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Old 30th December 2002, 01:00 PM   #11
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Join Date: Jul 2001
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Hi,

The FET is used to short an 22uF capacitor to ground. That way the cathode bias resistor is shorted by a capacitor: increasing gain. This can be done by a footswitch.

The topology is of the amp is in essence:

Gain stage ->
volume(some High Pass) ->
Gain stage ->
tone stack ->
Gain stage with Low Pass and volume control

Even when the first volume controll is all the way up, the second gain stage hardly or never clips partly due to the 130KOhm. I think the clipping comes from the third gain stage. The LowPass filter is very importand too!

Typical fenderisch is the tone stack BEFORE the clipping stage, but they needed some controll over the harsch clipping, so there's the Low pass just before the master volume controll.

gr,
Thijs
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Old 30th December 2002, 01:38 PM   #12
stigla is offline stigla  Norway
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Hmmm, yeah, okay.


But, my calculations will give me, for the LP filter right before the Mastervolume; with the 430k and the 1500pF, a -3db point at 247Hz! There must be something I am overlooking.

At what frequincy would it be wise to have such a LP filter at? 10kHz? 5kHz? 20kHz?

and the Highpass filtereing with the 0.0022uF and the 430k and the 50k pot gives a knee around 150Hz am i right?

I suddenly realised that EL84's are used here, and that's the reason for all the attunation after the gainstages...


thanks,

Stigla
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Old 30th December 2002, 02:20 PM   #13
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Ehhhhhhhh .. hmmmmmm something like
1 / 2*pi*RC ehhhhhhhhh where's that spice simulator again?

I guess about 246.75Hz That sound a little low, about 1KHz seems more likely.. hmmm I never really calculated the values before ... maybe we should consider R40 and R41 in parrallel so the resistance the 1500pF sees is about 50K, that would give 2.5KHz for F-3?

but usually around 500 to 1KHz works for me..around 1KHz you can get a irritating, very prominent harsch sound.. you should put some of your favorite guitar sound through your MP3 player and mesh around with your EQ... very instructive!

You are right about the EL84 needing only about 10Vrms but the reason for the attenuation is not to keep the output stage from clipping, but to keep the phasesplitter from clipping, well I think.

The attenuation at the first volume controll is part of the High Pass structure, needed to avoid mushy overdrive.

goodluck,
Thijs


PS Do try a Proco RAT type of pedal, it might be just what you need, for 15$ you can build one!
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Old 30th December 2002, 06:50 PM   #14
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I recommended the 12AX7 phase splitter for more gain with a 2 stage preamp. The classic Marshalls and Voxes used an AX7. The AT7 has much lower plate resistance and better drive capability, but that's not necessarily what you want in a crunchy guitar amp.

The CCS on the first stage looks like an interesting idea. I've never tried that, but I have used an IRF820 MOSFET wired as a source follower immediately following the first gain stage. This drives the tone stack with a much lower source impedance. Works great and very easy to implement.

The James tone stack looks good on paper, but I've always preferred the "rollercoaster" Fender/Marshall/Vox type tone stack.

In the classic "poweramp" distortion of the 1960's and early 70's, the output, phase splitter, and sometimes even the latter part of the preamp are all clipping. The problem is that with a pair of EL34 at 450v, you need to crank it to ear-bleeding volume to get that tone. Some people report good results using attenuators such as a Hot Plate or Power Brake. Not my cup of tea, but it may work for you. Try wiring the EL34s in triode mode. It will cut the output power from a nominal 50 watts to around 20 watts. I've had good results with this in my twin reverb, your mileage may vary.

If you go with the AB763 reverb channel, try reducing the 3.3Meg to 1Meg and a 470k grid resistor on the 3rd stage. I usually eliminate the 10pf cap in this case. Crunch City! You'll want to attenuate the signal after the 3rd stage so it doesn't hit the phase splitter too hard.

If you need a LP filter, take a look at the Vox AC30 "cut" control. This can tame an overly bright preamp and is adjustable for different guitars.

I like the Blues Junior too. A lot of its appeal to me is the glorious EL84 output tubes, lower B+ voltage, lower output power and 33k tail resistor (low headroom phase splitter). The reverb circuit is a bit weak though. By the way I simulated the 3rd preamp stage and LP filter section in CircuitMaker and came out with a -3db point of 3.74k.

Regards,
Mike
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Old 30th December 2002, 09:41 PM   #15
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-3db point of 3.74kHz ??? Great.. now I feel pretty stupid ...

About that Junior schematic.. can you calculate what stage will clip first? Your comment about the low headroom Phasesplitter interests me! Maybe the Blues Junior uses phaseplitter clipping?

I once simulated the whole circuit and found out that the 3rd gain stage provided the clipping, but I might have gone wrong somewhere...


gr,
Thijs
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Old 31st December 2002, 10:03 AM   #16
stigla is offline stigla  Norway
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Hello!

Last night I tried some of the ideas on my breadboard.

The diodes was the first ones out, and since I had no feedback, I put them in between the phasesplitter and the last preamptube.
I used 2 diodes, back 2 back, Si types. Wow, fuzz from the beginning of! Rectification city! It reminded me of my old Peavy Bandit 12... Hmmm, I then tried to add another tubestage, like in the Super Reverb. (removing the diodes...)

Hmmm, I wonder what it would sound like, if I used TUBE diodes... Well, anyway;

I first tried a 50/50 splitt of the gain, 2 470k in series, and the grid in the middle, with no cap over the "top" resistor. Yep, distortion is present, when I crank it! Sounded like a Crate, really thight and "heavy" kind of distortion. Not necceserily what I want. Increasing the Top resistor to a 2,2M gave me a nice transition from no fuzz to really nice "vintage" up to a thighter but not so heavy one at max volume. Though, still a bit harsh and a little muddy in the lows, but filters will help this I think.

