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Best hammond PT for push/pull EL84 amp

8/9/2004 8:07 PM
Best hammond PT for push/pull EL84 amp
Best hammond PT for push/pull EL84 amp??????? I looked and they have so many. I just want to make a 12 watt el84 amp. I have a PV Classless 20 and it really sounded like crap. I want to use a very simple fender design but with 2 el84's. The chassis and cab are small and I think a standup tranny would fit better than a fender type PT. Thanks
8/9/2004 8:58 PM
Mark dB

Don't be putting down the PV Classic 20 until you've tried it through a big cabinet! It's huge! Don't worry about the "16 ohm minimum load" requirement. That just means 16 ohms or less is OK. I've been running mine through a 4-ohm 2x15 Bassman cabinet for years... no problems.  
I based my first amp build on the Peavey Classic 20 four tube topology... 3 gain stages, a "concertina" split load phase splitter and EL84 push-pull output. I've been really pleased with the Hammond 1608 output transformer that I used. It handles the 18 watt output very nicely. It's an awesome amp. I like it way better than my Fender Hot Rod Deluxe and the PV 20.
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8/9/2004 10:44 PM

Thanks, but I need a power transformer.
8/10/2004 11:38 AM
272FX, if you're using an EZ81 rectifier.  
The heater voltage might run a little high. If it gets up to 7 volts, then use a 6.3v pilot lamp that burns the most current of any you can find. I think 250ma is about as high as they go.  
If you're not going to use an EZ81 rectifier, then you'll probably want a lower HT voltage. The EZ81 drops a lot of voltage, but it still comes out a bit on the high side with the 272FX. So if you're going to use a rectifier with a lower voltage drop, you'll need a lower HT voltage. In that case, look into the 270FX. The 270EX would probably work too.  
If you're going to use solid-state rectification or a GZ34, you might want to start out even lower, say with the 270DAX or 270DAZ.  
One thing I know from experience is that the 272FX / EZ81 combination works, keeping in mind what I said about the heater voltage.  
Or just get one of those Heyboer 18-watt Marshall power trannies from Graydon Stuckey. Then you don't really have to worry about the heater voltage, and you'll have 2 separate 6.3v windings, which apparently has some technical advantage over running the EZ81 off the same winding as the rest of the amp (although I've done that with no apparent problems).  
8/10/2004 12:18 PM
Thanks Shea
Thanks Shea, this is what I have, a PV classless or should I say useless 20. I want to use a simple fender long tailed PI and BF preamp. The power section is 2-el84's, I also have a webervst gz34 type copper cap. I don't know what type of bias yet but maybe cathode bias. I am very new to this but I want to make a low wattage practice and recording amp with a certain sound. The 20 and Fender champ amps just don't do it for me. Thanks again, any help is greatly apperciated.
8/10/2004 2:48 PM
Typically you need to roll off more bass when you're using EL84s than you would with other tubes. So, don't be suprised if you have to use smaller coupling caps in the circuit than you would in a Princeton or Deluxe Reverb.  
The Vox Cambridge Reverb sort of mates up the BF preamp to EL84s, with some significant tweaks (such as elimating the reverb/dry mixer stage). It's a really nice sounding amp -- it just doesn't get much attention because all the Vox fans are looking for British-made JMI Voxes, instead of American-made Thomas Organ Voxes. It's a sleeper!  
Here's the schematic:  
8/10/2004 3:39 PM

Great just what I was looking for!!!!! would the hammond tranny work with this design? I think I'm on my way to making what I want, thank you.
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