Stewart SL-21 Amp [new project]

[casssax]

A friend had this amp at his house and let me take it to fix it up.

It is a Stewart SL-21. I haven't been able to find out much about it. The closest I found was a Stewart SL-11 in a YouTube video.
https://youtu.be/s74Bo10OYf4

It's a push pull with two 6BQ5's for power, a 6CA4 rectifier, and two 12AX7's for the preamp section.
Input Transformer - PHILCO 32-10006-1
Output Transformer - PHILCO 32-7634
Other Transformer - BENDIX 125921 (on speaker)

This has definitely been worked on in the past. There are a lot of strange screws that don't match

I'm not sure what is going on with what looks like two different output transformers, the one on the speaker has some wires that aren't hooked up to anything. There's a hole in the speaker cloth but I don't see any thing wrong with the speaker so I think someone swapped the original speaker and maybe didn't know what to do with it so they just left it there.

It has what looks like two channels for input each with it's own volume and tone, each with two input jacks. I thought the input jacks might be high and low like a fender but they look like they are just two identical inputs. Both of the channels then go into the same in put of the first 12AX7.

It also has a tremolo with speed and intensity that has a foot switch to switch it on/off.

For some reason they used double gang pots for every pot. The posts are all just wired together except on the tremolo section where half the pot is not used at all.

There is no central ground. Everything is just grounded to the chassis where ever they needed a ground.

It works, and doesn't sound all that bad. But there is a hum that is there even when everything is turned down and it doesn't change when you increase the volume or adjust the tone.

I'm going to start trying to figure out the schematic but I thought I would ask if this looks familiar or maybe this is modeled after another amp that someone might recognize?

Something looks weird with the output transformer. There are three wires coming out of the top and I would think that the center wire would be the center tap, but it is wired to one of the output tubes. Does anyone know where I could get a layout for the output transformer? I looked around online but didn't find anything.

It looks like this should be fun to work on an keep me occupied for a while.




(Beer bottle for scale)

[eleventeen]

The OT (the one NOT mounted on the speaker) looks completely fried, flamed, smoked to me. Yet you say the amp works.

[sluckey]

I think that's just tar.

[PRR]

> a hum that is there even when everything is turned down and it doesn't change when you increase the volume or adjust the tone.

Is the filter cap more than a few years old?

I have a 1949 radio with constant hum as loud as the radio program. Yours is probably not much younger. Time to re-cap.

One possible reason for the second transformer is a "wrong speaker". If a 4-ohm died, and a 16-ohm was handy, some clever tech could have stuck a 4-8-16 winding in as an auto-transformer to make-up the diff. Unfortunately to confirm/deny this you probably need to take a primary-secondary turns-ratio measurement on the original OT.

What is "input transformer"? I don't even see one?

[PRR]

I don't think the two jacks each side are truly parallel.

Note though that A and B inputs go through volume and tone networks, then one wire to preamp, NO wires back to front panel except Trem connections. This amp is "always up", hissing. Most guitar amps let you fade-off the first stage gain and hiss. I also think it presents fairly low impedance to guitar. (This was sometimes done on cheap guitar+amp gift-packs, where a too-shrill guitar was compensated by a treble-suck amplifier.)

I admire the cheapness, and hate to see good old amps hacked-up, but this maybe begs for major preamp re-think.

The power stage is probably fine, just fix what's broke or faded-away.

Agree that I have seen enough "tar-ball" transformers to not reject this ugly thing on sight. One excellent Bogen OT looked like poop behind the paving truck.

[PRR]

The rotted-rubber 2-wire cord with knot in chassis hole HAS to go.

And is that a metal beaded-chain on the bench just behind the fuseholder terminals?

[casssax]

I think that's just tar.
The black OT is covered in some kind of black wax or tar.

Is the filter cap more than a few years old?
Based on the YouTube video of a similar amp I'm guessing that the filter cap is original and from the mid 1960's

It's 60MF @ 350V, 60MF @ 350V, 500MF @ 25V

That will get replaced.

What is "input transformer"?
The input transformer is a PHILCO 32-10006-1

You can get a good look at it in the 2nd picture of the whole back of the amp.

