John H Attenuator Build

[Byrdman]

I'm going to build a 50w, 8 & 16 Ohm Attenuator. Parts are coming in and if anyone is interested, I will post progress here. Here is my layout so far:

Based on the John H community project over at Marshall Forum.

 

[Byrdman]

Finally completed it after 6 months. Sounds great! Time to start saving for a Plexi now that I can handle the volume issue

 

[newholland]

whats the inductor for? (im assuming to roll high end off... but why?)

 

[Byrdman]

whats the inductor for? (im assuming to roll high end off... but why?)

Not 100% sure of this, but from what I remember it's there to make it a reactive load vs a resistance load that affects tone more. It also takes care of any odd high frequency issues/oscillations that can cause problems if present.

Reactive load attenuators do not suck as much of the high end as resistance load attenuators supposedly. It was in the plans, so I put it in there. I played my Marshall SC20H through my cheapo Harley Benton resistance load attenuator and compared it to this one today and this one is much better with my cranked amp.

Short answer "make is sound better"

Here is where I found the plans & parts list if you want to dive deeper

Simple Attenuators - Design And Testing

Background Passive attenuators are wired between the amp output and the speakers. Their function is to absorb most of the output power of the amp, feeding a smaller amount to the speaker itself. This allows the amp output stage to run at higher power, letting the glorious tone of a good valve...

marshallforum.com

 

[Chocol8]

TLDR: just build one, they are REALLY good attenuators.

The inductor is a big part of how it matches a speaker impedance curve (increasing impedance at higher frequencies) and matching impedance is why this design works and failure to match is why most commercial solutions “suck tone”.

I think the circuit theory came from a a paper Aiken wrote on reactive loads and John rolled with that basic circuit concept and expanded it into an attenuator with switched levels of attenuation.

There was a long thread discussing the load concept on TOP and obviously there is a lot more discussion about the attenuator design evolution and different models over on the Marshall forum.

 

[Alex Kenivel]

 

[Byrdman]

BTW, I think my all in cost was around $165, which IMO, is pretty reasonable considering what this thing does and how it sounds. They do sell a commercial version for around $365 if you don't want to build it.

Pretty easy build and I was a little intimidated when all the parts came in. If you take your time and follow directions, it's not bad at all except for my sloppy solder joints

 

[Chocol8]

If you can drill, screw, solder, and follow basic instructions, it's a pretty easy build and a very worthwhile one for anyone who has tube amps.

I originally built one in 2 ohms for Super Reverbs and tweed Bassman that don't play well with most commercial products, then I built another one in 8 ohm for other Fender amps and anything Marshall/other that has multiple output taps. For the 4 tube 4 ohm Fenders (Twin Reverb) I pull 2 power tubes and run at 50w 8 ohm, but I am thinking about building a 100w 4 ohm version, and maybe a 100w 16 ohm version. That would cover me for pretty much any amp that should end up in my possession.

 

[Byrdman]

@Chocol8 I have a question for you.

After playing this for about a week, I do notice a slight roll of on the highs when -14db is engaged. It may be doing it on the other switches too, but if it is, it's very slight and un-noticeable. Do you notice this on your builds as well? I have been countering this with an Eq and SD-1 with my Stomp XL.

I have a HB-PA100, a resistance attenuator, that I installed a switched cap "Brightcap" to make it brighter. Is something like this possible on the John H? Here is the schematic I used to make mine below. If this is possible, where would it go?

Thanks for any suggestions.

 

[Chocol8]

Short answer, no. I would check your values of R5 and R6 and also all your solder joints in that stage. Measure the resistors, don’t just check labels in case you have a bad one.

Also, if you can, try making a recording of something playing through with and without that stage on, preferably with a looper to make it identical. Then level match the recordings and listen to make sure it isn’t just the Fletcher Munson effect of knocking off 14 db which is a big chunk.

You could put a bright cap across R6, maybe in series with a resistor in the 4.7 to 10 ohm range so the highs still get some attention but not as much as the full signal, but I don’t think it should be needed.

Tagging @Johnh to see if he can chime in with any suggestions.

 

[Byrdman]

Short answer, no. I would check your values of R5 and R6 and also all your solder joints in that stage. Measure the resistors, don’t just check labels in case you have a bad one.

Also, if you can, try making a recording of something playing through with and without that stage on, preferably with a looper to make it identical. Then level match the recordings and listen to make sure it isn’t just the Fletcher Munson effect of knocking off 14 db which is a big chunk.

You could put a bright cap across R6, maybe in series with a resistor in the 4.7 to 10 ohm range so the highs still get some attention but not as much as the full signal, but I don’t think it should be needed.

Tagging @Johnh to see if he can chime in with any suggestions.

Thanks for the response, @Johnh responded saying about the same thing. Here is my response over at the John H Attenuator thread at Marshall:

Thanks for the information. It's probably my setup and my ears. I just switched out speakers in my 212 from G12T-75 to Greenbacks before it was finished, so this is the only cab I have used with it. Since Greenbacks are more mid-high frequency than the G12T's, that is probably what I'm hearing. It makes sense that the just the decrease in volume are affecting the efficiency some and I'm not ruling out FM effect either.

I have another 212 with G12T's at our practice location and another 412 at home that I have not even tried it on either with more balanced across the lows/mids/highs . I'll take it to practice next week and see if it's noticeable on that cab at gig volume.

Thanks, for the suggestions, but for now I'm going to just check my solder connections, resistor specs with my meter, and try it on my other cabs. I don't have a recording setup, I have all the pieces needed except good speakers, but before I do all that, I'll go the easier route first. I'll report back with my findings.

Thanks again!!

Let me know if you think I'm off base with any of this. Thanks again!!

 

[Byrdman]

Tried it on another cab and IMO it's probably just me not the attenuator