Silverface Princeton burnt resistor? What's the value? What else should I check?

[Fuzzy Slippers]

I was playing with my good ol' 5 knob (no reverb) Silverface Princeton when it seemed more quiet than usual but ignored it. About 30 minutes into jamming I could smell a burning component. I opened her up and found a burnt resistor.

When I went to take her apart I noticed a power tube seemed like it could have slipped slightly out of it's socket. The thing is I don't know if that slippage happened before or after I started to dismantle her.

I would like to know what value the the resistor is so I can replace it.

I would also like to know what else I need to check. I.E. why did this resistor fail?

I think the amp is a 68' but I can't remember for sure.

If you look at the pictures the resistor is in the upper left corner of the image. When looking at the board straight on its the upper most right.

I am in the united states.

[christarak]

G'day Fuzzy,

The resistor is a 1K 1watt between your first filter cap supplying the plates of your 6V6 via the output transformer and the screen supply. After you replace the resistor, I would recommend that you pull the 6V6's out and check voltage around the socket. Has the multi cap can ever been replaced? If it the original, its way over due.

Have you found a schematic? Here's one: http://www.thetubestore.com/lib/thet...-Schematic.pdf

[Fuzzy Slippers]

Hey buddy! Thanks! To answer your question: yep the multicap has been replaced. How do I check the voltage around the socket? What voltages am I testing for and where?

WHile I have it open and I'm buying parts online anyway should I got ahead and check the bias? Anything else?

Thanks for the schematic.

oh and could a loose power tube have cuased this?

[christarak]

The voltages are shown in the schematic. Please be careful of the lethal voltages here. I expect with no 6V6 tubes, the voltages may read a little higher. How's the replaced 1K resistor looking? Measure the voltages both side of the resistor and brush up on your Ohms Law. All good? If so, replace the power tubes and measure the Vdrop across the 1K resistor. If its more than 30v QUICKLY shut off the amp and ask someone smarter than me. :-).

After you replace the 1K resistor, try a known working set of tubes (not necessarily a brand new expensive pair).

[mikepukmel]

Check out that one wire, looks like the insulation might have gotten roasted as well. bend it a little and see if the insulation cracks and falls off.

[g1]

Just to note, the 1K does not feed the 6V6 plates, just the screens.
The most common cause of it burning like this is a bad 6V6.

[Fuzzy Slippers]

Just to note, the 1K does not feed the 6V6 plates, just the screens.
The most common cause of it burning like this is a bad 6V6.

How can I test to see if it's my power tubes? I don't have an extra pair laying around to test.

[Fuzzy Slippers]

Will report back when my resistor gets delivered.

[eschertron]

How can I test to see if it's my power tubes? I don't have an extra pair laying around to test.

First, visually inspect the tubes. If one was bad (making the lower than expected volume) it may have failed in a way that is visible.
Second, make a light-bulb limiter. In a pinch, any incandescent desk lamp will do. An essential tool for amp troubleshooting.
Third, after checking voltages with light bulb limiter in, and tubes out, note voltages with the tubes in. As christarak says, kill the power at the first sign of trouble.

I'm wondering if pulling one tube would cause the bias to run hot enough for the remaining tube's screen to draw excess current, to the point where this happens.
Or to avoid looking for zebras, I'd hazard a guess that at least one of the tubes' screens has shorted out.

[Fuzzy Slippers]

I'm back. I replaced the 1k resistor as mentioned above and the power tubes. All was well until just recently she seemed quieter. And today I was recording and noticed my usual awesome bass tone wasn't there. It was just so quiet and the speaker didn't seems to be adding the compression I was used to

How should I go about testing to see what's going on?

[eschertron]

So you have two sets of power tubes now? And both sets visually look OK? You can see if the amp behaves the same with either set installed.

Also, does the lessening of the volume happen after the amp has been on a while? Does it return to normal after the amp cools, or is it 'permanently' lower volume now? Do you have a voltmeter? There's a lot that can be learned about the condition of the amp with a few voltage readings. Without a meter, your best bet is to take the amp to a service tech.