Uncle Dougs Supro Tremo-Verb... Need Help!

[sluckey]

Take a few resistance readings. One probe on chassis ground, the other probe directly on the tube pin...

V1 pin 3
V4 pin 8
V5 pin 5
V5 pin 8
 

[Mjcopp]

Hey Platefire, I went thru the amp with the stick...no change. I did find a somewhat loose connection on the right lug of the volume pot but the vol. Simply decreased/cut-out when I moved the lug. Nothing increased the vol. At this point I'm wondering if I should buy new 12ax7s or gut the chassis and start over with fresh solder and wires? My problem now is I'm absolutely broke and spending more money on this project at this time is impossible. So either I fix what I have or it sits on my kitchen table for another 3 months. Could it be the tubes? Could it be the power transformer which is only rated for 70mA at 391v? Could it be the the cathode bias resister is incorrect? Could it be a filter cap(s)?

What say ye?

[Mjcopp]

Hey Sluckey, just noticed your post.
Resistance readings:
V1-3:  991ohms
V4-8:  4.37k
V5-5:  470k
V5-8:  677

[Platefire]

Well if you could pin it down to the preamp or the power amp, would narrow the search down. Then you would know where to concentrate on. You would have to be sure that you were doing it safe, but if you could make a jumper with an old guitar cord with a 1/4 jack on one end and alligator clips on the other end and connect the output of the preamp & Tone stack to another working amp. If you got good sound to the other amp out of your preamp, then you would know it was in your power amp section and you preamp section is good, or if the problem remained, then the problem probably was in your preamp. The output of your tone stack prior to the Phase Inverter would be a good point to tap into. Doug at this site also has info on building listening devices for tracing problems you might want to check out.

So once you got your test amp and connection cord ready, you could check after the tone stack and still got weak sound, go to prior to the tone stack and check. You could even use your test cord to take a pre-amp out of another amp/preamp and insert into the input of the phase inverter and check the power amp. You would just have to be super careful to attached you alligator clip to the grid where DC voltage is blocked by a coupling cap where only very low AC voltage exist. Do that by checking with your MM first.

Just another idea, trying to be helpful! If you decide to try that, I know the others will pitch in and cover stuff I probably missed in the process. You'll find it if you hang in there! Believe me, I've been there to the point where I would find myself laying awake a night re-thinking the circuit trying to figure it out.  It does do you good to take a little break from it, go do something else you enjoy and come back with a fresh perspective. Platefire 

[Mjcopp]

Thanks for the ideas Platefire. I'll think about it. Unfortunately , I don't have another tube amp and have no access to one. It is a clever idea and I will look into Dougs info on listening devices. I do understand the need to walk away once in awhile, and I have been doing that quite a bit lately. I have to tell you, I have taken on many projects with zero prior experience...just studied the project on my own and got her done. I never took an engine apart in my life until 5 years ago...didnt even know how a carb worked...but I bought an old 1957 evinrude 10hp outboard, took it apart down to the last nut and bolt, cleaned it all, fixed what was needed, put it back together and it started and ran like new! Since then, Ive done the same with a dozen others... always problems and challenges, but always a solution. Electronics has always been a mystery to me as well and I really want to conquer my ignorance of it. So I will be patient, I will never give up, I will get this amp to work someday. Perhaps I simply need to invest in extra tubes, an oscilloscope, etc...for that to happen?

[sluckey]

As a temporary test I want you to bypass V1 and connect the input jacks directly to V4b grid.

There's a short yellow wire connected between V4-7 and the board. Disconnect that wire from the board and stand it up so it cannot touch anything. Now solder a wire to the free end of that dangling yellow wire. Connect the other end of that wire to the junction of the two 68K resistors at the very end of the board. Don't worry about the shielded cable that's also connected to the two 68K resistors. This is the blue wire shown on my attached pic.

Now remove V1, V2, and V3 and lay them aside. We only want V4, V5, and V6 to be plugged into the chassis. Now plug your guitar into the high gain input and tell me what you hear. It's not gonna be really loud and that's OK, but describe how loud it is. And also describe any distortion you may hear.

