Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: GarlandKelley on January 24, 2024, 01:43:26 am
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Hey everyone,
I have been building a Hofffman 18 Watt Stout TMB into an old Philco radio. I thought I was finished today but when I plugged everything in, there was no input signal. The lamp turns on and all the tubes are glowing. I get a hum, sometimes a slight oscillation when adjusting the pots but no signal from the guitar input.
Let me preface by saying that I have scoured the schematics and checked my work many times. The output transformer, input transformer, choke, and tube sockets are all new. I know all the tubes are in good working order.
I completely understand that this could very likely be a bad solder joint or some other error on my end but I was hoping someone could offer some advice or point me in the right direction as to what the issue could be and how to remedy it. Am I correct in seeing that pins 3 and 8 of V2 are bussed together being connecting to the 820 ohm resistor?
Thank you for your help, I am a noob and everything is appreciated.
Garland
https://el34world.com/Hoffman/files/Hoffman_Stout.pdf
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Here’s a photo of the board and wiring, apologies for the mess.
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This is how you can find wiring mistakes
https://el34world.com/Forum/index.php?topic=17701.0
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In addition to the above, take your iron and re-flow all your solder joints (especially all the jumpering that's down low on the turrets). Some look kind of chunky, they should look shiny and smooth. I like to use a magnifying glass to examine all my solder joints. You'd be surprised what you see. When I build amps I test it in steps. Like Heaters first. Then B+. Etc. Waiting till the end to test everything makes it harder to troubleshoot. Did you check all your the voltage points against what is expected on a working amp?
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Am I correct in seeing that pins 3 and 8 of V2 are bussed together being connecting to the 820 ohm resistor?
That's correct, but I don't think the resistor on the board is 820Ω. Looks like a 2.2K to me.
The dangling 82K feedback resistor is in danger of shorting to the B+ dropping resistor. Needs some insulation.
Your layout really sucks. May be the worst I've ever seen. Sorry. Had to be said.
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I normally use Kester 83-4000-0000 SN60PB40 solder but it is unavailable so I had to use Kester 83-7068-1402 and it just does not flow like the other stuff and looks dull when dried no matter how carefully I solder. I am new to amp building as previously stated so how would I test voltage points?
Sluckey, while I want to say I appreciate your advice, your comment about the layout serves no purpose and in no way is helpful. What about it sucks? I followed the board layout provided and the wire routing was restricted due to using a donor radio chassis. The components barely fit on the board and have to be where they are in order to fit the chassis. I specifically stated that I am a noob to avoid snarky comments like yours but being the internet there is ALWAYS someone who has to state some sarcastic, unhelpful information. To clarify for you, that in no way “needs to be said”.
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When you have bare wires crossing like that in the pic above always use heat shrink (insulation) on the leads to prevent any short that could possibly happen.
If that Kester solder you used is lead free then you ain't going to like it. Always use the right stuff even if you have to wait for it.
Sluckey was just being honest. And what he said is true. What you did was take a layout that was meant for a clean chassis and try to adapt it to the old chassis and the result is really a rats nest of wiring. What he is alluding to is that it would be much better to have adjusted the layout, redraw it if necessary to adapt to the components being in different positions. We all know you are a beginner. I just installed one of these into a Vox AC15CC1 chassis and had to flip the whole layout vertically to make it work because everything was backwards from that layout. So I redrew the layout to work.
Sluckey is extremely knowlegeable and helpful so getting upset with him isn't goin to work very well for you. Just unwind yourself relax and the help will come.
now how about that 820 ohm resistor he pointed out?
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I'm sorry you took it that way, but it serves a purpose. There was nothing snarky about my remark. It was straightforward and to the point. It's a prime example of what not to do when building an amp. There's nothing wrong with the board. It's everything else that sucks. Things like putting the sensitive input jack right over the rectifier tube is crazy. Very long wires crisscrossing the chassis is another example. You have broken dozens of "things not to do" rules. It's like you have never seen the insides of an amp. I'm sure you will find that your layout is a major contributor to the problems that amp has.
Sometimes criticism is positive and sometimes it's not. But you can learn from both. I did not make that statement in a mean spirited way, but I did want to shock you into realizing just how big a mess you have made. I try to speak the truth but I'm also very plain spoken. I wish you luck but I'm not optimistic about the helter-skelter way you crammed that stuff into that Philco chassis.
