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bajaman wrote:Hi schemripper
You could try decreasing the 2 x 47k plate load resistors - try 22k for example when using the lower plate impedance 12 AU7 type tubes - A 12AT7 might work okay in here too.
Cheers
bajaman

I can confirm that a 12AT7 works in this stompbox design,since I have tried one in both my RTO stompboxes,you get a darker midrangey kind of sound with the stock 47k plate resistors.....

i will try this project from bajaman this coming year....happy new year to all....DIYer

thank you drNomis for the parts lay out...

it really helped a lot...

happy new year to all..

sweeplikevai wrote:thank you drNomis for the parts lay out...

it really helped a lot...

happy new year to all..

No worries at all,hope your Real Tube Overdrive build works first time,if not,let me know....

Got mine all working properly and am now having the 1590 bb box powder coated. Also building two more for a few friends. Thanks to everyone who helped and especially to Steve!!! I will post pics when the box is back from powder coating.

Alex

No worries Tonezone,how does your build sound?,I found that if you turn the Drive control up to the 3 O-Clock position,it starts producing a harsh sounding harmonic in the sound in contrast to the smoother sound you get at lower drive settings,I'm suspecting that the harsh harmonic is being caused by the IC clipping the signal a bit on one or both peaks,I'm going to see if I can fix that,and will post what I come up with......

no - it is not the op amp clipping
bajaman

bajaman wrote: no - it is not the op amp clipping
bajaman

Do you have any idea what it could be then,Bajaman?,it certainly doesn't sound like it could be something generated by the Valve....hmmmm...maybe I should post a screenshot of the waveform generated with the drive control at 3 O Clock?....



Here's a pic of what the waveform looks like,the wierd thing about it is that I only get it if I feed a signal into the input from a guitar,if I feed a signal in from my signal generator,I don't get this waveform,I'll post a pic of the waveform I do get with the signal generator....

Do you get this with your pedal Bajaman?....


Sorry that the pic is a bit blurry,I had to use one hand to operate the camera,and the other to pluck the string,not easy when you're right-handed....

cross post[/url] but I'm really interested in the answer.

Can the BAJA Real Tube Drive pedal (of the BK Butler Tube drive) be built using only +12v DC and -11v DC (inverter voltage loss)?

I recon It'll lose some headroom and/or some volume. Are there any other objections?

Do you get this with your pedal Bajaman?..

what exactly is the nature of the input signal ? - sine wave? or something else?
NO - I don't get a waveform like that when I input a 1KHz sine wave.

bajaman wrote:

Do you get this with your pedal Bajaman?..

what exactly is the nature of the input signal ? - sine wave? or something else?
NO - I don't get a waveform like that when I input a 1KHz sine wave.

The signal was actually from my Squire Strat,I had it set to the neck pickup,with the neck tone fully rolled off,I fretted the A string at the 12th fret,and plucked it,I just managed to get a pic of the waveform I got as you can see,when I bypass the pedal,the signal from the guitar looks like a sine wave on the scope screen....

The funny thing is,I don't get that waveform if I feed in a 1kHz Sine wave from my Hameg Function Generator,I'll post a pic of the waveform I get with a 1kHz Sinewave in the morning.....

I'm a bit puzzled as it surely can't possibly be an impedance loading issue,since the signal goes through a buffer before it gets boosted.....

Harold wrote:cross post[/url] but I'm really interested in the answer.

Can the BAJA Real Tube Drive pedal (of the BK Butler Tube drive) be built using only +12v DC and -11v DC (inverter voltage loss)?

I recon It'll lose some headroom and/or some volume. Are there any other objections?

It's possible,but you'll find that the Op-Amp will run out of headroom and start clipping on one peak of the waveform,and the tube won't work as well....

Here's some Scope screenshots of the waveform I get when I feed a 1kHz 1Vpp Sinewave into the input of the pedal,as you can see,the resulting waveform is very different to what I get with a signal from a guitar,note that it doesn't change much at all if the input level is increased....


Settings on the pedal were as follows:

Drive-At the 3 O Clock position.

Tone-Fully Clockwise.

Level-Adjusted to give 1Vpp output.

how about a picture showing it just starting to clip!

bajaman wrote:how about a picture showing it just starting to clip!

Can do,I still have all the equipment set up,I'll go and do that now and post a pic of what I see on the Scope screen....

Input signal was a 1kHz Sinewave from my Function Generator


Could you post some pics of what you see coming out of your pedal build,for a comparison?,thanks in advance Bajaman....


Okay here's what I saw on my Scope's screen after I adjusted the levels so that it was just starting to clip,I'm assuming that this is what you meant:

yes
Now - THAT is TRUE asymmetrical clipping
None of that silicon / germanium diode clipping rubbish
Wind up the gain or signal input level and post some more pictures
Ever seen a JCM800 clip
bajaman

ps: no need for me to post any pictures - that's my BRTO!

ps: love the reflection in the scope pics.

bajaman wrote:yes
Now - THAT is TRUE asymmetrical clipping
None of that silicon / germanium diode clipping rubbish
Wind up the gain or signal input level and post some more pictures
Ever seen a JCM800 clip
bajaman

ps: no need for me to post any pictures - that's my BRTO!

ps: love the reflection in the scope pics.

Oh,I get it now....,okay I'll go do that then,back shortly mate...


Okay,here we go,I took 4 more pics with increasing gain settings,clipping 4 is with the drive control set to maximum and the tone control set fully anticlockwise,looks like a triangle waveform,doesn't it?.....

This basically goes to show that back-to-back solid-state diodes can't really produce the same kind of clipping that valves can produce,the harmonics produced are different,mostly even-order harmonics which is produced by the asymetric clipping....


Do you mean the dual-channel version of the Marshall JCM 800?,I had a look at the schematic once,and it deffinitely uses a bridge rectifier as clipper in the high gain channel,Marshall had a different JCM 800 model that had two inputs,and a standard preamp system without any solid state clipping.

the JCM power output - not the preamp

bajaman wrote:the JCM power output - not the preamp

Ah,right,I'd imagine that we would get pretty much clipping on both peaks of the waveform,with a slight bit of asymmetry....


It just so happens that I can post a pic or two of what the clipping would look like,I have a Tube based Amp head that I built myself,and an 8 Ohm/50 Watt dummy load,I'll just set it all up and post a pic here....

The Amp Head's Power Amp is equivalent to a 50 Watt Marshall Head schematic,so I think it's a pretty good representation of what happens when you overdrive the output valves in a JCM800,or at least it is typical,the valves were biased to operate in Class AB1.....


Scope channel input sensitivity is 5V per 1cm graticule division....

nice pics but those tubes look like they are biased a bit on the hot side