Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: makingnoise88 on October 17, 2016, 10:35:31 pm
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Hi guys, This is my second build I'm using a hoffman turret board but have changed a few things.
I am using a deluxe reverb output transformer, and added a mid knob.
I got the amp all built but It is EXPLODING the fuse so I have done something wrong but I keep going over it and can't seem to find anything wrong. Im attaching pictures of the build. I didn't wire in the foot switch connecter as I want to use a 1/4 in instead of the standard RCA style jacks. If you have info on wiring that in as well would be helpful! Also I am using an LED instead of the standard lamp, I didn't have a 180k resistor on hand so I wired it to a 100k and a 68k to get me close. Is that to incredibly ignorant? haha as I'm sure you can see I am very new to all of this but I'm having a blast learning! Thanks so much for any help in advance.
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Hi,
Take all the tubes out and see if the fuse still blows. I assume there is a tube rectifier. I have not looked at a schematic. If you are still up I will stick around for a few min.
Cheers,
Billy
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I pulled all the tubes and The fuse still exploded!
Thanks for your help I really appreciate it
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ok...with the tubes out HT power only goes to the rectifier tube socket and heater voltage to all the tube sockets. So..the issue is in the power transformer it self or transformer wireing. I assume you are running it on 120VAC. Be sure you have the AC power to the transformer ( two black wires in most cases) wired correctly. If that is correct then disconnect the the heaters ( both green and yellow) and High voltage ( red wires and I assume a red wire with a yellow stripe to ground).
Bottom line, you need to test the power transformer without it being hooked up to anything. If it still blows the fuse then the power transformer is shorted and no good.
Be very careful messing with this and keep you hands off the chassis. I can not do CPR over the internet...lol
Cheers,
Billy
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haha this is true! I am sure I have it hooked up correctly As this was my first thought and I have been over it a dozen times! I will unwire the power transformer and I will test tomorrow. I will post and let you know whats going on then.
Cheers
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See you tomorrow.....it is after midnight here....
Cheers,
Billy
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And, when you have the PT secondaries unhooked for testing, make sure the bare winding ends aren't touching each other or anything else (even the chassis).
With everything unhooked, the fuse should not blow (unless the problem is a short on the primary side of the PT (like a shorted mains-voltage lamp or something - if you are using a mains-voltage lamp that is).
If the fuse doesn't blow with everything unhooked, then you will know to either look for a short from the B+ to ground somewhere, or a short from one of the heater pins/wires to ground or to something else.
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I see what appears to be a blue transformer wire connected to chassis at a transformer bolt. I bet that wire should not be connected to chassis.
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Slucky, the blue wire you see is blue/black it's the ground for the 120v
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Transformers don't need a ground for the 120V.
What PT did you use? Where did it come from? Can you provide a data sheet?
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Wow I feel really dumb....
The black blue is 100v white 120v and black is zero
With this being the case what would the solid black be for?
I need to swap the black/blue with the black and do what with the black? Seal it off?
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There are six wires on the primary. NONE OF THESE SIX WIRES WILL CONNECT TO CHASSIS. You will use the black wire. You will use the white wire. The other four wires will be taped off and not used.
The black wire will connect to your power switch. The white wire will connect directly to the NEUTRAL lug on the IEC connector.
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Connecting the primary to ground would surely do it! One of the reasons I told you to be careful what you touch.
Anytime I have an amp that is blowing a fuse I take extra precautions in how I touch things.
Good luck.
Cheers,
Billy
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Yes, I have made single ended fuse blowers and push-pull fuse blowers. Never made a primary fuse blower.
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Well I pulled the blue/black wire taped it off and it's still blew the fuse!
So I guess the next step is testing the transformer!
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Maybe you blew the PT with that 100v blue primary to ground...?
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Just so it's clear:
You feed your 120 VAC into two transformer wires. Black and white.
Any and all of the 4 unused primary wires are:
A) Not used for anything. Not connected to anything.
B) Not connected to the chassis
C) Not connected to each other, meaning INSULATED from each other.
My next test would be:
Either bring the thing up on a variac, OR light bulb limiter, in this case the light bulb limiter might actually be better; good excuse to build one if do not have such a thing and you want to do other builds or fix other gear.
Connect up the power transformer in temporary fashion (eg; tack-solder it) as if you were running your amp on 240 VAC.
That would be:
Leave alone the black wire.
Disconnect the white wire from wherever it is now connected and tape it off.
Wherever that white wire went, use the black/red instead with a temp solder connection, don't tie up your wires.
Now, you are connecting the tranny up as if it expects 240 VAC, but still of course only feeding it 120 VAC. That means it will be getting only half-powered. I really doubt you blew it up; trannies are quite rugged, the previous fuse(s) did their job and blew rapidly if not instantly.
