something wrong with my Audio innovations 400 MKII?

[Rooster]

It might have to do with the caps that are over the cathode resistor. As the AI 400 does not really have a preamp, I guess that the two caps over the resistor left and right of the ecc82 are the cathode bypass caps? The value is 0,01 K / 400 V. I have understood lowering the value of these caps will increase brightness. But these caps are already very small (1cmx1cm, MKT).
Would the 0,01 K caps be 100 uF? And then lower them to 47 uF would do the trick, or am I making an error?
There are also 100uf/10volts caps over (i.e. parrallel to) the resistors on the powerboard, but I had thought these were elco's? There are two for the left channel and two for the right channel. That makes sense since there are two el84 parrallel per channel for more power. Should these be altered in value also, or should these be left alone?

[Ianm2]

by all means try ithe couplers if you like them, my view on tweaking, a largely isolated one, but one I have come to by lots of listening and analysis of circuits, is that simply subbing components may make around 5-10% difference, perhaps like spices/flavourings on the end of a dish, but won't generally alter the tonal signature of the amp massively, as that is largely fixed by the unpredictable nature of things such as transformers, valves, circuit design, operating points, and the way the whole thing interacts with the powersupply, perhaps even mains, and speakers/sources. Its an holistic view.

But by all means keep plugging away, the only thing I can think of that will change it in one fell swoop is, apart from perhaps output transformers, which are probably quite good, audionote derived, and perhaps our friend andy grove, perhaps not, is reducing feedback a bit. I do find some audionote derived products to be voiced on the safe or boring side actually, in fact just about every item I have heard...viz speakers AZ 2, kit 1, kit 4, kit pre hmmm not sure, dac kit in its original form, (all interesting designs by other people actually....) which responded to modding well...strange, that a house sound, given my theory that sound is accidental, can't explain that totally yet.

I have just read your last post, the small ones won't be bypass decups, they'll be either stability or couplers, if they are not for coupling, do not touch at all.

the electrolytics 10 volters will probably, almost surely be the cathode caps, making them smaller will roll of bass earlier, however, a lot of designs use overkill for values of these, I personally don't think cathode caps in the scheme respond as well as couplers, but don't let me dissuade you if you want to try, you can't do much harm if you are careful,

I am not sure of the values, but assuming they are totally overdone on capacitance, which they probably are, certainly for the 'small valves', not sure if powers are as over capped, try halving the value and see if you like it.

again, oddly, I have found the mix of the components usually stock to be best sounding for balance, and I am sure they don't try myriads in testing, its done on cost, yet it usually turns out best...odd....but true.

Theroetically, power supply caps benefit from renewing, I have done this in practise a few times and not found it to be beneficial, with only regular replacements, some swear by exotics.

you need a schematic badly.

It sounds a little as though you are clutching at straws, not knowing where to begin or end.

Can I suggest before you do any more, carefully trace the circuit out on the underside, if you havne't done that b4, you will learn loads, its not that difficult, and you need to know what's what b4 proceeding further. If you can do that, and upload into your computer, and either host it on the web, or attach it here, other people may be able to suggest things I haven't.

A good photo or two may be useful to see, too.

One radical mod. is disconnecting the feedeback entirely, the amp will go louder, and it won't be 'voiced' by the compensation cap in the feedback loop, but there may be others to roll off the top end, the only problem is that it may highlight hums and noises, but the effect will be dramatic, again you need to know where these bits are b4 attempting.

I am not sure if its worth keeping an amp that you don't like the sonic signature to, if you want to mod and experiment, fine, but it may be better to find one that's more to your taste, and then tweak that for the better, instead of trying to improve a 'bad' amp, which may not be possible

[Rooster]

Thanks Ian for staying with me:-)!. I have a schematic, and the 100 uf/10 volters are indeed the coupling caps for the el84 (as the schematic shows). As the AI 400 has no real pre-amp but only a 500 k Blue alps and a ecc82, I guess these caps need replacement with the same values to say the least? There is a productiondate on them that says 09/82..... Would they not have 'died' over the years? They also are elco's, which can be used for coupling but C seems better? I agree on the mix of stock components, but I guess some parts just have had their best time after so many years?
Ofcourse I could experiment with values, but would need to know the likeliness of them dying, first.

