Ampeg B15 with dreadful hum Help

[freeformfx]

I'm working on a B15 that I rebuilt from a chassis missing both transformers among other things. I put it in a B18 enclosure with adapter baffle for use with a "15. The PT I used is a hammond 274BX and I remember putting alot of work into picking the best trannys for the amp. I have always had a problem with high B+ , at one point I think over 550v but I've got the voltage down since then (zener). Though heater vltgs are high, 6.9 and 5.4ac
My problem is two part... I have a dreadful hum coming from either channels volume controls, when the other channel volume is turned up at all while playing the other channel the hum is serious problem. With the other channels volume is down it's very useable and sounds GREAT. I wanna get rid of this hum and have been trying a long time. No ground loops I can find. The main chassis ground is at the input jack ground. The sound of the hum on channel 1 sounds like touching the live end of a guitar cable really like 60hz hum. But channel 2's hum is more muffled with a certain frequency you can sort of hear to it. maybe 120hz I'm not sure
And secondly, this may be the cause of the hum, I used my bias probe (1ohm to ground) to check the bias current on the 6l6's and the one i checked reads 73ma, wasn't sure that was accurate being C biased but Dividing Cathode voltage by resistor value also confirms this coming out to .148ma (.074 per tube) Is this why my hum problem is so bad maybe?. The amp is biasing way too hot?
I think I might try a 360 ohm 5 watt resistor I have here and see what I get but I'm a long way from 35-40ma as it is.
Any help with the hum issue is really appreciated. As is the bias issue

[Chuck H]

Well, the first order of business is the excessive dissipation. Do correct that. Then tackle the hum problem.

Hot, unbalanced tubes can hum like they don't know the words, but they would do it all the time. Not just when one channel is turned up a little. My feeling is that there must be a wiring error, a ground fault or a ground loop. You said that there doesn't appear to be any ground loops but they can be sneaky. If, for example, your input jack is metal and not insulated, the chassis is ground and the negative terminal of the input jack is grounded at a buss or star ground point (very common in most vintage and many modern designs), that IS a loop. Whether it hums or not depends on current between the two ground points. I'm just saying that it's not always easy to spot a ground loop. It does seem odd that the hum only manifests when the unused channel volume is raised from zero. I'm not sure how to interpret that, but I'm not looking at a schematic either (should always be provided with the question ).

[freeformfx]

cheapo 6SL7's causing hum?

I changed the cathode resistor to 360ohm and only brought the bias current down to under 60ma : ( So I'd figure going all the way up to a 680ohm or so Might bring to bias down in an acceptable range. I can't let this thing eat a nice pair of green Sylvanias. That i don't understand why they're not redplating running that high.
And I also forgot to mention Is it possible that my hum issue is from inferior preamp tubes. I bought 3 of those russian 6N9S's for like 3 bucks apiece. They can't worth squat i figure but I also figure they'd do the job at least. I only have one old stock 6SL7 and it's in the phase inverter position. I wonder what's the chance those el cheapo tubes could be causing the hum the whole time.
thoughts? sorry here's the schematic

I actually have that screen 1.5k resistor AnD two 1k screen resistors on the tubes so maybe I could remove the 1.5k, but that's not the problem were dealing with i'm sure

[freeformfx]

I did some experimenting, looks like it's gonna take around a 620 to 780ohm cathode resistor to get somewhere around 30-42 ma. that's my estimate based on averages, could be wrong but you get an idea. But this will still be at the risk of B+ and plate voltage (hopefully) of just over 500v. So it''s a difficult tradeoff.
I have to wait for one to get here so not much I can do til then. I figured it best to tackle the high current draw fist also but was I right to mess with it ( this amp was working great and being used periodically (hot transformer) is there something I don't know about measuring cathode bias... that measuring that high was ok ?

[Chuck H]

I didn't know the problem manifested with the new tubes as well as the new build. Always suspect the preamp tubes in this case. It's happened to me. Fire up a new build with new tubes and HUMMMM!... I looked for a mistake in wiring for half a day before I plugged in a different preamp tube and the hum stopped (DOH!). Now, in these cases, I always suspect tubes first.

It's still odd that the problem only happens when the volume is up on the unused channel. So, to find out if it's a tube or a circuit you should be able to pull one of the channels preamp tube. If turning up the volume on the tubeless channel causes hum in the other channel then it's probably a circuit problem.

If you have the two preamps arranged as "modules" in that each preamp has a ground point, try grounding them in the same place. If you have the filter cap feeding these stages grounded with the power supply, try grounding that filter at the same shared preamp ground.

If you have a scope you may be able to trace the hum to where it starts, if not to it's source. This may clue you in on any interactions and give you a better bead on where to look.

If you're willing to buy another rectifier tube you can drop some volts with a 5u4. If you're diode rectified you'll need to use zeners.

