Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: goldstache on May 25, 2021, 08:23:21 am
-
I’m having trouble understanding how to bias the pentode section of a 6bm8 as a transformer coupled reverb driver.
I’ve been playing with values and topology for a solid day and am not understanding how they derive the numbers in the data sheet.
For instance for a class A amp they state.
Va 300vdc
Ia 40ma
???????
When I use common bias formula
.9 x 7w / 285 (minus vk)
I get 22ma not 40ma. Is this for PP or something.
To be fair, I’m currently powering it with 350v into a 5k verb transformer.
Got about 345 (323v after cath deduction) on the pentode plate.
Screen is at 300v at idle.
I have a 1K cathode resistor 22vdc at K. I have a meter inline with the plate and am getting 18ma.
What am I missing here.
Data attached.
-
22V / 1k ohm = 22mA
by the looks of it if you change Rk from 1K to ~~~ 500 ohms
current should go up and plate volts should come down. redo the math at the cathode Vk /Rk = tube current
there is a point at which you just go in circles though
-
Va 300vdc
Ia 40ma
That's 12 watts! Surely you don't need that much for a little reverb driver. Your numbers seem more appropriate to me. How does it sound?
FYI, here is a simple cap driven reverb circuit that is proven and sounds good too...
-
Thanks y’all. Yeah, I understand the bias and that Ik will include about 5ma of current.
As the spec sheet says.
40ma x 300v =12w!!!
What’s the deal???
That sounds melty to me.
It sounds good and I’m using a trimmer for dwell, so drive current is more than enough.
Just wanna make sure I’m not over-diss’ing the internals.
Thanks for the cap driven variant. Tried several of those topologies and triode mode as well.
Last few builds were cap driven, so I’m looking to tackle transformer driven. I’m using simple mix resistor before PI. Works well.
Just can’t understand the math. And why it doesn’t equate to the data. Those specs stated make a welder.
-
I bet you could lower the B+ considerably and/or use a 2.2K cathode resistor and still have plenty of verb.
I wouldn't get too hung up on data sheet numbers. You might like these numbers better...
http://tdsl.duncanamps.com/show.php?des=6BM8
-
Sluckey, thanks. Is there a particular data sheet from TDSL you’d recommend? I’m seeing the same numbers.
Also, I see you set your cap driven send after v1a and the return gets the makeup stage post eq. I’m on board with that as my dwell trimmer is less than 50% rotation and there’s pretty good drive. I’m not after surf drip here, so perhaps I should try that.
Really like the mixed dry/wet to see a stage, gels them and brings em to life.
In an attempt to keep phase coherence, I opted for send/return right before PI. But now ya got me thinking. The make up stage 6bm8 is pretty good and the mixing resistors helped balance it out. But I wouldn’t mind a little more verb
-
No data sheet. I was just referring to the "Application Data" on the page I linked.
That's not my circuit. That came from this forum (or maybe TAG) about a year ago. There was a sound clip in the same post. I was impressed.
-
> , I’m currently powering it with 350v into a 5k verb transformer.
> Just can’t understand the math. And why it doesn’t equate to the data. Those specs stated make a welder.
The 300V-350V is your number. Where did that come from? Your convenience?
The DataSheet says 200V. No wonder you are at like twice the dissipation the tube maker intended.
V*I should be <= Pdiss.
V/I should be roughly Rl.
Pout of a well-balanced power pentode design will tend to be 0.4*Pdiss. (This spec says 0.5 which is exceptionally good even at high THD.)
You do NOT need or want 2.8W or 3.5 Watts to drive a reverb tank!! Altho people do....
Take the sheet-suggested design. Run your 350V through a resistor to drop 150V to get a 200V point. Total current is 35mA+7mA or 0.042A. 150V/0.042A is 3,600 Ohms and 7 Watts, so you want to splice this up out of several 10W resistors.
Yes, this is a LOT of heat. The 5k loading is perhaps unfortunate. The 12AT7 design 15k-22k RT suggests more like 300V 20mA or like half the heat. You could probably starve it down to 15mA or 10mA without suffering.
-
Excellent info.
Why do they give a 300v AF spec then? Attached the data I found as a reference.
It sounds good where it’s at. I agree I get around 18k for center load of OT.
But the 22k doesn’t sound nearly as nice and of course I’m brimming on that standard parts max dis of 3.5w (does get warm at my present bias)
5k sounds better as implemented. Better than the 8k tap on the same iron.
Also the draw is where I think they should be, gleaning from the data.
Still don’t understand their spec for 300v
40ma
??????
Totally agree that you don’t need more than a watt to drive a spring tank.
For shits, I’ll run off a lower node and see how it performs as clipped in.
If you were going for the drippy bang-arang like you get when a 5w tube is driving a tank, then low voltage jacked up current definitely will do it, and so will this.
Just trying to keep the tube alive and get it to sound good. Often not one in the same.
Thanks to all.
-
> Why do they give a 300v AF spec then?
Choices. You could aim for 300V 23mA. That wants a 13k optimum load. OTOH it could probably work 150V 46mA and 3k load. All these options ripple-through to choice of main power supply and what your transformer winders prefer.
-
Makes sense.
I see you are using
V x V / max dis for OT primary.
But they have stated
300v
40ma
5k
That’s when I started thinking existence is a lie!!! Hahaha.
Thanks for talking me down in voltage and from the ledge.
-
> But they have stated
> 300v
> 40ma
You can't push all the ratings at the same time.
An airplane might fly 3,000 miles, or carry 9,000 pounds, but not both at once.
-
6BM8/6F3P Reverb Driver Attempt 3 - reverb moved to first gain stage - YouTube (https://www.youtube.com/watch?v=QrrTKNlA0i0)
-
Tube Nit.
Sounds great, is that the v1a send in that Bax equipped schematic above?
-
I honestly don't know for sure? It might be? Not my amp.
With respect, Tubenit
-
This build went real smooth. Sounds good!
Thanks to all.
-Joe