Measure output volts on the various secondaries. They should be half of what's expected.
If it still blows the fuse and the secondaries are disco'ed from anything, then my opinion would change and shift towards yeah, you blew it up.
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Perfect! I'll try this tonight!
I can't thank you guys enough!
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It appears in the picture that black (0V/Common) is connected to what is ordinarily neutral in US house wiring. White (117/120V) is connected to one end of the fuse with the other end connected to what is ordinarily hot. Neutral connects to ground in a properly wired US house. With the 100V tap connected to ground, there would have been a complete circuit to run the wall power from the 100V tap to the 117/120V tap. With way fewer turns than from 0 to 120V, you would expect the load to blow the fuse. With the 100V tap removed from ground and all unused taps insulated, you would expect everything to be hunky-dory.
I would like some DMM readings before tearing everything apart. While disconnected from the wall, what is the DC resistance across the external pins of the IEC connector with the power switch on and a good fuse installed?
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Ok. So i finally have some time to sit down and test some stuff on this amp.
I have the power transformer completely unwired to test but as I said before this is my second build and I have no clue what settings I need on my multi meter or what I'm looking for. Does anyone have any resources on how to test this transformer?
Thanks in advance
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I suggest you read this page first:
http://paulrubyamps.com/info.html (http://paulrubyamps.com/info.html)
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Ive pulled the transformer completely and took some measurements!
Please let me know if I'm right or wrong in how I'm measuring and I will adjust.
DMM Setting = 200 ohms
Black and white wire = 05.8
Yellow wires = 00.4
Green wires = 00.5
DMM Setting = 2000 ohms
Red wires = 256
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That looks very in the ballpark of a power transformer. Both heaters are practically short circuits. The other readings are pretty typical, at least in my experience.
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I'm gunna reinstall and try again!
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Hook the primary up with a mains fuse from the Active/Phase wall wire, then the mains switch, then the Pt primary. Return the other end of the primary to the 'neutral' wall wire. Install the PT in the chassis, and have the chassis firmly bolted to the earth wire. Don't let any of the secondaries touch each other nor the chassis, nor anything else. Clip your insulated 'gator leads/test clips across each secondary winding end and measure the VAC across each winding when you flick the power switch on. Switch of the PT off each time you go to change the clips to another winding.
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Do like Tubeswell said.....
The only tweak I would add is...many multimeters don't like >600 volts. My old Fluke 77 is one. So, when measuring the HV winding, measure from one side to the CT, not across the whole winding. Do it again, other side of the HV winding.
In all cases, your test leads are CLIPPED ON, and your hands are nowhere near the thing.
If you are in a partially built amp and have line cord > AC entry > fuse, fine. Power it up that way.
With a raw tranny, I don't mind sticking the primary wires into the female end of an IEC cord. Why? Because even though it may seem hairy, it forces my hands to be away from the tranny wires, holding a rubber/plastic thing. I could understand if that seems scary for Hallowe'en.
It's also easy to do the "remote turn on" using one of those 6-way outlet strips.
Know that the voltages will be high since they are all unloaded. Thus, 6.3 volts could be 7, 7.2.
5 volts could be 5.8
That's why your HV winding (designed to be say 350-0-350 = 700) might be closer to 800 volts (750-775) when unloaded, esp if your line volts are highish. Some meters will not like that.
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Is it me or does he have one leg of the filament winding connected to ground in this picture?
Or is that a bung'd up way of doing an artificial CT mixed with a current limiting resistor?...either way, it ain't good
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Is it me or does he have one leg of the filament winding connected to ground in this picture?
I noticed that too, but it's kinda hard to tell from that angle. I figured I would mention it if he ever gets the PT primary hooked up correctly.
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Is it me or does he have one leg of the filament winding connected to ground in this picture?
I noticed that too, but it's kinda hard to tell from that angle. I figured I would mention it if he ever gets the PT primary hooked up correctly.
OK, I feel better knowing you're still lurking...
Even if it aint going to chassis, those resistors are just a dead short across that winding....I'll bet that LED was really bright for a split nano-second :huh:
While I was at it, I spotted this as well...
Nothing connected to bias supply (maybe that's what the BLUE wire is for- usually on new Hammond xfmrs)
But also, the bias supply is going through a front panel pot and I'm scratching my head as to why, and where those other 2 pot wires are going.
Is this a fancy master volume?...of course it is
I've been away for a while...maybe it's me?
EDITED- If it really is the Mojo PT posted in pic, then I'm obviously wrong about the blue wire
EDITED AGAIN- so as not to confuse OP
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I'll bet that LED was really bright for a split nano-second
Nah. Those current resistors for the LED are parallel 100K and 68K. That LED ain't never been lit. The only thing that's been bright so far is the line fuse.
