67 Fender Princeton Reverb tube/fuse problem

[GrumpyJoe]

A buddy brought his 100% original (even the tubes) to me last night. Said he was playing and it just died. I'm fairly handy at this sort of thing but not too hip on tube amps. His fuse had blown and I could tell that one of the 6V6s was cooked. So I pulled all of the tubes and put in a correct fuse and it didn't blow. So I told him to bring me some new tubes. Well he bought a whole set of mullards. I put the new tubes in and was getting ready to check the bias and the fuse popped again after about 2 minutes with the amp on. So I put in another fuse and pulled all tubes, hooked it up to my current limiter and the bulb stays dim. Powered the amp down and inserted the 5AR4 rectifier and turned the limiter back on and the bulb stays dim. Next I added the first 6V6 and here's where the trouble lies. With it hooked to the limiter, the bulb stays dim for the 1st 30-45 seconds and then brightens. No signs of arcing on the sockets. I cleaned and re-tensioned them anyway. Filter can cap is in excellent condition and shows what it should. Pin 7 and pin 8 of this tube is right at a full on short showing 0.3 ohms on my Fluke which means my heater pin 7 is 0.3 ohms when I have my lead on the chassis. This isn't right is it?? Any help would be greatly appreciated. Oh and it's the AA764 circuit. Thanks

[Leo_Gnardo]

Pin 7 and pin 8 of this tube is right at a full on short showing 0.3 ohms on my Fluke which means my heater pin 7 is 0.3 ohms when I have my lead on the chassis. This isn't right is it?? Any help would be greatly appreciated. Oh and it's the AA764 circuit. Thanks

No I'd expect chassis ground to filament resistance to read some ultra low resistance IF it's an amp that has a filament center tap going to ground.

I'm thinkin' your bias supply is suspect. Old bias filter caps often fail. Replace with a 47 to 100 uF cap, 50 to 100V. And make sure the positive end of that cap goes to ground. "It looks OK..." mmmm, no. After 50 years, time for a fresh one. Cheap insurance.

[GrumpyJoe]

Very true. The can shows the correct capacitance on all sections but I'll go ahead and look for a new one. I pulled the 50uf 50v bias cap and it's reading 60uf. Is that enough to cause a problem?

[Leo_Gnardo]

Very true. The can shows the correct capacitance on all sections but I'll go ahead and look for a new one. I pulled the 50uf 50v bias cap and it's reading 60uf. Is that enough to cause a problem?

20% over would be considered "in tolerance", otoh on some cap meters leaky caps read over value. For a buck & a quarter cap I'd swap in a new one and know I don't have to worry for another couple decades.

[Enzo]

It isn't the value that matters, it would be leakage. Your meter might read it with a volt or two. But when it tries to handle 50 volts, it may leak like a sieve.
If the heaters light up on the tubes, then your filament supply is working.

[GrumpyJoe]

Never thought about it that way. Yup heaters all light up. I'll change the bias cap and see what happens.

[xtian]

When power tubes fail they often burn the screen resistors.

[Enzo]

Before just changing the boas cap, try just measuring bias at the power tube sockets to see if it is way off or going away.

[GrumpyJoe]

Hey guys sorry it took so long to reply. Been extremely busy. Here are some of my voltage readings for the Princeton.

Plate voltage is 440vdc
Bias voltage at pin 5 of V6 is -30.8vdc
Bias voltage at pin 5 of V5 is -30.5vdc
Voltage across bias cap is 38.5vdc
Plate current of V6 is 25.2ma
Plate current of V5 is 23ma

[nevetslab]

With the 'full set' of Mullard tubes, did that include a new Mullard 5AR4 rectifier tube?

[GrumpyJoe]

Yes it includes a new 5AR4/gz34.

[nevetslab]

AS the mains furs only blows when you have one or both power tubes installed, I'd be looking at the power supply caps, located in the metal compartment below the chassis. I doubt if the bias cap is the culprit, as so little current is in that circuit (uAmps). This amp being a 1967 vintage, I'd suspect them next, since you have a new rectifier, new power tubes, new preamp tubes.

[GrumpyJoe]

I've replaced the can filter cap, all electrolytic bypass caps, etc. like I said in the initial post, the fuse only blows after the amp has been powered up for five or so minutes. I'm totally stumped

[SoulFetish]

I've replaced the can filter cap, all electrolytic bypass caps, etc. like I said in the initial post, the fuse only blows after the amp has been powered up for five or so minutes. I'm totally stumped

Hmmmm.... Do the output tubes show any sign of red-plating? If this is a consistent event, which you've confirmed each time you go through the start up procedure, I would go through the power up procedure again. This time, keep your meter on the grids and see if you loose your grid bias. If it drifts positive or goes open, your amp will start pulling significant current (although the tube usually red-plates as well, but sometimes it's tough to see in full light). I would pay close mind to the socket which had the blown tube, as this could be the reason the tube failed. After taking a good reading of the grid bias, I can understand the logic of crossing that off the list of culprits and not thinking anything of it. But, strange things are afoot at the bench when a piece of gear is under test.

For example, quick story: I just fixed a Marshall 6100; put all the switching under test; sign wave tested it; played it to test the audio; had put the chassis back in the head and had all but buttoned it up and put it away when I realized that I hadn't double checked the bias on the tubes that the amp came in with. So, all I did was pull the chassis to check the output tube bias and I notice that I was getting a B+ of 510V. This was suspiciously high for your typical Marshall designs. Seemed a lot like a reading of an unloaded secondary, and wouldn't you know, the tubes weren't drawing any current. After the total deflating feeling of thinking you were finally finished with the amp and solved it's issues, I had to go through and find what the hell just happened. Turns out the 470Ω resistor which feeds the screens all of a sudden goes open sometime between when I had just played the thing, put it in the head cab, and took it out to check the bias? How's that for timing. I mean, that job was soooo close to being one of the easy ones.

[GrumpyJoe]

I'm headed to the shop shortly. I'll do that and post the results. Thanks Soulfetish