Les Paul hates my amp

[clyde1]

Or should I say the amp hates the Les Paul, esp. bridge pickup. I get that trailing, hangover distortion on low notes with sharp pick attack. Topology is V1A into splitter resistor to CF, tone stack, PI, KT77's, pretty much standard 5F6A/Marshall. V1 is 100k (273vB+), 1.5k, .47 on the cathode. V2A goes to another gain stage then splitter resistor to CF. It's the single gain stage that does it, just that buzz that shouldn't be there, like string buzz. I've rejigged the power supply several times and this is about as high as I can get B+ to V1 and still tap off the 275v I want to see on the CF stage, which is also 100k, 1.5k. The ONLY thing that helps is a resistor to ground from the CF grid, 1M at the moment. The PI has the usual suppression cap and additional ones on the plates have no effect, nor does a Zoebel network on the output. It seems to be in the preamp and I'm out of ideas. I also bypassed the e-caps with film caps, no dice. It's not overpowering, but shouldn't be there I'd say. Can anyone bring over a scope?

[big_teee]

Or should I say the amp hates the Les Paul, esp. bridge pickup. I get that trailing, hangover distortion on low notes with sharp pick attack. Topology is V1A into splitter resistor to CF, tone stack, PI, KT77's, pretty much standard 5F6A/Marshall. V1 is 100k (273vB+), 1.5k, .47 on the cathode. V2A goes to another gain stage then splitter resistor to CF. It's the single gain stage that does it, just that buzz that shouldn't be there, like string buzz. I've rejigged the power supply several times and this is about as high as I can get B+ to V1 and still tap off the 275v I want to see on the CF stage, which is also 100k, 1.5k. The ONLY thing that helps is a resistor to ground from the CF grid, 1M at the moment. The PI has the usual suppression cap and additional ones on the plates have no effect, nor does a Zoebel network on the output. It seems to be in the preamp and I'm out of ideas. I also bypassed the e-caps with film caps, no dice. It's not overpowering, but shouldn't be there I'd say. Can anyone bring over a scope?

That doesn't sound like the standard 3 gain stage marshall with the 1.5k, .47uf on the Cathode setup.
if V2A goes to another G. Stge then would that not be at least 4 stages?
Doesn't sound like the JTM45 or 1987 Circuit.
So what are you running there, and do you have any pictures and Schematics?
T

[clyde1]

Sorry I should have been clearer. It's a channel-switcher, V1A is the only stage in front of the CF. V2A goes to another stage then the CF. It's V1A that's making the buzz. Pretty standard Bassman/Marshall circuit but yes,
the values may be different.

[big_teee]

Sorry I should have been clearer. It's a channel-switcher, V1A is the only stage in front of the CF. V2A goes to another stage then the CF. It's V1A that's making the buzz. Pretty standard Bassman/Marshall circuit but yes,
the values may be different.

I get it now!
I guess you have tried differnt Bypass Cap values, and different resistor values, tubes?
How about the standard .68uf 2.7k Cathode on V1A and 68k Grid Stopper?
My 2204 has a 100pf cap on the first stage from cathode to plate, for oscillation.
Looks like real simple ones stage stuff.
Clipping maybe?
Good Luck,
T

[clyde1]

Thanks for that. I did try 2.2 but I'll give the 2.7 a shot, don't have any .68's so I'll use the .47 or 1. I also have a 120pf cap on the anode resistor, equivalent to cathode to plate I believe. It has little effect.

[big_teee]

Thanks for that. I did try 2.2 but I'll give the 2.7 a shot, don't have any .68's so I'll use the .47 or 1. I also have a 120pf cap on the anode resistor, equivalent to cathode to plate I believe. It has little effect.

Yes, just to prove a point, you could remove the cap, and just vary the resistor.
So if no Cap and a 5k resistor, it cleans it up and makes it milder then you would just have to find the sweet spot, Maybe.
The bypass cap ups the gain. the bigger the cap value the more freqs are passed.
So with .47, or .68 you get that tighter bass, no flub that marshall is famous for.
If it is with the LP Bridge, It is probably the hottest voltage in milli volts, of your pickup choices.
So is the Pickup a High Imp. High Output model?
Sounds like your on the right track.
Good Luck,
T

[daz]

i'd get a .68uf before you go any further. It may seem like a tiny difference but i use a .68uf and i tried a .47 there once and it was a major change for the worse. It was fizzville. Especiallt if you keep going up to a 2.2 or 2.7k resistor. the bigger you go the fizzier it'll get. I use a 1.5k with the .68uf by the way.

[clyde1]

Actually I've settled on 1uf and 1.8k, sounds the best to me, not much difference to the .68 (have one), but not as fizzy as the .47. Good call. However I'm not able to mitigate the trailing buzz totally, still relying on the 1M resistor off the CF grid to ground. Lowering the plate resistor value helps but I don't want to go that route.

[loudthud]

The stage that directly drives the CF needs a smaller cathode resistor. Check the 5F6A, it's 820 ohms. This is because the cathode follower kills the headroom on the B+ side of the driving stage.

[clyde1]

Awesome and thank-you very much. I don't think I've ever heard that term before: "kills the headroom on the B+". How does it accomplish this?

[loudthud]

A 12AX7 needs 100V from plate to cathode at 2mA, otherwise it draws grid current. You can see this if you draw a load line of 100K when B+ is 300V. For maximum headrooom you would bias the plate to about 200V so it could swing +/- 100V. When you connect to a direct coupled cathode follower, the follower starts to draw grid current when the plate voltage trys to swing above about 200V. So to get some headroom you need to bias the plate to roughly 150V. You would see this if you had an oscilloscope.

[clyde1]

No go with the 820r cathode resistor. If anything it's worse because it's louder. I'm at a total loss. The resistor to ground off the CF grid is the only thing that helps. I've regrounded the tone stack to a different place, added some shielded wiring under the channel-switching relays and nothing makes any difference whatsoever. What on earth am I missing here?
Update: I raised preamp voltages again and some improvement. It is what it is I guess and I'll band-aid it.

[loudthud]

Why is the B+ (275V) so low? If you are worried about the 12AX7 rating of 330V, don't. That's plate to cathode voltage, not B+ voltage. B+ to V2 needs to be about 350V.

[clyde1]

Well, I'm at 330+ volts right now, which is an improvement. I do have the heater voltage elevated also, a necessity in these times of wimpy tubes. Unfortunately the 820r cathode resistor is not an improvement and I'm going to have to stick with 1.5k. This is a pretty subtle sound but I find it fatiguing, seems to be at the same frequency no matter where you play on the neck, although lower notes definitely make it more pronounced.

[loudthud]

Have you tried a higher B+ just for the cathode follower? It won't change the cathode voltage, but you will have more headroom.