I did point out a wrong value resistor you put on the board and pointed out a danger. Maybe you'll find that helpful.
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Apologies for being so hot. I’m frustrated because this has been a huge project with many labor hours including restoring the cabinet. I appreciate the explanation as to why the poor layout is more than just an aesthetic issue and could be the root or one of the main causes of the problem. I do respect the expertise of the users of this forum and appreciate the advice. That resistor is indeed a 2.2k so I must not have received an 820 ohm resistor in my BOM. Still a major mistake on my part as well as the other issues. Is this enough of an issue that it could be the primary issue? Thanks again.
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This is a video explaining my reasoning for the layout decisions. I am not sure how I could modify this and the board barely fits in the space on the underside as is.
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That resistor is indeed a 2.2k... Is this enough of an issue that it could be the primary issue?
It has nothing to do with the issues I heard in your first video, but it does need to be replaced with the correct value. The only reason I noticed it was because you asked, "Am I correct in seeing that pins 3 and 8 of V2 are bussed together being connecting to the 820 ohm resistor?" So, I looked and saw the resistor was the wrong value.
I think your best approach to troubleshooting is to follow the link posted by EL34. Once any wiring errors and/or wrong component values are sorted, the amp should pass a guitar signal. But I'm almost certain it will sound very noisy and generally crappy.
I get it that you want to keep the old looks of the radio. And that can be done and still have a good looking amp that works. But it could be a major rework. Several of the forum members (including myself) have converted old radios to amps. They don't have to necessarily look good but they need to follow some basic building rules. I already have some ideas for your project that will allow you to keep the front of the radio looking original and also house your amp. Looks like you will be accesing the amp from the back side of the radio cab so that makes it even easier.
But follow the link posted by EL34 first. If no joy, I'll share my ideas with you. BTW, I just drug this '46 Philco cab to the curb last week. Gone by the end of the day. :icon_biggrin:
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:laugh:
at an estate sale 15 years back picked up a '39, tore it down, finished, new speaker, drove it 300 miles, gave it way. The story from the estate, "was Dad's last project, he requested it ONLY be gifted to someone that "knew what it was"" made my contributions to his project, gave the same "instructions on gifting" and set it free again
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I am new to amp building as previously stated so how would I test voltage points?
Sometimes the project documents have schematic or layout that shows "typical" voltages. Sometimes you can get them from another person who built one. I haven't built one of these so I don't have one myself.
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Did you ever get that 5F1 Champ working?
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Hate to say it, but it's too bad you just didn't rebuild the radio. You already restored the cab.
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Sluckey,
Thank you for your help with this build and the 5F1 Champ build. I did end up getting it working and it sounds great! I’m hell bent on getting this one operational. These builds are used for recording in my home studio. I trust your expertise on this but want to see if I can get it operational before completely rebuilding. I included a video of that Champ on here.
AlNewman,
I know I’ve kind of violated the credo of antique restoration and antique radio aficionados. I got this radio for free and would not use an am/fm radio. I enjoy the process of restoring the radio cabinets and building an amp into them. I also love the aesthetic of these old Philcos in particular and like having a wide palette of amps to use for recording.
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Okay, I checked everything by printing out black and white board layouts and tracing/highlighting every path and component. I checked the values of every board component as well. I also reflowed solder joints. I am still not getting any input audio. The bad oscillations from the pots are gone but there is still a small amount of noise coming from the volume pot. Do you have any advice? The lamp is operating properly and all of the tubes are glowing without red-plating. I’m definitely getting internal audio to the speaker. The noise floor of the amp is low but I can hear the subtle crackle of life and get that noise when adjusting the volume pot.
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You missed something. Do it again.
Also check voltages at every filter cap and all pins on all tube sockets. Post your voltages.
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Do you mean retrace everything again as well as reflow solder joints or just reflow all solder joints again? This is embarrassing but how do I check voltages at those locations? I’m assuming the amp should be off, unplugged, caps drained, with no tubes when I check voltage? Thanks for helping me figure this out. Also, is the buss wire supposed to be soldered to the back of all the pots?
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I can hear the subtle crackle of life and get that noise when adjusting the volume pot.
typical sound for either bad solder joint, or DC on volume
NO, you don't need to solder the buss wire to the pots.