The rotted-rubber 2-wire cord with knot in chassis hole HAS to go.
Yes, I'm going to put a three prong cord on it.

And is that a metal beaded-chain on the bench just behind the fuseholder terminals?
That is an old saxophone neck strap from the 1930's
It won't be on the bench when I'm working on the amp.

One possible reason for the second transformer is a "wrong speaker"
That's a good point. I'm going to test the speaker tonight to see what the ohms are. I can test it with and with out the extra transformer.

When I get home tonight I'll post some sound files of the amp before I pulled it apart.

[sluckey]

Something looks weird with the output transformer. There are three wires coming out of the top and I would think that the center wire would be the center tap, but it is wired to one of the output tubes. Does anyone know where I could get a layout for the output transformer?

You can't go by the physical position of a transformer wire. IE, don't assume the center wire is the center tap. Use your ohm meter to verify it's wired correctly. Measure resistance between the two wires connected to pin 7 of each tube. Should typically be a couple hundred ohms. Now measure resistance between pin 7 of a tube and the third wire (that is likely connected to a filter cap). This resistance should be approx. half the first resistance reading. Repeat for the other tube pin 7. This should also be approx. half the first reading. If all these readings are as stated, the OT is wired correctly.

We usually refer to your "input transformer" as a power transformer.

Replacing filter caps will 'likely' cure the hum. It's unlikely that you will find a replacement can. I'd just use three individual caps.

[PRR]

> I'm going to test the speaker tonight to see what the ohms are. I can test it with and with out the extra transformer.

Ohm meter on a speaker withOUT transformer is reasonably valid. 3 Ohms DC is about 4 Ohms in the audio band.

You can not test transformer for Ohms this way; or a speaker with a transformer across it. Transformer is ideally zero Ohms for DC, and always much less than the intended load. Maybe 4% to 20% of design load, which doesn't really narrow it down.

You could if you built an impedance bridge. You need an audio source (probably in the 200Hz-600Hz area) with enough power to put readable voltage through maybe 10 Ohms resistance to the speaker system. Then you need math. Few people bother.

IMHO you should be measuring the transformer. 6V AC to one side of primary, read secondary voltage. This can compute (math again!) the turns ratio and the impedance ratio. Since we "know" a pair of EL84 will be loaded in about 8KCT, we can find the likely intended secondary load.

If you go as far as replacing the speaker, I'd just find one to match the OT and lose that little lump. It is barely big enough to be transforming 18 Watts even as an autotransformer. It is likely losing 20% of your precious power, and nicking the lowest notes.

[kagliostro]

I've an impedance meter that a friend give me as a present

something similar to this (in the function)

http://www.mcmelectronics.com/product/TENMA-72-6948-/72-6948




http://www.photonage.com.au/audio-speaker-and-transformer-impedance-meter-with-case-p-2556.html?zenid=d1359f3125944702e0346b0ba7c0c8b7




But I prefer to calculate the ratio between primary and secondary and apply a bit of math

Franco

[PRR]

Those seem to test at 1KHz.

Fine, but many guitar speakers will be well up from nominal impedance at 1KHz. There's a minimum between 150 and 600Hz, 400Hz is conventional for speaker impedance rating.

[casssax]

http://www.unitedsound.us/Xm12$3&4/StewartInitialDemo.mp3
Here is the recording I made before pulling the amp apart.

The first part is with the volume half way up and the tone at 12 o'clock.
The second part is with the volume all the way up and tone at 12 o'clock
The third part is testing the tremolo
and the fourth section is with the amp all the way up but turning the guitar up a little each time.
(Played on a Telecaster with the switch in the middle position and both volume and tone turned all the way up,
Recorded with a Sennheiser e906 with the eq in the flat position hanging over the amp near the edge of the speaker. )

I checked the output transformer. across the two pin 7's it reads 650ohms. Then from each pin 7 to the center tap it reads 305ohms on one side and 355ohms on the other. Looking a little closer at the wires I can see that the wires going to the tubes are black and white and the wire going to the filter cap is black/white.

