Any insight into this b@st@rd amp I have?

[Rafael]

Here is a schematic for a modified amp I'm working on. The amp is a bodge of a power amp section [to the right of the second volume control] and two separate preamp stages cascaded together to form the circuit to the left of the second volume control.

1] anything obviously wrong with this circuit?
2] any tips on perhaps simplifying the second and third stage biasing scheme?
3] without adding tone control, is there a way to improve the low/high balance in the amp? Right now it seems a little bass heavy with a variety of guitars I've tried [LP w/p100s, '74 SG w/stock humbuckers, Flying V w/aftermarket humbuckers, etc.]

Thanks in advance.

[David Root]

All those 100uF bypass caps have a lot to do with the bass, especially with HBs. Yet they are fighting against very small interstage coupling caps. Why? Are you trying to minimize phase distortion?

The cathodyne inverter could be simplified a bit. The gain stages are unusual, what is the intended result of the cathode arrangements?

[jjman]

You could reduce bass (with no tone controls) by:

Reducing the value of the cathode cap on stage 1. It's 100u? Try 1u or 0.1u.

Reducing the value of the .02 coupling cap(s.) Stage 1's should have the most affect. Maybe try .005 or .01.

Removing the treble reducing resistor/cap on the plate of the 3rd stage (47k-.0022u.) Although that might be there to eliminate something nasty.

Adding a bypass cap to either/both volumes. This reduces bass only when the volume is set to lower levels. Maybe 47pf or so. Or a bright switch.


I would do the 1st choice myself.

[LOUDthud]

The biasing on the second stage 12AX7 is suspect. You would like the plate voltage to be about 2/3 of the B+. With two 100K's for a plate load something around 2.2K would be right for a cathode resistor. The 68K in series with the .5uF cap could be replaced with a short to brighten the amp a little. This amp has plenty of gain so it should be relatively easy to add some brightness.

The .0022 cap and 47K resistor is giving you a bass boost (actually a treble cut) try removing it. This may cause the power section to oscillate at some settings of the feedback control so it may have to stay.

Any of the .02 caps could be reduced to reduce the bass some as well as the .047's in the power amp. I'd start with changing the .02 at the second volume control to .01 or .005. The 100uF cap on the first preamp tube's cathode could be reduced to 1uF or so to add a little brightness.

[Rafael]

All those 100uF bypass caps have a lot to do with the bass, especially with HBs. Yet they are fighting against very small interstage coupling caps. Why? Are you trying to minimize phase distortion?

The cathodyne inverter could be simplified a bit. The gain stages are unusual, what is the intended result of the cathode arrangements?

I'm in the process of replacing components and I think I may attack those 100uf bypass caps first. As for the original design, I suppose that minimizing phase distortion could have been a consideration. My needs are simpler, so I am looking at ways to pare this unit down into a usable guitar amp w/creamy distortion....mmm creamy distortion grglel

BTW what is an HB?

What would you suggest for simplifying the inverter? Or should I just gut it and steal a twead plan? As for th cathode arrangements, the stages came from other uses than they are being used for in the current arrangement, and I have considered changing at least the second stage to something simpler, perhaps just a clone of the first stage. I have enough gain to where I'm thinking of dropping in a tone section between stage 2 & 3.

The third stage makes me most queasy: I don't understand the intent of the cathode/FB circuit here at all, so I'm leery of changing anything here.

[Rafael]

The biasing on the second stage 12AX7 is suspect. You would like the plate voltage to be about 2/3 of the B+. With two 100K's for a plate load something around 2.2K would be right for a cathode resistor. The 68K in series with the .5uF cap could be replaced with a short to brighten the amp a little. This amp has plenty of gain so it should be relatively easy to add some brightness.

The .0022 cap and 47K resistor is giving you a bass boost (actually a treble cut) try removing it. This may cause the power section to oscillate at some settings of the feedback control so it may have to stay.

Any of the .02 caps could be reduced to reduce the bass some as well as the .047's in the power amp. I'd start with changing the .02 at the second volume control to .01 or .005. The 100uF cap on the first preamp tube's cathode could be reduced to 1uF or so to add a little brightness.

Someone else mentioned the small size of the cathode resistor in the second stage: I double checked it just now and it is a350. I can see replacing it with something larger, but I'm still curious about the function of the 68k resistor [BTW that is a 5/150 not a .5: that's the positive symbol you're seeing] you mention bypassing it: what would this be doing?

And now I'm noticing something else a little odd: I did just replace the coupling caps with [my error] .0022 instead of .02 and I did notice a bit of improvement in cutting the bass response but not as much as I would have expected.

[Rafael]

You could reduce bass (with no tone controls) by:

Reducing the value of the cathode cap on stage 1. It's 100u? Try 1u or 0.1u.

Reducing the value of the .02 coupling cap(s.) Stage 1's should have the most affect. Maybe try .005 or .01.

Removing the treble reducing resistor/cap on the plate of the 3rd stage (47k-.0022u.) Although that might be there to eliminate something nasty.

