1570B AMPLIFIER PROBLEMS

[DEGS1911]

Hi Folks! I hope someone might have an idea of what the problem might be with this amp.

-On a variac, the power is brought to 60 volts. The amp warms up and music is heard from the speaker and sounds fine.
-At 70 volts, a bass note sets off a loud low hum. The power must be cut immediately to protect the amp and the speaker.

-All tubes were replaced from a known working amp with no change in symptoms.
-All capacitors were replaced WITH THE EXCEPTION OF THE 6uF 1000V oil filled.
-The hum is NOT affected by the volume control.
-Removal of the 12AX7 allows electrical power to be brought to 115V but of course there is no sound at all.
-The amp, with its component replacements was working fine, but was then handled roughly after which the symptom appeared.
-The underside of the chassis was closely inspected with a magnifying glass and all connections appear to be tight and good.
-All screws and mounting hardware for the transformers was checked to tightness.
-The amp was blown out with an air compressor and tube sockets washed with contact cleaner.

Anyone have an idea? I don't presently own any test equipment except a volt/ohm meter.

THANKS!!!
-DANC

[bfish]

HI, DANC!
You caught me with one of those Green Monsters on the bench, and another in the wings, so I'll try to help out. You gave a good description, so I'm assuming you're aware of the correct safety precautions needed when working with lightning-in-a-can......I have a more-than-healthy respect for them, (borderline fear...).
That said, I've been lookin', and trying to determine what, if anything, would be most likely to fail, based on your description. If mechanical shock is what you meant by 'rough handling', tubes of course, and possibly the 6uf cap (C8) you mentioned, appear to be the only parts that might suffer much, if you rolled the amp down the stairs! Industrial is too mild an adjective for a 1570B! C8 also spans directly from 930V to ground, and if still the original, is nearing 30 years old, at best. Unfortuneately, a VOM won't help in testing for the breakdown/low leakage C8 would exhibit to cause this problem. Usually, when an old oil cap goes, it goes all the way, taking other valuable components with it.
At 60V line, everything's really just 'waking up' so many problems show themselves here, making us glad we use Variacs. Does the volume control operate correctly when you do get clean sound? Can you get to line V without hum with no input connected, and the 12AX7 in place?Are you using unbalanced (1&2) input, or balanced (3&4) with a transformer? Was a new 3-prong, grounded line cord installed lately? (I always include a 'ground-lift' switch with a new cord). Was the re-cap done with schematic or modified values? Poorly calculated mods can wait awhile before becoming problems.
That's all the guessing I'm good for now, good luck!
BFISH

[donaldpatten]

DANC
Sounds OK at low line Voltage...probably has low gain at 60 V .Could the negative feedback circuit have changed ? Or more likely is there a grounding problem ? A loose phone jack, a broken XLR connector , a barrier strip shorting to ground , a missing/loose screw that may be nessary between two sheet metal parts. Try twisting , flexing , jarring , thumppimg , dropping ( only one inch )the amp while it is on .
Don

[DEGS1911]

Originally posted by bfish:
HI, DANC!
You caught me with one of those Green Monsters on the bench, and another in the wings, so I'll try to help out. You gave a good description, so I'm assuming you're aware of the correct safety precautions needed when working with lightning-in-a-can......I have a more-than-healthy respect for them, (borderline fear...).
That said, I've been lookin', and trying to determine what, if anything, would be most likely to fail, based on your description. If mechanical shock is what you meant by 'rough handling', tubes of course, and possibly the 6uf cap (C8) you mentioned, appear to be the only parts that might suffer much, if you rolled the amp down the stairs! Industrial is too mild an adjective for a 1570B! C8 also spans directly from 930V to ground, and if still the original, is nearing 30 years old, at best. Unfortuneately, a VOM won't help in testing for the breakdown/low leakage C8 would exhibit to cause this problem. Usually, when an old oil cap goes, it goes all the way, taking other valuable components with it.
At 60V line, everything's really just 'waking up' so many problems show themselves here, making us glad we use Variacs. Does the volume control operate correctly when you do get clean sound? Can you get to line V without hum with no input connected, and the 12AX7 in place?Are you using unbalanced (1&2) input, or balanced (3&4) with a transformer? Was a new 3-prong, grounded line cord installed lately? (I always include a 'ground-lift' switch with a new cord). Was the re-cap done with schematic or modified values? Poorly calculated mods can wait awhile before becoming problems.
That's all the guessing I'm good for now, good luck!
BFISH

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[DEGS1911]

Hi BFISH!

