V4B - Voltage at C18

[Roman Ships]

I have been rebuilding my Ampeg V-4B ( 1971 I think )

I turned it back on and I believe everything is wired correctly. I am measuring only about 60 V DC at C18 Filter Cap. There are no tubes installed.

This doesn't seem correct. C20 measures fine, and on the neg. terminal it measures the same 60 V DC as that leads to the pos. of the following cap.

Also when measuring the hot lines exiting C17, 325V and 360V per the schematic, they all read B+ ( about 565-ish) will those change when tubes are installed? Or is something else wrong?

Thanks,

[Hangman]

sounds like something might not be wired correctly. do you have a picture you could post? it would be easier to see whats wrong.

The power supply always puts out higher voltages when the tubes are not installed. once the tubes are installed the voltages will drop

[Roman Ships]

I have some pictures but need to host them somewhere first.

I did find a simple error, I had R56 going between two positive ends of C20 instead of in parallel.

C18 is now more like 290 VDC. This seem closer? I was thinking it should still be the same as B+ ( 585 VDC ) or so.

C17 measures correctly. The legs that shoot off of C17, 325 VDC and 360 VDC, according to schematic, those all measure full B+ ( 585 VDC ) or close. Is that because I don't have tubes in yet?

I will get some pics posted.

[Hangman]

you should be able to add the pictures to your post as an attachment. If you click "new reply" there will be an attachments window below the message window, add a photo there. resize the images before you upload them so they are of reasonable size.

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sounds like you found the problem. C18 shouldn't measure the same as the B+

notice that R56 and R57 form a Voltage divider, the voltage across C18 should be exactly half of the B+. so you're good.

[Roman Ships]

Replaced all caps, added mid tone mod
Removed Hum Pot and replaced with two 100k resistors to ground
Added 1ohm R's from each cathode to ground with probe jacks

- added dual bias control
- moved R55


Dual Bias - Instead of altering R49, I built a second bias circuit and altered R35, R36 into a 75k + 50k trim pot.
I can't see any reason this wouldn't be an acceptable way of doing it. Am I correct?

I still get full B+ off all legs of c18, my assumption is that since I have no tubes in, there virtually no current and thus no voltage drop across those.

I put tubes back in and it popped the 6A Fuse. Not sure why that is yet.

Here are some pictures.

                    Rewired 1971 V4

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More pictures

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More Pictures

[Hangman]

Ok, until we sort this out, do not turn it back on.

I'm writing a quick reply with images.

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Here is a picture of one of a V4 that I recapped.

C18 and C20 are not in parallel as you have them wired, They are in series. i.e. the negative end of C20 goes into the positive of C18.
Additionally there should be 2 100k resistors, one going from positive terminal of C20 to positive terminal of C18 then one going from negative terminal of C20 to negative terminal of C18.

If you have 580V going across C18 and C20 right now, you're exceeding the voltage rating of those capacitors by a significant margin. They may be damaged.





I don't understand what you mean regarding the bias supply. When you say you " built a second bias circuit and altered R35, R36 into a 75k + 50k trim pot. " what does that mean? can you draw the schematic? If you use two bias supplies then you don't need a balance pot, so what is that circuit supposed to be doing?

[Roman Ships]

I have attached a schematic of my dual bias circuit.

I had read that if you bias tubes in pairs off of one bias supply that it can cause problems because it would unequally split the current. This makes sense in theory since the resistance to each branch would vary, but with such low current it seems like it wouldn't be an issue. Anyway, it seemed like I was reading that you should have separate bias supplies to bias tubes in pairs.

So I split after C16 and built identical bias circuits.
One feeds V5 and V6, the other V7 and V8.

I may have misspoke, C18 and C20 measure correctly. C20 is full B+ and C18 is about half.
C18 and C20 are wired in series. It may be hard to tell with my pictures. Below you will see which cap is which.

And the 100k R's are wired in parallel across each C20 and C18 per the schematic.

My "E" which comes off R51, and "D" which comes off between R52 and R51, both were reading full B+ with no tubes in. That was my main question, I figured they would drop once tubes were in. But tubes are blowing the fuse.

Hopefully these pictures clear it up and we can find the issue.

           

[Hangman]

So the R35 and R36 cannot be used to adjust the bias. Remember that the DC current going through the grid is very small so there will be only a small voltage drop across those resistors, if you want to adjust the bias you need to put something similar in place for R49.

So, you've boosted the values of the reservoir cap up to an effective 75uf. using two 150uf capacitors in series. I can see that now. I didn't see all of those caps you have over on the side of the amp by the power transformer.

its hard to say why your fuse is blowing, are you certain that the grids are seeing the bias voltage? double check power tube socket wiring. does the fuse pop right away? or does the amp warm up a little and then the fuse pops?

-Edit-
I should change my wording from R35 and R36 cannot be use, to should not be used simply to adjust the bias. it would be more effective to use the pot+resistor combo in place of R49. This way gives you a better sweep, and the load impedance for the phase inverter doesn't change with the setting of the bias.

[Roman Ships]

So the R35 and R36 cannot be used to adjust the bias. Remember that the DC current going through the grid is very small so there will be only a small voltage drop across those resistors, if you want to adjust the bias you need to put something similar in place for R49.

So, you've boosted the values of the reservoir cap up to an effective 75uf. using two 150uf capacitors in series. I can see that now. I didn't see all of those caps you have over on the side of the amp by the power transformer.

its hard to say why your fuse is blowing, are you certain that the grids are seeing the bias voltage? double check power tube socket wiring. does the fuse pop right away? or does the amp warm up a little and then the fuse pops?

-Edit-
I should change my wording from R35 and R36 cannot be use, to should not be used simply to adjust the bias. it would be more effective to use the pot+resistor combo in place of R49. This way gives you a better sweep, and the load impedance for the phase inverter doesn't change with the setting of the bias.

Finally getting back to working on it. The fuse blowing was just a dumb error, don't recall exactly, I had something grounded.

I fixed the bias circuits to put the pot + res combos in for R49.
All the voltages look good.

No sound. I was getting a faint fizzy sound for a bit... Messed with the eq and then nothing.

Anything obvious pop out to you?
I can post pics again, but it will be hard to follow probably.

Also, R49 is now 56k + 50k trim.
In both bias circuits, at full resistance I was only getting -52VDC approx...
That is, about 100k... With the trim down it would go down to about -42 VDC...

That seems wrong, I should be able to easily hit the -64V

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Update, sound is fine. I had been staring at it too long and had something hooked up wrong.

Stil a little puzzled by the bias voltage.
However, at about -52 V, I am running about 23 and 26 watts at the plates respectively.
Biased in pairs now.

Maybe I should swap out my 56k for a 75 k, my combo exceeds the original 75k for R49, I have tried to measure the effective resistance across the resistor + pot combo but it doesn't seem to read well.

[Hangman]

that is odd, did you replace C16? and if so did you make certain it was the same value? changing the value there can change the voltage.

if you haven't replaced it yet, replace it.

although I wonder if having two bias circuits coming off C16 has changed the behavior of the circuit. if you've already replaced the cap, maybe try increasing its value a bit. I"ve heard of people switching to a .1uf to adjust the bias range for KT-88s. apparently it works.