The Mule Driver - aka Fischer Meets Dumble

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didit
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Re: The Mule Driver - aka Fischer Meets Dumble

Post by didit »

Thanks for the suggestion. Will try one. Also have small stash of 1.5M, and was considering switching in/out with 1.5M fixed in series to get full range with perhaps improved control sensitivity. Believe there's also 3.5M pull in a missing misc parts box that will turn up someday. Another experiment.

Best .. Ian
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Re: The Mule Driver - aka Fischer Meets Dumble

Post by RJ Guitars »

More progress...

I went ahead and put in the KT66 output tubes. They really sound nice!!!! This is a great sounding clean amp and all my hopes have been rewarded with the tone this thing delivers. It's really got a low noise floor and is well behaved so far.

I finished up most of the wiring that I had left and now I can switch from standard to OD mode (I did put the #2 tube back in in case anybody noticed it is missing in my photos). The Colossal PS and Relay boards work wonderfully, that part of the project was really fun and I love it when something works like it is supposed to.

I have not put in a proper footswitch jack yet because I didn't have an insulated one here. If I can't dig one up in my boxes of stuff I'll create my own... that should be taken care of soon. I have a footswitch on the way and then I think that is going to sorta finalize the switching part of the equation. More than anything else I was delighted that I had the relay wiring sorted out and functional in the way I wanted it to be.

I added the negative feedback wire... haven't listened to the amp critically enough to tell if it makes a big difference or not.
max_lwedge wrote: Tue Oct 03, 2017 5:41 pm I really like the idea, the minimal loss but useful bass and treble are nice. I have been scheming on something similar thing based on a Supro, with its paraphase, 6973s and quirks. Looking at the schem, I am wondering if the volume pot in the circuit feeding the OD will be too interactive. A change in overall volume could make both the OD gain and master need adjustment. At low volumes, you might not have enough juice for the OD.
Max, I have a better understanding of your concerns here... the primary volume control does indeed have a big effect on the OD. I am contemplating going to a fixed setting or maybe a couple switchable settings for a fixed signal level to come from the first gain stage that is independent of the volume control... still thinking about this but I'm inclined to think I will like it better.

I did not especially like the OD tone I was getting but I have done nothing to tweak it yet... I will probably begin by doing some voltage rail adjustments. I'll take some more voltage measurements and post them in the next day or two.

Meanwhile, it's coming together but still very much a work in progress.
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xtian
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Re: The Mule Driver - aka Fischer Meets Dumble

Post by xtian »

I'm laughing because of the trouble I've been having with hum in my Rocket build. I've tried to minimize the number of ground paths to the chassis, with no success in reducing the hum, and here you've got no fewer than 13 chassis connections, and you report "low noise floor." Obviously grounding scheme is not my issue. Good work, Bob!
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M Fowler
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Re: The Mule Driver - aka Fischer Meets Dumble

Post by M Fowler »

Me too, I have one Rocket that keeps red plating one tube and I think I got it fixed it shows up again. Oh well one winter day it will back on the bench for the 10th time. :roll:
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Re: The Mule Driver - aka Fischer Meets Dumble

Post by tubeswell »

M Fowler wrote: Thu Oct 12, 2017 10:26 pm Me too, I have one Rocket that keeps red plating one tube and I think I got it fixed it shows up again. Oh well one winter day it will back on the bench for the 10th time. :roll:
Hiya Mark, does it redplate under heavy signal conditions? If so, see the blurb in Merlin Blencowe's 2nd edition 'designing pre-amps' book - Section 9.18 onwards, 'Effects of Bias on the LTP'?

If its just red plating at idle, it could be a leaky coupling cap on that grid.
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M Fowler
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Re: The Mule Driver - aka Fischer Meets Dumble

Post by M Fowler »

Thanks Pete I will have to investigate leaky cap.
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Re: The Mule Driver - aka Fischer Meets Dumble

Post by RJ Guitars »

xtian wrote: Thu Oct 12, 2017 6:39 pm I'm laughing because of the trouble I've been having with hum in my Rocket build. I've tried to minimize the number of ground paths to the chassis, with no success in reducing the hum, and here you've got no fewer than 13 chassis connections, and you report "low noise floor." Obviously grounding scheme is not my issue. Good work, Bob!
I see the humor... some of the grounds you see are simply the shield wires of the coax but yes, I have a lot of ground nodes. I largely credit not just good solid grounds but also the shielded coax as the key to the low noise floor.

I've done star grounding and Randal Aiken "style stage" grounding but in general I've found in my various builds that grounds that have their own wire to the chassis tend to be happy while occasionally components that share a wire to the chassis ground can tend to cause problems. On my early Rocket builds I adopted this approach when I was trying to chase down a hum. In my experience TW Rockets are a little fussy about sharing ground wires. Uncoupling the PS cap grounds and running each of them to independent ground location was often key to resolving those problems. Sorta like Aiken's approach but I don't worry about running a wire back to that PS node ground for every grounded component in that stage. I either have been very lucky using this strategy and never had any ground loops or maybe I am fortunate enough that they haven't caused any ill effects. FWIW - when using JJ can caps with the grounds tied together, I haven't seen same issue. I do have a theory about why...

One big thing I think matters is having good solid connections to ground which means using star washers and verifying that I have tightened every screw in the build before I put the cover on.

