Silvertone Low Preamp Plates

[Gollum]

I've got a 1484 (2 x 6L6) that I've had for several years now (my first tube amp at that). It's been very good to me all things considered. Sounds great. Everything works (though the reverb sure is lacking life).

Well, I've got a couple of other amps in the stable taking care of my main needs, so I finally get the chance to do what I can to give it back a portion of all the joy it's given me.

I'm a novice, though I've done my best to educate myself as much as I can. I know how to discharge capacitors, and how to stay relatively safe while working on an amp.

Ok, onto the amp.

6 tubes in the preamp. 4 12AX7's, and 2 6 6CG7's. All tubes are ORIGINAL. This might very well be the source of any "issues" the amp has (though is sounds great to me), so if I'm just a cheap tard that needs to go buy some tubes, feel free to let me know I'm an idiot.

So the first thing I wanted to check was plate voltage. This amp doesn't seem to have nearly as much gain as I've heard these things crank out (and I play LOUD), so I wanted to start there. Turns out I'm a bit low just about everywhere.

Most of the lower voltage preamp plates are spec'ed around 100V, and I'm measuring between 70-85 in most cases. I know that's not too far off, but what also concerns me is that the higher voltage plates are spec'ed at 225V, 215V,205V, and 160V. I know those aren't "high", but they're all at least 50 volts bellow spec.

There's no listed spec on the heater voltage I can see on the schematic, but I've got an extremely consistent 5.3V across all the heaters in the preamp (measured across the tube, not chassis).



So, am I dealing with old tubes that just aren't drawing the current they should? Or am I looking at old caps that are dry'ed up? I don't see any visible leakage from any caps, and no bulging. That being said this thing IS circa 65' or so.

I haven't checked the power tube voltages yet. Saving that for tomarrow. My lack of knowledge of which pin is which makes me weary without a pin readout in front of me.

Ok, so that's that. Any advice is welcome.

EDIT: Oh, I'm not sure how much this matters, but at the location I'm working on it, the wall AC jumps between 109.8-110.2VAC.

[MWJB]

Power tube plate voltages and heater voltages would be handy, at the moment we don't know whether it is a preamp tube problem, or a general, accross the board voltage issue.

Voltages (dc) at pin 3 (plate), 4 (screen) & 5 (grid) are what we need from the 6L6s.

Is 110VAC typical in the SF bay area? Sounds low, schem shows 117VAC. I would expect any 50's/60's amp to run more voltage than the schem suggests, not the same voltage and certainly not less voltage.

New tubes & especially filter caps (before they change themselves) would be a good idea, but probably not that relevant to your current issue.

[Gollum]

So that wasn't so hard.

Plate voltages (pin3) are 429 and 431.

Screen voltage (pin 4) was same on both tubes at 316

Grid voltage (pin 5) was same on both tubes at 30


Checked the heater voltages and they were at 5.6VAC. This was a little higher than the preamp when I checked it last night (which was at 5.3) so I checked again and they turned out to be at 5.5VAC.

So I looked at the wall voltage, which is now at 113.5AC... Random. I live in an old house, which could be a problem. Maybe I should only be working on amps with a variac with an output readout. Or maybe just a place with cleaner power.

Also I checked my rectification diodes (so cool to me, these tiny little diodes from my parent's childhood). There's 4 of them, and on the schematic it looks like they should have an output of 480V. The almost identical 1485 (4 6L6 model) uses the same diodes and reads 500V on the schematic. On my 4 diodes I'm getting 108,107,107,and 110 V on each one, reading 428V to ground at the last one (4 in series).

[MWJB]

Theres not much point in measuring the voltages, if we know that the Wall AC is down, this will throw off the bias etc & impact tube voltages further. 5.6VAC is low for the heaters, I'd expect a mushy, ill-defined tone. Nothing to stop you trying new tubes & recapping the amp (it's somewhat understressed at the moment), but you will want to check the final set up on full wall AC before playing out/at regular wall AC.

[guitician]

Plug it into a 120VAC outlet

If you can take the amp to a friends house that has 120vac at the outlet you may be in for a surprise as to the difference the 7VAC will make. That 1484 uses a voltage doubler on the secondaries to get the 480 VDC.

[Gollum]

Ok, thanks for the input!

Since I don't own a variac I'll try to find a location I can test this thing at with full wall AC voltage.

[Gollum]

So I haven't taken the amp anywhere else yet, but I have been measuring the wall voltage whenever I think to check, and I have the time to. I found a time that the AC measured 115 and figured that was a lot closer to the rated 117 on the schematic and figured I'd give my meter a whirl.

Well, the numbers barely changed. Probably less than a 3% difference.

In measuring everything more in detail tonight (though I'm practically sleep walking) I noticed that it seems every voltage I measure that's on the schematic is low. In fact, the plate levels on the first three tubes are actually the closest to being right.

I'll go back and record all my readings versus schematic once I'm not so tired, so in a few days. I'll post up a chart showing the spec versus my readings.

Roughly speeking though, the screen grid on the 6L6's are about 50V low, the plates on the same tubes are about 75V low. The 225V on the plate of V3 is about 60V low. The 215V on V5 is about 50V low on both plates.

The store just goes on from there. I thought maybe it was something in the cap can or that funny little transformer (which I'm not too sure of it's job in the circuit). But then I remembered that the voltage leading to the plates of the 6L6's was low too, and it was low from the exit of the diodes that lead to the output transformer. It's supposed to measure 480, and I'm only getting 405-420.

Everything seems to be between 40-60V low except V1 and V2 which are within 20V of spec all around.

