Amp Build From a Vintage Radio

[sluckey]

My idea was an experiment to see what kind of sag was involved with your PT and that 5Y3. It may not be a real world solution for you.

It's gonna get hot. Can you mount it in the original location?

[blown240]

The original posisition was the rear of the chassis.   Now the rear of the chassis is full of other parts.  I have a place to mount by the PT and filter caps.  I moved the caps so they are about 1/2 from where the resistor will be.   If need be I can make some kind of heat shield.

Are we talking 200 degrees or 800?  It makes a big difference when its sits close to 80 year old wood.  Or maybe I can find a place on top of the cab.  Maybe by the power tubes.

I like the amount of voltage your idea gives me.  What would be a more real world solution?

[blown240]

I was looking at the original location on the chassis and it has MANY cooling holes drilled on either side of the resistor.  So I ended up mounting it in the outside of the chassis.

Here is a pic:



I checked the voltages. 

Pin 8 on 5Y3 and the first 3 filter caps are all around 395v.   The last filter cap is around 210v.

So I plugged in a speaker and my guitar.  NOTHING.  You can hear a bit of a hum thru the speaker, but no signal.   I think I may have smoked the 6V6 tubes.  Will bad output tubes keep ALL the sound from coming thru?   Is there anything else I should check?

Thanks!

[John]

If it were me, I would first trace every wire and component by the schematic, and do that at least twice. Because, I checked and double checked the schematic on my first time build, and it wasn't till the 5th or so time thru that I caught my huge mistake, and it  should have been glaringly obvious (and would have to a normal person  )

[blown240]

Ya I have already done that a couple times.  When my voltages were still very high; I was able to get a faint signal thru it.  But now that the voltage is lower, I get nothing thru it.

Could it be that the higher voltage was forcing some signal thru the dead 6V6 tubes, and now that its lower nothing gets thru?  OR how can I test the 6V6s?

[blown240]

UPDATE:

I just got back from taking my amp to the local amp guy.  We tested all the tubes and they are all good.  Then we plugged it into a known good cab, and there is a signal getting thru.  It is a clean signal but VERY weak.  

Then we plugged in a Bias tester.  Its was a tester that plugged into the tube sockets, and then the tubes plug into it.  There was NO reading on the bias tester.  He suggested that I have the wrong screen resisters.  Then it dawned on my that the schematic that I am copying has 5881 tubes and I am using 6V6gt tubes.  Maybe I need to put a different value resistor to the power tubes?

Thoughts?

[sluckey]

I can't believe your amp guy didn't take the extra 5 minutes to "know" why he had no reading on the bias tester.

Fender used 470Ω screen resistors on almost all his amps. 6V6s, 6L6s, 5881s, all the same. If you used 470Ω screen resistors , you're fine. Something is still not right. Check voltages on ALL pins on both 6V6s and report your findings.

RE: the experiment with the 7750Ω power resistor.... I never meant to use that resistor as a 'fix' for lowering the B+ in your amp. I just wanted to see how much the B+ would drop when you added the additional load. The voltage dropped to 450V. That's an additional load of 58ma. IMO, 450V is still too hot for 6v6s, but it is very common to see 450V on a pair of 6L6s.

I don't know why you chose to put 6V6s in this amp, but here's my recommendation based on the power resistor experiment. Use a pair of 6L6s. And change the rectifier to a GZ34/5AR4. And get a meter that's capable of accurately reading voltage and resistance. $20 should get a cheap DVM that will get the job done. That Fluke T5-600 is a fine instrument but it's more suited for an auto mechanic or electrician. It just isn't intended for poking around inside an amp.

[blown240]

Would swapping out to 6L6 and a 5AR4 take care of the voltage issues, or would I still need to figure out a way to drop my B+?  Maybe the different rectifier tube would do that itself.  I initially used the 6V6 because I already had them.  And I was/am trying to keep the cost down on this project.