I measured the ohms of the speaker without the little transformer and it reads 6.5ohms so I'm assuming it's an 8ohm speaker. I don't know if it means anything but if I measure across where the signal goes in and then out of the little transformer I get about 1.5 ohms

I'm not sure that either the power transformer  or the output transformer are original to the amp. The center tap from the output transformer isn't long enough to reach the filter cap and has another wire connected to it. And the power transformer has an extra wire I just noticed that has been pushed back down below the chassis that is not taped off or connected to anything.

[casssax]

Not knowing what was going on with the output transformer, my original plan was to just hook the little transformer back to the speaker and then replace the filter caps and and anything else that needs updating or looking at.

But, yesterday I found a great deal on a variac on ebay and I should have it by the beginning of next week. (It's a General Radio, 2-Amp 120V Type V2 that someone has built a nice enclosure for [less than $50]).



Once I have that I should be able to figure out what the OT is and where to go from there.

Yesterday I put a three prong cord on and ordered the filter caps and replacements for the two old style caps in the preamp.

[PRR]

I don't see what the Variac adds to OT testing. You probably have 6VAC available, and a DMM which will read small ACV. Put 6V on the primary, read both primary and secondary. (Primary because your "6.3V" may be 6.9V unloaded.)

[casssax]

I tried to measure the winding ratio but I'm not sure about the results.

The variac I picked up was on my doorstep when I got home today so I used the 'Uncle Doug' method.

I hooked up the variac to the primary side with a meter attached and then hooked the secondary side up to my VTVM.
I turned the variac up until I was getting a reading of 1ACV on the VTVM.
When I do that I get a reading of about 116ACV on the other meter.

So 116:1 winding ratio. Does that seem right?
Or did I do something wrong?

[PRR]

116 across *half*(?) of the primary. 1V across secondary.

Voltage ratio 232:1. Impedance ratio 15.2:1. Assuming around 8KCT load on a pair of 6BQ5, this is a *500* Ohm winding.

What is the DC Resistance of this secondary?

[casssax]

I get about 0.5ohm if I measure across the secondary.

What do you mean by 'half of the primary'?

I'm measuring across the two wires that were each hooked up to pin 7 of the output tubes.
(The center tap is the black and white striped wire sticking off to the left in the picture above)

[PRR]

Sorry, a back-spasm is messing with my head.

116:1 voltage suggests 13,400:1 impedance, which comes out way too low for any loudspeaker.

To cross-check measurement technique, do you have any other OT you can measure? You do not have to take it out of the amp. Just be sure power is down, speaker is disconnected, speaker jack is not shorting the secondary.

[Paul1453]

I'm seeing like 33.5VAC on your meter, which is better.

Z =  1122.25  or  9K plate load with 8 ohm speaker.   

[JB]

His meter is on the 1.5V, not the 50V range.  It's reading 1V!

[casssax]

I measured the little transformer that was on the speaker. It has a 4:1 winding ratio.

I put the new caps in today and I think I will hook up the OT the way it was when I got it and just check to see if everything is working right.

At this point I'm starting to think about just getting an OT that is right for the amp/speaker.

[DummyLoad]

I measured the little transformer that was on the speaker. It has a 4:1 winding ratio.

I put the new caps in today and I think I will hook up the OT the way it was when I got it and just check to see if everything is working right.

At this point I'm starting to think about just getting an OT that is right for the amp/speaker.

IMO that would be the best option. use a 8ka-a. doug sells a nice unit for the 18W marshall. with the 18W heyboer, you're probably getting something that's 10x better than what got stuffed in it when manufactured. use a 50uF/50uF F&T for the B+ filters, the 500uF 25V is likely for the power tubes bypass: a 100uF or 220uF 25V axial will do the job there. 


--pete

[Paul1453]

+1 for new OT   

[casssax]

New issue:

I put the new caps in (including two .047 caps in the pre-amp section). Put everything back together the way it was when I got it and two strange things happened.

First, the first time I powered it on I heard a humming from the OT. I shut off the amp but just as I did I thought I heard an electrical sound like bzzzzt.

I thought the humming from the OT might be that I had only hand tightened the nuts holding it in place and it might be vibrating. So I tightened everything up and turned it on again.

