Silvertone-ish build

[showsii]

OK Good - I wanted to believe my eyes, they looked too big for 6V6GT's.

If you don't mind indulging my interests here - verify for me that I have this correct:

330V -- AC voltage going into the rectifier?
323V -- DC on the plates

Can you get the DC voltage going into the center tap on the output tranny? I think this is pin 8 on the rectifier tube.

Are you planning to follow the Silvertone layout? While you have that thing there and open, you might want to get some photos of the inside and outside views to use for references on your layout drawings.

If it's not too much trouble, I would be curious how many volts you have (AC) on the tube rectifier tube socket at pins 4 and 6 if you remove the rectifier tube.

Should be a fun project.

rj

Rj,

Yes that is correct..

However That Vac on pins 4,6 measurement was WITHOUT the Rec tube in. Forgot to mention that.

The measurement I got off of pin 8 is 333V, does that sound right?



I do not plan on following their layout, as of now. I have drawn up a layout on a turret board style design. As I'm coming right off of the rocket build, I have that in the back of my mind as far as a layout goes..(I am considering a Cap stack, is that crazy?)

I do not know how much layout will effect the tone of the amp. I plan on keeping everything original values consistent with the Bass amp design. Maybe a bright switch, (however I thought of something on a toggle switch to take out some of the boominess if that might sound cool, Low volume is where this seems to be a problem) It will be a smaller chassis, just one input, Volume, Treble, Bass

here is the layout so far.. Haven't re-worked the grounds yet however. (no heater wires either, Just a draft if you will)

I have been taking some gut shots though


What else do we need?


Ian

[overtone]

Ian, while you still have the amp:

1. As RJ said, get the voltage off the rectifier to the center tap of the OT, probably pin 8, but in the schematic pin 2. We want to know this because of the high voltage drop across the OT as noted in the schematic.
2. Verify that this amp really is running a 5Y3 rectifier.
3. Get the 6L6 screen voltages and did anyone add a screen resistor at a later date to each of the screen pins? (pin 4)
4. Get the voltage reading off the 6L6 cathodes, Pin 8. It would interest me, how far off the schematic voltage that is.
5. If you have any other voltages we may have missed but are noted in the schematic, check those too.
6. I guess to be really informed the house voltage coming into the amp would be useful.
7. Check the heater circuit voltage.
8. If you have time, trace the circuit with a colored pen against the components in the amp, this really helps to get familiar with the amp at another level. There may well be deviations from the schematic anyway.
9. Has this one lost the choke on the speaker already?
10. IMHO leave your layout aside right now and concentrate on milking the borrowed amp for all you can, while you can,

best, tony

[showsii]

Ian, while you still have the amp:

1. As RJ said, get the voltage off the rectifier to the center tap of the OT, probably pin 8, but in the schematic pin 2. We want to know this because of the high voltage drop across the OT as noted in the schematic.
2. Verify that this amp really is running a 5Y3 rectifier.
3. Get the 6L6 screen voltages and did anyone add a screen resistor at a later date to each of the screen pins? (pin 4)
4. Get the voltage reading off the 6L6 cathodes, Pin 8. It would interest me, how far off the schematic voltage that is.
5. If you have any other voltages we may have missed but are noted in the schematic, check those too.
6. I guess to be really informed the house voltage coming into the amp would be useful.
7. Check the heater circuit voltage.
8. If you have time, trace the circuit with a colored pen against the components in the amp, this really helps to get familiar with the amp at another level. There may well be deviations from the schematic anyway.
9. Has this one lost the choke on the speaker already?
10. IMHO leave your layout aside right now and concentrate on milking the borrowed amp for all you can, while you can,

best, tony

Ok here we go...

1. As RJ said, get the voltage off the rectifier to the center tap of the OT, probably pin 8, but in the schematic pin 2. We want to know this because of the high voltage drop across the OT as noted in the schematic.