Nothing connected to bias supply (maybe that's what the BLUE wire is for- usually on new Hammond xfmrs)
The blue (actually blue/black) in a primary voltage tap. There's a data sheet for the PT up above. He will eventually have to connect the bias supply to pin 4 or 6 of the rectifier socket.
the bias supply is going through a front panel pot and I'm scratching my head as to why
Look at the PR schematic and the truth will be revealed. :icon_biggrin:
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the bias supply is going through a front panel pot and I'm scratching my head as to why
Look at the PR schematic and the truth will be revealed. :icon_biggrin:
Ahhh, so that one was me....kinda
I'll go away with red face and check back for updates.
Please don't keep it on my permanent record... :icon_biggrin:
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No promises. This forum is RIGGED! :l2: :l2: :l2:
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Ok. So i was able to get the transformer hooked up correctly and power tubes and lighting up nicely!
As far as the led it was just floating in the air, I figured I would rewire it permanently after got the amp up and going.
I did forget the bias Tap so I added that from the 3w 100k resister to pin 6 of the rectifier.
So it looks like everything is correct at this point but I'm not getting any sound what soever so I'm going to go back over my schematic with a fine tooth comb and see if I find anything! If anyone sees anything please let me know! This has been an incredibly awesome thread I can't thank you guys enough!
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Your resistors for the LED are way wrong. Hoffman likes a bright LED and uses a 180Ω resistor. I find that about 1K to 5K gives me plenty of shine. But 100K and 68K wont even light at all.
Click on the link in my signature line at the bottom of this reply for a good method to verify that your wiring and component values are correct.
Measure the voltage on all 4 of the filter caps. Then measure voltage on every pin of each tube. Post these voltages (even to zero voltages). Hoffman has an online chart for this purpose that makes sharing your voltages easy. Or you can just post in a reply.
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Perfect! I'll do this. I want as bright of an led as possible! If I didn't use any resistor would that be a problem?
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Also make sure pins 4 and 5 are tied together on the pre-amp tube sockets (for 6V heater operation)
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I want as bright of an led as possible! If I didn't use any resistor would that be a problem?
Absolutely!
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So 180ohm is the best choice for the brightest led?
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> If I didn't use any resistor
No resistor, current goes to "INFINITY", LED blows up.
> 180ohm is the best
On 6V power it gives around 20mA, which is the Max rating of most small LEDs.
20mA in a modern LED is very bright. Many people favor 1K, 5K, even 10K or more.
But it would have to be Halloween Midnight to see an LED with 68K (~~0.05mA) resistor.
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Ok so I had sometime to sit down and take voltage readings across the amp and I've definitely found a problem!
My heaters are not lighting up and rightly so seeing as they are getting 1.1 volts ac. So here are all the voltage readings and if you can give me a starting point as what to look for and try that would be amazing!
Both 6v6s read the same.
6v6
1. 0
2. 1.1 ac
3. 1 .
4. 1 .
5. -25.0
6. -25.0
7. 1.1 ac
8. 0
V1. reads
1. 354
2. 9.6
3. 0
4. 1.1 ac
5. 1.1 ac
6. 0
7. 0
8. 0
9. 1.1 ac
V2 reads
1. 400
2. 0
3. 0
4. 1.1 ac
5. 1.1 ac
6. 400
7. 0
8. 0
9. 1.1 ac
V3 reads
1. 352
2. 0
3. 0
4. 1.1 ac
5. 1.1 ac
6. 352
7. 0
8. 0
9. 1.1 ac
V4 reads
1. 330
2. 0
3. 0
4. 1.1 ac
5. 1.1 ac
6. 1 .
7. 0
8. 0
9. 1.1 ac
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Take another look at the picture I attached in reply#26.
How did you resolve that wiring error?
(A close up pic would do wonders for our curiosity)
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I added the bias tap, but everything else's is the same. The diagram shows that wire going to the intensity pot for the trem.
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Measure voltage between pin 2 and pin 7 of a 6V6 socket. What have you?
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If your tubes are not lighting up...they can not work...and they thus cannot draw/pass current. That's why you have huge volts on your preamp tubes. Are you using a tube rectifier and is *IT* lit up?
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6.3 on both the power tubes are lighting up as well as the rectifier tube. So if 6.3 isn't passing through to the pre amp tubes that would mean a cold solder joint?
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Probably a bad connection of some kind, cold joint, broken wire, etc. Should be easy to find.
Measure voltage between pins 4/5 and pin 6 of each small tube socket.
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Reading 4/5 and 6 with ax voltage I get 0 across the board.
