My pair of Quicksilver 8417s are now biased to 90mA on one amp and the other has one soft valve, so biassing is 90mA on one and about 60mA 0n the other. I have won more valves on ebay, hope I can make a better match. Listening impressions will wait until a better match is possible.
Please, if anyone has access to new or good 8417 valves to sell I would like to buy them. Please send me a comment. I also found the following article about modifying the amps to use 6550 tubes, I guess the same mods would work for EL34 and KT88.
Well, here’s a paper on how to convert QS to 6550, etc, so you can decide for yourself whether it’s too complex for you or not.
The drive requirements for 6550, etc are different but well with the capabilities of the driver tube (eg I’ve built Mk3 Dyna mod using single 12AU7 as driver, there’s other examples out there).
Many of these note apply to other amplifiers using 8417.
Quick Fix for Quicksilver Amplifiers
The general circuit looks like it was borrowed piecemeal out of a Bogen MO-100 PA amp (not a bad place to start, actually). These amps have problems with output tube instability due to unnecessarily high grid circuit resistors on the 8417 tubes, and occasional arcing over of the 5AR4 rectifier tubes due to the input capacitor being 3 times the maximum size allowed according to the tube specifications. The manufacturer uses 1 each of 12AX7 and 12AU7 (or 12BH7) tubes, but only uses half of each tube, the other half just sits there and does nothing.
NOTE: Unplug the amp AND discharge 320 uF filter capacitors (you can use a screwdriver between the terminals of the capacitor, if charged you WILL get a spark!) prior to proceeding with any work inside the amplifier.
1. This mod will allow amplifiers originally equipped with Sylvania or Philips ECG 8417 tubes to use the GE made 8417 without unnecessary instability.
A. Find the 150K resistors connected to pin 5 of the 8417 sockets. Replace with 47K. The actual value is not too critical but the resistors should be matched to 2% or better (preferably better).
B. Change the coupling capacitors to .47 uF at 600V or similar. The originals were too low of a voltage rating (400V, should have been 600V) If bias drift on the GE 8417’s is still excessive, the resistor values may be made as low as 22K. In that case change coupling caps to 1.0 uF
2. This mod will allow use of either 8417, KT88 or 6550 tubes (probably EL34’s as well although we haven’t tried it.)
A. Change bias potentiometer to 33K or 47K.
B. Change resistor between bias pot and bias diode to 10K. C. Optionally, capactior in bias circuit may be increased (up to 220 uF) to replace original 60 uF 150V part.
D. Set the pots to full negative, install output tubes and rebias as usual. Note that if bias won’t stay stable you may have to replace 150K grid resistors with lower value (100K or in extreme cases 47K) (as noted above connected to pin 5 of 8417 socket). Defective tubes can cause the same problem, generally symptom of this is that one amp will set bias and other won’t. Chinese tubes in particular are most likely to have this problem, I’ve heard people having sucess with these by setting bias to half the usual current. Note that if tubes other than 8417’s are used, it may be preferable to make the feedback adjustable by installing a 1K or 500 ohm pot in series with a 100 ohm resistor to replace the original 562 ohm resistor, then adjusting feedback to taste. If 6550/KT88/EL34 will be only tubes, simply replace with (approximately) 270 ohms. Resistors and pots should be 1 watt parts.
3. This mod allows setting bias with a voltmeter instead of a milliammeter.
A. Install a 10 ohm (1/4 watt minimum) 1% resistor across the bias jack terminals. Now the voltage will correspond to the number of milliamps drawn across this resistor when jack is inserted. 1 volt=100 ma, 1.2 = 120 ma, etc.
4. (optional) This mod will allow more voltage drive and lower output impedence of 12AU7 or 12BH7 driver tube. (DNA to amps using 7247/12DW7 tube)
A. Install jumper between pins 1 and 6 of 12AU7 socket.
B. Install jumper between pins 2 and 7 of 12AU7 socket.
C. Install jumper between pins 3 and 8 of 12AU7 socket. The phase spliiter resistors can be made any value from 22K to 47K (as low as 15K with 12BH7) ohms, so long as they’re matched. Won’t hurt to experiment with different values, so long as you’re careful to get everything hooked back up right. Note that if this mod is performed, a tube with relatively well balanced sections is preferable.
5.(optional) This mod will parallel sections of 12AX7 tube. (not applicable to amps using 7247/12DW7)
A. Find 330K ohm resistor attached to 12AX7 socket. Change to 150K.
B. Find the 820 ohm resistor attached to 12AX7 socket. Change to 390 ohms.
C. Install jumpers (as noted above) between pins 1 and 6, pins 2 and 7, and pins 3 and 8, on the 12AX7 socket.
D. Operate amplifier and measure voltage drop between plate and cathode of 12AU7 tube. This should be in range of 100 to 150 volts. If too low, decrease the value of the 390 ohm resistor (attached to the 12AX7) until it voltage falls in range. If way too high (say over 200 volts), increase the value of the 390 ohm resistor. Note that if this mod is performed, a tube with relatively well balanced sections is preferable.If you don’t like the way it sounds, just undo these instructions.
6. This mod will allow adjustment of input impedence to better match preamps with a high output impedence (example- Dyna PAS3 or PAS2) A. Find the 100K resistor between the input jack and ground. Replace with higher value (between 220K and 500K, whatever sounds best..Dyna units seem to be happiest with 470 or 475K in my experience). Optionally, a 470 K or 500K pot could be installed to make input impedence adjustable, attach center lead of RCA jack to wiper (center terminal), and either end to ground. Pot should be linear taper.
7. This mod will eliminate 5AR4 arcing over problems. If arcing hasn’t been a problem, you can probably skip it. Don’t forget to discharge the 320 uF caps first!
A. Disconnect both leads of the choke.
B. Wire the positive terminals of the 320 uF capacitors together.
C. Disconnect the wire leading from 5AR4’s to the 320 uF capacitor. In place of the wire, connect the choke leads. Choke polarity is not important.
D. Connect a 500V electrolytic capacitor (any value from 22 to 100 uF should do fine) between the cathode of either of the 5AR4’s (pin 8) and ground (center tap of power transformer primary). Make sure you have polarity of electrolytics correct (positive end goes to 5AR4 cathode!)
E. Many people have reported good results using GZ37/CV378 tube in place of 5AR4, these have higher PRV and peak current ratings than 5AR4
7. This is mod suggested in Sound Practices. Apparently will reduce IM distortion, altho at cost of slightly lower power output.
A.Disconnect screen lead from pin 4 of output tube (8417,6550,or KT88,etc). Connect to pin 6. (assuming pin 6 is empty. If not install a small terminal strip and connect screen lead to ungrounded lug.
B.Install 80V, 5watt zener diode between pin 6 (or terminal lug) and pin 4. Banded end (cathode) must be connected to lead from transformer!