[Enzo]

Your schematic shows 36v on the cathode with 250 ohms. If that is to be believed - and with Joe Piazza drawings that is always taking a chance - then I get 144ma for two tubes, or 72ma per tube. And that would be darn hot for a 6L6.

[freeformfx]

I didn't know the problem manifested with the new tubes as well as the new build. Always suspect the preamp tubes in this case. It's happened to me. Fire up a new build with new tubes and HUMMMM!... I looked for a mistake in wiring for half a day before I plugged in a different preamp tube and the hum stopped (DOH!). Now, in these cases, I always suspect tubes first

ah So it can happen, especially with a super cheap cross# preamp tube then. I know the heaters are high but I've had em high before and never caused bad hum. I even tried the center tap on the PT instead of the pot and there's no difference and this is what makes me think it not a tube, the hum balance control has no effedt on the hum really at all.

I know that finding a loop can be a real PITA, but I don't think there's one. all my grounds go to the board and then on to the input jack main ground. Actually two , there's one off of the chanel 2 ground as well. But I took it off and it actually improved the hum when it's attached. I did do alot, new volume controls and mostly all new shielded cable, every connection made always goes exactly where the original went so a wiring error is hard to imagine. I could take some pics of how it's wired and post them if anyone wants to take a look, it's not a Picasso job as i just salvaged an old damaged b15 chassis. I didn't do a complete rewire I just went with what was there.
I'm going to order two old stock sylvania's I found and see if that leaves me in the same shape

[g1]

Do you have shorting jacks for the inputs, and are they working correctly?

[freeformfx]

Oh and I did try a 5u4 in there and it's' almost perfect for the voltage drop it gives, knocks off at least 25v. I just wasn't confident rolling with it because of the current draw of it. The PT gets real hot as it is.
plus I've been trying to bias it for a 5AR4 so I ordered a 680ohm, best I could get I'd rather have had 620ohm, and a 780ohm just in case. I wanted to try two 1.2k in parallel but couldn't find any in the drawers. If I went with the 5U4 I'd have to reevaluate my cathode resistor choice I'd reckon. Anyone here think PT could handle it

[freeformfx]

Do you have shorting jacks for the inputs, and are they working correctly?

No the first jack for the first channel is NOT a shorting jack as I believe it should be and the jack has a different color to the wafers but it really looks like stock wiring/soldering. But yeah the amp hums like a mother with no jack inserted. i wouldn't think this would be causing my hum issue though so I've not went through the trouble of swapping the jack out

[freeformfx]

Your schematic shows 36v on the cathode with 250 ohms. If that is to be believed - and with Joe Piazza drawings that is always taking a chance - then I get 144ma for two tubes, or 72ma per tube. And that would be darn hot for a 6L6.

Yes it was pretty true to that schematic of his , but yeah that's exactly what I had Enzo. Why weren't the tubes redplating? And is it ok to balance things out , plate voltage, b+ , current draw, dissippation by adjusting that cathode resistor up like I'm doing?

[Chuck H]

I could take some pics of how it's wired and post them if anyone wants to take a look, it's not a Picasso job as i just salvaged an old damaged b15 chassis.

There's a reason I don't show off build pics here My amps work great, but I'm pretty sure my solder work would raise some eyebrows. No worries. We've seen it all.

The way you describe it sounds like you may have daisy chained grounds. It only takes one long lead with a high current circuit tied to a low signal circuit to muck things up. Please post pics. I HATE following circuits with pics, but what are the options, right?

[freeformfx]

There's a reason I don't show off build pics here My amps work great, but I'm pretty sure my solder work would raise some eyebrows. No worries. We've seen it all.

The way you describe it sounds like you may have daisy chained grounds. It only takes one long lead with a high current circuit tied to a low signal circuit to muck things up. Please post pics. I HATE following circuits with pics, but what are the options, right?

right I feel what you're saying I'm guilty of that sometimes too , prob on this one. But it's the best option probably maybe someone will catch something I did wrong. get em up next chance I get.
Still wonder on opinions of the cathode raising method.

[freeformfx]

Here's some pics I know it's hard to see where stuff is going esp with the way Ampeg wired it. I left it all as was.

[freeformfx]

power supply / output tubes

I know it's hard to see how I have the output wired but I am fairly confident in my wiring in that area. The rest is hard enough to decipher with gut shots, so I figured i'd mention that. As is now with a 5AR4 with a 1000ohm cathode resistor they're idling at apprx 26ma each. With 538v on the plates Minus a 53v cathode voltage equaling an actual 485v on the plates (if I'm correct about that) isn't actually so bad.
Lower idle current hasn't had a reaction on the hum though.
So a 680ohm or maybe likely a 780ohm is what I'll end up using. The idle will come up more in the 30ma's and plate vltge will drop a a good bit putting my tubes more into safety range.
My screens are held a little low by that I added 1k resistors on the tubes in conjunction with that 1.5 screen dropping resistor, don't think that's causing any problems.
I don't mean to get off the subject of the hum just getting that dissipation under control. If my math is right is 12.85 now.