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Reading 4/5 and 6 with ax voltage I get 0 across the board.
Crap. My mistake. That should be "Measure voltage between pins 4/5 and pin 9 of each small tube socket."
Sorry
Be sure there is a jumper between pins 4 and 5 on all those little sockets.
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I should've figured that's what you meant. When testing individually I get 1 to 1.1 but when testing together this is zeros on every one. So I would've steam since V4 he's first in line for the 6.3 V it would be somewhere between 6V6 and V4 I reflow the Sauter with no change at all so I assume I would need to change those wires out?
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Take another look at the picture I attached in reply#26.
How did you resolve that wiring error?
(A close up pic would do wonders for our curiosity)
I added the bias tap, but everything else's is the same. The diagram shows that wire going to the intensity pot for the trem.
I was referring to REPLY#26
Sluckey got you straightened out on the LED resistor value, but how do you have it wired?
You should have created an artificial CT with 2 100ohm resistors (which is why I thought you had 50ohms feeding that LED :wink:)
Sluckey picked up on the values (100K and 68K - because he is forum "establishment")
Did you incorporate an artif. CT and how did you wind up wiring that area in the circle of my posted pic?
Here's an adjusted schematic snip:
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Take another look at the picture I attached in reply#26.
How did you resolve that wiring error?
(A close up pic would do wonders for our curiosity)
[/quote]
I added the bias tap, but everything else's is the same. The diagram shows that wire going to the intensity pot for the trem.
I was referring to REPLY#26
Sluckey got you straightened out on the LED resistor value, but how do you have it wired?
You should have created an artificial CT with 2 100ohm resistors (which is why I thought you had 50ohms feeding that LED :wink:)
Sluckey picked up on the values (100K and 68K - because he is forum "establishment")
Did you incorporate an artif. CT and how did you wind up wiring that area in the circle of my posted pic?
Here's an adjusted schematic snip:
[/quote]
Gotcha! Ok so I didn't know I needed an artificial Center Tap. I'll get that installed.
I haven't gotten to cleaning up that spot yet what I'm going to do is use the 180ohm resister that just came in and use heat shrink to make sure its all clean. I'll do that replace the heater wire between the 6v6 and V4 and let you guys know what happens.
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Alright, I found and fixed the broken wire! I'm getting 6.1 on my preamp tubes now! But I rewired the led with a 180ohm resistor and did the artificial ct and now I have no led at all!
Also no sound. If I turn the reverb pot I can get some hum but that's it.
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All tubes lit?
1-2 volts on preamp tube cathodes? (pins 3-8)
Redo your voltage chart from some posts back.
Early preamp tube has not 350 volts on plates...(pins 1, 6) more like 180-200. Not the 12AT7 reverb driver, that will have BIG volts on the plate.
Print out a schematic for PR and any place where you see a voltage reading, check your build against it. There no "V1", "V2" on the Fender schematic and thus there is not a super obvious correlation between what YOU happen to call V1, V2, V3, V4 (probably derived from the physical sequence of those tubes = perfectly logical) and the ACTUAL V1, V2, V3, V4. You have to get those elements straight in your thinking.
You know for sure that the first 12A_7 is almost certainly the first preamp. 180-200 volts on the plates, 1.x volts on the cathodes. You know that whichever tube has its cathodes and grids and plates strapped together and which should get rather HOTTER than the other tubes, with BIG volts on its plate is the reverb driver. Probably V2. You know the little tube closest to the power tubes ought to be half phase inverter and half trem oscillator. Probably V4. That leaves only V3, half is the the reverb recovery, the other half wet.dry mixer.
It is very, very easy to get confused to how the two tubes which are split function, V3, and V4, are allocated. Each 12A_7 tubes has "1-2-3" half and a "6-7-8" half. You must must must get the halves right and coherently right in your thinking. Yes?
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But I rewired the led with a 180ohm resistor and did the artificial ct and now I have no led at all!
LEDs are fragile. It's possible that it's dead after all that work on it. If it's still alive you probably have a wiring error or poor connection.
Those filament wires really need a secure mounting point such as a terminal strip. Floating around like that is inviting gremlins.
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I used a new LED so I'm not sure whats wrong. I'll try it again I guess.
Here are my new voltages.
6v6s read the same.
1. 33.1
2. 2.9
3. 395
4. 387
5. -24.9
6. -24.9
7. 2.9
8. 33.1
12ax7 closest to the 6v6 (V1?)
1. 163
2. 00.3
3. 3.2
4. 2.9
5. 2.9
6. 172
7. 5.6
8. 65.9
9. 2.9
12ax7 next in line (V2?)
