6G15 Reverb Unit Pre-Build Questions

[Wittgenstein]

I just finished building the chassis for my next project, a Fender 6G15 stand alone reverb unit. I have the circuit board drilled (waiting for turrets) following the Hoffman design (Circuit Board layout Hoffman). I just need to order the transformers, rectifier, and a few misc parts. I have a few questions however before I place my final order.

half wave vs full wave?
Fender originally used a half wave diodes to filter out the ripple before the choke. I have seen a few builds that use full wave with a bridge rectifier. Which is preferable, and does it matter based on which transformers I use? Does anyone who has heard/built an amp/reverb unit with these transformers have any input? The Hoffman parts list just lists 1 x Bridge rectifier, so I would assume they want you to use they one they sell (1000 volt - 3 amp bridge rectifier
B - Bridge rectifier- 3 amp/1000Piv rating, 3/4" X 3/4", #6 screw hole.)?

I also do not have the reverb tank, and was hoping for a suggestion there as well.

Which transformers to get? They are all similarly priced, so I am not too concerned with cost. I would like to have this sound as vintage as easily possible, and plan on purchasing some NOS 6K6 power tubes, but I would like to run this with the higher voltage 6V6s as well. I think, correct me if I am wrong, that all the OT listed below will work for both (7-7.5K ohms)

Weber
Power Transformer- WRVBPT
https://taweber.powweb.com/store/wrvbptsch.jpg
Primary: 120VAC; HV secondary is rated for 260V @ 100 mA with 6.3 VCT @ 3A for filaments.
no center tap on the HV, a full wave bridge rectifier is used, and gives about 370 VDC with no load.

Output Transformer - W022905M
https://taweber.powweb.com/store/022905msch.jpg
4 and 8 ohms output @ 7000 primary

Choke - W022699
https://taweber.powweb.com/store/022699sch.jpg
9H @ 120ma.

Weber Layout Diagram


Triode Transformer Bundle w/ Choke
Fender 6G15 Reverb Unit Transformer Bundle w/ Choke MADE IN USA
Power Transformer - PT-47609
Primary 120VAC, Secondaries 306V at 27mA, 6.3V at 1.05A.
no center tap on the filaments, half-wave diodes are used (I think?)

Output Transformer - OT-47605
Matches 7.5K 28mA plate load to 8 ohm reverb tank input.

Choke - 22699
Rated 4 henries at 90 ma DC. 110 ohms DC resistance (+-5%), 2500 volt DC

Triode Layout Diagram

Hoffman
Power Transformer
MOJO 779 - Uses a bridge rectifier
Primary: 100/120/220/240v
secondaries: 250Vac, 6Vac

Output Transformer
022905 - Champ output transformer
Pri: 7000 Ohm / CT: Sec: 3.2 Ohm: Power: 5 Watt

Choke
125C3A - 4 H at 50mA DC


Layout Diagram Hoffman


Diode Rectifier -1N4007
1000V, 1amp rating for B- power supply (2 or 3 at $0.25 each)

Any other tips or suggestions before I begin this build?

[Wittgenstein]

So I realized that I asked a few questions that have already been addressed here. I apologize for that. Here is what I have found in my digging.

I also do not have the reverb tank, and was hoping for a suggestion there as well.
I see where Bruce suggested The new MOD tanks:

P-RMOD-4AB3C1B ($18.00 at AES)
Reverb Tank - Mod, Equivalent to 4AB3C1B, Black Finish
Equivalent to 4AB3C1B, designed to sound like vintage tanks.
Type 4, 8 ohm input, 2,250 ohm output, long decay, 2 spring.

half wave vs full wave?
"...I suggest using a PT that supports FW SS rectifier is you theoretically get better filtering from a FW rectifier (because the time gap between maximum voltage pulses in a FW rectifier is 1/2 what it is with a Half Wave rectifier, so a FW rectifier reduces the work that the filter has to do to smooth out the ripple). Remember that anything you can do to reduce the risk of a high noise:signal ratio at the output of these units is beneficial. 220-0-220 to 240-0-240 puts you in the ballpark for a 310-340VDC B+, which is ideal for this circuit. The pan just needs to be a pan with an 8R input transducer and whatever 170R + impedance for the output transducer." (quoted from another thread)

Any other tips or suggestions before I begin this build?
Make sure the trannies are preferably well-shielded (preferably with end-bells).

