Hoffman 6G15 filter cap values and other differences

[prongster]

I’m thinking about a 6G15 for my next build, and since the 3 amps I’ve done so far we’re on eyelet boards, seems like this might be a good one to try a turret board on and use the Hoffman layout.

I’m curious tho as to the purpose of the larger filter cap values?

And seems like the bridge rectifier is a good way to go too…any other notable differences to be aware of…the standard transformers work with this layout right?

[sluckey]

I’m curious tho as to the purpose of the larger filter cap values?

Larger than what? Hoffman simply copied the Fender original except for the PT and FWB.

And seems like the bridge rectifier is a good way to go too…any other notable differences to be aware of…the standard transformers work with this layout right?

You need a PT that's appropriate to work with the FWB rectifier.

[EL34]

I need to update the documents for the 6G15
That was back when I stocked Sprague atom filter caps

Use the 47uf/500v FT caps now

I'll be updating all the documents soon

[prongster]

Larger than what? Hoffman simply copied the Fender original except for the PT and FWB.

You need a PT that's appropriate to work with the FWB rectifier.

Ok duh, I see that now re the filter caps.. I was looking at the Mojo layout and they use 22uF caps for some reason.

How does the PT need to be different for the FWB? I was thinking of using a Hammond P-T290WX…will that not work?

[EL34]

It's a full wave bridge rectifier PT
Mojo sells the correct one for a 6G15

[EL34]

I updated the filter caps to 47uf/500v on BOM and layout

The 6G15 parts list is here
https://hoffmanamps.com/MyStore/catalog/Amp_6G15Parts.htm

The build documents are here
https://el34world.com/schematics.htm#Hoffman_6G15_Reverb_Unit_

If you have viewed the PDF BOM or PDF Layout before, your browser cache will pull up the old document
If you still see 40uf/500v caps, you are looking at the old documents

Clear your cache files and images only like this in your browser settings menu
Then hit refresh on the library page
You should see the new documents

[EL34]

Here is the mojo reverb unit power trans MOJO779
https://www.mojotone.com/GA-5-Reissue-Rocket-Reverb-Unit-Power-Transformer

[sluckey]

I was thinking of using a Hammond P-T290WX…will that not work?

Should work just fine.

[prongster]

Awesome - is there something in particular about PT that makes it work or not work with the FWB?

[prongster]

I updated the filter caps to 47uf/500v on BOM and layout

Wow, that was fast - thank you! This is certainly helpful info. I'm still in the planning process but this will come in handy come time to order.

Very much appreciated!

[Willabe]

is there something in particular about PT that makes it work or not work with the FWB?

FWB doesn't use a B+ secondary CT.

[EL34]

is there something in particular about PT that makes it work or not work with the FWB?

FWB doesn't use a B+ secondary CT.

Also, the AC voltage will be lower than a non bridge PT
Here's the spec's on the Mojo779

I have used this for 6G15, Re-Vibes and tube pre amp builds

[prongster]

Ok the Mojo779 is in stock, but not the NOS771 OT. Looks like there are 2 possible Hammond equivalents, the 1750C & 1760CP.

1750C =  5,000/8,000 ohm pri., 3.2 ohm sec., 5W

1760CP = O/P 5W PRI=5/8K, SEC= 4/8 OHMS

Does either of these seem preferable? The 1750C is a little cheaper.

I’d just assume go with Mojo, but the Hammonds have been easier to get lately.

I’ve got the Hammond 194A choke already, so I think I’m set there.

[sluckey]

I like Hammond iron. Hawk has best prices.

     https://www.hawkusa.com/

[Willabe]

I think the stand alone reverb OT is supposed to be 8 ohms on the secondary, not 3.2 ohms.

The 3.2 ohms secondary is for the old style Champ amps.

[prongster]

I think the stand alone reverb OT is supposed to be 8 ohms on the secondary, not 3.2 ohms.

The 3.2 ohms secondary is for the old style Champ amps.

Thank you - I was wondering about that, being that (what I think is) the standard tank - 4AB3C1C - wants to see 8 ohms on the input. So sounds like the 1760CP is the one to get?

[sluckey]

I used this...

     https://www.tedweber.com/w022905m/

[prongster]

Looking more closely, it looks the 290WX is a replacement PT for the 63 Reissue unit, and the holes aren't the same spacing as the Mojo779 - which I assume fits the Mojo chassis (and that seems to be the only one readily available in the US), so I'll probably just get the Mojo 779 now.

I've seen some different layouts with a 100kl tone pot instead of 50kl - what difference does that make? Does it just give more fine control over the tone setting, or does it skew the control more to the low end?

[HotBluePlates]

... is there something in particular about PT that makes it work or not work with the FWB?

FWB doesn't use a B+ secondary CT.

And the secondary has half the turns, compared to a power transformer intended to use full-wave (non-bridge) rectification.

This matters because if you just tack on a bridge & disconnect the center-tap, this transformer will have twice the expected output voltage.  This may help explain it.[url]