in the middle, the UL tap is at 40% or whatever, and the plate tap is at the end of the winding...ALL on the same coil of wire! So if you apply a DC voltage to the center tap, the measured voltage will be nearly the same at the UL and plate taps!

[Firestorm]

So I guess you're all right: I need to get the power and output sections working first and I've already called on my amp repair guy to rewind the PT as well as the choke since it seems that it was taken down as well by all this shorting.

A rewind may not be the best choice unless the PT was high quality to begin with. Why not source a new PT with the proper specs for what you're trying to build? It has a choke? Real Fender 135 models did not. Voltages for PI and preamp stages were stabilized (and dropped) with a voltage divider of a couple of high wattage resistors.

[titser_marco]

So I guess you're all right: I need to get the power and output sections working first and I've already called on my amp repair guy to rewind the PT as well as the choke since it seems that it was taken down as well by all this shorting.

A rewind may not be the best choice unless the PT was high quality to begin with. Why not source a new PT with the proper specs for what you're trying to build? It has a choke? Real Fender 135 models did not. Voltages for PI and preamp stages were stabilized (and dropped) with a voltage divider of a couple of high wattage resistors.

Ah well, that's the best option I have right now given how long it can take for me to get a transformer shipped here. In any case, the working assumption now is that it's not even a Fender to begin with and I think I will have to rebuild the power supply from the ground up.

[titser_marco]

Wouldn't some pictures of the inside of this amp be in order?

especially the area around the power tubes, power supply, and bias supply?

TT

Will post when I can!

[titser_marco]

A carbon track along the wiring side of the socket or the tube side? Is there any way I can still use the sockets after cleaning them?

This is an image from a Mesa Boogie I bought that had several issues. One of which was very loose output tube sockets. This thing had been arcing for some time. IMHO, it's best to replace the sockets.

I think you're right. Sockets aren't expensive anyway, and I am glad to hear that my tech has 4 in stock.

[JazzGuitarGimp]

This is going to be a minor hijack of the thread. Apologies for doing it.

I think I misunderstand UL winding. I am operating with the understanding that 40% means the tap is at the point where 40% of the turns are on one side of the tap and 60% on the other side. (This is with respect to each half from the CT.) Since there is a direct relationship between turns and voltage, if the end of the winding puts out 300V, the 40% tap should put out 120V (or is it 180V). Where did I go wrong?

I just attempted some Google research, but found information that goes over my head. It all seems to come back to this article: http://www.r-type.org/articles/art-140.htm In that article, the winding information is at the bottom. It looks to me like the UL tap is at 20% of the turns.

I'm having difficulty understanding how impedance is a factor here. Even after allowing for some interaction between the tube, the plate, the screen, and the transformer, it seems to me the turns between the plate supply (end of the winding) and the UL screen tap are going to keep the screen somewhat below the plate voltage.

Please help me to understand this. I see that I should not have posted what I did about UL.

Thanks.

Hi Phil

It sounds like you're cofusing impedance with resistance. The typical 50W 6L6 pair output stage uses an OT with a primary impedance of about 4K, but if you've ever measured the primary DC resistance (for plate current calculation), you see that the DC resistance of the winding is about 50 ohms from each side of the center tap. Keep in mind that UL taps are closer to, not farther from the center tap, where B+ is applied. For a 50W, UL OT, you've got about 20 ohms from the CT to the UL leads, and another 30-ish ohms from the UL taps to the plate taps. At idle, a 50W amp is going to have about 5mADC on the screens. So, by ohms law, E=0.005x20, or, 0.100 volt drop away from B+. With a B+ of 450V, the screens will be at about 449.9V.

The plates, which at 70% dissipation, will be pulling about 47mA each, will be at:
450-(50x0.047)=447.65V

Edit: and it's just a bit more involved than I stated becaue the first 20 ohms of OT will be carrying both the screen and plate current, so the formulae look like this:
Voltage at the screen tap: 450 -20x(0.005+0.047)=448.96V
Voltage at the plate tap: 450-20x(0.005+0.047)-(30x0.047)=447.55V

HTH,
Lou

[Phil_S]

Hey, Phil ... the center tap is in the middle, the UL tap is at 40% or whatever, and the plate tap is at the end of the winding...ALL on the same coil of wire!

D'oh! <headslap> Thanks. This I get. I was lost, now I am found. Give the very low DCR (of the wire) between the tap and the end, it's got to be less than one volt difference.

There is a whole other world of discussion about how the PT reacts to the AC signal that goes across to the secondary winding and out to the speaker. I feel rather silly here. Thanks for setting me straight.

[pdf64]

Ha, we've all been to 'Doh', I'm a regular visitor.
Something to note is that in UL, the screen grid static voltage may be a volt or two higher than the plate, as there's a lot more wire on the OT primary to drop voltage, between the screen grid tap and the plate.

[gui_tarzan]

I'd like to see some up close, focused pictures of the guts. Maybe one helicopter shot to get the whole thing in the frame, but also shots of the power section and tubes.

[titser_marco]

I'd probably just rebuild the power supply section first while waiting for the sockets. Anyway here are photos:

[Phil_S]

I'm no expert on the topic as I am an amateur, not a pro tech, so I don't see this kind of thing on my own amps. If I saw those sockets in my amp, they'd be gone in a heartbeat.

I see some old electrolytic caps. Those should be updated because they are beyond expected life, making them an accident waiting to happen.

The diodes are old. That doesn't make them bad. I'd test them with my meter anyway to see if they are OK. The simple truth is that 1N4007 modern diodes cost 10 cents each. I'd probably go ahead and R&R them as a very cheap upgrade that can be done in just a few minutes.

I'm assuming you know about the disconnected red wire ;-}

[titser_marco]

I'm no expert on the topic as I am an amateur, not a pro tech, so I don't see this kind of thing on my own amps. If I saw those sockets in my amp, they'd be gone in a heartbeat.

.

Thinking along the same lines too, as this is starting to look like a complete rebuild anyway, so might as well get new and higher-quality sockets.

I see some old electrolytic caps. Those should be updated because they are beyond expected life, making them an accident waiting to happen.

Haven't checked that yet, but I don't think this is an issue at the moment.

The diodes are old. That doesn't make them bad. I'd test them with my meter anyway to see if they are OK. The simple truth is that 1N4007 modern diodes cost 10 cents each. I'd probably go ahead and R&R them as a very cheap upgrade that can be done in just a few minutes.

Yep, and I got a quartet of 1N4007s just this weekend precisely for this purpose. I looked up the DS1K data sheet and while it is rated for 600v, I don't think there would be any harm in replacing them with newer diodes with higher rating, i.e. 1N4007.

I'm assuming you know about the disconnected red wire ;-}

Yep, that is coming from the choke that got taken out (I suspect) by the arcing in the output tube area.