R.G. Keen's grounding method
http://music-electronics-forum.com/a...nding-amps.pdf

Thanks Tubeswell for the great 6G15 threads with build pictures. I see that you used:
PT - 240-0-240PT @ 100mA with a centre-tapped 6.3V heater winding
Reverb Tranny - I used a 5k5 to 8R 10-12W (but any SE Champ-type OT will do)
Choke - 4H 40-50mA
Accutronics type 4 pan (spec'd for the 8R reverb tranny)
Did you use the half wave rectified diodes due to your PT? I wonder if it is just best to use full wave to filter the signal as much as possible. I also found the conversation about using a 1:1 Isolation Transformer Box with 1/4" Jacks to lift the ground between amps and the reverb unit fascinating, but I guess I can worry about that after it is built.


So if I get the Weber or Hoffman PT then I can use a bridge rectifier, but if I choose the Triode PT then I would use diodes? Any suggestions on which trannys to get would be greatly appreciated. I would like to order the parts here in a few days.

[Wittgenstein]

I have been reading everything I can get my hands on to understand this build better, but I still do have a few questions.

1.) If I get the Weber or Hoffman PT then I can use a bridge rectifier, but if I choose the Triode PT then I would use diodes?

2.) Any suggestions on which trannys to get from above, would be greatly appreciated. I would like to order the parts here in a few days.

3.) The general consensus seems to be that using full wave rectification is preferable and easier on your filter caps, but I see many builds that use diodes. How much difference does it really make in the overall sound/tone?

4.) Any suggestions/ tips before I order the parts and start soldering?

[loudthud]

Seems strange that the Triode PT is only rated at 27mA. If it's true, use one of the others. The voltage is a little on the high side but it might drop down because of high resistance in the transformer.

The original Reverb used Half Wave recification. Why three diodes in series were used is probably because the diodes used were not very reliable. Half wave rectification puts stress on the core of the transformer, most transformers are not designed for it. You can get away with it if the transformer is used well below it's current rating.

If you want Full Wave Rectification, you can use one of the pre-made bridge rectifiers or four 1N4007s connected as a bridge. It doesn't matter to the transformer, none of the transformers seems to have a center tap.

The 3A 1000PIV bridge is overkill and a waste of money unless you don't have room for the four 1N4007s on your board. You can mount the bridge on the chassis or even on the board if you have room. It won't get very hot.

[tubeswell]

I used FW rectification (240-0-240). Make sure that all the iron is covered with end bells. Keep the pan input transducer as far away from the iron as you possibly can. Follow R.G.'s ground scheme to the letter.

[Wittgenstein]

I ordered all the parts last night that I need to finish this build. I ended up going with the entire Triode transformer bundle, the Mod tank mentioned above, and FW rectification; I bought a bridge and 4 x 1N4007s too see which would fit better. I hope to have it up and running in a few weeks, at which point I will post more pics.

[Wombaticus]

I ordered all the parts last night that I need to finish this build. I ended up going with the entire Triode transformer bundle, the Mod tank mentioned above, and FW rectification; I bought a bridge and 4 x 1N4007s too see which would fit better. I hope to have it up and running in a few weeks, at which point I will post more pics.

The triode bundle works just fine. The PT, I think, is not actually rated 27mA, rather that is approximately the total current draw you're likely to see in the finished unit. Full wave rectification works fine with it, and is much to be preferred. Voltages came out perfectly in my build.

[Wittgenstein]

Follow R.G.'s ground scheme to the letter.

I am not sure how a few spots are supposed to be grounded according to R.G.'s scheme. I have attached the a GIF showing the Hoffman layout that I used to make the board. I circled and lettered the spots where I have made changes to Hoffman's design to run every separate stage back to a central ground star. Could someone please look at this to let me know if I am making any mistakes? I have the board ready to drop in with whips, but I do not want to go any further until I know that I have it nailed.

I can only zoom in so far on your build tubeswell to see how you have yours run. I am most clueless what to do with the grounds for the input jack and pots. Any suggestions would be awesome.

