Hammond 272JX for JTM45 clone?

[casey73]

I haven't built an amp in years, other than a few small SE class A types that I sold on Ebay. I decided to biuld a JTM45 because is it a classic and I've always liked the "Hideaway" tone, and like the idea of a simple classic amp without channel switching, reverb and all the bells and whistles. It's like my desire to own a Forrest Green 68 Mustang with a 390, a 4 speed and American Mag wheels. A simple classic.

I picked up a nice aluminum chassis from Dirty Dawg amps on ebay and have the JTM45 turret board. I was pricing the power tranny at AES and a Hammond drop in replacement for the original (290LX) is $145! The Hammond 272JX has the same secondary HV at 300-0-300 @250ma and it's only $92. I also like that the 272JX is a top mount so I don't have to cut a huge hole in my chassis to mount it.

The only thing that concerns me a little is that the 6.3VAC filament tap on the 272JX is rated at 8 amps! Three 12AX7s and two KT66s come to just under 4 amps current draw on the filament winding. So I'm thinking the filament voltage might run a little high with half the rated amps on that winding. Or it may just run cooler. Hammond is known for their conservative ratings.

Any thoughts from those with JTM45 or other relevant experience is appreciated.

[Jazz P Bass]

Sounds to me like a wonderful plan
I wouldn't worry too much about the 6.3V winding.

[Chuck H]

+1
A higher rated secondary won't cause higher voltage. It just won't sag. That's a good thing.

I've had some problems with the Hammond 260/270 series power trannies and high filament voltages. But those have 115V primaries and modern wall voltages (in the US) are around 125V. I do think the transformer your talking about has a 120V primary. That should be fine. Again, a higher current rating than what is needed isn't going to cause the voltage to go above the spec voltage for the secondary.

[EFK]

FWIW, original JTM amps ran higher voltages than a 300/300 is going to provide with a 5AR4. I realize a number of aftermarket makers, MetroAmp in particular, supply 300/300 power transformers, but this is largely because most folks nowadays using JTM45 amps are doing so for the 'dirty' Beano tone and a lower power PT will get you there sooner on the volume dial. It's a response to customer demand, in other words. However - again, JMHO as someone who has built them both ways - the lower power does change the character of the whole amp and ALL of the resulting voltages, preamp and power amp both, are lowered if following the original schematic. The original PT were running in the neighborhood of 340/340 or 350/350, and believe me those 40-50 extra volts do make an audible difference.

[casey73]

FWIW, original JTM amps ran higher voltages than a 300/300 is going to provide with a 5AR4.

That's interesting. I assumed since the 290LX is sold as drop in replacements for the JTM45, the 300-0-300 secondary was true to the original. I did some looking around and found the schematic for the "bluesbreaker" JTM45 and it shows 440V on the plates. Guess I'll look around some more. I don't mind the idea of lower overall wattage or earlier breakup, but would you say that the lower voltage changes the character of the amp in a negative way?

[Chuck H]

IIRC the JTM45 used a 350-0-350 PT. With modern US wall voltage you'll want a 340-0-340 for a similar Vp (about 440V). A 300-0-300 will deliver about 380Vp. Which is definitely an audible change. For better or worse? Dunno.

Interestingly the Hammond Bassman RI PT is 348-0-348. And the standard Bassman PT is 330-0-330. I'm sure they did their research but since the JTM45 was modeled after the Bassman, and the VP is reported to be between 400V and 420V (Google search) It doea seem like they missed with a 300-0-300 for the JTM45 PT.?.

[casey73]

Since starting this thread, I stumbled onto an great find at a local guitar show in Tempe AZ. AES had a booth set up and were selling lots of parts that were returns, samples, discontinued items, etc., that could not be re-sold, and they were selling them at the show cheap! Because I'd been looking already, I knew what part numbers I needed and picked up a Hammond 290LX (drop in replacement for a JTM45, at Pacific Transformer TF-125 drop in replacement for a Bassman or Super Rev output with 2, 4, and 8 ohm taps, and a Hammond 194D choke (5h - 150ma). All this for just under $60. No returns but I did test the power tranny and it reads right but haven't loaded it yet. The other two were in sealed boxes.

The primary is 300-0-300 @230ma, (higher unloaded) so I know the plate voltage is going to be lower that the JTM45 from what I've read (unless I go SS, but then it would be another step away from a JTM45. And the output tranny is not a JTM45 tranny. The primary on the TF-125 I picked up is listed as 4230 ohms and intended originally for a pair of 6L6s. So I can either build with what I acquired, or find a JTM45 replacement output tranny. I'm leaning toward using the one I have (Bassman OT drop in replacement).

So here's the question now. I've built many amps over the years, but have never been real sharp on tube voltage curve charts and how it effects plate load compared to the OT primary impedance and have always built conventional designs. Will there be any audio difference if I used KT66s instead of 6L6s? I never used a KT66. And will running them at lower that usual voltages have any negative effect on the plate load and primary impedance being close to normal or at least acceptable?