Secondary voltage too high from power transformer DeWald L5000

[sabrefencer]

I am reviving a 1958 mono integrated amp. It is a DeWald L5000 Troubadour. Uses an EZ81 rectifier, two 12AX7s and two EL-84's. The problem I am running into is that the voltage out of the secondary of the power supply transformer that goes directly to the rectifier is very high. I checked this voltage before I did anything to the amp and it was about right (spec is 285VAC) without the rectifier tube in place. I also checked the resistance of the secondary winding from each leg to the center tap and those readings were also on or within 10% of spec. Given this I decided that the TX was working and carefully powered up the unit (DBT then Variac) and it produced output but at very low gain. After determining that I did not have paperweight, I ordered a new 4 section can cap from Hayseed and new coupling caps, few other electrolytic caps and then checked spec on every resistor and ordered new metal film if they were out of spec.
What I have done so far: I replaced the line cord with a polarized 2 wire cord, and am running the hot leg into a newly installed fuse holder (there originally was no fuse), then out of the fuse holder to the switch which feeds one of the legs of the primary of the transformer. The other primary leg is fed directly from the neutral of the line cord. I checked and double checked this wiring, comparing to before photos etc. i think it is electrically the same as before i touched it. The second thing i did was replace the can cap and two resistors that directly attached to the caps. So far that is it. I decided to power it up at this point to make sure i did not mess it up. DBT did not light (60w bulb), not sure if it should. I then brought it up on the variac, but not real slow to 117v with no tubes. I immediately checked the voltage at the secondary (no rectifier installed) and got 600v, which is more than double the 285V spec. Thinking that the lack of load might be the reason, i put in all the tubes, which brought the voltage down below 500v but still too high. Fearing that i somehow fried the transformer, i rechecked the resistance on the secondary, and all looks fine. Not sure what to try next. Any suggestions?

 

[nerdorama]

You are reading the entire secondary. Half of that will be seen by each diode in the rectifier.

 

[gadget73]

Voltages with no tube load will also be too high so don't read too much into unloaded readings.

and yeah, the transformer is most likely a 285-0-285, which is another way of saying 570 volts with a center tap. reading 600v on that with no load is not unexpected at all. Read it to ground, should be ~300vac which is reasonably close to spec.

 

[sabrefencer]

Rookie mistake. Thanks for the info, makes perfect sense. This amp is my "practice" project for more complex amps later. I am meeting the objecive which is to make mistakes and learn. Thank you for the quick response.

 

[gadget73]

something that tends to come up with old gear and service docs is sometimes things aren't spelled out because it was considered common knowledge. 60 years later, its not so common.

 

[zackthedog]

Rookie mistake. Thanks for the info, makes perfect sense. This amp is my "practice" project for more complex amps later. I am meeting the objecive which is to make mistakes and learn. Thank you for the quick response.

Rookie mistakes are a-okay. I make a lot of them, after 35 years of DIY. Good work on the project.

 

[sabrefencer]

All voltages now check out ok. I guess it matters where you stick the probe . However, still no output of any kind. I double checked the tubes and they test fine. I'm now replacing the remaining 4 electrolytics, including the coupling caps. Will get back to it tomorrow, and try to bring it up again with the new caps.

 

[sabrefencer]

This is embarassing, but if you can't take the heat don't post on the forum. With a good night sleep came back and found that one of the speaker wires broken off my banana to spade adaptor. That explains zero output. Fixed that, and whalla, sound. Actually, not bad. No hum or hiss, good volume. Now replacing the electrolytic and remaining resistors.

 

[gadget73]

if you had any idea how many times I've chased my tail for something as simple as a bad connection or junk cable you'd laugh. It happens, and often the best thing to do is to walk away and look at it again with fresh eyes once things stop making sense.