[blown240]

Here are the 6V6 voltages, These are all to ground with the amp on and tubes in:

Pin 1- 0
Pin 2- 29vac
Pin 3- 401vdc
Pin 4- 401vdc
Pin 5-  -43 vdc
Pin 6- 400vdc
Pin 7- 33vac
Pin 8- 0

And just for reference

5Y3 Pin 8- 402vdc

Bias Connections:

rec pin4 to 100k - 416vac
100k to diode - 193vac
diode to cap - -51
cap to gnd - 0


And I learned that it does suck to get shocked off a filter cap.   Glad it was unplugged and I wasn't grounded.   Still not as bad as when i got shocked off of the ignition coil on my old 66 Suburban, or when i got hit by a 277 stinger leg in a commercial building!

[terminalgs]

Would swapping out to 6L6 and a 5AR4 take care of the voltage issues, or would I still need to figure out a way to drop my B+?

6L6s have a higher max plate voltage than 6v6s:  500 vs 350.   5u4 or 5ar4 handles higher PT voltage as well.  you can remove that 7K resistor and let the 480v stand as is...

[HotBluePlates]

[Duplicating my post from the other thread, since we're talking about this amp.]

Here are the 6V6 voltages, These are all to ground with the amp on and tubes in:

...
Pin 2- 29vac
...
Pin 7- 33vac
...

I hope you really mean 2.9vac and 3.3vac for these pins. The nominal filament voltage is 6.3vac, but when measuring to ground, you'll see half of that at each pin, or 3.15vac. Each would be within tolerance is the decimal is in the wrong place. If not, your filament voltage is way wrong.

Pin 5-  -43 vdc

That may be too much bias voltage for 6V6's. That's roughly 1/10th of screen voltage, which should bias a 6V6 near cutoff. You'll want to experiment with reducing the resistance to ground across the bias cap to reduce the negative voltage and increase tube current.

The "easy" way to do that is to use a 100-250k pot, wired as a variable resistor, across the resistor to ground. Start at the position of maximum resistance, and slowly turn the shaft while monitoring tube current. You'll eventually land on your desired current. At that point, shut off the amp. Measure resistance across the parallel combination of stock resistor and pot, get the nearest standard-value resistor, and replace the stock resistor and pot with this new resistor to ground. Bias done.

[HotBluePlates]

On a side-note, I think you're learning for yourself why a number of us advocated leaving the radio in it's stock form, and just ripping out the RF portion of the circuit. It's easier and faster to let it be what it is, and just nudge it towards working with a guitar signal.

Kinda like trying to take a Volvo 240 and turn it into a Ferrari... 

But these are valuable lessons to experience. I think you're also learning a LOT about all facets of amps messing with this project.

[blown240]

On a side-note, I think you're learning for yourself why a number of us advocated leaving the radio in it's stock form, and just ripping out the RF portion of the circuit. It's easier and faster to let it be what it is, and just nudge it towards working with a guitar signal.

Kinda like trying to take a Volvo 240 and turn it into a Ferrari... 

But these are valuable lessons to experience. I think you're also learning a LOT about all facets of amps messing with this project.

Oh, I absolutely agree with you.  But also you need to know that I am having a great time figuring it out and learning.  Just like when everybody told you that you could NOT make a Volvo fast with a stock motor.  Well that was a fun project too.  And when I am done I am going to have something I can be proud of.  I already have 2 more projects/builds I want to once I get a little $$$.


I was just going over the schematic and layout for the 10th time and I found something I missed.  There is a jumper from pin 3 to pin 8 on the 3rd 12ax7.  Well I missed that jumper!   I just put it in and once my kids wake up I will see if that did anything.

[blown240]

Strange things are happening! 

I put the jumper in that I missed and now there is HUGE signal coming thru.  But its a terrible sound.  You can hear the guitar signal, but its very distorted (in a bad way) and there is lots of scratching, popping, and ringing.  And is seems that the controls dont do what they should.

I double checked my voltages and Pin 3, 4, & 6 are down in the 300s.  But my heaters really are at 29 and 33 volts!  Even with the speaker and guitar cables unplugged you can hear the amp ringing.  I am assuming this is the heater voltage being too high.

