Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: toneseeker on September 15, 2014, 07:46:02 am
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5F1 power up issue.
:BangHead:
I am building a 5F1 using a Hoffman board and small parts. I am using Classic tone transformers. OT 40-18031. Specks: http://www.classictone.net/40-18031.pdf (http://www.classictone.net/40-18031.pdf) PT 40-18019 Specks: http://www.classictone.net/40-18019.pdf (http://www.classictone.net/40-18019.pdf)
The only changes I have made is adding an output impedance selector a power switch/volume control and duel inputs.
I am not sure I got the input jack wiring correct.
On power up with only the 5Y3 tube installed this is what I get. Indicator light comes on. Filaments have 7 volts. 5Y3 filaments have 5.9 Volts. B+ goes to 407 VDC. Then after 15 seconds the power switch starts to smoke.
I connected the PT this way: From power cord Black to tip of fuse. Top of fuse to bottom of power switch. Top of power switch to PT transformer black. Power cord white to the other black of the PT. Green to star ground.
I am testing with a 300 watt light bulb in a current limiter. No glow from light.
Here is a link to pictures: https://plus.google.com/photos/102698715276172838187/albums/6059273778220801121 (https://plus.google.com/photos/102698715276172838187/albums/6059273778220801121)
Any suggestions are appreciated.
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"you do not have permission to view this album"
error message when clicking the provided link.
--pete
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Got same error as DL, what is your switch rating?, fuse rating?, I'm not sure but think 65-100w light is closer to what you should be using. should be able to search this site for that. Do a good visual pull the 5y3 and see if the results change, measure each plate pin(4 and 6) to PT CT. This will be AC volts, they should be about the same.
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Photos should be shared now.
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Look for a short to ground somewhere after one of the secondaries - could be a heater pin touching a tube socket mounting bolt or one of the lamp terminals touching the chassis or each other, or the B+ line to the reservoir filter cap touching the chassis underneath the board or some such thing. This is my 1st inclination (not having looked at any pic's 'n'all, but going by your description of the smoke)
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The switch is wired correctly. I suspect a faulty switch. Disconnect the two black wires from the switch and connect them together. Does the amp work?
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I am testing with a 300 watt light bulb in a current limiter. No glow from light.
Your bulb is too big, you want to use a 60w or 100w bulb.
Brad :icon_biggrin:
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He says the "higher the wattage the less the resistance." And we want (some) resistance, lower wattage bulb, to limit the current.
Brad :icon_biggrin:
Edit; Toneseeker did you delete your last post with the link to the Uncle Doug video clip, or did I goof and delete it?
I'm sorry if I deleted it. :BangHead:
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What model switch/volume control did you use?
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I deleted the link. Here it is again. This is his explanation:
https://www.youtube.com/watch?v=wRFRwOnLsZI&list=UUuR4hQTXkG_KxozLxwPzEjQ (https://www.youtube.com/watch?v=wRFRwOnLsZI&list=UUuR4hQTXkG_KxozLxwPzEjQ)
1.) The resistance of the cold filament is indeed 1.6 ohms but (and this is important) this resistance increases significantly as the filament heats up.
2.) The Photoflood bulb is rated at 250W, which means that the filament does not get hot enough to fully glow until it reaches its power rating of 250W / 120VAC = about 2.1A current flowing through it. At this temperature, the effective resistance of the filament is 120V / 2.1A = 57 Ohms. So the filament resistance varies from 1.6 Ohms cold up to 57 Ohms hot. As more current flows, the filament gets hotter and its resistance increases. This is why the bulb will only draw a maximum of 2.1A even though it is plugged into a receptacle that is capable of passing 30A or more, and this is why the bulb is considered to be a "current limiting" device.
3.) I selected a 250W bulb because virtually all of amps I deal with draw less current than the bulb at full brightness. For example, a Fender Deluxe Reverb draws about 1.5A @ 120VAC = 180W vs 2.1A and 250W for the bulb. The bulb may get a little warm but it should not glow when the DR draws its 1.5A through it. If it does glow, then the amp is drawing too much current and the glowing bulb not only warns me to unplug the amp, but its increasing filament resistance and the fact that the bulb will not pass more than 2.1A (instead of 15A to 30A if the amp were plugged directly into the AC outlet) protects the amp circuit from harm.
I hope this makes sense and clarifies the issue. Thanks for your excellent question.
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I bought the switch from tubedepot:
RV24A01F-10-15R1-A Alpha 24MM 1M Audio Taper Pot with Switch (1) @ $3.95 = $3.95
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OK, just glad I didn't delete it by mistake.
I watched the clip before you took it down.
I've never heard about anybody using that big of a bulb in their limiter. I guess he's using that large of a bulb so the voltages don't drop but we don't try and measure voltages when using the limiter just checking for shorts.
Once it passes the 'short' test, we disconnect it, then we check the voltages if needed.
In what you posted he says 2.1 amps can pass through the 250w bulbs filament, that's still a lot of current. Smaller bulb will pass less.