All the tubes were glowing and I didn't hear the humming from the OT. There was quite a bit of hum from the speaker which got louder (all the volume controls were turned all the way down). Then it started making a noise like a slide whistle and started feeding back, getting louder and louder until I turned the amp off.

I started checking things with my meter (after removing all the tubes) and found that there was a connection to ground on the speaker tap of the OT secondary. (I later disconnected the OT completely and found there is continuity between the ground and speaker tap of the OT. Not sure if that is normal or not). After disconnecting the OT I did the same test as before to check the winding ratio and got the same results.

There is also continuity to ground on all the heater wires, and also on both of the 120v wires from the PT (This goes away if I lift the center tap from ground).

Another spot I found continuity to ground was in the positive side of the 500uf/25V cap. This has a connection to ground through a 150ohm resistor but I think the ground is coming from the connection to pin 3 of one of the power tubes.

I planned on opening up my other amp and checking the continuity to ground in the same places to check to see what is normal and what is not.

As far as I can tell so far everything was hooked up correctly (but obviously something is not). Any advice about where to start checking or ideas about what could have happened?

Thanks.

[sluckey]

Don't use the continuity checker on your meter. Use the ohm meter functions instead. Continuity does not necessarily mean zero ohms. Continuity can be indicated by anything less than several hundred ohms on some meters. It varies with different meters.

[casssax]

I double checked everything and hooked it all back up and it seems to be working now. 

One reason for the feedback may have been the florescent light on my bench. I forgot how much that can interfere with the amp when it is on.

I'm going to do some recording of it again to compare to before I started. There is still a bit of hum coming from the amp, changing the caps didn't seem to effect it.

Now that I know that it's working I think I will order one of those heyboer output transformers. Once that is in there it will be easier to see how it sounds with different speakers. I'm not sure what the speaker in it now is, it might be a Quam but it doesn't say anything on it.

Then I can think about the preamp and EQ. The single tone nob doesn't really give you much flexibility.

[DummyLoad]

what are the numbers on the speaker underneath the transformer attached to the speaker?

--pete

[casssax]

The numbers on the speaker under the transformer are '5812071'
There are also other number '018001' and '137 445'
And then on the cone itself is printed 1223-GL

[DummyLoad]

the speaker is very likely a CTS (eia code 137) however the mfg. date is the real unknown, unless you can date other original components. the decade is unknown, as year digit 4 so either 54, 64, or 74, etc.. the production was 45th week: 137 445. on a hunch i'd say it's a 64. the speaker is def. a keeper if it works.

the series 5812071 is likely an OEM part number.

--pete

[casssax]

I got the new transformer and installed it.



Now when I turn the amp on, after about 30 sec. it starts to hum and the hum gets louder and louder (with all the volumes turned down) and then it starts making a loud squeal that starts stepping down in pitch like the overtone series getting lower each step.

All I did was to take the old OT out and put the new one in, and remove the little transformer from the speaker.
(Before I received the new OT I did some test recordings to see if the new caps made a difference and everything was working then)

Brown and Blue wire hooked up to power tubes. Red wire to the B+, Black to ground and Green to the speaker. (Orange and Yellow are taped off)
There is also a NFB (Orange/White) connected to the speaker.

Any ideas what would cause this or what to look for?

Thanks.


About the speaker: 64 sounds about right for the amp, even though I don't know if it is original. Thanks.


 

[Paul1453]

Look at your OT wires.   

Do you think you see anything that would cause your howling and hum noises?   

[DummyLoad]

disconnect NFB wire and test. if it works, swap OT primary leads: the blue and brown wires.


--pete

[casssax]

Disconnecting the NFB stopped the screaming noises from the amp.

I found this other post about the same topic. http://el34world.com/Forum/index.php?topic=14507.0
where it said:

*Disconnect the NFB
*Turn on the amp & turn it up to where you can hear some hiss (if you have a really quiet amp, plug in a guitar cord & let it hang, not plugged into a guitar, to generate some noise
*Now CAREFULLY reconnect the NFB while listening to the noise....I use an alligator clip wire connected to the hanging NFB resistor & touch it to the circuit while listening for volume change results.

Does the sound become quieter/less when NFB is reconnected? Then the OT primary is correct....NEGATIVE feedback is being applied.

