Octal 6g3

[jbones1]

Hi guys,
Long time lurker first time poster.

I'm in the planning stages of a new amp build and I'd like to get some opinions on my layout. The amp is basically a Fender 6g3 clone. Mods are: single channel, single input, no trem, and octal pre 6sc7's.

I'm planning on running Classic Tone 40-18066 PT and 40-18064 OT

Here is my layout below. Anyone see anything that is going to blow up?!

Thanks,
-Jon

[loudthud]

Diode is backwards in the bias supply, The 100K will get quite hot. Make room for a 2 watt flame proof. The pot is basically in series with the 100K. It may smoke. Why not use the bias tap on the transformer?

You haven't shown where the centertaps of the transformer are grounded. Be prepaired to re-do the entire ground if there is too much hum.

Where does pin 2 of the first 6SC7 go? The amp won't work without it. I didn't trace out the whole thing, there could be more problems.

[printer2]

Diode is backwards in the bias supply, The 100K will get quite hot. Make room for a 2 watt flame proof. The pot is basically in series with the 100K. It may smoke. Why not use the bias tap on the transformer?

You haven't shown where the centertaps of the transformer are grounded. Be prepaired to re-do the entire ground if there is too much hum.

Where does pin 2 of the first 6SC7 go? The amp won't work without it. I didn't trace out the whole thing, there could be more problems.

Talk about raining on a parade.

[jbones1]

Talk about raining on a parade.

Nah, I'm a newbie, and I came here for advice. I really do appreciate the help! This is in fact my 4th amp project, but my first scratch build. (the rest have been kits, 5f1, 5e3, and a Watts Supro clone.)

I've modified my layout to reflect your thoughts. Not sure how I missed that lead on pin 2!

I've never done an adjustable fixed bias set up before so I am going from other schematics I have. Does this modified version look better? The ground will go to the chassis bolt on the PT.

My understanding on the 6v center tap is that it shouldn't be grounded since the supplies are lifted with the two 100ohm resistors?

If anyone has any further critiques I am genuinely interested in hearing them!

Thanks again,
Jon

[Enzo]

Your 6v needs to be referenced to ground somehow. Two 100 ohm resistors is fine, grounding the center tap is fine. Don;t do both. The pair of resistors is creating what we call a virtual center tap. It is generally used for transformers that have no center tap on the 6v. DO one or the other. I have no reason to prefer the resistors, they are just extra parts to need and solder in place, when the center tap is already there and is one wire soldered to ground. But absolutely nothing wrong with either way.

Speaking for myself, I SURE wish we could have a schematic rather than the layout. No, I mean in addition to the layout. I think in terms of the circuit, and I have to trace out the circuit either in my head or on paper, when all I have is the layout. You say it is a 6G3 clone, but ditching a channel and an input and the trem and changing the tubes is not a clone, it is more of a tribute. So... And it makes it easier for us to see if your layout matches the circuit you are trying to build.

I just looked at your bias supply. You have the variable element on top, a 50k pot. The bottom is a 22k resistor. (top and bottom elements of a voltage divider.) This may well work, but I don;t like the method. If the wiper on the pot fails, your upper resistor defaults to 50k. That results in the LOWEST possible bias voltage. I prefer to make the lower element the adjustable one, then if the pot wiper fails, we are left with the HIGHEST posssible bias voltage. The amp will continue to work, though maybe with less than stellar tone. By defaulting to the lowest bias voltage, the amp will continue to function, possibly burning up your tubes in the process. The pot is under little or no stress, so WILL it fail? Probably not, but... famous last words.

[jbones1]

Your 6v needs to be referenced to ground somehow. Two 100 ohm resistors is fine, grounding the center tap is fine. Don;t do both. The pair of resistors is creating what we call a virtual center tap. It is generally used for transformers that have no center tap on the 6v. DO one or the other. I have no reason to prefer the resistors, they are just extra parts to need and solder in place, when the center tap is already there and is one wire soldered to ground. But absolutely nothing wrong with either way.

Speaking for myself, I SURE wish we could have a schematic rather than the layout. No, I mean in addition to the layout. I think in terms of the circuit, and I have to trace out the circuit either in my head or on paper, when all I have is the layout. You say it is a 6G3 clone, but ditching a channel and an input and the trem and changing the tubes is not a clone, it is more of a tribute. So... And it makes it easier for us to see if your layout matches the circuit you are trying to build.

I just looked at your bias supply. You have the variable element on top, a 50k pot. The bottom is a 22k resistor. (top and bottom elements of a voltage divider.) This may well work, but I don;t like the method. If the wiper on the pot fails, your upper resistor defaults to 50k. That results in the LOWEST possible bias voltage. I prefer to make the lower element the adjustable one, then if the pot wiper fails, we are left with the HIGHEST posssible bias voltage. The amp will continue to work, though maybe with less than stellar tone. By defaulting to the lowest bias voltage, the amp will continue to function, possibly burning up your tubes in the process. The pot is under little or no stress, so WILL it fail? Probably not, but... famous last words.

