New Amplifier Design - "Murder One" - Submini Pentode, Low Voltage

[bbmonster]

A question on the bias mod. Is it possible to stick a switch to get back to your original design? Could I just stick an SPST to disconnect the bias or would I need a DPDT to bypass the .1 capacitor after the volume pot along with disconnecting the bias connection? I would try this out but currently using the board to mount the standoffs in the enclosure so unable cut the connections and experiment right now till the epoxy hardens.

[frequencycentral]

A question on the bias mod. Is it possible to stick a switch to get back to your original design? Could I just stick an SPST to disconnect the bias or would I need a DPDT to bypass the .1 capacitor after the volume pot along with disconnecting the bias connection? I would try this out but currently using the board to mount the standoffs in the enclosure so unable cut the connections and experiment right now till the epoxy hardens.

Should be fine to do that. You can just use a SPST to disconnect the negative bias, as the volume pot would then become the bias. I don't think you would need to bypass the 0.1 cap. I did that on the breadboard yesterday and it seemed ok to me.

[bbmonster]

Wondering if you could help with a slight problem I have. I added the SPST switch to cut off the bias mod to get back to your original design for a more distorted sound. The problem I am having is that when I cut the bias, sound is good for a bit then starts to get really flubbery (or gated is probably the better term) after a few seconds when strumming chords. If I rest a bit, then the chords sound good again for a bit before flubbery again.  When I have the bias on, no problems at all, sounds awesome. Any suggestions that I could do?

Thanks.
-Bernard

[frequencycentral]

Ok, I think whats happening is that the grid is basically unbiased then, because the cap between the volume and the grid is blocking the volume from biasing the grid. So maybe you need to either bypass the cap when ground biasing (as you suggested earlier), or use a DPDT instead of an SPST with the second pole referenced to ground via a 1M resistor. Hey - I'm glad you think it sounds awesome though!

[bbmonster]

Thanks, bypassing the cap did the trick.
-Bernard

[frequencycentral]

Could I just stick an SPST to disconnect the bias or would I need a DPDT to bypass the .1 capacitor after the volume pot along with disconnecting the bias connection?

Thanks, bypassing the cap did the trick.
-Bernard

You were right then! How are you finding the two different modes now?



EDIT: If anyone want's a frequencycentral prototype, the original Murder One is for sale: http://www.diystompboxes.com/smfforum/index.php?topic=78320.0

[bbmonster]

I'm liking it. It's like how you mentioned before, the bias provides a cleaner sound and the original a more dirty sound. There is enough difference that I'm keeping the switch on.

I was just guessing on bypassing the cap, I have very little understanding of tube amps right now. I'm more of have little understanding of it, build it for even more understanding, if the build doesn't work is even better because you can learn more kind of guy.

Thanks again. I look forward to attempting more of your projects in the future.
-Bernard

[frequencycentral]

Interesting. I originally saw the grid bias mod as an improvement which superceded the original, but now it seems it should really be a stock switchable option. Maybe I should modify the schematic to that effect.

[Taylor]

Could anyone compare the sound of this amp to something like a Valve Junior? I have a 100w tube amp (I play bass) but I'd like something I can get to distort at lower volumes. Was thinking about getting a VJ, but if I can build something (without having to fool with high voltage) that would be better. Of course, I realize this amp will have less volume than VJ.

[frequencycentral]

I have a VJ. Wouldn't dare max it at home. I have the volume set at 10 o'clock generally, with pedals providing the dirt. Murder One is comparable at it's maximum volume, a bit more if you push it with pedals. But "Superfly" with a BM pushing it is anti-social.

[Renegadrian]

It seems that our good  pal here Rick is getting quite a lot of attentions!
http://www.jedistar.com/low_watt_amps_2.htm
Hey, I can say I know him!!! 

[frequencycentral]

I have a question with the biasing conversation that you and bbmonster had, which cap do you bypass when wanting a little more grit from the amp?

Here's the schematic for the version I'm building nowadays, with the biasing switch implemented:



The perf and PCB layouts earlier in this thread are based one the version 3 schematic, I haven't updated the layout artwork yet - I build these onto perf nowadays (as people seem to value my perf builds), so I kind have the version 5 layout on my head. Do people want updated layouts?

[panterafanatic]

it's because your perf work is amazing. would the switching make noise when changing the bias point or should i shut it off before switching how it is biased?

[frequencycentral]

it's because your perf work is amazing.

Thanks!

would the switching make noise when changing the bias point or should i shut it off before switching how it is biased?

The switching makes a little click, it's not a problem to flick the switch with the amp turned on.

[Custard]

Hi, this project looks pretty good and I'm going to give it a go, but still waiting (bad time of the year to order stuff) on a few parts.
An updated layout would be greatly appreciated.

Cheers,
Rob

[yesitsdrew5310]

One experiment I did was to use two 5672's in parallel (but each having it's own voltage drop resistor for the heater) for the power amp - louder and ballsier

Rick, I love the design you have here.  I want to use some of the ideas in my amp that I am currently working on.  I was using an LM380 (2.5W output amp IC) with a tube preamp section.  But I am intrigued with using a 5672 in the output side of things.
My question is when you were putting the two 5672's in parallel, did the values on the resistors with the bias switch change at all?
Last question, when you put the two tubes in parallel I know it cuts the resistance in half, in this case is the Fender Reverb Transformer 22921 still applicable?

Thanks for your time and insight!


Cheers,
Drew Brashler

[frequencycentral]

An updated layout would be greatly appreciated.

On it's way soon.........

My question is when you were putting the two 5672's in parallel, did the values on the resistors with the bias switch change at all?
Last question, when you put the two tubes in parallel I know it cuts the resistance in half, in this case is the Fender Reverb Transformer 22921 still applicable?

As I remember I just plugged a second 5672 into the breadboard, just isolated pin 3 for seperate heater supply. I didn't experiment with it much, but the biasing switch resistors wouldn't need to change I don't think. You're right, I think it does half the resistance, it still sounded good through Fender Reverb Transformer. I my experience you can get away with some quite extreme impedance mismatch.

[dimi3]

I been reading the whole thread regarding this amp, together with superfly , Rick I know you are buisy testing new stuff and everyday work, but is there any chance to get version 5 layout posted? please?.

I hope that I can join the murder family soon .

Keep up the good work.

[gigyas]

Hi all, this is my first post here, so please bear with me.
I built a Murder One using revision 5 of the schematic, minus the bias switch, just used the 100K resistor over to pin 5 on the 1044. It sounds fantastic except for one problem: it produces a soft hum at about 17KHz.  So I was wondering if anyone else had run into this, or if you guys had any ideas as to how to fix it.

Thanks in advance for the help.

[frequencycentral]

17KHz

........is towards the upper limit of human hearing. So it's a high pitched whine rather than a hum? If that's the case it could be that you haven't implemented the boost feature of the MAX1044, ie you need to connect pin 1 to +ve.

......and welcome to the forum, and congrats on your build - we want to see it!