Choke loading Anodes.

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Dave the bass
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#1 Choke loading Anodes.

Post by Dave the bass »

Now then, at a recent BassFest MrI bought round a choke loaded 6DN7 that biffed 6EM7 on the nose fair and square in the dynamics arena.

Not to be out-done by the Bedfordshire Bruiser I fitted a pair of cheap Honkers 100H (30mA) anode chokes into the first stage anode of 6EM7. I've kept the 120K resistor as advised and have a 10uF lytic to 0V at the point the 120K and choke meet, also as advised.

Boing and zoing! Yes the mids and HF have that lovely zest but wheres all me bass gone? It's there but attenuated. Its like the choke has created a mid range hump in the response.

The gain has increased slightly I think and the overall sound is very 'exciting' and detailed but ..... oh for my hefty bottom (cue Will!).

Ideas?

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Nick
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#2

Post by Nick »

Well, the 120k to 10uf will have -3dB at lass than 1hZ so that should be ok. But the Ra of the first part of the 6em7 looks to be about 40k, but a RL filter of 40k and 100H will have a -3dB of 60hZ, so you will be loosing a lot of bass.

You need more henries, or a CCS (I prefer L myself, but CCS is very good)
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#3

Post by Andrew »

Yes, 6EM7 is really too higher anode R to work well with choke loading, the high R means it requires a rather large choke to prevent bass roll off.

Can you post a close up photo the choke? It might be it could be regapped for lower current and hence run at higher inductance. What do you reckon Nick, 30mA choke is probably over kill here!

A stacked pair of the little Hammonds might actually be better.

cheers,

-- Andrew
Last edited by Andrew on Sat Jul 11, 2009 10:37 am, edited 1 time in total.
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#4

Post by Will »

Sounds like you need more Henrys and the valves anode res. could be too high for choke loading.
Try 2 in in series.
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andrew Ivimey
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#5

Post by andrew Ivimey »

hchello boysssss (I'm Marleine Dietriech for this posting - no I'm not, can't be bothered)

I missed this when it got off the ground.

6EM7 , nicer output anode resistor, much higher input anode resistor than the beloved 6DN7.


There are a couple of differences to my circuit to MrDtB's too. For the anode of the first stage I couldn't be bothered with stepping down the HT so its about 300volts into a Hongker 200H choke - its fine! For teh anode of teh second stage its parafeedly into the output. V.cheap claimed 5K primary output trafos as the parafeed 'choke' (can't imagine its very many Henneries) into old PP (Dansette Major type record player) trafos with the obligatory polyprop cap which is either 3 or 10mfd - sorry can't 'member.

Its a nice amp fa'shure and thank you MrDtB for the compliment. I have been whittling away at 6DN7 for a while now and think that just maybe try CCS for input anode of the first stage (Not my idea, I hasten to add!), but why not and then p'raps parallel the valves (like I did with 6DNPP a couple of years ago (but keep it as SE this time) just to get a little more welly. Tho' maybe with Lowther who needs it!

These valve types really are quite amazing if you are happy down around the 1 watt level. I am also full to bursting with praise for the 1626.
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Dave the bass
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#6

Post by Dave the bass »

Aaaaaahhhh, I see. Thank you so much for the advice gents.

The higher the Ra of a valve the less likely its due to be a happy bunny with choke loading its Anode.

For once (ahem) I've just read up on the anode resistance of various valves on TDSL, check me out eh.

So, the 1st stage in 6EM7 has an Ra of 40K like y'all say, not a prime candidate for chokey dokeys (unless I can find zigga Henry Chokes).

A 76 valve has an Ra of 9K5, much more realistic yes?

A 26 valve has an Ra of approx 7K6, even betterer?

A 27 valve hs an Ra of approx 9K.

A 10 valve has an Ra of approx 5K.

I notice a trend here... the older the design of the valve ... the lower the Ra.. 2nd thing I just noticed, older valves have less gain.

Just to check... Yesss..........an ECC83 has an Ra of approx 62K5, blimey. Don't ever chokey dokey an ECC83 unless yer a bat?:lol:

Just out of interest (and to make sure MrI isn't totally barmy) I checked the Ra of the first stage triode of 6DN7, lordy approx 9K. Hence the (part) of the reason MrI's little tiddler responds to chokey anodes and 6EM7 doesn't.

Damn, I'm 'bright' this morning.

Conclusion, Ra is 'quite important', I have soooooooooooooooooooooooooo much to learn.

Audio-talk. "the forum that can teach a monkey to build an amp" :lol:

Thanks Gents, really 'precciate it.

I'm gonna check out more Ra's. This is exciting.

DTB
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#7

Post by Andrew »

Dave the bass wrote: I notice a trend here... the older the design of the valve ... the lower the Ra.. 2nd thing I just noticed, older valves have less gain.

