18Watt.com

Had a bummer earlier this week. The faceplate I ordered at Modulus Amplification arrived 2 weeks after ordering. Sadly it is a mirror image of what I expected to get. Because I already drilled all the holes and added the power supply there were only 2 options:
1. Order a new , correct one
2. Reverse the amp. Attach the faceplate to what would have been the backside, meaning the pots would be positioned above the heater wiring.

I have seen Geoff's work and he did this on several amps before, so I figured I'd be all right doing the same. After all there are only 4 wires coming from the pots to the board, one that even travels to the grid of V2a. This wire will now only be like 6cm long, where it would have been 20cm shielded wire crossing the entre amp. Don't think the build will suffer from this configuration.

Sorry to hear about that. It's hard enough building an amp without something like that happening. Hope it works out OK.

Jack

It's alive!!! You were right Jack, about the excessive low end. I got it under control by disconnecting the cathode bypass cap of V1a and also leaving out the extra 820R 220uf network.
Sounds fat and creamy!! Awesome !!!
It's tweaking time though... i'd like to have more mids. Going to swap the .022uf cap + 25k pot for a .01uf and 50k pot, and go the jalapeno route... should I also change the 56k slopen resistor?
And how the F*** can I calculate the bias with the 12AU7? Plate voltage is 260V and Cathode measured 8.6V

Bieworm wrote: ↑

Fri 06/11/21 1:59 pm

It's alive!!! You were right Jack, about the excessive low end. I got it under control by disconnecting the cathode bypass cap of V1a and also leaving out the extra 820R 220uf network.
Sounds fat and creamy!! Awesome !!!
It's tweaking time though... i'd like to have more mids. Going to swap the .022uf cap + 25k pot for a .01uf and 50k pot, and go the jalapeno route... should I also change the 56k slopen resistor?
And how the F*** can I calculate the bias with the 12AU7? Plate voltage is 260V and Cathode measured 8.6V

Rob's calculator covers a 12AU7: https://robrobinette.com/Tube_Bias_Calculator.htm

According to that, you're at 4.6W, and 78% dissipation assuming I entered everything correctly.

I tweaked like I said. Put the tremolo TMB tone stack and pots in it. Added zener diodes to lower B+. All voltages are within 10%, so that's fine.
Here's the measurement data:

Soundclip:
https://www.dropbox.com/s/yesqsusvmsi9y ... 2.m4a?dl=0

that sounds good bieworm,well done!

Bieworm wrote: ↑

Sat 06/12/21 6:20 am

Soundclip:
https://www.dropbox.com/s/yesqsusvmsi9y ... 2.m4a?dl=0

Overall that sounds great! Are you using pedals too? There's a couple spots that might need attention:
1:00 to 1:20 is less articulate, and slightly muddied out.
1:35 to the end has a fast clicking noise. Is it a pedal or an oscillation? Or a fan?

Thanks,
Josh

JMPGuitars wrote: ↑

Sat 06/12/21 10:18 am

Bieworm wrote: ↑

Sat 06/12/21 6:20 am

Soundclip:
https://www.dropbox.com/s/yesqsusvmsi9y ... 2.m4a?dl=0

Overall that sounds great! Are you using pedals too? There's a couple spots that might need attention:
1:00 to 1:20 is less articulate, and slightly muddied out.
1:35 to the end has a fast clicking noise. Is it a pedal or an oscillation? Or a fan?

Thanks,
Josh

The clicking is my wireless router in the next room. Sometimes it does it harder than other times... annoying, yeah..but I'm used to it.
Any suggestions about the muddiness?
Was thinking about cold clipper at V2a...
This is where I landed:

Bieworm wrote: ↑

Sat 06/12/21 10:42 am

Any suggestions about the muddiness?

Sure. Experiment.

Nice work! Looks great, sounds great!

The way to figure bias is to first calculate the current through the cathode resistor: I = E ÷ R. In your case, it's 8.6 ÷ 470, or 0.0183 (18.3 mA). Next, use P = I X E to calculate the power dissipation in the tube. The voltage across the tube is 260 (anode voltage) minus 8.6 (cathode voltage), or 251.4V. Multiply by 0.0183, and the result is 4.6W, like Josh said. Dividing that by 2 gives the dissipation of each section of the 12AU7, or 2.3W. The tube is rated for 2.75W per section, so you're at roughly 84% of maximum dissipation.

