# Advice: Hammond PT for a Two-Stroke (Rev 2) Amp



## Todd MacCulloch (Mar 8, 2018)

I am a first time tube amp builder and I would like to use a Hammond PT to build the 2nd revision of the Two-Stroke Amp from Dave Hunter’s book

I am undecided which transformer to pick and I am looking for help from experienced builders especially ones that have built this amp.

The Tube Amp Network hosts a copy of the layout and the schematic

The book suggests one of:

Mojo760 
Secondary: 330-0-330 VDC @ 75mA
Filament: 6.3V @ 4A
Rectifier: 5V @ 4A

P-TF22772
Secondary: 325-0-325 @70mA
Filament: 6.3V @ 2A
Rectifier: 5V @ 3A

Weber 125P1B - W022772 Transformer
Secondary: 330-0-330 100mA
Filament: 6.3 V @ 4A
Rectifier: 5.0 V @ 3A

Or any transformer for a tube amp with:

Secondary: 310-0-310 to 330-0-330 VDC at least 70 - 80mA 
Filament: 6.3 V @ 4A (note: the schematic from the Tube Amp Network says 2.7A)
Rectifier: 5.0 V @ 3A​

*Hammond Transformers*

Starting with the Fender replacements at the Hammond classic 290 series I see :

Hammond 290 AX
Secondary: 325-0-325 VDC @100 mA
Filament: 6.3V @ 2.25 A
Rectifier: 5 V @ 3A

The filament voltage is low but according to the Hammond site this is the replacement for for Fender parts 125P1B and 022772.

Would 2.25A be too low for this amp (this would be powering the power lamp + 12ax7 + one of 6V6/6L6/EL34)


Hammond 290 CX
Secondary: 325-0-325 VDC @207 mA
Filament: 6.3V @ 4A
Rectifier: 5 V @ 3A

The filament amperage is correct but the secondary is amperage is high 207 mA vs the 100 mA or less for the other PT. Would this be a problem?

Hammond 290 CBZ
Secondary: 325-0-325 VDC @120 mA
Filament: 6.3V @ 4A
Rectifier: 5 V @ 3A
This seems to be closer to what I need according to the original spec.

Looking at the Hammond classic 200 series 

Hammond 272 DX
Secondary: 300-0-300 @144 mA
Filament: 6.3V @ 4A
Rectifier: 5 V @ 3A

The 300-0-300 is lower than the spec’d 310-0-310 minimum

Looking at the Hammond classic 300 series I didn’t see anything closer …


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## WCGill (Mar 27, 2009)

Hammond 291AEX, dual HV taps. I'd use the 275-0-275 tap with a 5AR4 rectifier and a choke.


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## Schmart (Jun 7, 2011)

I think the Hammond 290AX would fine. It also has 325V and 275V taps. The 291AEX is the export version that you can use at European as well as North American voltages. 

Even with an EL34 the current draw would be less than what the transformer is rated for and with a 6L6 it would be a fair amount lower. An EL34 needs 1.5A, a KT66 1.3A, a 6L6GC 0.9A, and a 6V6GTA 0.45A. The 12AX7 uses 0.3A, and a #47 Bulb uses 0.15 Amps.

I built a similar amp with a Heyboer 022772 that I ordered from Weber. The amp started out as a 5F2A clone but I changed it into something closer to a Two-Stroke with a 15W Hammond 125ESE for the output transformer. I also changed the rectifier to a GZ34 and plan on installing a choke in the near future.


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## WCGill (Mar 27, 2009)

Schmart said:


> I think the Hammond 290AX would fine. It also has 325V and 275V taps. The 291AEX is the export version that you can use at European as well as North American voltages.
> 
> Even with an EL34 the current draw would be less than what the transformer is rated for and with a 6L6 it would be a fair amount lower. An EL34 needs 1.5A, a KT66 1.3A, a 6L6GC 0.9A, and a 6V6GTA 0.45A. The 12AX7 uses 0.3A, and a #47 Bulb uses 0.15 Amps.
> 
> I built a similar amp with a Heyboer 022772 that I ordered from Weber. The amp started out as a 5F2A clone but I changed it into something closer to a Two-Stroke with a 15W Hammond 125ESE for the output transformer. I also changed the rectifier to a GZ34 and plan on installing a choke in the near future.


Yeah, 5F2A was all the rage awhile back. How loud was your build?


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## jb welder (Sep 14, 2010)

Those are good suggestions above, the 2A heater winding should be all you will need for this. Not sure why someone would have said 2.7A on the schematic, should be less than 2A like Schmart showed above.
The problem with going over-spec'd is you can end up with voltages that are too high. Lot's of guys run into this problem with builds, then end up having to find ways to dump the extra voltage. For example, if you used a 4A heater winding here, you might end up with 7V heaters or something, and have to bring it down to 6.3V again. Or if you use a 200mA HT winding instead of 100mA, your main B+ voltage will end up a lot higher than it should, so again you have to find a way to reduce it.
It's already bad enough that most transformers are built for 117 or 120V mains, when in many places it's getting closer to 125V out of the wall outlets. So you already start with voltages a bit higher than vintage, you don't need to make it even worse with transformers that are way over-spec, like that 290CX would be, for example.


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## WCGill (Mar 27, 2009)

Yeah I'd think 2A would be enough, esp. with a Hammond. Will a higher amperage rating boost voltages?


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## jb welder (Sep 14, 2010)

WCGill said:


> Yeah I'd think 2A would be enough, esp. with a Hammond. Will a higher amperage rating boost voltages?


Technically I guess you'd say light loading won't bring them down into line, so not exactly a 'boost' per se. 
The quoted voltages are under the specified load, so if there is a lighter load, the voltages will be higher. The unloaded voltage will be higher still. So light load will be somewhere between unloaded and fully loaded level.
Where ever possible I prefer the newer Hammonds that have the choice for 120 or 125V primary. The north-american power grid is always trending upward, I rarely see line voltage below 124V where I am.


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## Todd MacCulloch (Mar 8, 2018)

Thanks to everyone one who chimed in on Guitars Canada, Tube Amp Network, diyAudio, and TDPRI 

The consensus is that the Hammond 290 AX should be fine for the Two-Stroke.

Some other interesting observations:

“There have been reports that the 200 series can run hot, and can even buzz IIRC, so if you are the type who wants extra quality and is willing to spend more to get it, then the 300 series is better.”​
The 291 AEX is an “upgrade” version of the 290AX and in addition to supporting export voltages ...
“291AEX uses high grade 29M6 magnetic steel laminations for reduced losses, more surge output power, cooler operation & longer life.”​


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## Schmart (Jun 7, 2011)

The 290AX must have been upgraded last year when they did that new datasheet. I have a datasheet from March 22 of 2009 and the specs then were lower:

Secondary: 325-0-325 VDC @ 81mA
Filament: 6.3V @ 2A
Rectifier: 5 V @ 2A

Given that the specs now match those of 291AEX I wonder what they've changed.


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## Todd MacCulloch (Mar 8, 2018)

Schmart said:


> The 290AX must have been upgraded last year when they did that new datasheet. I have a datasheet from March 22 of 2009 and the specs then were lower:
> 
> Secondary: 325-0-325 VDC @ 81mA
> Filament: 6.3V @ 2A
> ...


Good question. Hammond's site doesn't mention the upgrade info anymore but various resellers have the exact same blurb -- maybe the resellers info is out of date...


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