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I’m assuming the amp should be off, unplugged, caps drained, with no tubes when I check voltage? Thanks for helping me figure this out. Also, is the buss wire supposed to be soldered to the back of all the pots?
No you want it powered on. If you're not comfortable with checking lethal voltages you might want to have a friend or amp tech help. What I do is clamp the negative lead of my multimeter to the metal chassis (or ground buss). Then I keep one hand at my side and only use the other hand to hold the positive probe to test LIVE voltage points. At this point I'd leave the tubes in as you've already had it powered up, and if there is damage to them, too late. I no longer solder the buss to back of pots. I just suspend the long buss wire in the air using soldered jumpers (20 ga bare buss wire) to the pot terminals to support it.
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One thing to remember when probing a live amp: Some of the places you touch with your probe can make a loud noise in the speaker. DON'T jump and accidentally short something out. When I was younger and more timid, I'd often clip my leads to each point with the amp off, then turn it on and let it warm up to measure voltage.
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Okay everyone,
I replaced that resistor and took all the voltage measurements of all tube pins and multicap. I had the speaker connected, all tubes in, powered on. I have attached a chart of all the ac/dc readings and some videos of odd things I noticed from my limited knowledge perspective. I hope I haven’t damaged anything. Any help is appreciated.
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you might be missing the milli in your AC volts
What is point A to ground, VDC measure???
WITH the PA tubes OUT, amp on, what does the plate pin on each tube socket measure to ground VDC
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Zero volts on V1 pin 1 will prevent guitar signal from reaching the speaker.
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yeah you seem to be getting power to the second triode in v1 but not the first
so something is a miss a round here, maybe
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Should I be reading voltage at that 500p that connects to pin 1 V1?
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Should I be reading voltage at that 500p that connects to pin 1 V1?
yes
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That may be the issue or one of the main issues. I’m not reading any voltage on either side of that.
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B+ voltage comes from turret A, goes through 100K to turret B which is jumpered to turret C. The red wire on turret C takes the B+ voltage to V1 pin 1. Now you find what is broken in that simple ABC chain.
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That 100k resistor is reading 511 on one side and nothing on the other. Is that just a bad resistor?
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The lead coming from turret A reads 511 but the lead going to turret B reads nothing
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That 100k resistor is reading 511 on one side and nothing on the other. Is that just a bad resistor?
Probably. But it could be a bad solder connection. Or turret B is grounded to the chassis. Use your ohm meter to check if B is grounded.
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What setting would I use and what reading am I looking for?
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Just replace the resistor.
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Turret B was grounded to the chassis, lol. I’m getting signal from the input but something is still definitely wrong. I’ll upload a video. You rock!
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Pin 1 V1 now reads 417 AC. Pin 2 still reads zero but it sounds like I’m picking up AM radio when I touch the probe to the pin. The volume pot affects this radio interference as well. Pin 3 now reads 2V AC. Pin 7 is still 0 but very noisy when I touch the probe to the pin. Sluckey, some of this may be the consequences of my poor layout like you had warned me but I can tell there is still something wrong. I feel like I am so close and greatly appreciate your ongoing advice, everyone.
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Does anyone have an idea of what might be happening in the above video? Also, should pin 2 be receiving any voltage? I’m wondering if pin 2 is related to the noise problems on the volume pot and possibly the overall issues with the overall tone? Does any of this sound like tube issues to you?
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are you trying to go viral? I go to the tube couple times a year to "find" that magic bolt to take a fender off
since you're there, look up "basic troubleshooting tips for guitar amps" or some such wording
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Pin 1 V1 now reads 417 AC... Pin 2 still reads zero...Pin 3 now reads 2V AC... Pin 7 is still 0...
Pin 1 and pin 3 should be DC, not AC. The voltage on V1 pins 2 and 7 are supposed to be zero. Nothing wrong there.
I suppose there could be some more errors in the amp but the elephant in the room is YOUR LAYOUT! No amount of logical troubleshooting will fix that.
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Disconnect the green wire from the feedback resistor. Any joy?
Here's my proposal to fix your layout. Remove all the wiring on the tube side of the chassis. OK to leave the filament wires. Relocate the controls to the right side. Label the tubes as shown. Now rewire the amp. That board was designed for this layout. The board is only part of the total layout. It has to work with the tubes and controls as one layout. The wires will be much shorter and more directly connected between the board, tubes, and controls. No more long crisscrossing wires to act as antennas to pick up all kind of noises and interference.
