New Build Problems

[Ed_Chambley]

I just completed a 18 watt preamp with cathode bias El-34's.  I checked with a bulb limiter.  everything ok, even played a little with it.  Everything seemed ok, so went to full power to begin checking voltages.  The rectifier tube got hot and flashed and popped the relay ( I use 2 amp relays as opposed to fuses).  The tube was very hot.  I pulled all the tubes and checked the rectifier voltages and I get 315vac on the reds and 5 vac on the yellows.  Put the rectifier back, same thing happened.  Tried a few different tubes, same thing.  What would be the next step in troubleshooting, I cannot get any voltages without the rectifier.

[Leevi]

I suppose you are using fixed bias.
Check that the bias circuit is working correctly e.g. by measuring the negative voltage on power tubes (pin 5).
/Leevi

[John]

The flashing and popping sound as if something is shorting across the socket for your tube. Maybe something almost making contact, needs the full juice to jump the gap?

[sluckey]

How much B+ voltage do you get with the rectifier tube in? Tried another rectifier? Does the amp work at all? Got a schematic? Got some pics?

[Ed_Chambley]

The B+ at pin 8 rectifier tube using a 5u4gb is 315vac and of course at the standby switch.   It is not fixed, it is cathode bias.  Just as soon as you filp the standby it makes a deep buzzing sound.  This is with only the rectifier tube in.  I have tried many different tubes.  This is the amp where I used a 18watt preamp and a Chiefton Power Section.  The tube flashes inside and I see no arching across the socket.  I will get some photos later, I ain't much with a digital camera.

I am getting 320 on both reds, 320 on pin2 and 315 on pin 8 (vac).  In cathode bias I do not understand where the amp gets its DC other than filter caps.  C1 and C2 are a dual 50uf/500v  and I have a choke connected across.  Hammond 194, a hefty choke.

[John]

Pin 2&8 should be showing DC voltage, not AC. You have AC going into the tube, but  DC coming out. At pin 8 you should be showing ~ 385 vdc.

Hate to ask a dumb question, but you do have the red wires going to 4 and 6, and the yellows to 2 and 8, right? Oh, and the CT  is grounded?

[Willabe]

In cathode bias I do not understand where the amp gets its DC other than filter caps.

The rectifier tube, 5U4 in this case is a dual diode tube. This is where the AC coming in is changed/rectified to DC going out.

Pin 8 should have DC on it. But to get a stable dcv on it you need a filter cap on it too. 20uF or more would be fine.

[Ed_Chambley]

Some Photos.  I know I should have vdc on pin 8. here is a photo or 2.  Also it is center tapped.  I may need to add a cap.

[Ed_Chambley]

More Photos

[Ed_Chambley]

Couple more

[Ed_Chambley]

I used this layout and substituted the octal sockets.  If I add a cap, how should I place it?  Should it be between standby and pin 8?

[LooseChange]

Sorry if this was mentioned.... Have you tried this with only the rectifier tube and the B+ wire from pin 8 disconnected from the rest of the circuit?
How much B+ filtering do you have on that first node?

[Ed_Chambley]

Sorry if this was mentioned.... Have you tried this with only the rectifier tube and the B+ wire from pin 8 disconnected from the rest of the circuit?
How much B+ filtering do you have on that first node?

I am not sure what you are suggesting.  If I do understand, I have disconnected the b+ from pin 8, but this disables the B+ entirely.  Going from pin 8 to standby to the left side of a dual cap can which feeds the OT, a choke connects the other side of the dual cap can which then feeds what I would consider the first node.

I now have a new problem.  Now I have a dead short somewhere.  This is very weird.  When I firest fired it up with the bulb limiter, I played the amp for 5 minutes or so.  Nothing got hot.  When I went to full power, the rectifier tube flashed and blew the fuse.  Now if I reconnect it to the bulb limiter with a known good rec tube, it shines bright off standby.  I thought I should get dc current from the cap can, but not so.  I can hook up another cap from ground and hoook a wire to pin 8 and get 297dvc.  Just cannot figure out the problem.  Should I move the choke as it connects the dual cap can together.

[Willabe]

When I went to full power, the rectifier tube flashed and blew the fuse.  Now if I reconnect it to the bulb limiter with a known good rec tube, it shines bright off standby.

