Silvertone 1474 Resuscitation

[erobillard]

Greetings,

I picked up a Silvertone 1474 Twin Twelve combo amp in the sort of shape that tells me someone got frustrated - cut cord, broken fuse holder, missing reverb tank, no knobs. On the upside, the Jensens are intact and in great shape, the transformers seem fine, and it doesn't look like any repairs were attempted in its past. Several tubes appear original, all electronics intact including the red Planet Lyticaps and blue Sangamo caps, none showing blistering or other damage. Sockets are in good shape, resistors measure within reason. All in all a good candidate for a first project.

I did replace a badly blistered and physically leaking Pyramid PIO cap with a Sprague. Replaced the fuse holder and 3W fuse. Replaced the cut cord with a 3-prong and grounded the green to a post I added to the metal casing. Found and repaired a broken solder joint from a cap to a post. And shorted the connection where the reverb tank would be wired in. So far, so good.

I replaced the speaker wires, and this triggered my first question - how should they be wired? The old ones were not all connected so I have clips holding things in place for the moment. The output transformers each have a black and yellow wire leading to a 3-contact row of terminals where the blacks are connected to the middle, grounded node, and each yellows to the outer posts. From here I have wires running from the ground to one side of each speaker, and the yellows to the other post on each speaker. The two connections on each speaker are also "bridged" with a choke mounted next to the speaker inside the cabinet. The chokes don't appear in great shape, and I don't have any reason to presume them good or bad. If any of this is unclear I could draw a diagram.

So what's confusing me is that my multimeter shows continuity between both either speaker post on either speaker, and any of the 4 posts and ground. Everything is electronically connected to everything else. Of course, we're talking about metal posts on a metal speaker, so it does seem obvious that "yes, there is continuity," but still, something screams "this is wrong." How's this work? Do I rebuild the 3-contact row of terminals and ensure each is isolated from the others? Did the chokes come correctly wired? Why are these speakers different from every other pair I remember looking at?

Tonight I wanted to start reforming the remaining caps, so I put in the preamp tubes, left out the rectifier tube and output tubes, and powered up on a Variac at about 25V. I also have an inline circuit limiter (the Weber-style 100W bulb in series). No issues so far so I brought it down after a bit and added the rectifier tube back in. Will let it run an hour or so and then repeat at 50V. Until I figure out whether the speakers are wired correctly for load I won't crank it any higher.

So am I on the right track? All suggestions, comments, corrections and guidance more than welcome.

Cheers,
-Eli.

[erobillard]

Attaching all photos to this post. If you have trouble viewing these please try: https://1drv.ms/f/s!AsTlnbsw-ik6lv5oMUoxLvB6dxYVrw

Speakers before rewiring - metal on metal connected by metal. With posts bridged by the chokes (bottom).


Internal view, left side


Internal view, middle section including PT


Internal view, right section including the leaky PIO cap (back right)


Original tube misconfiguration, sockets are: 12AX7, 6CG7, 12AX7, 6CG7


Rectifier and (4) power tubes: 5U4GB, 4x 6L6G, 6AU6

[Enzo]

Photo is a bad link


So you have two output transformer secondary windings, which by their nature have VERY low DC resistance. Each one is grounded. SO that means there is a VERY low resistance from any OT secondary wire to any other. And whether the speakers are connected or not, that low resistance remains.

Try to get away from "continuity" and use resistance. A dead short and a 6 ohm speaker winding will both have continuity.

[J M Fahey]

Continuity is a VERY CRUDE measurement.
And I bet those "metal posts on a speaker metal frame" are actually insulated from it, look closely and you´ll see that said terminals are not straight riveted to the frame but separated from it by cardboard/fiber washers.

[erobillard]

Continuity is a VERY CRUDE measurement.
And I bet those "metal posts on a speaker metal frame" are actually insulated from it, look closely and you´ll see that said terminals are not straight riveted to the frame but separated from it by cardboard/fiber washers.

All photos are attached to a message above, now in JPG format. Or you can find a more complete set of "before" shots and schematics here:
https://1drv.ms/f/s!AsTlnbsw-ik6lv5oMUoxLvB6dxYVrw

Wouldn't it be nice - metal on metal. I did check another speaker that does have "isolation" at the terminals (random modern Celestion) and sure enough, continuity everywhere, test and learn. Since I have tested the speakers with another amp and they worked fine, I'm going to assume they're presenting load unless someone sees a problem with the way I've described it.

To check impedance I believe the speakers would need to be detached from the source, no?

