New Member - New to Amp Design.

[bassetrox]

Hi there,

I'm new to the forum, just in the process of designing my first build.

My name is Rowan, I'm from the UK, I'm in my 20s and a passionate guitarist. I'm probably going to ask all sorts of stupid newbie questions, but believe it or not, I'm in the middle of an electronics degree, so I have *some* basic knowledge. However, my course doesn't really ever deal with overall designs like this amp attempt, rather with electron level and an increasing emphasis on coding/digital design. Of course, it never mentions valves! I need some advice from people who've been working on guitar gear for a long time!



My Aim

The amp (attached) is my attempt at a low wattage TMB controlled amp with Master Volume. I cobbled it together from the poweramp circuit for an 18watt push pull amp with 2 EL84s. Then I adapted a jcm800 style preamp circuit to fit the front end. I then added attempted to switch out one of the 12AX7s to allow for reduced preamp gain, hopefully more plexi-like.

The main aim of the amp is for me to learn some of the practical ways that circuit elements affect tone. I'm probably going to buy a ridiculous amount of DPDTs and switch things in and out as I please. I envisage switching out of that master volume at some early stage so I can investigate the effect that it has. Might also try diode clipping (taboo?) and driving the opamps harder with a FET...

I'd appreciate some opinions on the schematic that I've butchered together.

Please don't be afraid to be critical or technical in responses.

From my degree stuff, I understand all the building blocks, Pushpull OPs, basic amps, the equalisation controls. I'm just not very confident at putting the elements together...

Main question about attached schematic

Ive got a couple of queries... The main one is this: the amp that I took the Power stage from had a 470k pot for the volume control just before the phase inverting stage (it was a 2 knob vol/tone amp). In mine I've used a 1Meg pot a la JCM, I'm not sure if this could cause problems. I'm sure I should know this but I don't!!

Any other comments you've got on the schematic I'd appreciate.

Also:

I've got Morgan Jones' Book Valve amplifiers, and Ive just discovered Merlinb's book on this forum, and ordered it from amazon, are there any others you would recommend?


Wow that was an essay!
Thanks a lot for your time,

Rowan Smales

[txstrat]

Hi Rowan and welcome to the forum.
I don't wanna talk you out of your design of switching, but what you're looking for is probably something like this: http://sonicdeli.com/ThunderTweakWeb/info/P-Matic2.pdf (schematic on page four)
Hope you like it.
Switching a tube in and out of the circuit does work, but isn't always the best way.
The PI and power stage could be adopted from your design.

Hope this helps

Matt

[Wilder Amplification]

Looks like a cathode biased Marshall 2204 to me...probably class A push pull or just low powered class AB.

[loudthud]

Not a good idea to leave V1a pin 2 floating when that stage is switched out. Add a 1 meg to ground there.

[Hardtailed]

Not a good idea to leave V1a pin 2 floating when that stage is switched out. Add a 1 meg to ground there.

This, I'd even try to completely ground the grid if possible.

You might want to look into a Marshall 2203/2204 schematic. Instead of taking the 2nd stage out, there is a 2nd input jack (the "Lo" input) that taps in after the first stage. The first stage is automatically grounded by way of the Hi jack being unused, and the switching scheme also ground the output of the first stage. Simple and effective.

In my amp I went once step further and added a switch that parallels the 2 stage instead of cascading them. It meant using a dual gang pot for gang (although separate pots would also work). I like the paralleled more way better than the single stage mode. My next build won't have the Lo jack.

[Wakculloch]

V1b also needs a decoupling cap.

[Bruce / Mission Amps]

There will be too much preamp gain with what you have so far but, here's a few other minor things I'd do to start it up. Then you can cut gain out as you proceed so the amp will be more usable.

[unparalleled]

Welcome aboard.

Heaps of circuits here:
The Blue Guitar
This is a good site for valve specs:
EL 84, Tube EL84; Röhre EL 84 (EL84)
Just google radiomuseum **** (valve number)
Good luck welcome to the fun.
Don't forget one hand in your pocket when probing live chassis!

[bassetrox]

There will be too much preamp gain with what you have so far but, here's a few other minor things I'd do to start it up. Then you can cut gain out as you proceed so the amp will be more usable.

