Silvertone 1344-a gut shot

[munkeyboy]

Hi guys.  Long shot, but does anyone have a gut shot of a silvertone 1344-a?   Actually a 1344 would work, but the 1344-a would be better (has two vibrato controls instead of one).  I have the schematic here: https://el34world.com/charts/Schematics/files/Silvertone/Silvertone1344.pdf, but somethings aren't making sense to me.

What I actually have is an Danelectro Super Vibravox (actual model unknown), but it seems very similar to the 1344-A.  I'm guessing it was a precursor to this silvertone.  There is zero info out there on it.  The tubes are the same (tho i'm unclear on the 12sc7 vs 12sn7). The component values are close, but not the same.  I'm not the best schematic reader, so I'm having a hard time figuring it out. 

When I got it, there were three loose elec caps.  I wired them back up best guess based on the 1344 schematic (or looking at disturbed solder points) to a point where i can turn it on.  I wanted to turn it on to see where the base line is before I start replacing caps (in case bad transformers etc).  The only sound I'm getting is an obnoxiously loud hum that doesn't change with volume control or removing preamp tubes.  No input signal is getting thru (or i can't hear it due to loud hum).  I'd like to be sure I didn't wire these loose caps in the wrong spot.

Here is the gut shot of my danelectro

https://photos.app.goo.gl/Xo974PnHVXcQqFA67

[munkeyboy]

FYI, i've already wired in 3 prong and removed the death cap.  Only change i've made.

[PRR]

DanElectro made amps. Sears contracted to have the SilverTone name put on. There may be differences.

> unclear on the 12sc7 vs 12sn7

Different pin-out. (Also very different gain, but they may not have known how much gain to give.)

[shooter]

It's winter, so a week with a meter, your ballpark schematic, Doug's Diy-layout, paper n pencil, walla, nice layout, errors n corrections made to the schematic.

Parts on order 

[munkeyboy]

DanElectro made amps. Sears contracted to have the SilverTone name put on. There may be differences.

Yes, i'm a fan of Danelecto/Silvertones.  I have a few.

> unclear on the 12sc7 vs 12sn7

Different pin-out. (Also very different gain, but they may not have known how much gain to give.)

I'm confused at to the schematic itself and the wiring on my amp.  The original schematic for the 1344 looks like it had a 12sc7, but scribbled on the top of the schematic is 12sn7.  My amp has the 12sc7 and has 12sc7 scribbled on the chassis, but the actual wiring looks to be 12sn7.  That or they are using pin 1 to just hang connection off of and i'm not deciphering correctly.  The latter is probably the case.  I'll need to review the pin outs for both and attempt another pass at following the wires.

[munkeyboy]

It's winter, so a week with a meter, your ballpark schematic, Doug's Diy-layout, paper n pencil, walla, nice layout, errors n corrections made to the schematic.

Parts on order 

Agreed, i'm excited.  I was more excited when I won it on ebay, but then it arrived in pieces thanks to horrible packing (or the lack of).  Not sure I can save the cabinet. 

[munkeyboy]

Here is my first pass at reverse engineering the Danelectro Maestro based on the silvertone 1344 schematic.   Found the model of the amp hiding under some grime.  No series X in the model, so I'm guessing first generation.

There are some things that are different.  Most resistor values are different, tho close.  It also doesn't have 1 of the 3 8uf caps.   The tremolo circuit added a speed pot.

This is my first attempt at modifying/adding to a schematic.   If anyone has time, I could use someone's eye to review the tremolo area circled in yellow.  This is the biggest section that i modified.  I'm having a hard time wrapping my mind around C16.  The layout on the fiberboard doesn't make sense to me and I don't think i have it correct in the schematic.

I also have a very rough layout attached (missing some grounds/power/speaker), but should be fairly good for the tremolo section.

[PRR]

C16 is just a coupling cap. The trem oscillator makes a (subsonic) signal, mixed with DC; we want the signal without the DC.

The green line under that area shorts-out the 100K plate load; this can't be right.

I do not know what the green 1Meg in the power stage bias is for.

[munkeyboy]

C16 is just a coupling cap. The trem oscillator makes a (subsonic) signal, mixed with DC; we want the signal without the DC.