What say you?

[92Volts]

Hey Platefire, I went thru the amp with the stick...no change. I did find a somewhat loose connection on the right lug of the volume pot but the vol. Simply decreased/cut-out when I moved the lug. Nothing increased the vol.

Did you re-solder that connection? I'd make sure it's solid and maybe reflow other connections on the pots to be sure-- even if moving those connections didn't change enough to suggest they are your "main" problem.


In my experience a bad tube will USUALLY reveal itself with weird DC behavior. Yes, overall operation of a tube is more than you can measure with a simple DMM but if it draws current and biases correctly (which you can measure) you're likely good... I had a 12AU7 which was passing low-volume signal. Well, it turns out it was pulling excess current which could be determined by a low plate voltage, and a burnt plate resistor. Considering your voltages look reasonable I think it's not a bad tube.

[Mjcopp]

I say...after following your instructions, Sluckey, I notice that the volume has decreased, but the distortion is still present when picking the low E and A strings, two or more notes at a time (even picking soft), or picking with any force. It does seem less distorted picking high strings soft, but overall it seems like a lower volume with the same crappy tone. I also notice a double note or echo when plucking low open E or A. But yes, strumming an E chord...lots of distortion and muddy tone. Thanks for the help. I cant wait to see what you have up your sleeve next?

92volts, yes I did re-solder the connection. Thanks.

[sluckey]

So, it appears that the problem is somewhere in the V4b and V5 circuit. Leave it connected as is until you fix it. Ain't much left, 4 resistors, 3 caps, couple tubes. Change tubes first since they are easy. What speaker are you using? Do you know that it's good?

I can't be certain but it sure looks like you have a white wire connected to V5 pin 6? If so, that wire should be connected to pin 5. Check that.

[sluckey]

Verify that V4 pin 4 and 5 are connected together.

[Mjcopp]

Sluckey, the speaker, (Weber 12F150 8ohm/25 watt/5k imp.) is great. Just tested it in a Marshall AVT 20...loud and clear! I did already replace the V5 tube with a brand new tung sol 5881 two weeks ago. I interchanged all 4 12ax7s in V4 after bypassing V1 (no change). I doubt they're all bad. Should I try a new 12ax7 anyway? I have not replaced the 5y3...should I order one?

White wire is connected to V5 pin 5.
V5 pins 4-5 are connected and show continuity as well as 6.4 vac
Thanks.

[sluckey]

I did already replace the V5 tube with a brand new tung sol 5881 two weeks ago.

But have you tried another tube since you installed that cathode bypass cap correctly three days ago?

What did you do with the extra wires on the OT?

[Mjcopp]

No I have not changed the V5 tube since replacing the caps. Do you think the tube could be damaged from the cathode bypass cap being installed backwards? The extra O.T. wires - I placed wire nuts over the end of each wire and pushed them under the circuit board.

[davidwpack]

You need to eliminate the tube as a problem. If it's not the problem you'll have a spare. I would use shrink tubing or electrical tape rather than wire nuts due to vibration.

[Mjcopp]

Ok, I'll order a tube and report back when its installed.

[Mjcopp]

Hey guys...with the preamp bypass still connected, I replaced the power tube with a known working tube and noticed no change. I also tested it without the bypass and the volume was still incredibly weak, muddy and distorted. Sluckey, would you suggest I now try replacing the remaining 4 resistors and 3 caps? Is it possible the filter caps were damaged?  I've read so many warnings and protocols to drain the filter caps before working in the chassis, yet every single time I shut off the switch or unplug the amp, the caps immediately lose all voltage and never read more tha 1-5vdc. Is this typical or indicative of a problem?

[sluckey]

I think your problem is most likely a wiring error or poor soldering, especially with the grounds. But I'm out of gas and my eyes are bleary from looking at your pics. There's nothing else I can do without having the amp on my bench.

[shooter]

with the preamp bypass still connected

configure it "bypassed", find an old CD player that has volume, jack it in, play music, full volume, then dial back to "clean".  If you can't get reasonably clean music, I'd rebuild V4b and V5.