Yes a direct short, that's why the bulb is glowing.

Ed, divide and conquer.

You might have killed the HT on the PT. Try taking an acv reading from the 2 red PT wires with out the rec. tube pluged in. If good then try just hooking up and powering up just the PT, HT, 5U4, and 1 filter cap, no _ other _ tubes/no B+ current draw. Forget the choke for now and all else down stream in the B+.

Try it with the limiter first and then if it's OK try it with out the limiter, if OK, then try adding the choke, and so forth.
  

[Ed_Chambley]

When I went to full power, the rectifier tube flashed and blew the fuse.  Now if I reconnect it to the bulb limiter with a known good rec tube, it shines bright off standby.

Yes a direct short, that's why the bulb is glowing.

Ed, divide and conquer.

You might have killed the HT on the PT. Try taking an acv reading from the 2 red PT wires with out the rec. tube pluged in. If good then try just hooking up and powering up just the PT, HT, 5U4, and 1 filter cap, no _ other _ tubes/no B+ current draw. Forget the choke for now and all else down stream in the B+.

Try it with the limiter first and then if it's OK try it with out the limiter, if OK, then try adding the choke, and so forth.
  

That is exactly what I am doing now.  Good news.  PT all good.  I am calling it quits for tonight, but the Dual cap is leaking and I did get a spark from the ground post.  I built this out of parts I had laying around.  The only "new" part is the 50/50 FT dual cap.  I have 320vac from both reds and 5.8 no tube.  Heaters read good as well.  Hooked up a single 100uf/475 to ground and ran a wire back to pin 8 and got a reading of 295vdc.  Tomorrow I will hook it to one side of the dual and we will see.

Thanks for the guidance.

[sluckey]

Use your eyes and your ohm meter to find the short on the B+ rail.

Your board appears to be supported by only two 1/4x20 bolts on the tube side? Is there any support on the pot side? How high does the board sit above the chassis?

There's a lot of plastic coated solid wire in this amp. Is that low voltage control wire from a thermostat or sprinkler system, or similar application?

( I use 2 amp relays as opposed to fuses). 

Are you sure?

[phsyconoodler]

Quote:"Also it is center tapped."

 Hope you didn't ground the rectifier center tap!!!

Spend some time and clean up your wiring mess.It will save you lots of time when troubleshooting a problem in the future.

[Ed_Chambley]

Use your eyes and your ohm meter to find the short on the B+ rail.

Your board appears to be supported by only two 1/4x20 bolts on the tube side? Is there any support on the pot side? How high does the board sit above the chassis?

There's a lot of plastic coated solid wire in this amp. Is that low voltage control wire from a thermostat or sprinkler system, or similar application?

( I use 2 amp relays as opposed to fuses). 

Are you sure?

The wire is pvc, 20ga solid core copper. Not low voltage, but how could that cause a dead short? Current leaking out of the jacket.  The 2 amp relay states 2 amp.  I get them from weber.  I have them in a few builds.  I like them, but I can put a fuse holder in just as easily, the hole is drilled large enough.

There is support on the tube side, you cannot see it, but there is a board cut off the same material mounted with epoxy to the chassis.  It gives me enough room to run the wires under with plenty of wiggle room. The board is 1/4" from the chassis.

Problem with using eyes and meter, I get no reading at the first filter cap and I cannot understand why.  It is a straight shot from pin 8 when I bypassed the standby switch.  I am assuming the short is there, but I have not checked it.  I have a farad meter and will check it shortly.

[Ed_Chambley]

Quote:"Also it is center tapped."

 Hope you didn't ground the rectifier center tap!!!

Spend some time and clean up your wiring mess.It will save you lots of time when troubleshooting a problem in the future.

It is a very small chassis.  The wiring is pretty direct.  Suggestion of how to clean it up?  The board runs from end to end of the chassis, so the option of making pretty tie-downs around the board weren't very feasible.

[sluckey]

Not low voltage, but how could that cause a dead short?

The insulation burns up easily with a soldering iron, exposing bare wire. Just an observation. Probably not related to your problem.

The 2 amp relay states 2 amp.  I get them from weber. 

