6G15 Fender Reverb unit build questions

[Raidermofojeff]

I'm building a 6G15 rev unit and have some questions for techs or knowledgeable people who have built one of these things. I am using the layout from Triode electronics and transformers from them, chassis built by me, and I sourced the parts.
These are the layouts I'm working off of.
http://site.triodestore.com/6G15ReverbLayoutV2.pdf
http://site.triodestore.com/6g15a.pdf
I'm building version 1 but with the sideways mounted chassis like version 2.
A little more straight forward have the filter caps on the main board inside.

My issues are regarding isolating jacks

1-The layout shows Switchcraft type Jacks for the input and output. I am not grounding at the input and certainly not the output and have installed isolation washers on both. (OK?)

2-I also intend on using Isolation washers on the foot switch jack. (OK?)

3- We also have 2 RCA jacks for reverb tank send and return and I'm thinking I should also isolate those?

Please share your thoughts and experience on these particular issues, I thank you in advance.
If you built one and didn't isolate any of this stuff and it's quiet, tell me about it.......
Did Fender isolate this stuff? All or some?

 

[Winnie Thomas]

1-The layout shows Switchcraft type Jacks for the input and output. I am not grounding at the input and certainly not the output and have installed isolation washers on both. (OK?)

****** Ground the input, but not the output

2-I also intend on using Isolation washers on the foot switch jack. (OK?)

******* Don't do that, it needs a ground to operate

3- We also have 2 RCA jacks for reverb tank send and return and I'm thinking I should also isolate those?

****** The cables need to be grounded to avoid unwanted noise.

 

[Jeff Gehring]

If you are looking to avoid the ground loop hum that will result from blindly building a 6G15 per Fender's old layout, but with the addition of a grounded AC cord green wire connected to the chassis, you might try building the unit with what is called an isolated ground buss. Fender does this with the current production tube reverb units, and it does work to minimize the hum that would otherwise result.

The problem arises from the fact that the reverb unit chassis is grounded with the AC ground wire, and the circuit and signal ground is also connected directly to chassis. This then is connected via a shielded patch cord to an amplifier, whose chassis is also tied to AC safety ground and circuit/signal ground. What ends up happening is that the difference in potential between the reverb unit's chassis and the amp's chassis appears as an input signal to be amplified by the amp.

This is defeated in the Fender isolated ground scheme by insulating ALL jacks on the reverb unit, and collecting ALL the circuit grounds to an internal buss which is NOT connected to the chassis. Then a 16 ohm resistor and a few other safety components are used to connect the isolated buss to the reverb unit chassis. In the reverb unit the ONLY connections to the chassis should be the following:

• the AC cord green wire, and
• the 'low' side of the 16 ohm resistor and other safety components.

NOTHING else should connect to the reverb unit chassis. You don't need to insulate the pots, just don't use the pot cases as a ground. It is not hard to implement this scheme when you are building a new unit, it is not so much fun to do it as a fix/mod after a unit is already improperly built.

Here is a diagram that explains the function of the isolated ground buss. The extra safety components I mentioned are just a pair of rectifier diodes, both paralleling the resistor, but with the cathode of one diode connected to the isolated buss, and the cathode of the other diode connected to the chassis -- they together limit the DC potential difference between the isolated buss and the chassis to one diode drop, about 0.6V. The other safety component isn't really for safety, it is to eliminate RFI/EMI by decoupling the isolated buss. It is a 47nF cap connected between the isolated ground buss connection at the input jack sleeve lug, and chassis ground.

Sorry to be so long, but it is what it is. I have a layout drawing that I modified to show an isolated ground buss installation in a 6G15 if you'd care to see it. Here's that diagram I mentioned earlier:

 

[Mattbedrock]

Wow, that is a cool design, Jeff G.

I built mine pretty much stock and it came out pretty quiet. Probably just dumb luck. Here's a link to my build thread, RMFJeff.
https://www.thegearpage.net/board/i...hread-6g6-blonde-bassman-6g15-reverb.1281827/

Best of luck and keep us posted!

 

[Raidermofojeff]

Thank you for the responses!
Winnie, I know these things need to be grounded, I was more questioning should I isolate them at their location with ground wires running to a buss, should of been more specific.

Jeff, I would very much appreciate a copy of your layout, as I do your thoughtful response.
I am somewhat familiar with the type grounding you describe, would it be the same diode value as the rectifiers in the amp?
I am still concerned about jack grounds, and would guess your layout has them running to the buss. By all means, post as long as you will, I will try and soak up all I can.....Much respect..

