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 Post subject: Heathkit W-5M amplifier rebuild
PostPosted: Jun Tue 13, 2017 2:07 am 
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Location: Wood River, Ill.
I picked up one of these amplifiers from an estate over the winter, along with the matching WA-P2 pre-amps. Other than being dusty, they both appeared to be complete and in pretty good condition. The amplifier has the Peerless #16309 output transformer and a pair of KT66 output tubes. It looked like a good project, so I began by looking for obvious signs of problems, checking for shorted transformers and choke. The filter caps looked to be of fairly recent vintage, but some of the paper wax caps looked original. I initially powered up with a dim bulb and variac. and got output. No hum and was able to get a signal thru it, although somewhat distorted.

I basically rebuilt it, replacing all the paper and filter caps, out of tolerance resistors, and replaced the tube sockets with porcelain sockets. I replaced components one for one and kept the wiring in the same location as built. I had no plans for making any modifications, wanting to keep wired as stock. It looked like the previous owner may have been having some issues with the feed back from the audio output transformer because he had 2 different 1 watt resistors in the feed back line; the stock 4.7k (disconnected) and a 1k wired in. When rewiring I installed the stock 4.7k.



When powering up and testing, I found that any signal over about 600mV (1k hz) the amp would start to oscillate with a buzz instead of a nice sin wave output. Disconnecting the feedback from the 16 ohm output tap, the buzz would stop. It sounded much better, had higher output, but the sin wave signal was distorted (as looking at it on the scope. The sin wave input looked good thru V1 thru V2 into both KT66 grids, but the output was distorted.

I am not sure where the problem lies. I tried several values of resistor, including the 1K the PO had wired in. I am assuming that he must have had the same problem, and was experimenting too with the feedback circuit. I have gone over the wiring with a fine tooth comb, and everything appears to be correct. The tube voltages appear correct, or at least close, except at the V1 input tube, where the plate voltage on the first section is running around 100 vdc instead of 88 vdc. Same for the 2nd section of V1, where the grid is wired direct to the first section plate.

I am thinking the problem is in either the KT66 output tubes or the output transformer. Both high dollar items! I don't want to start throwing good money in after bad. I have no other chassis lying around that use the KT66's, and my tester will not check these tubes. I don't have any known good extra KT66's lying around either. If the transformer is shot, that is a deal breaker, as far as I am concerned.

I am open to ideas, at this point. Thanks for reading.https://goo.gl/images/BeRcOa

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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Jun Tue 13, 2017 3:48 am 
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What did the no feedback distortion look like?

Have you balanced the output tubes?

What load do you have on the outputs?

I'd recheck all the components in the output section.

Btw, removing the feedback will give a higher output. That's how feedback works. It exchanges amplitude for lower distortion.


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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Jun Tue 13, 2017 3:50 am 
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Test the KT-66's as 6L6's .

Ohm the output transfomer to make sure one side is not open .

The small peerless output transfomers are know to be a weak point of that amp , the larger peerless output transformers are generally , good . Don't remember the part numbers off the top of my head.

Edit
The Peerless 16309 transfomer is the one with the high failure rate

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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Jun Tue 13, 2017 1:18 pm 
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Flipper & Batt, thanks for the response.
Flipper, the no feedback output looked like a pure sine wave in the 0 to 180 deg of the trace, in the 180 to 360 half of the wave form there was a smaller "hump" going positive back to negative then going positive again. I have balanced the output tubes, and I am using an 8 ohm speaker on the 8 ohm tap. I understand the lower output when the feedback is present and expected it.

Batt, I was not sure about testing the KT66s as 6L6s. I will give that a shot. I have checked the resistances on all the transformers and choke, finding no opens or shorts to the case on any of them. I am not sure what values of resistance to expect, if there are shorted turns. It looks like the output transformer may have been replaced at one time, as some of the leads were very short. None the less, I will check them again and note the values. I will also go over the output section again, but I used good quality new components and checked them as I installed them.

When they start to oscillate, with feedback connected, the amplitude of the oscillation is very high, compared to the normal signal. The signal looks good and undistorted before oscillation begins. I didn't get a voltage on the oscillation, as I shut it down very quickly, before getting a reading. As I said before, I think the PO may have been fighting the same issue.

