8417 tube data Replacing 8417 with 6550 or KT88

You will need to be an experienced test technician to carry out these repairs.

This is historic data that we found on the internet. Although offered in good faith, there are no guarantees with this information. It is “as found” and “as is” so that you, an experienced technician, can carry out your own investigation.

The drive requirements for 6550, etc are different but well with the capabilities of the driver valve.

Quick Fix for Quicksilver Amplifiers

The general circuit looks like it was borrowed piecemeal out of a Bogen MO-100 PA amp. These amps have problems with output valve instability due to unnecessarily high grid circuit resistors on the 8417 valves, and occasional arcing over of the GZ34/5AR4 rectifier valves due to the input capacitor being 3 times the maximum size allowed according to the valve specifications. The manufacturer uses 1 each of ECC83/12AX7 and ECC82/ 12AU7 (or 12BH7) valves, but only uses half of each valve.

NOTE: Unplug the amp AND discharge 320 MFD filter capacitors prior to proceeding with any work inside the amplifier.

1. This modification will allow amplifiers originally equipped with Sylvania or Philips ECG 8417 valves 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.

B. Change the coupling capacitors to .47 MFD 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 MFD.

2. This modification will allow use of either 8417, KT88 or 6550 valves & possibly EL34.

A. Change bias potentiometer to 33K or 47K.

B. Change resistor between bias pot and bias diode to 10K. C. Optionally, capacitor in bias circuit may be increased (up to 220 MFD) to replace original 60 MFD 150V part.

D. Set the potentiometers to full negative, install output valves 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 valves can cause the same problem, generally symptom of this is that one amp will set bias and other won't. Some people had success with these by setting bias to half the usual current. Note that if valves other than 8417 are used, it may be preferable to make the feedback adjustable by installing a 1K or 500 ohm potentiometer 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 valves, simply replace with (approximately) 270 ohms. Resistors and potentiometers should be 1 watt parts.

3. This modification 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 modification will allow more voltage drive and lower output impedance of ECC82 or 12BH7 driver valve. (DNA to amps using 7247/12DW7 valve)

A. Install jumper between pins 1 and 6 of ECC82/ 12AU7 socket.

B. Install jumper between pins 2 and 7 of ECC82/ 12AU7 socket.

C. Install jumper between pins 3 and 8 of ECC82/ 12AU7 socket. The phase splitter resistors can be made any value from 22K to 47K (as low as 15K with 12BH7) ohms, so long as they're matched. If this modification is performed, a valve with relatively well balanced sections is preferable.

5.(optional) This modification will parallel sections of ECC83/12AX7 valve. (not applicable to amps using 12DW7/7247)

A. Find 330K ohm resistor attached to ECC83/12AX7 socket. Change to 150K.

B. Find the 820 ohm resistor attached to ECC83/12AX7 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 ECC83/12AX7 socket.

D. Operate amplifier and measure voltage drop between plate and cathode of ECC82/ 12AU7 valve. This should be in range of 100 to 150 volts. If too low, decrease the value of the 390 ohm resistor (attached to the ECC83/12AX7) 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 modification is performed, a valve with relatively well balanced sections is preferable. If you don't like the way it sounds, just undo these instructions.

6. This modification will allow adjustment of input impedance to better match preamps with a high output impedance (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 potentiometer could be installed to make input impedance adjustable, attach centre lead of RCA jack to wiper (centre terminal), and either end to ground. Potentiometer should be linear taper.

7. This modification will eliminate GZ34/5AR4 arcing over problems. If arcing hasn't been a problem, you can probably skip it. Don't forget to discharge the 320 MFD caps first!

A. Disconnect both leads of the choke.

B. Wire the positive terminals of the 320 MFD capacitors together.

C. Disconnect the wire leading from GZ34/5AR4's to the 320 MFD 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 MFD should do fine) between the cathode of either of the GZ34/5AR4's (pin 8) and ground (centre tap of power transformer primary). Make sure you have polarity of electrolytics correct (positive end goes to GZ34/5AR4 cathode!)

E. Recommended. Replace the GZ34/5AR4, with a GZ37. GZ37 have higher PRV and peak current ratings than GZ34/5AR4.

7. To reduce IM distortion, but lower your power output. Disconnect screen lead from pin 4 of output valve (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. THEN install 80V, 5watt zener diode between pin 6 (or terminal lug) and pin 4. Banded end (cathode) must be connected to lead from transformer!

Data for 8417 valves can be found at this link http://tubedata.itchurch.org/sheets8.html