It's a circuit breaker, not a relay.

The board is 1/4" from the chassis.

Dangerously close to the solder lugs on that dual cap can!

I am assuming the short is there, but I have not checked it.  I have a farad meter and will check it shortly.

Why haven't you checked it? Farad meter is useless in finding a short circuit. Use your eyes and your OHM METER. You should find it in 5 minutes.

[Ed_Chambley]

Not low voltage, but how could that cause a dead short?

The insulation burns up easily with a soldering iron, exposing bare wire. Just an observation. Probably not related to your problem.

The 2 amp relay states 2 amp.  I get them from weber. 

It's a circuit breaker, not a relay.

The board is 1/4" from the chassis.

Dangerously close to the solder lugs on that dual cap can!

I am assuming the short is there, but I have not checked it.  I have a farad meter and will check it shortly.

Why haven't you checked it? Farad meter is useless in finding a short circuit. Use your eyes and your OHM METER. You should find it in 5 minutes.

Correction, I have a circuit breaker.
I have traced everything.  I now have a diode socket in the rectifier.  Lots cheaper than my nice tubes.  I have 438vdc at pin 8.  On the other side of the cap can I have 386vdc.  Everything is hooked up except the OT.  The OT is causing the short which I did not expect as it worked at first.

Initially, I did use a meter to check the cap and got nothing. Now I've got one red wire when I hook it to the cap, I get the short and a loud humming.  Later I am going to check to see if I have it wired correctly to the speaker out.  Knowing me, I may have it backwards.  Could this cause these symptoms?  If so, why did it work then not?  Also, the layout shows connecting it to the first side of the dual cap where I have 438vdc.  This would be prior to the choke.

I have made a cover for the cap can to isolate it from the chassis and board, I just had it removed for the photos.

[sluckey]

The OT is causing the short which I did not expect as it worked at first.

Follow the short. Leave the OT red lead disconnected. See if you have the short on pins 3 of the output tubes. If so, disconnect the OT leads from the tube sockets. Is the short still on the red lead or is it still on a tube socket?

[Ed_Chambley]

The OT is causing the short which I did not expect as it worked at first.

Follow the short. Leave the OT red lead disconnected. See if you have the short on pins 3 of the output tubes. If so, disconnect the OT leads from the tube sockets. Is the short still on the red lead or is it still on a tube socket?

I have no short on pin 3 when connected with red disconnected.  No short with pin 3 socket with wires disconnected.  ohm readings from b+ connection to brown plate lead 76, blue 82.  I read somewhere that if you were in 10's to a few 100's, the windings are good. Over 1 meg, dead OT.  Don't know how true this is, but I thought I would check it since I had everything disconnected.

[sluckey]

Is the dangling red OT lead still shorted to chassis with the plate leads disconnected? If so, the OT is bad. If not, something was likely boogered where the red lead connected to the cap can.

We're talking about a short, but there is likely some resistance value. What is that value, even if it is zero ohms?

I have 438vdc at pin 8.  On the other side of the cap can I have 386vdc.  Everything is hooked up except the OT.

IOW, you have 52 volts dropped across the choke? I'm concerned about that. There is typically only a small voltage dropped across the choke. 52 volt drop means a fairly large current through the screens or else where downstream. However, this may be normal since the plates have no voltage on them.

[Ed_Chambley]

Is the dangling red OT lead still shorted to chassis with the plate leads disconnected? If so, the OT is bad. If not, something was likely boogered where the red lead connected to the cap can.

We're talking about a short, but there is likely some resistance value. What is that value, even if it is zero ohms?

I have 438vdc at pin 8.  On the other side of the cap can I have 386vdc.  Everything is hooked up except the OT.

IOW, you have 52 volts dropped across the choke? I'm concerned about that. There is typically only a small voltage dropped across the choke. 52 volt drop means a fairly large current through the screens or else where downstream. However, this may be normal since the plates have no voltage on them.

I got tired last night.  I will disconnect the plate leads again and see if just connecting the red lead causes a short.  If so, I understand it is a transformer short.  I hate that if it is the case.  It is a MM OT.  I don't have much money in it, but still.  Have you ever tried to repair a shorted winding?