332V

2. Verify that this amp really is running a 5Y3 rectifier.

Yes it is running a 5Y3GT Rectifier

3. Get the 6L6 screen voltages and did anyone add a screen resistor at a later date to each of the screen pins? (pin 4)

It looks like there is a 68ohm resistor coming off each screen (pin 4) of the 6l6 going to the junction of the 1.5k 1watt and 20uf filter cap.

V4-316.8V
V5- 316.9V


4. Get the voltage reading off the 6L6 cathodes, Pin 8. It would interest me, how far off the schematic voltage that is.

There is a 22uf 50v bypass cap connected in parallel to the 250ohm resistor in this amp. Does this add more gain compared to the schematic?

V4-23.2V
V5- 23.2V

5. If you have any other voltages we may have missed but are noted in the schematic, check those too.

Plate of V1b Pin 1- 112V
Plate of V1a Pin 6-113V

Plate of V2b Pin 6- 118V
Cathode of V2b Pin 8- .967

Plate of V2a-Pin 1- 121V
Cathode of V2a Pin 3- 27.5V

Plate of V3 (PI) Pin 1/6- 146V

Plates of 6L6GC- Pin3- 323V

6. I guess to be really informed the house voltage coming into the amp would be useful.

115Vac

7. Check the heater circuit voltage.

Heater voltages checked out.

8. If you have time, trace the circuit with a colored pen against the components in the amp, this really helps to get familiar with the amp at another level. There may well be deviations from the schematic anyway.

Will do!

9. Has this one lost the choke on the speaker already?

Yes, there is NO choke. It is a head unit.

10. IMHO leave your layout aside right now and concentrate on milking the borrowed amp for all you can, while you can.

Point taken!

[overtone]

That all falls into place pretty well. I am thinking this is cannot be more than 13 Watts plate dissipation for each 6L6, but I have not had breakfast yet!
I'm also pretty sure the PI is actually the V2b. Your V3 is another driver stage pair before the output tubes. You find that on some bass amps and it looks good to me.

One more job please:
can you still get the B+2 node voltage, that is the one marked 345V between R36 + R37?

[overtone]

one more thing, the 68 Ohm g2 screen resistors are such a low value that it is hardly worth the bother - could you double check that value?

[showsii]

That all falls into place pretty well. I am thinking this is cannot be more than 13 Watts plate dissipation for each 6L6, but I have not had breakfast yet!
I'm also pretty sure the PI is actually the V2b. Your V3 is another driver stage pair before the output tubes. You find that on some bass amps and it looks good to me.

One more job please:
can you still get the B+2 node voltage, that is the one marked 345V between R36 + R37?

woops on the PI! good to know though.. so the PI is a 12ax7?

Ok the B+2 voltage is 323V

So, How do we go from here..

What is some math that I can do to get the PT and OT values?


Thanks

Ian

[overtone]

Ian, a few things don't add up now.

Looking at the schematic there is a 1k5 resistor "R37" dropping the voltage from B+1 360V down to B+2 345V. I am not seeing that kind of a drop on your posted voltages which have both B+1 and B+2 at 323V. Has R37 been taken out? Or maybe you took the B+2 reading "before" R37?

It may hang together with the above point, but to my mind the 64 Ohm screen resistor does not check out. My intuition tells me to expect that value to be hundreds of Ohms.

The math nearly always boils down to Ohms law I=V/R
Sometimes it is written I=E/R, but my grandfather taught me the law when I was nine or ten and I stick with V for Volts.

If you let Ohms law work for you, let's say to check the theoretical current "being pulled" though the g2 screen resistor, then you need to plug in some of your readings from your posts. I will walk you some of the way:
323V B+2 into 64 Ohms dropping to 317V gives us 6 Volt drop.
I = 6V / 64 Ohms = 0.94 (which x 1000 = 94 mA)
Now take a look at the 6L6GC data sheet and tell me what you find the maximum mA for the screens to be.
Then do the same calculation for 640 Ohm screen resistors, mull it over a bit, then get back to us with your conclusions and any questions.

[overtone]

Ian,
how long will you have the Silvertone for?
Do you know how to check the ratio of the OT?
best, tony

[showsii]

Ian, a few things don't add up now.

