Reverb 6G15 clone doubts

maybeoneday

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Hi guys,

I am about to build a Reverb Unit using the Triode Electric schematic but I got few doubts about the PT I should be using.
Being in Europe and operating in 230/240V I found the Hammond 261D6 to be as close as possible to the original specs.
Here come the doubts about the grounding as the Hammond got the CT for 6.3V and HT.
Also I guess I am overthinking but it is the first time (in my anyway very short experience about amps) that I see the second red cable of the PT secondary that goes to the main ground. Wiring it like that, should I not get a short to ground?

How do you think is the best way to ground the amp using the Central taps?
How about to follow the grounding of Rob's 5E3 layout?

Thank you in advance!
 

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sudogeek

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I've built and modded a number of these over the decades. I would recommend:
This schematic by Tubeswell on this forum. There are a number of build threads on TDPRI which will be helpful. This includes a "hum blocker" which is very useful.
I use a Mojo 779 transformer (250V red-red) but given Euro voltages, the above suggestion of Hammond 290WEX should work fine.
Use a bridge rectifier instead of the half wave rectifier in the vintage 6G15 schematic. (With a bridge rectifier, the Mojo 779 hits the proper B+ voltages right on.)
Consider adding a 4th tube buffer/boost a la the silverface tube reverb.
Good luck and have fun.
 
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Lowerleftcoast

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I guess I am overthinking but it is the first time (in my anyway very short experience about amps) that I see the second red cable of the PT secondary that goes to the main ground. Wiring it like that, should I not get a short to ground?
This PT with center taps will work... and a PT without a CT will work.

This is a very simplistic description of what is happening in a very small nutshell. Don't quote me, hah.

The PT is fed with 240VAC on the primary winding. The secondary winding will put out ~500VAC (red wire to red wire) when it is loaded in the circuit. The rectifier will separate the upper half of the waveform and the lower half of the waveform taking ~250VAC from each side of the secondary (red wire to CT). When the 250VAC is rectified into VDC, the voltage will be similar to a battery (DC). The B+ will be like the positive side of the battery and the CT will be like the negative side of the battery.

Try to *imagine* the secondary of the PT and the rectifier and the reservoir cap, (all three) as the *battery* that powers the circuit. (Just like a battery positive and negative.)

Think of powering a flashlight (torch). There is a battery and a load (bulb). The metal case (chassis) holding the bulb does not have to be connected to the battery for the flashlight (torch) to work. (A wire can do that.) It is your choice to use the chassis as a return path for the current or use a wire.

In our audio circuit each tube is a load. It is more complex than a flashlight (torch) where the bulb is the load, but it is very similar (just more bulbs, I mean loads). The DC is doing most of the work. Just like the flashlight (torch), the DC really doesn't need to be connected to the chassis and the chassis does not have to be a return path for the current.

That said... there is *some* AC we are dealing with and in an audio circuit we want to get rid of the AC we do not want to hear. We have some AC ripple current that is leftover from rectifying the AC to DC and we have some AC signal current from our guitar pickups. We want to connect the DC negative to the chassis (reference to ground) to allow the unwanted AC to return to ground through the power cord ground. (Like an actual rod driven into the dirt.)

What we want from the chassis...
We want a safety ground so we don't get zapped. We also want the chassis to act as a shield to keep out radio frequencies and other hash.

Of course it is much more complex than this. There is a lot to learn and that is where the fun is.
 

Lowerleftcoast

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I reviewed the Triode Electric schematic. Imo, you would be much better off with the one suggested by @sudogeek

The Triode Electric displays a half wave rectifier which will be comparatively noisy. A full wave or bridge rectifier would be superior.

The ground scheme of the Triode Electric might create a ground loop when connected with other equipment. The one sudogeek recommended should keep that from happening.

I would make a small change to the schematic sudogeek provided. I would connect the HT CT directly to the reservoir cap to keep that distance as short as possible. See below.