[Deric]

Long shot, but...


Bad speaker cable?  Speaker jack wiring looks OK in pick but hard to tell - maybe double check that too.

[shooter]

Long shot, but...

we're into the long shot area now 

[Platefire]

Wow! I've been out of pocket about a week fully expecting when I got
Back here this to be up and running. I wish I could lay hands on it!
Love to troubleshoot, nice challenge. Something simple I'm sure--just
finding it? Platefire

[shooter]

I did a quick look, might be wrong, V4b shows a gain in the ballpark of 50?
If so, your bias at 30ish, a 1 Vrms in should yield something close to max clean drive into the 5881, and a 6V6 should be screaming QIUT!!!!!!

[Mjcopp]

I going to rebuild V4b and V5 and may re-wire the grounding scheme...we'll see what happens?
Does anyone have a answer to my question regarding the filter caps? I've asked twice and no response, but I am curious....
 Is it possible the filter caps were damaged?  I've read so many warnings and protocols to drain the filter caps before working in the chassis, yet every single time I shut off the switch or unplug the amp, the caps immediately lose all voltage and never read more tha 1-5vdc. Is this typical or indicative of a problem?
Thanks.

[shooter]

the caps immediately lose all voltage

I don't see bleeders in the schematic so;  with power off, OHM from B3 tap to ground, should be many thousands, charging to many, many thousands of OHMS

[pompeiisneaks]

I going to rebuild V4b and V5 and may re-wire the grounding scheme...we'll see what happens?
Does anyone have a answer to my question regarding the filter caps? I've asked twice and no response, but I am curious....
 Is it possible the filter caps were damaged?  I've read so many warnings and protocols to drain the filter caps before working in the chassis, yet every single time I shut off the switch or unplug the amp, the caps immediately lose all voltage and never read more tha 1-5vdc. Is this typical or indicative of a problem?
Thanks.

The only way to know would be to swap them out.  They should discharge pretty quickly if the tubes were warm and conducting well.  The way they can keep charge is if you turn the amp on, and shut it off pretty quickly with no bleeder resistors (I didn't see any on your schematic, if I recall). Did you replace them with the entire group?  You'd had them upside down everywhere else, not sure if those were as well? 

~Phil 

[sluckey]

I doubt the filter caps are damaged unless they were installed backwards at some time, in which case I would expect some ot them would have exploded.

Your amp has a cathode biased output tube. This puts a heavy load on the power supply and will quickly drain the filter caps when you turn it off. This is normal and nothing to worry about. If you want to see the filter caps hold a charge when you turn it off, just remove all tubes except the 5Y3. This removes the load from the power supply. Now the caps will hold a charge a long time. If not, then you have faulty caps or a wiring error.

[pompeiisneaks]

Also I was just looking at the filters again, and not sure I'm fully understanding the wiring.  I'm attaching a screenshot of the last filter and how it's connected.  Two questions:

1. it loos like you have two resistors there, was that due to not having 1 of the right values so you just jumpered two? (it's not a big deal, and not uncommon, just wasn't on the schematic).
2. This looks to me like the positive side of the capacitor I've highlighted has two black wires coming off of it, one goes to the 270k resistor that's intentional per the schematic, but the other seems to go over to ground?  I can't quite make out what that's actually doing?  if so, I'd think the amp couldn't work as all B+ is dumped directly to ground, but just not sure what I'm seeing.  (This is part of the reason that Sluckey I think is indicating his eyes hurt,  There are a ton of wires going on there and the images are still quite unclear, blurry and dark, if you could put the amp in a very brightly lit room, have someone with a high resolution camera loan it to you, or phone camera etc, and take some more close up shots of each area it may help. 

Also I'm attaching a picture of a completed amp I did with the solder blobs on the turrets, to show you what generally well soldered connections should look like.  Most of yours seem like there's almost no solder in the turret hole.  This could be causing serious problems as well. (don't get me wrong, mine used to look the same way, it takes time soldering to get better, and this pic isn't my best work either, I get better each time I solder)

~Phil

[sluckey]

one goes to the 270k resistor that's intentional per the schematic, but the other seems to go over to ground?