Assuming there are no more errors on the board, the amp should work fine if you follow my proposal. And it will be much more serviceable. Easier to troubleshoot. I know it's a lot of work but it will be faster than trying to make a working amp out of what you presently have.
Oh yeah, read the manual for your meter and learn how to use it properly.
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Sluckey,
Very much appreciate your advice/guidance/time. That rework is totally doable. I was worried I was going to have to redesign my own turret board. I had considered placing the controls on the side like that but made the decision for the back solely for aesthetic reasons. When I disconnect the green feedback wire, the noise floor of the amp is better until that threshold on the volume pot. The guitar signal is still distorted in the same way that is not proper and once I reach that particular threshold on the volume pot, the noise returns and there is a high frequency squeal that is constant. Do you think performing the rework first is advisable? I would like to pinpoint the problem first but understand that may be or is very likely related to the layout issues.
Garland
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The squeal is only with my guitar volume turned up as well, disappears when volume is down.
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Could this be related to the tip of the input jack with the 33k resistor going to pin 2 of V1?
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Design rework in a nutshell, move input and controls to the side of the chassis next to the tubes and reverse the positions of power tubes and preamp tubes?
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input close to input, PA away from input stuff, yep
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Awesome sauce. Thanks again everyone. I’m doing the rework now and will post the results when I’m done.
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> once I reach that particular threshold on the volume pot, the noise returns and there is a high frequency squeal that is constant.
This IS layout. It is just like setting up a vocal system with the mike aimed at the speaker. It is fine but meek at very low gain settings. It howls loud at some higher gain setting.
Amps do this too but the electric fields can "howl" supersonic. You don't hear the scream but it mucks-up normal sound and throws strange hetrodyne birdies at odd frequencies.
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Okay, changing the layout fixed the nose issue. The tone controls seem to be working properly, the volume pot is working correctly, and the amp is nice and quiet. I damaged the 33k resistor that goes from the top of the input jack to pin 2 of V2 so I currently have a 22k resistor in its place to test the amp. That resistor may be the problem I’m having but it still doesn’t seem like the amp is sounding the way it should. I get some breakup towards the top of volume range but the overall volume seems low and it doesn’t seem to crunch the way I was expecting this high gain 18 watt amp to breakup. I’ll wait to share a video once I replace the resistor but do you have any idea why I might be having this issue?
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Noise issue*
I should clarify that I don’t have another 33k resistor currently so I substituted the closest value I had for now. I also swapped power tubes and that didn’t change anything. I swear I’m almost done with this build and then I’ll be out of your hair for a long time, lol.
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This is not a high gain amp.
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overall volume seems low
this is where scopes n good meters comes in handy, we all have a pretty good idea of "what low volume sounds like"
scroll through your contacts, look for that one geeky dude that just might have a scope, barter your way there, amp, speaker, siggen app.
short of that, post a new set of VDC readings;
PS taps
preamp plate and cathode
the cathodes of the 84's
EDIT:
assuming this is the build?
https://el34world.com/Hoffman/files/Hoffman_Stout.pdf
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Gotcha, I must have been mistaken. I had read that this a high gain variation of the classic Marshall 18 watt. Maybe just “higher” gain than the original. I’m sure it’s partly due to the fact that I’m playing single coils through a single 10” alnico but I’m getting more crunch and volume from that 5F1 Champ I built with an 8” ceramic. I have rolled all of the tubes minus the 5Y3 rectifier because I don’t have an extra right now. I watched a video of someone playing their 18 watt stout build and it was definitely way beefier and aggressive. Granted, they were playing through humbuckers but it seemed pretty significant.
Yes, that is the build. I’ll get new VDC readings posted ASAP.
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this a high gain variation of the classic Marshall 18 watt.
once you get the amp sorted out, a simple "workaround", most every self biased amp i've built takes pedals well, throw a tube-screamer, or gain-pedal up front.
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I'd like to see a hi-rez pic of the amp with the new layout.
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Garlandkelly,
I won’t pretend to be an expert here. I am an amateur too. There are lots here that live and breath this stuff that can’t fathom what it’s like to not know what we don’t know. It looks like you’re getting some great advise and following which is making progress for you.