The green line under that area shorts-out the 100K plate load; this can't be right.

I do not know what the green 1Meg in the power stage bias is for.

Thanks for looking PRR!

That green line is what had me worried too, I'm just not looking at the layout (or the real board) well enough i think.  It's hard to get under that board without clipping all the lines... which is why i made the layout so I know how it goes back.  Already have a replacement fiberboard with turrets waiting.   

That green line is from C16 to C12 in the layout.  If it didn't connect there, but continued on to R19 it would make sense. I'm thinking it should be c12 attached to R19/R20 and c16 attached directly to the other side (pin 8) of r19. (see attached)

Yeah, that 1M in the power stage is odd as well.

It could be that I got a few things wrong in that section.  I'll have to take the plunge and cut the wires so i can see under that board properly.

Did the pots make sense in the tremolo section?

[munkeyboy]

well I started snipping wires and that section is wired as i have it.  Doesn't make it right i guess.  Maybe why this amp was discarded.  There was work done on this and some of the work was pretty sloppy.   That connection could have been made by mistake by some tech or other.

By miracle of photoshop, i made an xray of sorts...  composite of the underside and top of the board.   Hard to see some of the other lines, but this section around c16/c12 is pretty clear.

[munkeyboy]

So,  I've go this guy working.  The resistor values looked like prewar values and had to hunt to get them (hunt wasn't too bad, tubesdepot had them).

The output is weaker than I expected and a lot of gain/distortion.   Having not worked with 12sj7 and 12sc7, i'm not sure what I'm looking for voltage wise.   Especially, I don't understand the heaters on V1 and V2.  It looks like neg DC current is going to them, not 12.6vac.  The schematic backs that up, but I can't grasp why the heaters aren't connected to 12.6vac like the other 12sj7 (in the trem section).  Reading the datasheet, i'm guessing these can take ac or dc, but then why is one heater -9 and the other -17.

The power transformer is bigger that the 270/270 in the silvertone 1344.   Looks like 350/350.   So my B+ voltage is 318, 250, 245.  That last one being significantly higher than the schematic's 150.

Attached are the latest schematic based on the silvertone 1344 and my layout to go with it.   Below is my tube voltage chart.  Any insight is much appreciated.

V1 12SJ7
1:
2:0
3:
4:-0.5
5:
6:27
7:-9.5vac
8:8.45

V2 12SC7
1:
2:99.3
3:
4:
5:89.2
6:1.3
7:-17vac
8:-9.5vac

V3 6V6
1:
2:6.5vac
3:310.2
4:248
5:-16
6:
7:
8:

V4 6V6
1:
2:6.5vac
3:can't get reading
4:246
5:-16
6:
7:
8:

V5 12SJ7
1:
2:6.6vac
3:
4:-0.8
5:
6:37.5
7:6.8vac
8:85.4

V6 6Y6
1:-15.5
2:6.6v
3:164
4:172
5:-14.9
6:-17.7
7:
8:-15.5

[PRR]

All heaters can take DC. AC is usually cheaper, but prone to hum. In this amp they got so much DC flowing in 6V6es and 6Y6 that they ran the lowest-level tube heaters on this DC, also acting like a cathode bias resistor.

If both tubes are clearly 12V types, they should have the =same= heater voltage. (May not be quite 12V, that is OK.) But for that to happen in a series-string they must both be the =same= nominal heater current. Get data for the types you have, and types you should have (if different) and compare. Someone may have found a type that would be "equivalent" in a solid 12V job, but of different current so the two heaters are not sharing equally.

[munkeyboy]

FYI in case it matters:  V2 (12SC7) is being fed the -17 in to the first heater pin, then that going out to V1 (12SJ7) at -9v to the first heater pin.  Finally V1 dumps from the second heater pin to ground.

Both documents state that they are 12.6v and 0.15 amps.

Also (or related), i don't think 8v to the plate is right for V1.  Pretty sure it should be upwards of 90vdc.

