Ampeg V4 questions on cap value

[DrGonz78]

Here are the requested voltage checks on those pins of the power tubes.

V301 Pin 4 = 352v Pin 5 = -32.2v
V302 Pin 4 = 354v Pin 5 = -32.2v
V303 Pin 4 = 354v Pin 5 = -32.2
V304 Pin 4 = 352v Pin 5 = -31.95v

From comparison the schematic says the screen voltage should be 338v, so it is a bit higher.

[g1]

Suggest you try adding resistance in parallel with R216 (39K) till you can get a reasonable idle current.

[DrGonz78]

So by adding parallel resistance I will be reducing the over all value at R216, correct? Any idea what type of bias trim pot would be a good one for that location? Or I guess for now what value resistor would you start with to get the bias cooler? Sorry lots of questions...

[g1]

For an adjustable option, you could remove the 39K, and in it's place put a 20K resistor in series with a 25K pot (wired as rheostat).
That would give you a range of 20K (1/2 value of original) up to 45K.
Otherwise, I would start with a 39K in parallel with existing R216 and see if the bias is now too cold, then try again with higher or lower value in parallel as required.

[52 Bill]

For an adjustable option, you could remove the 39K, and in it's place put a 20K resistor in series with a 25K pot (wired as rheostat).

Here's Enzo rule #37, never put a pot in a bias circuit that will totally remove the bias voltage if the pot fails.

Add the pot in series with the bleeder resistor R215. That resistor is pulling down the bias voltage to the final level.

[DrGonz78]

Oh ok I see there is a bit of a debate here... G-one when you were saying R216 I was not looking at the schematic and in my head was envisioning R215. Bill is stating place bias pot in series connected to R215. What happens if the pot goes open with the in series connection? Also, what would happen if the pot went open with a parallel connection? Perhaps it is just more common the pot would short out closed circuit.

[The Dude]

If the pot is in series with R215 and goes open, your bias voltage actually goes more negative. Your tubes will go towards cutoff (colder) instead of red plating. If the pot goes open in parallel with R216 you'll be where you are right now- no changes made to the circuit electronically. R215 and R216 make up a simple votage divider circuit. The only time I've seen a pot short was because solder fell inside of it.

Edit: I wouldn't put a pot in series with R216. If the pot goes wonky, you lose bias altogether.

[g1]

Thanks guys, I see the error of my ways. Forgot about the pot going open issue. Varying R215 rather than R216 is the better way for a pot. For a fixed value adjustment, I think adjusting R216 value would be ok.

[g1]

Dr.Gonz, a little more background about these changes fyi in case you're not that familiar with tweaking these circuits. As was mentioned, R216 and R215 form the voltage divider, R216 being a voltage dropper, R215 being like a load on the circuit.
Decreasing R216 value will give colder bias (more neg. voltage), but you would need to increase R215 to get colder bias.
For myself, when tweaking the circuit by trial and error, I find it easiest to tack resistors in parallel. I can do it either from component, or solder side of the board.
In this case, the bias needed to be more negative (colder). Putting a resistor in parallel with R215 would make the bias hotter, so that wouldn't work. Putting a resistor in parallel with R216 would make the bias more negative, so that's what I suggested.
Making it adjustable with a pot was an afterthought, and an oversight on my part.
Hope that helps.

[DrGonz78]

I actually am understanding this bias circuit better now thanks guys!! So as I adjust the bias circuit will that start to raise the B+ voltages?

Edit: ooops sorry Just realized G-one answered that back on post #30

[DrGonz78]

One last question before I proceed further... I noticed that at R215 there is -600v at the junction where you measure bias supply voltage. When you flip on the standby it becomes -28.5v (-32.2v in my case) and the amp is stable. I have read that it is the grounding scheme while in standby mode that puts voltages in strange places. Is this correct? If all is well with that then do I need to be concerned with the type of bias pot that I install that it can handle this type of voltage while in standby?

[The Dude]

Dr.

I'm going to guess you took that measurement to chassis ground? That wouldn't be the actual voltage across the pot itself. Measure ACROSS R215 instead of referencing to ground. That will tell you the approximate actual voltage there at standby. The cap across R215 is a 100V rated cap, so there couldn't possibly be -600V there. Whatever voltage is there will be further "split" when you put the pot in series with the resistor.

[g1]

As you mentioned, on standby there is no ground reference. So that ghost voltage would appear on either side of the pot, with a net voltage of 0 across the pot, as suggested by the sleep-meister .

[DrGonz78]

Okay I see it clearly now! I knew there could not be -600v there but my meter referenced to ground was picking up "Ghost Voltages". That all occurred earlier while working on the amp some nights ago and had me wondering. It is so much better that I ask these questions as I have so much to gain, even if I seem a tad bit ignorant of basic theory. Thanks you guys for the heads up and now I feel better about all that.

Edit: BTW I found that horrible album cover post to be pretty funny...!! The sleep meister lol The dude still abides right?

[g1]

abidezzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz
Nighty night