Bassman Overdrive 50/100

[kcstratslinger]

Hey guys. I'm a fairly new member and first time OP. I have really learned a lot from this site lately. I am a recent EET graduate with a good bit of experience building pedals and a fair amount of experience with amps (enough to know how not to kill myself). I just got a job working at a local repair shop. I am thankful to be working at such a place and look forward to my new journey in life that is audio electronics. I thank you all for the time and energy spent helping out people, like myself, in need of advise.

I am working on a design for an amp that I am hoping will consist of:
- AA864 Bassman Preamp/PI (Added Mid pot)
- 70s Dumble-style overdrive channel
- Lead boost in front
- Twin Reverb Transformers and Power amp
- Presence Pot
- Possibly 50W/100W switch (unless just pulling 2 tubes is the same thing)
- Ability to use EL34 or 6L6
- All stuffed into SF/BF Bassman Head/Chassis

Questions:
1- How to implement a 50W/100W switch. Is it even necessary?
2- Power supply resistor value calculation. What values will give me the desired voltages stated in my layout?
3- Is there a difference between Bassman and Twin choke? If so, which one is appropriate for this build?
4- Where is a good place to get a custom drilled/labeled bassman chassis and faceplate?
5- Is there a way to wire tube sockets to accept 6L6 and EL34 tubes? I am only experienced with bassman-style 6L6 output.
6- I have been trying to figure out a way to add the master volume bypass to a relay/stomp. In the layout I am using two relays (Metro boards linked to stereo jack) to switch in/out the lead boost and dumble circuit. When I did the layout I just wanted the master bypass on a toggle switch on the amp. Now I am thinking it may be cool to have it on a stomp as a volume boost. What would I need to do to add a third relay for the master volume bypass?
7- Does it matter how the 33k and 1M at the input are arranged? Could the 1M be placed after the 33k? That would make the switching easier.
8- Do you have any thoughts/improvements for my schematic/layout?

Any useful advise is greatly appreciated.

Schematic
https://www.dropbox.com/s/pa9grtwti4htd ... c.png?dl=0

Layout
https://www.dropbox.com/s/2azz42ak4qxmx ... t.png?dl=0

[Haze13]

Well... You really have to learn a few things about electronics and how the tube amps work, before you start with your build. Without a simple understanding you'll probably will burn some thing (OT Transformer for tubes) and it will cost you a lot of money, and it's not healthy working with high voltages without understanding about what is going on inside the amp.
Voltage(V) * Current(I) = Resistance(R). Power = (I^2)*R = (V^2)/R = V*I. It's for the second question, for a start.
Chokes with the same Inductance may have different the maximum value of the current that can pass through it, like 6V6/EL84 amps have smaller chokes than a EL34/6L6...
6l6 and El34 can be fitted in the same socket, but have to biased differently... Different cathode current, maximum plate dissipation, screen resistors...
33k resistor or 68k resistor have to be placed at the socket (better than on the input jack).

Google for a Valve Amplifiers 3rd eddition by Morgan Jones. Leave alone parts about phono preamps and things that you don't need for a guitar amp. Grate Book. All the answers that you need are there.

[romberg]

...

I am working on a design for an amp that I am hoping will consist of:
- AA864 Bassman Preamp/PI (Added Mid pot)
- 70s Dumble-style overdrive channel
- Lead boost in front
- Twin Reverb Transformers and Power amp
- Presence Pot
- Possibly 50W/100W switch (unless just pulling 2 tubes is the same thing)
- Ability to use EL34 or 6L6
- All stuffed into SF/BF Bassman Head/Chassis

This is all *very* ambitious if it is your first build. I would recommend building a vanilla/stock bassman first. Once it is working, you could then modify it using some/all of your ideas. The danger is if you deviate too much from a known/working topology then it is very difficult to track down where things go wrong. If you add modifications one at a time you can easily see if the modification works or causes problems elsewhere in the circuit (that used to work).

Questions:
1- How to implement a 50W/100W switch. Is it even necessary?

Pulling two tubes will get you from 100 watts to 50 watts. You should also adjust the tap on the output transformer to reflect the load correctly to the two remaining tubes. Doing all this with switches I suppose would be possible. But...

The difference between 50 watts and 100 watts is a whopping 3db. This is barely audible. Swapping speakers can often make more of a total volume difference. And 50 watts into a 4x12 will already peel paint off the walls.

2- Power supply resistor value calculation. What values will give me the desired voltages stated in my layout?
3- Is there a difference between Bassman and Twin choke? If so, which one is appropriate for this build?

Check out this software application. I found that it did a pretty good job estimating my last build. You will need to be able to estimate/calculate the current draw of your various tubes.

http://www.duncanamps.com/psud2/

4- Where is a good place to get a custom drilled/labeled bassman chassis and faceplate?

I get my custom faceplates from Sandy at Precision design:

http://www.precisiondesignin.com/

If it were me, I'd try and find a stock chassis you can modify to suit you needs my adding a few extra holes. Aluminum is way, way easier to drill/punch than steel. A full custom punched chassis takes alot of time to layout. And the places I've looked at charge for this.

5- Is there a way to wire tube sockets to accept 6L6 and EL34 tubes? I am only experienced with bassman-style 6L6 output.

I'm not 100% sure on this but I think the trainwreck express can use both of those power tubes. I'd research that circuit.

7- Does it matter how the 33k and 1M at the input are arranged? Could the 1M be placed after the 33k? That would make the switching easier.

In general it is best to mount the grid stoppers (33k) right on the tube socket. And it is also best to ground the input jack (1M) right by the jack. Deviations from this may or may not work.