DTB
Dave, check out 5687, ECC88, 5842, 6C45pi (aka 6S45pe), 437, 6N6P, 6N30, ECC99 and lots more including many late pentodes when triode connected.

cheers,

-- Andrew
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#8

Post by Nick »

OK, now you have found ra and gain, there is one more number you need, the transconductance or gm. This is a measure of how the current flowing through the valve changes with a change of voltage on the grid. (thats why it is transconductance, how a voltage changes into a current, another name is mutual conductance).

Now first big thing, as its a measure of current (amp) against voltage (volts), we already know a relationship between volts and amps, its called ohms law, and the ratio of current and voltage in a conductor is V/I = R where we call R the resistance in ohms. In a valve we want to measure the same thing, but in this case its the other way round, how the current varies with the voltage. so I = V / R, where 1/R we call the conductance and is the recriprical of a resistance, and just for fun it used to be measured in mhos (ohm backwards), or now in siemens.

Its the ma/v measure you will see on valve sheets, and is what we talk about as the gm of the valve.

Now the second big thing on a Sunday morning, you know of anode resistance (ra), you know of gain (µ), now you know of the transconductance (gm), now the magic formula that holds for all triodes is

gm = µ / ra

So if you just have two, you can always calculate the third.

So when you hear Andrew L and myself wittering on about valves with high gm, thats what we mean, valves that can have high gain AND low anode resistance. Most valves have lowish gm, which means you can have high gain and a high ra (ECC83) or low gain and low ra (76) but not both.

It might not click and make sense at first, but it will, and I just wanted to plant the seed :-)
Last edited by Nick on Sun Jul 12, 2009 10:20 am, edited 1 time in total.
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#9

Post by Dave the bass »

Andrew wrote:
Dave, check out 5687, ECC88, 5842, 6C45pi (aka 6S45pe), 437, 6N6P, 6N30, ECC99 and lots more including many late pentodes when triode connected.

cheers,

-- Andrew
Gadzooks, you brute! My world turned upside down!

5687 Ra = 2-3K (depending on Va I think)
ECC88 Ra = 2.5K
5842 Ra = 1.8K
6S45pe Ra= its all in blummen Rushen but I think about 3K5
417 Ra = 1.8K (couldn't find 437)
6N6P RA = 1.8K (?)

.....Ooooooo.... I see. I retract my age/gain/ra observations :oops:

I have soooo much to learn...etc etc etc etc :lol:

DTB
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#10

Post by Nick »

.....Ooooooo.... I see. I retract my age/gain/ra observations
No, you are still right, look at the age of the valves Andrew mentioned, and you will find they are all a generation (or) two after the 76. And guess what, they are all high gm valves :-)

If you just want low ra, look up the 6c33c :-)
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#11

Post by Dave the bass »

Nick wrote:
No, you are still right, look at the age of the valves Andrew mentioned, and you will find they are all a generation (or) two after the 76. And guess what, they are all high gm valves :-)

If you just want low ra, look up the 6c33c :-)
:shock: 130R ish? Blimey.

I did notice when looking at 'compactron' data sheets the 'power valve' section generally has lower Ra than the smaller one.

I'm out for a skootle just now but I'll be back, don't you worry, more swotting up later. promise.

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#12

Post by Andrew »

I thought the 6C45 was lower than that but its not important, that main thing is you get the idea and now Nick has thrown you in at the deep end....well you have no excuse. Some of them can even make a nice spud amp.

Now, if I were you, and its what I have decided to do with my 6EM7s, is forget the first valve and try one of the above instead with your nice choke, 5687 might be very nice. This is all very Barker-esque, doffs cap to Mr. B.

Have a think on this as its not as simple as it first seems as you have to juggle the DC coupling and now that both valves are not in the same envelope you get all the problems that the Monkey is designed to avoid.

cheers,

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#13

Post by andrew Ivimey »

and a few others besides ... which don't make for good drivers. (obscure ruskies apart the 300B weighs in at 800ohms)

If you hear me wittering on about trasnconductance I am usually talking to myself and perhaps its better to avoid me.

But although I came to what transconductance was many years ago with op.transconductance i.c.s which became the heart of any Electro Harmonix stomp box, it was from the Lynne Olson artickles that ended for me with 'Amity'. It was his way of saying (what others would say too) about the early generations of valves with their under 10K rA and the 'modern' generation epitomised by the 5687 which although is great, it is generally agreed that the early ones sound so much sweeter - oh, and of course there are other reasons e.g. you pump one amp of heater current through a 26 and it is lovely. You pump 1 amp of heater current through a 5687 and ... larks! but the modern titchy valves just don't sound so good. The otherwise very splendid and worthwhile 6922 has too much 3rd harmonic distortion, I am told... and so on.

its a lovely day today
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#14

Post by Dave the bass »

OK. Swotting again.

Now then, this Transconductance measured in 'mhos', I've seen that detailed in some Data sheets. If a unit of resistance is 'negative ohms' it becomes an indication of 'gain' or amplification yes?

Just checking I've got this bit right before going on.

Not quite as bright as Sunday what with it being a Monday .....

DTB
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#15

Post by andrew Ivimey »

Monday might be the high spot of the week; cool fresh morning of brightest summer and al :D l
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