OR....Does the sound actually get LOUDER?? If louder (even if just slightly), then your OT wires need to be reversed....you are applying POSITIVE feedback to the circuit.

-- Geezer

So I did the test after disconnecting the NFB. The way I first had it wired up re-connecting the NFB made the loud squeal again.
I reversed the wires to the power tubes and now re-connecting the NFB makes it quieter.

I cut the wires on the primary side to so they are not all over the place. A little more cleaning up of the rest of the wires to do and then I can start thinking about the preamp section.

Thanks for the advice.

[casssax]

The amp that keeps on giving 

After getting the new OT sorted out I did something I should have done when I first got the amp.
I started checking the voltages.

Turns out that the PT is also not what it should be.
When I checked the heater voltage it is only 5.5v
The high voltage seems to be fine, around 295-300.

So my questions are:

What effect will running 5.5v into 6.3v tubes have?

Could I just get a little 6.3v transformer and hook it up (in parallel with the PT I think) for the heaters?

I don't really feel like paying for a brand new PT for this amp.
I might try to find a used one on ebay.
or if it's doable I can get a 6.3v/4A transformer from Mouser for about $12

Thanks.

[casssax]

I created a schematic of the amp and measured some voltages.

Now I have to think about what needs changing. I'll add a cathode bypass cap and resistor to the first stage (pin 8 of V1).

I've been comparing this to the 18W Marshall schematic from this site
http://el34world.com/charts/Schematics/files/marshall/marshall_18watt_schem.pdf

Based on that I'm wondering if I should remove the 180k resistor on pin 3 of V1 which connects to the cathode bypass on pin 3 of both of the power tubes.
And then just put a regular cathode bypass on pin 3 of V1.

I'm also thinking of moving the volume and tone after the first gain stage so the input goes straight to the first tube.
(I didn't draw the other input jacks or the second volume and tone control on the schematic just to keep it simple for now. they all connect at the output of the volume pot)

There is also one cap that I'm not sure of the value. The markings on it say:
RMC
100
N750
20%

[HotBluePlates]

I've been comparing this to the 18W Marshall schematic from this site ...

Based on that I'm wondering if I should remove the 180k resistor on pin 3 of V1 which connects to the cathode bypass on pin 3 of both of the power tubes. ...

The connection between the 180kΩ resistor and pin 3 of the output tubes seems like a bad idea. It is almost certainly the series feedback resistor from the OT, and that feedback is being injected at the cathode of V1 pin 3.

Based on that I'm wondering if I should ... put a regular cathode bypass on pin 3 of V1.

Do you want feedback around your output stage? If so, unhook the 180kΩ from the output tube cathodes, as noted above. Then do not add a cathode bypass cap from V1 pin 3 to ground.

If you don't want negative feedback around the output stage, just yank the 180kΩ altogether.

I created a schematic of the amp ...

V1 pin 6 seems to be missing a plate load resistor.

V1 pin 1 plate load (1MΩ) seems improbable. If it's correct, it and the 10kΩ cathode resistor may be "a sound". Unless you're ditching or modifying the negative feedback loop around the output section, you can't change that 10kΩ cathode resistor at V1 pin 3 without altering the feedback. If you add a bypass cap across the cathode resistor (with no other changes), you're shorting the feedback signal to ground, and might as well remove the 180kΩ resistor.

So you might decide how closely you want to copy the 18w schematic.

[casssax]

I had some mistakes in that schematic. I think I've fixed everything now (I hope).



I added the plate load resistor to V1 pin6, it was mistakenly going to ground on the original. And I realized that the 180k resistor is not connected to the cathode bypass of the output tubes at all. I also added the other inputs/channel.


I measured the plate current and voltage and I'm seeing 27-29ma and 327-325V so about 9Watts.
I think I can bring that closer to 12 if I lower the resistor from 135ohm to about 82ohm.

The tubes are who knows how old so I ordered two matched pair of power tubes and I'll see if that makes a difference before changing anything.

I put everything back in the box today and found that it's dead quiet with the volume turned all the way up and the guitar turned down.
I'm having second thoughts about making any big changes in the amp. I may just try tweaking the tone stack and the tremolo a bit.