Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tschwarz on September 05, 2021, 11:12:41 am

Title: Sanity Check Negative Feedback - ECLL800 Schema
Post by: tschwarz on September 05, 2021, 11:12:41 am

Hi all,

found a very exotic tube in my stash (ECLL800: PI triode and TWO power pentodes) - a full PP power amp in a single bottle. I looked around for app notes and schemata and found the attached phono amp. Something about the routing of the NFB signal into a fully bypassed cathode on V1B looks fishy. Wouldn't this just eliminate feedback completely?

Thanks for enlightening me
Thomas

Title: Re: Sanity Check Negative Feedback - ECLL800 Schema
Post by: sluckey on September 05, 2021, 11:51:24 am

That's not right. Look at this PR schematic to see how it should be done...

     https://el34world.com/charts/Schematics/files/Fender/Fender_princeton_reverb_aa1164.pdf

Title: Re: Sanity Check Negative Feedback - ECLL800 Schema
Post by: tschwarz on September 05, 2021, 11:57:20 am

Yep, Thx! That makes sense!
...

Title: Re: Sanity Check Negative Feedback - ECLL800 Schema
Post by: PRR on September 05, 2021, 01:41:54 pm

ECLL800 are very rare and may be worth enough on eBay to buy an amp you can fix in the future (or on a weekend gig).

Title: Re: Sanity Check Negative Feedback - ECLL800 Schema
Post by: tschwarz on September 06, 2021, 07:17:46 am

Quote from: PRR on September 05, 2021, 01:41:54 pm

ECLL800 are very rare and may be worth enough on eBay to buy an amp you can fix in the future (or on a weekend gig).

...nah, I reckon the fun to be had from playing with such a strange tube would far outweigh the ~€45.- the ECLL800 is valued at the moment.  :laugh: Two full-sized 6W power pentodes mounted back to back with a tiny triode system on top of one of them, sharing its cathode. The datasheet shows an A/B configuration producing 8.5W - practice amp material...