1. 146.7
2. 00.1
3. 1.4
4. 2.9
5. 2.9
6. 144
7. -33.3
8. 1.5
9. 2.9
12at7 Although I'm using an ax for right now.
1. 389
2. 0.1
3. 6.6
4. 2.9
5. 2.9
6. 389
7. 0
8. 6.1
9. 2.9
12ax7 last in line (V3?)
1. 143
2. 0
3. 1.4
4. 2.9
5. 2.9
6. -141
7. 0
8. 180
9. 2.9
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Are you aware that LEDs have a positive terminal and a negative terminal?
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Here are some general thoughts.
First, have you posted a schematic of this amp anywhere...or is it supposed to be a conventional AA964 or AA1164 Princeton Reverb?
Second, you show your 6V6 voltages. There are two general ways of biasing these two tubes. "Bias" is a negative voltage applied to pin 5, grid 1, that partially shuts down the tube, chokes it off...otherwise it/they will overcurrent, pass too much current, redplate, and burn up. One way is to have an actual power supply that, in contrast to (almost) all the other voltages in the amp generates a NEGATIVE voltage. The cathodes of the output tubes are connected to ground which is zero volts, and the negative volts produced by the "bias supply" is applied to the first grids in the 6V6 tubes. There may be (usually is but does not have to be) a control which changes the amount of neg volts applied to these grids. The signal produced by the amp, which has been amplified many times over what comes out of your guitar, is ALSO applied to these grids. The warbly TREMOLO signal is ALSO applied to these grids. This is known as (even if there is variable control to change the bias voltage, "fixed" bias. ++
The second way to bias the tubes is to insert a resistor underneath their cathodes, between the joined cathodes and ground. That resistor is usually 150-250 ohms and is usually a power resistor. When current passes through those tubes, a voltage drop occurs across that resistor. This causes the cathodes NOT to sit at zero volts, but to sit at maybe 20-30 volts. This method is known as "cathode" bias.
One way or another MUST be used, or the 6V6 tubes will overheat and burn up.
If you have fixed bias, the separate supply...then the cathodes are going to sit at zero volts and we will see the neg -25 volts on each pin 5, the control grids, the first grids. If you have cathode bias you WON'T have the -25 volts on the control grids, you'll have zero volts or very low (at idle) volts. instead, the CATHODES of the 6V6 sit at POSITIVE 25 volts. It is the RELATIVE voltages we are concerned with. The grids have to be -25 volts NEGATIVE relative to the cathodes. Whether that occurs with real negative volts, negative with respect to ground, OR, by raising the cathodes 25 volts and letting the grids sit at zero, either way is OK. But not both.
Per your voltage chart, you show the 6V6 cathodes sitting at 33 volts (let's call that the 20-30 volts) which can ONLY occur if you have the resistors under the cathodes and the cathodes are NOT connected to ground. But then...you show the -25 volts on your grid 1 = pin 5.
This cannot be. Or I should say, this can not work.
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MOST people would label V1 as FARTHEST from the 6V6, the first preamp tube, "first" meaning the first tube your guitar signal passes through. I believe that would be the most common "convention". Using that practice, the 6V6 tubes usually have the highest "V" numbers, though this is not written in stone.
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Your voltages seem generally reasonable other than what I said about the 6V6 grids and cathodes EXCEPT FOR your "last in line" which is not going to work with -141 volts on its plate pin 6. I would normally assume that is a typo EXCEPT you also show 180 volts on pin 8 cathode. Those volts are way wrong. You should have 180 (could be anywhere from 160-200) on the plates pins 1 and 6, and 1.x on the cathodes, pins 3 and 8. Where the heck are you even getting NEGATIVE 141 volts?
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Once again....is there a schematic for this build you can post?
*******************
So I am pointing out two anomalies in your voltages. Let me add that EITHER ONE of those could prevent the amp from making a peep of a sound. In the case of the odd 6V6 volltages, those tubes have in effect negative (33 + 25) volts = neg 58 volts on their grids which, if it didn't totally shut off the output tubes (render them incapable of passing signal or passing current) would be darn close. Maybe the amp would be 10-15% as loud as it should be it it made any sound at all.
The voltages on the preamp tube would completely foil its operation. It would not pass signal nor collect $200.
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Thank you! VERY insightful
This was just a hoffman princeton reverb Build so I used his schematics and layout. I tried to post it but Its not letting me.
I do have a bias trem pot but I wasn't sure how to check the bias or what to set it to!
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I'm attaching the schematics I'm using.
I'm still not quite understanding the biasing because what I see here in this schematic and what eleventeen said aren't connecting in my head haha. I have done exacticly what's in this lay out and schematic! So what did I do wrong and how do I fix it!