Onward with the results of the slight power up. Some tube heaters appear to be working, some not so much. One of 4 output tubes lights, and one 12AX7 lights of 4 pre-amp tubes (2x12AX7, 2x6CG7). No sign of life - or arcing - from the rectifier tube, since some DC is making it to those heaters I believe it's functional on some level.

Next thoughts are to replace the output and rectifier tubes, and check the wiring to the heaters in the meantime.

So the remaining electrolytics - replace 'em all? Believe I've read it's worth attempting a reform first, or at least to measure for DC leakage (which I haven't learned yet, links to good articles welcome). There's one compound cap I presume is the/a filter cap, labelled 2x10uF 450V. Can this be replaced with a pair of 10 uF 500V Spragues with a shared (-)?

Looking at the "before" photos it's evident some bozo put tubes in the wrong sockets - 6CG7 in a 12AX7 socket and vice versa. While I can hope this occurred after the cord was cut, would that cause further damage to diagnose? [Update: checked tube schematics and the only difference in pin function is 9: internal shield on 6CG7 vs heater mid-tap on 12AX7. Heaters will both be 6.3V, unless the 12AX7 is wired in series, in which case its heater is 12.6V.]

Cheers,
-Eli.

[Tom Phillips]

Please attach your photos directly to the discussion thread. This will prevent the information from being lost to dead links in the future and make it easier for forum members to help you. When members contribute to a discussion part of the incentive is that we are building a data base that will be useful for many years to come. With supporting information attached to the thread you will get the best overall support. There is good information about the attachment uploading process at Reading and Posting Messages if you need.
Welcome to MEF,
Tom

[erobillard]

Brief update to document the original caps used in the Silvertone 1474:

Brand	Type	uF	Max Voltage
Pyramid	Paper-in-oil (PIO)	.5	200
Planet Lyticap	Electrolytic	40	450
		16	450
		12	450
		10-10	450
		3x10	75
Sangamo	Bipolar	.05	600
		6x.02	600
		4x.01	600
		.1	400

[Tom Phillips]

Just to be sure that you know but that 9 pin tube with the white film at the top is dead. The white is the getter inside the glass. It turns white like that when the tube has lost vacuum.

I didn't see that you said you had a schematic for the amp. Attached is a reference copy if you need it.


Note that, at low line voltage, the rectifier tube will not function so your approach to forming the caps by starting at a very low line voltage will not work. When I do that I temporarily substitute a solid state replacement for the rectifier tube. Then monitor the B+ to verify that the caps are getting the forming voltage I expect and to make sure they never see too much DC voltage. (The SS rectifier will result in higher than normal B# is you crank the Variac up to full line voltage)

[erobillard]

Thanks Tom. Had the 185.10410 schematic as a PDF (OneDrive link above), yours may be a tad more crisp. There are slight differences or possible errors from it to mine (185.10411), e.g. tube 2 is a 12AX7 in the schematic, where my chassis is marked for a 6CG7. I've also seen a corrected pair with just the pre-amp stage, both with and without a new reverb tank and minor mods by forum user nevetslab.

I've just ordered tubes and hope to get the fresh set in by next weekend, thanks for the tip on the milky 6CG7. Also replacing the output tubes and 6AU6 as a matter of course.

I suspected the reforming wasn't working. When I went to drain the caps today I took a quick reading and found little to no charge anywhere. I found a solid state replacement at TubeDepot.com for $9.95, ordered. Is it an option to reform the caps by applying DC directly to each cap (while measuring for leakage)? Or I have a working 1484 head; anecdotally the circuits are the same or similar, aside from the 1474 being tube rectified. Unfortunately the 1484 schematic doesn't show the voltage expected on the filter caps. Either way I guess I'm stuck until the tubes arrive.

Questions about the schematic:
- Could you or someone confirm the .05uF cap coming off the ground switch should be clipped now that I've converted to 3-prong? Seems a worthy goal to remove the ground switch entirely.
- The two largest caps are closest to the rectifier - can those be assumed to be the filter caps? If so I think I'll order Sprague Atoms to replace them and try my luck to reform the rest.

Cheers,
-Eli.

[DRH1958]

Regarding your speaker question, the speakers pictured in post #1 are wired correctly, with the chokes wired in parallel to each one. Those Jensen P12Qs are standard for this amp or at least that's what came with mine anyway. They are 8 ohm.

Yes, remove all wiring associate with the ground switch, including that .05 uF cap now that you have the proper 3-wire cord installed.