Matt, I actually have been to your thread, I usually do a lot of looking around before posting this stuff.. I also do my own chassis, in this case using 5052 Aluminum .063......17$ and labor..Havn't tryed brazing though!.. Also have a HF dovetail jig, looks exactly like yours...... Must be different though, mine only does half blind.....unless I missed something

 

[Jeff Gehring]

Hey, Matt, if you want to hear the ground loop hum of a unit built without the isolated ground buss at its worst, plug the reverb unit into an amp with an instrument cable, plug in the power cords of both the amp and reverb unit, turn on the amp, and crank the volume all the way up. The reverb unit doesn't even need to be turned on. Then, while listening to the hum, unplug the power cord of the reverb unit only. The hum that goes away is the ground loop hum. It's not objectionable if you always keep the volume on your amp low, but if you are a player that likes to crank the amp and run the volume from your guitar, it is a problem. If the reverb unit and amp are plugged into the same outlet that helps suppress the problem. I like your finger-joint jig in your build thread, BTW!!

Oh, and the circle in the diagram with a sine wave inside is a schematic depiction of a voltage source, which represents the instantaneous voltage difference between the reverb unit chassis and the amp chassis.

Here's the layout of a Weber 6G15 modified to show an isolated ground buss scheme. On the drawing it refers to a brass tube used as a ground buss; it really doesn't need to be that, it can be whatever wire you want to use for your buss. The brass tubing is a little easier to solder than a large copper conductor.

Just to reiterate, with an isolated ground buss, the only connections to the chassis should be the AC cord green wire and the low side of the 16 ohm resistor, diodes and RFI/EMI decoupling cap. All other grounds go to the isolated buss. When you are all done building the unit, a good indicator that it was done properly is if you take an ohmmeter and measure between the chassis and the sleeve of the input jack. This should measure as 16 ohms; if it is a short, there is a screw-up somewhere.

 

[Raidermofojeff]

Thank you very much Jeff.......I feel more confident in my direction...I have been known to play loud! That .047uf cap off the input doesn't appear to be polarized-correct?

 

[Jeff Gehring]

Yes that's right. It's just a 47nF polyester cap, no polarity expressed or implied...

 

[Mattbedrock]

Ok Jeff, I ran your experiment. Your right, the hum is there. I just never really noticed it. It only is significant when the amp is cranked. I have my reverb right now plugged into my Brownface Super that I rarely run above 4 or so. At that level, it is barely noticeable. Then again, I am deaf as a post after too many years on too loud stages.

Thanks for the great info. Maybe someday I'll retrofit mine. Maybe.

 

[Jeff Gehring]

Matt, it's kind of like a tree going to the bathroom in the forest, if nobody hears it, did it happen? If the hum isn't a bother with the setting you usually run the amp at, I'd say there's no need to bother about doing a retrofit, really. When I was designing the Revibe for Ted Weber, the first prototype didn't have the isolated ground buss. We both thought it was fine, and they tooled up to do the kits that way. But that residual hum kind of grated on my nerves. I know a lot of players who use reverb units who do crank their amps, so for all later builds that I did of the Revibe and of stand alone reverb-only units, I incorporated the isolated ground buss to take care of that problem.

 

[Raidermofojeff]

My next question is location and orientation of the OT and choke.......Should I line em' up just like on a Fender, how I believe Matt did it (my preference), or is that problematic? I can see how the Weber design goes(or Jeff's design?), but i have already drilled the wire holes per Triode version....(which I think is the same as Fender)....the Triode layout only shows the inside of the chassis...

 

[Jeff Gehring]

Here's a picture of a Fender, the PT goes inside the chassis and the choke and reverb drive transformer go on the outside as you see them here. Not the greatest shot for showing the location, but I'm sure you get the idea. The cores on the choke and drive transformer are lined up perpendicular to the long axis of the chassis. Works fine.

 

[Raidermofojeff]

Actually IMO a pretty good picture---being able to see all 3....I knew the Fender placement, but it struck me odd having the OT and PT facing the same direction. Maybe not an issue being on opposite sides of the chassis. So I thought I'd ask, just in case ......just laid it out- Thank you very much once again...

 

[Jeff Gehring]

If you are using an aluminum chassis, you might rotate either the PT or the OT 90°. If it's a steel box, it's not much of an issue.

 

[Raidermofojeff]

My chassis is 5052 .063 Aluminum, pretty stout, Guess I'll rotate the OT..
Very interesting to hear of different characteristics with metals. If steel is more forgiving with oscillation, this is something I'll consider for the future...