Thanks again.

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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Jun Tue 13, 2017 2:24 pm 
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If it doesn't oscillate with the feedback disconnected , they may have the flipped the two plate leeds of the output transfomer and the the 43% tapped leads and the feedback is in phase . See if you can find an assembly manual to see which leads go where .

I would try a different output transformer , the have also been known to have shorted turns .

I live drown the road in Alton , if you need an extra set of eyes let me know .

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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Jun Tue 13, 2017 5:59 pm 
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Hi Mark,

You are, indeed, close!
I do have the assembly manual and did wire according to the manual, but could have screwed up. The colors are hard to read after all these years, so could have messed that up. I will take a look at that when I have a chance. Thanks for the advice and the offer to have a look. I do not have another transformer, so it may have to go on the shelf until another comes along. I understand that the transformer that I have is not the more desirable one, anyway. I may just have to put it on the "want" list.

Thanks, again.

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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Jun Tue 13, 2017 6:01 pm 
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N9whh wrote:
Flipper & Batt, thanks for the response.
Flipper, the no feedback output looked like a pure sine wave in the 0 to 180 deg of the trace, in the 180 to 360 half of the wave form there was a smaller "hump" going positive back to negative then going positive again. I have balanced the output tubes, and I am using an 8 ohm speaker on the 8 ohm tap. I understand the lower output when the feedback is present and expected it.

Batt, I was not sure about testing the KT66s as 6L6s. I will give that a shot. I have checked the resistances on all the transformers and choke, finding no opens or shorts to the case on any of them. I am not sure what values of resistance to expect, if there are shorted turns. It looks like the output transformer may have been replaced at one time, as some of the leads were very short. None the less, I will check them again and note the values. I will also go over the output section again, but I used good quality new components and checked them as I installed them.

When they start to oscillate, with feedback connected, the amplitude of the oscillation is very high, compared to the normal signal. The signal looks good and undistorted before oscillation begins. I didn't get a voltage on the oscillation, as I shut it down very quickly, before getting a reading. As I said before, I think the PO may have been fighting the same issue.

Thanks again.
Would be better with a picture of the scope trace but, if I read you right, that sounds a lot like crossover notch distortion but, at that output level (deduced from the input voltage), there shouldn't be any, at least not that pronounced. It doesn't sound like bias is right (too low). Does it have the output zobel (47 ohm || .1uF) installed?

Note, you can't measure full power continuously because it's cathode bias (the cathode bypass caps will charge up shifting the bias point) and tying to do so will produce the crossover notch (bias shifts to where the tubes conduction no longer overlaps). But it should be able to handle .600Vrms before that happens, which leads me to believe bias is low (bad tubes would produce a similar result). Still, it shouldn't go into spurious oscillation in any event (which is why I asked about the zobel).

The crossover notch can induce spurious oscillation (if the amp isn't compensated for it) because the notch contains high frequency components where the phase shift causes feedback to become positive. That's why it goes away with no feedback.

Note, there are other things that can cause spurious oscillation but they don't fit the symptoms.


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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Jun Tue 13, 2017 11:07 pm 
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http://www.angelfire.com/vt/audio/heathw5m.jpg.

Resistors R-17 & R-23 go to pin 5 of V-4 and V-3 respectively , disconnect both from pin 5 and solder a short extension on them , then connect R-17 topin 5 of V-3 , and R-23 to V-4 . This will flip the phase of the amp , it will cure the oscillation if the output transformer is out of phase , or make it worse if that is not the problem .The output transfomer has an internal zobal , that can go bad .



TT-035-OT would be a good replacement for the original transfomer


http://www.transcendar.com/push-pull-tr ... -watt-p-p/

http://www.edcorusa.com/cxpp30-6_6k or this edcor

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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Jun Wed 14, 2017 2:34 am 
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Hi Flipper and Mark,

Well, I blew it!!! Either I missed seeing an open winding when checking, or it was marginal and opened up after I checked! Either way, the output transformer is shot. One of the screen taps is open between the tap and the center tap. The plate to center tap measurements are no where near equal either, with one side being about twice that of the other, measuring from the center tap. No matter, now, looks like I will be looking for an output transformer.