[munkeyboy]

Yeah, I'm missing something.  Something is drawing power.  Putting the MM on the V1 plate when first turning on amp,  voltages read over 200v, then drop slowly for a few seconds, then drops to the teens once one of the tubes(i'm guessing) warms up.  I've tried swapping v1, with no success.  I'm wondering if it's the 6Y6.  I have no spare to test.   But vibrato doesn't work either except to introduce more hum when turning up strength.   I'll swap known tubes that I have tomorrow (6v6, 5y3) to see if that helps, otherwise, i've done something wrong.

[munkeyboy]

Still working on this (tube swaps didn't help).  It is plenty loud enough actually, about what I'd expect from 6v6.  And it sounds okay if i wanted all that gain.  Very difficult to get cleans unless i back of the guitar volume to about 1 or 2.   I have a bias check tool on order as well as grounded input switches, new cts pots and 12sc7.  I hope proper bias and grounding will help with some of this (save for the tremolo).

Question.  Is it the field coil that takes so long to power up and what drains a lot of power (I know it acts as a choke of sorts)?  It takes way longer than my other tube amps to start producing sound.  It takes about 35+ seconds to "warm up".   I've got 200v+ on v1 until whatever it is that happens at that 35 second mark, then it drops to about 12 to 15v.

2nd Question, related to the first (sort of).  What is this 6Y6 doing?   What benefit does it provide?  There is some relation to the field coil and the tremolo section.  If I replaced the field coil with a choke/1500ohm resistor and a PM speaker, would this tube still be needed?  I'm guessing that I might wire the tremolo directly back to the output tubes like other circuits.

Thanks!

[shooter]

whatever it is that happens at that 35 second mark

heaters are hot, tube is sucking -e.

how big a jump you get @ V2 plate vdc, when you have V1 out?

what happens on the cathode of V1 when the plate hits the dirt?

have you swapped V1

[munkeyboy]

Hi Shooter

how big a jump you get @ V2 plate vdc, when you have V1 out?

A couple of measurements:

V2 (12SC7) on start of amp
[pin]2: 436
5: 433

V2 on final warm up
2: 103
5: 93

V2 warmed up without V1
2: 288
5: 289

V1 (12SJ7) on start of amp
[pin]6: 425
8: 445

V1 on final warm up
6: 31.7
8: 10.7

V1 warmed up without V2
6: 291
8: 298

what happens on the cathode of V1 when the plate hits the dirt?

Pin 5 is 0v throughout the process.

have you swapped V1

Yes, with V5.   Maybe both are bad, I don't have another 12sj7 yet.
It could be V2 is bad as well.  I have two on order to swap out with.

It could also just be bias.  IDK why, but that is still a concept that I have a hard time wrapping my brain around.

For the tremolo, on my second pass thru highlighting the paths thru the schematic, i found a wiring error.  Fixed it, but still no tremolo effects.  However, there is also no bad hum introduced on turning up the speed/intensity which seems good.

[munkeyboy]

So let me know if i'm measuring the bias incorrectly.

Plate DC on 6v6 (V3): 312.2
Output Transformer DC: 320.4
Voltage Drop on V3: 8.2
Output Trans to Cathode Resistance: 166.1

8.2/166.1 = 0.0493 or 49.3 ma

Plate DC on 6v6 (V4): 313.6
Output Transformer DC: 320.4
Voltage Drop on V4: 6.8
Output Trans to Cathode Resistance: 143.7

6.8/143.7 = 0.0473 or 47.3 ma


That seems pretty bad if I'm doing this right.

[PRR]

> It takes way longer than my other tube amps to start producing sound.

The preamp heaters are powered by the DC current in several big tubes. So launch is a 2-step process:

1) hot-up the 6V6es so they flow current
2) that hots-up the 12Sxx so they wake-up

With late-1950s+ heaters this will be about 11+11 or 22 seconds. If you have very vintage tubes it could be more.

Further the small tube may be under-heated(?) so will take a little longer than usual.

However leaky e-caps can also delay things.

[munkeyboy]

Bringing back this thread.  I took a break from this for a while while I worked on other amp projects.   The time did me good, I've learned a little.   I have everything working, even the vibrato... but i still get odd distortion.   Best I can explain it is that there is a clean signal, but there is a light layer or an echo of fizzy distortion/static on every note.  I've swapped all the tubes, tried an external speaker, nothing changes save for playing with treble/bass pots can help a little.