Looking at the schematic there is a 1k5 resistor "R37" dropping the voltage from B+1 360V down to B+2 345V. I am not seeing that kind of a drop on your posted voltages which have both B+1 and B+2 at 323V. Has R37 been taken out? Or maybe you took the B+2 reading "before" R37?

It may hang together with the above point, but to my mind the 64 Ohm screen resistor does not check out. My intuition tells me to expect that value to be hundreds of Ohms.

The math nearly always boils down to Ohms law I=V/R
Sometimes it is written I=E/R, but my grandfather taught me the law when I was nine or ten and I stick with V for Volts.

If you let Ohms law work for you, let's say to check the theoretical current "being pulled" though the g2 screen resistor, then you need to plug in some of your readings from your posts. I will walk you some of the way:
323V B+2 into 64 Ohms dropping to 317V gives us 6 Volt drop.
I = 6V / 64 Ohms = 0.94 (which x 1000 = 94 mA)
Now take a look at the 6L6GC data sheet and tell me what you find the maximum mA for the screens to be.
Then do the same calculation for 640 Ohm screen resistors, mull it over a bit, then get back to us with your conclusions and any questions.

Tony,

Here are some pictures of the output section. I have circled the screen resistors, or what I think are the screen resistors.. It is hard to determine what color that middle band is. I took my DMM and measured them and it came to be 70ohms, so I dunno.

Also, there is a picture with where I circled the 1.5k 1watt resistor (R37?), and circled the point where I took the B+2 voltage right at the cap can.

What can you gather from these pictures?

Also it looks like the Max signal screen current of the 6l6GC is 22mA..

hmmmmm something seems wrong then..? ha

The continued help is greatly appreciated.

Ian

[showsii]

Ian,
how long will you have the Silvertone for?
Do you know how to check the ratio of the OT?
best, tony

Ill have it for a while actually. Probably as long as I need it.


No I do not, I would love to learn though!

thanks

Ian

[overtone]

... I have circled the screen resistors, or what I think are the screen resistors.. It is hard to determine what color that middle band is. I took my DMM and measured them and it came to be 70ohms, so I dunno.

Also, there is a picture with where I circled the 1.5k 1watt resistor (R37?), and circled the point where I took the B+2 voltage right at the cap can.

What can you gather from these pictures?...
Ian

Great photos this time! My eyes make those resistors to be 68 Ohms too.
The shiny carbon comps look recent to me. By the way, the 1K5 at the cap can looks too small to me - but it may be the perspective in the photo.
What I cannot quite make out is how that second 1k5 resistor in the second picture is connected. If you follow the components from the cap can do you have this network going on:
cap can > 1K5 on the can > 1K5 on a free pin at the 6L6 socket > from here one 68 Ohm to each 6L6 screen pin 4?
That would be kinder to the screens, somewhere under 4mA.

Going to give you the OT measuring thing in a second post.

[showsii]

... I have circled the screen resistors, or what I think are the screen resistors.. It is hard to determine what color that middle band is. I took my DMM and measured them and it came to be 70ohms, so I dunno.

Also, there is a picture with where I circled the 1.5k 1watt resistor (R37?), and circled the point where I took the B+2 voltage right at the cap can.

What can you gather from these pictures?...
Ian

Great photos this time! My eyes make those resistors to be 68 Ohms too.
The shiny carbon comps look recent to me. By the way, the 1K5 at the cap can looks too small to me - but it may be the perspective in the photo.
What I cannot quite make out is how that second 1k5 resistor in the second picture is connected. If you follow the components from the cap can do you have this network going on:
cap can > 1K5 on the can > 1K5 on a free pin at the 6L6 socket > from here one 68 Ohm to each 6L6 screen pin 4?
That would be kinder to the screens, somewhere under 4mA.

Going to give you the OT measuring thing in a second post.

This flow as you stated is correct.

cap can > 1K5 on the can > 1K5 on a free pin at the 6L6 socket > from here one 68 Ohm to each 6L6 screen pin 4

The orange wire coming off the cap goes to that free pin where the other 1k5 and the 68Ohm resistors connect.