Mark, thanks for the information on the transformers. I may give one of those a shot as I do not want to give up on this. Is one better than the other? In the mean time, I have a friend who claims to have a stash of Heathkit parts, and I will be checking with him first. It would be nice to have an original Peerless, but this is not something that I am going to resell anyway. This is for fun, and my own enjoyment, so the hunt begins.

I am at least happy that I was at the very least barking up the right tree: in thinking it was somewhere in the output stages. Not what I wanted to find, but it was well worth the education.

Oh, in answer to your question, Flipper, the zobel was in place. (Hell, I didn't even know what a zobel was until today, having never heard the term. I had to look it up. At least I learned something new today.)

Thanks again, guys for all the help and the education. I will report back if or when I find something.

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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Jun Wed 14, 2017 3:17 am 
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N9whh wrote:
Hi Flipper and Mark,

Well, I blew it!!! Either I missed seeing an open winding when checking, or it was marginal and opened up after I checked! Either way, the output transformer is shot. One of the screen taps is open between the tap and the center tap. The plate to center tap measurements are no where near equal either, with one side being about twice that of the other, measuring from the center tap. No matter, now, looks like I will be looking for an output transformer. .....
Sorry to hear the transformer's shot but the good news is you found it.


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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Jun Wed 14, 2017 3:55 am 
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N9whh wrote:
Hi Flipper and Mark,

Well, I blew it!!! Either I missed seeing an open winding when checking, or it was marginal and opened up after I checked! Either way, the output transformer is shot. One of the screen taps is open between the tap and the center tap. The plate to center tap measurements are no where near equal either, with one side being about twice that of the other, measuring from the center tap. No matter, now, looks like I will be looking for an output transformer.

Mark, thanks for the information on the transformers. I may give one of those a shot as I do not want to give up on this. Is one better than the other? In the mean time, I have a friend who claims to have a stash of Heathkit parts, and I will be checking with him first. It would be nice to have an original Peerless, but this is not something that I am going to resell anyway. This is for fun, and my own enjoyment, so the hunt begins.

I am at least happy that I was at the very least barking up the right tree: in thinking it was somewhere in the output stages. Not what I wanted to find, but it was well worth the education.

Oh, in answer to your question, Flipper, the zobel was in place. (Hell, I didn't even know what a zobel was until today, having never heard the term. I had to look it up. At least I learned something new today.)

Thanks again, guys for all the help and the education. I will report back if or when I find something.


The Transcenders are better but more expensive .

Mark

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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Jun Sat 17, 2017 7:16 pm 
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Chris - If you find that this amp is causing you too much stress and you want to just get rid of it, I'll give you my address..... (I didn't figure so, but I thought I'd try :D )

I had one in higschool that I got at a thrift store (about 1992) for $10. They had a second one there, but I didn't know they were mono. I've kicked myself ever since I found out for not buying the second one. Also had to sell it about 10 years ago. I should have just lived on ramen noodles for a while. Oh well.


battradio@ wrote:
TT-035-OT would be a good replacement for the original transfomer


http://www.transcendar.com/push-pull-tr ... -watt-p-p/


How do you order those?

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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Jun Sat 17, 2017 8:04 pm 
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TPAairman wrote:
Chris - If you find that this amp is causing you too much stress and you want to just get rid of it, I'll give you my address..... (I didn't figure so, but I thought I'd try :D )

I had one in higschool that I got at a thrift store (about 1992) for $10. They had a second one there, but I didn't know they were mono. I've kicked myself ever since I found out for not buying the second one. Also had to sell it about 10 years ago. I should have just lived on ramen noodles for a while. Oh well.


battradio@ wrote:
TT-035-OT would be a good replacement for the original transfomer


http://www.transcendar.com/push-pull-tr ... -watt-p-p/


How do you order those?