I don't know what a very cold bias sounds like, but I need to check that anyways.  Problem is, I just can't wrap my head around the bias section of this amp.  I'm guessing this is fixed bias amp, but it doesn't look that similar to say the AB763 bias.  I suspect it has to do with the 6y6/vibrato.  If i put in a bias pot(s), where would the best place in the circuit to drop them in?

Also, looking the silvertone 1344 schematic attached, the resistor values highlighted there in green are quite high (430k).  Could this actually be right or should they be 430R?

Thanks in advance!

[shooter]

I'm guessing this is fixed bias amp

to make educated guessing look at the PA cathodes, if they go to ground they're "fixed" (rules always have exceptions though :)

If you want to make it a keeper, add a small "mod", put 1 ohm R's from each cathode to ground.  They make biasing way easier.

you need a more inclusive schematic so we can help with bias circuit

[jjasilli]

@shooter: full schematic is in the 1st post

there is a light layer or an echo of fizzy distortion/static on every note
 1.  Always suspect tubes 1st.
 2.  My faulty memory tells me that this may result from inadequate B+ filtering.  Or, maybe bad connections in the B+ filtering circuit.
 3.  NFB might help.
 4.  A scope or listening amp may help pinpoint where this fault originates.

[shooter]

Thx JJ, didn't have my coffee yet 

I'll agree V1 is a good start based on;

nothing changes save for playing with treble/bass pots can help a little.

old ceramic caps, sketchy shields in/out of TS.....

[munkeyboy]

Thanks guys

I'll agree V1 is a good start based on;

V1 meaning the first preamp tube?   Could the PI be too hot?  I've replaced that with 3 different tubes, but all 3 are quite noisy when tapped.

nothing changes save for playing with treble/bass pots can help a little.

old ceramic caps, sketchy shields in/out of TS.....

I know I should know what "TS" means but I don't  
The amp and components were so bad and disassembled when i got it, it was easier to replace everything.  Even build a new turret board.  Unless I missed one or two, all the caps, resistors, pots and jacks are new.   Not to say there aren't bad ones.  I pretty much rebuilt the circuit from the schematic but used original values that were in the amp.  Which is probably where the issues is.  I'm sure I've overlooked something or have a bad value somewhere.  The original amp had the same circuit, but with slightly different resistor values throughout (i.e. instead of 430k, it had 390k or instead of 56k, it had 75k).  I might need to just re-work the resistor values just like the schematic.

@shooter: full schematic is in the 1st post

there is a light layer or an echo of fizzy distortion/static on every note
 1.  Always suspect tubes 1st.
 2.  My faulty memory tells me that this may result from inadequate B+ filtering.  Or, maybe bad connections in the B+ filtering circuit.
 3.  NFB might help.
 4.  A scope or listening amp may help pinpoint where this fault originates.

1. I have swapped so many tubes around that I think bad tubes may not be the issue, but biasing might.  3 sets of 6v6s, 3 different 12sc7 and swapped the two 12sj7s around.  I think i even played with a few different 5y3 and a new 6y6.
2. Ok, that could be. With 120v maybe the filter caps need to be bigger?
3. I can try that
4. Good idea, i have a homemade prob for hooking up to another listening amp.   I also have a scope, but I have no idea how to use it or if it even works right... pretty sure it doesn't.

[jjasilli]

TS = ToneStack

[munkeyboy]

Also I had mentioned this before but forgot until now.  The PT is much larger that the silvertone 1344 in the schematic . I have 350vac vs the 270vac in the schematic.  Makes me wonder if this was meant for 6L6 instead.  Anyways that's why i kept the voltage dropping values at the original 8k (vs 5.1k) and 75k (vs 56k).

[shooter]

are new

see post about coffee 

put the 1 ohmers in, measure the mV across it and you have idle PA tube current.

many "things" can cause problems, solve 1 at a time, solving them all at once gets messy.

[shooter]

still playing catch-up;
from PRR;

All heaters can take DC. AC is usually cheaper, but prone to hum. In this amp they got so much DC flowing in 6V6es and 6Y6 that they ran the lowest-level tube heaters on this DC, also acting like a cathode bias resistor.