[phsyconoodler]

Follow Sluckey's lead here.It is unlikely the OT is shorted,but you need to follow the steps.
  That 52v drop is interesting.Something has to draw that amount of voltage and if there are no tubes to draw current,where is the draw?
 

[Ed_Chambley]

With every wire disconnected from the OT, I get a short when connected to the B+.  Bad OT.  Contacted Mercury Magnetics today and they told me their transformers are really though and usually don't short before something else breaks.  They said ship it to them and they would check to see if it was defective.  Probably a waste of time, but I asked if a rewind was cheaper than buying new and they said it was more expensive.  This makes no sense unless they are not making them anymore and just selling them.  In my mind app the parts other than winding cost something.  Seems like rewinding should be cheaper.

The OT is 4900/8ohms.  This build is a cathode bias PP el-34 and maybe run kt-66's.  If I get another OT I would like the option of multi-taps.  Anyone have a suggestion for an OT that will fit this bill.

[phsyconoodler]

You are having some difficulty here I can see,but a rewind is ALWAYS more money cause the labour cost goes up.
  You are still missing something cause like MM says they don't short out for no good reason.
It's likely something you are missing which is not hard to do even for a seasoned repair guy.I'd hate to have you get a new OT and have the same issue.
 

[Ed_Chambley]

You are having some difficulty here I can see,but a rewind is ALWAYS more money cause the labour cost goes up.
  You are still missing something cause like MM says they don't short out for no good reason.
It's likely something you are missing which is not hard to do even for a seasoned repair guy.I'd hate to have you get a new OT and have the same issue.
 

I agree, I would hate to buy a new OT and it be something else.  The amp was working with the current limiter and sounded good.  Somewhat distorted.  I wish I had another ot that was not installed in another amp.  But what else can explain connecting 1 wire from the ot at c1 or c2 or c3 with all the other wires disconnected and get a short.  When I had everything connected, if I touched the red wire to a cap I could hear it pop in the speaker with the tubes in.  With no tubes, I get the short.

[Ed_Chambley]

Here is the deal.  I disconnected the black and green from the speaker out.  Short disappeared.  I instaled all the tubes.  I get 401vdc on both plates of the el34's. Problem is I get -401vdc from the speaker green and the black.  Negative.  I do not understand this, but it seems like I am getting plate voltage to the speaker out.  Any ideas?

[Willabe]

Ed, don't run that OT with out a speaker load or power R of the correct ohms on it.    

[Ed_Chambley]

Ed, don't run that OT with out a speaker load or power R of the correct ohms on it.    

I know what you mean.  Flyback.  It was only disconnected long enough to see the short disappear the I shut down the amp.  I have never had a speaker connection shock me before.  It bit me a little, so I put a MM on it.  I was going back through everything.  Checking sockets, voltage.  Everytime I disconnected the red, by short disappeared.  I have checked the wiring looking for something wrong.  There are only 5 wires.  Red, blue and black.  Black ground and green 8ohms.  I cannot find anything incorrect except when I connect the OT.  Put a 10 ohm 15 watt resistor.  Lamplimiter lit up the room.  Psyconoodler suggested more testing.  The only thing I know for sure is if I hook up the OT I get a short.  With the plate leads disconnected I get 401 vdc on each.  I connect them, I get a short and have no B+ at all.  Touch the speaker out and get a shock.  This aint right.  It's like it is hooked up backwards.

What could it be if not the OT?  Forgot to mention if I have the speaker wires connected I get no plate voltage.  I had then disconnected for about 5 seconds.  If that kills an OT, it was ready to go anyway.

[sluckey]

Test the OT. Disconnect all five wires. Use your ohm meter. Put one meter lead on the red OT wire.

1. Put the other meter lead on the blue wire. What is the resistance reading?
2. Put the other meter lead on the brown wire. What is the resistance reading?
3. Put the other meter lead on the green wire. What is the resistance reading?
4. Put the other meter lead on the black wire. What is the resistance reading?
5. Put the other meter lead on the OT shell. What is the resistance reading?

I want 5 meter readings, even if the meter says OL or infinity, or whatever.

Now remove the meter lead from the red OT wire and reconnect to the black OT wire.