Thanks Enzo

Here is the schematic I've been working from. It is a single channel version of the original 6g3 Fender schematic I pulled off the net. I've obviously made some changes to it in the layout. New preamp tubes, beefed up filter caps, modern OT and PT etc.

On the 6v center tap: In my previous fender builds I always ground lifted because it was my understanding that this is a quieter way to wire these amps? If it doesn't really matter you are right it seems like just using the CT would make more sense.

As far as the bias setup. Do I just need to swap the pot and resistor locations or do any of the other components need reorganization?

-Jon

[Enzo]

The bias pot was just my own bugaboo, the other guys may think I am overly prissy. If it were me, I'd make the 22k the variable and the 50k a resistor. I would not use those values however. You will have to decide on values. More likely than not, your upper resistor could become the 22k and the lower resistor could be your 50k pot.

[Enzo]

Fender used the two resistors instead of center taps in a zillion amps because two resistors cost less than adding a center tap to the transformer. Not because it worked better.

[Bruce / Mission Amps]

Bias Supply:
We have NO idea what power transformer you are using, with what Hi-V secondary AC voltage is being delivered, and with all the tubes installed.... so this is all speculation
but if it is close to a real 6G3, I think when you actually get around to building this, you'll find that a 25K trim pot, (with respect to ground) sitting UNDER a 6k8 to 10K resistor will give you better results.
Use a 2 watt 100K resistor for safety and at least a 47uF-100v reverse biased cap.

[jbones1]

Bias Supply:
We have NO idea what power transformer you are using, with what Hi-V secondary AC voltage is being delivered, and with all the tubes installed.... so this is all speculation
but if it is close to a real 6G3, I think when you actually get around to building this, you'll find that a 25K trim pot, (with respect to ground) sitting UNDER a 6k8 to 10K resistor will give you better results.
Use a 2 watt 100K resistor for safety and at least a 47uF-100v reverse biased cap.

Hi Bruce,
Thanks for the advice.

The PT I'm planning on using is:

660v@120mA
50v Bias

6.3V@3A with center tap

5v@3A

[Bruce / Mission Amps]

OK. The 50v tap is relative to the center tap of the high voltage winding.
If the transformer is spec'd correctly, the 330vac from center tap to either outside winding will be higher then this when you build the amp because... you will not be drawing 120ma form this PT.
Therefore I suspect the 50vac bias tap will be slightly higher too.

Anyhow, a +50vac bias tap is pretty high for a 6G3... after rectification and filtering, you'll be dealing with something around negative 65 to -75vdc.
That is quite a bit considering you probably only want around -40vdc.
So, it seems to me that your bias supply will be a bit experimental until you find the right combination of resistors for the pot and divider.

[jbones1]

OK. The 50v tap is relative to the center tap of the high voltage winding.
If the transformer is spec'd correctly, the 330vac from center tap to either outside winding will be higher then this when you build the amp because... you will not be drawing 120ma form this PT.
Therefore I suspect the 50vac bias tap will be slightly higher too.

Anyhow, a +50vac bias tap is pretty high for a 6G3... after rectification and filtering, you'll be dealing with something around negative 65 to -75vdc.
That is quite a bit considering you probably only want around -40vdc.
So, it seems to me that your bias supply will be a bit experimental until you find the right combination of resistors for the pot and divider.

Right, so this is why I originally had forgone the 50v bias tap altogether and just bled off of the main voltage supply for the bias ala the original fender design. (But with an adjustable pot) Does this seem like a better way to go? I don't mind doing a bit of experimenting, but as I say I'm a bit of a newbie when it comes to some of this stuff
Thanks,
Jon

[HBamps]

I have built almost this same amp with a couple changes. This set up will give you possibly way more gain than you will need hitting the PI. It can take a full tone stack between V1 and V2 (maybe the octals have less gain).

For example, I put a full TMB tone stack there, have 100k's on V1 and V2, voltage divider on V2 (like you), and a .005 coupling cap to PI. Plenty of low end and gain.

[jbones1]

I have built almost this same amp with a couple changes. This set up will give you possibly way more gain than you will need hitting the PI. It can take a full tone stack between V1 and V2 (maybe the octals have less gain).

For example, I put a full TMB tone stack there, have 100k's on V1 and V2, voltage divider on V2 (like you), and a .005 coupling cap to PI. Plenty of low end and gain.

Hmm...
Not sure what to do about that? I'm building this into an old radio chassis so I don't have room for all extra knobs. Was planning for input, volume and tone. I might be able to do one more pot. Any ideas? maybe a gain control?

I do want a fairly gainy amp but not if its going to be uncontrolable!

-Jon

[HBamps]

Hmm...
Not sure what to do about that? I'm building this into an old radio chassis so I don't have room for all extra knobs. Was planning for input, volume and tone. I might be able to do one more pot. Any ideas? maybe a gain control?

I do want a fairly gainy amp but not if its going to be uncontrolable!

-Jon

Just try it and see. You can always tweak after. The schem above already has the voltage divider on V2. Just my personal option but I would drop the V1 plate R and lower the input cap to the PI to around .005. Both very easy adjustments later.