That's not ground. It's labeled as "B3" on the layout. Voltage has been checked at that point and is OK.

[pompeiisneaks]

I get that it's B3, and it's not the wire over to the one that I saw, it's the other black wire running away that I can't quite make out.  But again, I'm sure you're right because of the voltages, otherwise it would be shorting B+ to ground directly and boom. 

Just can't see in that image at all.

~Phil

[sluckey]

Yeah, you can't tell much in that pic. But there is another close-up pic that shows those wires much better. They are really blue, not black.

[Mjcopp]

Well gentleman, I have returned once again with the amp build from hell. After rebuilding V4b and V5, (removed all components & old solder then re-wired with new grounding scheme) I am back where I started - no change at all in sound or voltage readings. Very low volume and muddy tone with much distortion. I totally understand if you guys want to pass on this one...the wiring is  a pain in the rear to follow, the pics aren't very clear, etc... But I did my best to take some new pics if anyone is interested.

At this point all I can think to try is replacing the power transformer, rectifier, filter caps, pots or maybe then, rewiring everything exactly like uncle Doug did (strict point to point with no circuit board). I hate the thought of spending more money shot-gunning, but don't know what else to do?

If the problem is strictlty confined to the power amp section, (V4b-V5) as Sluckey determined, then what is next other than replacing possibly bad components?

[sluckey]

Show us the new pics. Maybe something will show up. Check your speaker jack.

[Mjcopp]

I'm trying to post pics, but they are all barely over the max image file size. I'll keep trying.

[sluckey]

If you can't get the file size down then upload then somewhere on the cloud and just post a link pointing to them.

[Mjcopp]

Circuit board

[Mjcopp]

Control side

[Mjcopp]

V1&V2

[Mjcopp]

V3 & V4

[Mjcopp]

V5

[sluckey]

I didn't find any new clues in your new pics.

I see a lot of soldering iron scars on your wiring, including the shielded cable. The center conductor insulation on the type shielded cable you have is very easily melted. I would replace all that shielded wire with fresh regular single conductor wire just to eliminate defective shielded wire from the suspect list.

Still high on my suspect list is wiring errors and poor soldering workmanship. What kind of solder are you using? And what kind of soldering iron? I can't say that either is the source of your problem but I will never be able to eliminate either from the suspect list just by looking at your pics. Take a look at the pic in the following link. Look closely at the soldering connections, not just on the board, but on the sockets, jacks, pots, etc. Not saying the wiring and soldering workmanship are the best example in the world, but if your soldering and wiring looked like this, they would not even be on my suspect list at all.

     http://sluckeyamps.com/VAC15/big_guts.jpg

At this point, I would need to have this amp on my bench to figure out what's wrong. I will continue to follow this thread and will comment if I see or read something that may provide another clue.

[Mjcopp]

Sluckey, that layout is beautiful. I am presently gutting my board, will order new parts and design a layout based on your style. When all is done I'll post a pic. I am using a Weller 40 watt iron but the solder is a 20 year old roll. I'll buy a new one. Thanks again.

[sluckey]

Solder age is not a factor. All my solder is at least 25 years old. But the type of solder is very important. You need lead/tin with a rosin core. 60/40 or 63/37 are common. Don't use any plumber's solder, especially acid core.

[Mjcopp]

Hey guys, I rebuilt my circuit board and re-wired everything with proper solder and cleaner layout. (See pics below.) Unfortunately no improvement. Wow! WTF AM I MISSING? Still low volume, muddy tone and nasty distortion. At high volume, tremelo and reverb now thump and squeal. Ok, so I bypassed the preamp, connecting input directly to V4-7 grid as Sluckey suggested, with same result - quieter but stil distortion and bad tone. So if problem lies with power section, then where is it?

