Guitar Gear Forum

Stick any large value resistor (470k+) between the treble pot and the master volume.

Increase the ltpi bias resistor from 470r to something in the low kohms and you'll get the squish at lower volumes. I use 3k3 on my superbass with a badcat 30 power amp and that makes it a little less stupid

_ej_ wrote:I should get one of those

Got a recommendation for a decent one? Or a better MV circuit? This is just like my old JCM800s, the volume knob really only adjusts from 75%-100%

I know the brand has a reputation for fire . But the Bugera attenuator is actually very good. I have a Weber Mass 200, a SPL Reducer and the Bugera here. The Bugera hangs with both of them for the most part. If you go to extreme attenuation settings like 80% or higher, the Bugera sounds a bit stiffer than the other 2 but for lower settings they sound the same.

Another option would be to get a reactive load and go DI or into a power amp. Obviously the Power Station does that in one unit, but you could make a cheaper version of it with a dedicated load box and a small SS power amp. I found that SS power amps are actually ok for this type of application since you just need clean power at low volumes, all the actual tone is being generated by the amp.

_ej_ wrote:I should get one of those

Got a recommendation for a decent one? Or a better MV circuit? This is just like my old JCM800s, the volume knob really only adjusts from 75%-100%

I like using my Two Notes Captor to make the MV on my amps less of a hair trigger.

Hotplate ordered

Zozobra wrote:Stick any large value resistor (470k+) between the treble pot and the master volume.

Increase the ltpi bias resistor from 470r to something in the low kohms and you'll get the squish at lower volumes. I use 3k3 on my superbass with a badcat 30 power amp and that makes it a little less stupid

I have always wondered what the resistor between the treble and master does. I tried the resistor after the mast pot trick, but that didnt do anything. Always thought about this trick on my 2203 clone...

GuitarBilly wrote:

_ej_ wrote:I should get one of those

Got a recommendation for a decent one? Or a better MV circuit? This is just like my old JCM800s, the volume knob really only adjusts from 75%-100%

Another option would be to get a reactive load and go DI or into a power amp. Obviously the Power Station does that in one unit, but you could make a cheaper version of it with a dedicated load box and a small SS power amp. I found that SS power amps are actually ok for this type of application since you just need clean power at low volumes, all the actual tone is being generated by the amp.

That is my tube amp setup these days. One of a few tube amps into a Fractal X-Load, into a MXR 10 band EQ, into a desktop mixer (to run parallel rack effects units in the sends) and then into a Matrix GT1000FX into 2 cabs in stereo. Sounds huge at low volumes.

So the Hot Plate let me do a full test of the amp. Fully cranked all the controls with no issues. No oscillations or weirdness, overall I'm really stoked.

There is one real issue that remains, with the Vol-Tone channel I get radio interference when the TMB channel master is up. My guess is its a combination of the bad ground layout and the lack of shielded cables on the inputs. Both will be fixed in the next few days.

There is also a tone issue. I'm getting buzz/fizz in the low end. You can particularly hear it on palm mutes. Fizz doesn't change with the EQ controls but seems to get worse the higher the gain. The clip below I think highlights the issue pretty well. At the end I switch over to the V-T channel, its not as noticeable but I still can hear it a little. Any thoughts?

https://on.soundcloud.com/RpiAm

Settings for the clip, the master was dimed during the TMB clip:

Can't comment on issues but I think it still sounded rad

spawnofthesith wrote:Can't comment on issues but I think it still sounded rad

This fuckin attenuator is my favorite gear purchase in a long time dude. Getting those glorious cranked tones at bedroom volumes is amazing.

_ej_ wrote:So the Hot Plate let me do a full test of the amp. Fully cranked all the controls with no issues. No oscillations or weirdness, overall I'm really stoked.

There is one real issue that remains, with the Vol-Tone channel I get radio interference when the TMB channel master is up. My guess is its a combination of the bad ground layout and the lack of shielded cables on the inputs. Both will be fixed in the next few days.

There is also a tone issue. I'm getting buzz/fizz in the low end. You can particularly hear it on palm mutes. Fizz doesn't change with the EQ controls but seems to get worse the higher the gain. The clip below I think highlights the issue pretty well. At the end I switch over to the V-T channel, its not as noticeable but I still can hear it a little. Any thoughts?

https://on.soundcloud.com/RpiAm

Settings for the clip, the master was dimed during the TMB clip:

PXL_20230324_162818170.jpg

Oops, just seen your DM now. It could be a number of things, which isn't a helpful comment
Do you have a schematic?