This is very close to one of the "Typical Operation" scenarios in the Brimar datasheet. They show +250V anode, -8.5V on the grid (fixed bias) and current of 10.5mA. That's slightly over 2.6W, so you're right on the money.

Again, you can tweak the tonestack in software using Duncan Monroe's calculator. This will show you instantly what change to expect when any of the components are modified. His calculator is an indispensable tool for tweaking the tonal quality of the stack without spending days on the bench. The URL below contains a list of software available from the site. Note that this software is designed to run on older systems like Windows XP. Duncan's site is also where I go to look up datasheets when I'm too lazy to pull a tube manual off the shelf.

http://www.duncanamps.com/software.html

The muddiness Josh mentioned sounds to me like too much overdrive. I don't hear a frequency response issue.

Jack

TriodeLuvr wrote: ↑

Sat 06/12/21 11:53 am

The muddiness Josh mentioned sounds to me like too much overdrive.

I agree, but it could be either too much overdrive, or too much bass in the part of the circuit being over-overdriven. Given what we know about where the circuit started, I might guess the latter, but he needs to experiment anyway.

JMPGuitars wrote: ↑

Sat 06/12/21 12:55 pm

TriodeLuvr wrote: ↑

Sat 06/12/21 11:53 am

The muddiness Josh mentioned sounds to me like too much overdrive.

I agree, but it could be either too much overdrive, or too much bass in the part of the circuit being over-overdriven. Given what we know about where the circuit started, I might guess the latter, but he needs to experiment anyway.

Next on the list is the 10k cathode resistor on V2a , lift the cathode bypass cap on the PI and see where it takes things.
Or should I start with putting a .0047uf at the plate of V1a to V1b ?

Bieworm wrote: ↑

Sat 06/12/21 1:53 pm

JMPGuitars wrote: ↑

Sat 06/12/21 12:55 pm

TriodeLuvr wrote: ↑

Sat 06/12/21 11:53 am

The muddiness Josh mentioned sounds to me like too much overdrive.

I agree, but it could be either too much overdrive, or too much bass in the part of the circuit being over-overdriven. Given what we know about where the circuit started, I might guess the latter, but he needs to experiment anyway.

Next on the list is the 10k cathode resistor on V2a , lift the cathode bypass cap on the PI and see where it takes things.
Or should I start with putting a .0047uf at the plate of V1a to V1b ?

The PI is already operating with gain below unity. Removing the bypass cap will mostly serve to unbalance it. I'd work on the bass a little more like Josh suggested, or reduce the gain somewhere. You could change the 470K just after V1A to 680K or 820K, and/or change the 10M in parallel with the gain pot to 2.2M or even 1M. Changing the V2A cathode resistor to 10K will probably make it impossible to ever play the amp clean. That puts the 12AX7 on a really shallow part of its curve.

Jack

How about the .022uf coupling caps from PI to the 12AU7? I could try .01uf there? The tremolo TMB has that value there...

Bieworm wrote: ↑

Sun 06/13/21 1:50 am

How about the .022uf coupling caps from PI to the 12AU7? I could try .01uf there? The tremolo TMB has that value there...

Sure, that will roll off the bass at those grids. You could also reduce the value of the 0.47uF at the input jack. That might be a good place to experiment with a .01uF.

Are the grid stoppers actually 100K? I know a 12AU7 doesn't have much Miller capacitance, but that seems excessively high to me. I need to throw that in the simulator later to see how much they're rolling off the highs.

Jack

The simulator says the -1dB point is well above 20 kHz, so no worries there. The 100Ks are intended to prevent grid blocking when the amp is overdriven, but I wanted to see if they might be creating a response problem. It all looks fine to me.

Jack

TriodeLuvr wrote: ↑

Sun 06/13/21 8:36 pm

The simulator says the -1dB point is well above 20 kHz, so no worries there. The 100Ks are intended to prevent grid blocking when the amp is overdriven, but I wanted to see if they might be creating a response problem. It all looks fine to me.

Jack

Thanks Jack

Need Help here... there is a sub octave note on the amp. Where do I look?
When I increase gain it's less audible. ( not on the recording)
I tried adding a parallel filter cap to get 55uf on the 1st filter, but no help in that..
It is on several different speakers, so that's not the problem.
Also tried different preamp tubes, but kot the 12AU7 power tube.. I don't have a spare

https://www.dropbox.com/s/i2jmhgc0wi1do ... 5.m4a?dl=0

Tried parallelling all 3 filter caps with an extra 22uf.. no difference