[Ianm2]

Originally Posted by Rooster

Thanks Ian for staying with me:-)!

1/.and the 100 uf/10 volters are indeed the coupling caps for the el84 (as the schematic shows).

2/ As the AI 400 has no real pre-amp but only a 500 k Blue alps and a ecc82,

3/ There is a productiondate on them that says 09/82.....

4/ They also are elco's, which can be used for coupling but C seems better? I agree on the mix of stock components, but I guess some parts just have had their best time after so many years?

5/ Ofcourse I could experiment with values, but would need to know the likeliness of them dying, first.

1/ decoupling actually. coupling caps are as a rule in the signal path, valve to valve, viewed on the schematic horizontally, usually, decouplers are top to bottom, valve to ground, or psu to ground, vertical, if its drawn properly.

2/ all amps have a preamp as such, its either a volume control and selector, or active, are you saying this has an ecc82 before the main amp? in which case it would be considered to be a real preamp.

3/ did AI go back that far? my,

elcaps are usually 'wet' which dry with age/heat, (usually, tho the effect is a gradual reduction in capacitance, and whilst perhaps not dying totally, they become geriatric and derepit which isn't very pc these days.)

and can go pop dramatically at times, certainly wouldn't hurt recapping the elcaps in the psu with same values/voltage ratings, those on the circuit MAY be altered in value

4/ ??? sorry?

5./dying? provided voltage ratings are ok, they will be ok lifewise.

Can you somehow get the schematic up here, or email it to me? I can then tell you which to alter safely and which ones values may be altered or not.

For an el84 amp, Peter's kel84 is very nice sounding indeed, and a simple circuit to understand and tweak, too, no active preamp or phonostage as the 400 tho'. They come up secondhand fairly regular if you can't stretch to new, but aren't TOO good for repeat modding as the track will lift afer a few go's, I don't know if that's been improved in the latest version, and the mains tx was a bit small/buzzy/hot b4, too.

Sometimes tho', old components actually make an amp more interesting, as the values alter in unpredictable ways, so they can add character, and very old, 40-50 yr old amps can still sound wonderful with aged components, and these aren't hi tech ones, too, another of my theories says progress is bad for sound, modern materials, usually man made, older ones, perhaps less refined, and natural materials, sound better. just a theory, as is most of my spewing.

[Rooster]

Hi Ian, if you give me your email I'll send the schematics, which I have here in digital form. Send your email to kiphaan@home.nl. The schematics are for a MKI version, mine is slightly different with the three ecc83 valves for phono and one ecc82 for preamp. But the parts are the same for the poweramp. You'll only miss the ecc82 board, which has the two ERO green coupling C's (0.47uF/630V) and the two 100/10volt elco's (and a bunch of resitors).

Thanks, Hans.

P.s. Ad.1. your right ofcourse vs coupling/decoupling;
Ad.2. signal goes through selector, volume and ecc82 and then on into poweramp;
ad.3: AI mkII 400 should originate from 1996. Cant imagine they would use parets that old, but nevertheless its on the outside of the 100 uf/10V caps.
ad.4:I must have read it wrond, sorry;
ad.5. thats a relief'

Best would be then to replace the elco's in the powersupply. There are two smaller ones 1000uF/250V and one large blue one with various values and functions. I'll wait for that untill further contact.
The values of the 100uf/10V might be the ones to alter the value from, but I'll wait after you have studied the schematics.

[Rooster]

Hi, I have replaced (allmost) all elco's on the powerboard and the result is remarkable! Voices are clearer and above the music. In the next few days I'll have them all replaced. The change-over from 100uF/10V to 68uF for shift in frequency didnt do the trick, it became much louder and the volumepot was not functioning properly any more. So I put the 100uF back in (new ones) and replaced them all around the pre- and powerboard. So far so good!