Building a new Fender style Deluxe Reverb Amp AB763 from scratch with photos

[Planobilly]

Hi everyone,

This is my first time at trying to build an amp from scratch. I am sure I will have some questions along the way!!...lol I repair amps but I don't have much understanding of how they get designed. The main reason for building this amp is to learn more about design and how they actually work.

Here is the basic schematic I am working with. I am sure along the way I will make changes to the design.

http://schematicheaven.net/fenderamps/deluxe_reverb_ab763_schem.pdf

I will insert a few photos of the project to date as soon as I figure out how to do that!! ...lol

Be back in a bit.

Thanks,

Billy

Here are three photos of how I started the project. I guess they are to large to insert as attachments. I could use a little help in understanding how to get photos and schematics to appear in a post...if that can be done.

https://www.flickr.com/photos/planobilly/shares/vPJ90c

[singapol]

This is a proven commercial design,why would you want to change it or reinvent the wheel?

[Planobilly]

This is a proven commercial design,why would you want to change it or reinvent the wheel?

There are a few things that are not included in this design. Master volume control for one. I intend to install a post phase inverter master volume control. I also what to include a switchable solid state bridge rectifier along with the tube rectifier. I have also given some thought to install a Mosfet based power scaling arrangement. I have also given some thought to rearranging the grounding layout for possible better noise reduction. As I stated, I am building this amp to learn about design and I am using this basic design as a starting place for further experimentation.

Cheers,

Billy

[wgrant]

If you are interested, you can purchase the components for the original design in kit form from Mojotone.
http://www.mojotone.com/kits/BlackfaceAmpKits_x/Mojotone-Blackface-Deluxe-Reverb-Style-Amp-Kit_2
It could save you some time and money. I'm not sure of the quality of the parts.
I'm new to electronics and was considering an attempt at building their Tweed Champ Kit to start.
Good Luck.

[Planobilly]

Hi wgrant,

Thanks

Actually I have a basic stock of all the parts  to build these amps. I need to order a output transformer and a choke or two.

That is one of the questions I have. The choices are Mercury Magnetics...a bit too pricy...Magnetic Components...the least costly...and Heyboer which is a little more than Magnetic Components.

Anyone have any thoughts on these brands?

Cheers,

Billy   

EDIT: BTW Grant, the Champ is a very good amp to use for you first build. Not too complex and does not cost a ton of money. If you build that amp kit I would be glad to help you if you get stuck at some point.

[singapol]

Hi wgrant,

Thanks

Actually I have a basic stock of all the parts  to build these amps. I need to order a output transformer and a choke or two.

That is one of the questions I have. The choices are Mercury Magnetics...a bit too pricy...Magnetic Components...the least costly...and Heyboer which is a little more than Magnetic Components.

Anyone have any thoughts on these brands?

Cheers,

Billy   

EDIT: BTW Grant, the Champ is a very good amp to use for you first build. Not too complex and does not cost a ton of money. If you build that amp kit I would be glad to help you if you get stuck at some point.

Hammond?

[Planobilly]

Hi singapol, thanks

Yes, I looked at Hammond also. As there are people from all over the world on this site, I thought perhaps they could know about transformers that were made in the far east or other countries other than the United States for example. Because transformers are heavy I am not sure if shipping cost  from other parts of the world would exceed any possible savings.

Cheers,

Billy

[Planobilly]

If you are interested, you can purchase the components for the original design in kit form from Mojotone.
http://www.mojotone.com/kits/BlackfaceAmpKits_x/Mojotone-Blackface-Deluxe-Reverb-Style-Amp-Kit_2
It could save you some time and money. I'm not sure of the quality of the parts.
I'm new to electronics and was considering an attempt at building their Tweed Champ Kit to start.
Good Luck.

I have built a few of the kits from both Mojotone and several others. In general the quality of the parts have been ok. The sellers of these kits try to keep the price within reason so you will not find the very best components that exist in the kits. For the most part, the parts they provide are good enough. To get better parts starts to get expensive pretty quick. For example most of the kits come with Alpha potentiometers which cost me about one dollar each at wholesale. Really better potentiometers such as P.E.C cost me around ten dollars each.

Bottom line...the quality of the kit parts will certainly be fine for a first build. Also there is no reason you can not upgrade things in the future.

