Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: choosebronze on July 25, 2019, 12:59:02 am
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Spent the weekend building my Dual Lite (with MV). I got it powered up yesterday. The "Marshall" side works. Haven't had a chance to crank it yet.
The AC15 side has no sound. That helps me narrow things down to the AC15 preamp. Voltages through the amp look about right. I used a 290-0-290 PT, and my voltages are a bit lower than Sluckey's throughout. B+ is 293V which seems pretty low. Most voltages are about 10% lower than the schematic. Voltage on the EF86 pins all look good though.
I ran through Rob Robinette's troubleshooting page. I do hear pops when probing the plate of the EF86. The EF86 has a nice glow. I assume the EF86 is functioning properly. I don't have a sub & nobody sells them locally. I ordered a spare but it won't be here for a few days. I'd like to have smarter people think about this before I start re-wiring. I went through checking continuity and voltages to see if I could find a short, all I found is a couple things I don't understand:
1. At the Brilliance switch I do not have continuity between the two sides of the switch. True in all positions. Seems to me that means signal can't get through the switch and that's probably my issue. I ordered the right switch (I don't know how to test if I was sent the MBB one). I do have continuity between the EF86 plate and all pins on the in side of the switch, so that side's tied together properly.
2. I'm getting weird readings on the 5V PT secondary. Between pins 2 & 8 on the rectifier I have 3.28 VAC. I disconnected the wires and got the same readings between the yellow wires. My DMM has done some weird things lately, but it keeps giving me the same number on this. What happens if that's accurate and a rectifier doesn't get enough voltage on the heaters? Does it just not smooth the voltage well? Does it make the B+ voltage lower? Am I missing something here?
Anyway, sorry for the long post and my ignorance. Thank you for any help you guys are able to give. I've reached the point where things I'm looking at don't make sense anymore so I should probably take a day or two off and come back to it.
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1. There should be no continuity between the sides of the switch. The signal has to pass through a selected cap to get through the switch. But you may have a wiring error with the switch. You can easily bypass the switch to see if that's the problem. Just connect a wire (or a gator clip lead) from the right side of that .1µF cap to the top of the volume control. You don't have to undo any of the switch wiring. Got sound now?
2. Since you have B+ I suspect your meter may be lying to you. Otherwise the PT 5VAC winding is bad. Test your meter by measuring the 6.3VAC between pins 4 and 5 of an EL84. What reading do you get?
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You're always right, Sluckey. I jumpered around the switch and the AC15 channel works. I'll rewire the switch. I actually bought 2 since they were cheap. I might just start the other from scratch. I followed the close-up drawing you posted somewhere. On the "in" side, all the cap lugs are tied together, and that is tied to the common lug, while the "out" side the common lug just goes to Volume. Just making sure I have that right.
I thought I would have continuity between whichever two pins on the switch were closed. These rotary switches confuse the heck out of me.
As for voltage, the 6.3V secondary reads at 6.28VAC on the EL84 heater pins. So maybe the 5V winding is bad? If it were, would it have an audible effect? Steve, I read your post in another topic about how you can't reliably estimate B+ based on a multiplier, and I understand why, so I'm not going to say my B+ with a 5Y3 should be 290 * 1.1.... but I would expect a rise greater than ~3V. Could that be an effect of an underpowered rectifier?
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I thought I would have continuity between whichever two pins on the switch were closed.
That's true. But that does not mean you will have continuity between the two red wires (see layout) on the switch.
As for voltage, the 6.3V secondary reads at 6.28VAC on the EL84 heater pins. So maybe the 5V winding is bad?
It would seem so. Which PT do you have?
Pull the 5Y3 and connect a 1N4007 between pin 4 and pin 8 of the 5Y3 socket. Connect another 1N4007 between pin 6 and pin 8 of the 5Y3 socket. The diode cathodes tie together on pin 8. What is your B+ now?
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Thanks, I'll try the diodes when I get home and post voltage. The PT is the Mojotone 18w. Just to make sure I'm not an idiot, the white wire (6V) should not be grounded, right? I've just got it in a wire nut. Yellow and yellow/white to the rectifier.
