I need your genius to stop this noise!

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I need your genius to stop this noise!

Postby ioginy » Mon Jan 24, 2011 10:42 pm

Good day all. I recently built a single channel version of an Ampeg B15N68.
http://www.prowessamplifiers.com/schematics/ampeg/B15N68_Portaflex.html
The only differences are that I used 12at7 input, a 5U4 rectifier, a 33uf-450V (this could be part of the problem) instead of 30uf-600V first cap, and moved the standby from the CT to the classic fender position between the Rectifier and the 33uf Capacitor.

When I turn it on it yells at me. Like a deep(ish) (not a squeal) hum, but really loud. I tried switching the plate connection on the OT with no change. There is also no change when I adjust volume or tone. There is actually no change when I plug in an instrument either, in fact there appears to be no input coming from the instrument at all.
All the tubes are new except 5U4 (which works in another amp) and the 12at7 which I tested in other amps as well as a tube tester and it checks out.

There is no noise when the standby is on, however when I turn the power off, the noise comes back and then fades out again as (i assume) the caps drain... all while the standby is on.

I have only checked the bias voltage (-49) so far. I'll check the rest of the voltage points tomorrow for reference, but I figured i'd throw this thread up first in case anyone had any ideas.

I am totally stuck. If anyone could give me some direction I would be very thankful.
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Postby dcgillespie » Tue Jan 25, 2011 6:52 am

As a basic diagnostic effort, first see if it will act the same way with the 12AT7 input tube removed. If it still does it, then with the phase inverter/driver tube also removed. This will at least help localize the noise aspect of your issue so you at least know where to focus your efforts on for that.

For the standby issue, I'd first verify that in fact the B+ voltage completely disappears when the standby is turned on -- and watch it when you then turn the power off. Very strange that the noise disappears with the standby on, but then reappears when the power is turned off! That means some B+ is somehow becoming available again? (or more likely, never completely left) If so, your meter should show that.

The one thing your change to the standby switch did was change the way the bias was applied to the output stage. The original approach killed the bias and the B+, where as you are killing just the B+. It's conceivable then, that if the tubes are still receiving some sort of B+ with the standby switch engaged -- but at a lower level -- then with the bias still applied, the output tubes would be in a state of cut-off, eliminating all sound. Killing the power under that condition then removes the bias, allowing the tubes to conduct again for a short period before the B+ completely dies away, bringing the sound back for a short period.

Either way, it would seem that you have two separate issues -- the noise, which is always there when the amp is operating, and then the behavior of the standby switch operation. Both with take different approaches to resolve.

Gotta keep us posted on this one!


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Postby ioginy » Tue Jan 25, 2011 9:43 pm

ALRIGHT! I did some looking and here are my findings.

First I had the plate and screen swapped on the 12AT7, fixed that. Now I am getting a response from a guitar when it's plugged in. Sadly I discovered I wired the tone pots and volume pot in reverse, easily fixed (I really should write down their orientation).

I pulled the tubes and when the 12AT7 is pulled, the hum changes tone slightly, but not volume. When I pull the 6SL7, the hum completely stops. This made me happy.

I tested all the voltages, which i'll post below, but I discovered something interesting while doing so. I was getting 0v on one of the 6L6 plates. I checked the transformer and confirmed that I was getting voltage to one plate, but not the other.
The thing about that is the transformer I am using is this one:
http://pacifictv.ca/schematics/scott210c.jpg
The b+ voltage is 519V. I am getting the same out of the "plate of V9", but nothing from the "plate of V8". Oddly I am getting 519V from the "screen tap not used" directly beside the "plate of V9". I tried hooking the other plate up to the screen tap, thinking that the diagram might be wrong, but there was no change in the hum. I have a fear that the transformer is pooched, but it was working perfectly when I pulled it from the donor unit.

Here are the voltages.

6l6 (1) plate +519
Control grid -47
Screen grid +515
Cathode 0

6l6 (2) plate +0 (or 513 from the screen grid)
Control grid -47
Screen Grid +515
Cathode 0


6SL7
Pin 1,2,3
plate +371
grid 0
cathode +4

Pin 4,5,6
plate +242
grid 0
cathode +1


12AT7
pin 1,2,3
plate +178
grid 0
cathode +3

pin 6,7,8
plate +102
grid 0
cathode +1


B+ Voltage
A- 513v
B- 509v
C- 454v
D- 470v

Well... that's all I've got. I have a deep fear that I'm going to have to buy a different OT, but if anyone has any other ideas, I'd love you for it!

