Please Critique My New Schematic.......

[AJD]

Hi All,
A year and-a-half ago I built my first amp from scratch. Basically it was a Fender Princeton 5F2A with a few tweaks here and there a'la Dave Hunter's Two-Stroke. I am now designing my next amp and I would appreciate any feedback regarding my current design. I have attached a schematic in PDF form. This new design is a two-channel amp, the clean channel being the same design as my Princeton clone and the second channel being the overdrive channel. As I have never designed an overdrive circuit before I am particularly interested in comments regarding the overdrive channel circuit topology. I also would like to know what you think of the power circuit (particularly the filter cap sizing and arrangement), and whether a choke would be of benefit in the power circuit. Please feel free to chime in with anything else that might jump out at your more experienced eyes too. Thank you all very much. Aaron D

Oh, yeah.....If I were to add an effects loop would unbuffered by adequate , or would it be advisable to use buffered?

[Jana]

I would suggest that you stick with a known design and get more experience before you try to "design" your own. There are a lot of issues with this schematic that lead me to believe that you don't currently have the knowledge to attempt a design from scratch.

[dynaman]

Yeah, don't build it as is.

The voltage divider for the elevated heater reference is funky. Generally a 10:1 ratio for the resistor values is what you should be looking for. I use 1M/120K (bypassed with cap). That fetches me around 40VDC, depending on B+. I've not done the math, but I believe your combo of 220K/220K will give you a LOT more, prolly smoking the bypass cap.

There are lots of fellas here that are way more experienced than I am with switching schemes, but it seems kinda scary to me to be switching high plate voltage on the first gain stage. Since your not changing cap value, why not put the .02 coupling cap BEFORE the switch?

I'll let someone else who has more time to trace the circuit chime in here. Try posting at Hoffman's for some brutally honest critique.

[AJD]

I would suggest that you stick with a known design and get more experience before you try to "design" your own. There are a lot of issues with this schematic that lead me to believe that you don't currently have the knowledge to attempt a design from scratch.

The reason for posting was to try to get feedback from more experienced builders so that I can learn, even if this idea never gets built. The clean channel was taken from a proven design (5F2A) and works like a charm. So in my mind the questionable area is the O.D. channel, no? So what are some of the issues that you see? Thanks.

[AJD]

Yeah, don't build it as is.

The voltage divider for the elevated heater reference is funky. Generally a 10:1 ratio for the resistor values is what you should be looking for. I use 1M/120K (bypassed with cap). That fetches me around 40VDC, depending on B+. I've not done the math, but I believe your combo of 220K/220K will give you a LOT more, prolly smoking the bypass cap.

There are lots of fellas here that are way more experienced than I am with switching schemes, but it seems kinda scary to me to be switching high plate voltage on the first gain stage. Since your not changing cap value, why not put the .02 coupling cap BEFORE the switch?

I'll let someone else who has more time to trace the circuit chime in here. Try posting at Hoffman's for some brutally honest critique.

Thanks for the feedback. I actually have been using that voltage divider in my Princeton for over a year now with no problems, but I will definitely check the voltage that it is seeing. If I recall correctly it did not read high.

The reason for the two coupling caps was that it would allow me to use a different value for each channel, should the need/desire arise. The DPDT switch would probably be a relay (like in a Dumble). Would that be o.k. or still a bad idea? Thanks.

[dynaman]

Don't know if a relay could handle the voltage. Try Metro too.

[AJD]

Don't know if a relay could handle the voltage. Try Metro too.

Thanks. Will do. I just checked out the Hoffman forum. I've never spent any time there, but I will look for help there also if I don't learn enough over here. Thanks for the tips.

[Jana]

What do I see? Too numerous to mention all of them, a few are:

elevating the heaters to 200 volts and using a 100 volt cap on it.

switching the plate voltage with that relay/switch

uncertainty as to using a 68k resistor or a choke
and a 10K or a choke

AC switch in the neutral line

the tone controls and master volume in the OD section--think it will work?

lack of B+ filtering--motorboat special

the funky 1/2 power switch arrangement on the screen grid. The switch just puts a 1K resistor in parallel with the OT primary. Is this what you intended? It's only a 5W resistor--the EL 34 with smoke that in a hurry.

Seriously, do you understand what you have drawn?

[John_P_WI]

A few more from memory:

Most important, where is the chassis ground?

As said, no DC blocking on the switch. Throw a large value blocking cap in there.

After the switch, there is no ground ref / grid leak resistor on the grid of the following stage. Pop pop

I also think there is a lot of gain in the OD section which could lead to grid blocking for that great sticky farting sound. Might get some fizz too.

When your presence is "turned off" do you want to defeat the neg feedback?

Also as Jana said, the volume / tone in the OD is funky.

