Help with bad distortion

[k-man]

Hi,
Newbie here. I am having trouble with my Carr Slant 6 (modded by Red Plate amps) and was hoping for some assistance. If I turn the clean channel up past noon, I get this buzzy distortion that phases in and out after the initial pick attack. It only happens on the wound strings.

I thought it was blocking distortion, but I have tried the following to no avail:

1. Changed the phase inverter tube (and subsequently all the tubes).
2. Increased the grid resistors on the power tubes (cathode bias from 10 k to 100k and fixed bias from 1k to 4.7k).
3. Decreased the power tube coupling caps from 0.1 uf to 0.047 uf.
4. Decreased the power tube grid bias resistors (I think Red Plate increased them to 220k, I lowered them back to 100k like Carr originally used).
5. Checked the preamp for the clean channel and phase inverter for leaky coupling caps (not sure if I did this right - I lifted one end of all of them at the same time, and then turned the amp on and measured the DC voltage on each one using an analog meter. I didn't get any readings).
6. Added a grid resistor to the phase inverter.
7. Added Zener clipping diodes to the grid bias resistors.
8. Tried moving wires with a chopstick, but most of the wires were glued in place by Carr so this was not very productive.

I am at wits end with this. When the amp was shipped to me from Red Plate, it must have taken a hit from the shipping company because the power transformer is dented like so:

[IMG:800:600]http://i46.photobucket.com/albums/f111/ ... G_0746.jpg[/img]

[IMG:800:600]http://i46.photobucket.com/albums/f111/ ... G_0745.jpg[/img]

Could the power transfomer be damaged and somehow creating a parasitic oscillation or something? I figured it would either work or not, but I don't know what else is causing the problem.

Thanks.

[M Fowler]

I would call Henry at Redplate, I know he is more than willing to help out and since he did the amp he would know what direction to start in.

Mark

[billyz]

I doubt if the dented PT is the problem. But take the end bell off and see if what is any damage was done the the core. Have you tried different speakers ? Not being familiar with the slant six, is it a cathodyne PI ? is a 6v6 pr output tube ? did it do this before the redplate mods ?

[k-man]

I doubt if the dented PT is the problem. But take the end bell off and see if what is any damage was done the the core. Have you tried different speakers ? Not being familiar with the slant six, is it a cathodyne PI ? is a 6v6 pr output tube ? did it do this before the redplate mods ?

I did try a different speaker. Same problem.

Long tail pair phase inverter. There are two pairs of 6v6 power tunes, one pair is fixed bias and the other pair is cathode biased.

Before I had sent it to Redplate, I had screwed around with the lead channel and noticed it on the lead channel then. Since it was modded, I don't hear it on the lead channel (the one that was modded), but now it is on the clean channel (which wasn't modded). I don't remember ever hearing it on the clean channel until recently.

Sorry if I'm a little unclear, I had it modded a year ago and haven't had time to play it much since then.

[Structo]

Wow!
That end bell took quite a hit!

I never ceases to amaze me the kind of abuse that these shipping companies do to electronic items.

[billyz]

Sounds like a job for an oscilloscope. Could be a lead dress issue or bad coupling cap on that channel. My first mind told me Phase invertor though. I would definitely talk to redplate about it.

[k-man]

Unfortunately I don't have an oscilloscope.

I did measure the voltages. On channel 2, I don't know what the voltages should be, and I couldn't access the last stage without taking the board out. The first number is what I measured, and the number in parenthesis is from the schematic:

V1A (Preamp Ch. 1) Plate = 212V (202V)
V1A (Preamp Ch. 1) Cathode = 1.71V (1.45V)
V1B (Preamp Ch. 1) Plate = 208V (210V)
V1B (Preamp Ch. 1) Cathode = 1.72V (1.65V)
V2A (Reverb Recovery) Plate = 232V (230V)
V2A (Reverb Recovery) Cathode = 1.64V (1.6V)
V2B (Effect Loop Driver) Cathode= 213V (215V)
V3A (Preamp Ch. 2) Plate = 194V (?)
V3A (Preamp Ch. 2) Cathode = 1.52V (?)
V3B (Preamp Ch. 2) Plate = 189V (?)
V3B (Preamp Ch. 2) Cathode = 1.58V (?)
V4A (Preamp Ch. 2) Plate = 217V (?)
V4A (Preamp Ch. 2) Cathode = 1.88V (?)
V4B (Preamp Ch. 2) Plate = ? (?)
V4B (Preamp Ch. 2) Cathode = ? (?)
V5 (Reverb Driver) Plate = 382V (375V)
V5 (Reverb Driver) Cathode = 5.7V (6.0V)
V6A (PI) Plate = 238V (251V)
V6B (PI) Plate = 243V (251V)
V6 (PI) Cathode = 122V (105V)
6V6 Plate = 393V (387V)
6V6 Fixed Bias Grid = -31V (-35V)
6V6 Cathode Bias Cathode = 29V (30V)

Do those offer any clue? I don't know how far off the voltages can be before they affect the circuit.

[k-man]

I'm pretty sure the problem is in the phase inverter. I measured 100VDC on the grids of the PI. Does that sound right?

Also, there is 125VDC on the cathode of the PI, but it should be 105V according to the schematic. I'm not sure why it is off. The bias resistor measures 470ohms like shown on the schematic. The tail resistor measures 29k which is close to the 27k shown on the schematic.

[joCCe]

Is that a head or a combo?

Joakim

[Structo]

Is this your amp?

http://www.youtube.com/watch?v=_4hyvvoPn_U

[k-man]

Combo.

That's not me, but my amp is the same.

[joCCe]

Combo.

That's not me, but my amp is the same.

I had the exact same problem with a super reverb and spent hours trying to figure it out. It was a cold solder joint / partly loose ground wire on the speaker jack. The jack cover (flat style) held it in place, but as I turned up the volume and hit the bass strings, the notes buzzed. Sounded like blocking distortion. It happened during shipping. Anyway, check it if you haven't.

Edit: But I guess you shipped the "amp" only, since the transformer is dented? Does it happen when the amp is out of the chassis?

Joakim

[k-man]

It happens with the chassis out of the combo. I have tried a different speaker cab as well and it still does it.

I will check the wiring to the jacks, thanks for the suggestion.

[joCCe]

Build a simple stethoscope and listen to the amp. Hook it up to a load and go through the amp step by step.

Bottom of this site for instructions: http://www.el34world.com/Hoffman/tools.htm