In a related note - the last two big hum issues I had to sort out were caused by 1) a bad Russian 12AX7 and 2) a bad JJ EL34. On two separate and very different amplifiers both of these tubes could hum a tune all by themselves. They individually worked me over before I swapped them out and viola - the hum was gone??

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Re: The Mule Driver - aka Fischer Meets Dumble

Post by norburybrook »

M Fowler wrote: Thu Oct 12, 2017 10:26 pm Me too, I have one Rocket that keeps red plating one tube and I think I got it fixed it shows up again. Oh well one winter day it will back on the bench for the 10th time. :roll:
Mark, I had the same problem when I built my one and only rocket. I was using a matched set of JJ El84's. I bought some Russian military EL84's from Russia via ebay and problem solved. Been running like a champ ever since.

M
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Re: The Mule Driver - aka Fischer Meets Dumble

Post by JazzGuitarGimp »

norburybrook wrote: Fri Oct 13, 2017 8:46 am
M Fowler wrote: Thu Oct 12, 2017 10:26 pm Me too, I have one Rocket that keeps red plating one tube and I think I got it fixed it shows up again. Oh well one winter day it will back on the bench for the 10th time. :roll:
Mark, I had the same problem when I built my one and only rocket. I was using a matched set of JJ El84's. I bought some Russian military EL84's from Russia via ebay and problem solved. Been running like a champ ever since.

M
I can't imagine having a Rocket and wanting it to run like a Champ! :-)
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Re: The Mule Driver - aka Fischer Meets Dumble

Post by RJ Guitars »

I like the sound of a Champ... but I want my Rocket to sound like Brad Paisley
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Re: The Mule Driver - aka Fischer Meets Dumble

Post by RJ Guitars »

I went forward with the addition of another relay to allow me to directly switch the full signal from the first gain stage into the OD section. I can dial down the signal with the trimmer pot before it enters the the 2nd tube and it works extremely well... except for one thing. I get a huge "pop" when I switch into OD, but not going the other way. I did find another thread where they talk about this but would like to get the input from those that might have seen this or can see the cause of it. This is new turf for me and I need some schooling. Help please!

http://ampgarage.com/forum/viewtopic.php?f=6&t=28118
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Re: The Mule Driver - aka Fischer Meets Dumble

Post by RJ Guitars »

I found that if I dialed down the OD to zero and the Normal channel to zero, I had no pop. After I did some more studying, it occurred to me that I needed to tie these two nodes together with some sort of high resistance bleed resistor. I chose a 1 Meg resistor and Viola, that did it. It did not work completely on the first cycle but once I did that first cycle it was quiet from then on. I'm not sure if I just needed to give it more time to balance the differences or if it really needs to cycle one time. At any rate, it's progress and I can go forward while I think about this some more. I'm going to finish up with the addition of the boost and bright features and maybe even put a front panel on it and prep a cabinet in anticipation of road testing it.

After I got rid of the pop I played through the amp going back and forth between Normal and OD. I'm still struggling to really enjoy the preamp distortion but I did find one spot that had a nice Dumble quack to it that had some potential. More to come...

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Re: The Mule Driver - aka Fischer Meets Dumble

Post by andresound »

norburybrook wrote: Fri Oct 13, 2017 8:46 am
M Fowler wrote: Thu Oct 12, 2017 10:26 pm Me too, I have one Rocket that keeps red plating one tube and I think I got it fixed it shows up again. Oh well one winter day it will back on the bench for the 10th time. :roll:
Mark, I had the same problem when I built my one and only rocket. I was using a matched set of JJ El84's. I bought some Russian military EL84's from Russia via ebay and problem solved. Been running like a champ ever since.

M
I had the same issue with red plating and “motor boating” and a tube set swap cured the problem. That was 2 years ago. Amp is still going strong. Also a rocket
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Re: The Mule Driver - aka Fischer Meets Dumble

Post by RJ Guitars »

M Fowler wrote: Mon Aug 21, 2017 12:34 am That looks great Bob. So 200R is enough to bias those KT66's?
Nope... finally looked at voltages and calculated the current yesterday and those dudes were looking at early extinction for sure. I knew I was pushing them hard but I sorta forgot about using a higher voltage PT than my Rockster builds and things were way too hot! Almost a factor of 2 in over-current... I am thankful for the kindness of cathode biasing and the self regulating thing that prevents redplating to some degree. I replaced that bias resistor with a 470R unit and now were looking at 44mA on each tube.

This has clearly been a riveting project for you all... well maybe not so much based on the way the discussion seems to sorta routinely drift off. But I thought it was a very cool idea and I sold a package to a co-creator who also thought it was an awesome idea. At this point I will go ahead and finish/document and time will tell if it was worth the effort or not. That channel switching thing is very cool and I do get two very distinct voices with a great clean sound and some Dumble like tones out of the other channel.

The amp is sounding quite good now although I'm just starting to dial it in. I clearly need to look at those voltages and tweak things but I'm going to use it as is for a while and see how it sounds. The OD switching scheme and has worked out great (quiet and smooth) and I'm thankful for some new tools and skills I learned there. I'm sorta trying to get it wrapped up over the weekend because I've got other work to do so I put it into a blemished cabinet to make it gig ready... we'll see how it does...

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Re: The Mule Driver - aka Fischer Meets Dumble

Post by Mark »

All that is needed now is audio clips of the amp.
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