Introduction and First Question

[Tugboat]

All,

I have been a part time lurker for a while and finally pulled the trigger and got a SEX kit before Christmas.  Assembly was fairly straight forward.  I have built two guitar amps in the past, both using turret board layouts and tube rectifiers, so the point to point wiring method, diodes and RCA input jacks of the SEX kit are something new.

Check-out ran into a problem right off the bat.  When measuring the resistance of the RCA jacks center pins they remained constant (89K left, 94K right) no matter the volume knob position. (Each side was measured to ground not to each other)  Other resistance checks that should vary with volume knob position checked out OK ( 0 - 89K L , 0-94K R).  I have gone over the wiring of this circuit portion (Jack > pot> TP 18/19 or 38/39) several times and do not see a reason for the problem.  Drains are connected together at the pot end only, are not touching each other, chasiss etc.....  Looking at pictures at Wardsweb's build site it appears that I soldered the jack center pin correctly, however as I have not use these types of jacks before I may be mistaken.  Is it possible to "overfill" the cup?

Only other resistance measurement that was off was at 20 & 40.  Values were 247 and 249 respectively as opposed to the 295 listed in the build manual.  Any need for concern or further investigation on this?

Any thoughts or tips appreciated.  Look forward to getting this sorted and enjoying some music.

Thanks, - Dave

 

[Paul Joppa]

The input resistances are an error in the manual; your readings are correct.

T20 and T40 read the resistance of the output transformer. Make sure you are using the correct wire (brown, 0.625W); if so then you are good to go. These inexpensive transformers are not precision parts, though they do sound darn good.

Time to check the voltages!

 

[Tugboat]

Thanks,  I really couldn't see any reason that the resistance at the RCA jacks would change with the pot postion but wanted to make sure.

Voltage checks did not turn out well.  Pretty sure I have a grounding issue somewhere.  Getting good voltage readings at the power entry connections when the amp power is off, but when I turn the amp on the voltage drops to around 50V.  Did some quick checks, and nothing jumped out as obvious.  I will get at it again tomorrow after doing some on-line research.

Thanks for the help,
Dave

 

[JC]

I can't help but wonder if you are having the same issue as an earlier poster, IE: The amp either isn't plugged into a grounded outlet, or the point you are using for reference (meter's black lead) is somehow not connected to circuit ground and safety ground.

So, when you refer to a Voltage dropping to around 50 Volts, it will help to know precisely what you are measuring: What is the black lead of the meter connected to, and what is the red lead connected to? And, do you know that the outlet the amp is plugged into is indeed wired completely and correctly?

 

[Tugboat]

Did not have time to do any troubleshooting during the week, but have been at it again this morning.  First check was for outlet grounding.  I bought a tester at the hardware store and checked a few outlets including the one I am using and they all tested OK for ground, polarity etc...  I have been running through a light bulb based current limiting device and the socket on that also checked out OK for ground and polarity.

With the power cord plugged into the power entry module measuring from the ground tip of the cord I recorded the following readings: 

to safety ground screw and tab - 0.1 Ohms
to the power entry ground - 0.1 Ohms
to the line of the power entry module - open circuit
to the neutral of the power entry module- open circuit
to any of the ground bus locations - 0.1 Ohms
to any place on the chassis - 0.1 Ohms
to Xfmr 1 - open circuit
to Xfmr 2 - open circuit
to Xmfr 4 or 5 - 370 Ohms
to C1, C3, C4 - 0.1 Ohms
to C2 - 0.7 Ohms
to C5  - 0.7 Ohms

With the power cord plugged into the power entry module measuring from the neutral prong of the cord I recorded the following readings:
Open circuit to any ground point measured above
to Xrmr 1  - 11.7 Ohms
to Xfmr 2 - 0.2 Ohms

With the power cord plugged into the power entry module measuring from the line prong of the cord I recorded the following readings:
Open circuit to any ground point measured above
to Xrmr 1  - 0.2 Ohms
to Xfmr 2 - 11.7 Ohms

Plugged amp into current limiter and measured the following voltages with amp off:
  Line at entry module to ground tab - 120.6V
  Line at entry module to neutral - 120.6V
  Neutral at entry module to ground tab - 0.008V
  Line at entry module to Xfmr 1 or 2  - 120.6V

Turn the amp on and the reading are:
  Line at entry module to ground tab - 43.6 V
  Line at entry module to neutral - 43.6 V
  Line at entry to XFMR 1 - 0V
  Line at entry to Xfmr 2 - 43.6 V
One thing that seems odd other than the voltage readings is that they decrease over time.  If I leave the amp on for a minute and take the same reading instead of 43.6V I will get something in the mid 30V range.

One thought is to disconnect the secondary connections on the Xfmr and fire it up, then if this section checks out OK hook up the heater circuit, if OK left HT, etc...  Seems like a good divide and conquer approach.  Could someone confirm that this should not harm the PT.

Any other thoughts, comments, additional readings to take would be helpful.  I am stumped.
Thanks,
Dave

 

[JC]

I must be missing something, but what is it that is stumping you?  I am unsure.

