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Quicksilver 8417 bias runaway?

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I recently picked up a couple of Quicksilver 8417 amps (the originals, not the GE version). One of them is exhibiting runaway bias current after warming up for a few minutes. I've backed the bias pot all the way down, but using the ammeter supplied with the amps, I hear a crackle then the ammeter starts to climb quickly (and pegs). The amp blew a fuse the first time this happened (I was listening, not monitoring the bias current). I swapped the 8417 tubes between the amps and the problem did not go with the tubes.

Does this sound like a bad cap? Or do I have more expensive problems to resolve?

Thanks for any info,
Doug
 
Lets see if I can post a schematic
 

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Doug

I own several versions of the Quicksilver 8417 amplifier. There is NO difference other than the selection of the feedback resistor and several other resistors. The feedback amount remains the same between the different vintages. As far as their being a difference that allows you to use either the GE or Phillips tube I will cry BS.

I would suggest never changing tubes when there is a problem of this nature. The 8417's are costly and hard to come by. Just my .02 I will move aside while Frank answers your question. The fix is easy.
 
Well, I did get the caps and put them in myself. The amp now works fine. The guy at the shop I frequent had them (0.22 uF rather than 0.18, but he assured me that was OK); no one else had them.

Thanks for the help and encouragement. I just wasn't properly motivated. (I like Scotch!)

Doug
 
diyAudio Senior Member
Joined 2002
Hi,

Well, I did get the caps and put them in myself. The amp now works fine. The guy at the shop I frequent had them (0.22 uF rather than 0.18, but he assured me that was OK); no one else had them.

Forgot to mention it...
Ideally the caps should be 600V MKP types or better and matched within 1% for both monoblocks.

If you can get hold of reasonably priced polypropylene film & foil cap such as MIT, RelCap or TRTs give them a try.
No doubt in my mind there are other good ones but at least I know these are excellent.

Best policy is to change the caps in both blocks, you don't want to end up with two disparate channels.

In case you plan on modding the amps to take the GE 8417s and change the bias scheme, don't forget that, since the gridleak resistors are substantially lower, you'll need to use higher value coupling caps (0.47µF to 0.68µF/600VDC).
It doesn't hurt to put in these values now even if you plan on using the GEs later on.

Instructions on how to adapt the amp for use with the GE8417s can be found here:

UNCLE NED'S ADVISE.

The complete schematic can be found here:

QS 8417

Cheers,;)
 
diyAudio Senior Member
Joined 2002
Hi,

Joe,

Please explain the differences between the Phillips and the GE version.

I actually never saw a Quicksilver that came factory equipped with GE tubes.
What I can attest to however is that there simply was no way to set bias with the GE 8417s without them drifting like mad. Reaching for the tube with your hand alone made the meter swing full tilt already.

Surely, changing the global NFB on the amp won't do anything for the bias of the tubes.

I have found only two resistors that are different in the amps.

If those are the gridleak resistors of the output tubes then that's all there should be.
Usable values for these range from 47K to 68K, the lower one being the safer bet here.

One area that I don't think we have discussed yet is the 470K input resistors; with the current tendency for rather high capacitive I/C cables that highish value may sometimes be the cause of an unwanted high frequency roll off.
I often change this value to 100K or 220K depending on the customers' system balance and I/Cs.
Give it a try if you like.

Cheers, ;)
 
The saga continues....

Last night while playing, one of the 8417 tubes arc'd and blew the fuse on one of the Quicksilvers. I know this can't be a good thing; however, I've seen some mention that it wasn't uncommon with this particular tube (the brand on this particular set is RCA). Does this sort of thing do collateral damage, or should I just have to replace the 8417's? I know that won't be an easy task, either.

I haven't had time to take the cover off yet, but there wasn't the telltale sign or smell of burning electronics.....

Thanks, Doug
 
diyAudio Senior Member
Joined 2002
Hi,

I know this can't be a good thing; however, I've seen some mention that it wasn't uncommon with this particular tube (the brand on this particular set is RCA).

Hmmmm....Once you have a good set of tubes this shouldn't happen.
Unfortunately the 8417 never had a good rep in this department probably due to the main interest going toward the new semi-conductor thingies at that point in time.

Does this sort of thing do collateral damage, or should I just have to replace the 8417's?

Normally with the QS amps having fixed bias you won't have damage but you best check nonetheless before popping in another victim errrrrrrr....tube.
Anyhow, fuses pop for a reason and it could happen that the tube has lost bias for a while make it go into "runaway".
I'd keep an eye on the amp while it's on the bench to see if it's stable.

Some general advice for biasing the 8417s in a QS:
- let them warm up for 15' at least .
- go easy on the pot, changing bias gently while keeping an eye on your DVM.
- don't go near the glass bulb of the tube with your hand(s), some have such high gm that they'll react to almost any stray field and that includes your body.
- always make sure not to mix tubes from different manufacturers
within the same monoblock. Same batch if possible but I realize I'm probably asking too much given their scarsity.
Brandnames are meaningless, when in doubt about pedigree ask here or a reputed vendor.

Anyone running these monoblocks using original manufacturer filter caps, be aware that you should replace them before they give serious trouble: 10 years + in a hot environment is asking for trouble.

If you can and are keen on your amps, replace electrolytics with polypropylenes or PIO: they should last "for ever" and should improve sonics audibly.
Good brands are Unlytic, Solen to a lesser extent and ASC.

If you don't want to throw money at them but still be safe, excellent low ESR caps are available even at 500VDC, Philips, Epcos and JJ are just examples of what can be had at reasonable cost.
Do NOT bypass these caps with smaller filmcaps unless you want a disjointed sound.

Make sure all sockets make for a tight fit and are chemically clean, a lot of problems and a loss of HF extension come from bad contacts.

Hope this helps,;)
 
Follow the advise of Frank in the above post.

However, I haven't had good luck with RCA's. Maybe I had a bad set of them. Bias setting I don't believe has been talked about.
Instead of the 125mA I set to 100mA. My distortion analyzer didn't show any difference between 125mA and 100mA bias setting. It will make the 8417 tubes last longer also and I didn't hear any difference in the sound quality.

Joe
 
Hi guys, my friend was complaining about his Quicksilver Mono (120W / ch, I think the V4 Mono) that eating up tubes and sending them to hell (red plates). He lost 3 of his JJ already, 3 of his original tubes that came with the amp.

He says, it biases right, but then after a few weeks (one was months), it kills the power tubes, and took the G2 resistor with them (says the tech who looked into it).

Any ideas what's happening? Darn, this is a $$$$5K amp!
 
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