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Yellow and yellow/white to the rectifier.
That's correct. The white is not used. Just for giggles, what voltage do you measure between the white and yellow? And white and yellow/white?
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Well, I usually don't find out I'm out of parts until I reach for them. Surprise, I'm out of 1n4007's. I can stop by a shop tomorrow and get more.
White & Yellow/White: 3.34v. That's the same number I'm getting on Yellow & Yellow/White now. So maybe those are crossed/shorted/tied/something internally?
White & Yellow: 6.65v
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The transformer was tapped wrong during manufacturer. What you have is a 6.3VAC with center tap. What you should have is a 6.3VAC with a 5V tap. That winding is wound wrong.
I wonder if the manufacturer just got the leads connected to the wrong windings. Let's see if the 6.3 winding for the other tubes is wound correctly. Measure the voltage from each green lead to the green/yellow lead. I'm hoping your green/yellow is connected to chassis. What are the two voltage readings?
In any case, Mojo should replace that PT free of charge. Talk to them. Refer them to this thread. The Mojo guys should know Doug very well.
Most people use that transformer with an EZ81 rectifier tube so the yellow and white wires would be used to provide 6.3VAC and the yel/wht 5v wire would be capped off. You may prefer to switch to an EZ81. You would need a noval adaptor plate to put in the octal hole. I would not be surprised to find that Mojo has a whole batch of PTs just like yours. :icon_biggrin:
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Green/Yellow is to the chassis. The green to green/yellow readings are 3.32VAC and 3.33VAC. I talked to mojotone on the phone yesterday and they seemed like they have no problem replacing it. I go real slow, I try to take my time and pay attention. I'm just glad this wasn't a mistake I made!
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Get it replaced ASAP. I would even request they check the 5v tap on the replacement to ensure it really is 5v
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Well I got the new PT today and got it all hooked up. This amp screams. I love the crunch and I'm glad I put in the MV. I've got a few new questions though:
1. The big one - I was playing for a while on both channels. Sounded great. Then I was playing hard & the AC15 side just cut out. It was an instant drop to no signal. I switched over to the 18w side and it sounded fine. Switched back to the AC15 side and it was working fine. Easily replicate-able, happens after a few seconds of hard playing on the AC15 side. There's also been some crackling sounds on the AC15 side. Made me think it's the DPDT channel switch. But after doing this a few dozen times I noticed that after it cuts out, I switch to the 18w side, and the 18w side feels like it's at half volume, or muffled, or something for maybe 2-3 seconds and then it comes up to volume. That seemed like something the switch wouldn't have much control over. But I can try a new switch I guess, it's cheap and easy.
2. This amp is hot as hell. Holy moly. I don't think I've ever felt tubes or a chassis running this hot. Aside from the typical "yea, tubes make heat," is that what I should be expecting? Forget about how long I could my finger on the power tubes, I can't even touch them for an instant. No red plating though, everything looks fine.
3. Last week I happened to see a 150w incandescent lightbulb at the store so I bought it. I haven't seen many over 60w lately. I've never used a light bulb limiter. I started this amp up and played it. Then I remembered the lightbulb, built the limiter, and took out all the tubes to startup from scratch again so I could see what that process looked like and how the bulb reacted, for the future. The bulb never dimmed at any point of my startup. Does the bulb mean I have an error somewhere in the power wiring?
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IF the bulb did not dim, you have a short in the power wiring...
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Okay we can cross one of those off. Like I said, I'd never used a limiter before which means I've never used the one I built. Figured maybe I wired it wrong. So I grabbed a 3 prong extension cord off the shelf, cut it, and wired it right into the bulb socket like the pic in Sluckey's info doc. Now the bulb does glow dimly when the amp is powered up. I never see a bright bulb when I power on, it's only dim. But after reading Rob Robientte's site, sounds like that's probably because I'm using a high wattage bulb.
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Now the bulb does glow dimly when the amp is powered up. I never see a bright bulb when I power on, it's only dim.
That's good. Time to put the lamp limiter away and plug the amp straight into the wall.
