Blowing fuses

[gui_tarzan]

I was starting to question the balance pot. Shouldn't the .1 caps be showing the same voltage coming off the PI socket?

[martin manning]

With that positive voltage on the empty V9 and V10 sockets I'm wondering about the board being conductive. Did you say the fuse will hold with all of the output tubes removed? If so than you could poke around on the board with your meter. If you get any indication of voltage where it shouldn't be then I'd get the high-voltage wires (B+1, 2, and the choke connections) off of the board and temporarily connect them up in the air.

The PI plates may or may not have the same voltage due to the difference in the two 12AT7's triodes. Measure voltage on the balance pot with the tubes out too. Leaking HV could be getting to it.

[gui_tarzan]

So I poked around the boards and I get a consistent 1.8-2.0v on the main board. Funny thing is, I get between 1.0 and 2.0v at different spots on the bias board, but 3v when I test near the outside-most HT connection. The main board is really warped, which is nothing new but there must be a good half inch or more rise at the highest points between the screws.

[martin manning]

Warping is a good indication that the board has absorbed moisture. Sounds like the rectifier board is conductive too.

[gui_tarzan]

That's what I'm starting to think. Does anyone make replacement boards for these? I've made my own but they don't look as good as a pro would do.

[edit] Never mind, I know several vendors have board material, I was just wondering if anyone made direct replacements.

[edit] I found those too. I can't see where this would be worth completely rebuilding, but I'll call the shop tomorrow and see if the customer wants to do that.

Thanks for all the help guys, I've read about conductive boards but hadn't run into one until now.

[martin manning]

I think it's a standard Twin Reverb board layout. Hoffman has fiber or G10 replacement boards available. Before going there maybe you can prove that that is the cause by lifting the HV lead wires.

[gui_tarzan]

Connect them to what? The diodes? If the board is conductive that won't make much of a difference, will it?

[martin manning]

I'm suggesting you de-solder the connections to the eyelets with high voltage on them and join them up in the air, off the board. In the layout below, the blue circles indicate the potential candidates (no pun intended). The upper right one is probably the most suspect since it's the closest to the V9 and V10 power tube grid connection.

[Leo_Gnardo]

I'm suggesting you de-solder the connections to the eyelets with high voltage on them and join them up in the air, off the board.

I do this kind of "above the board" fix quite often, but at the other end where a little DC leakage causes scratchy volume & tone controls, also upsets the bias of preamp tubes and introduces unwanted noises.

Use tie strips if you want to make it permanent and neater than just "air-boarding" if you try the technique and find success. Never have had to trouble shoot a leaky board by doing a complete replacement much to the relief of customers and their wallets. Pick a - now unused - eyelet, solder to it the tie strip mounting lug, and don't use that lug for your circuit. Antique/CE carries 'em in many configurations, Mouser has 'em too just not so many varieties. FWIW I find the 5-lug strip to generally be most useful. And it's not too radical, consider all those Matchless amps built on tie strips.

[martin manning]

Soldering a terminal strip to an eyelet is a good suggestion Leo. I like 5-lug strips too. They are easily modified into 2, 3, or 4-lug strips with either left or right off-set just by clipping them with diagonal cutters.

[Stevem]

If you have a 600 to 1000 ohm 5 watt or better resistor sitting around you can lift the pin 8 ground connection on two of the output tubes, tie them to one end of the resistor and then ground its other end.
Next ground out the tie point of the two 220K resistors off the PI.
This will let you atleast idle the amps output stage being Cathode biased and the the fuse will hold and you can then figure out what's going on.

[Leo_Gnardo]

Soldering a terminal strip to an eyelet is a good suggestion Leo. I like 5-lug strips too. They are easily modified into 2, 3, or 4-lug strips with either left or right off-set just by clipping them with diagonal cutters.

Right-o Martin. I buy a bag of 100 at a time from Mouser, lasts me a couple years. The price, even with the 100-break, ow! Used to get these for pennies... have to justify the price now approaching $1 each with "it's still cheap considering the problems they solve."

I'm liking your suggestion too Stevem, this amp sure has a bizarre problem & I hope to see a solution here in case I run across one like it.

Just had a mini brain wave - I think it was suggested in another comment - if there's leakage on the rectifier/bias board - at the AC terminal end where the transformer leads are attached - that could bring about an unexpected increase in bias voltage. You may want to rebuild that board on tie strips or perf board, see if that doesn't sort it out.

[JerryFJA]

Were the power tubes actually new or just another set you had? Have you tried a third set just in case the new tubes had a bad one?

Check the 100 ohm resistor on the tail of the phase inverter and check the solder joints there to ground. Seen a lot of Fender amps with a broken solder joint here that will cause issues with the bias.

Any arcing or burn spots on the power tubes sockets?

Try a different OT.
Jerry

[martin manning]

Check the 100 ohm resistor on the tail of the phase inverter and check the solder joints there to ground. Seen a lot of Fender amps with a broken solder joint here that will cause issues with the bias.

The only way I can see the PI ground affecting power tube bias is if the PI coupling caps are leaky. The anode voltage will rise if no current is flowing, and of course pulling the PI tube would have the same effect.

Any arcing or burn spots on the power tubes sockets?

Valid question.

Try a different OT.

I can't think of any reason to suspect the OT is causing this. What's the logic there?

[Firestorm]

I've seen a '70s Fender where extreme board warping pulled the ground connection for the 100R loose from its solder blob on the ground bus. The PI then is grounded through the OT secondary via what used to be the feedback network. Might not that cause assymetric feedback related oscillations? It seems odd that the grids of V 9-10 behave almost oppositely to the grids of V 7-8.