I will wait in suspense....

Ian

[overtone]

That is what I was hoping! Now for the last last major part of the puzzle, regarding the OT used in that old Silvertone 1483:

We can guess that the Silvertone OT was wound to provide a 6600 Ohms reflected load with the 8 Ohm speaker, but there is room for leeway. I think you started to see that in your thread about the Rocket OT taps. Some builders regard the OT rating as a kind of tone control and move beyond the data sheet specs. While you have the amp it would be great to find out to what ratio the OT was wound and how that works to get the sound you like.

I will try to keep this simple, it may look involved, but it is a doddle to do. An experienced tech with a Variac can get more accurate results, but the way below will get you in the ball park quickly.

First some ghetto theory: consider the OT to be a hunk of iron with two different windings, a bit like big resistors, side by side. One side is wound more than the other and that is why when you measure the Ohms you get different readings on each side. The ratio of the two voltages Primary / Secondary is what you need to know. This is the Voltage Ratio.

Math:
the Voltage Ratio squared x the speaker impedance = the reflected load "Raa" (Ra for an SE amp)

Using the following ghetto method you can grab the voltages on each side:
1. Take a 9V AC power supply, an old wall-wart will do fine. Connect the 9V AC to the primary windings of the OT, which are where the Plates of the output tubes are connected. Nothing goes to the CT. Now measure the voltage across those two primaries. This is Vin____AC. (Probably around 11.5V)
2. Measure the voltage on the secondaries giving you Vout_____AC (it will probably be a fraction of a Volt)

The Voltage Ratio (VTR) = Vin / Vout

…for example
on a dumpster OT I measure 11.5 Vin / 0.4 Vout = 28.55 VTR
with an 8 Ohm speaker I will get a reflected load of:
28.55 x 28.55 x 8 Ohms = 6520
which is pretty close to the Raa 6600 shown in the data sheets for 6L6.

If you have been paying attention you will see that if you connect the wall wart to the speaker side you will have a very high voltage on the primary. That way would be a bit more precise, but I recommend the wall wart into the primary to keep thing safe.

Anybody still reading this far?

You can fudge the rating of your OT by mixing and matching different speakers impedances or taps. For example, suppose I have found an OT from Grandpa which is marked for 4 Ohms:
I put in 11.5V (Vin) and get 0.39 out (Vout).
So I have a VTR of 29.5 (11.5 / 0.39)
29.5 x 29.5 x 4 Ohms = 3481
not much use for 2 x 6L6 in PP, but if you connect a 8 Ohm speaker load you have:
29.5 x 29.5 x 8 Ohms = 6962, which is a little over - really close enough.
BUT,
the question that I would now have is: can Grandpa's iron actually handle the current from the 6L6 tubes? With no current handling data for the OT it will be a shot in the dark, possibly followed by some smoke.

It is a similar dilema for the OT which you asked the forum about in a post earlier. That one has no current handling data but that I think you should get information that from Hammond.

Or someone here knows that the 1750H "keeps the smoke in" with 2 x 6L6 in PP,
or you can take an informed guess from a bit more math...

[M Fowler]

I see Weber was offering this OT:

Better known as that funky Sears Silvertone Output transformer, I just had to do this one after I heard a Sears Silvertone 1484 amp. 50 watts, 4K : 4,8. Great overdrive tone, really juicy. This is a through-chassis mount.

[sneakers563]

Did you guys ever figure out the turn ratio of the OT? I'm about to embark on a 1483 build from scratch.

I've seen that Weber OT, but I believe they came up with that for a planned 1484 kit that never materialized. The question is whether the 1483 had the same 4K primary.

Incidentally, if showsii is still listening, how effective is the tone stack for guitar? It seems quite primitive. It looks like they used the same stack in the 1474, but they switched to a Baxandall design for the 1484 and 1485.

For my build, I'm thinking about swapping in a different stack on one of the channels, either the 1484 stack or the ubiquitous blackface Fender stack.