[Wittgenstein]

Edit: It does not show it on my attachment, but the power cord ground will be bolted to the chassis right where it comes in, and then it gets its own wire back to the star ground.

[Wombaticus]

I can only zoom in so far on your build tubeswell to see how you have yours run. I am most clueless what to do with the grounds for the input jack and pots. Any suggestions would be awesome.

I frankly can't be arsed to take the time just at the moment to decipher the Hoffman layout and your diagram, but the best practice is that grounds for things like pots should go back to the ground side of the power supply cap that feeds them... then if star grounding, each cap's cathode connects to the star ground point (which is not necessarily, and often not, the chassis attachment point). The point of confusion for circuitry connecting two triodes (including master volumes, tone controls, etc) is usually deciding what belongs to which one. The basic rule is that the coupling cap is the dividing line. The input jack should be grounded to the chassis very close to the jack itself (use an insulated jack though) and this should be your single chassis reference for the ground. The cap feeding the input valve is often connected to this too.

If you have a moment, read Merlin's excellent free chapter on grounding... it will never be a mysterious topic again: The Valve Wizard

[Wombaticus]

Edit: It does not show it on my attachment, but the power cord ground will be bolted to the chassis right where it comes in, and then it gets its own wire back to the star ground.

You are referring to the safety ground/earth.

This has nothing whatever to do with the circuit ground, and it should not be attached to the star ground. It should have its own dedicated connection to the chassis. In a properly functioning amp there will be NO current flowing through this connection. It is strictly for safety to ensure that the chassis and everything attached to it remains at ground potential in the eventuality of a fault condition that puts B+ on the chassis.
Usually the best place to make the unique ground reference for the circuit ground is right at the input jack, not at the entry point for the IEC jack earth.

It's worth noting that there will also be no current flowing through the circuit ground connection to the chassis. It is strictly maintains a reference for the circuit to ground potential - in a properly conceived ground scheme, it is not a ground current path, and indeed, ground current should never be routed through the chassis.

[Wittgenstein]

You are referring to the safety ground/earth. This has nothing whatever to do with the circuit ground, and it should not be attached to the star ground.

That is how I have done it on my previous builds, but I was trying to follow R.G. Keen's grounding layout (attached), as this circuit is known for being noisy if not properly laid out. I have read Merlin's grounding chapter about seven times and I feel I am still struggling.

I did find Tubeswell's schematic for the 6G15 (attachment 2) which definitely helps me see the various stages that need to be connected.

[tubeswell]

AFAICR without looking inside the unit just now, for the reverb unit, I used isolating input and output jacks and isolating RCA jacks for the reverb pan cables. All the grounds via separate ground return wires. I pretty much followed R.G.'s grounding scheme and it works pretty well.

However, there's no guarantees it'll work first pop. You build it and you troubleshoot it until you're happy with it. (I'm a persistent troubleshooterer. Even if it takes months.)

[Wittgenstein]

Update: I finally had some time last weekend to finish wiring this reverb unit up. By the time I finished building a light bulb limiter for initial power-up, and measuring the B+ without tubes it was very late, so I did not have much time to mess with it. After seeing good voltages at the bridge rectifier, I threw in the tubes, connected the tank, and hooked it up to my buddies 2-stroke amp we recently built.

It does not appear to be working however. When on it passes the guitar signal through to the amp, but there is no reverb. The mix, dwell, and tone knobs do not seem to affect the sound very much. I do not have a foot switch yet, but I was under the assumption that the unit would be permanently on without it.

I need to go back through and take voltage measurements to get a better idea of what is going on, but my main question is...... isn't it a good sign that it is at least passing the guitar signal through to the amp, even if clean? I'm thinking it should be something simple that I can (fingers-crossed) fix easily.

[tubeswell]

It does not appear to be working however. When on it passes the guitar signal through to the amp, but there is no reverb. The mix, dwell, and tone knobs do not seem to affect the sound very much.

Yes the bypass channel through the cathode follower stage sounds like its working okay. The problem will be in the 'wet' side. As well as (all) the wet-side voltages, check the reverb pan cables and transducer coils for continuity with your R-meter. And check the signal path for continuity