affordable valve company not responding

Nice speakers LT

 

Thanks, Tannoy Cheviots (HPD315a). I've heard only good things about D700's also.

 

Well, I tried to check the voltages and currents. Problem is, the readings didn't make any sense. For one thing, the grid 1 -ve voltage was actually +490V. The grid 2 +ve 100-250V was actually -190V. Also, the Svetlana EL34's supplied with the amp have the layout for 8 pins but pin number 6 (+ve anode) isn't actually there (the valves only have 7 pins).
I tried to check the +ve DC current at the cathode but heard a faint buzzing from the amp, so decided to call it a day (on removing the meter, the buzzing stopped).
The amp is still working fine, but beyond checking some values, I didn't want to electrocute myself.
I checked everything with all the EL34 valves removed and placed the meter probes into the respective pin sockets (-ve lead on the grounded chassis). Was I meant to leave the valves in and check from underneath (via the ceramic valve bases)?

BTW, in the circuit board underneath, there are actually 4 trim pots....I assum these are the pots for each valve. They were all set to the same rotation (amount), but I didn't want to delve any further, so don't know what the bias settings would be.

Long and short of it?....I think I'll just get a matched quad from an internet valve shop and hope they aren't over/under run too much.

 

Gaffer,

Take a nudie pic, of the amp that is. I dont know that amp but Id have thought it would be set-up with a resisitor to ground from the cathode, its this that you can use to take your readings off. Much safer!

Somewhere theres a picture of a ming da amp showing the internals, it wasnt that long ago. That clearly shows a typical set-up, Id have thought yours should be quite similar. Have a look.

 

Thanx T-Bone. I did take the opportunity to take picces of the internals. I'll stick them onto my flashdisk and upload them from work (broadband as opposed to 9kbps via mobile I use at home ).
The internals are similar to the mingda/icon40/primaluna etc etc.The only difference between these amps I could see was the amount of circuit board as opposed to hard wiring used.
Anyways, I'll try to stick them up sometime 2moro if I can.

PS, yep, had a look at the piccies on my camera again, there is a resistor to ground in there, connected to the anode (fortunately I took a macro-close up piccie of the valve bases too, so you'll see what I mean).

 

I think from memory, the avc amp has 3 trim pots each side, 2 will be for the bias, and one will be for the ac balance, which should be equal, on each side, I don't know much about that, its pushing the limits of my knowledge.
I coudln't tell from drawing the circuit, it got a bit fiddly up there,so I gave up, best to leave the bias alone and just get a matched quad.

you may have more luck than I in working it out.

you have the measurements/pins wrong if you are getting those readings, should be something like

anode 490 volts
grid 3 is the same as cathode
grid 2 around 485 volts
grid one, -45 volts or so
cathode, not sure not many volts.
typcial bias measured across cathode resistor will be around 30-40 millivolts but could be wrong, its been a while since I played with this fella

you need to stick the black end of the voltmeter in the speaker terminal black on the back of the amp, and just probe with the red for voltages.

 

We'll have to see the piccies to be sure on this one. In the mean time take a reading after the resistor and see what you get. Dont forget to read the value of the resistor before you turn on, it'll prob read 10ohm if it is for the purpose we're looking for. Also, remember to set your DVM high and work your way down the scale, as LT said your looking for something between 30-40mA, this would read 0.XX when your down to around 20V DC or similar.

 

Yes. The k pin value was 9.5 ohms (or thereabouts).
I'll reduce the photo sizes (as they're 5 megapixel) to about 800x600 and put them on imageshack if I can.

I've re-assembled it so I'll post the photo's first and you can see what you think before I try again.

Lt Cdr Data - definitely only got 2 pots per side (so I assumes one pot per valve), but I may have missed something. You'll see on the photo's.

 

well here's a link to lims website.

it does indeed show only 2 pots, the blue things at the edge...that's strange, I recall mine having 3...ah well perhaps I am wrong.

note the size of the choke on the left, the one I had was tiny!! no doubt he put a big one in for the fotos.

I think its a disgusting attitude personally to insist on buying valves only from him, after the warranty goes, you are free to do what you wish.
professional companies would still offer servicing.
There are some serious shysters around in the audio area.secretive, unnecessary and unethical.

http://www.affordablevalvecompany.com/el34buil.html

 

LT,

Im impressed that you've managed to navigate you way round that site of his!

Maybe gaffers is a simpler amp than yours? Maybe your was a lim signature etc etc, who knows what strange things he does.



That one in the pick should be straight forward to bias.

 

Well I'm surprised Lim stuck a photo on the web (wouldn't want someone to "reverse engineer it" would he?)

Any ways, that's my amp alright, all caps/resistors/chokes etc seem to be the correct size

 

yes, its odd, I have no idea why he is so secretive, all the resistor values are on the pcb and cap values, too.

