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SEP Tube amp hum has developed, at least I have found out from where. How do you do it?


JL Sargent

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My little 6BQ5 SEP amp has developed a hum on one channel. I'm using it with an active setup driving my horns. It started humming a few days ago on one side. I just knew it was one of the output tubes so I swapped it out to no avail. Getting louder and real annoying I lauching a real hunt. While listening, I turned the amp on its side and began tapping the caps underneath with a pencil. Sure enough an old Archer .047 600v barrel cap seems to be the culprit. With a couple of good pencil raps the noise totally went away. I have ordered orange drops to replace all the audio path caps. How do you hunt down bad components? I'm sure there are much more sophisticated ways of doing it.[;)]

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Tapping on components as you have done, shaking wires, using cold spray to expose thermal intermittents, are all ways to isolate certain problems in circuits. If none of these methods yields results then signal tracers, multimeters, and oscilloscopes should be employed. Check power first, and make sure that no AC is present on DC power busses. Then start at the input and work toward the output stage, observing when the voltage readings vary from spec, or when the signal is lost or becomes distorted. You are trying to isolate the offending section of the circuit, and then pinpoint the component causing the problem.

Of course, be very careful when poking around live circuitry. Some tube equipment has 400 DCV or more, besides 120 VAC.

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What don said.

Re-capping and replacing resistors along with other out of spec components will save you the hassle of troubleshooting in the first place. Or at least keep it to a minimum when powering it back up.

Sound like it's time to freshen up the Maggotbox. :)

For the sake of clarity, your Magnavox amp is a single-ended 6BQ5/EL84 pentode connected amplifier. Or SEP.

This would be a SET amp. It is a single-ended amplifier using a 2A3 triode. Single-ended triode.....SET. Not pentode.....

post-6643-13819511709494_thumb.jpg

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To check coupling caps, first determine which end of the cap is the high voltage end and which is the low voltage end, turn the amp off and unsolder the low voltage side, connect your meter by clamping the negative lead to the chassis and the positive lead to the low voltage side of the cap, turn the amp on, the D.C voltage reading should be under 100 milivolts if the cap is ok, anything over .5 volts indicates the cap is leaking D.C. and should be replaced.

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Thanks for the great replys guys. Ok, so I have a SEP.

Mike, did you build that SET amp in the photo? Real sweet looking unit.

db101, I'll give that test a try on this cap.

Is leaking DC likely causing the hum I hear? Tapping on the cap also changed the tone momentarily.

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Thanks for the great replys guys. Ok, so I have a SEP.

Mike, did you build that SET amp in the photo? Real sweet looking unit.

db101, I'll give that test a try on this cap.

Is leaking DC likely causing the hum I hear? Tapping on the cap also changed the tone momentarily.

It's possible. The cap may have a bad connection on either end, or is just pretty much shorted out.

If you are using the amp with all the old coupling and de-coupling caps, it would be wise just to change them out. The amp will just get more noisy over time. The aluminum multisectional can capacitor will have two or three sections for de-coupling (filtering) for the power supply, with a (20?) microfarad lower voltage section for the cathode bypass of the output tubes.

The aluminum can cap "should" be ok, if you only hear hum through one side. The section for cathode bypass to the output tubes can be swapped out for a cheaper axial electrolytic of around 100 microfarad @ 35 volts, and mounted underside. It will help.

The coupling caps between the plate sections of 6EU7 to the grids (G1) of 6BQ5/El84, should be a ceramic at .01 or .047 microfarad. They could be a molded plastic axial type or a wax axial type. Those are the coupling caps to get rid of.......

The barrel or axial cap you mention tapping on, might be part of a original tone compensation network. (I think, ain't sure) either the network is connected and related to the pre-amp tuner, and possibly a rumble filter for records, or compensation for the open baffle nature of the speaker drivers in the consoles.

All lot of those parts in that network can be just deleted......

Check through all the resistors for tolerance with a DMM, or a volt/ohm meter. Pay heed to the cathode resistor for the output tubes, and power supply dropping resistors.

Do you have a schematic?

Yup, that's my 2A3 amp. I befriended a retired EE years ago with a bad audio habit, especially with speakers. He had all the parts scrounged up from over a year or so to build the amp, and then eventually sold it to me for 400 bucks.

It was my first scratch built amp, but it was like a kit. Had the chassis, iron, tubes, oil caps. I had to drill holes, buy sockets, hardware, and wire. I did all the layout myself.

He did help with orienting the filament transformers for the 2A3's on the chassis, and answered a billion questions.

He powered each one up (at a time) with a variac, and then connected the filament xformer's secondary to a scope. He then moved the filament xformer about the chassis, monitoring the scope waveform for the least interference. Once found, that's where they were mounted.

I'm pretty certain this is why the amp came out with 1-2 millivolts of filament line noise from AC heating on the first take, the amp doesn't use null pots either. 1-2 millivolts isn't bad for direct AC heating I think......He still says I was just lucky....

The PS transformer is a old Gramer brand tube television transformer. It has a couple 6.3 volt windings. One for the 6EA7 of reasonable size, and a heavy duty 6.3 volt winding for the 6AU4 damper diodes. You find a old tube TV from the 50's or early 60's for free, pinch the PS transformer.......I got a pile of them.

The amp originally used TJ fullmusic 300B's with 2.5 filaments. I swapped to 2A3. He come up with the driver circuit on his own. It's a 6EA7 with a Allied brand interstage transformer bridged with a cap to the 2A3. It has Scholl 10 watt OPT's. It is a good sounding triode amp. I couldn't do worse for the 600 bucks I put into it.........

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I've read all that about three times so far. Thanks alot!

Mike, so it would seem I can eliminate the few barrel caps from the circuit anyway? I also noticed these are "dart" barrel caps where the ceramic coupling caps are not? I certainly want to clean it up as much as possible. I'm gonna go back and look at that picture you posted of yours a while back. Thanks again.

ON EDIT: Looking at the photo of the underside of your Maggie SEP amp it appears there are two barrel type caps needed?

Do you know a source for a nice clean schematic for these amps with tuner and tone circuits removed?

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I've read all that about three times so far. Thanks alot!

Mike, so it would seem I can eliminate the few barrel caps from the circuit anyway? I also noticed these are "dart" barrel caps where the ceramic coupling caps are not? I certainly want to clean it up as much as possible. I'm gonna go back and look at that picture you posted of yours a while back. Thanks again.

ON EDIT: Looking at the photo of the underside of your Maggie SEP amp it appears there are two barrel type caps needed?

Do you know a source for a nice clean schematic for these amps with tuner and tone circuits removed?

This is the best I can do for a clean known schematic. The voltages indicated may not necessarily come out exactly the same, as long as they are not way too high. Once going, determine the total dissipation of each output tube and make sure it is in within it's ratings. 70-80% of the total dissipation rating is good......

Mike

post-6643-13819511938572_thumb.jpg

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Just to follow up on this. I replaced all the audio caps on this unit with orange drops and I still had the tone?!? So looking at the schematic Mike provided and I realized the suspected culprit was a side circuit of some sort as Mike had suggested and I snipped that newly installed .047 uf cap out and the tone is gone. I also noticed the amp is has noticably more gain now than before. I've been listening for a while and all sounds real good and amp appears happy. This was a cheap upgrade that really makes a difference. All the help with this appreciated.

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