8417 triode el.. Mod??'s

[lusso]

If I haven't looked hard enough I apologize in advance.

I have a pair of the dreaded as well as the much loved Quicksilver 8417 monos. These currently have Philips 8417's in them and are the six tube version.
I'm very much a newbie when it comes to tubes and a novice when it comes to electronic mods.

Please forgive my ignorance in advance.

I'm would like to convert my amps so that I can use KT88's, 6550's, 8417's....

This is the recipe that I would like to use: http://www.triodeel.com/8417.htm

I would like to use the best pots, resistors, caps(within reason).

For example I need either a 33k or a 47k pot for the bias but Triode only advertises on their website a 50k can I use this or can I use any 33k or 47k single turn 1w pot? Does the voltage matter?

I'm also planning on putting a feedback pot in as well. Does it matter where (physical location) I put it?

The reason for the change is:

One of the amps is exhibiting runaway bias which is probably a bad tube.

8417's are hard to find and a bit temperamental

I want to turn the amps on and be able to relax not wait for something to pop

I've searched everywhere about this and Ned's mod seems to be the most respected.

Any suggestions are welcome. I'm not in a huge rush since I have back up amps.

Please advise
David

I've attached pics of the internals

[Sal Brisindi]

Sorry I don't have an an answer for you, I have a pair of Dynaco Mark VI's that used 4 each. I purchased the amps without 8417 tubes... I finally managed to snag 10 NOS 8417's. I would have loved to modify the amps to use KT88's but I did not want to modify them, and trust me, I did search the internet to see if anyone modded the Mark VI's to use KT88's

Sal

[lusso]

Sal,
Thank you.

When I'm finished I will have eight 8417's for sale 4 philips and 4 RAM. I will test and post.

David

[Sal Brisindi]

Sounds good David,

Thank you,
Sal

[HollowState]

Yo Dave,
I just read the Triode Electronics article and they are essentially correct, but for the bias pot values. Potentiometers (pots) are not made in values of 33K or 47K. 50K is the standard value and the size to use. Use a 1 or 2 watt pot for reliability. Voltage rating of the pot is not of concern since it's low anyway. (under 100v)

You may put the feedback pot most anywere convenient, just keep it away from the power transformer. But try to keep the wiring short.

This is a topic that has been discussed here before. I'd recommend reading this in it's entirety. You'll find the schematics on the second page.
Quicksilver KT88 mono 60watt amp schematic

Note that the input capacitor is too big for the 5AR4. Old American and European tubes might survive, but modern Russky and Chicom tubes don't stand a chance. And when you buy output tubes, buy them from someone reputable like Jim McShane.
Jim McShane Home Page

[lusso]

Sal,
8417 for sale

I have 7 not 8

all test in the good area

Let me know if you're interested
David

[Original Burnedfingers]

First of all the comment on the power supply cap being too big.

A slight modification in the power supply by adding some diodes will cure the problem of 5Ar4 life being too short. I added the diodes to my Quicksilver 8417's years ago and they are still running strong. By adding the diodes they take the strain off the 5Ar4 and the 5AR4 acts simply as a slow way to turn on the B+.

The published article on changing the Quicksilvers to KT88's or 6550's works well. I did one conversion and did nothing but change the bias circuit and the grid resistors. Your runaway bias may very well be a coupling cap problem and not a tube problem.

Check the Quicksilver threads and you will find my diode modification.

The best resistors, caps, and so forth will provide little difference over stock. I had one with all the best parts in it and A/B it with a stock amp and very little difference.

Do change the amp to individual tube bias.

[Original Burnedfingers]

Attached diode modification.

After making this mod I have not lost a single 5AR4 and the amplifier has been running for years now.

With respect to the feedback pot.... If it were me I would install them as a temporary means of establishing what sounds good to your ears and then replace them with a fixed value resistor.