6G15 grounding with Hum Loop Blocker.png
 

maybeoneday

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A lot to read and thank you for the explanation @Lowerleftcoast !!

The bad news is that I already bought the 261D6, anyway from the mod of Lowerleftcoast i understand that I can make use of the CT.

To recap all info I gathered it should be kind of the schematic below:
1652864100449.png


Is it something like that or there is a ZAP on the way?
 

Lowerleftcoast

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Below shows the schematic for a bridge rectifier. It does not use the hardwired PT CT. It creates an artificial HT CT. If you use this type of rectifier, heat shrink and tuck away the hardwired PT CT. IOW do not use both the hardwired CT and the artificial CT.

1652864100449.png

Below shows wiring diodes to form a full wave rectifier using the hardwired HT CT. This is similar to the rectifier on the schematic sudogeek provided. (That schematic shows capacitors in parallel with the diodes. The capacitors help to smooth the switching action of the diodes to make it quieter. Capacitors can be placed in a similar fashion parallel to the diodes of a bridge rectifier to make it quieter as well.)

1652864100449-2.png
 

maybeoneday

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Ok then.

Thank you for the help, I will follow this grounding.

@sudogeek what do you mean with add a 4th tube buff/boost?
 
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maybeoneday

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I reviewed the Triode Electric schematic. Imo, you would be much better off with the one suggested by @sudogeek

The Triode Electric displays a half wave rectifier which will be comparatively noisy. A full wave or bridge rectifier would be superior.

The ground scheme of the Triode Electric might create a ground loop when connected with other equipment. The one sudogeek recommended should keep that from happening.

I would make a small change to the schematic sudogeek provided. I would connect the HT CT directly to the reservoir cap to keep that distance as short as possible. See below.

View attachment 984515
just to be sure, should I connect the buss bar to the HT CT and to the noise buffer scheme (0.1 cap, 15ohm res) and then to the chassis?
 

Lowerleftcoast

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just to be sure
If you follow that schematic, the only electrical attachments to the chassis will be the power cord safety ground, and the buffer. I would have the buffer ground terminate near the input jack.

(Note: The ground bus is isolated from the chassis. The jacks are isolated from the chassis. The schematic does not show the reverb tank. The tank chassis should act as a shield. It will need *one* attachment to the 6G15 chassis.)
 

maybeoneday

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If you follow that schematic, the only electrical attachments to the chassis will be the power cord safety ground, and the buffer. I would have the buffer ground terminate near the input jack.

(Note: The ground bus is isolated from the chassis. The jacks are isolated from the chassis. The schematic does not show the reverb tank. The tank chassis should act as a shield. It will need *one* attachment to the 6G15 chassis.)
It means the tank should be grounded to the chassis but not to the buffer ground, right?

Does the unit needs a pedal or can I just bypass the pedal jack to be always on?

Also from the full wave rectifier comes out only 194VDC
 
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Lowerleftcoast

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It means the tank should be grounded to the chassis but not to the buffer ground, right?
Yes just to the chassis. The chassis and the reverb tank metal surround act as a Faraday cage. It is just like shielding the cavities in a guitar.

Usually a guitar amp will have RCA jacks which are not isolated from the chassis. The shield of one of the RCA cables will connect the reverb tank metal surround to the amplifier chassis.

Does the unit needs a pedal or can I just bypass the pedal jack to be always on?
You can have it always *on*, or you can put in an *on/off* switch to use your hand.
Also from the full wave rectifier comes out only 194VDC
It should be more than that if there is 250VAC on the PT secondaries.
 

Ronno25

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This thread is really helpful. I'm following along here, as I'm trying to fix an annoying hum I have in my 6g15 that increases as the mixer knob increases. I suspect it has something to do with my grounding of the reverb tank in my build, which you guys were just talking about.

The way I have mine grounded is that the rca input AND output of the tank are floated at the chassis and sent to the buffer bus.

