Blues Deluxe to ODS #183 Thread

[pompeiisneaks]

OK here are some stats I gathered today.

First off, one of the oddest things was when I had the oscillation happening, and I touched my oscilloscope probe to the V1a grid, the noise goes away completely! (This is the second stage of V1 in this amp, V1b grid is input).

Here are some of the voltages from the amp:

BD to ODS Voltages:

B+ 423
B+2 420
B+3 402
B+4 322
B+5 318

V1a
A 185
C 1.95

V1b
A 185
C 1.97

V2a
A 190
C 1.95

V2b
A 191
C 1.95

V3a
A 269
C 58.8

V3b
A 271
C 58.8

V4
A 420
S 420
C 0.0

V5
A 421
S 421
C 0.0

Here are the scope shots I took. I put in about a 400-500mA 1kHz sine.

Clean9vol.png

- Clean volume at about 9/12

CleanMaxVol.png

- Clean at max volume.

DriveLow.png

- OD at lower volume

DriveHigh.png

- OD at higher volume

DriveMaxMasterUp.png

- OD at max volume with master volume raised

MoreDrive.png

- Another look at drive.

V2bGrid.png

- V2a grid... (filename is wrong, this was the second stage of the OD side, I checked the other grid and it was normal not chopped

It seems like there's some kind of high frequency positive feedback happening, if you see how the clean channel max volume suddenly CLIMBS but only in the top of the curve... in a notch. The voltages also seemed maybe a touch high to me, but not sure.

Any thoughts as to what this means? Also chopsticking by either touching the grids of V1 a or b makes a change in tone, and touching the board in that general area seems to as well, but only changes the tone, it doesn't make it worse or better.

Treble impacts the frequency of the horrid oscillation tone, the rock/jazz switch do as well, and the mid pot does a 'little' bass does nothing. The other oddity is how in the drive channel, at V2B, the top side of the sine wave is hard flat but the bottom is fine? Is that normal for the OD channel?

I should clarify the clean scope readings were done with OD off on purpose to see how the clean channel reacts alone.


Edit: also going to add a line of description to each screenshot of the scope. Another important point is that I took these readings on one of the power tubes grids, and with MV down pretty low, thus the millivolt readings on them. I was just trying to find an easy spot for the readings. I think I also did this at the output of the first triode and didn't see any difference in the signals...

~Phil

[JazzGuitarGimp]

You might try adding a small capacitor from V1A grid to GND; something in the 10 - 47pF range. Does the oscillation happen all the time now, even with nothing plugged into the input?

The small cap should be added right at V1 socket, not at the same net on the board.

[talbany]

Phil
The problem with those type of oscillations is it permeates throughout the entire amp

Pull V1 tube out completely what does it do?

Tony

[pdf64]

Does the setting of the VR4 vol control affect the oscillation?

[pompeiisneaks]

You might try adding a small capacitor from V1A grid to GND; something in the 10 - 47pF range. Does the oscillation happen all the time now, even with nothing plugged into the input?

The small cap should be added right at V1 socket, not at the same net on the board.

There's no oscillation at all if there's not a guitar plugged in, but any guitar, single coil or humbucker does cause it to happen.

I'll look to try that. I think I have a 50pF somewhere.

[pompeiisneaks]

Does the setting of the VR4 vol control affect the oscillation?

Do you mean the volume for the channel? (in this amp I'm guessing VR1-3 are the tone pots for treble/mid/bass, and then the volume is the 4th). If I dial the main volume down enough, then yes it stops the oscillation. Basically if I keep the normal volume down around 9/12 it's clean on OD (It's 'clean ish on the clean channel but a bit driven, instead of super clean like I've been told it should do right up until almost max volume). when I push either the VR4 volume above 9/12 with the drive up around that same level, it starts oscillating with OD engaged, so basically it's a balancing act. If I dial either Drive, VR5, or volume VR4 below about 9/12 it works fine, but then somewhere around there, either drive or volume cause the oscillation to happen. This happens at very low master volume levels. like 2/12.

~Phil

[pompeiisneaks]

Phil
The problem with those type of oscillations is it permeates throughout the entire amp

Pull V1 tube out completely what does it do?

Tony

I'll have to retry, I think it went away, yes.

I.e. it seems to somehow originate with V1. but if I pull V2 obviously I lose all drive anyway.

