Hiwatt dr103 Bias voltage to little

[Leo_Gnardo]

"I'd try either changing it, or temporarily clipping another one across it to see if anything changes."

Yeah, but if the cap is partially shorting and pulling the voltage, what will clipping in another across it tell us?

What Randall said ^^^. If that bias cap is an original part, should be changed out for a fresh one anyway on account of age. Cheap insurance.

[The Dude]

If it is partially shorted, your right............ it will tell us nothing. That said, the most common failure mode of electrolytic caps is that the ESR goes high and/or the capacitance value goes low with age. I'll bet my lunch money it's not near shorted. I offered that test because it's a quick easy test- no soldering involved. If it doesn't prove anything, then we move on to other theories- to include a shorted cap.

[drewl]

Had an Ampeg B15 last week with a bad bias cap dragging the bias voltage down.

Change it and test it.

[nosaj]

Had an Ampeg B15 last week with a bad bias cap dragging the bias voltage down.

Change it and test it.

No kidding especially considering how much one cap costs versus what a bad one can do. Change it and test it in the round file can if it makes a noise when it hits it bad.

nosaj

[J M Fahey]

36VAC tells me you have -50VDC at first cap node, way more than you need to cold bias those EL34.

Now if you waste them with piss poor voltage divider design, I can not help you.

Please post *actual* voltage divider values in your amp.

Just as an example, and supposing you have the values shown on post #2:

a) short the 1k resistor.
If that gives you the shivers, replace it by a 100 ohm one, although it´s not needed

b) replace the top 47k resistor by a 10k one ... what is your range adjustment now?

No, Zeus won´t fulminate you with lightning, there is nothing sacred about old circuits, they used what they thought best at the moment but of course they can be tweaked if needed.

[nosaj]

36VAC tells me you have -50VDC at first cap node, way more than you need to cold bias those EL34.

Now if you waste them with piss poor voltage divider design, I can not help you.

Please post *actual* voltage divider values in your amp.

Just as an example, and supposing you have the values shown on post #2:

a) short the 1k resistor.
If that gives you the shivers, replace it by a 100 ohm one, although it´s not needed

b) replace the top 47k resistor by a 10k one ... what is your range adjustment now?

No, Zeus won´t fulminate you with lightning, there is nothing sacred about old circuits, they used what they thought best at the moment but of course they can be tweaked if needed.

Also todays voltages are a little larger. 120 vs 115 or 117
nosaj

[Ampzone]

36VAC tells me you have -50VDC at first cap node, way more than you need to cold bias those EL34.

Now if you waste them with piss poor voltage divider design, I can not help you.

Please post *actual* voltage divider values in your amp.

Just as an example, and supposing you have the values shown on post #2:

a) short the 1k resistor.
If that gives you the shivers, replace it by a 100 ohm one, although it´s not needed

b) replace the top 47k resistor by a 10k one ... what is your range adjustment now?

No, Zeus won´t fulminate you with lightning, there is nothing sacred about old circuits, they used what they thought best at the moment but of course they can be tweaked if needed.

The bias circuit was stock Dr103 47k / 100uf 63v cap, then the diode with a 1k on dc side. When I get home today I will first try a new fresh cap, then your idea of a 100ohm in place of the 1k and drop the 47k to a 10k. I did try adding a 25k trim pot with a 100k removing the stock 47k and that got me up to -43vdc on the grids. I need to get closer to -47-50vdc

[Helmholtz]

with a 1k on dc side

Stock has 1k on AC side. It reduces peak cap charging current.

[Ampzone]

Stock has 1k on AC side. It reduces peak cap charging current.

aha, that is changing to 100ohm right?

[Helmholtz]

aha, that is changing to 100ohm right?

I was speaking of the same 1k resistor as Juan above. It is shown in the example circuit above as well as on the stock schematic on the AC side. Actually it doesn't matter if it's on the AC or DC side as long as it is in series with the diode. I don't think reducing to 100 Ohm will have any noticeable disadvantage.

I just wanted to make sure we agree on which side is AC.

[Ampzone]

I was speaking of the same 1k resistor as Juan above. It is shown in the example circuit above as well as on the stock schematic on the AC side. Actually it doesn't matter if it's on the AC or DC side as long as it is in series with the diode. I don't think reducing to 100 Ohm will have any noticeable disadvantage.

I just wanted to make sure we agree on which side is AC.

Ok so I changed the original 100uf bias cap to a new one and swapped the 1k for a 100ohm. To be clear the 1k was in series ON the DC side. Doing this raised the Neg DC voltage from -42vdc to now -48vdc dropping the MA across the EL34's \ ranging from the new 26ma to 32ma draw. So I am not sure which caused the increase, the cap or the resistor or a combination. So now we can get a good range.

I hope the stock 1k was not dong something necessary?

[Ampzone]

Wanted to mention the screen resistors are stock 100ohm 1w. With the b+ at almost 508 now would a change to say 500ohm to 1k 3-5 watt screens be wise?