6. Put the other meter lead on the blue wire. What is the resistance reading?
7. Put the other meter lead on the brown wire. What is the resistance reading?
8. Put the other meter lead on the red wire. What is the resistance reading?
9. Put the other meter lead on the green wire. What is the resistance reading?
10. Put the other meter lead on the OT shell. What is the resistance reading?

I want 5 more meter readings, even if the meter says OL or infinity, or whatever.

Please answer these ten questions directly. No other info is needed to determine if your OT is faulty.

[phsyconoodler]

Guys you can turn on a tube amp with no load and not worry too much about it blowing up the OT as long as their is no input signal.It's good practice to have a load on it anyway,but it won't blow up if you accidentally forget the speaker.

 Ed, MM wiring can be convoluted so make sure one of your plate or even the center tap wire isn't hooked to your speaker jack!!!

  Color codes may be different with MM.
If you understood more of how the OT works it would be easy to figure out with just an ohmmeter as Sluckey is showing you.

[Ed_Chambley]

Guys you can turn on a tube amp with no load and not worry too much about it blowing up the OT as long as their is no input signal.It's good practice to have a load on it anyway,but it won't blow up if you accidentally forget the speaker.

 Ed, MM wiring can be convoluted so make sure one of your plate or even the center tap wire isn't hooked to your speaker jack!!!

  Color codes may be different with MM.
If you understood more of how the OT works it would be easy to figure out with just an ohmmeter as Sluckey is showing you.

I did some of the checks he mentioned, I will do the rest and report.  I have a hookup diagram from MM  It is straightforward.  I attached it.  Doesn't get much simpler.

[jerrydyer]

I throw this in because i forgot to ground speaker jacks once. Are your speaker jacks grounded?

[jerrydyer]

nevermind..looks like old style grounded to chassis as soon as they are bolted in. 

[Ed_Chambley]

Test the OT. Disconnect all five wires. Use your ohm meter. Put one meter lead on the red OT wire.

1. Put the other meter lead on the blue wire. What is the resistance reading?
2. Put the other meter lead on the brown wire. What is the resistance reading?
3. Put the other meter lead on the green wire. What is the resistance reading?
4. Put the other meter lead on the black wire. What is the resistance reading?
5. Put the other meter lead on the OT shell. What is the resistance reading?

I want 5 meter readings, even if the meter says OL or infinity, or whatever.

Now remove the meter lead from the red OT wire and reconnect to the black OT wire.

6. Put the other meter lead on the blue wire. What is the resistance reading?
7. Put the other meter lead on the brown wire. What is the resistance reading?
8. Put the other meter lead on the red wire. What is the resistance reading?
9. Put the other meter lead on the green wire. What is the resistance reading?
10. Put the other meter lead on the OT shell. What is the resistance reading?

I want 5 more meter readings, even if the meter says OL or infinity, or whatever.

Please answer these ten questions directly. No other info is needed to determine if your OT is faulty.

Since you are asking for a resistance reading to the shell, I should unbolt it from the chassis?

[Willabe]

I would say no, he just wants to see if any of the windings are shorted to the shell and/or to each other.

[Ed_Chambley]

Test the OT. Disconnect all five wires. Use your ohm meter. Put one meter lead on the red OT wire.

1. Put the other meter lead on the blue wire. What is the resistance reading?
84.0
2. Put the other meter lead on the brown wire. What is the resistance reading?
75.1
3. Put the other meter lead on the green wire. What is the resistance reading?
78.6
4. Put the other meter lead on the black wire. What is the resistance reading?
78.5
5. Put the other meter lead on the OT shell. What is the resistance reading?
It puts a 1 in the far left, no reading

I want 5 meter readings, even if the meter says OL or infinity, or whatever.

Now remove the meter lead from the red OT wire and reconnect to the black OT wire.

6. Put the other meter lead on the blue wire. What is the resistance reading?
6.3
7. Put the other meter lead on the brown wire. What is the resistance reading?
152.1
8. Put the other meter lead on the red wire. What is the resistance reading?
78.1
9. Put the other meter lead on the green wire. What is the resistance reading?
.9
10. Put the other meter lead on the OT shell. What is the resistance reading?
1 in the left side, no reading

All ohms

I want 5 more meter readings, even if the meter says OL or infinity, or whatever.