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Your 6V6 cathodes are held off ground by a mere 1 ohm resistor. This is a "modern" adaption for the sole purpose of making a BETTER bias measurement. What is a "better" bias measurement? It is a measurement of ACTUAL CURRENT through the output tubes. When we look at an original Fender schematic we see a voltage reading on the G1's, the pin 5's of the output tubes. You could set your amp up (meaning, tweak the bias control) such that that voltage was present on the pin 5s of the 6V6s. Now, go get 5 other sets of 6V6. What you would find (assuming you had a way to measure it) is that the CURRENT through the various pairs of 6V6 would vary. Probably not much, but some. Conclusion: We set the bias VOLTS to a level and ASSUME that that average voltage level will, by partially shutting down the 6V6 pair, produce a through-current for any random 6V6 that will have the amp operate OK and not fry the 6V6. The VOLTAGE reading is quite the indirect measurement. We're REALLY interested in the through-current, we actually do not care about the volts. Knowing the through-current allows us to bias the tubes cold; which would make them last longer, or, hot, which would make them sound a bit grittier and wear them out sooner. Or middle, just right. Your choice.
So, by installing those 1 ohm resistors (you should understand that 1 ohm is practically nothing in tubeland, it is closer to a short circuit than anything else. We can use ohm's law E = IR to read the current through the tube MUCH better than surmising something indirectly-indirectly from a goofy G1 reading. With those R's installed, every milliVOLT atop that resistor is a milliAMP through the tube. We're usually looking for 17-25 mils through a 6V6, in that range. If you find an older pair of 6V6GT from WW2, maybe you want to crank the bias colder to reduce current to preserve the tubes. If you have anew pair of JJ's and like a more distorted sound, crank up bias and run the tubes hotter.
I should say that we HATE to measure current directly, because it requires interrupting the circuit (cutting and then reconnecting wires) AND it forces current through our meter. In the old days, this is what destroyed meters.
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So the real question is, if your 6V6 cathodes are reading 33 volts (not MILLIvolts, but volts) a mere 1200 times what the tubes can handle, and are held off ground by a 1 ohm resistor, to get 33 volts on the cathodes, those tubes would have to be passing 33 amps, which would launch them into low-earth orbit trailing wisps molten metal, glass, and plastic. Ergo, there is no way you can have followed the schematic AND find 33 volts on the cathodes. (the power supply cannot supply 33 amps, not to mention TWICE 33 amps for two tubes, but your tubes would be cooked in near-zero time)
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Do you see how all three of 1: bias volts, 2: guitar signal, and 3: trem signal all meet up at 6V6 g1?
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I had just assumed he probably had his meter set to auto-range and he simply overlooked the small mv indication on the display.
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Elevteen. Thank you for that explanation!
Ok so pin 1 & 8 is 33.1 millivolts but pin 5 & 6 are - 24.9
So I need to figure out where the negative bolts are coming from
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So I need to figure out where the negative bolts are coming from
Those are good negative bolts and they are coming from your vias supply. :icon_biggrin:
He was referring to the big negative readings you had on a couple small tubes. I'm pretty sure those are just operator error readings.
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Slucky, I'll check that!
I'm still not getting any signal from the amp but if I turn the reverb up with out the tank plugged in it starts giving me a hum and if I turn the trem up I can hear the the speed and the intensity working. But no guitar signal at all.
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This amp is a bit more complicated than the 5F2A you did back in April, but the troubleshooting procedure is the same. Verify that the amp is wired properly. Click the link in my signature line at the bottom of this post for a good procedure to verify the wiring. When it's wired right it will work.
We've already spotted a couple problems and they have been corrected. You just haven't found all of them yet. Keep looking. You'll get there.
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Slucky,
You sure know how to give a guy some confidence!
I'll go over it again tonight! One question if I have a jumper wire under the board wrong is there a way to test for that? So I don't have to unsolder everything and lift up the board.
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Yes. Use your ohm meter to check for zero ohms between the two turrets involved. There are only three underboard jumpers on that board. Should take about three minutes to verify. But it will take a lot more time if they ain't right. :icon_biggrin:
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Jumpers are correct! Praise God!
Now for everything else haha
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I haven't had time to get back to the amp until today.
I have gone over the amp with a fine tooth comb and and found a wrong connection on v2 where the reverb trans connects. anyway after that was fixed i haven't seen anything out of the ordinary. I do have a very small broken amount of sound coming through the amp with it turned up all the way. if I turn the term up it works and the speed works as well but if I turn the amount up to much there is a loud hum that starts and I will have to turn the amp off to get it to go away.
Ive switched the wires on the jack with no difference. So any direction on where to look would be greatly appreciated here are some updated pictures for reference!
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Those pics are way too small to be useful.
