Thinking about a cut-rate "champish" design. A few Q's

[armstrom]

I'm contemplating trying to build an amp head based on the 5F1 champ schematic. Since the component count is so low I'm going to try to save some money on the iron and by using SS rectifiers. I'm trying to find the cheapest power and output transformers I can that will get the job done. I've come across the "flea power" transformers sold by ted weber and I'm wondering if they will do the job (at $12 for the PT I really hope it'll work!)

Here's the data sheet: https://taweber.powweb.com/store/lopo_pt.jpg

So, here are my questions:

Will the 275V CT secondary be enough voltage for a champ? I know the +B on older champs were often quite high... Would I have to change any of the component values drastically to use this PT?

How about the current capacity? is 50mA @ 275v enough? I'm pretty sure I'm safe with the filament side since the 12AX7 and 6v6GT together only need .75 A of heater current. Should be ok with the 1A supplied by the PT.

This may sound dumb, but I need to use a bridge rectifier without the center tap, correct? As far as I can tell from the PT data sheet the HT voltage is 275V between the two red leads of the secondary. If I used a two-diode rectifier along with the CT I would only end up with 137V. In other words, is this PT the equivalent of a 137-0-137 ?? The normal champ PT seems to be 300-0-300 so I have to avoid using the center tap if I want to get anywhere near the voltage of the standard tranny.

This one is a bit out there.... I'm considering trying to build the amp using sub-mini tubes. I would replace the 12AX7 with a 6112 dual triode and the 6V6GT with a 5902 sub-mini pentode. I'm fairly confident that the 6112 will pretty much "drop in" to the standard circuit, but I'm concerned about how to adapt the power amp stage to work with the 5902. Can anyone provide some guidance on what might need to change? I'm also pretty sure the resulting amp wouldn't sound much like a real champ, but it still might be interesting

Finally, can anyone suggest a good cheap OT for the 5F1?

Wow, that turned out much longer than I had hoped.. thanks in advance for any advice!

-Matt

[Chuck H]

Sorry. That PT won't work. To get the voltage you need you'll be using a bridge rectifier. A bridge rectifier uses ALL of the secondary ALL of the time and therefore can't deliver the goods for current. Basically, you can have either the voltage of the bridge OR the current of the full wave. No rectifier delivers both. If you could use a bridge rectifier to up the voltage without any loss in current capacity, why would anyone ever use anything else?

Your design being class A will be drawing max current at idle. Probably about 50mA. So even if you could get 50mA from that transformer using a bridge (which you can't) it would be at it's max output ALL THE TIME. You would almost certainly get excessive sag and the PT would run hot. You really do need a PT capable of 70mA or better to get acceptible performance IMO.

Chuck

[armstrom]

Thanks for the feedback. I got a similar response on another forum so I guess that PT is out of the question. One thing that was suggested to me though was to try the PT for the standalone reverb unit. That one only costs $25 and has a 260V (non CT) secondary rated at 100mA. Should do the trick I believe. I was also given a link to a cheap OT for the Champ (less than $20) TF103-48 Single Ended Champ Transformer with 4/8 Ohm Output
Seems like I'm on my way