Hiwatt L50R buzzzzzzzzzz

[miltown]

I have been looking at this amp for a week. I know . The buzz is definitely in the pre stage. So I measured for AC on the plate and caps and it is not any higher than like .022 volts. I was using my DMM. I have a DSO nano scope and it can't AC couple so I really can't see ripple . I guess to check for AC is that were I look on the plates and the filter caps? Sorry just at a loss here. I can't find a decent schematic not even from Hiwatt so anyone's knowledge would be helpful. Sorry kinda vague here please ask me question and I'm sure I can answer them.

[g1]

Yes the AC check of the filter caps with the DMM would tell you if you had bad ripple. .022VAC is good (no ripple). You measured with one probe to cap, other probe to ground, yes?
"can't find a decent schematic". Many of the official Hiwatt schematics were hand drawn, so this may be as good as it gets:
http://drtube.com/schematics/hiwatt/hwpre4.gif
Not sure about the scope you are talking about, but you should be able to use a coupling cap. in series with the probe (to block the DC so you can look at AC signals riding on DC). Make sure the voltage rating is high enough for the DC you are working with.

[Enzo]

You are locked in laser-like on ripple being your p[roblem, it seems, but it probably is not ripple.

The B+ serves the power amp first, so if the screens are free of ripple, so will the preamp be. More likely you have a missing ground reference for the preamp heaters, or a grounding problem elsewhere in the preamp, or a bad tube, or unshielded tube that should be shielded, signal line running through strong AC field, etc.

Isolate the problem: DO ANY of the controls have ANY effect on the sound of the buzz? VOlume, tone, whatever? COntrols that affect it are after the source, and controls that have no effect on the noise are before its source.

Even a crummy schematic ought to show you the basic signal path. If the signal passes more or less through V1, V2, V3, V4, then pull V1. Noise stop? Then pull V2. Noise stop? Etc, finding out which tubes seem to be involved. Doesn;t mean teh tube is bad, though it could be, but it helps identify which stage is picking it up.

[miltown]

Yeah I did the coupling cap on the probe. Is it the + side to the signal and the - side to the probe and then the ground on ground? I used an eletrolytic 40uf @450 volts. But this scope doesn't have AC coupling just DC and someone told me I need AC . I also have that schematic and one Hiwatt sent me. I'd like a power supply schematic. I feel it is in the power supply because when you turn the Master volume it is scratch sounding like a dirty pot. I can also ground pin #2 on the pot and that will kill the buzz but on pin#3 input I can't kill the buzz.

[miltown]

Thanks Enzo I did all that. The Master control and Mid and tremble will effect the buzz.The mid/tremble not as much as the master volume though. I have replaced the tubes. It is really in the V1,V2 area it stops when I pull V3 which is the phase inverter. This is not a hard wired amp as some of them are the tube sockets are on a PCB board. This is one that was actually made by Hiwatt not one that was made in Modesto, Cali. Don't know if you know the story on these amps but some guys were making their own amp and putting the Hiwatt name on it. It is pretty much the same just no PCB board it was hard wired like it should be.But this is not one of them. This is the actual Hiwatt one. I talked to Tony Imbesi the guy who was helping the guys who were build the copy amp. He said the rectifier diodes which I replaced or ripple at the plates or caps and I'm not seeing anything and all componets check out good atleast the cap and the plate resistors. Thanks for the post again Enzo I'm just kinda stumped/frustrated so I can't seem to explain myself accurately and these post help me get it out so to speak.

[g1]

Seeing as there is no power supply schematic available, I'm wondering whether maybe V1 & V2 have DC heaters while the other tubes have AC heaters.
If so, could be faulty filtering in the DC heater supply.

[g1]

re: scope decoupling cap. You should be using a non electrolytic for this, maybe something like .047 @ 600V.

[Enzo]

V1 has two triodes, so ground a clip wire to the bottom end of the cathode resistor, NoT just chassis somewhere, and use it to ground the grid of each triode. Does grounding that grid kill the hum in either place?

See that line from the first grid of V2 where the wiper of VR4 is connected that goes back to the input jack? It connect at the jack to a ground cutout. WHenever nothing is plugged into the jack, that grid is grounded. Keeps the amp quiet when nothing is pluggeed in. Find that wire and disconnect it from the V2/VR4 end. In fact, if it is a trace on the pc board, I'd cut it, as close as possible to the grid/pot. That wire can act like an antenna. And easy to restore if not the problem.

[miltown]

Guys give me a moment to check stuff out. Been working crazy mad.