I am going to take out the big voltage dropping resistor to see if that brings my heater voltage back down.   I really hope that I dont have a bad PT.

[HotBluePlates]

Get the tubes out until you figure out that heater issue. 10x too much voltage on the heaters will kill tubes.

[sluckey]

Here are the 6V6 voltages, These are all to ground with the amp on and tubes in:

...
Pin 2- 29vac
...
Pin 7- 33vac
...

Those filament readings are bogus! Your PT has no center tap for the filament winding and your schematic shows no artificial centertap. Therefore, your readings to chassis for the filament AC voltage are meaningless. The correct way to measure your filament voltage is between tube pins. For the 6V6s put one meter lead on pin 2 and put the other meter lead on pin 7. Now what voltage does your tester say you have?

[blown240]

Good Point!  I will have to double check that.

Can anyone point me in the right direction to start figuring out why its making all these crazy sounds?  I already checked all my solder joints and grounds.  It all looks pretty good.  Next I am going to take a small wooden dowel and tap all the components to see if I can find any that are bad.  I figure if I tap something and it makes the sound worse, or better, than thats something to investigate further.

[HotBluePlates]

Can anyone point me in the right direction to start figuring out why its making all these crazy sounds? 

That's what we're trying to do... 

Hoffman's rule applies here: "If it was wired right, it would be working right now."

There's got to be wiring errors, etc going on. You've found one already, which resulted in a big change/improvement. There are almost certainly others. The best way to proceed is find and resolve all the issues.

[sluckey]

Can anyone point me in the right direction to start figuring out why its making all these crazy sounds?

    1. Remove the 7750 ohm resistor.

    2. Disconnect the NFB loop.

    3. Measure tube voltages and put the results in Doug's tube chart so we can see them.

What about the filament voltage???

[blown240]

OK. 

1.  I removed the 7550 resistor.
2.  I unhooked the feedback loop from the speaker jack side.
3.  Heater voltage is 6.
4. Here are my voltages: http://www.el34world.com/charts/valve/ValveData.php?e=view&f=11927

Looks like there may be a problem with Valve 2.  But I dont really know...

I am going to see about getting some 6L6 tubes and a 5AR4

[sluckey]

V2 and V4 are messed up. Divide and conquer. Fix V4 first.

Compare to the voltages on your green schematic. Your Fluke tester will not be able to read the grid voltages on V4 but it should read the cathode voltage OK. Looks like V4 cathodes may not be connected to ground properly. Do a resistance check with amp off--- V4 pins 3/8 to chassis. Should be approx 10K to 12K.

[blown240]

Holy Crap You Just Found Something!

I deleted the presence control from the original Bassman schematic and never put in the ground!  Looks like I need a 4700 resistor to ground.

Also I got a 5U4 and two 6L6 tubes.

EDIT:
  I installed the 4.7k resistor to ground from what would have been the presence knob, also the new tubes.  Now I am able to get an improved signal thru it, even though its still terrible.  At this point we are making some good progress.

Also, I may have wires the pots backwards.  Does this matter other than them working backwards?  Looking at a layout of an amp, Does it assume you are looking at the back of the pot, or the side with the stem?

EDIT 2:

I hooked up the feedback loop again to see if there are any changes.  There arent any differences.  Also I looked at how V2 is wired and I dont see anything jump out at me as being wrong.

[blown240]

Here are my new voltages:

http://www.el34world.com/charts/valve/ValveData.php?e=view&f=11930

filter caps:
1=401
2=394
3=393
4=210

Also I notice that when I turn the volume knob, I get odd vibrations and squeels from the amp.  Even with the speaker and input unplugged.

[sluckey]

Your new voltage readings look OK except for V3-7. No way can V3-7 be -37v! Check that again.

Also, I may have wires the pots backwards.  Does this matter other than them working backwards?  Looking at a layout of an amp, Does it assume you are looking at the back of the pot, or the side with the stem?

Doesn't matter. The typical Fender layout is viewed from the back side of the pots.

Keep an eye on the PT. That 5U4 needs 3A filament current and the original 80 tube only required 2A. I'd be more comfortable with a 5AR4.

