Amb builder wannabe seeking more experienced heads

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Amb builder wannabe seeking more experienced heads

Postby GiF on Tue May 06, 2014 12:51 pm

Hey all. I've been getting pretty heavily into valve amp concepts over the last month or so and I've been working on the design for what will hopefully be my first build. I'm planning gutting an old Calrsbro 100w PA head I have to do this so parts of design have been based on Carlbro's own circuitry, with the preamp and power supply based on other designs found around the internet.

I've got a decent working knowledge of what everything in the schematic does, but my actual electronic theory level ain't all that. So I was hoping I could throw it open to inspection and see if anyone could pull me up on any part of the design and maybe plug any holes in my understanding.

https://dl.dropboxusercontent.com/u/300 ... -Build.pdf

Thanks for your time. :cheers:
GiF
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Re: Amb builder wannabe seeking more experienced heads

Postby PeterS on Wed May 07, 2014 1:04 am

Hi
R14 won't work. Since you're educating yourself I'll let you figure out why, but if you get stuck just ask.

The caps in your bias supply are backwards.

Not sure I understand your presence control.

There's a lot of gain there, so you'll have to be careful about layout or it will oscillate. This is a complicated amp for a first build. Not trying to put you off, but you'll have to tread along patiently and methodically...
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Re: Amb builder wannabe seeking more experienced heads

Postby GiF on Wed May 07, 2014 7:07 am

Hi Peter, thanks for taking a look.

R14... Having another scan over my collection of other schematics I'm thinking that the impedance is too low? Other amps use a 1M Log in that function.

The bias filter caps are just a schoolboy error. Copying stuff out without checking what I'm doing. :oops: The presence control is borrowed from the "Heather" schematic from Weber Kits, I may have redrawn it incorrectly. The preamp & tonestack design is by John Chambers of Champ Electronics who is actually local to me and has done repair work for me & friends in the past (he rewound the mains tranny in the amp I'll be using for this project). Unfortunately he doesn't seem to be answering emails of late.

And yeah I appreciate this is a very "sink or swim" project. But I'm looking to build an amp that I can use in my normal band projects. Something like one of the many 18W Plexi kits that are available would probably be a much simply entry point but to be honest an amp like that would only really be used in my bedroom, and I don't spend that much time playing in there! The band I play guitar in is very loud, my normal rig is based around a Sound City 120 PA. :p I plan to build this carefully and in stages, starting with the power supplies, ouput stage & drive/phase inverter. Following these instructions to the letter. I figure that if I get this far, then I'll at least have a functional slave amp, and a good point from which to troubleshoot any issues that arrive with the preamp.
GiF
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Re: Amb builder wannabe seeking more experienced heads

Postby ThisLifeILead on Wed May 07, 2014 2:43 pm

If you are gutting a PA head, why not just "rearrange" the preamp. I did this a bunch of times and it worked out great in all cases. upgrade the bias supply, strip just the preamp circuitry and rebuild. Its a wonderful and economical way to learn. (you can always completely gut and rebuild later)
High Gain stuff is quite frustrating and requires a well planned and executed layout. i built an ax84 High-Octane using their layout and it was still noisey. took me months to sort things out.
just my two cents.
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Re: Amb builder wannabe seeking more experienced heads

Postby GiF on Wed May 07, 2014 3:13 pm

I had considered that, but to be honest the original eyelet board isn't in the best condition and it takes up all the open space inside the chassis. I'd feel more comfortable starting from scratch. There's not much to it in any case, 2 triodes to buffer the 4 inputs, 2 more bracketing the tone stack and the phase inverter. To do any useful mods I'd have to strip most of the board anyway.

There's no reason why I couldn't work gradually from both directions I'm thinking. Get the power stage working, then add the 1st preamp stage. If all's well at that point I can build in the extra gain stages and see what happens. Maybe even make them switchable. :jam:
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Re: Amb builder wannabe seeking more experienced heads

Postby PeterS on Wed May 07, 2014 4:13 pm

Turning R14 down all the way grounds the cathode of the preceding stage. I would put a cap in between the cathode and the pot, and also possibly turn the pot the other way around so that the signal to the PI is taken from the wiper.
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Re: Amb builder wannabe seeking more experienced heads

Postby GiF on Wed May 07, 2014 5:40 pm

PeterS wrote:Turning R14 down all the way grounds the cathode of the preceding stage. I would put a cap in between the cathode and the pot, and also possibly turn the pot the other way around so that the signal to the PI is taken from the wiper.


Obvious in hindsight really. :?
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Re: Amb builder wannabe seeking more experienced heads

Postby ThisLifeILead on Wed May 07, 2014 8:10 pm

GiF wrote:I had considered that, but to be honest the original eyelet board isn't in the best condition and it takes up all the open space inside the chassis. I'd feel more comfortable starting from scratch. There's not much to it in any case, 2 triodes to buffer the 4 inputs, 2 more bracketing the tone stack and the phase inverter. To do any useful mods I'd have to strip most of the board anyway.

There's no reason why I couldn't work gradually from both directions I'm thinking. Get the power stage working, then add the 1st preamp stage. If all's well at that point I can build in the extra gain stages and see what happens. Maybe even make them switchable. :jam:



Ya one step at a time. Sounds like you got a good grasp on things. have fun and keep us posted!!
Peter has helped me on more occasions I can think of!!
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Re: Amb builder wannabe seeking more experienced heads

Postby Unit_1 on Wed May 07, 2014 9:51 pm

i vote for foot switchable !!! :mrgreen:
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Re: Amb builder wannabe seeking more experienced heads

Postby GiF on Thu May 08, 2014 7:23 am

Just ordered in the parts anyway.

Let the games begin!
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Re: Amb builder wannabe seeking more experienced heads

Postby pdf64 on Thu May 08, 2014 12:05 pm

A light bulb current limiter would be a really good idea.
See USA version http://ampgarage.com/forum/viewtopic.ph ... 6df7f634de
As would a scope, signal generator and dummy load.
And time to learn how to use them!
Pete
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Re: Amb builder wannabe seeking more experienced heads

Postby GiF on Thu May 08, 2014 1:54 pm

I'll probably build a basic dummy load & pick up an old scope at some point. You can get them pretty cheap on eBay these days. The learning to use them is, as you say, the hard part!
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Re: Amb builder wannabe seeking more experienced heads

Postby GiF on Thu May 08, 2014 2:46 pm

A dim bulb limiter is definitely called for however. Question though: Incandescent bulbs in high wattages are getting tricky to get hold of in the UK. Would a pair of bulbs wired in series do the job as well?
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Re: Amb builder wannabe seeking more experienced heads

Postby PeterS on Thu May 08, 2014 9:59 pm

No but a pair in parallel should work...

A signal source is also handy, I have an ancient Heathkit, but I also built R.G. Keen's quick and dirty test oscillator, it's a bit handier:
http://www.geofex.com/FX_images/q&dosc.pdf

Peter has helped me on more occasions I can think of!!

You know the saying, an expert is someone who's made every possible mistake in a narrow field :D
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Re: Amb builder wannabe seeking more experienced heads

Postby GiF on Fri May 09, 2014 4:37 pm

Well I managed to successfully fix & re-bias a friends busted Sound City 120 today , so I've got at least 1 repair job under my belt before embarking on my first full build. :lol:
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