Brad :think1:
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Thanks for all the tips yesterday!
Last night I changed the light bulb to 60 Watt. Pulled the filament wires and re-soldered. Checked for any shorts on the tube sockets. Then turned the power on. After 15 seconds, when b+ started to grow, the light went on and got very bright. Turned off the power.
If someone could look at the input jacks I would appreciate it. My next plan is to take out the dual jacks and volume/switch and put in one input jack, switch and separate volume.
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After 15 seconds, when b+ started to grow, the light went on and got very bright.
How bright? Would you say it was as bright as a fully powered 60W bulb? Even if the light reaches 1/2 brightness, you have a problem with the B+. Pull the 6V6. Does the light get dim? If so, you have a problem in the output stage. If the light stays bright you have a problem in the B+ filter stages.
A normal indication with a cold 60W bulb and a cold 5F1 amp would be... Turn power on and the light briefly glows somewhat bright (nothing like a full bright bulb)and fades to a dull glow as the tube filaments warm up. Then the dull glow will increase slightly as the B+ builds up and the output tubes begin drawing cathode current. The lamp should never reach 1/4 full brightness for a properly working amp. Once you have a normal lamp limiter indication, it's time to plug the amp straight into the wall. Leaving the amp connected to a 60W bulb will really starve the amp. It may pass a guitar signal, but it will not be happy and all voltages will be low.
If someone could look at the input jacks I would appreciate it. My next plan is to take out the dual jacks and volume/switch and put in one input jack, switch and separate volume.
Your input jacks appear to be wired correctly --- IF --- they are switchcraft jacks.
You still have not said if the amp works at all. What kind of sound if any does the amp make? Have you actually had a guitar plugged in yet?
Disconnect the NFB wire that goes to the board at the speaker jack and leave it disconnected until the amp is working.
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I am still at first power up with just the 5Y3 installed. No 6v6. The bulb is at full glow. I will pull the NFB and check the board again.
Thanks!
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I am still at first power up with just the 5Y3 installed. No 6v6. The bulb is at full glow. I will pull the NFB and check the board again.
No need to pull the NFB. Make sure all three filter caps are discharged. Then check for low resistance from each filter cap positive end to ground. They should all have a high resistance reading. A low resistance reading on any of them would indicate a bad cap or a wiring error.
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Sluckey, you hit it out of the park!
First cap 10K, middle 0.3, third 20K. Does that mean that I fried it? I will recheck wiring.
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Sluckey, you hit it out of the park!
First cap 10K, middle 0.3, third 20K. Does that mean that I fried it? I will recheck wiring.
Not fried. But there is a dead short on the middle cap. Unsolder the positive end of the middle cap and lift away from the board. Recheck resistance of the positive end to ground. If it's still very low, throw the cap away and get another good one. But, if the resistance is now high, look for a short on the board or pin 4 of the 6V6.
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the cap reads 13m. That seems low correct?
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the cap reads 13m. That seems low correct?
Are you saying 13 Megohm? If so, that's very high. Measure resistance from ground to the board terminal where the cap positive lead normally connects? Is it still shorted?
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Ok. That is 0.3. I will look for a short. Thanks.
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That is 0.3.
Then you mean 300K?
Brad :icon_biggrin:
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I'm pretty sure he meant 0.3Ω, or about as low as his meter will read.
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Look under the board at these resistors. Does one of these leads touch the chassis?
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Steve has a point.
If you pour too much hot solder down a lug and it contacts the chassis, you have a problem.
If a chassis screw comes up from below and touches a lug, also a problem
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> After 15 seconds, when b+ started to grow, the light went on and got very bright.
Means you have a short on the B+ line.
> If someone could look at the input jacks I would appreciate it.
The input jacks can't cause this symptom. Maybe no-signal, but not smoke.
Limiter lamp size..... a 250W will let any small-medium amp run at nearly full power.
If you are getting SMOKE, you want much less current (lower Watt lamp) so it does not smoke so fast. Certainly for initial smoke-test, 60W should be a good value, even though it may not bring the happy amplifier up to 3/4 full voltage. It won't play well, but it should play, without smoke. *Then* you can go with the 250W, or straight to naked full power.
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Thanks so much for all the tips. I found the short under the board. When I attached the OT, I put the bolts under the chassis. It was just tall enough to touch the board. I will reverse them tonight and test.
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There ya go
That's why I put the screw heads under the board and the nuts on the outside of the chassis
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That's why I put the screw heads under the board and the nuts on the outside of the chassis
I do the same and I like to use standoffs that will give me at least 1/4" between the top of the bolt heads and the bottom of the eyelet/turret board.
Brad :icon_biggrin:
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I hope Uncle Doug reads this thread!
The original schematic has a 2 amp fuse listed and I dropped it down to 1 amp thinking that would be ok
http://el34world.com/charts/Schematics/files/fender/Fender_champ_5f1_schem.pdf (http://el34world.com/charts/Schematics/files/fender/Fender_champ_5f1_schem.pdf)
You guys think it should be dropped down to a 1/2 amp fuse?
