Buzzy distortion in Marshall style preamps?

[Mark]

Hi

I have posted this in technical section as the amp in question is a Marshallized AX84 P1 amp. I have tried a few different things with the amp to no avail, has anyone come across this sort of thing?

Here is the link to the discussion.

https://ampgarage.com/forum/viewtopic.p ... 65#p468865

[GlideOn]

I don't have a P1 but I have an SEL, both based on the AX84 site.

I posted a reply as to what was going on with the Power Amp section? Only saw a preamp schematic.

It's a single ended amp with no phase invertor, no negative feedback. Its' inherently going to be a sort of buzzy, hairy and gainy amp.

EL84s are especially known to be buzzy when distorted near their limit, could be a voltage thing, could be a bias thing.

There are few things however to try out in the Power Amp section that could smooth it without screwing up the preamp tone, if you're game.

But need to see some sort of layout or schematic

[cdemike]

Based on the master volume comment in the 1st post with scope shots and the scope shots in the last page as the thread current stands show there's something at least happening in the preamp. That said, the comment about calibrating your equipment may change the analysis some, but you're hearing what you're as far as what happens when the master is turned down, so I think you probably could gain something by tweaking the preamp. Your first thread actually got me thinking about a plexi style build I completed last year, so I did some of the tweaks I suggested in my comment on the 3rd page of the thread to pin down values and see how it sounded to my ear. Granted, my amp is very different (P-P output stage using KT88s being one major point of departure), but increasing the second gain stage's cathode resistor to 2k2 and making the cathode bypass cap on that same stage switchable was an interesting experiment. I actually kept the amp with the higher value cathode resistor since it improved the headroom and smoothed the overdrive. Removing the cathode bypass cap via the switch makes the amp much cleaner, but it wasn't a good fit with what the amp has going on elsewhere (very loose filtering, saggy power transformer, KT88s). I also lowered my cathode follower's cathode resistor by paralleling other resistors with the stock 100k and ended up liking a configuration totaling 63k in conjunction with the raised cathode resistor: very smooth but without losing the snarl and bark that I expect from Marshalls.

If I'm understanding things, the amp is single-ended and basically uses the same output section as the P1, right? In other words, there's no phase inverter to tweak?

[GlideOn]

With consideration that this is the Marshall forum, let's compare some schematics and incorporate the 2204 which is what the P1/SEL is taking inspiration from:

JCM800_2204.gif

Hi-octane schematic.jpg

IMG_0715.jpeg

Also reposted yours for reference.

Right off the bat, there's a wide swing in voltages, with yours unsurprisingly being the lowest.

Marshalls typically have their preamp voltages in the high 100s or low 200s.

AX84 amps are a decent 40v-50v lower, which from experience makes for a much "browner" sound.

However too low and you get exactly what you describe - buzzy, harsh distorted tone, especially with EL84.

Yours current layout is only giving 130v ish for V1 and V2 which is very low for a 12AX7. Some designs that are considered "starved voltage" in hybrid amps are close to 120v, so yours could be having similar results.

Your power transformer isn't the stoutest if it is only giving 250v ish volts for the B+ line.

I would consider lowering the B+ Droppers. Most AX84 schematics don't show the main B+2 being higher than 1.2k, is that yours as well?

Just looking at your (preamp) schematic write-up, that B+3 dropper could easily be 4.7k or even 1.2k to get some voltage back into the cathode follower. If you prefer the "browner" tone, you can save it for just the V1 by adding an additional B+4 dropper of similar value after the cathode follower if there is room (early Marshall 2204 had an extra dropper too!). Make sure to keep these 1w rating and above for longevity.

[cdemike]

Agree with the voltages analysis -- good eye. I generally like to bias the DCCF as close to the datasheet maximum 200V for 12AX7/ECC83, so I'd adjust accordingly if possible, including GlideOn's suggestion of the 4k7 or 1k2 B+3 dropping resistor; I'd probably go gradually as far as voltage testing goes, but would play test the extreme ends (i.e., stock vs 1k2 or 1k) to see if that gets you in the direction you'd like. From there it might be worth checking into further adjustment but I actually think this might make more sense as a starting place ahead of my suggestions since those start to get into things that I've never seen in a stock Marshall schematic.

[pullshocks]

With consideration that this is the Marshall forum, let's compare some schematics and incorporate the 2204 which is what the P1/SEL is taking inspiration from:



Right off the bat, there's a wide swing in voltages, with yours unsurprisingly being the lowest.

Marshalls typically have their preamp voltages in the high 100s or low 200s.

AX84 amps are a decent 40v-50v lower, which from experience makes for a much "browner" sound.

However too low and you get exactly what you describe - buzzy, harsh distorted tone, especially with EL84.

Yours current layout is only giving 130v ish for V1 and V2 which is very low for a 12AX7. Some designs that are considered "starved voltage" in hybrid amps are close to 120v, so yours could be having similar results.

Your power transformer isn't the stoutest if it is only giving 250v ish volts for the B+ line.

I would consider lowering the B+ Droppers. Most AX84 schematics don't show the main B+2 being higher than 1.2k, is that yours as well?

Just looking at your (preamp) schematic write-up, that B+3 dropper could easily be 4.7k or even 1.2k to get some voltage back into the cathode follower. If you prefer the "browner" tone, you can save it for just the V1 by adding an additional B+4 dropper of similar value after the cathode follower if there is room (early Marshall 2204 had an extra dropper too!). Make sure to keep these 1w rating and above for longevity.

The OP hasn't checked in for almost a month, but after reading your response I checked plate voltages in a project I have going. The first stage was even lower than the OP had on his amp. I was all the way down at 113v (B+ 197v). Easy to see why, I had a whopping 24K dropping resistor in there. Oops!!!! After tacking on a couple smaller resistors in parallel to bring the dropper down to 1.7K, B+ was up to 278v and plate voltage to 161v. Played it briefly. Seems to confirm yours and cdemike's comments.
Distortion was kind of ratty before. Much better now.
I highly encourage OP to check this.

[Mark]

Sorry about not checking in, I had a few pressing matters that I needed to attend to.

I increased the rail voltage to the preamp valves by using a different power transformer, the amp sounded better after that, but it was still buzzy. I don’t know why but I thought to try different speakers again. I did this initially and the amp was buzzy regardless of the speakers used. This time the amp was clear sounding through all the speakers except it’s own one. The funny thing is the speaker sounds fine clean and with a bit of drive, but once you have quite a distorted timbre the speaker sounds buzzy. I found it unusual that the other speakers didn’t sound less buzzy when I first started work on it. I guess it really was that bad.

My favourite speaker with the amp is V30, oddly enough I tried a 4x12” cab with V30 and a 2230 and I didn’t like it much, go figure?

[GlideOn]

Another fine example of how a speaker can break or make an amp!

Happy to hear yours was able to work out.

[Mark]

Another fine example of how a speaker can break or make an amp!

Happy to hear yours was able to work out.

Thanks for your help, I was glad the speaker was the problem as I was running out of options.