Scope reading; dummy load vs speaker inconsistency - Page 1
Forum section: electronics / beginners

author: JHAnd
author_rank: Contributor
timestamp: November 19, 2023, 09:33:14 pm
content: Hi,I'm troubleshooting a tube amp that has severe buzzing at 100hz.When probing with my 4ohm dummy load attached I get no reading of the noise (ground clip on amp ground, probe on one side of dummy load). Connecting the two speakers (4ohms) I get a reading showing the 100hz clearly (ground clip on amp ground, probe on one of the speaker terminals).This is probably a stupid question, but I can't make it work with the dummy load, and I know it is supposed to work.Is there some voodoo going on or am I making some kind of noob mistake? Probably the latter.I'll post some pics of my setup.Pic 1: dummy load attached to ampPic 2: dummy load

author: CaptDon
author_rank: Super Contributor
timestamp: November 19, 2023, 10:12:33 pm
content: Maybe Blue isn't the Hot lead? Try probing the other side of your dummy load. Make sure the dummy load reads 4 DC ohms between the tip and sleeve of your plug. Should be easy to figure out. Speakers have an 'impedance' but your resistors don't so possibly the amplifier is 'motorboating' when connected to real speakers (although I doubt it). Some of those aluminum body resistors are marked in a strange way. I have some marked 20R and they ARE NOT 20 ohms as one would think.

author: Thunderer
author_rank: Regular Contributor
timestamp: November 20, 2023, 02:49:04 am
content: Schematic?Tried to power the heater in DC? For some tubes, powering the heater in AC is guaranteed hum.Now, was it buzzing like this before?PS: you don't see any noise on the dummy because of your scope (in)sensitivity. On the speakers, most likely some high SPL types, the noise is easily more present.

author: JHAnd
author_rank: Contributor
timestamp: November 20, 2023, 07:11:34 pm
content: I think you were right... I was probably not probing the hot lead... but I'm quite sure I tested both sides, so it might have been some connection that wasn't right in combination. Anyway now I'm getting a reading, thanks!I think I was assuming that output transformer secondaries were floating and because of that either side was probe-able (would that be correct for a floating secondary?)I see now on the schematic that one side is of the secondary is tied to ground.Is the OT secondary always grounded in amplifiers?https://schematicheaven.net/fenderamps/fender_bassman70.pdf

author: JHAnd
author_rank: Contributor
timestamp: November 20, 2023, 09:01:10 pm
content: Quote from: Thunderer on November 20, 2023, 02:49:04 amGood idea about powering the filaments with DC... however the filaments would yield hum (50hz), I have buzz (100hz). Buzz usually means bad filter cap.

author: CaptDon
author_rank: Super Contributor
timestamp: November 20, 2023, 10:37:36 pm
content: Usually the A.C. powered filaments are powered by a center tapped filament winding and as such becomes a non-significant source of induced hum. On an old tube type amplifier hum is usually the first indication of bad filter caps. Most guitar amps were not really 'hum-free' to begin with. They always tended to be noisier than perhaps a home stereo type amplifier. Generally capacitors are much physically smaller today and you can probably get twice as much capacitance at the same voltage as the old filter cans. Also, be aware that in many guitar amps the cabinet itself has metal shielding and the amp may hum when out of the cabinet.

author: Thunderer
author_rank: Regular Contributor
timestamp: November 21, 2023, 01:23:30 am
content: This schematic is not the center tap for the heaters, but has a "hum" balance setting.Anyway, if this unit still has the 47nF for the ground switch, better remove it, it is a killer component. Killer for the user!If you want to keep the ground switch, at least put a new and the best quality (rated 400Vac minimum) capacitor there.

author: JHAnd
author_rank: Contributor
timestamp: November 25, 2023, 09:03:47 pm
content: Just wanted to post the solution to the amp buzz even though that was not my main question...The 100hz buzz was solved by replacing the tube with a matched set and then using the "tube matching" pot on the back of the amp to remove the last bit of buzz.I did not have to replace any filter cap when doing this... it's dead silent now.Thanks to all of you for your input!

author: vk6zgo
author_rank: Super Contributor
timestamp: November 26, 2023, 12:11:22 am
content: Quote from: JHAnd on November 25, 2023, 09:03:47 pmYeah, about that.In the old days, filtering a high voltage high current supply was difficult, as it required large values of capacitance & seriously big filter chokes, all of which cost money.Luckily, designers devised a "cunning plan".It was easy enough to filter DC supplies for the low level/high gain, (but low current draw) stages, so they filtered the supply in two stages, with fairly poor filtering for the output stages, & good, (but needing smaller & cheaper RC components) filtering for the lower current stages.With the output stages, they took advantage of the inbuilt common mode rejection of push pull stages to cancel the 100/120Hz ripple down to a very low level.The inherent matching of "off the shelf" tubes in those days meant that once the balance pot was set, the rejection stayed in spec for years.Eventually, one tube would lose emission before the other one, & 100/120Hz hum would become audible.At my old work, we had a number of these type of amplifiers mounted along the wall of the transmitter hall in large, efficient speaker boxes, so when one of the EL34s died, the 100Hz from that amp wasveryaudible!We would swap a new amp in, take the faulty one into the workshop, chuck in a new pair of EL34s, tweak the balance, & that was usually that!

author: vk6zgo
author_rank: Super Contributor
timestamp: November 26, 2023, 12:16:40 am
content: Quote from: Thunderer on November 20, 2023, 02:49:04 amIndirectly heated tubes should not need DC filaments, if they are within spec for heater/ cathode leakage.Unfortunately, many tubes sold today are rejects of some kind, sometimes from a factory or in other cases "pulls" from equipment, which were probably removed because they were out of spec.