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Crap. Where did my post go? Doug, you're the wrong Doug. 'Uncle Doug' was the guy in that video that insisted on using a 250W (or larger) bulb in a current limiter. He has several amp related videos on YouTube.
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Crap, I must have hit modify instead of quote
I did that to a PRR post also last week
Sorry about that
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Where did my post go?
That post is worth rewriting, IMO.
I think you explained the problem with using too large a wattage bulb very well, it would be helpful for guys in understanding a light bulb limiter and setting to rest 'Uncle Doug's' (not our host :laugh: ) youtube post.
Brad :icon_biggrin:
Edit; In fact I think (if you rewrite it) you should add it to your web site on the light bulb limiter page. That youtube post is gonna have some guys burning up some amps/PT's from/by his own instructions that they ~think~ are being protected but in reality are not. :w2:
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Shouldn't even a 2 amp blow with a dead short? I have blown a couple 2 and 3 amp fuses without damaging any amp parts.
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That depends on where the dead short is. Look at the schematic.
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So if the fuse and switch were both on the hot and neutral was straight thru, assuming the switch was rated more than the fuse, the fuse shoulda worked? I have seen many things fryed an the fuse was saved, just never spent lots of time wondering why, now I have time to wonder. Thanks
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I don't think the switch smoked at all. That would make no sense. I think the switch is still OK.
I believe the 10K resistor sitting directly below the switch is what produced the smoke. Look at the attached pic. 400v B+ from the rectifier is on one side of that resistor and the other side of that resistor is shorted to ground. That 10KΩ 3 watt resistor is trying to dissipate 16 watts! That's the real smoker. And since smoke rises, it only appeared that the switch was smoking.
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Look for a short to ground somewhere after one of the secondaries - could be a heater pin touching a tube socket mounting bolt or one of the lamp terminals touching the chassis or each other, or the B+ line to the reservoir filter cap touching the chassis underneath the board or some such thing. This is my 1st inclination (not having looked at any pic's 'n'all, but going by your description of the smoke)
My hunch was correct then
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thanks Steve, I seen the resistor/short but I didn't follow the smoke! Makes way more sense now.
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Toneseeker, got it working yet?
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Ok, just had time to start further testing. Using the light bulb limiter all voltages are lower than expected. I do get sound with a small amount of hum.
Then I plug into the wall directly. After caps charge up, I get a high pitch squeal from the speaker. Can't check voltages yet. The wife is sleeping.
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I get a high pitch squeal from the speaker.
That's probably a NFB issue. Temporarily disconnect the NFB wire from the speaker jack that goes to the board. If the squeal disappears you will need to reverse the OT primary wires. Then reconnect the NFB wire.
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Thanks, that did it. The amp has very little hum at full volume. It is a little to clean for my taste. What would you suggest I change to get more breakup.
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Glad you're up and playing! Try going to the achieve forum further down the list from this one, a couple posts in you will find a list of like 20-30 questions, that I still ask! Read everything, try as much as you can, then come here and have these folks get you back on track!
My2cents, add a 1 knob tone stack and play with the 2 cap values, try and get a sense on how they change your sound, then keep going, you will wind up with "your" amp and knowledge that would be hard to learn at $100 a credit hour.
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Thanks, that did it. The amp has very little hum at full volume. It is a little to clean for my taste. What would you suggest I change to get more breakup.
Put a 22µF cap across the 1.5KΩ resistor that connects to V1 pin 3. Positive end goes toward the tube.
Or you could put a cheap ratty 8" speaker in it like Fender did.
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--pete
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totally insane unusable gain. never tried it - it should work in theory. tweak 680R to personal taste. (560,750,820,910,1K etc.)
--pete
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Thanks, that did it. The amp has very little hum at full volume. It is a little to clean for my taste. What would you suggest I change to get more breakup.
Turn it to 12.
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It is a little to clean for my taste. What would you suggest I change to get more breakup.
Try adding cathode bypass caps on V1A and or V1B
Change the plate resistors on V1 to a higher value sort of like a Gibson GA5 which has 220k plate resistors
http://el34world.com/charts/Schematics/files/gibson/Gibson_GA-5.pdf (http://el34world.com/charts/Schematics/files/gibson/Gibson_GA-5.pdf)
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totally insane unusable gain. never tried it - it should work in theory. tweak 680R to personal taste. (560,750,820,910,1K etc.)
--pete
Okay, I really don't understand what is going on there.....can someone please enlighten me?
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You have to look at the layout pic to see all the stuff Pete is suggesting. He modified the NFB circuit so he could put a bypass cap on the cathode of V1B. He also put a bypass cap on the cathode of V1A. Both of those bypass caps will significantly increase the gain of both halves of the 12AX7.
In addition, he's suggesting tweaking the 680Ω resistor he added to the NFB circuit. Making it larger will lower the NFB which will also increase gain.
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Okay, that helps. I did look at the layout....that's the problem! I understand schematics a little better than layouts, so I couldn't quite picture what the changes were doing. Thanks!