[blown240]

I will grab a 5AR4 when I order more parts.  Its not worth the $15 of a new tube to risk the PT.

I will double check the voltage on V3-P7.  But that does lead to the new ground and 4.7k resistor that I put in.  I wonder of there is a problem...

Also, why would my volume know be doing such odd things?   I think I did end up using 500k pots for volume and treble, and a 50k for bass.   I may have the original volume pot from the radio still...

[blown240]

OK  so V3 Pin 7 is actually -37 volts. It changed when I put in the 4.7k resistor to ground for V4 cathodes.  What would cause that?

[sluckey]

I suspect that V3-7 does not have a path to ground. It should measure approx 1 meg to ground (with treble pot set to max CCW). But I don't think your tester can measure that much resistance. Use some clips to temporarily connect a 1meg (heck, use anything between 100K and 2meg) directly between V3-7 and chassis. What's the voltage now?

[blown240]

With the 1m resistor in place it reads from -32 to 0v depending on where the treble pot is set.

The treble and volume pots are both tied into this tube.  I wonder if maybe the tube is bad?

EDIT:

I swapped my tubes around and nothing changed.  The weird sound I am getting is coming from my power tubes based on where the volume is.  The volume has to be all the way up, or the tubes scream at me.

Also, I double checked everything on the schematic and a layout of the original amp I am copying and other that the bass pot being wired backwards (I fixed that) everything seems to be wired correctly.  But I am still at -35 on t3-p7.

EDIT 2:

Are these black caps polarized (I dont think they are)?



If they are polarized, and are installed backwards, could that be causing the problem?  The lightning show in my power tubes must be tied into the negative voltage on valve 3 pin 7.

This is the preamp board, ground for those caps is at the bottom of the picture:

[terminalgs]

The volume has to be all the way up, or the tubes scream at me.

do this:   ground the grid on the last 12ax7 stage of each preamp.  on  the "bass" channel, that would be the pin that the center lug on the treble knob is connected to,  and on the "normal" channel, its the pin the volume pot is connected to.   ground the grids with a jumper wire of something.    It the amp gets quiet,  you have a problem in the preamps (disconnecting one of the two jumper wires at that time, will tell you which preamp).   

If the amp stays noisy,  its either the PI, the output section, or the OT.

at that point, I might connect  the two grids of the 6L6s together with a .1 cap, and with another  .1 cap,  connect to one of the 6L6's grid to  ground.  For the purpose of troubleshooting,  this will isolate the problem to either the PI or the power section.    In a healthy amp, this would be like having the volume knobs at zero.

Here's one other thing:  are your input jacks both properly wired switching jacks?  when you don't have an instrument plugged in,  the signal should be grounded.

[sluckey]

With the 1m resistor in place it reads from -32 to 0v depending on where the treble pot is set.

Plain and simple. It ain't wired like your green schematic.
Or your voltage tester is lying.

Get a decent/cheap DVM.

[blown240]

This amp only has the bass channel.  I grounded the center pin on the treble pot and still got noise.  It just didnt change when I turned the volume or treble.

I only have 1 extra .1 cap, I have three .01 caps and a 100 uf cap.  Is there any other way to test this?

There is only 1 input jack and it is not switching.  I grounded the 2 terminals together for testing purposes.

[blown240]

I will try to get a DVM tonight or tomorrow.  Its gonna be a harbor freight one though,  thats all I got the $$$ for.

EDIT:

I went to HF and got a DVM.  I tested several resistors and checked its voltage against my Fluke and it seems accurate.  My other meter wasnt lying. I am at -32 on pin7 of tube 2.    Time to trace my schematic AGAIN!

[blown240]

A couple more strange things!
 
I was testing the pots in my amp and for some reason the volume pot was acting strange. It would read from 0-250-0 as you turned it.  I replaced it with a known good 500k.

Then when I was testing it, and turned it on standby from the ON position, I got a big spark from the choke.  It was big enough that it killed my standby switch for a bit.  I had to cycle it on and off several times before it would latch off.  I also took the choke out of the circuit.

