Super Lead V2A Cathode Resistor

[65greg]

Hi All,
I have a 1969 metal face 100W Super Lead. It was a "basement find" and had been heavily modified in the 80's...

I removed all the mods (hard wired presence, Master volume, effects loop) except for one.

The one mod left is someone strapped a 1.1K resistor across the 820 Ohm cathode resistor on V2A (the last gain stage before the cathode follower) effectively making the resistor value 470 Ohms. That also has the 0.68uF cap across it.

On my bench, I tried it with and without and it seemed to increase the breakup and I liked it - so I left it in.
I then used it at band practice and the amp seems to break up too early and doesn't seem to have much headroom. For example, using a 1x12 cab, attenuator set to -3dB, inputs jumped, volume 1 around 12:00 and Vol 2 around 10:00, the amp has quite a bit of dirt, when I hit my overdrive the volume won't increase like I think it should...

My questions:
What does a resistor value that low do? I understand it sets the bias point of the tube and will cause it to clip more asymmetrically.
Does a value that low clip so heavily it kills the headroom?
If I left it, would it damage the tube?

Thanks in advance for any insight!
Greg

[Haze13]

This resistor sets the "Headroom" for the voltage that goes to that tube. If it's too small than one half of the input signal (look at this as a Sine wave - one half is positive, another is negative) is being cut since the tube is fully open - 0 voltage on the grid.
Example: Bias of the tube is -3 volts ( grid is lower than the cathode by 3 volts) => In theory the max input voltage 3V Peak-to-Peak. If the voltage in the Input will be 4Vp-p that one side of the wave will be clipped, and the other will be normal.

[65greg]

Haze13,
Thanks for the reply!

I got back into the amp to remove the mod today and found I made a mistake...

I assumed it was an 820 Ohm resistor hiding under the 0.68uF cap but it was actually a 10K

So - what was actually on there was a 10K in parallel with a 1K which had drifted to 1.25K so the total resistance was around 1.1K

Would this do the same thing just clipping the other side of the waveform?

I'm going to wire in an 820 and double check the values on the rest of the board.

Greg...

[neikeel]

I have seen a 10k in that spot before, it makes the amp very cold on the second stage (a factory mistake when a 1k was intended?).

A 1.1-1.2k is pretty close to optimum for that stage.

I would look at the rest of the amp and your set up.

A healthy 100w Superlead will be pretty loud even at 2 on the volume knob (before the 5nF bypass cap on the vol 1 pot kicks in)

If you post some links to clear pictures maybe we can help if there is another issue

[65greg]

Ok - so I went through the board today and here's what I found:

All 4 of the 1W power resistors (10K on the schematic) are 8.2K. They look original - maybe they were out of 10K resistors that day? I read the two connected end to end are commonly 8.2K + 10K. I wonder if its worth changing the three to 10K? Probably wont change the plate voltages too much...

Vol 1 is missing the .005 cap

Other than that all the values seem ok for all the other components.

Greg..

[Haze13]

In guitar tube amps +/-20V or so on the B+ (in the Preamp) would not make any difference, so don't worry about resistors... Listen to the Neikeel, he worked and played on vintage marshalls more than I saw Ri and Solid states.

[Carbia]

I have seen a 10k in that spot before, it makes the amp very cold on the second stage (a factory mistake when a 1k was intended?).

I restored a '69 Super Lead some months ago with that mistake.
10k in V2A factory installed (original red paint on the solder joints)

IMO, with 10k there, the amp sounds like shit

[65greg]

So I used the amp again at rehearsal last night without the attenuator. I think the amp is working fine. Its super loud and has plenty of headroom.
I did change the resistor to 820 Ohm but I don't think it made much difference.
Tone is great but I guess I have some work to do to find the settings that work best for me with the attenuator (if I use it) and overdrive pedal's'.

This amp has the NFB wire connected to the speaker jack. With my 16 Ohm 1x12 I tried driving it with the selector set to 4 Ohms and it definitely sounded better to me like that. If I have it correct that means less NFB and less Volts to speaker.

Greg...

[shakti]

When you are mismatching like that you are presenting the output tubes with a much higher (x4) primary impedance. They will be running much less efficiently, and you will reduce output volume and also bandwidth, and it will sound a little cleaner. You do run the risk of flyback voltage from the speaker to the OT with a high mismatch like that though. Most likely you will be fine, but it's something to consider.
Negative feedback will be lower when it is taken off the 4 ohm tap, which will increase gain/distortion a little and make it just a touch looser/more ragged in the bass.

[65greg]

Thanks for the tip about the flyback voltage. I never really looked into that.
I stumbled upon this article which was enlightening to me...

http://peavey.com/monitor/pvpapers/Chapter7.pdf

I guess I'll use the attenuator for now and match the load!

Greg