Mike

[kcstratslinger]

Thanks guys. I appreciate your responses. I already know the Watts Law and Ohms law. As well as much more complicated electronics math. I have an old bassman (AC568) that I have already done many, many things to. It has been an AB165, AA864, 5F6A, 1959 Super Lead, 2203, SLO100 Lead Channel, and a Dumble. I have also done a PPIMV and channel switching via relay. I use turret boards and the original has been removed to keep in it it's original state. No holes have been drilled in the chassis. I have basically used the amp to try out many different circuits for cheap.

All of these builds were only done up to the power tubes with original rectifier, transformers and power tube compliment. All have been built with low noise and no oscillations. Currently the amp is the AA864 and Dumble section (Still need to add relay). I love how it sounds. It sounds similar to a dumble circuit but fatter with less mid range. Great for Sabbath riffing and blues!

Haze13. Thanks for info on chokes. That's something I will have to read more about. As well as Other things outside of the preamp. But I'm up for the challenge. As for the 33k/68k, i have read that they should go on the socket. In Fenders they are on the input jack. I am weird about parts just hanging off of others, so I will put them on a tag board to the socket (saves more board space as well)

Romberg. Thx for info. I think I will just leave the power switch out of the circuit, seems pointless. Awesome link to Duncan Amps! I have the TS calculator, but didn't know this existed! The reason I want a custom chassis and faceplate is because I don't want to drill in a vintage chassis. Guess I could try to find a modern one.

The switching at the input is throwing me. I know the tubes need to have resistance at the grid to stay stable. Should I just put a 33k and 1M on each of the first 2 tubes and do the switching directly at the jack?

[kcstratslinger]

I have acquired the Valve Amplifier book and already know the stuff in the first chapter. Unfortunately, my school skipped over tubes and jumped straight to the more "relevant" technologies (ICs, BJT, FET, Digital...etc). So I will start on chapter 2. Thx for that Haze, this book will be VERY helpful.

romberg, I downloaded the PSU software, but it is a little confusing and there is no manual to explain it completely. The first thing that threw me was the capacitor resistance. I found out this is ESR and can vary by the cap manufacturer, so I will leave it alone for now (or just use general values). What I am stuck on now is the "current tap". I'm unsure what to use for the values. I know how to calculate current at the plate (12ax7) and I assume this is one of the values. What would the other value be for a 12ax7? Grid current? Also, would you simply add the currents together for multiple tubes at the same node? As for the power tubes, I have no idea of their operation. I was just going to copy the Twin Reverb output. That's something I have to read more about. In the meantime, would you happen to know the current values of (or how to calculate) 4 6L6 tubes running at 450V Class AB (ala Twin Reverb)? (Cathode to ground, 470ohm to each Grid #2, 1.5k to from PI to grid #1, Plate to OT 450V). I figure the Grid #2 current is simply 450V/470ohm=0.957A for each tube. Is this one of the values needed for the PSU software? I have started reading about Push/Pull in the book that Haze recommended, but I haven't fully absorbed it yet.

I know I have a lot of questions and an answer to all of them may be a lot to ask for. I'm just trying to gain as much knowledge as possible from more experienced builders. Once I am able to understand power supply and power output I think I will have a pretty decent grasp on the complete functionality of the basic AB Push/Pull guitar amplifier.

Thanks a bunch

[romberg]

romberg, I downloaded the PSU software, but it is a little confusing and there is no manual to explain it completely.

Welcome to the wonderful world of electronics! This was our first lesson in junior electronics lab. All the manuals are gone 'cause in the real world all the manuals will have been lost years ago!

The first thing that threw me was the capacitor resistance. I found out this is ESR and can vary by the cap manufacturer, so I will leave it alone for now (or just use general values).

Yea things like ESR and DC resistance of inductors are sometimes hard to find. So, just google and make educated guesses. Alot of components used to build amps (tubes, caps, transformers, etc) are pretty short on specs.

What I am stuck on now is the "current tap". I'm unsure what to use for the values. I know how to calculate current at the plate (12ax7) and I assume this is one of the values.

The real way involves drawing a load line for each triode and then finding the cathode current (which should be most all the current the tube draws from a node). I think most 12ax7s only draw 1 or 2 milliamps. For ballpark guessing these I found it helpful to look at a few of the example circuits included with the software. With the example PSU simulation and the actual circuit schematic you should be able to see how the simulation was setup.

What would the other value be for a 12ax7? Grid current? Also, would you simply add the currents together for multiple tubes at the same node?

Yep

As for the power tubes,

The plate current at idle (what you measure to bias the tubes) is pretty close. Say 39mA for el34s at 450V. Screens typically draw around 5mA at idle (which is all the PSU2 is emulating).

I have no idea of their operation.

http://www.valvewizard.co.uk/se.html

Merlin also has a book on tube pre-amps written specifically for guitar amps and a power supply book. Both are short technical enough for you to design something yourself and focused on guitar amps. I highly recommend them. The pre-amp book explains how both triodes and pentodes operate. So, it is very pertinent to power amps as well. I think it would be a good idea for you to study some material like this before purchasing a bunch of parts.

Mike

[kcstratslinger]

Thanks again romberg! I have been reading the Valve Wizard site. Best source for tube amps that I have found. I will probably buy the pre-amp book. But the power supply book is out of print. I can only find three copies online. Two of them are on Amazon for $600 and another one somewhere else for $1200! WTF!? The website alone is very informative though. The guy I am working for has some good reading on PSU as well.

I will post all my progress as I go if anyone is interested.

[qtcustom]

I would suggest a trip to the amp garage to look at the Dumble modified bassmans over there. The mods that you're talking about are actually all fairly standard ideas, but I imagine it would be helpful to you to look at some implementations and schematics of mods of this type that were successfully implemented.