If both tubes are clearly 12V types, they should have the =same= heater voltage. (May not be quite 12V, that is OK.) But for that to happen in a series-string they must both be the =same= nominal heater current. Get data for the types you have, and types you should have (if different) and compare. Someone may have found a type that would be "equivalent" in a solid 12V job, but of different current so the two heaters are not sharing equally.

I'm hung-up on bias type, the schematic is sketchy, are the filaments acting as a hybrid fixed/self bias, or is the -20vdc the bias, tempered by filaments 

[munkeyboy]

Yah that is what has me confused too.

[munkeyboy]

put the 1 ohmers in, measure the mV across it and you have idle PA tube current.

many "things" can cause problems, solve 1 at a time, solving them all at once gets messy.

PA=preamp?  Or if you are talking about the power tubes, i have the hoffman bias checker.  I just don't know what value is right because I don't understand what kind of bias this is.   Assuming this is an AB amp and a fixed bias with 280 on the plates, I'm a little cold @ 22/26mA instead but not bad.

The amp isn't really worth much, especially in the condition it's in.   So if a few more weeks go without resolving this, i'll probably just build a new 5e3 circuit or something using the existing iron.

[shooter]

PA = power amp

20mA is alittle cold, but wouldn't mess with it based on, well the unknowns 

I used the "same iron" in 15 - 20 proto builds, considered it my "college lab parts" 
the bonus, using them in real builds that leave 

[munkeyboy]

I used the "same iron" in 15 - 20 proto builds, considered it my "college lab parts" 
the bonus, using them in real builds that leave 

Yep.  I have quite a few builds under my belt using repurposed iron and a few waiting for this danelectro to be done.  One 5e3 I actually sold/traded for partscaster... guy loves it.  Currently enjoying an bell carillon pa repurposed as a 6v6 tiny terror.  A little cheaper than buying new iron I suppose, plus each one is unique.

[sluckey]

still playing catch-up;
from PRR;

All heaters can take DC. AC is usually cheaper, but prone to hum. In this amp they got so much DC flowing in 6V6es and 6Y6 that they ran the lowest-level tube heaters on this DC, also acting like a cathode bias resistor.

If both tubes are clearly 12V types, they should have the =same= heater voltage. (May not be quite 12V, that is OK.) But for that to happen in a series-string they must both be the =same= nominal heater current. Get data for the types you have, and types you should have (if different) and compare. Someone may have found a type that would be "equivalent" in a solid 12V job, but of different current so the two heaters are not sharing equally.

I'm hung-up on bias type, the schematic is sketchy, are the filaments acting as a hybrid fixed/self bias, or is the -20vdc the bias, tempered by filaments 

The series filaments are connected between the PT CT and ground. Think of the filaments as a resistor. The current flowing through the PT CT also flows through the filaments (resistor). This causes a -20V drop across the filaments which is enough to lite'em up. The -20v is also used for fixed bias for the output tube grids. This type bias circuit is known as "back bias" (google it). This has nothing to do with cathode bias resistors.

I never cared for back bias circuits. The voltage is totally dependent upon the amp draw for the B+. If the output tubes draw less current, the bias voltage will decrease which in turn will cause the output tubes to conduct harder. Crazy. And this particular circuit uses the preamp tube filaments for the resistor. If the output tubes current decreases too much, the preamp tubes won't have enough current to lite up properly. Sears used a lot of penny pinching accountant types for clever engineers.   

[shooter]

penny pinching accountant types for clever engineers. 

 

  GE used the same bean counter EE's
ya gotta  love the creativity though 

[munkeyboy]

This type bias circuit is known as "back bias" (google it).

I googled it, https://www.aikenamps.com/index.php/what-is-back-biasing

But for a man (me) without a collage degree, much less an engineering degree, this is a bit over my head.   Ugh, not sure I want to continue down this rabbit hole :)

Sears used a lot of penny pinching accountant types for clever engineers.

Tell me though, how does this save money?

[sluckey]

By using the 12??? tube filaments as a resistor you save the cost of a big power resistor. By using the back bias circuit you avoid the cost of a diode (pretty significant back then).