Thanks for taking the time! I kind of suspected C8 and I think I'll start by replacing it. GE has one available that is 6uF 1500V. Some say the oil filled caps last forever, but it is true that they are at least 30 years old and I think these are considerably older. That's alot of years being slammed with alot of volts.

-The volume control does work at 60V before the hum presents itself, and the fidelity is great.
-With the 12AX7 out, I can get the unit up to line voltage with no hum.
-Two caps that were replaced were not exactly to spec: C7 and C11 are now 180uF 400V.

Anyway, I'll start with C8 and let you know how it turns out. Thanks again Bfish!!
-danc

[DEGS1911]

Originally posted by donaldpatten:
DANC
Sounds OK at low line Voltage...probably has low gain at 60 V .Could the negative feedback circuit have changed ? Or more likely is there a grounding problem ? A loose phone jack, a broken XLR connector , a barrier strip shorting to ground , a missing/loose screw that may be nessary between two sheet metal parts. Try twisting , flexing , jarring , thumppimg , dropping ( only one inch )the amp while it is on .
Don

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[DEGS1911]

Hi donaldpatten!

Thanks for taking the time to help me out! I think I'm going to start by replacing C8, but I'll also inspect the items you listed including wires, screws, connectors etc. Have a good one!

-danc

[bfish]

DANC,
CDE has a relatively new line they call "elecrofilm", with very impressive specs for non-polars and there's a 7.5uf/1200VDC in the lineup that I may try out (Mouser # 5984-UNL12W7P5K). I'd like to hear from any here with experience with them. Just make sure you don't end up with a cap intended for motor starting, check both DC AND AC ratings.
I don't mean to hijack your post, but I feel obliged to raise an issue that the younger members might not be aware of. The capacitor under discussion, as well as all other OIL-FILLED caps (including NOS and even some of those wonderful, potted transformers) produced prior to 1977, should be assumed to contain polychlorinated biphenyls (PCBs). As long as the oil stays safely contained (no leaks) it poses no risk, though it has been recommended for years that any devices suspected of containing PCBs be removed from service, and given to hazardous materials professionals for proper disposal. To find out where you can take such items locally, check the Yellow pages under haz. matl., or ask someone at the nearest Fire Dept., as they must also be prepared to deal with such issues. Generally, individuals can drop off limited quantities for free, but commercial quantities can be very expensive to get rid of. For more info, visit;
http://www.atsdr.cdc.gov/tfacts17.html
Back in the 70's, this was a Big Deal, as the number of products containing PCBs was staggering. with power companies affected most, due to the millions of oil-filled transformers they had in use. Since then, you seldom hear about PCBs, asbestos, or any of the other nasties of that era, but a reminder that some of this stuff is still around won't hurt any of us!
BFISH

[bfish]

DANC,
CDE has a relatively new line they call "elecrofilm", with very impressive specs for non-polars and there's a 7.5uf/1200VDC in the lineup that I may try out (Mouser # 5984-UNL12W7P5K). I'd like to hear from any here with experience with them. Just make sure you don't end up with a cap intended for motor starting, check both DC AND AC ratings.
I don't mean to hijack your post, but I feel obliged to raise an issue that the younger members might not be aware of. The capacitor under discussion, as well as all other OIL-FILLED caps (including NOS and even some of those wonderful, potted transformers) produced prior to 1977, should be assumed to contain polychlorinated biphenyls (PCBs). As long as the oil stays safely contained (no leaks) it poses no risk, though it has been recommended for years that any devices suspected of containing PCBs be removed from service, and given to hazardous materials professionals for proper disposal. To find out where you can take such items locally, check the Yellow pages under haz. matl., or ask someone at the nearest Fire Dept., as they must also be prepared to deal with such issues. Generally, individuals can drop off limited quantities for free, but commercial quantities can be very expensive to get rid of. For more info, visit;
http://www.atsdr.cdc.gov/tfacts17.html
Back in the 70's, this was a Big Deal, as the number of products containing PCBs was staggering. with power companies affected most, due to the millions of oil-filled transformers they had in use. Since then, you seldom hear about PCBs, asbestos, or any of the other nasties of that era, but a reminder that some of this stuff is still around won't hurt any of us!
BFISH