 

[Jeff Gehring]

The deal is that steel is a ferromagnetic metal, and will provide a measure of magnetic shielding in addition to electrostatic shielding. That's why they put the PT inside the chassis, to take advantage of the magnetic shielding that the chassis provides. Aluminum has no ferromagnetic properties at all, and so provides only electrostatic shielding. There are trade-offs with everything; aluminum is a better conductor than steel, and so is a better electrostatic shield material...

 

[Raidermofojeff]

I don't know why, but this hasn't worked very well for me......... I used a 3 1/2 inch piece of 3/4 inch brass round bar mounted below my input jack, and it hums more with the resistor and diodes grounded to chassis than just a wire. The hum isn't too bad until the gain on the amp is turned up and the amp ain't high gain...The mixer knob tends to vary the frequency of the hum, but not change the volume of it noticeably.. I've chop sticked around without any solution...
Worse case was when I had it grounded by the pilot light like the Triode layout.....Wouldn't recommend that....
Triode never responded to either of 2 inquiries I made, apparently I didn't spend enough to warrant answering questions....
I've had this thing going for a while, and I haven't yet tried it on either of my two ungrounded amps, I'll get back on that one......

Hate the thought of it, but I might need to consider moving my OT around...... or maybe use of an aluminum chassis was a mistake....

 

[Raidermofojeff]

I plugged it into my 63 Deluxe today and the reverb build actually made the amp a little quieter, which is a delight...... I've probably over emphasized the hum issue......
I plugged back into the amp I've been playing most of the time lately, my grounded AX84 P1EX build...... and the hum isn't really very bad.... I'm picky! It probably hums about as much as a decent vintage unit..... Overall the time and money was well worth it, even if I never get it perfect....
Bottom line, I love this thing!

Two things I may do when I get the gumption to open the amp back up.......
#1- the 2 100ohm resistors grounding my light/heaters are going to chassis at the bulb......I'll run them to the bus that grounds by the input jack.....
#2- Install a ground lift switch to the output jack......

Another thing, is with my Marshalls I usually run an Airbrake build I made a few years ago...
With the attenuator in place it actually quiets the Marshalls a little...... and hopefully with the reverb in the path, it will benefit from this as well.....

Peace

 

[slorinczi]

Here is a diagram that explains the function of the isolated ground buss. The extra safety components I mentioned are just a pair of rectifier diodes, both paralleling the resistor, but with the cathode of one diode connected to the isolated buss, and the cathode of the other diode connected to the chassis -- they together limit the DC potential difference between the isolated buss and the chassis to one diode drop, about 0.6V. The other safety component isn't really for safety, it is to eliminate RFI/EMI by decoupling the isolated buss. It is a 47nF cap connected between the isolated ground buss connection at the input jack sleeve lug, and chassis ground.

Wondering if I could tack a question or two on here. I've just built a standalone reverb unit, loosely based on the classic Fender design, but incorporating an old Hammond "necklace" reverb tank. The unit sounds good, but has more hum than I'd like (sounds like power supply to my untrained ears).

I already implemented the isolated buss ground as recommended in Jeff's earlier post, but I'm curious about what I can do to lower this hum. One source recommends grounding all electrolytics to one point and everything to another (on the same buss bar); another source recommends grounding all electrolytics associated with the preamp section to one lug, those associated with output to another.

I'll play with lead dress further (reverb is crammed inside a very tight little chassis) and I'm thinking it'd be a good idea to change the fake 100 Ohm center tap of the heater circuit to a proper elevated supply (and bypass cap?) as there's a cathode-follower 12au7 added to this build.

But assuming the work and layout are at least reasonably clean, I'd love to hear any further ideas for lowering this noise floor....

 

[eolon]

The deal is that steel is a ferromagnetic metal, and will provide a measure of magnetic shielding in addition to electrostatic shielding. That's why they put the PT inside the chassis, to take advantage of the magnetic shielding that the chassis provides. Aluminum has no ferromagnetic properties at all, and so provides only electrostatic shielding. There are trade-offs with everything; aluminum is a better conductor than steel, and so is a better electrostatic shield material...

Also, these is so much room in the chassis between the PT and the circuit. There is physical isolation between the circuit, which minimizes coupling. I built a 6GRV, the earliest model (with half-wave rectifier) and had no problem with noise or grounding, although I did make a brass grounding plate for the pots. Here: https://hardway.quantum-foam.com/fender-reverb-unit-6g15---6grv.html

Just FYI,
Best Regards,

Don