@Lowerleftcoast from what you're saying what I have done is incorrect and that I should have either the input OR the output rca jack grounded directly to the chassis, but not both, and have the other rca jack floated and sent to the buffer? Or should I have the other jack floated and not sent anywhere?

Thanks for clarifying.

Great thread. I hope I'm not stealing it, definitely not trying to!
 

Lowerleftcoast

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but not both
Yep, if both the shielded cables are connected to the chassis and the reverb tank chassis it will make a ground loop. Lifting one side is all that is needed. With one side lifted it breaks the loop but still maintains a shield over each wire and the tank chassis is the shield for the tank internal wiring.

Here is an image from Blencowe showing a loop. Essentially the reverb tank will take the place of the pictured *shielded cable* in Fig. 15.3a. The 6G15 chassis is the *chassis* in the same figure.

upload_2020-4-16_8-45-2.png
 

sudogeek

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As far as the 4th tube, I would consider that an advanced modification - whereas if you look at the schematic I suggested, a "hum blocker" is a pretty simple bolt-on mod. It basically is elevating the entire 6G15 circuit above 0V ground, much like you would elevate the heater ground to reduce hum.

[If you check out the schematic of the silverface tube reverb unit, you will see a 4th tube - a 12AX7 wired with parallel triodes. This can act as a buffer to reduce "tone suck" and to further reduce noise. It's not necessary as a properly constructed reverb unit with the hum blocker is very quiet. You can wire it with a pot in lieu of one of the resistors to create a variable boost. In my current unit, I have a push-pull pot which switches the 4th tube in and out of the circuit as well as regulates a boost when the pot is in. I usually keep the 4th tube active and dial this in to about a 20-30% increase in output V. This unit is very much a test bed but, in general, I prefer the sound of the silverface units. The tone suck can be minimized by other methods during use of the 3 tube vintage units. So, forget I mentioned it.]
 
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maybeoneday

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Yes just to the chassis. The chassis and the reverb tank metal surround act as a Faraday cage. It is just like shielding the cavities in a guitar.

Usually a guitar amp will have RCA jacks which are not isolated from the chassis. The shield of one of the RCA cables will connect the reverb tank metal surround to the amplifier chassis.
I bought a RCA cable which has separate ground, I will connect it from tank to 6G15 chassis.

You can have it always *on*, or you can put in an *on/off* switch to use your hand.

It should be more than that if there is 250VAC on the PT secondaries.
Indeed, on the layout says it should be 300VDC.
Looking at some online full wave rectifier calculator the 180VDC output are correct for 280VAC input, if so how do you guys get 300 on B+?
 
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maybeoneday

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I put together what discussed so far, let see if I got it right. See attachment.
 

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Lowerleftcoast

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let see if I got it right
I have changed a few things to closer match the schematic sudogeek supplied.
You forgot to include the PT CT. I show it connected to the reservoir cap negative.
I moved the heater CT to elevate it to the 6k6 cathode voltage. (Alternatively this could terminate at the 6K6 pin8. It is the same place electrically. Use whichever seems more convenient on your build.)
I added the ground from the input jack.
I moved some of the ground connections so they stay with their respective ground nodes.
IDK the configuration of your chassis. The *buffer* board could be moved to the right, on this layout, to keep the wires shorter.
(If you are designing the eyelet board, I suggest moving the last filter cap to the location indicated in lavender. If this is a prefab eyelet board just leave it as is.)
The power cord typically has the *hot* line fused before the switch. I did not change it on this layout.

2022-05-20 at 07-03-58 6G15.pdf.png

Looking at some online full wave rectifier calculator the 180VDC output are correct for 280VAC input, if so how do you guys get 300 on B+?
There seems to be something wrong with the calculation. The rectified AC will show as higher DC (estimated at 1.41 x the VAC).

Since your PT has the CT, the voltage to use with the calculator would be measured from one of the red HT wires to the red/yellow CT wire. (This will be approximately half the voltage from red to red.)

Of course use best safety practices when working with high voltages.
 




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