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Bug bombed the garage last week but I guess that doesn't work on Gremlins... Went out to troubleshoot this morning and the problem changed. Plugged straight into the wall. This morning I had 1) no audio on the AC15 side, which was working but cutting out yesterday. 2) if I start the amp up on the 18w side I have faint audio for about 20 seconds and then it fades to silence. Swapped V3, no change.
Came back this afternoon and it's more like it was yesterday. The 18w side has been working for about 15 minutes now with no issue but I'm not super confident about it. The AC15 side has done all 3 of these things within the last hour 1) no sound, 2) sounds fine but cuts out when I strum hard, and 3) let me play for about 5 minutes with zero issues.
Weird things I've noticed: there are some very strange intermittent sounds. I did get a very brief hint of almost motorboating, so maybe I need to swap the OT leads? But that doesn't explain why signal would just drop on the AC15 side. There has been some background squealing noise and crackling, especially when adjusting the pots. Adjusting the 18w volume pot can introduce noise when I'm playing the AC15 channel. Maybe that's just a scratchy pot that needs to go.
Then the Gremlins came again. First the amp did one big whirr. Hard to explain. It sounded like my window AC was turning on, but it wasn't. Through the speaker it sort of started like a faint noise and then sounded like an electric motor revving up. Then these noises started. https://soundcloud.com/jaredcohen/duallitenoise/s-v3SCt - This is the amp, hooked up to a speaker. There's a 1/4" cable plugged in to the amp, and I'm just holding the other end of the cable in my hand (I'm not touching the plug, no guitar connected). The sounds are changing as I move my hand around, and adjust the pots. I'm totally stumped.
Edit to attach my voltages
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no guitar connected
you're letting all the bugs in! your amp is now a receiver, NOT an amp
what happen with NO cable plugged in?
I'd just hardwire the bottom ch n make it work, Then move to the other, it's lots easier to eat a whole pie 1 piece at a time :icon_biggrin:
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I can't argue with that. Thanks Shooter. I hooked the 18w channel up and played it for ~30 minutes. No problems when playing. I do have noise when adjusting the Vol, Tone, and MV pots. And a squeal if the MV is all the way up. I'm guessing this is because I didn't use shielded cables for the MV (I wasn't sure how long "long" wires were). I can swap out those with shielded though.
With the MV turned down, I do have high-end loss. Any recommendations on a different value for the tone cap to get more treble through without turning into an icepick?
Assuming the shielded wires fix the squeal, I'll wire up the AC15 next. Thanks again guys for all the help. I can feel I'm close. I just want to play some loud rock music.
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Which channel switch are you using?
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A heavy duty on-on dpdt.
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Tonight I hard wired the AC15 side right to the input jack. No audio. There's a faint hum which gets louder with the volume knob. The Brilliance pot gives an audible pop through the speaker with every click.
I grabbed a chopstick and with touching any component, or tube, or even the chassis itself, there's a sort of metallic noise. Hard to describe. Maybe like gently tapping two screws together. But if the Brilliance switch and Volume pot are audible in the output, my problem is before them, right? That narrows it down to a handful of components. I was dumb and ordered an EF86 replacement from Europe. Should be here any day, maybe it's just a bad tube.
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If the 18W preamp works fine then it's most likely the EF86 or the wiring around it.
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while ur waiting, try putting a gatorclip on the "right" side the .1uF coupler to the "top" of volume pot to jumper out your rotary switch for testing
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I'm losing my mind... I got the new EF86, put it in, and hard-wired the AC15 channel (and I jumpered around the rotary). No audio. Not even a hum or hiss when cranked. Total silence. I reconnected the 18w channel that I know works, which now also had no audio, and I started probing. I found 318 VDC at A, 303 VDC at B, and 0.75 VDC at C. On the 8.2K resistor the leg to B was 303v, the leg to C was 0.75v. I start thinking maybe it's the resistor or the C filter cap.
Then I probe "B" on the board again to verify the numbers, because it seemed wacky. On contact I hear a little crackle and that mechanical whirring noise through the speaker again, and the input is live. It almost sounded like something "charging up" in a sci fi movie for about 3 seconds. I took the probe off, the amp worked fine. Power the amp off and back on, same thing. Back to low voltage on C.