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Try these
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more
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Any idea of a starting point?
I'm so lost at this point. Ive gone over and over the schematic and layout but I don't see anything wrong yet I still am not getting sound really at all! any help would be greatly appreciated.
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Well I seem to have a few broken wires!
I used pre tinned teflon wire and had given me one heck of a headache!
I'm going to pull it all and start over wiring everything in! What is the best wire to use?
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That should be fine, the only thing you want to be wary of is not to nick the wire when stripping it, it can create a weak spot and after a few bends/moves it may break. Just try to replace the wires that are broken, or resolder them after stripping if possible. Verify continuity on each lead by touching to the turret and the other side away from the soldered connection to ensure you get a continuity beep. If not somethings still not connecting well. That's another quick easy test to do.
~Phil
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Well I've rebuilt the entire amp and still no sound haha
I've been over and over it, I know I'm simply missing something!
If anyone sees anything please let me know!
I'm uploading images of the amp and voltages that I've taken.
Thanks for all the help so far! I am detumained to figure this out!
Also I am using a larger output transformer ( deluxe trans ) instead of a normal Princeton output trans. And you may notice a pot that is in line and grounded but not hooked up to anything that will become a mid knob after I get the amp running properly!
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More pictures
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Where to start! There are so many bad voltage readings in that chart. That indicates multiple wiring errors. Too many to logically troubleshoot at this time. You must double, triple, quadruple check your wiring and correct those errors. Click on the link in my signature line at the bottom of this reply to get started.
Hoffman's first law... If it were wired correctly, it would work. I don't mean that in a smartass way. But it's true. You have the amp in hand and are in the best position to discover wiring errors. So I repeat, Click on the link in my signature line at the bottom of this reply to get started.
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You are right!
Ok I'll go over everything again.
I am gunna get this thing right one way or the other haha
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I know your tired and want to get her up and running BUT take your time, go slow so you don't miss anything.
You might be missing the same mistake(s) over and over again.
It will be worth it in the end when you get her straightened out. :icon_biggrin:
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Thanks wilabe, that's really encouraging!
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What Sluckey said. You have a ton of wiring errors.
On your 12A_7's the big volts show up on pins 1 and 6, the plates. With the exception of the 12a_7 closest to the 6V6 tubes, the cathodes (pins 3 & 8) should be 1-2 volts.
On V1 you have this flipped, but OTOH, if you were really feeding 361 volts to pin 3 (a cathode.....in a tube where there can't be more than either 100 or 150 heater-to-cathode volts or the tube will blow up) you should be blowing up that tube right now. As in visible sparks, inside the tube, and this could well have created a situation where the guts of that tubes are now welded into a fuse-blowing short. So do not overlook the notion that you are reading the tube pins wrong. Are all your 9 pin sockets aligned the same, eg; the "keyway" the space between pin 1 & pin 9 are all faced the same way?
There is a way to check wiring that I sometimes use that is different than what has been detailed elsewhere, multiple times on this forum. It is not as procedural nor as comprehensive but it is in some ways more definitive.
I clamp one lead of my ohmmeter onto V1, pin 1. A plate. Power is off, of course, and has been for a few minutes. I now go measure ohms to node "D" of the power supply and I either see 100K ohms or something is wrong. Not maybe, not "could be", I read 100K or something is wrong. I now go measure to pin 6 of that same tube and I either see 200K (the series sum of 2 - 100K resistors) or something is wrong. I now measure to node "C" and I either see 110K (100K plate R plus 10K node C-D resistor which I am assuming is 10K) or something is wrong.
I follow every resistive path that pin 1, V1 goes to, until the path is blocked by a cap (which will not pass DC)
Even all the way back at V1, I should be able to get to (eg; measure a resistive path from) the plates of the PI, by going through plate resistor, node resistor, node resistor, plate resistor.
It is EXCELLENT practice to follow those paths, predict the ohms you should measure, and get that number or not. It will help you understand the topology of the amp.
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Am I seeing things wrong, or does the speaker jack have wires on the wrong lugs? I think I don't see a connection to the sleeve/ground lug...
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Kind of looks like it, huh?
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I think I see a jumper between the sleeve and switch lugs.
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output put jack is wired as follows.
Green wire from OT to the Tip, Black wire from OT goes to Ground and Sleeve then the feed back loop is connected to the Tip as well.
I did find a couple of things wrong with the amp. some wires hidden under the board and a couple of hookup wires in the wrong spot. now that its all fixed I'm still not getting any sound.
So i decided to feel around with a chop stick and see what I can come up with.
Everything is dead quite until I hit the hook up wire from pin 1 and 8 the cathode of the 6v6 closest to the rectifier tube, that is hooked up to a 1ohm resister on the board.
