211 amp SRPP driver
 

Hi, I got a Chinese built 211 SE amp that uses 5687 SRPP as a driver. I always used same Philips 5687 for past 5 years and never had any problems. Recently tried Tung Sol and had one fail soon with loud cracking sound. After looking into the shematics I got from the manufacturer I noticed that heater to cathode voltage is MUCH higher then max spec from 5687 tube data (roughly 160V vs 100V in the spec)! What is strange is that I used same Phillips tubes for 5 years with no problem.
Any way, I am now wondering about the best aproach. One simple possible solution would be lovering voltage drop accros the 5687 SRPP by bringing the top plate voltage down to, let's say 230V and bottom cathode up to -80v; this would make top cathode be around 80v so both would sit confortably within +/-100V max spec for heater to cathode.
What I am not sure is how would this effect the actual circuit and ability to drive 211 tube.
As I already did lots of modifications on this amp I am ready to go much further and redesign second stage if there would be a better solution sugested; even using an IT would be not out of question.
Please have a look at attached shematics; any sugestions are welcome!

upload new schematic with missing small triodes heating circuit , then we'll know exactly what to do

The heating circuit is there in the PS part; you will note 3.15-0-3.15 on the transformer secondary; it is simple 6.3V ac with centar grounded center tap.

Whats the voltage,

Between the heater and ground?

Ie I guess the 3.15 0 3.15 powers the srpp and the EC88?
So what is the voltage between 0 and ground?

Regards
M. Gregg

sorry - didn't saw it

anyway - make resistor divider from +390V to gnd , made of 330K/1W connected to 390V and
82K (small resistor will suffice) to gnd , put 10uF/350V cap across lower resistor (82K) , and connect heating CT to dividing point

that way heaters will be elevated to some 75-80V , problem solved

Or float,

with a 0.1uf polypropylene 600v from 0 to gnd.
The divider is better sometimes floating can overcome other problems. :)


Regards
M. Gregg

Yes, both input stages use comon heaters; center tap is grounded, so volatage between 0 and ground is 0V.
The problem with elevating heaters, as you sugest is following:
In SRPP stage the top cathode is at 160V, bottom cathode is at -150V!
This means, total difference between two cathodes in 5687 tube is 310v, basically making impossible to have less then 100V cathode to heater difference if keeping curent operating voltages for SRPP.
This is the reason I got really confused and am open in possible re-design, if necessary...

uff

forget everything I wrote above

doing several things in same time often doesn't help in seeing everything on schmtc .... :(

it seems that small additional xformer is must - having 2 windings , resulting that each small triode position is having dedicated heater winding

then - appropriate resistor divider nets for each one , putting fil of first stage to app. +50V , lower one in SRPP to some - 100V and upper one in SRPP to some +100V

having lower one SRPP triodes in one envelope , and upper one in another envelope is absolute must

Hi, no problem, thanks for trying to help!
The real problem here is that to SRPP stages are just 2 sections of same dual triode, 5687, so they share same heater....
This is why I am asking about advice on possibly droping the SRPP top plate voltage and rising the bottom cathode in order to get two cathodes at around +80v and -80v; heater at 0V should then work fine.
What I do not know and need a help with is how will this influence SRPP performance as a 211 driver?
Other option is to just keep running it vay over the spec using the Phillips 5687 tubes which somehow survived this for last 5 years....very strange as they are run over 60% higher then max spec...but they never had a problem...

that amp needs something as extended rework

try to find Ongaku schematic and compare general principle

problem is that - if you decrease voltage for SRPP - stage will not have enough headroom for driving 211 ; without thorough analysis I can\'t say more and I\'m sorta short in time these days .......

one thing is sure - if you do what I wrote regarding additional xformer , heaters will be where they need to be