I'm completely stumped.
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> I probe "B" on the board again .... On contact I hear a little crackle and that mechanical whirring .., and the input is live.
You have a bad solder joint.
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Thank you PRR, I'm sure you were right. All the joints looked okay but I re-flowed the ones around there and I'm back to where I was. The 18w channel works fine when directly connected, the AC15 doesn't. It's only a few components, and all of those joints look fine. Maybe I'll just start pulling components to see if one is bad.
Could someone explain why touching a bad joint with a DMM probe seemed to kickstart the connection? Was that just a fluke?
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Post some hi rez pics. Maybe we can spot something.
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Thanks Sluckey I'll try to get some good photos tonight. I saw one other solder joint that I thought mostly looked good but figured it couldn't hurt to redo it, so I did. Joint was on the board & nothing to do with power.
When I powered the amp back up it still didn't work but I heard a noise coming from inside the chassis, sort of like crackling wax. Then I smelled burning paper. Then I unplugged the damn thing. I assume any combination of wax/paper smells/noises is a transformer issue.
I don't understand how I have new problems every time I turn the thing on. I've never felt so defeated by a machine.
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Did you build this amp using my layout?
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Did you build this amp using my layout?
Yep pretty much exactly. Added the MV the way you suggested. Only other change I can think of is using a separate 120v pilot light.
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Apologies for the terrible lead dress, and for my habit of using scrap wire which can make tracing through a photograph difficult. Because of the Hoffman size limits here's also a link to the enormous photos at full resolution: https://www.dropbox.com/sh/srbpma6743lv1m8/AABd6Ui1vNggWtiPQ07C-oRCa?dl=0
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More
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A couple more
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Blobby.
You should be able to "see" the wires under the solder so you know the solder "wet" both wire and terminal.
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Alright thanks PRR, I'll reflow it all. I've been working with lead-free solder for the first time and I hate it. Wife didn't give me a choice though.
With the internal crackling sounds and burning paper smell I'm scared to turn it back on. Should I be? I got way too bummed out about this amp today so I might take a few days off and come back to it fresh.
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For what it's worth I disconnected all the PT secondary connections and put power to it. No smells, no sounds, and the secondary voltages all read correctly. So hopefully I was just shorting something but didn't fry anything inside. I'm going to take a few days off and then basically re-solder the whole amp.
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working with lead-free solder
I've been soldering since b4 my 1st kiss :laugh:, that solder sucks! (unlike my..:)
you need more heat, better heat sink on sensitive parts. Get a heap filter mask, a charcoal exhaust n 63/37 solder :icon_biggrin:
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i am no where near an expert on soldering, but, FWIW, If you are not using it, get some flux and use it. Makes all the difference in the world. Also I find that a screw driver type tip works better. Wet the tip, put a small amount of solder ball on the tip, touch it to the joint a way you go..I have found that a little of solder goes a long way.
and what Shooter said.
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put a small amount of solder ball on the tip
leave the tip clean, no ball, put the tip at the "junction" applying heat to ALL "parts" of that junction, count 1000, 1001, at 1002 bring in solder to that "junction", let it flow as you finish 1003 everybody off, walla. don't touch or bump anything for 3-4 seconds as join cools.
flux is great, but pre-positioning, knowing exactly where to apply heat so everyone gets hot equally is key. I like to unwind about 5 ft of solder so you're not tugging a brick, also I might move the amp around 4 different times to find the sweet spot.
after 10,000 joints, you'll be able solder from muscle memory while you're be-boobin to the music :icon_biggrin:
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I've been working with lead-free solder for the first time and I hate it. Wife didn't give me a choice though.
If she insists on lead-free then let her do the soldering? After trying to make a few good joints she'll let you go back to the old stuff. :laugh:
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a few good joints she'll let you go
do whatever you want :icon_biggrin:
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Thanks everyone. I took a couple weeks away from it to let the stress go, and hopefully come back with fresh eyes. FWIW my wife's pretty rad. She never says a word about me spending a lot of money on pointless hobbies like this - so if she wants me to use crappy solder that's probably safer, whatever. We compromised for this amp, though. I got the OK to use lead solder in the garage as long as they kids aren't around. Awesome.