[munkeyboy]

Ah makes sense

[shooter]

 

I took all the EE classes, even got a pedigree (not EE :) and it's a pretty wonky circuit so you're doing fine 

[Saransk]

A lot of the Danelectro/Silvertone/Airline, etc. amps were Danelectro designs and used the preamp heaters for the cathode "resistor"
I have a Danelctro 89 that takes about 15 seconds to make a noise - no need for a standby switch.
the best thing about this is you get DC on the preamp heaters with almost no hum - not to shabby for a 1954 amp.

The wierdness is in the way the tremolo is done
The 6L6's are cathode biased, but there is an extra stage in the PI circuit to allow for bias tremolo
the 12AX7 is the PI but it drives a 6SN7 that in turn drives the 6l6's in the output.
The circuits are in the EL34 library.

[PRR]

This amp is, for all practical purpose, "self biased, cathode resistor".

Yes, the resistor is inside the preamp tube and its waste heats the little tube.

> If the output tubes draw less current, the bias voltage will decrease which in turn will cause the output tubes to conduct harder. Crazy.

Exactly the same as self-bias. It tends to find a happy spot; rather it reduces tube variations to about half what they would be without cathode resistor(heater).

In *radios* this "resistor" would *also* pass the semi-steady current of all the tuner tubes. This makes it "more-fixed" since the 6V6 current is semi-swamped by the sum of the other tubes. In g-amp work the little tubes work at low current so we might call it "95% self 5% fixed" which is shaving hairs.

There is a start-up issue that the heater is low-R when cold so at 10 seconds when the 6V6 starts sucking good it has little bias. You may see a strong dip in B+ when the 6V6 comes hot. But that lasts just a second or so before the small heater's R comes up. And we know we can abuse tubes a LOT for a few seconds.

[munkeyboy]

This amp is, for all practical purpose, "self biased, cathode resistor".

So does that mean I would be safe in using the "Cathode Bias" result of robrobinettes calculator instead of the fixed bias result?   If so, I'm way cold.   at 280 plat voltage, i'm getting between 22mA and 26mA instead of 40 something.

https://robrobinette.com/Tube_Bias_Calculator.htm

[shooter]

Thx for the schoolin, cut n pasted in the backbias folder.

as I lernt, you can calc based on self bias, but "adjusting self bias, typically is replace Rk based on math, you have no Rk, you have filaments

[sluckey]

This amp is, for all practical purpose, "self biased, cathode resistor".

What schematic are you looking at? I'm looking at this one...

     https://el34world.com/charts/Schematics/files/Silvertone/Silvertone1344.pdf

Cathodes are tied directly to ground. Grids are fed a negative voltage, although developed through a back bias circuit. Where do you see any cathode resistor?

[PRR]

Yes, back-bias. Note that the CT of the PT does not go to ground. It goes to heaters of two 12V/150mA-heater tubes.

For 6V6 cathode current to get back to the PT CT it must flow in those heaters.

Heater is just a "wire wound resistor". Yes it runs hot enough that its resistance varies (significantly!), but if the preamp/PI work at all it is sure to be 850-950 degrees and pretty constant.

The two audio 6V6es probably run 74mA-105mA; the trem driver at say 50mA; say 3mA all else. So 128mA-158mA. This is close-enough to the 150mA needed by the heaters (these tubes work far below their max emission so don't need to be full-hot).

Here's some scribble. First notates the unusual "ground" point, and highlights the cathode "resistor". All current (except insignificant 6V6 grid current) MUST flow in the heater/resistor. Second sims the situation so I can draw it clearer than SilverTone's draftsman. (I'm out of 6V6 and too lazy to wire three, so you get one 6550 which at these voltages is very near a 3-pack of 6V6. A bit more, and there's PT DCR, so I dropped some R in the plate to get G2 voltage down and cathode current nearer the 6V6 trio.)

[sluckey]

I get it now. That's a sneaky way of looking at it. Thanks.

[DummyLoad]

cap is drawn with polarity reversed. please see markup schema attached - am i off base? 


--pete

[sluckey]

I agree.

[PRR]

Yes. (Note the original plan does not say.)

[munkeyboy]

cap is drawn with polarity reversed. please see markup schema attached - am i off base? 


--pete

Hmm, well i'm gonna have to look at what I put in there then.   

[munkeyboy]

So i had that cap in backwards just as that schematic did.  However, flipping it didn't change much.