Cheers,

Billy

[calexanian]

Looks fun there. I used to sell chassis to Mojo for their deluxe reverb kits. (Not the terrible old ones, but the ones before the ones they have now) They soon after got into the fabrication business for themselves and have some beautiful products now. Way more authentic than our shop could build at that time anyways. Please take note on where the grounds go and ground routing and lead dress. It is critical in fenders for stability because ground eddy currents and wire routing and lead dress were done for stability and noise reduction reasons. You CANT!!!!!! use single star grounds for everything or it will squeal like a banshee! Additionally add some series resistance with the reverb driver tube. The tubes today go into grid rectification and cause a scratchy tick in the note decay by loading down that signal line. Oh also watch out for the LDR in the tremolo circuit. You have to play with the resistor value in parallel with the neon and the bias of the tube driving it because if it clamps on and off too hard the ignition of the neon will cause a little tick of noise in the amp. 

[Seekonk]

I would put a cathode resistor in the output section so you can set the bias.  A little localized feedback wouldn't hurt either.

[Planobilly]

Hi calexanian,

I am sure you would laugh at the issues I went through building the chassis. Writing the G Code for the mill was a reminder of why most people would just buy a chassis...lol I wanted a chassis I could remove each panel and I needed it to fit a case I already had. Well...at least the cost was low.

I have to run out for a while with the wife. I will read all the post carefully later tonight.  I would like to understand your comments in detail concerning the grounding schema.

Thanks for the feedback.

Seekonk...I am not sure I understand your comment about the cathode resistor. The feedback comment I do understand...I think...lol

See you guys in a while.

Billy

[Planobilly]

This is the material list I think I want to use.

chassis...My design and made from 1/8 aluminum
Transformers... Heyboer
Tubes...JJ 12AX7 and 12AT7 with matched and balanced inverter tube, JJ 6V6S matched within 1mA
Ceramic tube sockets
Precision Electronic Components pots
F&T electrolytic caps
Cornell Dubilier polypropylene caps
I have on hand silver mica caps but I don't remember the brand
LED Pilot Light which I make with the resistor and diode in the shrink tube
Face Plate...I don't have this figured out yet...the ones from kits won't fit my chassis
IEC plug
Jacks..Cliff jacks from England
Texas Instruments MOSFETs for the power scaling circuit
Resistors...Have not made up my mind yet...this is a sticky subject in any case...lol...your point of view welcome
Toggle switches...Carling until I find something I like better

 In general, I am not trying to build a amp that closely matches the  sound of the Fender original.

Any ideas or thoughts welcome.

Cheers,

Billy

[Seekonk]

There is a bias voltage pot that must be set for output tube current.  The easiest way to do that is to measure the voltage on the cathode resistor to common.  Most tube amps have this resistor.  This one does not.  At the factory they probably measured total current at the standby switch with it open.

[Planobilly]

Hi Seekonk,

I assume you are talking about using a large one ohm resistor to measure plate current. I have bias probes that connect to my DMM to do that normally. Well, unless I am missing something and don't really understand what you said.

Cheers,

Billy

[calexanian]

There is a bias voltage pot that must be set for output tube current.  The easiest way to do that is to measure the voltage on the cathode resistor to common.  Most tube amps have this resistor.  This one does not.  At the factory they probably measured total current at the standby switch with it open.

They generally did not bother connecting a meter. They just put the amp under load and brought up the plate current until the crossover distortion went away on a scope, then took the amp into clipping and if the crossover distortion came back turned the plate current up a little more, then brought it back to clean rated output and made sure the tubes were not melting, and out the door it went. People get way too caught up in bias voltage and plate current when the whole point is to not have any crossover distortion during normal use and sound reasonably good. Its class AB. Get over it! hahahahaha

[Seekonk]

By the time you can see crossover distortion on a scope it is already too late.  Then again it is a guitar amp.  A little localized feedback is never bad and reading the voltage on a resistor at least gives you some metric to start from.  It turns the amp into its own tube tester.  1A X 1 ohm is 1W.  I wouldn't call that a big resistor and you will never get close to 1A.  You opened it up to the Peanut Gallery, do what you want.

