Burning through Pilot Light Bulbs

[Mitch]

Hello,

This is my first post here, and I’d like to thank everyone for the great resource! I just finished building my two stroke amp from the TAN kit (thanks Andy!), and it sounds great – everything I hoped for.

The problem is that it burns out the pilot light bulb every 10 minutes or so. Since the kit came with 2 bulbs, I’m out of bulbs now. I measured all the voltages that were mentioned at the end of the Two Stroke chapter in Hunter’s book, and all the numbers are within the range allowed for in the book. I’m measuring 6.3V AC at the pilot light holder. I used the 100 ohm resistors supplied with the kit to make the artificial center tap from the pilot light holder to the grounding terminal bolted on via the PT corner bolt.

I’m worried that this might be indicative of a more serious problem, and obviously it would be great to have to feed the amp a new light bulb every 10 minutes.

The only writing on the bulb is “AML-45”

I couldn’t find any mention of this problem elsewhere on the forum, and I’d be grateful for any ideas or advice.

Thank you!
Mitch

[Mitch]

OK – now things are getting a little weirder.

Since my first post, I’ve been playing the amp some more (without the pilot light bulb) and I’ve noticed a couple of other strange things.

1. After playing for a five minutes or so, the sound level slowly fades out (sounds like it would if you turned off the amp while playing). Within 5 – 8 seconds, the sound returns – as if nothing had happened.

2. Most recently, it has started making a “static” sort of noise that sustains after a chord is played – until I mute the guitar strings and then all sounds stop (including the static sound). The only similar sound I’ve heard before was with a Fender Super Champ that needed replacement power tubes.

At this point, I turned the amp off to do some more research about what might be going on.

Thanks for any possible insight!
Mitch

[Robin]

Welcome Mitch,
I’ve not encountered the pilot lamp issue previously. The artificial center tap should have no impact on the heater circuit (other than to help make it balanced and quiet).

As for the fading in and out and crackling, I would start trouble shooting by using a chop stick to carefully poke around the circuit and see if you can find a loose or marginal solder joint. Pay particular attention to the tube socket lugs and areas that heat up and cool down. Double check all the solder joints in the heater circuit. When the sound fades out, are the tubes still on (filaments are still glowing)? Post some pics of the chassis if you can.
Good luck,
Robin

[Mitch]

Thanks Robin!

I’ve had a bit more time today to play through the amp and poke around a bit with a chopstick and a multimeter.

First off, I couldn’t replicate the powering down problem from yesterday, even though I left the amp on for over an hour on two different occasions, and was able to play through it for about 20 – 30 minutes during each of those periods.

I bought some more 6.3V bulbs from Radio Shack, and that seems to have solved my pilot bulb problem – the replacement bulb is still alive and well after being lit up for more than one hour. Andy thought maybe the bulbs I received in the kit were from a bad batch – that seems consistent with what I saw today.

As far as the “static” sound, it was much reduced today, and I was only able to hear it on a couple of times. On one such occasion, it continued on even after I completely stopped playing, and I was then able to get the chopstick and poke a bit. When I poked on the socket where the output transformer is attached (Pin #4 on the power tube), the static stopped.

I don’t think the problem is a cold solder joint, but I’m wondering how tight the tubes should be in their sockets. The Power and rectifier tubes on my amp seem pretty loose. They don’t fall out on their own, but I can very easily “rock” them back and forth in the sockets with minimal pressure (compared to the preamp tube). Could the static problem and the previous powering down problem be due to tube pins that are loose in their sockets? Can the sockets be tightened up?

I don’t know if these voltage readings are helpful at all, but here is what I’ve measured:

Green heater wires at pilot light, power tube, and preamp tube: 6.4V AC
Power tube plate (pin 3): 373 V DC
Power tube grid (pin 4): 338 V DC
Preamp Tube plate (pin 1 and 6): 152 V DC

These numbers are a little higher than the expected range given in Hunter’s book, but are still below the values he gives as being at the upper limit of acceptable.

I am trying to add pictures of the chassis using the “attachments” function but I do not see them appearing here in my message. After I select the photo, it returns me to this text screen, and I don’t see the pictures added to my message.

Thank you again for your time!
Mitch

[Robin]

Those voltages are all fine, even higher would be ok. The sockets can be tensioned (usually).
I use a dental tool to gently push the metal sleeves toward the center of each hole. A tiny screwdriver would work as well. Sometimes a socket just won’t tighten up, in which case, it’s time to replace it, but with new ones, you can probably just massage them a little and be good to go.

[Mitch]

Thanks Robin!

I’ll try tightening the pin sockets a little bit and see if that helps get rid of the little bit of “static” I’m still hearing.

Do you know how folks usually post pictures to this forum? I see there is an “image link” in the tool bar above this text window, and an “add file/Attachment” tool below the text window. I didn’t have any luck with the Attachment feature, and I don’t have a ready way to upload photos to a different URL for linking them. I looked for, but didn’t see a FAQ or a “sticky” forum posting dealing with this question.

