Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: JustTheDad on June 12, 2022, 08:17:34 pm

Title: Is this PT sufficient for a SE EL34 and 8-10 watts
Post by: JustTheDad on June 12, 2022, 08:17:34 pm

I have a Hammond 270DAZ good for 260V CT, so 365 after the diodes, and 104ma.  A 158M choke with 200ohm resistance and a 15w SE OT with a 5000 ohm primary.

I want to filter the entire supply through a pi filter, so if there's 100ma, the pi drops 20v and I would get 345V on the plate of an EL34.   Will the PT and choke be sufficient to get 8 or 10 watts from an EL34, or do I need a lot more current than the 100ma the PT and choke are rated for?  I'm confused about whether the ratings are for average ma or the max, and I also can't figure out my idle current. 

Thanks

Title: Re: Is this PT sufficient for a SE EL34 and 8-10 watts
Post by: thetragichero on June 12, 2022, 10:30:30 pm

quick glance at datasheet points to yes: https://frank.pocnet.net/sheets/129/e/EL34.pdf
this tool is helpful too: https://thesubjectmatter.com/calcptcurrent.html

Title: Re: Is this PT sufficient for a SE EL34 and 8-10 watts
Post by: tubeswell on June 12, 2022, 11:51:07 pm

Quote from: JustTheDad on June 12, 2022, 08:17:34 pm

158M choke with 200ohm resistance

Is that 158mA? (or 158-Msomethungelse?) The choke wants at least 100mA DC rating

Title: Re: Is this PT sufficient for a SE EL34 and 8-10 watts
Post by: JustTheDad on June 13, 2022, 08:25:47 am

The choke is rated for 100ma and 10H at 200 ohms.  Might be 195 ohms.

Looking at the sheet and that calculator (thank you for the links), I'll be biasing for an idle current of around 70, 10 for the screen, and 20 for the 12ax7's (3).  Just makes it for idle current. 

Is a Hammond PT rated for the average current, or is 104ma the max it's rated for?

Title: Re: Is this PT sufficient for a SE EL34 and 8-10 watts
Post by: dwinstonwood on June 13, 2022, 09:02:41 am

The Hammond datasheet says 122.4mA Max; 520VCT with a 104mA load:

https://www.hammfg.com/files/parts/pdf/270DAZ.pdf

I think I'm reading that right.

Title: Re: Is this PT sufficient for a SE EL34 and 8-10 watts
Post by: PRR on June 13, 2022, 01:46:07 pm

> 20 for the 12ax7's (3).

How do you need three dual-triodes for one EL34??

Anyway your 12AX7es are surely working at 1mA-1.5mA per section, not 3mA.

A single-ended audio stage sucks the "same" current silent or full-roar.

Title: Re: Is this PT sufficient for a SE EL34 and 8-10 watts
Post by: JustTheDad on June 13, 2022, 01:58:01 pm

The way I drew it up, it's a parallel triode first gain to drive pot to first tube of the second 12ax7, then tone stack then second half of 12ax7#2, then Kevin O'connor's effects loop.  I have one sitting around and it requires both halves of a 12ax7.

There are obviously more efficient ways to do it, but I could buy those, so I figured I'd have some fun