This schematic needs a little house cleaning / organizing. It might be better to show a filter string with nodes A B C ect and plate r's with A B C etc instead of crossing lines everywhere...

Personally, if you want a quieter higher gain amp, I would lose the tube rectifier. Being single ended, you may want a lot of filtering on the first stage.

If you want to learn amp and 2 channel amp architecture switching check out The Ultimate Tone from Kevin O'Connor and TUT 3 for building tips.

[AJD]

What do I see? Too numerous to mention all of them, a few are:

1)elevating the heaters to 200 volts and using a 100 volt cap on it.

2)switching the plate voltage with that relay/switch

3)uncertainty as to using a 68k resistor or a choke
and a 10K or a choke

4)AC switch in the neutral line

5)the tone controls and master volume in the OD section--think it will work?

6)lack of B+ filtering--motorboat special

7)the funky 1/2 power switch arrangement on the screen grid. The switch just puts a 1K resistor in parallel with the OT primary. Is this what you intended? It's only a 5W resistor--the EL 34 with smoke that in a hurry.

8)Seriously, do you understand what you have drawn?

Thanks Jana. That is all very helpful. Yeah, I am inexperienced and it shows. No shame in that and I am trying to rectify it.

1) Easy enough to remedy.
2) Suggestions?????????
3) Are there rules of thumb for application/sizing of a choke? Some Princeton amps use them, and some don't. I know what they do, but not whether one should or should not be employed? How is that determined?
4) I'll move that over to the hot.
5) I don't know. That is why I am asking. What should I be looking for?
6) How should this be remedied? Larger value caps? Which one/ones? How much? How can I determine this/these values? Employ a choke?
7) That element was pulled from somewhere as an effective half-power modification, though I can't remember where at the moment. The resistor value would have to be upped for sure. Bad idea overall?
8- Not entirely, but with the help of this forum, I hope to. That is why I am here.

Thanks again. Sincerely, A.D.

[AJD]

A few more from memory:

1-As said, no DC blocking on the switch. Throw a larger value cap in there.

2-After the switch, there is no ground ref / grid leak resistor on the grid of the following stage. Pop pop

3-I also think there is a lot of gain in the OD section which will lead to grid blocking for that great sticky farting sound.

4-When your presence is "turned off" do you want to defeat the neg feedback?

5-This schematic needs a little house cleaning / organizing. It might be better to show a filter string with nodes A B C ect and plate r's with A B C etc instead of crossing lines everywhere...

6-If you want to learn amp and 2 channel amp architecture check out The Ultimate Tone from Kevin O'Connor and TUT 3 for building tips.

1- Pardon? Could you please clarify? Not certain what you are referencing? Larger cap value where exactly?
2- Doh! What would you recommend value-wise?
3- How would you remedy this?
4- Yes, that is what I intended.
5- I agree. Good idea.
6- I have a few books, but not that one. I will definitely try ro get a copy of that one.

Thank you very much! I'm learning already.

[John_P_WI]

AJD,
You are too fast to respond! I modded the list a little. Most importantly where is your chassis ground? You just made a toaster here and you will be the toast, especially switching the neutral.

I will try to answer more later.

About the DC blocking cap, you could put something like a .1u between the plate and the switch to block the DC. To be honest, I did not look to closely at the frequency response of your circuit, I don't have time right now...

John

[ampdoc1]

I concur with the other responses. You absolutely do no want to run your plate voltage thru the relay. Stick in a large value cap ahead of the relay.

So far as the OD circuit goes, that may sound great! But I'd suggest putting in a "typical" D*mbl* OD. I've mated that with a Trainwreck circuit, and it works very well. Pretty much the layout you already have.

ampdoc1

[AJD]

AJD,
You are too fast to respond! I modded the list a little. Most importantly where is your chassis ground? You just made a toaster here and you will be the toast, especially switching the neutral.

I will try to answer more later.

About the DC blocking cap, you could put something like a .1u between the plate and the switch to block the DC. To be honest, I did not look to closely at the frequency response of your circuit, I don't have time right now...

John

Ha! I just happened to be online at the same time. I'm not usually so prompt.
The chassis ground isn't drawn (a huge oversight), but it exists as built in my first amp. I'll be sure to add it to the schematic. Thanks.

Thanks for the cap sizing recommendation and placement too. I had not intended to build a toaster.

[AJD]

I concur with the other responses. You absolutely do no want to run your plate voltage thru the relay. Stick in a large value cap ahead of the relay.

So far as the OD circuit goes, that may sound great! But I'd suggest putting in a "typical" D*mbl* OD. I've mated that with a Trainwreck circuit, and it works very well. Pretty much the layout you already have.

ampdoc1

Thanks AmpDoc!