Bass Power Amp

[Leevi]

I'm planning a bass power amp and drew the following schematic for it.

The idea is to keep the sound as clean as possible even on high volume.

1. I think the ultra linear taps should be used for screen?
Is it recommended to use any resistors there?

2. I'm using the VOX PI. Do you think some other PI might be better for this?

3. Is there anything in the circuit I should change/add/remove?


/Leevi

[Willabe]

Hi Leevi,

I think that maybe  -48 on bias isn't gonna be enough to tame those 6550? IIRC  a good ball park # to shoot for on OP tube grid bias is 10%
of the screen voltage, so you would need to have some extra left over to still be able to bias the OP stage a little cooler if you wanted to (you said you wanted it clean) or if you get a hotter set of tubes.  So if you end up with 450 vdc or so on the screens then, 10%= -45 vdc, not much room left to play around with. Maybe I have this wrong? OTOH it would be easy enough to wire the -bias supply up as a vol. doubler to get the extra -voltage . Just a thought.

    Brad         

[PRR]

220K grid resistor is NOT legal for two 6550!!

Book says 50K each a staggering 25K per side of a quad. You can cheat this a little. But go study some four 6550 stage-amp plans, steal the whole driver and grid coupling scheme. You should probabaly be thinking 12AT7, 47K per plate, 75K-100K per grid-pair.
http://www.el34world.com/charts/Schematics/files/fender/bassman_100_schem.pdf
http://www.el34world.com/charts/Schematics/files/fender/BASSMAN_135.pdf
http://www.el34world.com/charts/Schematics/files/ampeg/ampeg_vt22_vt24.pdf
http://www.el34world.com/charts/Schematics/files/ampeg/Ampeg_V4&_VT22_RevG.pdf


Use _individual_ 1 ohm cathode resistors under the 6550. 280mV (280mA) could be 70mA each, or 90-90-90-9mA, one tube lame and the other three over-heated.

Add a GROUND to the bias supply or it will not work!

Change the first 10K resistor off the bias winding to 2K or so. The first cap will charge to -65V which is ample. Then tinker the other 10K and 47K to give -57V to -50V, a good range for smoke-test; then refine to give your final bias with the pot shorted.

Book says bias this condition with -48V, but tubes vary, and we often run stage-amps cooler than book.

UL does not want screen resistors.

[Leevi]

220K grid resistor is NOT legal for two 6550!!

Book says 50K each a staggering 25K per side of a quad. You can cheat this a little.

OK

You should probabaly be thinking 12AT7, 47K per plate, 75K-100K per grid-pair.

Now the question is which one of the PIs supports best my goal of clean sound

Use _individual_ 1 ohm cathode resistors under the 6550. 280mV (280mA) could be 70mA each, or 90-90-90-9mA, one tube lame and the other three over-heated.

Add a GROUND to the bias supply or it will not work!

Yes

Change the first 10K resistor off the bias winding to 2K or so. The first cap will charge to -65V which is ample. Then tinker the other 10K and 47K to give -57V to -50V, a good range for smoke-test; then refine to give your final bias with the pot shorted.

I just put some values there. I have noticed that the bias supply must always be tuned to right range.

Thanks
Leevi

[sluckey]

Take a look at the Sunn 2000 or model T. They were both simple but very popular and successful bass amps using quad 6550s.

http://www.el34world.com/charts/Schematics/files/sunn/sunn_model_t.pdf
http://www.el34world.com/charts/Schematics/files/sunn/Sunn_2000s.pdf

[Leevi]

Sunn seems to use a resistor on the plates of the power tubes.
What is the purpose of that?
/Leevi

[PRR]

> look at the Sunn ....model T

Thanks. The 6550 Sunns I looked at were Dynaco drivers; the T is very Fendery.

> resistor on the plates of the power tubes

Two parallel tubes against stray wiring reactances can get into cross-oscillation at radio frequencies. You can't hear that, but they can't handle audio smoothly while also screaming full-power radio waves.

The radio impedances are small and 50 or so ohms is enough to break-up such trouble without significant waste of audio power.

[Leevi]

Updated schematic based on the discussions above.
The PI is now a combination of Sunn and Fender and should probably be modified.



/Leevi

[sluckey]

You REALLY need a NFB loop for this clean sounding bass amp.

[Leevi]

Insert Quote
You REALLY need a NFB loop for this clean sounding bass amp.

OK, I'll try the NFB anyway. I think the need is very much depending on the level of the input signal?
In my case the input comes from an SS preamp.
/Leevi

[Danskman]

You REALLY need a NFB loop for this clean sounding bass amp.

Really? an NFB loop with an UL-tapped OT?
I thought it was an heresy, doubling the NFB effect ... Am I wrong??

[sluckey]

Really? an NFB loop with an UL-tapped OT?

Dynaco, Sunn, and Fender thought it was good.

[Leevi]

I think the NFB is needed if you can not manage the distortion or oscillation in other way.
IMO the amp sounds better without NFB.
/Leevi

[chocopower]

UL does not want screen resistors.

i´m working in 2xEL34 UL amp what already had 1K screen resistors.
Plate is 320v and screen 315 vdc.

What can i wait if get them out?

More volumen?
Less compresion?
 

[Ritchie200]

Hey Leevi!

Have you considered maybe a cathodyne with paraphase driver design like the Marshall Major/Williamson?  It's a UL design that will give you lots of clean womp.  The driver design is perfect for your application.

Good luck!
Jim

[Leevi]

Thanks, I have to check that.
/Leevi

[kagliostro]

Hi Leevi

Here are Famous Brand solution to your question

Kagliostro

[Leevi]

Thanks Kagliostro
It looks quite complicated.
/Leevi

[kagliostro]

Hi Leevi

I've sent a PM to you

Kagliostro