driver stage for ppp 6550's

[heinrichs]

hello!

Tubes are quite new to me... but i always wanted to build a tube amp. then a month ago i got a great deal for some parts, some big OPT a PT, 8x6550we, 2xecc82 and some pcb's.

so.. now i have a working tube amp.. but i kinda have the feeling that one ecc82 as driver per side isnt quite up for the job.

i dont have the exact schematic on a computer yet, but it looks alot like the first one on this page, but with different component values ofc.

velleman K8010 kt88 monoblock tube amp,el34,ecc82,ecc88,kt88,300b,845 direct heated triode,mkp, ...


so my question is:
is the ecc82 suitable to drive 4 6550? and if not, then i need some ideers to what to do instead? and i would love some schematics with the ideers.

if needed i will digitalise my schematic and post it tonight.

[SY]

How is the output stage configured (i.e., triode, UL, pentode)? Output stage operating conditions (i.e., B+, p-p load, idle current)? That will determinie requirements for the driver.

[heinrichs]

it is UL,
b+ = 380V
p-p load, is this the impedance of the opt? if so it is 1.75Kohm
bias around 87mA at the moment

i use a resistor between the cathode and ground for my bias, so does that mean that i should subtract the voltage across that resistor to get my effective b+?

[SY]

Yes, you do, but this is good enough for estimation. Your bias at those conditions is likely to be about -30V, which means that the drive signal needs to be 60V peak to peak max. Input capacitance works out to something less than 100pF. This is really not a difficult load at all- a good, moderately high transconductance triode configured as a split load inverter is more than adequate.

Your other alternative is a long tail pair input coupled to cathode followers, but that's more complex than needed. If you can provide higher B+, the same level of complication will allow you to configure the output tubes as triodes in AB2 and get 150 watts easily.

[heinrichs]

so are you saying that my ecc82 is just fine for the job? or that i should look for a different tube? to use in my split-load topology? (with the rest of component modified ofcourse)

and if i wanted a more powerfull amp coupled in triode mode, i could make a "long tail pair input coupled to cathode followers" driver stage, and up my B+ closer to what the tubes allow?

just want to be shure i understand you :-)

and thanks for the help.

[SY]

In terms of drive, the ECC82 as a split load inverter will work adequately. It is not, however, the most linear choice of tube! At the same operating point and with minimal socket changes, you could substitute a 6CG7/6FQ7. An ECC88 will have even more drive capability, but the operating points and component values will have to be moved around quite a bit.

The take-home point here is that your output stage is not a particularly onerous load, except psychologically- it just LOOKS like all those tubes would be tough to drive.

[heinrichs]

excelent!

and thank you again.

[richwalters]

Becareful interpreting the datasheet for 6550 series; there many about. Your'e fine with values for auto/cathode bias. An ECC82 cannot drive without distortion driving parallel p-p fixed bias grid leaks when datasheet values give each tube around 50K ohms. There is some leaway on this but this is nasty load for the driver stage.

richy

[SY]

Originally Posted by richwalters

Becareful interpreting the datasheet for 6550 series; there many about. Your'e fine with values for auto/cathode bias. An ECC82 cannot drive without distortion driving parallel p-p fixed bias grid leaks when datasheet values give each tube around 50K ohms. There is some leaway on this but this is nasty load for the driver stage.

richy

At 50k, the load ends up at ~17k, which is not too terrible for a split load inverter, especially an ECC88.

[ChrisA]

Originally Posted by heinrichs

...
i dont have the exact schematic on a computer yet, but it looks alot like the first one on this page, but with different component values ofc.

velleman K8010 kt88 monoblock tube amp,el34,ecc82,ecc88,kt88,300b,845 direct heated triode,mkp, .....

This first one appears to be a simple and forgiving design. I bet it sounds good too. You can push the tubes more but then you add complexity.

The other schematics below show how to add a long tailed pair phase splitter but that adds another tube and many more parts. then you might think about making the bias of each tube independently adjustable and switches to allow use on ultralinier or triode modes but the amp becomes complex. Keeping the parts count minimal and running the tube conservatively is best for the sound quality.

Simple things you might do are, check if the phase splitter is balanced, are AC volts the same on both outputs, adjust resisters until it is. I'd do this style of fine tuning rather than major changes.