And finally just now as I turned it to standby from ON, i got a spark from the resistor between filter cap 2 and 3.  Now its smoked.

This is getting VERY frusterating.


At this point I have several issues:

1.  Why is there -34 volts on tube 2 pin 7?
2.  Why are my tubes screaming at me?
3.  What is causing a huge spark when I go to standby from ON?
4.  Is my choke dead from sparking when I went to standby?
5.  Is my standby switch bad?
6.  I need a new 4700 1w resistor

[John]

If it was me, with my VERY limited knowledge (just so you know I'm no expert) I would unhook the PT from the rect. tube, so you're only turning on the PT. Plug into your light bulb limiter - if you don't have one already, make one, very cheap, pays for itself the first time something's wrong. Make sure there's no issue with the PT, nothing shorted to ground. If not, plug into wall outlet,  Check your heater voltages.

Then, re-hook to your rect. tube. If possible, only have your B+ "power rail" connected. Check again with limiter. Again, check your heater voltages.

If it were me, somewhere in there I'd find at least one mis-wire. (at least, I did on mine  ) Hope this helps... remember, divide and conquer.

If all that's okay, hook up the next little "section". 

[sluckey]

If it were me, somewhere in there I'd find at least one mis-wire.

+1 +1 +1

I suspect the white wire is not connected correctly.   

This project seems to have regressed quite a bit in the last 24 hours. Hang in there.

[blown240]

Things seem to have gone down hill since I found that the volume pot was acting up.  I think that there may be a ground issue now since my preamp grounds all went thru a wire bus on the back of the pots. 

I am going to be out of town for a couple days, so it may be nice to have a couple days off from the amp.   

When I get back the first thing I am going to do is reground the whole thing, see if it still sparks.  Then go from there.

[John]

so it may be nice to have a couple days off from the amp.

Yep. I found my big mistake because I didn't have time to fret over it for a few days, went back, re-traced (yet again) and thought "how did I miss THAT???". Good luck!

[blown240]

Thanks guys!

I really do appreciate all the help.  I know that I will get this thing straightened out, its just a matter of patience (from my wife). 

[blown240]

I got a tiny bit done today.  Mostly just re-grounding.

I used a 12gauge speaker wire between all the pots with both ends bolted to ground, one with a ring terminal, and the other to a ground block that is bolted to the chassis.  I soldered all the preamp grounds to it, along with the grounds for the shielded wires (preamp board to power board & input jack to preamp board).  The ground that would have gone to the presence knob is also soldered to the speaker wire buss.  The other side of the ground block also has the ground for the 3rd and 4th filter caps.

Here are some pics:







The PT, Power tubes, Bias, and filter caps 1 & 2 are grounded to the PT bolt along with the power cord ground.



I need to get more solder and a new soldering iron before I do more.

On another topic, the 12" speaker fit NICE:



Leaving town for work in the morning, so its on hold till Saturday at least.

[blown240]

I was comparing my schematic with the original fender schematic and I may have discovered a mistake.  

Here is the original schematic:



Here is the schematic that I built:



And here is one showing a change that I may need to make.  I think that by deleting the normal channel, I may have removed a b+ feed into the first power stage cap.  The red shows the modifications from the one I built.



Would this be better?  Maybe even correct?  And does the red line need a resistor in it?  Maybe a 220k.

[terminalgs]

be sure to connect "B" ("B" inside square,, not B+)  to "B" (the left choke lead),   you originally had that connected to the wrong side of the the 4700 R. 

you don't need the red line.   the last 12ax7 stage of preamp  gets its B+ from the "B" rail  (follow the 100k plate resistor),   not the "C" rail.  if you follow the original schematic,  from "C" rail to the plate of that 12ax7 section,   there is about 1.1M of resistance in between.

academically speaking:
if you used that red line, you absolutely do need a resistor in there,  otherwise the 20/600 filter cap at the bottom end of that red line will look like a short to ground for the AC signal voltage.  you'd more or less be giving that 12ax7 section two plate resistors,,   I don't know what you'd end up with really....


also,  12gauge wire is overkill,   especially for anything that doesn't carry any DC current (like volume pot grounds).   Don't feel that if you have a ground problem, 12g is going to fix it. even the 12ax7 cathode grounds have current measured in milliamps.   you can use 22awg, or 26awg..