I drew it out myself, there's nothing strange or proprietary about it, just a normal amp design.

Sadly I have a host of schematics I would like to share with people on the web, but no scanner so can't upload them...its going to be a lifetimes work to make public info which ought to be.

 

I keep meaning to draw out my dynavox although I dont think there much between that and the WAD EL84. Gaffer, those caps are just crying out to be changed, a very easy amp to work on by the looks of it!

 

Well here are the piccies:

Firstly, notice the subtle modification to the fascia. It started out as an accident when I spilled some jam on the front (was eating a donut too close ) and ended up wiping it up with some isopropyl alcohol. Then the bloody Logo started to come off...NOT impressed. So I made my own to replace it, and took the opportunity to correct the Knowlegeable Lim's mistake of calling it a Class A amp when, in actual fact, it's a class AB Push Pull Ultralinear. So off came the innaccurate "Class A" and on went AVC (stenciled via some WHSmiths transfers). You can also see my home-made tube rings for the pre-amp 5670W's:


Here is a much larger detailed interior shot. This should give you most of the info:


Finally, here are even closer macro shots of 2 of the trim pots and 1 of the valve bases. The socket which has the black lead soldered onto the large grey resistor actually isn't used as a socket because the EL34's that plug in have no corresponding pin here (position 6):




PS T-Bone : Which ones? there's loads of 'em in 'ere

 

T bone will tell you but the caps to change are the 4 bottom blue ones on each side, these are coupling caps in the signal path, personally I wouldn't bother myself, I have done too much fiddling with valve stuff, it changes it a little, but some are over hyped like paper in oils and not worth the cash.
jupiters and teflons are the best.
RS arcotronics polypropylenes are as good as hovlands and mcaps which I don't rate, having tried virtually all caps.

no probs doing the biasing with 2 pots,

the black lead goes to ground., so you are measuring the volts across that big resistor, makes no difference if you put the black on your meter on the black lead or black speaker terminal.

hope that helps

I drew out the dynavox roughly, I have a sneaky feeling that phase splitter, yes its very similar indeed to the kel 84, cannot drive the el34s properly. It will cause a bit of treble roll off.

 

Yep, as Lt says, the 4 blue caps on each side (you can ignore the top blue one). Ive used the RS caps mentioned but I always seem the go back to the LCR ones from maplins holiday camp, I found they do make a difference but would spend £20 on one cap. Its a very straight forward layout, a good idea is to start with the signal coming in and follow it around the board, you'll get a good idea of whats doing what then.

Biasing is as simple as LT says you should have no probs but Id still buy a matching pair or quads.

LT, the dynavox is worth trying with a pre I know some prefer it used as a straight forward power amp. Im quite happy with it as it is. Ive had another play with the 6u8's and Im amazed at how much difference various ones make, much more than I would expect, would this back-up your theory? If ive time over the wkend I might wire it up in triode and see what that does.

 

Thanx for your advice guys.
I'll check out the bias first and see about matched quads before I do any further tinkering

 

Right, here are the values:
Heater = 6.3V @ 1.6A but I measured 0V
Anode = 470V measured (meant to be @ 100mA - not measured) ?Res = 2kOhm?
Grid 1 = -43.5V measured
Grid 2 = 470V measured (meant to be @ 15mA - not measured)
Grid 3 = 0V measured
k = 95 ohm resistance measured

voltage measured across resistor ? = 0V

pins on valve = 1, 2, 3, 4, 5, 7, 8 (ie NO cathode pin present)

sockets in base = 1 & 8 soldered together (grid 3 & k).

I wasn't able to measure any currents.
So is the bias 0V then? (or -43.5?)

EDIT: I've decided to go for matched quads for the EL34's, and replacement Northern Electric 2C51's for the pre-amp section when I do eventually need new tubes (as the amp still seems to be working nicely even after 3 years of use ).
Thanx or your help LtCdr and Tbone.

 

This is very old noos I know, but am having correspondence with someone who's got what looks like setting-up problems with an AVC amp, though originally looked like heater supply fault. Sympton is way down on power on one channel, and stays on the same channel if if valves swapped over.

No pin 8 is the cathode. Pin 6 is always missing on EL34.

Yes, 1 = suppressor grid, 8 = cathode.

I'm assuming those currents are for two valves together, otherwise waaaay too high for one, that would make the anode power dissipation 47 Watts!!!! The absolute max. for an EL34 is 25 Watts.

For a HT supply of 470V, therefore, the anode current must not exceed about 50mA. That's about 0.5V across your big cathode resistor. Which I'm guessing is meant to actually be 10 Ohms but have to allow for tolerances.

 

Hi, i can supply M.F. service manuals but only on the process of the customer looses the warranty, witch i think is fair, if you liten to other clones they all sound different, the sound of any audio eq. is very personal, the after sales service is more important,
nando.