 

[sabrefencer]

Restoration completed, at least for now. This was a perfect 1st resto project, simple, one channel, 5 tubes including rectifier, no tuner, not heavy etc. I used this a practice for a Fisher 500c that I will be tackling soon - I have been reading the Fisher threads for a year in prep (not in a hurry). The end result on this DeWald sounds pretty good for what it is, which is 12 watts with about 1% distortion, more like a radio than true hi-fi, but not bad. Here is what I did to it: 1) Removed paper SN and inspection labels for later re-attachment, and cleaned out a heavy layer of dirt, dust and even a dead bug (see pics). 2) Removed and lightly cleaned tubes, and tested on my Hickock 600A, all tested very good - all Mullards, no need for replacements. 3) Checked every resistor against band values and schematic, identified 16 out of 27 that I wanted to replace. 4) Ordered replacement parts: New 4 section can cap (20-20-20-100 uF) from Hayseed Hamfest, replaced black beauty coupling and other caps, coupling replaced with Orange drop 716 series (.47 uF/600V) and others with IC polypropolene. Resistors ordered were mostly metal film, but did not realize I was getting the tiny ones (electrically fine, but a PITA to work with). 5) Reworked the input power: replaced the line cored with a polarized (2 prong) cord, replaced the auxiliary AC outlet with a fuse holder (in same hole, no drilling required) and ran the hot lead through the fuse, to an added terminal lug strip, then to the power switch and then to the primary of the power transformer. I put in jumper on the terminal lug, where I planned to install a CL 90 thermistor as an inrush current limiter, but not getting any pop on power up, so have not installed the ICL, but it can easily added. 6) Replaced the can cap and related resistors, replaced the coupling caps and related resistors, and the remaining caps and resistors. I powered it up a few times along the way to make sure I was not breaking things as I went. All went fairly smoothly, except for the measurement glitch and broken external speaker wire. There are a few nagging issues. When I was checking the resistors, there were several places where the unit did not match the schematic, all in the pre-amp section around V1 (12AX7) and V2 (12 AX7). The most nagging difference are the plate resistors for the pre-amp (1st stage) on V1. The spec is for 100k resistors to Pin 1 and Pin 6 which are the plates of the two stages of the tube. I found 470k resistors in place, which tested fine. I replaced them with new metal foil 470k resistors and while checking voltages, got about 53 vdc instead of the 40 and 45 vdc spec. I then pulled one side of each 470k resistor and jumped in 100k resistors, and the voltage went up top 163vdc. I quickly put the 470k's back and thats how it is now. I tried to find data on the plate resistors but if it is in the tube data sheets from GE and RCA, I could not find this spec. GE specifies 300v and RCA 330v as maximum plate voltage. QUESTION, do I need to worry about running slightly higher plate voltage than the original amp spec (since it is still way below the tube max, I assume it is ok.

 

[sabrefencer]

My computer froze before i got to finish this and post pics.

 

[sabrefencer]

Continued: there are a few other resistors that do not match the schematic, so I am not sure if someone has been in there (certainly not recently), or if the Sams Photofact schematic is wrong, or if the design chagned during production (or all three). When i get some time I will do a detailed trace of the preamp section. Anybody with thoughts on the plate resistors described above?

 

[sabrefencer]

Before photos

 

[sabrefencer]

During and after photos:
Replace outlet with fuse holder and add terminal strip with jumper/spot for thermistor:

Can cap r&r:

New caps and resistors:

Completed underside:

 

[sabrefencer]

After photos:
Completed topside:

Pretty face:

A 1958 mono amp needs a lonely 1957 speaker (another recent very local CL find): JBL C36.

Thanks for reading and looking.

 

[gadget73]

do the resistors appear to be non-original? If they look like all the rest, likely its just a change somewhere along the line. All else being equal a 470k plate resistor should give more gain, but without knowing what tube and where in the circuit I have no idea if thats needed. Usually it would also need a cathode resistor change and possibly to be fed from a higher voltage source to make up for the loss over the larger resistor.


Most of the Sams voltages are specified at 117v line voltage and give 15% or so tolerance. If you're at higher line voltage everything will be higher, but its still within usual running tolerances. I wouldn't worry too much about it.