Hiwatt Custom 50 Biacrown era Loud Scratchy Master Volume..Help

[vmazz]

Ok I rigged up a signal through my computer using Ableton Live and a synth(faux organ) track I just recorded a sustained C note pretty much a sine wave I think. Using that signal I recorded these AC voltages on these tubes and these pins (plates and grids)
V1
1-.006
2-.002
6-4
7-.046

V2
1-.61
2-.027
6-15.2
7-.6

V3(PI)
1-9.1
2-.36
6-9.2
7-.028(while testing this grid the signal almost becomes inaudible)

V4
1-.0002
2-.0009
6-.36
7-.026

On the power tubes I checked the plates and grids after the 22k stoppers and the plates had 7VAC and grids had 9VAC.
If anyone can give me any advice ..Ive never tested ac voltages like this, not sure what is normal. Thanks!

[g1]

For starters, it is important to know the level of the signal at the input jack, and how you have the controls set.
50 to 100mV signal at the input is fairly common test level. Controls set to noon but volume at 2 or 3 with master full are common.
As far as measuring at the power tube plates, be very careful, there can be voltages higher than your meter or probes can handle.
That being said, with 9VAC at power tube grids, 7VAC at the plates is wrong.
With no signal, what are your DC voltages at pin4 of power tubes?

P.S. Anyone care to comment on the lower half of V4? Is it having to do with crosstalk from the other half of the triode?

[vmazz]

With no signal the plates have been around 490-495 DC.

[g1]

Not the plates, the screens (pin4).

[vmazz]

Thanks for your help G1. I followed your suggestion and used Master on full all else 12 oclockish and volume on 2-3. Also checked screens (pin 4) of power tubes with no signal passing through, they were 475 VDC with 490 VDC on the plates.
I took new measurements and tested, the signal AC was 46 mv

V1
1-.0008
2-.002
6-3.89
7-.046

V2
1-4.3
2-.186
6-57
7-3.6

V3(PI)
1-98
2-41
6-76
7-.92

V4
1-.01
2-.043
6-41
7-11

and plates of power tuubes 58VAC and grids 92VAC

I hope this can help narrow it down to a cause. Thanks all!

[The Dude]

Do you have a bias probe? I've just skim read the thread and I don't see where the bias voltage or current were given (I could have missed it). If you have a bias probe, can you tell us what the bias current is? If not, can you tell us what the DC voltage is at the anode of the bias rectifier (where it says -40V on the schematic)?

[vmazz]

The bias voltage at the anode of the bias rectifier is about -39 VDC.

[The Dude]

Well, that blows the theory I had rolling around my brain. I'm with g-one though. There should not be more signal on the output grids than there is on the plates. It would seem to me that your problem lies right at the output stage. Does this thing have the original output transformer or has somebody fiddled with it?

[g1]

A few things you could re-check as they seem a little off.
First, in case any settings changed, check that V2 pin6 is still 57V, if not, fine tune so it is.
Then recheck V4 pins 7 and 6, the gain seems low.
Also recheck PI pins 1 and 6. Then pin5 of each power tube. These should roughly agree, and your last measurements showed more at power tube grids than PI plates.
Also, are you running into a dummy load or what?

[vmazz]

I will test this evening per your advice. I'm running into speaker load Stock fane speaker. 8 ohms?

[g1]

Using the speaker is fine. Just be prepared to get blasted if the amp starts working all of a sudden.
Make sure that the speaker and speaker cable are good, if possible, try another speaker and spkr. cable to be sure. Or try the ones you are using with a different amp.

[vmazz]

Ok...You guys got me on the right track. To my great surprise( and Horror!) I decided to double check the speaker impedence.
In order for me to test the amp out of the cab(its a combo single fane speaker)I had used a speaker extension cable which I had rigged up a while back. Checking for DCR on the speaker tabs gives me about 6.5 ohms, checking through the cable ...about .8 ohms!
I stupidly used some cheap right angle plug which apparently shorted itself. So I was pretty freaked that I may have blown the OT running on such a light load.
FIxed the plug with a Switchcraft and now it works ..loud as I could imagine 50 watts could be! Clean and loud.
Still a little bit of scratch on the pots. These pots seem to be a weak link on this amp. They have the plastic stems and generally crackle .
Im thinking I wil replace them all with marshall pcb mounts(they are all pcb mount) or I could wire them off the board.
Thanks to all for the help getting me down the right track
What made you wonder about the load on the OT G1?

[The Dude]

That's good news! Being unaware that you had changed your speaker cable, I was heading towards a bad OT. A piece of wire is much cheaper.

I can't speak for g-one, but your DC voltages all seemed right and your signal levels seemed within tolerance up to the plates of the output tubes, so I was thinking we had problems right there at the end.

Try cleaning the pots with a good cleaner/lube like Deoxit (available at Radio Shack). That might save you some hassle. If you do decide to change the pots entirely, I would not go with the current Marshall style pots. They are as bad or worse. If you're going to go through all of that effort, put something good in there- maybe CTS.

[vmazz]

Thanks for the help and advice Dude. I think ill try the deoxit first and then go with offboard cts pots. What do you think of Alpha pots? Tubes and more carries them and Ive used them on several builds.

[g1]

So I was pretty freaked that I may have blown the OT running on such a light load.

Actually, a short is a very heavy load, a high impedance is a light load. A tube amp doesn't mind a shorted load (eventually the power tubes will). So you were lucky the cord shorted rather than went open, an open could cause OT failure. This is why Fender and other amps use a shorting jack for the speaker out, if you forget to plug in your speaker cable, the jack shorts, protecting the OT. Marshall does not use this system, if you forget to plug your speaker cable in, the amp runs into an open circuit (no load). This may be part of the reason Marshall's seem more prone to OT failure than Fender.

What made you wonder about the load on the OT G1?

As Dude mentioned, everything else seemed ok except the power tube plate voltages. It was leading up to a possible bad OT but I thought it would be best to check all other options first. Actually it was lucky, I hadn't even considered that a short at the secondary would reflect back such a low impedance to the secondary that it would kill the signal at the plates. I thought there was maybe a measurement error and it would be useful to check the usual suspects.