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Trying to choose Hammond transformer set

5/8/2003 2:14 AM
Trying to choose Hammond transformer set
I've decided to try to build a smaller wattage UL with no NFB amp from scratch, chassis and all. I'm looking at the 270FX and the 1615. The PT is rated for 150mA and is 275-0-275. The OPT has 5000 primary Z and is rated for 15 audio watts. I'll be using the vintage voltage adapter, so the voltage will not be increased.  
The RCA manual gives an AB1 example with 360v to the plates of 6L6's. I'm guessing this is about where my plate voltage will be. However, the screens only had 270v B+. Idle current was given at 44mA per tube. Power was 18 watts for an a-a load of 3800, and 26 watts with an a-a load of 6600. Maximum current was around 132mA for the 6600 load and 140mA for the 3800 load. I think with a 5000 load figuring 135mA will be accurate enough.  
I don't know how the UL connection will effect the power output compared to the pentode connection with the lower screen voltage. I'm shooting for maximum power out before clipping at 25 watts, not less than 20 and not more than 30.  
Does this sound about right for this tranny set? The only other factors would be the current draw on the preamp section. I suppose if I went to some higher drawing stages like a 12AU7 or a 6N1P and then throw in a direct coupled CF driver to the output tubes, I might opt for the 270HX, rated at 200mA. It's only a few dollars more and is one half inch more in height and length. However, I don't know how the two trannies compare in weight, and I *am* trying to keep the weight down on this.  
Any comments, feedback, opinions, suggestions, etc. would be appreciated. Thanks.  
5/13/2003 7:12 PM
OK, just a simple "bottom line"
Will the 1615/270FX matchup working with two 6L6/EL34 tubes give me at least the power of a Deluxe Reverb? Let's assume typical 12AX7 stages.  
If I add CF drivers and perhaps higher current drawing gain stages, will I need to up the current capability of the PT?  
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5/13/2003 8:13 PM
Matthew Springer
Re: OK, just a simple "bottom line"
What kind of bias scheme do you have in mind? Fixed, Cathode? The amount of power you get out of the output stage will depend on whwere the tubes bias themselves. The relationship between peak audio power and idle power is complicated atbest, so don;t get to terribly hung up on the actual number. Keep in mind a 22W amp can be voiced just a "loud" as a 30W amp. In fact it's highly unlikely you'd hear a difference until you really ran them flat out. Loudness and power aren't always easily related. typical rule of thumb is that an amp has to be 10x more power to be x2 as loud.  
As far as power tranny rating goes, I usually figure 1mA per input triode stage and 40mA per power tube for medium power amps. You won't ever actually burn that much under normal operation, so you probably won't need to pad at all. You can almost assume the preamp and reverb dections will eat almost no curent and just figure on the output stage. A "high" current stage in the preamp isn't likely to draw current like the power stage. I just built a 2x6l6 amp with reverb and trem (4x12a_7) and used a 372BX. I think for output, I used a 1620, IIRC.  
The bigger limit is going to be the heater current. Since this is always burning, it's good not to underestimate this number, but you should be able to run pretty close to max rating on this without any problems especially if you don't use the 5V tap for tube rectifier. Just don't go over.  
Plate voltage is tricky. The hammond 275-0-275 @ 115V will give you 388V Peak if your household current is really 115V, which it almost never is. You probaly get about 400V "no load" out of you supply whihch will settle down to ~370V or so under load. again, tubes handle voltage stress much better than current stress, so it's more important to properly bias, than calculate ahead of time.  
The "vintage voltage" adapter is just a step down transformer, you don't need to use it. If you don;t like the plate voltage, you can drop it in more creative ways.  
Hammond output trannies are rated conservatively, so you should be able to use a 20W tranny for a 30W guitar amp without too many worries, provided you don't bias it into oblivion.  
5/15/2003 2:15 AM

Thanks, Matthew. It will be fixed bias. I guess I'm being a little too paranoid about current requirements. I just looked up the OPT for a Deluxe Reverb and it weighs something like 1.5 lbs. The Hammond 1615 weighs 3.5 lbs. This is still a pound more than David Allen's over rated OPT he sells for the DR. I have a feeling the Hammond PT might have less sag than the stock DR tranny. It's a 320-0-320, but from what I understand it sags like crazy when you push the 6V6 tubes, so there you go. The 270FX has 5 amps of heater current, so there's plenty even for EL34 tubes with a typical preamp section.  
I think these two will be fine for a smaller amp.  
5/15/2003 4:28 PM
Matthew Springer

I went down exactly this path when I was designing my Cornell Reverb (Tweed Deluxe PA with the princeton Reverb front end and PI, post-PI matser). Same fetaure list as a Carr Rambler, but with a MV.  
I haven't poked around in the guts with my scope too much, but even with my cathode biased amp with a Solen fast cap as a bypass cap and a Solen power supply there is NO compression whatsoever to my ears when the master Volume is full up.  
The Hammond trannies are tough to beat for the price. I've used to 270 series trannies in two out of two amps and I get about 410V no load with ~390 volts normally which is pretty much rock steady. I actually added a switchable 100 Ohm 25W resitor in the B+ feeds for this amp to try and give it more sag and it's still a very "sharp" amp. Have no fear with a Hammond power tranny and a medium power amp, they're forgiving and very tough to blow up.  
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