All wiring and solder redone. Input and speaker output jacks isolated with fiber bushings. New Power tube tests good. Speaker tests good. V4 12ax7 good. All resistors replaced with new. All filter and non-electrolytic caps replaced with new. Output transformer replaced with new. The only components that have not been replaced are the following:
5Y3 tube
Power transformer
Power Switch
Pilot light
Fuse holder
Input jacks
Pots
Sockets

Latest voltage measurements: (schematic specs in parenthesis)
B+1: 380 (360) vdc
B+2: 371 (350)
B+3: 303 (240)

V1-1: 221 (180)
V1-2: 0
V1-3: 2.3 (2)

V2-1: 168-175 (140)
V2-2: -1 to -1.6
V2-3: .5 to .6 (.4)
V2-6: 297 (240)
V2-7: 0
V2-8: 2.28 (2)

V3-1&6: 367 (350)
V3-2&7: .035
V3-3&8: .45 (1.2)

V4-1: 169 (120)
V4-2: 0
V4-3: 1.04 (1.2)
V4-6: 175 (140)
V4-7: .13
V4-8: 1.8 (1.6)

V5-3: 374 (360)
V5-4: 371 (350)
V5-5: .37
V5-8: 31

V6-2&8: 379 vdc
V6-4&6: 337 vdc

All heater voltages: 6.5 vac
All ground connections test good for continuity.
Schematic check against physical layout.

Could the 5y3 cause this? The PT? Improper ground somewwhere? Do footswitch jacks need to be isolated with fiber bushings?

(680 ohm Cathode bias resistor was removed when images taken.)

Thanks again for any advice.

[Mjcopp]

Pics

[Mjcopp]

Pics

[Mjcopp]

Pics

[Mjcopp]

Pics

[Mjcopp]

Link to schematic:
https://m.box.com/shared_item/https%3A%2F%2Fapp.box.com%2Fs%2Fu945p1s2dv7ffrok96ux52lseya2rjv4

[sluckey]

That looks a lot better than the first set of pics. I still can't see the problem from looking at the pics. The only voltage that raised an eyebrow is V5 pin 8. You have roughly twice the voltage that is shown on the schematic. Could be you are using a 6L6? Try a 6V6 instead.

There are three things that all must be in agreement... The schematic, the layout, and the actual build. I suspect one of those three does not agree. I'll assume the schematic was proven by U. Doug. So, that leaves the layout and actual build. Is it possible that your layout does not agree with the schematic? Did you draw the layout based on the schematic?

Of course it could be a faulty component, but I don't suspect any of the components you have not already replaced. At what time did you replace the OT? Was that before or after you fixed all the caps that were installed backwards?

[Mjcopp]

Hey Sluckey, yes I drew the layout based on the schematic. Ive checked it multiple times. It is possible Ive made a mistake, but U. Doug didnt use a board, just terminal strips and it looked very confusing. I am using a 6l6. U. Doug designed the amp for either 6l6 or 6v6. I thought the 6l6 had a richer tone.

The v5 pin 8 voltage is interesting. The original Supro schematic shows a 6v6 with a cathode voltage of 17 and a 330 ohm resistor. U. Dougs data showed using a 5881 tube, 5y3 rectifier and a 654 ohm bias resistor: a plate current of 45.4mA, a cathode to plate voltage of 333v, and a plate dissipation of 15.1 watts. If my calculation is correct, that is a bias voltage of 29.6v.  With a 6v6, 5y3 and 756ohm resistor he got 349v, 34.3mA, 11.96watts and I calculate a bias voltage of 25.6v He wired it up and both sounded great, but yeah, Ive tried to get the bias voltage closer to 17v, but the current jumps into the 90mA range. (And still sounds like crap)

The OT was replaced before I replaced the caps. I probably only ran the amp for a total of 10 minutes with the caps in backwards. (1-2 minutes at a time.) How could this really damage the OT? Just curious.

Spending more money on components that dont remedy the problem is becoming tiresome, ( and I believe my wife will conspire some evil deed against me if I do. So maybe a new OT, 6v6, or layout/wiring mistake I dont recognize?