Thanks again for your help,
Mitch

[Robin]

I’m having trouble posting photos today as well, I think Andy has a work around for it.

[Andy]

Yeah, the image thing is hit and miss. The whole system we use here is a bit outdated and the issue is one where I can’t update without breaking something. I have on a to do list to migrate to a better platform but it may cost a bit so I’ll have to hold off a bit.

A couple other things to check on the static issue.

1. Make Sure all the tube, transformer, and most important, speakers are mounted really tightly.
2. Check to see if perhaps the speaker is causing the static issue. I have seen this in some Renovo Amps
3. Check the Feedback connections – this is the 68K resistor that couples to the output jack. I have had some wierd static issues (usually only on one note, ie E all up and down the board) that were presence/feedback related.
4. Check the output grounding. The jack needs to be in contact with the chassis. I had an issue where I used plastic jacks and there was no ground connection. I forgot to wire the negative to ground and had static issues

Maybe those will help. Glad you found out about the bulbs. I’ll make sure to get replacements for what we have in stock!

[Andy]

Tested the attachment. I added the File per the box below, but did not “insert” it. It shows up in the post, at least as far as I can tell.

[Mitch]

Hi Andy,

Thanks for the list of things to try. I tightened up the speaker mounting screws a little bit, and checked the tightness of the transformer and tube socket mounts. As I mentioned before, the static issue seemed less pronounced yesterday, and that trend continues into today. I can definitely live with what I have going right now – the amp sounds really great, and I’m having lots of fun playing it.

I am still unable to get the Attachment feature to work. When I click on either “Attachments” or “Add File” in the box below, I get an “upload file” window that lets me select images from my hard drive. When I select one, it looks as if it will upload, but when I click the “open” button from that window, the window goes away and I’m back to the “reply topic” screen, but no image has uploaded. If it works for you, but not for me, it may well be something weird on my end (I’m a newish laptop that came with Windows 8, and I’m not used to this operating system).

Again, thanks Robin and Andy for lending a hand and helping out the new guy. This is my first time building an amp, and you guys have been awesome!

Mitch

[Robin]

Check your private messages

[beelzebum]

Just to let you know, #45 bulbs are for 3.2V, #44 are for 6.3V. The wrong bulbs might have been accidentally mixed into the batch. As for a slow fading away of sound, that is usually due to a problem with a grid leak resistors. I would check the value of the leak resistor on your power tube, and also that it is well grounded. Static could also be caused by poor grounding on the grid leak. Just a suggestion though. I forgot to mention that the screen resistor acts the same as the grid leak, in that it can also add static and drag the tube into cutoff.

[Mitch]

Thank you Beelzebum,

I unfortunately had to return my copy of Dave Hunter’s book to the library – it was overdue. I tried to do some research online to figure out which resistors are the grid leak and screen leak resistors, but I’m still scratching my head a bit.

I know the cathode resistor is the 500 ohm resistor in parallel with the 24uF capacitor (coming off of pin eight), but I’m not sure which resistors would be the grid and screen leak resistors. Is the grid resistor the 1.5 Kohm resistor connecting pins 5 and 6 on the tube socket? The 220K ohm one on the eyelet board between power tube pin 6 and ground? I think that pin 4 is the screen grid pin, but I can’t tell which resistor would be the screen grid resistor.

I’d appreciate any assistance in locating the exact resistors you are referring to. Thank you for your time and expertise!
Mitch

[beelzebum]

Sorry, there are quite a few schematics floating around for the two stroke. I assume in yours they did not have any screen stopper resistor. This would normally connect between B+ and the screen on pin 4.
The grid leak would be the one on yours connecting pin terminal 6 to ground. The one from pin 5 to 6 is your grid stopper.
So, what the grid leak resistor does is reference the control grid to ground. If it is too high of a resistance, or if it comes loose from ground, then you start to build up a negative voltage potential between your coupling cap and the grid. This will cause your tube to cut off. And if there is a loose connection, then it can introduce noise. As for the screen stopper, this is mainly to protect the screen from drawing too much current, but it can also introduce noise. And if it disconnects from B+ then it can also drag the tube into cutoff.

[Mitch]

Thanks for the additional information!

I don’t have a resistor between B+ and pin 4, so you are correct about that.

I checked the grid leak resistor, and It measured right at 220Kohms, just like the schematic calls for. I also re-soldered the connections between pin 5 on the power tube and ground just to be thorough.

I still have a little bit of the static noise at times. The static starts as the guitar notes start to fade – like a strange static “tail” on the notes. Sometimes the static fades out, and sometimes it sustains more or less indefinitely (until I play more on the guitar).

The static seems to be more of an issue at higher volumes.

When the static is sustaining, it stops if I gently poke pin 4 or 5 on the power tube socket with a chopstick.

Thanks again for the assistance!

Mitch

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