5F1 BUILD BLOWS PREAMP TUBES

dig22

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Hi,
I built this 5F1 tweed champ and it fired right up. Sounds amazing. I played it for about and hour and no problem. The next day, no sound. I checked the tubes and the 12AX7 was smokey on top and cracked on the side. I replaced tube and played for another hour hour. Again, sounds amazing, no problem. The next day, same thing. No sound. The tube was in exact same condition as the first blown tube. Smokey on top, cracked on the side. I'm thinking, uh oh, I got a problem.

I've uploaded a few pics of the blown the and amp.

Here are the voltages as best as I can read them with me crummy meter.

I used weber transformers.

5Y3:
pins 2 & 8: 389
pins 4&6: jumps between 12.1 & 15. I have a feeling I'm using meter wrong on these)

6V6:
pin 3: 374
pin 4: 331
pin 5: 18.8 - 19.5 (jumps)
pin 8: 20.3

12AX7:
pin 1: 179
pin 3: 1.45
pin 6: 178
pin 8: 1.4

What are the correct voltages for this amp?

Thanks in advance!
 

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dan40

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Your problem is most likely not related to the voltage in the circuit. Tubes usually exhibit this issue when the glass envelope fractures which allows oxygen to enter and the silver getter flashing turns white. Your socket may be putting undue stress on the 12ax7 causing the glass to break. Did you happen to get too much solder down into the socket pins when soldering? Does the tube feel excessively tight when inserting it in the socket?
 

milocj

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@dan40 could be on the right track. I was taught to always put an unloved tube in the preamp sockets when soldering them to help keep the pins of the socket straight and to help keep any solder from running down inside of them.

It may not hurt to put a tube in the sockets and lightly touch up the solder on each pin to help relieve any stress that might be currently "soldered" in as a crooked socket pin.
 

dig22

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Your problem is most likely not related to the voltage in the circuit. Tubes usually exhibit this issue when the glass envelope fractures which allows oxygen to enter and the silver getter flashing turns white. Your socket may be putting undue stress on the 12ax7 causing the glass to break. Did you happen to get too much solder down into the socket pins when soldering? Does the tube feel excessively tight when inserting it in the socket?

I'll check for solder. Also, I twisted the 4&5 pin together. Could that be it?
 

dig22

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@dan40 could be on the right track. I was taught to always put an unloved tube in the preamp sockets when soldering them to help keep the pins of the socket straight and to help keep any solder from running down inside of them.

It may not hurt to put a tube in the sockets and lightly touch up the solder on each pin to help relieve any stress that might be currently "soldered" in as a crooked socket pin.

I'll try that. Thanks! Do my voltages seem ok?
 

dan40

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Also, I twisted the 4&5 pin together. Could that be it?

Most likely. The pins are purposely made to be a bit loose inside of the socket so that they have a bit of "give" when inserting a tube. If the pins aren't able to move a bit when the tube is inserted, the tube's base will be stressed and cracks will occur in the glass.
 

corliss1

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I don't "twist" any pins but I do face 4 and 5 toward each other and push them together a bit - makes it much easier to get multiple wires in there. I'd cast another vote for solder down in the pins/pressure on glass/whatnot.
 

dan40

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Your voltages look fine. Most tube amps will run fine on a wide range of voltages. Nothing in your numbers look odd and if the amp is playing and sounding good, the problem most likely lies in the socket.
 

dig22

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Off Topic... does anyone know if a 7408 is an acceptable substitute for a 6v6 in 5F1, 5F2a or 5E3.
I came across a quad set.

Thanks
 

dig22

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Most likely. The pins are purposely made to be a bit loose inside of the socket so that they have a bit of "give" when inserting a tube. If the pins aren't able to move a bit when the tube is inserted, the tube's base will be stressed and cracks will occur in the glass.

The tube is a tight fit. Must be it. I''l work on it next weekend and report back.

Thanks to all.
 

Jon Snell

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Your photo shows a classic case of stressed glass usually caused by rough handling or forced insertion to a socket with stiff/stuck recepticals.
If the socket recepticalse are bent to make wiring 'easier', this is damage common, especially on thinner glass or valves that were not heat treated before sealing the base to the glass tube.

Possibly the crack started at pin 4?
5F1 BUILD-101-stressed glass.jpg

A lot of Chinesium 'Gold plated' valve bases are extremely hard and if not left with a bit of movement for the pins, will crack the glass.
 

dig22

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I swapped out the socket and so far so good. The ceramic socket I removed was an extremely tight fit.

New question for this build. It sounds great. And is fairly quiet when dialed to zero, but there is still a slight hum. I'm thinking it's the grounding. But I'd like to hear other opinions before s tart disconnecting and reconnecting.

On this build, I used a brass plate behind the controls.
1. I attached the striped wires from the p.t. with the ac cord ground wire and attached those to one of the transformer bolts. Should I attach one of those striped wires to the brass plate?
2. I connected a wire from the ground lug on output jack to the chassis.
3. Should I correct either #1. or #2. above?

Thanks again.
 

VintageSG

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A slight hum is normal. Do not go grey chasing down that which cannot be heard when playing.

The protective earth shouldn't ( ideally ) be connected to one of the transformer bolts, but should have its own point. I can't recall why it's frowned on, but there are a myriad reasons why a dedicated power ground is a good thing. A bolt with star washers either side, passing through the chassis, tighten a nut to 'ft' then add a star washer, a tag, another star washer, a plain washer then another nut, again, 'ft' and never think about it again.

Have the layout for the G3. Note the PE and other ground points. They're close together physically too. Too many ground points can equal noise.
 

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dig22

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A slight hum is normal. Do not go grey chasing down that which cannot be heard when playing.

The protective earth shouldn't ( ideally ) be connected to one of the transformer bolts, but should have its own point. I can't recall why it's frowned on, but there are a myriad reasons why a dedicated power ground is a good thing. A bolt with star washers either side, passing through the chassis, tighten a nut to 'ft' then add a star washer, a tag, another star washer, a plain washer then another nut, again, 'ft' and never think about it again.

Have the layout for the G3. Note the PE and other ground points. They're close together physically too. Too many ground points can equal noise.


Good to know thanks. When I do that, should I bother attaching one of the striped transformer wires to the brass plate or leave it?
 

Lowerleftcoast

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You can try connecting the red/yellow CT from the transformer to the negative of the first filter cap. You can also try connecting the green/yellow heater CT to either one of the places shown below. This location is know as elevating the heater CT. No promises for reducing the hum but these two changes might help reduce hum.

Inked5F1 BUILD-109-IMG_0868_LI.jpg
 

dan40

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Glad the new socket got you up and running again. As the fellas mentioned, a little hum in these amps is quite normal. The power amp is single ended and does not have the ability to cancel hum in the same manner as a push pull circuit. It may help to try a few different preamp tubes in v1 as they can also cause a bit of hum unless you find a really quiet one. The grounding changes the fellas mentioned are definitely worth trying but don't be surprised if you still have a bit of hum afterwards.
 




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