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At high volumes (12 o'clock and up) my Jolida 302b produces a slightly grungy, congested sound. It is relatively minor near 12 o'clock, appearing as distortion and congestion on the most complex, loud, and low music, and it increases as the volume increases. I'm guessing at a few possible causes:1) This is just the way the 302 is.
2) I've damaged my amp or tubes (It has been run without speakers for a few seconds when a terminal broke off a cable and was also hotswitched once).
3) This is a result of the modifications preformed (They were done by an Jolida Authorized modder)
4) I need different tubes. I'm using EH front end and Svetlana EL34's
5) It's actually my speakers. I haven't had the chance to hear either of my components with other gear, but when I was emoing my speakers with a different amp at a dealer I didn't notice this.Any thoughts on the cause of this or possible solutions?
Follow Ups:
That is the way the amp is. There is too much gain on the amp.
My 502A is the same(change resister to use EL34s).The amp is at full power with 500mv input but most CD players output at 2-2.5v.
Alex,This is not good. Did you alter the sensitivity of the inputs? Or lower the bias on the gain stage tubes?
I suppose a change to a smaller coupling cap could limit the voltage placed onto the EL34's---with the "coupled" change to the tone.
Twisty---BTW---I sent a little blurb on bias supply voltage stability. A tech can alter this negative voltage, to make the EL34 more overbiased (colder). Or you can bias the tubes to a colder point---where you can get 3/4 volume, without heavy distortion.
Otherwise, I'd suspect others on this board may know of some tricks to altering the bias on the gain stage and phase inverters. And reduce the input sensitivity.
Good luck.
I'm willing to accept that this is the nature of the amp. We're only talking about a moderate degradation in quality. If the amp is a '10' at low-to-moderate volumes then at the highest comfortable listening volume in my 14'x14' room it's an '8'. At this point I'm a bit past 12 o'clock on the volume pot into 90db speakers.At the price of this amp, I expect tradeoffs. If my music is reduced in quality when it becomes painfully loud, I can deal with it.
I'm not making light of this issue or portraying it as insignificant, and I do wish my music was more clear at 'rocking out' levels. However, I expect a $1000 product to include trade-offs and weaknesses. Even in light of this, I consider the 302b to be a truly great product for the price.
However, if there is a remedy, either tube rolling (5751 for preamp or in cd player?) or a repair to my unit, I'd be ecstatic. I'm willing to live with this, but I'd love to remedy it.
T,Be sure that those power tube anode plate DO NOT turn orange, at any time while listeining. You may need to turn down the bias (colder) to keep the tubes happy.
A potentially ballistic power tube can do a bunch of damage to the amp.
If indeed the gain is "too high", I might be tempted to put a 5751 in the 12AX7 sockets and see what happens. It may help.Second, I might contact Bill Baker (Response Audio). He's "Mr. Jolida" from what I can see, and while I have no direct connection with him, I'm more and more impressed by the way he contributes to this forum (and his courteous manner!). He's obviously a guy who wants to help. He may know something that he could pass on that helps in this case.
I got my Jolida 502A amp from Bill/Response Audio, Bill is great and knows alot about Jolida and tubes. I use to ask him way too many questions.
The amp is at full power at 12oclock and start clipping a little after that. I change the bias resistor to use both EL34 and KT88 power tubes. The bias pot is still adjustable. I did not change any resistors on the pre(gain) and inverter on the amp.
m/t
T,It's sounds like perhaps the amp is biased too hot. Like early clipping, at volume. May want to check this and if you have the tools---the plate voltages.
While you are checking the bias, see if the tubes are matched (at least the push-pull pairs).
Now, this is going to sound nutty---but did you change the gain stage and phase inverter tubes, recently? It's easy to mix up the position of the two and overdrive the amp. Just a thought.
Thanks, but not of these seem to pan out.Bias is correct 40mv.
I do not have a tube tester, but tubes were purchased as a matched quad and have their matching test readouts printed on the boxes.
Preamp and driver tubes are correctly positioned.
T,If both channels are crapping out on you, at the same time---and it ain't the bias or gain stage (or variety of different input sources)---How old are your electrolytic caps, in the main power filter supply?
Sometimes, when these goes dry, the amp can lose the "oomph" it once had. Especially, at volume.
Also, how's the wall outlet's VAC? A bit too low can also cause early distortion.
All of the gear is less than 2 years old. I did just switch from 8 ohm to 4 ohm speakers. Of course I'm now using the 4 ohm terminals. The bias should remain the same (40mv), yes?I think there is a brighter orange glow to the power tubes. The bias reads correctly.
I don't have the gear to check the wall VAC. (can I really just plug my Rat Shack multimeter into the outlet?!?)
Nothing's really 'crapping out'. It's just a bit of grunge/congestion/distortion at volume.
Perhaps unlikely, however source with higher than amp specification input voltage can cause distortion when turned up.
I'm using Jolida's 100 cdp and 302b amp, so that's probably not it, but thanks.
T,Your idle bias current may be in range, but it sounds like the plate voltage is too high.
Orange colored power tube anode plates is a sure sign of too much current passes through those tubes.
If the plate voltage is in range, check the resistance of the dropping resistor, which the manufacturer palced between the cathode(s) and ground. It may have drifted way off, from nominal---leading you to bias incorrectly.
Also check the values of the swamper, grid, and global feedback resistor(s)---for drift. And any pico-farad range cap in these region, used to stabilize the amp.
Finally, now I know this sounds silly, but try to use another method to measure the idle bias. Like a BIAS PROBE or BIAS KIT. The circuitry in your amp which allows you to bias may have gone bad. Or if you are using an external DVM. Try another one.
The DVM you are using may need calibration.
Good luck. Tell us how it turns out.
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