It was scratchy and the amp started making a weird oscillating sound. I removed and replaced the wire with a new one and its still making the same noise. Any idea on what to look for?
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I'm back to tinkering around with this a little more.
After some more research I'm thinking it my possibly my filter cap.
The amp is motor boating along with a 60 cycle hum!
I know this is rare in tube amps but when I built the amp the first time I had one of the 220v lines going to ground ( YIKES )
So i don't know if I could have possibly messed it up.
Anyway what is the best way to test this?
ground to the chassis and then test each lead?
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"Motorboating" is a single, specific thing with a single specific cause: The NFB taken from the speaker is the wrong phase and is POSITIVE feedback, a characteristic of an oscillator. There are 2 or 3 solutions, all of which accomplish the same thing. The usual thing done is to flip the blue and brown wires from the output transformer from where they go now to the opposite tube.
Thus, when we build an amp, we do not cut those leads to length and tuck them in neatly; we leave them long in case they have to be flipped; because this is one of those things that is unpredictable.
At the Fender factory, this WAS predictable, because they built a load of identical amps using a pile of trannies from one supplier. ALL those trannies used the blue for the winding start and brown for the end (or vice versa) and so could simply be color coded, blue to this 6V6 and brown to the other, and be counted on to be properly phased.
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First thing to come to my mind when I hear "motorboating" is filter caps.
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First thing to come to my mind when I hear "motorboating" is filter caps.
What's the best was to test this? Ground to chassis and then test each lead?
I would still be testing in DC correct?
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Use a spare filter cap (20µF to 40µF) and clip leads to bridge the filter caps one at a time. One clip lead to chassis. The other to the positive terminal (or lead) of the suspect cap. No need to disconnect the suspect cap for this test. If you find that the motor boating goes away and/or the hum gets less, change that cap.
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Back to this again :BangHead:
I decided to pull everything out and start from scratch AGAIN!
Did that built a very clean looking amp went over everything two and 3 times and I have a nearly audible sound if its cranked all the way! I can tell the term works because you can slightly hear that as well.
Here are my voltage readings with a few notes on them as well.
I'll attach a few pictures as well.
http://el34world.com/charts/valve/ValveData.php?e=view&f=19915 (http://el34world.com/charts/valve/ValveData.php?e=view&f=19915)
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Pictures there is a mid knob in line but not hooked up!
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More pics.
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I'm starting to think that it could be the OT?
Is there a way to test that hooked up?
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Which voltmeter do you have? Brand and model?
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GE digital multimeter
GE252A
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Never heard of it. Even Google can't find any info. Can you post a pic of that rare bird?
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Sure thing!
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> GE252A
GE2524 --- dammit, numbers matter.
Home-store low-price 3-1/2 digit meter.
Manual (http://www.homedepot.com/catalog/pdfImages/29/299adba4-e224-40d4-bcbf-0ad272acb166.pdf), note it displays bad, this is pre-press pre-trim image.
Key specs attached.
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> GE2524
$16 on Amazon (https://www.amazon.com/GE-Multimeter-6-Function-Non-Recording-50953/dp/B002DNG63C).
2.7 out of 5 stars
5 star 22%
4 star 11%
3 star 22%
2 star 0%
1 star 45% (!!)
Standout review:
"Fail. I do not understand what is wrong with this meter, but it's been nothing but trouble. ..., some of the erratic readings were repeatable, producing the impression that there was a problem with the circuit, but when tested with a Fluke, the power was perfect."
I have had luck with real-low-price DMMs. This may not be a lucky one.
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Makingnoise 88,
I greatly admire your tenacity and perseverance with this build!! With that kind of dedication, I am confident you will get this amp up and running. Keep it up! Don't give up! Be safe working on the amp and you'll get there. The reward will be worth it all.
You have some great forum guys helping you out! Some of the really knowledgeable and gifted individuals. Sluckey has gotten me thru more bone headed mistakes than anybody on this forum, so your in good hands with these guys.
I am rooting for you and watching this thread! Remember to post the success in your subject line when you get it all done " Princeton Reverb blowing fuses - RESOLVED"
With respect, Tubenit
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Ill look for a new MM!
But as for an update I got it working!
I was grounding my input to the TIP instead of the ground lug :cussing:
But after getting that right its up and running!
Its a bit quieter than it should be but I need to bias is. So once thats done I'll check it all again.
I hooked up the reverb really quick but didn't seem to get anything out of it so i'll have to check that after work as well!
Guys thanks so much for all your help. I'll post an update after I bias and check out the reverb!
As always if you guys see anything or have any suggestions I have greatly appreciated it! The knowledge Ive gained and gaining is mind blowing! Thank you thank you thank you!