I'll report back after checkign on that again.

~Phil

[pompeiisneaks]

I forgot to post back, I have the amp on with the oscillation happening, and I pull V1 and it's silent.

I also tried a 47pF cap and if the oscillation was 'just' starting, it would stop, but when I turn the vol and drive to max it comes back anyway. I also figured I look at any other areas I'd 'not done' and in the bluesmaster there's a 1M resistor from the output of the V1a coupling cap into the OD switching area, that I just left unpopulated as that doesn't exist on the #183, but adding a 1M there didn't change anything...

not sure what else to check. Do those voltages all look good as well? Why the odd waveforms? I did get a frequency of the oscillation when I put the scope on the signal, it shows up at like 2.45MHz. I also can make it disappear if I connect the scope probes directly to the anodes of V1 on either side, like getting that 10M ground reference is getting rid of it? ( I think its 10M for a standard scope probe, but I also think Ihad it in 10X mode which may be 100M.)


~Phil

[xtian]

I also can make it disappear if I connect the scope probes directly to the anodes of V1 on either side, like getting that 10M ground reference is getting rid of it? ( I think its 10M for a standard scope probe, but I also think Ihad it in 10X mode which may be 100M.)

Excellent observation. Try adding a 1M grid-to-ground reference to see if it silences the oscillation the same way your probe does!

You mention a 1M resistor you omitted. Which one (schematic compoment ID)?

[pompeiisneaks]

I also can make it disappear if I connect the scope probes directly to the anodes of V1 on either side, like getting that 10M ground reference is getting rid of it? ( I think its 10M for a standard scope probe, but I also think Ihad it in 10X mode which may be 100M.)

Excellent observation. Try adding a 1M grid-to-ground reference to see if it silences the oscillation the same way your probe does!

You mention a 1M resistor you omitted. Which one (schematic compoment ID)?

R8. It's the resistor between the clean channel and the OD channel.
I did try using some leads to ground and touching that resistor to the spot it's missing and it didn't remove the noise. I'll try on the other grid(s). Oddly, I think there should be a 1M to ground on the input side, as I've got that on the jack, unless something's wrong there? I'll double check that as well.

~Phil

[pompeiisneaks]

I tried the 1M to ground on either pin and it didn't help. it got smaller, but didn't go away completely, and then when i turned it up more it returned. There's a 1M reference on the input too, I tested it and it's there.

~Phil

[JazzGuitarGimp]

Hi Phil,

I think it may be advantageous to replace the wire that goes from the board to V1, pin 2 with shielded cable. Ground the shield at one end only.

Also, starting at the input jack, can you tell me the functions of all the switches and pots? And if there is shielded cable used on any of these runs, which ones are they?

Thanks,
Lou

[xtian]

R8. It's the resistor between the clean channel and the OD channel.
I did try using some leads to ground and touching that resistor to the spot it's missing and it didn't remove the noise. I'll try on the other grid(s). Oddly, I think there should be a 1M to ground on the input side, as I've got that on the jack, unless something's wrong there? I'll double check that as well.

R8 is just a DC leak to prevent pops when the OD relay switches.

V2 triodes get their grid-to-ground references thru the TR2 and VR6 pots.

[pdf64]

I’m pretty sure it’s a layout issue caused by the discrepancy in the relative position of V1 between the HRD and the Blues Deluxe.
There looks to be maybe 1” between the input stage grid terminal and the wiring for the 2nd plate of the overdrive circuit, so 4 cascading CC gain stages totalling maybe >100dB gain with a wide bandwidth. It’s bound to have a low margin of stability.
Moving V1 socket so as to be immediately adjacent to its wiring points on the board is one solution. Putting a screening plate between V1 wiring and the board is another.
A bit of scrap sheet metal could be grounded to the chassis and roughly bodged into place, to temporarily serve as the latter and so prove the hypothesis.

[JazzGuitarGimp]

Phil,

Did you use the 183 phase inverter and GNFB resistor values verbatim? I see the 183 wants the GNFB resistor tied to the 8-ohm secondary whereas the BM and Blues Deluxe both specify the 4-ohm secondary. Did you ties it to the 8-ohm secondary? If not, you might want to give that a try as it will decrease the overall gain of the amp.

Cheers,
Lou