- Posted by danlaudionut (A) on November 06, 2009 at 10:27:12
In Reply to: RE: 300b with Tangos need help posted by timothygurney on November 06, 2009 at 09:11:32
Tim
How about this.
Nice gain and works good with your interstage.
DanL
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In Reply to: RE: 300b with Tangos need help posted by timothygurney on November 06, 2009 at 09:11:32
Tim
How about this.
Nice gain and works good with your interstage.
DanL
Thanks for this MR. DanLoudAudioNut.....excuse the spelling...hhehe
I like the whole thing...battery bias...the 12k resistor from the 300B anode....all in 2 stages...I believe in 2 stages so my last bit was direct heated pentode to the output 2a3...battery biased..nice, but some hum....I have always wondered about the 6gk5...interesting tube...
a 12BZ7 might also get you there....very interesting....
Good Luck in your pursuits,
-3db
Very nice Dan, I like it, however, don't you think that cathode bypass cap on the 300B is a little low? The 3Db down point is about 7hz, which puts it about 1.5Db down at 28hz. For those of us that go full range with our amps, it might not be enough. For example, my speakers are flat to 30hz. I have a 300B amp with a 3.5K OPT, and a 1K ohm cathode resistor, there was a noticeable improvement in bass when I went from a 47uf bypass cap to a 120uf cap.Don't mean to rain on your parade, it's really a nice idea. I like the fact that you wouldn't need a preamp stage. Battery bias, and voltage regulation on the driver stage as well, very kewl.
twystd
Edits: 11/06/09
it is the parallel combo of the cathode and the cathode R that needs bypassed( and this will be a fairly small resistance; 1/gm ).
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
From what I understand the impedance that needs to be bypassed is the impedance of the tube plus the load/mu+1 in parallel with the cathode resistor, which in this case would be in the neighborhood of 500 ohms. Am I wrong?
twystd
Twystd
You are right I stand corrected.
It should be about 63uFd for 5Hz F3.
DanL
I messed up, it would have only been 3/4Db down at 28hz with the 47uf cap. I need to think more before I post.
twystd
Twystd
Sorry, still not right.
2 octaves above F3 with a first order filter
will give you a flat response.
A factor of 10 before the phase is flat.
2 octaves above 7Hz means that
at 28Hz the frequency is flat.
Like I said basic filter theory here.
DanL
Well, I guess theory is just that. I have 300B amps (Welborne Laurels), they have 3.5K OPTs, and a 1K ohm cathode resistor. They came with 47uf Solen cathode bypass caps. I changed them for 120uf Solens (that's what I had on hand), and noticed a very pronounced increase in bass. I've since gone to four 30uf 160V MBGO PIOs in parallel.
The math says (Rp+Rl/mu+1)//Rk is the impedance we need to bypass. So (600(Rp)+3500(Rl))/(2.77(mu)+1)= 1087. Rk = 1000//1087 = 520.8 ohm impedance. I think we can both agree on that. At 7hz C=1/(2*pi*7*520)= 43.656uf so according to the theory, a 47uf cap should be flat at 28hz. Why do my 30hz (they fall like a rock after that) capable speakers reproduce so much more bass with a 120uf cap? I guess it must be my imagination, theory says so.
I usually use 3hz as the frequency of interest which would indicate about 100uf. From previous posts, I see Tre' likes to use 2hz, which would indicate about 150uf. He uses 2hz as he claims the phase is correct. I don't know about the phase thing, but it is interesting, and I will look into it.
twystd
how on earth do you fit (4) of the MGBO's in your Laurel ?....do you have a picture of the inside ?....I am in Brushy Creek....maybe do a listening session at yours and / or mine ?....thanks -Al
Not only do I have the 4 MBGOs inside the chassis, but I also have a Lundahl choke for the choke loaded mu stage, a potted vintage filament choke, and batteries for the battery bias. On the top of the chassis, in between the power and output transformer, I'm installing a huge 100uf 1000V MBGO for the power supply, it's even bigger than the transformers.
Almost the entire wooden sides of the chassis have something attached to them. Now I'm trying to see if I can fit one more 20uf 430V MBGO to replace that Solen cap, that is the last power supply filter cap for the driver/mu stage. I'd send you a photo, but they are apart right now, pending the permanent attachment of the big MBGO on the top.
A listening session sounds great, always like to meet another audiophool in the area. I have a couple of other audio buds in the area. We get together from time to time to listen to different projects, perhaps you'd be interested as well. Send me an email through the asylum, so I have your email address, and we can do a listening session.
twystd
yikes....that'll be something to see....email sent....thanks....
Twystd
I think you may be hearing the phase change
which accounts for the increased bass.
When bass is out of phase you lose impact
and so the bass is duller and muddy.
Sounds like you should stay with 3Hz
so that your 30Hz phase is flat.
Same for high frequencies too.
Design for 200KHz F3 for flat phase
on your treble.
DanL
Dan, do you know where I might learn more about correct phase?
twystd
Dan
Try the 120uf bypass..You figuring a flat response at 28hz assuming you are 500 ohms with the added parallel impedance of the tube.This is why I measure but I would try 120uf and I am almost sure you will like it..You can always take it out.
If the power supply waveform isn't pretty,neither is the sound in most cases.
Michael
I would think it would be more likely that
the impedance of the OPT is lower at
the lower frequencies and not the tube.
DanL
Dan
you are right but its still allowing the tube to see a lower imp with the combo of the cathode resistor and the OT...At any rate,try the 100 or 120uf bypass and tell us what you think.
If the power supply waveform isn't pretty,neither is the sound in most cases.
Twystd
According to my calcualations
the F3 point is 4.7Hz.
Even with a 7Hz -3dB point then
the -1dB point would be 14Hz
and it would be flat at 28Hz.
First order basics here.
DanL
...what is the LF saturation frequency of the OPT?
It's smart to have the prior electronics rolling off a little before this point - because saturated OPTs really hurt the bass tonality. I'd be very surprised if the OPT can go to 7Hz at near full power.
Regards, Allen (Vacuum State)
I admit I don't know, as I didn't measure it, but my ears sure liked it.
twystd
6GK5 mu of 78.
No need for a line stage. Good, low Rp of 5400 and pretty linear looking tube.
I just might have to try that. Thanks.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
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