Adding a cap in parallel with the 2,2M caused serious oscillation, with any value, (well, any reasonable value...) so I abandoned this, gridstoppers didn't help.

I also tried a ECC82, with the intend of having less gain, and thus needeing less attunation, but this was no luck, I liked the 'AX7 better. Heck, I even tried a 9002 (mu of 25), but I still fancy that nice transition of the 'ax7 from no to full crunch.

But I think I'll give the 9002 another try... just for fun. If using a 12AX7 I will have 1 unsued section... The 9002 is a single triode on a 7-pin base

One potantial problem I see is all the noise I got from adding that extra stage... I had the Volume before the extra stage, and so when the mastervolume at 12'o clock, the noise was very high. Even with a 2,2M to 470k attunation and the preamp volume at MIN.

Today I will try various filters and try to tame the crunch a bit. Any suggestions as of how I can implement a LP filter. My circuit looks now allot like the Super-Reverb.

I'm considdering to have a switching option to add that extra stage to get crusch, and when I don't want it, I can just bypass it...

Anyway my intent is still to have 2 channels, like in the Super-reverb, One Normal, and one Tremolo / clean / crunch channel.

The only AC30 schematic I've got is a bit messy, so I cant find any "cut" controll... BTW, the phasesplitter in that amp seems to have a small headroom, with a 33k and a 7,5k tail resistor... Hmmm.... maybe the AC30 uses phasesplitter overdrive? I think so, if we considder that it have only one stage in front! If this amp distort at all, I have never had the luck to hear one... Live that is, didn't the Beatles use thisone allot?


Boy, this is great fun!!

Thanks for all the help guys!
Happy new year!


Stigla
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Old 31st December 2002, 07:28 PM   #17
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Thijs said:
-3db point of 3.74kHz ??? Great.. now I feel pretty stupid ...

Don't feel bad. This is more complex than it looks. You have to account for the tube's anode resistance, 100k plate resistor, coupling cap, and 50k pot in addition to the 430k/1500pf.

About that Junior schematic.. can you calculate what stage will clip first?

This depends on the setting of the master volume control. If it is cranked all the way up, then most likely the output stage will clip first then the splitter then the 3rd stage. If the master is turned down then the 3rd stage will definitely clip first. So the Junior can dial in the "classic" poweramp distortion and/or preamp distortion. I think that is part of the reason why it has been a very successful amp for Fender in spite of the less-than-inspiring reverb circuit. Plus EL84s sound great, you really have to go out of your way to build a lousy sounding EL84 amp. EL84s sound great in hi-fi amps too.

Your comment about the low headroom Phasesplitter interests me!

This is based on the concept of preventing "blocking distortion" which sounds like garbage. An excellent explanation of this can be found here:
www.aikenamps.com under the tech info - advanced section
Here, Aiken advises adjusting the phase splitter so it clips at about the same time as the output stage. He advocates series resistances, but another way is to increase the tail resistor for less headroom in the splitter. The advice about lowering the coupling caps and grid resistors is excellent too.

Regards,
Mike
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Old 31st December 2002, 09:27 PM   #18
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Stigla said:
The only AC30 schematic I've got is a bit messy, so I cant find any "cut" controll

Vox made some beautiful, great sounding amps, but their schematics are a mess, as are the insides of old AC30s.
The cut control is located after the phase splitter coupling caps. It consists of a 250k audio pot in series with a 0.0047uf cap.
See http://www1.korksoft.com/~schem/voxamps/ac301960.pdf
The Beatles did use an AC30 for their earlier stuff. A classic example is "I Feel Fine." Totally delicious guitar tone

Adding a cap in parallel with the 2,2M caused serious oscillation

Not good. Probably a lead dress issue. or a grounding issue. These kind of problems can be a real beast.

One potantial problem I see is all the noise I got from adding that extra stage

OK, since you've added a 3rd preamp gain stage, it's time to reconsider adding some negative feedback. It will make a lot of that noise go bye-bye. Also will give you an excuse to add a presense control. by the way, how much attenuation do you have between the 3rd stage and the splitter?

Regards,
Mike
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Old 1st January 2003, 01:30 AM   #19
stigla is offline stigla  Norway
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Quote:
by the way, how much attenuation do you have between the 3rd stage and the splitter?
Currently, I've got a 100k/270k attunation, due to my recent Clean channel experemintation. In fact, I had forgot all about it... The real load should be 220k/50k or something. (Tremolo pot)

Guess this will attunate the noise too.... I'll also try to add feedback. Hmmm, presense control... I'll see into that as well. I do find the sound to be a little on the dark side.

Heh, funny how it all works out... My first thoughts about building an Guitaramp; "I'll make it simple, yeah, real simple..."


Stigla
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Old 1st January 2003, 02:07 PM   #20
stigla is offline stigla  Norway
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Quote:
The real load should be 220k/50k or something. (Tremolo pot)
No, thats wrong... Wooops! I dont have any attunation in the Vibrato channel... the last tube just drives the 50k Tremolo pot, and then theres a 220k'er and straight into the phasespliter... Though, I belive maybe I have to take the input impedance of the phasesplitter and the output impedance of the "normal" channel into account, yes?

About a lopass filter: What if I add a large griddstopper at the grid of the 3'rd tube, say, 100k to 330k or something, I would then get a LP filter with the input capacitance... Or do the LP filterering have to be AFTER the tube? (filtering the actual crunch, and not just the frequencies hitting the distorting tube?)

regards,

stig
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