Here is some things that help me. This isn’t everything, but is a start:
Low volume could be several things. I would suggest starting by getting some wooden chopsticks and carefully poke around listening for pops and anything that may point/resistive to a bad solder connection. Check your pots for DC voltage with your meter. DC voltage can destroy pots which can cause low volume. Get a highlighter and highlight with your layout using one color for AC and one for DC so you know what is where. Having DC where you shouldn’t is good to know when chasing problems. And from there mark things you have checked on your layout. It’s easy to think you checked something but didn’t. Marking it helps to make sure you are checking things systematically and can recall on your prints.
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Lots here that live and breath this stuff
:laugh:
I blame it on my upbringing, got paid 25 cents to hold the coil wire while Dad gave the starter rope a pull, learn't valuable lessons; money isn't that important, understanding the "power of GOD" is valuable knowledge
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BTW, I just drug this '46 Philco cab to the curb last week. Gone by the end of the day. :icon_biggrin:
I had that exact model of '46 Philco, about 30 years ago, they were a fairly common model. I picked it up at an auction for around $15, and the cabinet was about like new.
Other than needing a re-cap, the only issue was a dry-rot speaker cone. It's an ED design, so my "hot fix" was to cut the magnet structure from the speaker, and screw it to the cab inside acting solely as the PS choke.
Then I replaced the speaker with a 12" PM paper cone type, pulled from a 60s console hifi. Worked dandy, nice sounding radio.
Iirc, it used a pair of 7C5 for the PA. Does that sound right to you?
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There was no chassis with this cab. The PA used P/P fixed bias 6V6s and had a bunch of loctals for the radio circuit. Schematic available at RadioMuseum.org. This 46-1209 model's speaker grill tilted out to give access to a turntable. I found it on the curb, brought it home and studied it a couple days. I had the idea of refinishing and putting one of my amps in it. I cleaned the cab enough to see that the wood was cheap birch and birch plywood so I came to my senses and drug it back to the curb. Not willing to put the time and money into refinishing a plain Jane cab. Besides, I don't have floor space for any more amps.
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Turns out my issue was a cold solder joint on the jumper from pins 3 and 8 of V2. I swapped that resistor for a 33k and it sounds awesome! I’ll be getting a couple NOS 12AX7s for V1 and V2. I just have some extra Groove Tube 12AX7s in there now which sound fine but nothing like a good set of RCAs or Mullards.
I cannot thank you all enough for your help and guidance during this build. Sluckey, I wanted to thank you in particular for taking the time to draw that new layout for me and for taking the time to analyze my build and provide guidance. The amp is so clean and quiet and I am so happy with it both in terms of aesthetics and tone.
I love the almost Vox chime it has with the EL84s and Weber Silver Alnico 10”. It also has that nice Marshall crunch of course. I’ll be getting more well equipped both with tools and knowledge and promise I will return more competent next time.
The last question I have before I leave you all alone is, am I able to run two Fender style lamps off the heater wires while running the neon lamp from the line and neutral? I’m hoping to use the neon lamp on the rear of the chassis and use the fender lamps to illuminate the front dials of the radio itself in the original lamp positions.
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I'd like to see a hi-rez pic of the amp with the new layout.
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I meant to attach the photo to that last post. I left the wiring I had on the pots to avoid having to require those. The soldering is still definitely not the prettiest thing.
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There a couple of things I’m still going to touch up like the extra length of buss wire and some other wires that are longer than necessary. I wanted to wait on the final details until I figured out about the lamps. The. I’ll close up the chassis and mount it back on the cabinet.
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am I able to run two Fender style lamps off the heater wires while running the neon lamp from the line and neutral?
yes
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Awesome, thanks! Typos from my autocorrect.
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I’ve built the Stout in an old Gibson GA15-RVT chassis and cabinet and can speak from experience that the TMB tone stack definitely wipes out a fair amount of gain compared to just a single tone control, however, I still do get a good amount now. Maybe it’s got something to do with voltages?
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This council isn't about your present job, it is for the next time you go to build an amp using an old chassis that was planned for one other use
Dismant all from the chassis you want to use, use an aluminium sheet to cover all the top of the chassis, then drill new holes in proper position
The same you can do on the part of the chassis that are on the front and on the back side where you can drill the correct number of new holes for pot and other stuff
Doing that can save you from a lot of problems
Franci