I started poking around today to start fresh, and I noticed the heater wires coming from the transformer, which were twisted, were melted together. I don't know if this goes back to the burning smell and melting wax sounds a while ago. No idea how it happened, and it's not like I touched it with the iron, because it's basically the entire length of the wires melted. I couldn't tell if the insulation had melted together enough for the wires to actually touch so I pulled the melted wires apart. There are some parts of wire now missing insulation, I assume I can just repair that with pieces of heatshrink. With all PT wires separated and disconnected, the voltages are all correct, so I feel good, but does anyone have an idea how I managed to melt those wires together?
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how I managed
the 90% answer, they were exposed to max current min efficiency :icon_biggrin:
I wouldn't put tubes in till I see 6vac across the filament pins, tube side
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but does anyone have an idea how I managed to melt those wires together?
I do! Because I've done that before. Somewhere along the filament string you had a short between the two wires. The insulation will melt from the point of the short circuit all the way back to the PT. If you are unlucky, it will even melt the enamel insulation on the actual PT winding wire. You should be able to look at the filament string and determine exactly where the short is. Insulation between the PT and the short will be melted. Insulation beyond the short will be fine.
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Thanks Sluckey, I guess that explains the burning smell. I was probably smelling wires melting together...
Is the enamel insulation something I should worry about? Like I said all voltages read okay right now.
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Well, I would not worry about it, but I would always keep it in the back of my mind. If the current/heat was enough to melt plastic, was it also enough to melt that very thin enamel (or varnish) insulation on the magnetic wire that is inside the PT? You've got to ask yourself one question: 'Do I feel lucky?' Well, do ya, punk? :wink:
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Well, do ya, punk? :wink:
By the way, no. No, I don't. Ha. Not on this amp, anyway. "Dual Lite" is a good name, but I think I'll name mine "Murphy's Law."
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There's a simple way to test that PT to see if there is a short on the filament (or any other) winding. I've never tried this before but I trust RG Keen. Pretty smart guy!
If I had just melted the insulation on my filament string, I'd definitely give this a try. BTW, how long was the amp on before the filament wires melted? Did the actual PT leads melt too? Have you found the short?
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Thanks for the PDF. I'll give that a shot. Ditto on trusting RG Keen.
No idea how long it was on before melting because I didn't realize they melted until long after the fact. Might've only been a few seconds, might have been much longer. I turned it off as soon as I smelled something. And yes, the green PT leads melted together. The leads as they go back into the paper of the PT look fine, but where they were twisted, they're melted. Haven't had a chance to look for the short yet, hoping to get some free time tonight.
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Okay! Lots of work. PT checked out fine. Re-flowed all connections on the board that looked iffy. Wired the channel switch back in. All voltages looked good. Powered up with tubes, and the 18w channel works, the AC15 doesn't. It only took me 2 months to get right back where I started! :laugh:
The 18w channel does seem quiet, even with the MV dimed, and it was crazy loud before, so I'm going to recheck the new MV wiring. I used RG174 and attached the shields to Lug 1 on the MV pot (ground). Any problem with that?
This dang AC channel has me stumped. I've triple checked all the wiring around the EF86, and I jumpered around the rotary switch which didn't help. Same with a second tube. Maybe I just need to take everything off the socket and start the whole channel fresh because whatever it is, I'm not seeing it.
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> This dang AC channel has me stumped.
Do you see any DC voltages on it?
DC voltages don't find all faults, but it's a start.
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Checked all voltages. Yes, I see DC on the AC15 channel. Readings are lower than the original throughout the amp, I think bc the PT's only giving 280vac, but the voltages are all fairly close everywhere. That all looks good. Completely rewired the EF86 socket. Still nada.