Transcendar Transformer Corp
1590 Lana Way
Hollister, CA 95023-2552 | view map
(831) 636-0404
transcendar.com

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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Jun Sat 17, 2017 9:00 pm 
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Michael, I am not quite ready to throw in the towel! Lol

Mark, I have been looking at the Transcendar TT-035-OT and it is for a 6k impedance and has no screen taps for the UL connection. The TT-039-OT has the UL taps and has a higher impedance than the 035.

The Peerless 16309 has a 10k primary, from the information that I can gather. I don't know how critical the primary impedance is or how I would compensate for it, if it is even required.

I checked the Edcor transformer and they are only rated for 10-15 watts. The Peerless is 25 watts.

Perhaps I am missing something?

Thanks.

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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Jun Sat 17, 2017 9:04 pm 
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battradio@ wrote:
Transcendar Transformer Corp
1590 Lana Way
Hollister, CA 95023-2552 | view map
(831) 636-0404
transcendar.com


I went to their website from your last post, but I don't see any way to order from them. Do you have to call them or do they have some online ordering that I'm missing?

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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Jun Sat 17, 2017 9:32 pm 
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http://www.edcorusa.com/cxpp45-66k

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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Jun Sat 17, 2017 9:34 pm 
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http://www.edcorusa.com/cxpp45-66k
http://www.edcorusa.com/cxppseries
http://www.edcorusa.com/cxpp30-10k

http://www.transcendar.com/push-pull-tr ... -watt-p-p/

(831) 636-0404 use their phone number to order
transcendar.com

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Last edited by battradio@ on Jun Sun 18, 2017 2:34 am, edited 2 times in total.

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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Jun Sat 17, 2017 9:47 pm 
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N9whh wrote:
Michael, I am not quite ready to throw in the towel! Lol

Mark, I have been looking at the Transcendar TT-035-OT and it is for a 6k impedance and has no screen taps for the UL connection. The TT-039-OT has the UL taps and has a higher impedance than the 035.

The Peerless 16309 has a 10k primary, from the information that I can gather. I don't know how critical the primary impedance is or how I would compensate for it, if it is even required.

I checked the Edcor transformer and they are only rated for 10-15 watts. The Peerless is 25 watts.

Perhaps I am missing something?

Thanks.


The proper impeadance for KT-66's is 6.6 K ohm CT Ultraliner

http://frank.pocnet.net/sheets/084/k/KT66_GEC.pdf

http://frank.pocnet.net/sheets/086/k/KT66.pdf

Both the Data sheets say 7 K ohm CT ultra linear , Also the B+ on the W-5m is way too high , i was going to run the one's i had choke input to lower the B+

http://www.bunkerofdoom.com/xfm/Stancor ... 4_0008.jpg

A stancor A8072 would be near perfect , but they sell used $150 +

I have a pair of Eico HF - 20 output transfomers , that are 7 k ultra linear , but won't fit on the heathkit chassis .


the old acro sound catalog recommends a TO 300 6.6k ct for KT 66

http://technicalaudio.com/pdf/Acro_Prod ... s_1955.pdf

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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Jun Thu 22, 2017 1:17 pm 
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I don't know what happened to my last post, but will try to reproduce it from memory. I decided to do a postmortem on the output transformer last night. When I opened it up, the core slid right out of the case. Obviously, it had been removed before. The tar potting compound was loose and in there in pieces, that were easily removed by hand. The core had been wrapped in black vinyl tape, very neatly, but I am guessing that it didn't come this way from Peerless. Measuring the windings with the ohm meter, I found a very broad mismatch between the primary center tap and the plate leads. Same was found between the screen taps and the center. I am assuming there are some shorted turns inside.

There is no doubt in my mind that the transformer is shot. The previous owner probably came to the same conclusion, and put it back on the shelf. I decided that I might as well go for a new transformer and placed an order for one of the Edco CXPP-30-10k. It looks like it comes closest to the original Peerless. I'll have to figure out how to mount it to the chassis. I would like to find a way without having to add any extra holes in the chassis, but I think I can find a way. Either way, will find out in about 6 weeks.

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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Jun Thu 22, 2017 2:48 pm 
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In the long run you're better off with a new transformer , The old one was a time bomb even if it .

I have a bad one here , we can dissect to see how it was originally potted , i want to place an interstage transfomer in the case anyway .

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