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In Reply to: RE: 6B4G Single End amp in a new suit ? posted by pix on April 02, 2017 at 23:01:29
What are the resistors at the grids marked "332R"? If they are wirewound precision, they should be changed to carbon composition. I also suggest that you remove the 10,000uF electrolytic at the cathode of the driver. Not sure 120 ohms is appropriate for that tube in triode mode. Does this schematic represent a commercial product?
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Buy Chinese. Bury freedom.
Follow Ups:
I am not sure about the grid stoppers. But due to the size of them I hardly think they are ww-resistors.
Still I have ha hugh stock of HOLCO-resistors, even though not carbon composition I think they are suitable for this application.
No, this is not an commercial Product. I bought it from a friend whom designed it severa years ago.
Initally my idea was to use some of the fine parts in it for my own design. But after using it for some weeks, I decided it was better to make new chassis for this one. And in the process, also make some improvents (if possible).
Whats worry me most is my fairly hi Zout frpm my sources, driving a fairly low Zin at this amp. Ideas such as replacing the 25K volyme for a 100K dito came to my mind. And also try the E280F as a pentode for decreasing the miller cap.
Anny ideas around that?
"even though not carbon composition I think they are suitable for this application."No, they're not. Only carbon composition is suitable for use as a grid stopper. All others risk creating oscillation that can be difficult to identify and troubleshoot.
No reason to have a 25K input Z. Remove the network and install a 100K pot to ground at the grid. Also, if you're interested in a pentode front end, the EF86 is one of the best. Internally shielded, stable, low distortion, minimal microphonics, etc. I won't use anything else.
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Buy Chinese. Bury freedom.
Edits: 04/04/17
Ok. I will change to carbon composition resistors.
Due to the input network I have a 100 K stepped ladder switch too try.
How high does the grid leak need to be for the E280F in triode mode?
I'm not familiar with the E280F specifically, but 100K is a safe value for a grid resistor.
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Buy Chinese. Bury freedom.
For the input stage( or any other, for that matter), as low as possible w/o adversely effecting the preceding stage. I like a linestage that can drive low loads...and for amplifiers, have run the grid resistor as low as 8kOhms on its input stage. Makes for an interesting linestage testing rig; few will work well like that...:)
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
There are lots of good pentodes to use *AS* pentodes. The lower gm and plate dissipation of the small, nominally 'signal' ones require a fairly high value plate load resistor to deliver adequate gain. When working into the grid of a power triode, it is not only the Miller of the voltage amp, and its loading from the volume pot, but the Miller-amplified capacitance of the final you'll have to worry about.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
So, what you are saying is that using the 6B4G as a penthode could cause drive loss compared to use it as a triode?
Or do you mean if changed to another driver tube?
I don't think it is possible to use 6B4G as a pentode( though arranging them in Cascode is perhaps possible ). The late production 6AV5 Sylvania tubes were sold as 6B4's and I do not know where the connection of g2 to anode was made.
There are lots of small and medium-sized pentodes to use for voltage amplification. Try a small TV sweep tube...:)
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Isn't the B+ across the 6B4g supposed to be 250vdc? Using a 10 ohm resistor drops the entire B+ across the tube and at 50ma,he is at full dissipation for that tube.If it was setup for fixed bias,he could control the current at least when using a 10 ohm cathode resistor.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Some people uses 250V across the 6B4G (and often 60mA Idle current). This would give 15W Plate disipation. However, the OPTs often used is 2500 ohm load.
In this case the Lundahl OPT is a 3000 ohm making the designer to chose a slightly higer voltage. And to stay with the 15W dispation, the Idle current caused of that decreased to 50mA.
The 10 ohm resistor at the 6B4G cathode is just for measure purposes.
50mA current through it drops 0,5V which is negligable in this case.
You are good.I actually run a pair of my Fisher 100 amps which are identical to the 80AZ amps that take EL37s.I had built an outboard supply for separate filament transformers biased with a 1.2k 10 watt cathode resistor.It is actually running at 290v across each tube but these amps sound incredible and I'm making 16 watts output per amp.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
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