Thanks
Cory..
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Postby azazello » Wed Jan 26, 2011 3:53 am

After
1 kohm/10 wt You must have about 300-350 v, but in p.B for screen grids
You have more 500v. Maybe 1 kohm/10 wt is shorted? Or You have wrong connections in OT? 513v in p.B will broke second cap 40 uF, that is for 500 v. Maybe it is warm and semybroked?
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Postby dcgillespie » Wed Jan 26, 2011 7:46 am

Ioginy -- Before you condemn the OPT, check to see that it is not of the type that has two separate B+ leads as part of the primary connections. The schematic implies that is the way this particular transformer was built, although it could simply be the way the schematic was drawn as well. Still, it was not uncommon for the better transformers to employ a separate B+ connection for each half of the primary winding, to allow for current monitoring of each tube. Acrosound, Chicago, Stancor, Peerless, and others routinely produced transformers like this throughout the golden age of audio. If your transformer has two B+ connections, but only one is connected, that would produce the scenario you are experiencing.

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Postby ioginy » Wed Jan 26, 2011 3:33 pm

Hmmm, i seem to remember a lead wire going from the b+ lug to the unused point on the lower left side (in diagram). I wondered what it was for and then dismissed it (silly). I have a feeling that may be the other b+ point. I'll give it a check when I get home.

I was a little curious about the screen tap though. I fugure those would be for wiring in a UL configuration. However, it read the same voltage as I was putting into the b+ tap. Shouldn't it have read 40% lower?

Also, the schemtaic for the Ampeg b15 doesn't have any screen resistors to the screen grids. I was thinking it might be a good plan to throw some 220ohm-5W resistors in there. I figured I'd ask before doing it. It just seems a little kinder to the 6L6's.

Thanks.
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Postby azazello » Wed Jan 26, 2011 4:32 pm

In Your case You can use res. 220 ohm/2-5 watts.
If You'll use OT UL transformer, UL taps will have U = 40-43 % from +U/central tap/ and You don't need res. 220ohm
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Postby ioginy » Wed Jan 26, 2011 10:32 pm

ALRIGHT! I hooked up the mystery lead on the OT and got a bad noise, tried swapping the plate leads and viola, it sings wonderfully! Sadly it sings wonderfully very quietly. I re-checked voltages all around they are very very very much lower. I then checked the voltages with the bias at it's max and at it's min. (i put an adjustable bias, another difference I forgot to mention).

Here are the voltages I got, the two numbers are from the max / min of the bias.

(1) 6L6
Pin 3 356/241
Pin 4 352/240
Pin 5 -46/-18
Pin 8 0/0

(2) 6L6
Pin 3 356/242
Pin 4 353/240
Pin 5 -46/-18
Pin 8 0/0

12AT7
Pin 1 133/103
Pin 2 0/0
Pin 3 2.3/1.5
Pin 6 83/78
Pin 6 0/0
Pin 8 1.1/0.7

6SL7
Pin 1 0/0
Pin 2 265/193
Pin 3 3/1.9
Pin 4 0/0
Pin 5 184/142
Pin 6 1.2/0.7

B+
A 356/239
B 353/238
C 332/226
D 320/218

Sorry for the pin reference instead of the pin characteristic, it's how I jotted them down in my note book.

So, there you have it. Something is definitely pulling a lot of voltage and I can't seem to figure out what. Aside from that, the amp sounds really really nice, a little too much distortion when dimed, but I'll work on that when I get the volume sorted.

Thanks in advance for the help.
Cory
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Postby azazello » Wed Jan 26, 2011 10:55 pm

Sorry, my mistake.... number of coils of UL tap is 40-43% from all coils for plate....!
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Postby ioginy » Wed Jan 26, 2011 11:01 pm

It's cool, I'll deal with the UL taps when I get the amp working properly.

As a side note, I remembered that I had the amp running through my "light bulb tester" which was sucking up a HUGE amount of power. When I plugged it in properly my voltages jumped up to where they should properly be. I am going to do a double check on the voltages again though, as the amp is still rather quiet. Full volume and I can sit beside the speaker and be completely comfortable. It's loud enough that I wouldn't put my ear to the cone, but if I did, I wouldn't be overly uncomfortable either. None the less, the quiet issue still hangs over my head.