Is my power transformer not so happy with me? I'll let you guys educate me a bit because I'm not sure what to conclude. The amp is still "ok" and sounds the same as it has for the last 5 years, so I'm not too worried about it dying overnight, but I'd like to know that it's giving me the best of what it has to offer (even know these are heavily criticized, and I understand why).

[MWJB]

Low voltage - other than low wall AC, which we know you have - could be down to old filter caps. If voltages are low, things are not OK.

[Dave Curtis, dB AudioTech]

Low voltage - other than low wall AC, which we know you have - could be down to old filter caps. If voltages are low, things are not OK.

I'd definitely R&R those, and see what a new set of output tubes does for it.

[guitician]

Always check the line voltage while your checking the amps voltages. And check it at the transformer primary wires, in case the power switch is going bad.

[Gollum]

Ok, here's the measurements I just took.

Wall AC = 110V
AC at accessory outlet = 110V
AC at "on" switch = 110V
AC at fuse = 110V

Everything looks like the wall AC is getting to the transformer ok.

(tube numbering is per schematic on schematic heaven for the 1484)

Format:
Tube Number
Rated Voltages - Measured Voltages

V1 Plates (12AX7)
100V - 88.9
100V - 86.8

V2 Plates (12AX7)
93V - 85.4
93V - 86.0

V3 Plates (6CG7)
100V - 98.5V
225V - 173.8V

V4 Plates (12AX7)
160V - 143.5V
55V - 57.0V

V5 Plates (6CG7)
215V - 173.2V
215V - 185.0V

V6 Plates (12AX7)
205V - 86.0V
120V - 86.5V


V7 (6L6)
Grid -36V - -30.4V
Screen 340V - 313V
Plate 475V - 420V

V8 (6L6)
Grid - 36V - -30.0V
Screen 340V - 310V
Plate 475V - 422V


That reverb V6 tube seems really low.... The reverb "works" but doesn't really, like most probably know of these amps. Meaning it turns on, and I can hear it making a difference, but it could scarcely be call "reverb". More like "slight noise addition".

[guitician]

I just took a look at the schematic and the 1484 has two secondaries both wired as stacked voltage doublers. That would quadruple the change in the primary voltage. A 7VAC change at the primary may be as much as 30VDC at the top of the quadrupler.

[Jag]

Is my power transformer not so happy with me? I'll let you guys educate me a bit because I'm not sure what to conclude.

I seriously don't think you can make any reasonable conclusion with your wall voltage being so low. Getting 117-120volts into the primary will change your measurements. I think alot of it will fall into line under the correct conditions.

. . . (even know these are heavily criticized, and I understand why).

I have a '57 Silvertone 1395. It is my absolute favorite amp for bluesy playing. It's really funny how many people really want to hate that amp because of what it is but, as soon as I wind it up the offers to buy start flying. I've got three or four oddball amps that absolutely kill everything else they go up against. Of course, they all required the basic maintenance to give up the goods. don't let the critics sway your opinion!

[Gollum]

Ok, got it measured tonight at a much better level. Wall was measuring at 118.5, but for some reason at the fuse and switch the amp was seeing 116.4. Still a heck of a lot better then what I was getting before.

Here's the new readings:

Format:
Tube Number
Rated Voltages - Measured Voltages

V1 Plates (12AX7)
100V - 89.5V
100V - 87.4V

V2 Plates (12AX7)
93V - 88.2V
93V - 87.3V

V3 Plates (6CG7)
100V - 102.9V
225V - 183.3V

V4 Plates (12AX7)
160V - 149.1V
55V - 57.6V

V5 Plates (6CG7)
215V - 182.1V
215V - 173.0V

V6 Plates (12AX7)
205V - 90.5V
120V - 75.3V


V7 (6L6)
Grid -36V - -32.1V
Screen 340V - 329V
Plate 475V - 446V

V8 (6L6)
Grid - 36V - -32.1V
Screen 340V - 330V
Plate 475V - 448V


So the readings are much better, as you guys were thinking they would be. But some are still a bit low from spec. I guess I should consider myself fortunate since it's 100% original. They're all close enough to a 10% tolerance that I guess I shouldn't be "worried" by any means.

The only one that concerns me is the reverb circuit. It's a wopping 44% and 63% of what spec is. That's pretty low. This doesn't seem to chance when I swap out the tube. Any suggestions on where to look for any issue? (not that I'll ever likely use reverb)

EDIT Actually, looking at the schematic again V6 is in the tremolo circuit. Woops... The tremolo seems to be working fine though, am I missing something here?


To Jag ~ I wasn't meaning to diss that amps like I might have come across. I was just stating that I understand why some people don't like these amps much. I get plenty of comments regarding my tone to know that I'm doing at least something "right" to some measure. This amp has never really held me back, just hasn't always been the perfect match for a given situation. It's got plenty of mojo going for it, and when people see that it has the original silvertone labeled tubes their jaw usually drops.


Oh, and on a semi-related note. The voltage at the wall is my house having issues apparently. I started wondering if it was a localized issue on one side of the house so I started doing more thorough testing. Turns out with every little thing I turn on in my house the voltage drops. I'd been working on the amp with a halogen lamp on the same outlet... that alone causes a 3V drop, though it is almost as many watts as my mini-fridge.

It's an old house with no original grounded outlets, just 2 prongs on every wall except where 3 prongs have been retrofitted (and probably without a good ground).

So it's not the power companies fault. Most likely just poor wiring that isn't up to the task of running anything high wattage. I might be in a fire hazard right now....

[Steve Conner]

Have you checked your voltmeter?