slowmotion said:Hi 7N7
I have the schematic of that amp on a big poster
on my wall for inspiration,
found the schematic on Robert Willis' site
( via Bas' site )
I love the interlock switch on the case on that amplifier; the idea being to prevent operatives frying themselves with the 1.5kV
They should have seen me testing the 1.2kV supply I had - 16 BY127 diodes configured in a bridge, choke input, so to test it I had to run it into four 47k 50W resistors bolted to an aluminium sheet. All I had was the DVM, an antique milli-ammeter and the floor in those days - no variac and a load of wires trailing everywhere.
A case of shut your eyes and hit the switch.
Those resistors got very hot very quickly!
BEGINNERS! Don't try this yourselves!!!
7N7
I luv 813's
and have an 813PP in construction, something similar to Steven Robinson's. It's sort of running at the moment, as I'm waiting for parts to arrive from overseas.
Current configuration is 12B4A -> LL1635 -> EL34 triode -> RC -> 813 , all stages balanced. I'm waiting on a pair of LL1660 as the second interstage, and some big PP PSU caps to replace the stacked Mallory's I have in place at the moment. The driver stage could end up being one of a dozen flavours from a 12b4, 6C4A, 6L6, 807, KT88, 2A3, 300B or anything else I find in the box.
The 813's will be run somewhere around 850-900 Va-k, with cathode bias at about 100mA.
I also just scored some huge chokes (200mH at 20A) so I think I'll do an LCLCLC filament supply, to keep the noise and hum down.
This is my "fun" amp as I have another I use mainly, but initial testing with the 813PP, even with too much hum, is excellent, far better than even I thought it would be.
Can I have an anorak too?
and have an 813PP in construction, something similar to Steven Robinson's. It's sort of running at the moment, as I'm waiting for parts to arrive from overseas.
Current configuration is 12B4A -> LL1635 -> EL34 triode -> RC -> 813 , all stages balanced. I'm waiting on a pair of LL1660 as the second interstage, and some big PP PSU caps to replace the stacked Mallory's I have in place at the moment. The driver stage could end up being one of a dozen flavours from a 12b4, 6C4A, 6L6, 807, KT88, 2A3, 300B or anything else I find in the box.
The 813's will be run somewhere around 850-900 Va-k, with cathode bias at about 100mA.
I also just scored some huge chokes (200mH at 20A) so I think I'll do an LCLCLC filament supply, to keep the noise and hum down.
This is my "fun" amp as I have another I use mainly, but initial testing with the 813PP, even with too much hum, is excellent, far better than even I thought it would be.
Can I have an anorak too?
It'll be two very big and heavy monos.slowmotion said:Brett,
Sounds like a big amp, is it to be
one for stereo or two for mono
What are you using for outputs?
Current OPTs are Lundahl LL1620PP wired for 6k:8, as my speakers are about 10 ohms. I'm using them because I have them. Later, I might get Bob at Lancroft to custom wind some for me if I'm feeling rich.
Grid chokes...now there is something that tickles my fancy! As does all Iron in valve amps come to think of it.
Definitely not a mainstream product yet. But I have the feeling more and more people are turning to it...Maybe the new fashion after parafeed or DRD's?
Don't have any experience or knowledge of why they work? To me it seems like one is shorting the grid to earth..
Maybe some diyaudio luminaries can shed some light on this?
But what I can tell you is that most people report great improvements in sound quality after installing one.
Definitely not a mainstream product yet. But I have the feeling more and more people are turning to it...Maybe the new fashion after parafeed or DRD's?
Don't have any experience or knowledge of why they work? To me it seems like one is shorting the grid to earth..
Maybe some diyaudio luminaries can shed some light on this?
But what I can tell you is that most people report great improvements in sound quality after installing one.
Guys,
The grid choke is a very low DC resistance to ground (cathode) for the grid circuit, whereas it presents a very high AC resistance for the audio signal to develop across.
If you are using a tube that requires a low grid circuit resistance, then they're good. Also using them with fixed bias is recommended.
BUT, the inductance of the choke must be high, otherwise you can actually have a loss of gain at low frequencies.
The grid choke is a very low DC resistance to ground (cathode) for the grid circuit, whereas it presents a very high AC resistance for the audio signal to develop across.
If you are using a tube that requires a low grid circuit resistance, then they're good. Also using them with fixed bias is recommended.
BUT, the inductance of the choke must be high, otherwise you can actually have a loss of gain at low frequencies.
Thanks Joel,
But I get the felling that I should have done my homework again...I mean I don't even know the purpose of the grid leak resistor..I always forget adding it to my schematics...also forgot it in my first ever power amp...result...no sound..what I understood then was that you want to get the grid a little negative or something..
Anyway I'll do my homework and come back to post some questions..
Cheers,
Bas
But I get the felling that I should have done my homework again...I mean I don't even know the purpose of the grid leak resistor..I always forget adding it to my schematics...also forgot it in my first ever power amp...result...no sound..what I understood then was that you want to get the grid a little negative or something..
Anyway I'll do my homework and come back to post some questions..
Cheers,
Bas
Grid Chokes Etc.
I'll bet that these chokes are expensive.
In any case most of the valves that require small values of grid leak are large. Given that they are large and in cases such as the 813, are triode-strapped beam tetrodes, or pentodes, there will be substantial anode-to-grid capacitance.
This capacitance has to be driven.
Since in my view, glass is always preferable to iron (usually cheaper and always better-looking) I would rather use a cathode follower if necessary. This will happily drive both the capacitance and the 33k grid leak that I customarily use on my 813s. In fact even say a 6SN7 cathode follower would easily drive something more challenging than this.
In fact I am currently driving 813 from the anodes of a 6BX7, which seems fine - except that I am going to re-jig the amp before long with another experiment.
7N7
I'll bet that these chokes are expensive.
In any case most of the valves that require small values of grid leak are large. Given that they are large and in cases such as the 813, are triode-strapped beam tetrodes, or pentodes, there will be substantial anode-to-grid capacitance.
This capacitance has to be driven.
Since in my view, glass is always preferable to iron (usually cheaper and always better-looking) I would rather use a cathode follower if necessary. This will happily drive both the capacitance and the 33k grid leak that I customarily use on my 813s. In fact even say a 6SN7 cathode follower would easily drive something more challenging than this.
In fact I am currently driving 813 from the anodes of a 6BX7, which seems fine - except that I am going to re-jig the amp before long with another experiment.
7N7
Re: Re: Grid Chokes Etc.
Well, the choke may well solve the grid leak problem, but it does not address the capacitance issue.
If the driver stage is good enough to drive the capacitance properly, it is probably good enough to drive the grid leak too.
Of course in marginal cases then every little bit helps, but it is better to avoid marginality I think
7N7
Joel said:
Well, now that all depends on who you ask!
I use a $10 choke with good results.
And I wouldn't be so fast in claiming an additional tube used as a cathode follower is preferrable to a high inductance, low DCR choke.
Well, the choke may well solve the grid leak problem, but it does not address the capacitance issue.
If the driver stage is good enough to drive the capacitance properly, it is probably good enough to drive the grid leak too.
Of course in marginal cases then every little bit helps, but it is better to avoid marginality I think
7N7
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