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SurfGuitar101 Forums » Gear »

Permalink 6g15 reissue rebuild questions!

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Would this rectifier layout work...

image

jbennett wrote:

So, is this the full wave rectifier circuit that I'll
need with my current PT?

image

Correct

If so, in the original unit there is ONE wire that
leaves the rectifier and goes to the choke, and the
caps under the doghouse. With a full wave rectifier the
two wires that leave the rectifier circuit would then
go to the same eyelet on the board, right? Wrong?

Wrong. Only one wire goes to the business (+ve) end of the first (reservoir) filter cap. The other wire from the rectifier goes to the ground side of that filter cap (the 'bottom' end of the 'load' in that picture). Which way around you do it will determine the polarity of the DC rail. One way makes it a B+, the other way makes it a "B-". And it is imporatnt to pay attention to what you get, because with electrolytic caps, the polarity of the caps has to be right w.r.t. the rectifier, or the caps will explode, so...

For a conventional B+ (as oppposed to a B-), the wire from the junction of the two diodes that are joined at their cathode (banded) ends, goes to the +ve side of the reservoir filter cap, and the wire from the junction of the other two diodes' anode (non-banded) ends, goes to the -ve end of the reservoir filter cap.

Check this site out - http://www.freewebs.com/valvewizard/

He who dies with the most tubes... wins

Last edited: Feb 09, 2012 23:29:16

jbennett wrote:

Would this rectifier layout work...

image

No. It will blow up the PT. See my previous post

He who dies with the most tubes... wins

Thanks again.

So, with the load in-line...

image

That wouldn't explode, would it?

and thanks for the VW link. Looks like essential reading.

Last edited: Feb 10, 2012 06:39:04

jbennett wrote:

Thanks again.

So, with the load in-line...

image

That wouldn't explode, would it?

That's correct. (FWIW in a conventional schematic, the filter caps are usually drawn the other way up - with the +ve end at the top, altho' in the amp layout, the +ve end is usually closest to the bottom of the chassis. Just sayin'.)

He who dies with the most tubes... wins

Last edited: Feb 10, 2012 10:54:49

Thanks. And yes, the cap would be oriented +up in the doghouse.

So, next question...

With the stock transformer, we've got:

black wire coming in from the fuse.
White wire coming in from the switch.
then the two green wires go out to the pilot light and on to the heaters.
and then the two red wires go into the rectifier.

On the original PTs there was a red/yellow & green/yellow wire that went to ground on the chassis. So, no grounding wires coming from the transformer. I guess the transformer grounds out through the capacitor (as seen in my crude drawing above?

jbennett wrote:

So, next question...

With the stock transformer, we've got:

black wire coming in from the fuse.
White wire coming in from the switch.
then the two green wires go out to the pilot light and
on to the heaters.
and then the two red wires go into the rectifier.

Yup

On the original PTs there was a red/yellow &
green/yellow wire that went to ground on the chassis.

The red-yellow was the grounded side of the HT (high tension) winding (which you do with 1/2-wave rectification). (See the original 6G15 schematic where one end of the HT winding is grounded)

The green-yellow was the centre tap for the heater winding, which you ground. (You can elevate the heaters, and hence eliminate a potential source of hum otherwise induced by heaters, by grounding the centre tap to Pin 8 of the 6V6/6K6 socket, which puts the mid-point for the 6.3VAC swing on heater winding at the same voltage as the output tube cathode).

For a PT without a centre tap on the heater winding, you make an artificial ground reference either with:

1) 2 x 100R resistors, one from each end of the heater winding*, going to ground (or alternatively to the output tube cathode as described above); Or

2) a 250R 'humdinger' pot with each end of the pot going to a respective end* of the heater winding and the pot wiper going to the grounding point/output tube cathode. This allows you to fine-tune the cancellation of hum from the heater circuit.

  • the usual connection point for these resistors/humdinger pot is from the terminals on the 6.3V lampholder.

So, no grounding wires coming from the transformer. I
guess the transformer grounds out through the capacitor
(as seen in my crude drawing above?

See my discussion above about ground referencing for the heater winding.

For a FW bridge rectifier, the ground reference is the node where the 2 diodes which are joined by their anode (non-banded) ends, are connected to the ground return at the -ve end of the first (reservoir) filter cap.

He who dies with the most tubes... wins

Last edited: Feb 10, 2012 11:42:34

I modified this before I saw your response. I'll revise it incorporating your new info...

image

Last edited: Feb 10, 2012 11:43:22

I think this is a photo of what you are talking about with the two resisters to ground/heaters:

image

image

Last edited: Feb 10, 2012 15:53:25

Yep. The top photo is those 100R going straight to the cathode for ground elevation on the output socket pins (between Pins 2 and 8 on the one side, and between Pins 7 and 8 on the other side)

He who dies with the most tubes... wins

So, I'm wondering now, is it both or just one of the other. The top photo shows the two resistors (thanks for clearing up exactly what pin is what in that photo, it's hard to tell) and the bottom shot is of the same resistors, but running from the other end of the heaters at the lamp going to the chassis ground.

And all of this is happening on the 6K6 octal socket.

Do both of these sets of resistors have to be installed in order for the hum to be defeated?

I'll modify my drawing with the resistors on the heater wires.

And, again thank you.

J

Last edited: Feb 10, 2012 19:39:19

jbennett wrote:

So, I'm wondering now, is it both or just one of the
other.

One or the other. Not both

And all of this is happening on the 6K6 octal socket.

The output/reverb driver tube socket whether it be 6K6 or 6V6 or 6F6 or... any acceptable tube that will work in that position

Do both of these sets of resistors have to be installed
in order for the hum to be defeated?

Nope - just one or the other

He who dies with the most tubes... wins

Last edited: Feb 10, 2012 20:18:03

image

So... that might do it?

(hard to see I know but I've got a resistor going from 2-8 and another from 8-7)

And that's a pair of 100 ohm resistors (brown/black/brown/gold).

Tomorrow I drill my board and mount the turrets. I'll post some photos.

Thanks.

Last edited: Feb 10, 2012 21:54:08

jbennett wrote:

So... that might do it?

(hard to see I know but I've got a resistor going from
2-8 and another from 8-7)

And that's a pair of 100 ohm resistors
(brown/black/brown/gold).

yes that will be fine

He who dies with the most tubes... wins

Two boards done...

image

Looks about right

He who dies with the most tubes... wins

Here is my drawing for the doghouse...

image

If there is enough room inside the chassis next to the main board (and I'm guessing there probably will be), why not mount the filter caps inside? (Its easier to get to than a doghouse).

He who dies with the most tubes... wins

There is enough room. The choke and OT are on the backside of the chassis and they are no taller than the 1.25" high doghouse covers. I think it will fit fine. I want to make this as accurate as I can to the original reverb units and it really only requires two larger holes with rubber grommets around the edges and four small screw holes.

AFAICT (which is Irish for 'a faict' Big Razz ), the main reason fender used a doghouse was to make more space inside the chassis. My theory is that he got so used to this modus operandi in building his amps, that he just started doin' it all the time. So even though there was plenty of real estate inside the chassis in the original 6G15, even with the PT mounted inside the chassis, the caps still went outside.

image

(Although I think you could still have fitted the caps inside)

The only reason I suggested it is that its easier/slightly-less-dangerous to work on an amp where the filter caps are inside the chassis with the rest of the circuit. You don't have to keep turning the amp over with the doghouse cover off to put meter probes on the filter caps etc.

He who dies with the most tubes... wins

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