Just trying to learn what kind of readings are normal, I probed Pin 9 (input) of the EF86. Got a hum through the speaker which was controlled by the Vol/Brill/Cut pots on the AC channel. Ok, that means if signal gets to the tube, it'll get through the rest of the circuit, right? I bypassed the switch and connected the input jack directly to AC15 input resistor. Still no sound. Replaced the resistor, no dice. It feels like I'm close, but I must be wrong.
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I probed Pin 9 (input) of the EF86. Got a hum through the speaker which was controlled by the Vol/Brill/Cut pots on the AC channel. Ok, that means if signal gets to the tube, it'll get through the rest of the circuit, right? I bypassed the switch and connected the input jack directly to AC15 input resistor. Still no sound. Replaced the resistor, no dice. It feels like I'm close, but I must be wrong.
I agree, the problem seems to be between the input jack and the grid of the EF86. Next thing to do is replace that shielded cable. I would just use a regular hookup wire to test. If the amp comes alive then replace with shielded wire.
BTW, looks like you may be using RG-174 like Doug sells? The plastic insulation for the center conductor is very fragile and will melt very quickly from too much iron heat. You have to be aware of this fact and get the iron in and out quickly and don't let the cable move until it is cool. It's also a good idea to not have any bends in the center conductor that puts tension on it while soldering. The insulation is almost guaranteed to melt and expose the wire in this case. I hate RG-174 just because of the low melting temp of that plastic insulation. I switched over to RG-216 a long time ago just for this reason. It has teflon insulation. Cannot melt it with a soldering iron.
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Steve will get you to the problem if you hang with him. If I had this problem I would build be an audio probe, plug it into the output of a CD player and use the probe to inject the signal into the circuit starting at the PI and working back to the input until I found where I was loosing the signal. P.S. one whisker from the shield of the RG-174 can ground out your signal.
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Thanks for confirming Sluckey. There's a local store that I bet sells RG216. That's dual shield cable right? Do I just cut back the outer shield and leave it, and then ground the inner shield like I have been w/ the 174? Maybe it'll be obvious once I see it.
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Not saying the RG-174 is your problem. But you've replaced everything else around that EF86 with no joy, so it's time to replace the RG174. I would temporarily replace with common hookup wire to see if it fixes the problem. Then go get some RG-216 at your leisure. And of course you could just use another piece of 174, just be careful with it. Heck it's working fine in the other channel. And I do have lot's of 174 in earlier projects so I'm not totally poo-pooing 174. Just bragging on 216! :icon_biggrin:
This is RG-216. Same size as RG-174, but tough as nails!
(http://sluckeyamps.com/VAC15/14.jpg)
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> temporarily replace with common hookup wire
+1
Signal with a little hum or squeal beats heck out of no signal at all. And confirms/denies the bad-cable diagnosis.
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You're geniuses! Connected the input directly to Pin 9 and it worked. Sounds great. There's a local store that sells most kinds of cable by the foot, so I can just go buy some RG216 and redo everything, I'd rather have the peace of mind.
On the MV wiring, I disconnected the shielding from ground, that brought everything back to full volume. I gather I wasn't supposed to have the shields grounded, though I'm not sure why. I'll let you guys know once it's all wired, but thank you, thank you, thank you, it seems like everything is on the right track finally.
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The store didn't have RG216 so I tried again (carefully) with 174 and the amp is up and running!! This thing has a ton of gain. I'm excited to play some crunchy tunes. Thank you so much for all the help, especially you Sluckey. I'm off to go play guitar but I'm sure I'll be back soon if something goes wrong. :icon_biggrin:
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It's a fun little amp. Fun to build and fun to play! Still one of my favorites. I seem to prefer the Vox channel. Glad you got it sorted out. Hope you have fun with it.
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I finally had the time to get this chassis into an enclosure and I just wanted to share the result. This was the first time I've used fabric (inspired by Benson Amps) instead of a tolex-like material. Fabric was so much easier! I don't think I'll ever go back to tolex. Thanks again Steve for this great project. It was a lot of fun and sounds great. And saved me from having to build an AC15 and an 18w =)
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> I've used fabric
OW!! My Eyes!!!
Good thing you made the biggest 4-8-16 flag I have ever seen.