Until I get the new voltages, I measured the first B+ lead at 495, as a reference to my other voltages.

Any ideas would be greatly appreciated.
Cory.
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Postby azazello » Thu Jan 27, 2011 3:43 am

495 v is good. U p.B will about 15-20 v less, because currents of splitter, preampl and second grids across 1 kohm.
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Postby dcgillespie » Thu Jan 27, 2011 8:06 am

In determining the issues behind it "singing quietly", you need to assess whether:

1. It has inadequate gain and is therefore not driving the amplifier to full power, or

2. It has plenty of gain to drive the amplifier to full power (overload), but the power is not being delivered to the speaker.

In the first scenario, the sound will be clean, but just not enough of it. In the second scenario, the sound will also not reach expected levels, and also be distorted.

These are two very different problems, and are approached in two different ways, so determining which is the case at hand is the first order of business.

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Postby ioginy » Thu Jan 27, 2011 12:12 pm

Well, I can definitely push the amp into distortion. It starts to break up around half volume and then progressively turns grittier as the volume goes up. The volume does however go up through out the range of the volume pot. I have found with some amps that after it goes into distortion, volume stays pretty stagnant but the distortion gets dirtier. This is not the case with this amp... so far. Ideally I would like it to be as clean as possible, as it's a bass amp, but a little maxed out volume distortion is NEVER a bad thing (love)
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Postby ioginy » Thu Jan 27, 2011 11:40 pm

New info! I tried, on a whim to jumper the tone stack out and it definitely kicked the volume up a bunch, I'm not sure though if that is just due to less resistance (a la mesa boogie mod) or if it is a part of the problem. I do notice though, when I flip either of the hi boost or low boost switches, everything gets crackly if I pump the amp. The tone changes a lot with the low boost, but not noticeably with the bright boost. I was thinking of maybe swapping the 500pf for a 250pf cap there to bring it up a bit, however that's a thought for another day, unless someone wants to throw their 2 cents in there. None the less, that is what I have so fr.

I measured the voltages again. I set the bias to -50 (as per the spec from the schematic) and ran my ipod through the amp to give it a bit of a load.

B+ off filter caps.
A: 445, B: 432, C: 384, D: 407

These all looked pretty good to me.

6L6 (1)
3 - Plate: 236
4 - Grid 1: 440
5 - Grid 2: -43
8 - Cathode: 0

6L6 (2)
3 - Plate: 265
4 - Grid 1: 435
5 - Grid 2: -48
8 - Cathode: 0

Now is where I noticed a problem. I measured aprx 440V at the b+ tap on the OT, got mid 200's on the plate taps and was getting low 400's on the un-used screen taps. I thought again, maybe I had messed up a bit with the OT wiring and swapped the grid leads to the "screen taps". When I tried the amp with a guitar it sounded REALLY flubby and bad, I measured all the voltages on the OT and I was getting mid 200's on both the screen taps and the plate taps now. I swapped the leads back, tried again, and the amp is back to it's quiet wonderful tone, and the voltages regained their off kilter readings. High on the screen taps and REALLY low on the plate taps.

I have a feeling this is where my problem is. None the less, for posterity, here are the readings from the other tubes.

6SL7
2 - Plate: 316
1 - Grid: .09
3 - Cathode: 5.4
5 - Plate: 175
4 - Grid: .04
6 - Cathode: 1.59

12AT7
1 - Plate: 152
2 - Grid: 0
3 - Cathode: 3.1
6 - Plate: 88
7 - Grid: 0
8 - Cathode: 1.52

All these appear to be decent, though in the back of my mind they seem a little low. Then again, I might just be over thinking things.

Anyone got any ideas?
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Postby azazello » Fri Jan 28, 2011 2:33 am

You wrote:
6L6 (1)
3 - Plate: 236
4 - Grid 1: 440
5 - Grid 2: -43
8 - Cathode: 0

6L6 (2)
3 - Plate: 265
4 - Grid 1: 435
5 - Grid 2: -48
8 - Cathode: 0

maybe really data are?:
plate - 440
G2 - 236
G1 - -43
C- 0

Plate - 435
G2 - 265
G1 - -48
C - 0 ????
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