Hi guys!!

Awesome response, wish Id found this forum earlier.

Bruce, cheers for the ammendments... Can I be really cheeky and ask for a justification? Just so i know 'for next time'.

I understand the coupling capacitor youve added
I probably seem really thick, but why the resistor to ground on V1a, this is better than grounding it?
Why the extra power point?

Thanks again.

Hi Hardtailed, Id looked at the 2204 (infact thats where most of its ripped from) I didnt go with the 2 input amp cos though its nice and simple from a playing/switching I want to keep everything on the same circuit as it were. I think I'll understand whats going on on the circuit more this way!

Paralleling the first 2 stages with a switch was another thing I'd considered, but hadnt got round to working an implementation, have you got a schematic for your build, or your next planned one?

txstrat: thanks for that link, I'm gonna sit down and digest it later!

Thanks everyone else too!

Any responses to the 1Meg pot query?

Cheers,
R

[bassetrox]

Also, is there a basic safety thread stickied anywhere? Its my first time dealing with over 12V and I dont want to blow my hand off after leaving a lethal charge in a capacitor!

[d95err]

Why the extra power point?

If lots of gain stages share the same powersupply node, you can get feedback through the powersupply which can cause oscillations. You can typically get away with using the same node for two gain stages in series (or two in parallel). Three sharing the same node is asking for trouble.

Low gain amps (e.g. the 5e3) can get away with fairly bad filtering, but in high gain amps, it's critical.

Also, the earlier a gain stage is in the circuit, the more sensible it will be to noise. Any noise at the first gain stage will be amplified again at the second and again at the third, etc... Every powersupply node in the chain improves filtering, so the more you have, the less powersupply noise you'll get.

[bassetrox]

Thanks!!

If lots of gain stages share the same powersupply node, you can get feedback through the powersupply which can cause oscillations. You can typically get away with using the same node for two gain stages in series (or two in parallel). Three sharing the same node is asking for trouble.

[Steve Conner]

Safety FAQ: http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#safety

1 Meg pot: Doesn't really make any difference, you'll lose a little high end compared to a 470k. (Because of Thevenin impedance and Miller capacitance, if you want to put your electronics degree to work )

I work at a UK university, and no, we don't teach tube amp design. It would be hard to justify nowadays.

[unparalleled]

Also, is there a basic safety thread stickied anywhere? Its my first time dealing with over 12V and I dont want to blow my hand off after leaving a lethal charge in a capacitor!

We know by now you're not stupid!
BUT I can send you a small parcel with a 1000uf 600V charged Cap if you really want to remove yer hand!
This forum seems to have more than it's share of concerned members who actually care about the welfare of new members!

First time I have put up a H/Volt warning to a newcomer, There is no sticky warning to my Knowledge, just genuine dudes concerned about HVDC and damp fingers.

Welcome aboard.
This is a great Forum.
There are some bright guys here and they are quite forthcoming with their knowledge.

[Hardtailed]

Hi Hardtailed, Id looked at the 2204 (infact thats where most of its ripped from) I didnt go with the 2 input amp cos though its nice and simple from a playing/switching I want to keep everything on the same circuit as it were. I think I'll understand whats going on on the circuit more this way!

Paralleling the first 2 stages with a switch was another thing I'd considered, but hadnt got round to working an implementation, have you got a schematic for your build, or your next planned one?

Here's the schematic of my amp, with the whole deal (2 inputs and the switch). I never use the low-input cause the paralleled (aka jumpered) mode is so amazing.
You'll notice my 2nd stage is hot biased (820R instead of 10k cathode resistor) with a bypass cap, so it's more like a modified Plexi than a true 2204.
I've removed the bypass cap on V2a since though (as per a 2204) cause it was too bright.
I've also increased the fizz cap (the small cap that links both anodes in the PI) to a 100pF, subtle difference but I like it.

Oh, and I've since removed NFB (it's a on a switch actually), I like it better that way. Which means my Presence and Depth knobs are now useless.

Edit: I leave the exercice of removing the Lo input to the reader, it's pretty easy
You'll notice I have a power supply node for every preamp tube (that's 5 in total if you add power tube plates and screens). I'd never share nodes between preamp tubes on a high-gainer, but that's just me.