[blown240]

Perfect thanks.  Hopefully this is why there is a negative voltage on the 2nd preamp tube pin 7.

[blown240]

At this point I have several issues:

1.  Why is there -34 volts on tube 2 pin 7?
2.  Why are my tubes screaming at me?
3.  What is causing a huge spark when I go to standby from ON?
4.  Is my choke dead from sparking when I went to standby?
5.  Is my standby switch bad?
6.  I need a new 4700 1w resistor

OK, so I fixed the filter cap wiring as suggested.  Also I sorted out some of the issues.

1.  Voltage is now -24 on pin 7 of tube 2.
2.  The tubes are still screaming at me.
3.  The spark is gone, must have been a ground issue.
4.  Choke seems fine.
5.  Standby switch seems fine, but I got a replacement if need be.
6.  I replaced the resistor with a new 2watt.

Bias is -44 now.

Here is a video clip of my tubes screaming at me.  The change in the sound of it is from me turning the pots.  The sound stops when all the pots are turned all the way up.  At the end of the video where the tube noise starts and stops several times I am turning the volume from 10-9 and back and forth.  The other noise in the background is my 1 year old.


http://youtu.be/3_tf3uhBGl4

[John]

I could be totally wrong,and often am. But, it looks to me as if you have arcing going on in your tubes. If so, that is Not Good.

[blown240]

Ya maybe I do, all I know is that it did the same thing with the last set of tubes as well.  So I am assuming its in the amp, not the tubes.

[terminalgs]

1.  Why is there -34 volts on tube 2 pin 7?

something is wired incorrectly.   look at the schematic,  where would -34 volt come from?  it should be 0.   there is no negative voltage supply for the grid there.  something is wired incorrectly there..  pull tubes, turn on and figure out why its not 0v.

[blown240]

I traced the schematic again and I dont see anything jumping out at me.   The only thing is that I dont have a switched output jack that automatically grounds itself when its unplugged.  I used a jumper wire to ground the 2 terminals together and the amp got a little quieter, but the negative voltage didnt change.  It goed from 0 volts when the pots are at full; to -27 when the treble out is at 0.

Also there is no ground switch before the PT, but with a 3 prong cord you dont need it right?

[John]

No ground switch with 3 prong plug, right.

If it were me, I'd look really hard at your treble pot wiring. It seems like it's acting like a bias pot. Something you might find useful is to get one of those 5-pack of colored Sharpies and color up some of the wires to make it easier to trace.

[blown240]

Is it possible that a bad or mis-wired OT can cause this?  I have retraced the treble knob 10 times, and its correct as far as I can tell.

[sluckey]

Is it possible that a bad or mis-wired OT can cause this?

It's highly unlikely. I think it's much more likely that you'll find wiring errors, or your wiring has poor solder connections, or that ground bus ain't connected to chassis very well, or something associated with those white wires. I really suspect multiple problems. Since you are sure that V2 and the treble pot are wired correctly, I suspect a poor connection somewhere between pin 7 and chassis.

I want you to make a few resistance checks with amp turned off. Clip one meter lead on chassis. Clip the other meter lead directly on the following tube pins. V2-8 and then V2-7.
I want one number for V2-8 and I want two numbers for V2-7 (one with treble pot max CW and another with treble pot max CCW.

Also, how is that ground buss connected to chassis? Is that chassis painted on the inside? If so, be sure to scrape off the paint at any point where you make a connection to the chassis. Don't try to solder to your chassis. Use screws, nuts, and star washers (or kep nuts) to make chassis connections.

And one final thing. Connect a jumper wire to chassis and connect the other end directly to V2-7. Be careful to get the right pin and only the right pin. Turn the amp on. Does V2-7 read zero volts now?