Building a 5F1 (OT question)

[94Tremoverb]

So which amp is probably genuinely class A?

The Selmer Tru-Voice Zodiac Twin 30. It uses a pair of EL34s in cathode-bias, running above their rated maximum dissipation, and does operate (just) in Class A - although it doesn't quite produce 30W, more like 25. I'm sure there may be others too, but the first indication is that the power output is much lower than you would expect from the power tube set - Class A is inherently inefficient, which is why it's rarely if ever used when designers are trying to build a good loud amp that isn't too expensive.

I figured it was worth a shot asking about the pilot light thing being that I'm going to be doing a fair amount of mods to this thing anyway.

If the one in the amps is like the noise reduction bulb in the Hotplate, a lamp in series with a resistor across the speaker output will work - the problem is that it will be completely off until you're up to a fair volume.

 

[EtherealWidow]

Thanks, Don for the insight! And so this gives me an idea. Instead of modding the amp, I could hook up a little box that goes in between the speaker out and the actual speaker equipped with a light bulb. If I wanted to use it with my Express 5:25, how big would the resistor need to be? I use the 8 ohm jack. From what I've heard, the 5:25 is more like 13 ish watts, and with my particular amp I'm using 2 12AU7's in the pre so the volume is much lower than a stock one.

 

[212Mavguy]

212MavGuy: Would you say that the punch and harmonic detail is due to the OT and the tube swapping capabilities are more due to the PT? Could the harmonic detail also be accredited to the PT? Would I need a bigger PT to run 6L6's or 5881's? The manual that tubedepot hands out says that all you need is a solid state rectifier, and for the solid state recto you should bulk up the filter caps.

I would say that changing out the PT did in fact contribute to more punch and harmonic detail, but not as much as the OT. I did put in a larger capacity PT that I got from eBay that fit in the same chassis cutout as the stocker in my Silverface. It was from Allen amplification. They still sell them. It puts out more secondary voltage as well as more current than stock. No you do not need a solid state rectifier. I use a 5v4ga. I yanked all the guts out of mine and built up a blackface champ turret board using a grounding bus instead of grounding to the chassis. I mainly use mine as a head running into a variety of 2/12, 1/15, and 2/15 cabs. It runs a lot of 6v6's too hot, but I have a pretty good collection of vintage tubes to choose from. Real 5881's and just about all 6l6's sound great in it. For giggles, I have stuck in a 5ar4 rectifier and run an el34 in it, (after connecting pins 1 and 8 on the power tube socket) that sounds really loud and good. Yes, the bias point is different for el34's, but who cares about being precise as long as it sounds great and is reliable, which in this case it does and is pretty much bulletproof.

With all this said, I was not willing to be true to the original design, I wanted my own sleeper amp and tone, and I got that in spades. There is still a lot of basic champ tone in it. I used some fancy hifi parts for resistors and cathode bypass caps and tweaked those bypass cap values to taste, bigger filter caps, (3 @ 30 mfd F & T with no ill effects to the rec tube) and lifted the ground from the bass pot so that the tone controls basically don't work any more, but the amount of gain available is huge now. There is no negative feedback hooked up. It's clean up to maybe 9:00 on the volume knob, and gets insanely loud when cranked. Fun to look at faces when I twist that volume knob... :twisted: It kinda has a short man's complex, it wants to be a Marshall Jubilee stack so bad...I can't run it full out on stage unless I run it through the 8" speaker in the cab, even with just a 6v6.

 

[EtherealWidow]

Ah. I need to stop being so broke. That sounds awesome. It's pretty much like a watered down univalve. Tube versatility and crazy simplicity. Sounds like fun.

 

[212Mavguy]

Yep, a little one and a bit more trick pony with attitude. But getting back to the idea of the thang, a champ is a good place to start with smelling the solder fumes, be it 5c1, 5e1. or 5f1 from scratch or modding a lowly silverface. Kinda like having an old VW Beetle that smokes the tires for a hundred feet.

 

[EtherealWidow]

Sounds sweet. I'm getting all excited about this little thing again. Thank you everyone for your help.