The only place I've found that has this schematic is in this book: https://www.amazon.com/Neptune-Bound.../dp/1574242393 I don't have it but there was a preview of the book and lo and behold, of all the schematics in the book, the one for this amp was shown so I took a picture of the two pages and taped them together. The attached schemo is the best I can do. It's a photo of a photo, real low tech
EDIT: It does have pretty good resolution, so if you magnify it and print, at least it shouldn't get blurry on you.

Also note that the two 12AX7 heaters are part of the 6L6s cathode bias supply circuit. I suppose it's an effort to get DC on them for reduced noise. Don't compare this amp to the 1484 as you alluded to in post #9, there are too many differences.

[erobillard]

DRH1958, that is the absolute grail, and a map I can read. You are going to make the day of many a 1474 owner as they stumble in here. From the chokes to the tubes, everything in its place as near as I can tell. Smells like, victory.

Realizing what different beasts the 1474 and 1484 are. While similar in the output stage, that's about it. A bit more research shows the 1484 and 1485 are much more similar, except for the output stage which is basically doubled in the 1485 to handle the difference from a 2x12" cab to the 6x10". Given the 1474 has that same doubled output stage for its Twin Twelves, I really can't wait to get it back in shape.

Lost this entire reply (and then some) when forced to login again. Argh! Had retyped G Weber's thoughts on the 1474 but they don't need repeating. He doesn't dig running 12AX7 heaters off the cathodes of the output tubes because the +/- drifts in current make it hard to dial in a tone and keep it. Not my concern today.

No further update, about to hit the workbench and get rid of the ground switch.

Thanks for the help everybody, in that hungry stage without anyone to bother in person about this stuff, you've been the best.

[Tom Phillips]

...Had retyped G Weber's thoughts on the 1474 but they don't need repeating. He doesn't dig running 12AX7 heaters off the cathodes of the output tubes because the +/- drifts in current make it hard to dial in a tone and keep it...

That "G Weber" thought makes no sense to me. Can't fathom what the author of that statement is talking about. If it is an accurate statement, it is based on a misunderstanding of electronics theory.
Reluctantly but necessary for sanity,
Tom

[erobillard]

Take it or leave it, here's the quotation from p.212 of All About Vacuum Tube Guitar Amplifiers, "[The 1474] features a cathode-biased output stage that strings the filaments of a couple of preamp tubes together and connects them to the cathode circuit of the output tubes in lieu of a cathode resistor! As clever as this may seem, it is a bad design because the output tubes' current will fluctuate some, causing the filaments of the two preamp tubes to become either too hot or not hot enough, which will negatively affect the tone." I don't think my assumption about the effect he's saying this would have on tone is too far off, but hey I'm relatively new to this. Until I can hear how the tube responds to changes in the input (assuming the effect is audible), I can't imagine a point in choosing one side or the other.

Success with removing the ground switch and death cap. Hot black from the cord now runs through the fuse, and neutral white is now connected to the line back to the transformer that had been connected to the ground switch. Had already built a new post on the chassis for ground, may yet run a line from it to a mount on the PT to provide a shorter route to ground should it arc. Also identified a 16uF cap that had been replaced with a 12uF (C30 in DRH's schematic).

Considering replacing all the Sangamo caps with 600V orange drops or the like after seeing this assessment (Silvertone in the Shop! | Sunnyside Amps). First I'll figure out how to test for leakage using the one I pulled off he ground switch. Who knows, maybe these will be magic Sangamos that don't fail like every one of the others.

[DRH1958]

That "G Weber" thought makes no sense to me. Can't fathom what the author of that statement is talking about. If it is an accurate statement, it is based on a misunderstanding of electronics theory.
Reluctantly but necessary for sanity,
Tom

Yes Tom and erobillard, I have this book as well and have read this statement. With the heaters in parallel with R53, 220 ohm, 5W bias resistor and both of them in series with R52, 39 ohm resistor, but having all this bypassed by C23 10uF, how would current through the heaters change? I suspect the cathode bypass cap would keep the current relatively stable since it's basically a short to AC, so current through the heaters remains relatively constant with changing signal, just like bypasing a preamp cathode keeps the bias voltage constant there. Am I in the ballpark with the answer? An unbypassed cathode resistance would be a different story I would think. But I have the stock tubes in the amp from 1962, so there can't be much detriment to the heaters. And the amp still sounds great. It really nails that trashy, early 60's blues distortion when cranked.

[g1]

The fact that Weber uses the phrase 'in lieu of' indicates he is not even aware there is still a cathode resistor there. That alone says enough for me.
I suppose there may be some slight variation in the voltage between idle and full tilt, but if it's enough to impact the tone, then maybe we should worry about matching heaters as well.