Please answer these ten questions directly. No other info is needed to determine if your OT is faulty.

[sluckey]

The OT is bad, shorted between primary and secondary.

This is an uncommon failure, especially for a new transformer. Hopefully MM won't claim you boogered the wiring.

[Ed_Chambley]

The OT is bad, shorted between primary and secondary.

This is an uncommon failure, especially for a new transformer. Hopefully MM won't claim you boogered the wiring.

Its not new.  I got it in a box of things for 50 bucks.  I have used 3 PT out of that box and many parts, so I don't feel so bad.  I will send it back, but you know how guarantees on electrical parts are.  You got a suggestion on a replacement?  EL-34 and maybe KT-66.

[sluckey]

You got a suggestion on a replacement?

Yeah. Buy something from Doug.

[Ed_Chambley]

You got a suggestion on a replacement?

Yeah. Buy something from Doug.

I buy most everything from Doug, but this is a different beast.  Cathode El34.
Like this one
40 watt 2 6l6 2-4-8 ohm output
Output for Super Reverb, Tweed Bassman, Bassman heads
50 watts with 2, 4 and 8 ohm secondary taps. Great upgrade for Pro Reverb, Bandmasters, etc.
This is the transformer that is being used to make Vibroverb's out of Bandmaster reverb heads.
Note: This transformer is physically larger than a pro reverb or bandmaster output.
If you are upgrading, you will probably have to drill some new mounting holes.
Two 6L6 power tubes
Part number 018343

Or do I really need this much.  Problem with Doug's Trans, most are not 8 ohms and most of my cabs are.  The Marshall Trans he has $$$$.  Are they that good?

I don't think the deluxe will be enough, do you?

[sluckey]

I'd get something advertised for 2 x EL34s, or 3400Ω:4/8/16Ω

[Ed_Chambley]

Called Hybour.  They are making me one in the same specs as the Chiefton.  I would think that will work.  Not a bad price.  Alden said that transformer was a little on the special side.  Mercury quoted $319 for it.  $78 shipped from Hybour.  They make good stuff and they are very easy to deal with.  He said it was actually 3800 ohms and 4-8-16.  He explained why it was different.  Has something to do with separate resistors and cps for each tube.  It was over my head, but there is something else inside the transformer beside simply windings.  I forget what he said, but I am sure you know.

[phsyconoodler]

It's 'HEYBOER' 

 Never heard of anything different,certainly separate resistor/caps don't have anything to do with it.Sounds like a normal transformer for a Marshall to me.

[Ed_Chambley]

It's 'HEYBOER' 

 Never heard of anything different,certainly separate resistor/caps don't have anything to do with it.Sounds like a normal transformer for a Marshall to me.

I will report back when I get it, but he said he has a 3400ohm 4-6-16 which was for marshall's like 800's, but this one is 3800 ohms.  Not much different, but there are 2 additional parts inside.  I forget, but they shipped it today and invoiced me and I have no account with them.  Said I could return it if it did not have the tone I wanted.  Risky way of doing business, but I guess they are looking for ways to increase business.  I have used many H"E"ybour transformers.  Mostly PT's tho, but I have never had a problem with one.  Internet rumor is Doug gets his from Heybour.  They are not chinese for sure.  Shipping weight is over 8lbs.  I might put it into a JTM-45 build I bought from someone who built it with cheap parts and use that one in this build since I built this from used parts.

[Ed_Chambley]

Got the new OT.  It is just a copy of a Drake Plexi.  Anyway, got it hooked up and I am having problems.  I have 450vdc at c1 and c2 445vdc.
v5
1-0
2-3.7vac
3-445
4-445
5-0
6-445
7-3.7vac
8-0

v4
Same

V3 which is not the PI I am getting no readings except on the heaters

v2 the PI only reading is from the plate resistors 30vdc

v1 nothing

Only tube in place is a ss rectifier.

Any ideas where to look.  How can there be so much current on the grids?

[Ed_Chambley]

Nevermind!  I don't have the resistors in the correct place.   
I basically have nothing for dropping resistors. 270 ohm.  Looks as if I need 10k on each g1 and at least 2k on each g2.