[Planobilly]

Hi calexanian,

There are many issues with tube electronics that are not well understood by many who deal with them. I include myself in that group...lol

There are many so call guitar amp technicians that don't even have a scope or have one but have no idea how to use it. You mean that little bend in the sine wave in the middle is not supposed to be there???...lol

The fact that a set of push pull output tubes are "well matched" at idle ( well matched to me, meaning they will run at less than a one or two mA difference) does not guarantee that they will be balanced or set properly  at higher gain.

Vacuum tubes by their very nature are difficult and costly to manufacture consistently. The only way to get a set of tubes that will perform well is to have a very large  number of them to select from.

Having the expectation that one can buy a set of well matched NOS tubes from a seller who only has a few tubes to try and match even using a  pretty good tester like a Hickok 539C is wishful thinking in my opinion. There are commercial companies that sell tubes who have better test equipment but even that only get you so far. The only affordable tube tester made in today's world is the amp that they have to run in.

To further complacate the issue, at the end of the day, the only thing that really matters is the subjective opinion of the user of the guitar amp. Competent electronics people can lead the horse to the water but should not try to force the horse to drink...lol

Due to the fact that you mention issues related to star grounding, please , if you have time explain in more detail the issue and why it exist in the AB763.

Cheers,

Billy

[Planobilly]

By the time you can see crossover distortion on a scope it is already too late.  Then again it is a guitar amp.  A little localized feedback is never bad and reading the voltage on a resistor at least gives you some metric to start from.  It turns the amp into its own tube tester.  1A X 1 ohm is 1W.  I wouldn't call that a big resistor and you will never get close to 1A.  You opened it up to the Peanut Gallery, do what you want.

Hi,

Crossover distortion can happen in all push pull class AB and class B amps. The bias point controls the amount of time  the two tubes are in a very non-linear part of their operating curves. Here are two photos at different bias settings. Crossover distortion occurs when the bias point is set to low as in the first photo. Most everything in electronics is a compromise. So finding a bias setting that produces the sound that someone likes is their choice of where to compromise.





Please don't think I am disregarding you comments about the use of a 1 ohm resistor in the cathode to ground position as that circuit is very commonly used. In fact, it become very useful in the few amp designs that become unstable with the use of bias probes. What I meant by "a large resistor " was that  the heat dissipation ability has to be observed and set to provide a safe value to make sure it does not fail. That resistor is the only path to ground. Generally that would be about two watts. The physical size of the resistor is dependent on the type such as carbon comp, carbon film, or metal oxide and so on.

Cheers,

Billy

[calexanian]

The proper answer to this would require more time than I can afford at the moment. It happens to be my birthday! hahahaha. Most amp "Experts" are generally either electronic technicians of any form, or even good musicians. They are just people with strong opinions! I am in the middle of writing an multi part in depth article for how tubes really work. After that I will turn my attention to Guitar Amplifiers.

[Planobilly]

Happy Birthday!!!!!

[Seekonk]

Never said to use a 1 ohm resistor.  That seems too low to serve any purpose.  This all reminds me of selling my old mono EICO 6L6 amp. Cleaned the parts out down to the frame, used the power transformer for something else (everyone had gone stereo) but it still has a massive Peerless output transformer that was twice the size of the power transformer.  That ought to make a good guitar amp.  Should put that on ebay.

[Planobilly]

Update,
Building the turret board and the placement the components in the chassis has required a bit of rethinking and adjustment. I changed from the original power transformer to another one which will require an adapter plate as it has a smaller cut out dimension. More adventures in G Code for the CNC mill...lol I have yet to master the tool offset issues yet. If you want to mess things up, use a computer. If you really want to mess things up, have that computer direct the actions of a four axis milling machine...lol

I still have not made up my mind about the placement of the filter capacitors. Fender typically placed them on top of the chassis and contained them inside a cover plate on a small board. I may try to place them (there are five 16mf 475V caps) inside the chassis under the potentiometers...not sure yet.

Here is a photo of the almost finished turret board. The transformers will arrive tomorrow and I want to place everything before I drill the mounting holes in the turret board. There has been more time and work involved in defining a step by step build process than I assumed there would be. The electronic issues are proving to be easer to deal with than the logistics of the construction.




If I had this to do over again, I would not try to use a head cabinet that I had laying around. It would have been much better to build a new cabinet to fit a slightly larger chassis. Space limitations in the old cabinet have caused me to make compromises which are problematic.

At the end of the day everything comes down to the issue of money...lol

Cheers,

Billy