[miltown]

OK back in action. So grounding the grids does nothing. Just adds some radio static.The last point that can be grounded to stop the noise is pin 2 on the master volume pot pin 3 the input does nothing to ground but pin 2 the output will stop the buzz.

[Enzo]

All you need to do is post in the old thread, and right to the top it goes, it hadn't even left the first page.

Hey reposting about an old post Hiwatt L50R buzzzzz. So I am stumped . The latest is I have checked all coupling caps in the second stage V2 and all are good. I have removed the wire that supplies the signal to VR10 which is the master control volume and the buzz is still there. When I ground T2 on VR10 the buzz is still there when I ground the wiper the buzz is gone. So sounds like it is in the V2 area but how come when I disconnect the signal from VR10 the buzz is still there? The buzz I should also note is only present about 5 to 10 on the volume control. It gets quiter at 10 but is still there. Here is the schematic of the pre-amp stage it is really all that is available. http://drtube.com/schematics/hiwatt/hwpre4.gif . I am about a click away from just taking it in to get repaired but want to see if anyone has any last suggestions.

Does turning VR10 to zero kill the buzz? Does it matter which channel is selected? Even with that feed wire to VR10 disconnected? Does the reverb setting affect the noise, as in does all the way off kill the buzz? When the relay selects VR10, that wiper is connected to the FX loop wiring and the reverb drive. One of those could be involved.

Those FX loop jacks can;t be wired like the schematic says.

If you ground the top of VR10, you report the hum remains, but with the top of the pot grounded, sweep the control all through its range. What does the hum do? Does it rise evenly? Does it peak at mid travel of the control and taper off at the ends? Is the back cover of the VR10 grounded?

And when you ground something, where are you connecting the ground end of your clip lead?

[miltown]

So yes turning VR10 down will kill the buzz. When turning VR10 up the buzz starts about 3 is low until until mid and then increases then at about 9-10 lowers and becomes more of a hum. Both channels are affected. Both VR1 Rhythm volume and VR5 Lead volume are slightly affected. What I mean by that is you can turn them up and at about 8 I will pick up a radio station and then at ten it will be gone that is with VR10 at mid and full up. The buzz does not increase in volume though when using VR1 and VR5. VR 8 which is bass in the lead section is scratchy. VR10 case is grounded by the mount to chassis but no leads are soildered to the case. Reverb no affect on buzz . VR10 wiper goes directly to V4 pin 7 . As for grounding the top of VR10 the buzz is pretty much the same but when I get to 9-10 the hum that was there is gone so basically at the top end of the taper its gone. As for my ground I used the cathodes on V1,V2 when checking grids and VR10 and I also used the chassis ground when checking grids and VR10. Also I should not at the top of VR10 along with the Return/Send is 220k ohm resistor that goes to the top of VR11 which is the Reverb. Grounding the leads on VR11 has no affect.

[Enzo]

That may be a clue, the reason I asked about the hum diminishing when the control is maxed is that when the wiper is in the center, it has the highest resistance away from the rest of the world. SO if there is something about the pot itself picking up hum, it will be loudest in the middle. Is tehre good low resistance between the rear cover of VR10 and chassis? If so, we don;t need to add a wire. Not all rear covers are electrically connected to the bushing.

VR10 wiper goes direct to V4 grid? Then the schematic linked does not match your amp?

I believe your connection there, but is anything else connected to either end of that wire? And how long is that wire and where is it dressed? Can you move it away from other wires? Parallel wires interect the most strongly. A signal wire running alongside an AC wire will pick up the AC field. It may look nice to bundle up wires, but if we bundle AC power wiring and signal wiring, hun results. DOn;t know if that is the case here, but it happens.

[miltown]

Yeah that is why I was calling that schematic crummy. It is close but there are bypass caps that are not on there and it really goes south after VR6 and VR10 the Master volume. The ground at the pots is .3 ohms . There is not other connections to that grid on V4 just straight from the wiper on VR10. Also this circuit is on a PCB not hard wired. So no chance of parallel wires. The tube sockets are mounted right to the PCB. The power supply/rectifier is solid state . I remember reading a design article a few years back and it was stating that it was not a good idea to use tubes with a solid state rectifier, I can't remember why they were saying that. I don't quite understand the idea of Master volume here is it a mix or sum of the regular signal and the send/return signal? I don't see how it recieves VR1 or VR5 aside from the send/return.

[miltown]

I just checked the chassis ground to PCB and it reads 1 ohm when the amp is on and .2 when off. Is that a normal situation. All other grounds are the ok.