ok I Bought it ... what now :D

[Richard]

Hi George, So do we read this as good news or is there more work to be done? How quiet is your normal setup with these speakers?

The input shorting plugs are important on the bench partic with this amp (presumably due to high bandwidth). I mentioned them end of post 6. Found this out with mine on the bench. I was using load resistors and burnt my hand on them with nothing connected and no signal in. It must have been in oscillation from local rf and would probably have been huming like yours if i'd been using speakers for loads. Oscillation can also throw the voltage measurements, yours seem ok but worth bearing in mind.

Rich

[Grog]

i had a crimson battery powered pre amp connected to the inputs, with a signal generator connected to the pre.
i can only guess it was an earth loop type hum?
it's good news, the hum is acceptable, i'd like to improve it, but it's probably an awful lot of work for a smallish improvement.
the yamaha dsp-Ax1 was/is dead silent.. i know it's not really an audiophile amp but my main system is upstairs, and i don't have suitable corners for the klipsch upstairs.

it was about 10:30pm when i got it hooked up in the system, and only had a quick listen at low-ish volume, the mid and treble was sooo much better then the yam, smooth lovely ulta detailed without being fatigueing.
i was however a little disapointed with the lower-mid to bass response, it seemed a littel slow and wolly, and on more busy music like Muse Bliss, it seemed to loose composure and sound a little lost.
apart from the sowters (they currently cost 350% more than the amp cost me) is there any improvements you'd suggest?
whats reccomended for the 4 smaller valves?

[Ali Tait]

You need to give it at least half an hour to warm up before critical listening.

[Richard]

Indeed, let it warm up, but also give yourself a week to get used to the new presentation. It might be that once you get used to it you'll find the ss amp somewhat sterile sounding.

If you want to tighten the bass this can be done by changing the value of bypass caps C17/18. Move in large amounts, it will only affect the bass roll-off, they are 4700uF standard. Easiest would be to fit pcb pins where the caps go to save lifting the pcb all the time, and then start with, say, 470uF, then 1000 and 2200 for finer tuning if needed.

Rich

[Black Stuart]

Richard's suggestion to use PCB pins is something that, in hindsight would have been great to know about when the old WAD forum first started - I cannot rec. this highly enough to those with PCBs.

When you are done with experimenting and have reached a 'I'm happy with this' stage, then and only then take out the PCB and solder all your choices into place properly.

[Grog]

i'm going to give a week or so then swap back, to fully assess the difference.

lowering the bypass caps will increase the degenerative feedback at the cathode? will this mean lower gain at LF with a lower cap value?

it's currently got sylvania input valves and telefunken ecc82's the writing has almost all come of the telefunken, the 'ecc82' has totally come off one and it looks a bit more like 'ecc83' on the other, can't be sure as the '2' is mostly missing.

i cant remember if it's the ecc or ip valve but i turned the amp on (still a little warm) and they flashed over blue. i presume thats just because the HT come in instantly?

[pre65]

Stuart-are you suggesting that PCB pins are not a "proper" way of permanently securing components ?

[Richard]

Originally Posted by Grog

i'm going to give a week or so then swap back, to fully assess the difference.

lowering the bypass caps will increase the degenerative feedback at the cathode? will this mean lower gain at LF with a lower cap value?

it's currently got sylvania input valves and telefunken ecc82's the writing has almost all come of the telefunken, the 'ecc82' has totally come off one and it looks a bit more like 'ecc83' on the other, can't be sure as the '2' is mostly missing.

i cant remember if it's the ecc or ip valve but i turned the amp on (still a little warm) and they flashed over blue. i presume thats just because the HT come in instantly?

Could be but I've not seen blue flashing with small valves. Watch the ecc82 next time at cold switch on and see if it's a white flash from the end of the heater filaments which caught your eye and is normal with many of these. I'd be tempted to get a spare set of small valves anyway.

lowering the bypass caps will increase the degenerative feedback at the cathode? will this mean lower gain at LF with a lower cap value?

