Quicksilver 8417 bias runaway?

[KaDe]

Hi Arnold, how did your friend place them ?
They need at least 6 inches ventilation space to the sides and also 6 inches above them.
- btw there are no 8417 inside -

[kevinkr]

Recent JJ KT88 have not been tremendously reliable, if this is what was in the V4 I would try the Electro-Harmonix KT88EH, KT90EH or 6550EH. Shoot Jim McShane an email and see what he has to say.

[bear]

Dear Quicksilver & VTL users,

You're gonna keep blowing up tubes, no matter what tube you put into those amps, or VTL amps for that matter (very similar, eh?)!

Why?

Simple, they are in a "rating war" for maximum output power. Do you really need 1.0 to 1.5 dB more power than the tubes are rated for? Will you hear it? Well, it does make for better advertising specs.

The problem is (assuming no leaky grid coupling caps) that the screens are being run way way too hard, on peaks they are substantially beyond design maximum, even for NOS USA vintage tubes!

(typically the B+ is also near the hairy edge of reality too...)

I've repaired a number of these amps, ALL have the same problem. The tubes have short life, and tend to run away and some (literally) burst into flames.

To fix, reduce the maximum screen dissapation so that at full power into load the screen current is NOT over the actual max for the tubes you have in the amp. Usually that is a single resistor in series with the screens.

Hint: if you put a dummy load on the amp, run it to full power and look carefully at the inner structure of the tube (you can on most typical output tubes) the screen should not be glowing white like a lightbulb filament.

That will keep them from blowing up and having short lives.

It will ALSO reduce the power back to the normal rated power for the tubes.

My advice is to get over the "loss" of power. If you really need more power, then you have the wrong amp.

Consider building your own amps, seriously.
These old circuit designed amps are very lacking in so many ways compared to what is possible from tube amps that it isn't funny.

At least adjust the screen current into range.

_-_-bear

[Original Burnedfingers]

Bear,

I took your advise and set up the dummy load and ran the amps till they clipped. I saw now abnormal signs or any problems or white screens with the Sovtek KT88's in them after 3-4 min on the load. Mine are converted 8417 mono block amps running KT88's.

[GMuffley]

The Sylvania tube manual lists a maximum grid circuit resistance of 47K for a 8417 operated in fixed bias. The Quicksilver uses 150K, presumably to make it easier for the Cathodyne phase splitter. Me thinks dropping the value of the grid circuit resistance would lessen the biasing problem, but you would probably want to increase the idle current for the phase splitter section.

[arnoldc]

KaDe, yes they have very good ventilation and they're the newer Quickslivers.

Kevin, he's looking for EH now.

Bear, maybe that explains why his g2 resistors are toast as well?

[bear]

Burnedfingers,

Suggest measuring the current through the screen resistors for good luck. I think they made the new "KT-88s" pretty overbuilt, from what I am told. Dunno. Measure the current at full load (also what freq??).

Arnoldc,

Perhaps that is why. A clue to be sure.
Don't overlook the potential for parasitic oscillations.
Again, the easiest thing is to just measure the current through the screen resistors - or if there is none there, put a small one there and measure the current.

_-_-bear

[jekman]

Hello, I'm new to this forum and I have followed this discussion from the beginning. I also have a pair of Quicksilver 8417s. Recently one of my amps which currently has one RCA and one GE 8417 tube has been getting out of bias. I talked to Mr. Sanders of Quicksilver on the telephone yesterday and he said that 1) ideally I should use tubes from the same manufacturer on the amp 2) I should set the bias at 90 when cold and 3) GE 8417 tubes tend to run away as stated by others in this thread.

Now for my question, I have an opportunity to buy a couple of new old stock Westinghouse 8417s. Does anyone know if these are good tubes like the original Philips or Sylvania or are they going to be like the GEs ?

John

[Original Burnedfingers]

I highly suggest that only Sylvania tubes be used. I had a learning curve with different brands of 8417 tubes and found the Sylvania to be less likely to run away. I have a 3lb coffee can of 8417 tubes that are going to stay right in the can. The amp sounds real close to the 8417's using KT88's So I have been using KT88's for years.

In order for the 8417's to work without run away they have to be matched perfectly and as you asked the grid resistors need to be changed to make the amplifier more stable. Another modification would be the bias circuit as individual tube bias is necessary with the 8417's to make sure one doesn't flame out.

As Mr. Sanders mentioned that you set the bias LOW when cold. Do not walk away from the SOB because the minute you do there will be trouble in River City. After the amplifier is warm 30-60 minutes then run the bias to about 100mA for the pair.

Do not get talked into an EL34 replacement because they totally suck and the price is too much for the small abount of work done in my opinion.

Did you just get these amps off ebay or were you trying to sell them there recently? I saw a pair with mismatched output tubes recently.

[GordonW]

I have four NOS 8417s, all Sylvania-manufactured. The labels on three of them say "International Servicemaster" (and they're in boxes that say International Servicemaster), and the fourth says General Electric (and is in a replacement GE box, as the original was destroyed by moisture in storage)... but ALL FOUR of them have the CHARACTERISTIC LABEL scheme on them that ONLY Sylvania used- looks like this:

8417
USA

(in black ink, on the side of the tube)

BTW: These are GENUINE 8417s, and not "fraudulent relabels", as some have apparently found with some 8417s from "jobber vendors" (such as Int. Servicemaster or IEC or such). I have verified that they have the PROPER SIZE AND SHAPE plates, for Sylvania 8417s. That, along with the factory-printed Sylvania type code (as described above) definitely settles it as correct.

One note- all four test as new on the Sencore Mighty-Mite (full emissions, and no shorts). I don't have an 8417 setting chart for the Hickok TV-7A/U here, so I can't test them in there... if anyone has the settings for such, I'd love to run them through the Hickok...

Jekman- Email me if you like ( gwaters@inbox.com ). I don't have anything to use these in now, so I can stand to sell them, if you need them...

Regards,
Gordon.