Power amp design

[chunkitup]

I want to build a simple rack mounted mono power amp modelled after the JCM 800 power section. It will be 50watts-EL34. I have a Hammond PT 300V CT at 150mA-Is that enough current?
For the design:
Do I just have the input run into the Volume then into the Phase splitter then into the power tubes exactly like in the JCM800? Is there anything I should be aware of? Is it that simple?

[Chris - CMW amps]

Current is pretty ok I guess, maybe a little low, I mostly use overrated PTs.

Regarding the design: just remove all "pre-amp-stuff" and maybe raise the node-resistor in the powersupply to the B+ from the PI a bit to get the same sound and feel.

[Tom Phillips]

IMHO that the transformer you have is quite a bit underrated. 300V @ 150ma is only 45 Watts of power. To run a 50W amp I would design a power supply section that could supply at least 100W. In addition, with 300VCT you will need to use a FW bridge rectifier. The FWB requires a higher secondary winding current rating than when you are using a FW center tap rectifier.
Cheers,
Tom

[Tiago]

Be sure to leave some space to an extra 12AX7 after you builded it.

Chris, don't get me wrong, I'm not saying the opposite of you just need to call atention for something. I've builded a power amp and I found that to be able to use it with stompboxes (100mv devices) I needed an extra triode before the PI. What I did was to add a 12AX7 an placed a gain stage at the input an after that a cathode follower to driver the volume pot, so I could use a lower value pot so it won't affect to much my tone whatever the position of the pot is.

This might be complex and prone to error, so build the PI/Power amp first and leave space to have the option to add it in the future.

[chunkitup]

I think I stated it wrong. The PT is 300-0-300V and is center tapped. I wont need a bridge rectifier. Would it be better for me to use a tube rectifier and EL84's? The tube rec would be needed to get the voltage down to safe EL84 use of course. I like playing heavier music so would the tube rec help or hurt the playability?

[Tom Phillips]

Sounds like a good idea. You could build a nice 20 to 25W amp with that PT. However, think about what you really want / need. You started out wanting a 50W power amp. The build will be a significant amount of work and it may not be worth compromising your goals just to use the PT that you happen to have on hand.

Cheers,
Tom

[chunkitup]

That's what I will do then. I just finished a SLO PP 20w combo that is almost too loud, almost! I never used a tube rect. but I dont think I will have any problems. Would you recommend a 5AR4/GZ34? I just happen to have a Lenco 25w OT so the 20w project would actually be better. The OT was labeled as a Deluxe Reverb swap, so do you think it will be alright in this application?

[Russ]

I think the 300v mentioned is 300v AC, which when rectified with a diode rec will give you around 420VDC. So 420VDC @ 150 ma is 63w.

Also, there's a hammond PT, the 272FX, which has the same specs as mentioned, 300vac CT @ 150 ma DC, and it has a VA rating of 146 watts. (Is this the PT?)
So it seems to me that this PT should be OK for a 50 watter.

Russ

[chunkitup]

That is exactly the Hammond tranny.

[Tom Phillips]

I think the 300v mentioned is 300v AC, which when rectified with a diode rec will give you around 420VDC. So 420VDC @ 150 ma is 63w.

Also, there's a hammond PT, the 272FX, which has the same specs as mentioned, 300vac CT @ 150 ma DC, and it has a VA rating of 146 watts. (Is this the PT?)
So it seems to me that this PT should be OK for a 50 watter.

Russ

OK But I still think you need more than a 63W B+ supply to run a 50W output capable power amp. My rule of thumb is double the output power plus some more for the other circuits. Let us know how it works out.
Tom

[chunkitup]

Tom, Good advice! I already have a 20w OT from another project that I will use in this amp so I will heed your words.
Thanks

[Russ]

Actually, I'm also working on a 50w el34 design, but I ended using the next PT up in the hammond line, the 272HX - 300vac @ 200ma. But I didn't have the PT laying around, I had the choice of buying it, but I was real tempted to use the 150ma model - saves $, weight, and space. The reason I didn't was because I heard some people say it was too small and most of the schematics I saw used the 200ma and even the 250ma model. But, there was 1 or 2 shchems that had the 150ma. I suppose if you're playing cranked at 10 all the time then the 150ma might get stressed. I to would be currious to find out how the 150ma works out. Let us know.

Russ

[bob p]

Marshall transformer schematic:

[StevieP]

You could build the amp with a 150ma PT but it will likely not deliver 50W of power to the speakers.
Look at an EL34 data sheet.
Assuming you implement a fixed bias Class B design with about 400 volts on the plates, the plates draw 100ma each at 50-ish watts output and the screens demand about 25ma each. Add to this preamp tubes and you are inclined to use a 250 ma PT. The theoretical current demand could be more than this, but I think most will agree that the output power requirement fluctuates under normal use so your 50 watt amp will operate nicely with a 250 ma PT (particularly with that under rated Hammond iron).

[rathmann]

What about a quad of EL84s?

Voltage might be a bit high for them, and the PT may still be ever so slightly underrated for full tilt operation. Perhaps throw a fan in there to reduce tube and transformer stress.