Yes, it will increase degenerative feedback at LF as caps have a reactance (impedance/ac resistance) which increases as frequency drops and with moving to smaller cap values. So the smaller value C17/18 bypass cap will stop some of the degenerative feedback at LF being shorted to ground and so cause less output at LF in the way we understand a normal cathode bypass cap to work.



There is a second effect though caused by the bypass cap C17/18 also being the ground connection for global feedback divider resistor R40/41. If the cap is lowered in value it will have greater impedance to the feedback signal at LF in that leg of the divider, so the divider will cause greater global feedback at LF, which will lower gain at LF and lower output impedance at LF (hence help damping). Clever eh .

To see this more clearly try looking at the circuit from an ac point of view at different low frequencies. Draw the reactance value on the cap and see it as a resistor;

At 20Hz the 4700uF cap becomes a 1.69 ohm resistor added to the 6.8R resistor to make the divider leg. A 470uF cap would be a much larger 16.9R added to the 6.8R and so increase the amount fed back at that frequency.

Try it a few times at different frequencies (which gives different reactance values) and with different cap values (also gives different reactance values) and all is revealed,

eg, at 50Hz the 4700uF becomes just 0.677R having little effect, whereas the 470uF is still a significant 6.77R etc.

Reactance calc to play with,
http://ourworld.compuserve.com/homep...Bowden/XLC.htm

Rich

[Grog]

i'll give it a week or so then try the mod above. so from whay i gather above, the gain at LF will drop but the admping will improve.

I checked the blue flash last night, it's the ecc82, and it looks like a gas flame / corona effect. a few seconds after switch on the blue kind of fills the valve then disappears?

I did read somewhere (HFW diy supplement 3 ish) that the 6sn7 would be much better than the ecc82?
i think i should try some new small valves, what would you reccommend? as i think i said i've currently got telefunken ecc's and sylvania 6au6 (or equiv).

[Richard]

Originally Posted by Grog

i'll give it a week or so then try the mod above. so from whay i gather above, the gain at LF will drop but the admping will improve.

I checked the blue flash last night, it's the ecc82, and it looks like a gas flame / corona effect. a few seconds after switch on the blue kind of fills the valve then disappears?

I did read somewhere (HFW diy supplement 3 ish) that the 6sn7 would be much better than the ecc82?
i think i should try some new small valves, what would you reccommend? as i think i said i've currently got telefunken ecc's and sylvania 6au6 (or equiv).

The cap mod - yes as you say - it depends on how you find the bass is not right at the moment. You said, a little slow and woolly. Slowness is often a perception with lower bass that's why smaller speakers seem quicker. Bass can also be a problem for some rooms and limiting it may be a better idea. Ditto woolly if there is more of it and its lower it may not seem so easy to follow bass lines that were easy to follow before. Only way is to try it, loose a little lower bass and tighten the damping and see if it helps.

The valve shouldn't flash blue like you describe at all so swap it. (If it's a repeatable problem swap it to the other channel to see if the flashing moves with the valve proving it to be the valve at fault.) The ecc82 are phase splitters so get the best matched-section quality you can. I tested the section balance and tried loads of different nos. Best were Mullard box anode CV4003. (They aren't best in all amps but are in this amp imho). Next best are British short plate Mullard or Mazda (Thorn) then other European like Amprex and RFT. US types, Philips, and 5814 types, are reliable but dark sounding. The Telfunkens should be superb but not if they're worn out. Again, I tried many 6au6 inc Mullards but this time found US Sylyvania/Philips or GE 6au6 are best. It could be that your small valves are letting it down/worn out and they're cheap to swap so go there first.

Both Mullard ecc82 are here, http://www.langrex.co.uk/sales.html getting pricey now, but the RFT at £8 are good. They'll have 6au6 so ask them which they've got, they may have some CV2524 British, Brimar I think, which are very tight and bright. Clean nos pins though with a fibre glass pencil as they will be black.

Rich