A farty low end as the gain goes up could be blocking distortion, which can be fixed by either lowering the coupling caps, or adding a large(ish) grid stopper to the next stage (100-470k, larger rolls off more highs).

It could also be the poweramp farting out if you have this up loud. Basically cathode bias amps (especially el84 amps) can get a swirling/muddy tone to them as the when the power goes up, the cathode voltage swings up and causes crossover distortion. Now a lil crossover distortion is part of the Marshall tone, so you want just a smidge. The Paul Ruby mod and Zener cathode bias clamp mod can help with this a lot and is easy to do. Check this rather long video on it

Schematic is here:

https://www.mojotone.com/Amp_Kits/Briti ... S4yNy4wLjA.

Did you hear what I'm talking about in the clip? A few people have mentioned it might just be EL84 grind, if thats the case it'd be a bit better if I turned down the MV right?

I think you have a few different things adding up to get the swirl/fizz/flub thing going on. Without a scope its going to be tricky to pinpoint it, so here are a bunch of reasonably quick and easy things to try. You may just need one of these, you may need all of them

1) It could just be blocking distortion in the preamp. -it probably isn't this though.
Fix this with a grid stopper on V2B. Try 220k. This will roll a bit of sizzle off too, which isn't necessarily a bad thing with el84s. Solder this directly to the pin on the socket then run it to the wire back to the two 470k resistors in the attenuation network.

2) It could be bias excursion in the poweramp.
EL84s are very easy to overdrive. They don't take much drive at the grid to get to full tilt. This also makes them more suceptible to very audable bias excursion/crossover distortion as the grid to cathode voltage increases as the power demand increases, which biases the tubes cooler thus introducing crossover distortion. On EL34s it takes several 10's of volts to get to full spank. EL84s are like 10-12V, so a 3-5V excursion is a big deal.
Fix this with a Zener diode across the 130r cathode bias resistor on the EL84s (Chuck H zener clamp mod). The schmatic indicates the idle voltage here is 12V, so a 13 or 14V Zener is appropriate here. Cathode points to the tube cathodes. This will also reduce blocking distotion and farty low end.

3) Similar to 2, there could be harsh grid clipping of the EL84s and that can make nasty harmonics. The Paul Ruby mod fixes that. It's a Zener and regular recifier dioide in series to ground across each of the 470k grid leak resistors on the EL84s. Description/discussion here: https://www.18watt.com/storage/18-watte ... fo_311.pdf
As per the video I posted, you can also try adding a 100k resistor across the PI outputs (i.e., in parallel with the 47p cap) instead of doing this mod. You're more likely to have a spare resitor than a bunch of Zeners kicking around.

Finally, the amp doesn't have any negative feedback in the poweramp, so it's always going to get a little farty

I have access to some very nice scopes at work so if you can give me some pointers on what to look for i'm happy to dig in.

I'm going to look into some of those changes you listed this week. What do you think about adding negative feedback?

If I understand you right, this would be an implementation of 1 and 2, correct?

Yep, that's right. 2) is probably going to do most of the heavy lifting, so start there. Then try 100k in parallel with the 47p across the PI outputs.

Zozobra wrote:Yep, that's right. 2) is probably going to do most of the heavy lifting, so start there. Then try 100k in parallel with the 47p across the PI outputs.

13v 5watt zeners are on order and I have a 220k so I'm just throwing that on there.

Thoughts on implementing a negative feedback loop?

Adding an NFB loop isn't particularly hard and a little bit might not hurt with the weird flub/fizz if the bias clamp etc doesn't help. It will stiffen the amp and clean it up though so be mindful of that. I'm on my phone at the moment but can add the bits to the schematic to show how to do it if you want?

Let me try these diodes and we'll go from there

Zozobra wrote:Adding an NFB loop isn't particularly hard and a little bit might not hurt with the weird flub/fizz if the bias clamp etc doesn't help. It will stiffen the amp and clean it up though so be mindful of that. I'm on my phone at the moment but can add the bits to the schematic to show how to do it if you want?

So I made some changes.

I implemented a star ground in the preamp and added shielded wires to the inputs. Didn't help the noise floor but it did reduce the amount of radio the amp is picking up. The V-T channel has no radio and the TMB channel has a tiny bit with the amp cranked. I'd like to reduce this further but I'm not sure what else I can do.

The grid stopper and diode definitely changed the weirdness in the low-end but honestly it just made it more distinct. You can hear it pretty clearly in the clip below. Its both on the palm mutes and the long ring out. Any thoughts? I have access to scopes at work if I need to dig into this more, I just need to know what I'm looking for.

Good news is the amp really does sound fuckin' great.

https://soundcloud.com/ejendres/loose-farts