Ampeg VT-40 Redplating Solution?

[neddyboy]

Sorry guys, I used a part number from the schematic I have on paper, not the one I attached. I'm referring to the 68k resistor next to the diode in the output bias circuit. And yeah, Merlin's math and the theory are several hundred feet above my head!

I apologize for all the questions. I spend a lot of time trying to figure stuff out before I ask the forum for help, but I generally don't have enough info/knowledge to get decent search results. Thanks everyone!

[Chuck H]

I failed to attach one of the documents I intended to. I edited the above post, but it sounds like you have the right resistor in mind.

[neddyboy]

Hoo boy. I pulled out the coupling caps to replace them, and I could finally see the value on one of them: 0.008mfd! The other is correct. Anyway, a matching pair of 0.33mfd caps are now in place. How did it even work?! I'll try to install the adjustment pot at lunch.

[g1]

The coupling cap going in to the PI is a .01, which they might use .008 for. It's for sure not that one?
It would still work with the wrong value coupling caps, just the frequency response would be altered.

[neddyboy]

I used the board markings and the schematic markings to find the 2 caps, and they're both marked as .33. There is another cap on the board. It's a tropical fish cap and I haven't figured out the specs. Maybe the one you're referring to? I have it as 0.1 tho, not .01.

[neddyboy]

Oh, I see that .01, sorry G1. EDIT: Yes, I see both the .01 and the 0.1 caps. All is well. I'm guessing some previous owner thought the difference between .33 mfd and .008 mfd wasn't a big deal.

[neddyboy]

Another noob question: are the 1 ohm resistors I installed to measure bias intended to be left in place or removed after I'm done? Doesn't seem like 1 ohm on the ground would make any real difference, but I'm not sure. EDIT: I added a 33k resistor in series with the 68k and took measurements again. The voltage drop is down to 14mv. Does this mean I need to reduce the resistance?

[Chuck H]

What voltage drop, where? What is the bias voltage now?

You know now how to calculate bias current. Less voltage across the 1 ohm means less current, which should mean more - bias voltage. You want maybe 22 watts per tube.

[neddyboy]

Voltage drop over the 1ohm resistor going to ground. I'm getting 20mv now for bias voltage. Chuck, can you answer my question about the 1 ohm-to-ground resistors I put in? I assumed they were temporary, but are they meant to stay in the circuit?

[Chuck H]

You can leave them in. Many builders include them from the start.

Your plate voltage will have increased with lower current draw so you need to remeasure that for a bias current calculation. You're probably somewhere around 11W or 12W right now. That would seem a little too cold to me for kt88's. So you'll want to reduce the value of the added series resistor from 33k to, maybe, 15k. Or, just put a 25k or 50k pot in series wired as a variable resistor and adjust it for 22W per tube. Then take the pot out and measure it. Put in the nearest standard resistor value.

How is your current in each tube matching up now that you have matching PI coupling caps?