If you are using a light bulb in the AC mains line to limit the current, it will definitely drop some Voltage with the amp turned 'On'.  With the amp turned 'Off', there will be no current flow through the light bulb, and it won't drop any Voltage at all; hence, your meter reads the full 120+ at the inlet to the amp.  I can only guess that the light bulb is dropping slightly more Voltage as its filament warms up, so the resultant Voltage at your amp drops slightly.   

An interesting experiment might be to configure your test set-up so that you can measure the Voltage across the light bulb.  Then, let everything get nice and cool.  Then, watch the Voltage across the light bulb as you go through turning the amp on and letting it warm up.

What you should find is that the Voltage drop across the light bulb, added to the Voltage measured at the Amp, will always add up to your line Voltage of 120+.  Obviously, if you can borrow another meter to measure both things simultaneously, this experiment can go more quickly!  But, you can do it in sequence, too, just let everything cool completely between measurements.

The reason for the cooling off periods is this:  The filament of the light bulb and the filaments of the tubes are both basically tungsten,  and tungsten filaments  have remarkably more resistance, the warmer they get.  Also, as mentioned above, the power supply caps, when they get charged up, don't want to draw any more current until they discharge.  With the SEX, there are bleeder resistors to drain them slowly when the amp is turned off, but once they've drained out, they will want a lot of current when the amp gets turned back on.

It is very good that you got an outlet checker and did these additional measurements.  Having a good safety ground on equipment with exposed metal parts can be vital.

Again, I fear I have missed the issue you are having, so please feel free to re-state it, and I will do my best to give you a better response.

 

[Tugboat]

Thanks for the comments.  I agree that a voltage drop is expected across the light bulb with current flow, but from 120V to 40V seems a bit much.  The bulb warming up probably explains why the voltage "decays" over time as you suggested.  My current limiter has a full power switch which bypasses the light bulb.  I tried to power up the SEX with the bulb bypassed, but blew the fuse instantly. 

I didn't mention it in the earlier posts, but the bulb was glowing fairly bright with the SEX powered up which would indicate a fault.

As a quick experiment I ran a copy of a 1959 Fender Princeton I built on the current limiter.  The amp has a 5Y3 tube rectifier, 6K6 power and 5751 pre-amp tubes.  At half volume the light bulb barely glowed, and the amp sounded fine, so the voltage must have been pretty close to normal.

Off to the hardware store to get some more fuses.  I think the next step will be to de-solder the heater and HT leads for each channel and fire it up.  If it is OK bring one set of secondary power feeds on at a time to try to isolate the part of the circuit where the trouble is.

Any other thoughts welcome and thanks again.
Dave

 

[JC]

Yes, the light bulb being bright after the initial turn on surge is not a good sign.  Still, I don't see any indication of problems with your Mains wiring (again, I could be overlooking something), so the next suspects would be the orientation of the various PS diodes and filter caps.

Have a good look at those before you unhook anything, just in case you can spot something easy.  Other than that, the only thing I would say is that, given what you've done so far, I think your approach going forward is a logical one, because I expect that you are going to find your problem on the secondary side of the power transformer.

 

[Paul Birkeland]

So when you power up the SEX amp w/o the light bulb, you blow a fuse?  If it is that immediate, there are several common causes:

1.  One of the large diodes soldered directly to the power transformer to rectify the heater voltage is backwards.  Generally if you find a backwards diode like this, you will want to remove it and replace it.

2.  A power supply capacitor is inserted backwards.  Check against the photos in the manual, also being sure to look at the little blue electrolytic cap by the power transformer.


Your idea to start lifting connections isn't a bad one.  I would just remove one wire from each of the high voltage windings, then test the power transformer/heater circuit as one unit at first.  It can be pretty tough to remove all the diode connections to the power transformer, so that is best left for last.  

-Paul

 

[Tugboat]

I think I have found the problem.  I have one of the 220uf / 250V capacitors backwards on each channel.

Time to dig through the parts bin to see if I have any spares and fire up the soldering iron.

Thanks for the help.
Dave

 

[Paul Birkeland]

It is pretty important that that pair of caps in each channel is the same.  You could use 100-300uf @250+v in this position.  If it was my SEX amp, I would send Eileen an e-mail and see if she will send you a new pair of caps. 

-Paul

 

[Tugboat]

Couldn't find anything on hand, so I took a chance and re-installed the two caps correctly.  Amp powered up, tried some music and it sounded very good.  Flipped the current limiter off and blew the fuse instantly. 

Will take a set of voltage measurements and go through the manual and Ward's photo build again.

Hopefully only one more hurdle to get it up to 100%.  Getting some satisfying sound while on the current limiter provides the light at the end of the tunnel.

Thanks for the help,
Dave

 

[Paul Joppa]

Just a note, electrolytics are damaged by reverse voltages. Lifetime wil be shorter even if they seem to work, and the maximum voltage may be reduced, i.e. it is possible that teh cap worked at the lower voltage with the current limiter, but failed at full voltage. You'd want to put fresh ones in even if they old ones did seem to work, since the lifetime of the reversed ones will be seriously impaired.

 

[Tugboat]

Thanks Paul,

Amp is shut down until the new caps arrive later this week.  Hopefully by the end of the weekend I will be up and running.

Dave