Adding a bypass cap to either/both volumes. This reduces bass only when the volume is set to lower levels. Maybe 47pf or so. Or a bright switch.


I would do the 1st choice myself.

I'm with you: next up, downsizing the 100u to something smaller. I have seen a recommendation that they be like 25s, and looking at a Tweed Deluxe schemo that's what's on those. What is the effect of dropping them to 1u or less?

Also, I think it's an easy test to see if the R/C bypass around the 3rd stage plate supply is workable. BTW I just noticed an error: that cap is actually .0002! Tiny! Hypersonics?

And the amp actually looses highs as the volume is cranked, so bypassing the volume control wouldn't really help. The second volume control may become replaced by a tone control anyway.

[Andy Le Blanc]

I've seen some thing like the second stage before
its a kind of feed back
I think I saw it in a magnatone
increase the 68K to 220K
and the chathode resistor to 470 wire wound
really tight sound .... little solid state like
I played with it a bit
it cancels power supply ripple and noise
every thing else can be tweeked to your liking

[drz400]

The biasing on the second stage 12AX7 is suspect. You would like the plate voltage to be about 2/3 of the B+. With two 100K's for a plate load something around 2.2K would be right for a cathode resistor. .

Exactly what I was going to say
at least 1.5K for better bias

I hate that style of schematic, is that two 100K's in series for the plate resistor?

[klingo]

if these 2X100k's are connected in the middle to the 68K sending local feedback to the cathode. then the 5/150 cap short feedback in the guitar frequency range (10hz reactance is about 3k). I would prefer a 5uF/250v here

I assume the 68k/350 to be a voltage divider, setting the cathode at a "steady" positive voltage (around a volt or so)
it act also as a cap bleeder...safe

it's common practice to kill very high frequency before a concertina/cathodyne phase inverter because of the mismatched output impedance/frequency range.

with a 470k on the 3rd stage plate you are not far form direct coupling to the phase inverter( depending on the 3rd stage cathode resistor)
orange did something like it
http://users.telenet.be/orangefg/OFG_SC ... hem_74.gif
http://users.telenet.be/orangefg/OFG_SC ... post74.jpg
in this case the 3rd stage anode voltage need to be less than the 1/3 of the phase inverter supply (275v/3...under 91v)
lowering the (6k8) 3rd stage cathode resistor will lower his anode (quiescient) voltage

the 100uf working with the 6k8 cathode resistor is huge (or might be a "huge cap trick"...)

HB could be humbucker pick-up

[jjman]

You could reduce bass (with no tone controls) by:

Reducing the value of the cathode cap on stage 1. It's 100u? Try 1u or 0.1u.

Reducing the value of the .02 coupling cap(s.) Stage 1's should have the most affect. Maybe try .005 or .01.

Removing the treble reducing resistor/cap on the plate of the 3rd stage (47k-.0022u.) Although that might be there to eliminate something nasty.

Adding a bypass cap to either/both volumes. This reduces bass only when the volume is set to lower levels. Maybe 47pf or so. Or a bright switch.


I would do the 1st choice myself.

I'm with you: next up, downsizing the 100u to something smaller. I have seen a recommendation that they be like 25s, and looking at a Tweed Deluxe schemo that's what's on those. What is the effect of dropping them to 1u or less?.......

Less amplification of lower frequencies. 100u is overkill. 25u is "normal." 1u and below for less low end.

[Rafael]

The biasing on the second stage 12AX7 is suspect. You would like the plate voltage to be about 2/3 of the B+. With two 100K's for a plate load something around 2.2K would be right for a cathode resistor. .

Exactly what I was going to say
at least 1.5K for better bias

I hate that style of schematic, is that two 100K's in series for the plate resistor?

Yes. BTW I just drew this with my limited artistic ability using ExpressSCH which I have no experience using, plus I'm left handed. What's an example of a style of schematic you'd consider having superior clarity? Thanx

[drz400]

The biasing on the second stage 12AX7 is suspect. You would like the plate voltage to be about 2/3 of the B+. With two 100K's for a plate load something around 2.2K would be right for a cathode resistor. .

Exactly what I was going to say
at least 1.5K for better bias

I hate that style of schematic, is that two 100K's in series for the plate resistor?

Yes. BTW I just drew this with my limited artistic ability using ExpressSCH which I have no experience using, plus I'm left handed. What's an example of a style of schematic you'd consider having superior clarity? Thanx

well, to make it easier to read and understand
..
A) if you make B+ on the top of the page always run your B+ to the top. First Stage you have it running up, second down, You should avoid putting positive voltage down unless you are Fender LOL! It is also the same voltage as the previous stage, it could just run there towards the top since now the area where it meets the 68K is confusing.

B) If you use junctions at intersections, do it everywhere there is one, so there is no mis-understanding. For instance some people could see that as the 68K running to the junction of the two 100Ks? I see it as it does not but is 68K to the 5uf to ground whihc could have been routed on the left side if it is not connected.
Which way is it?

C) Parts locaters designaters would be handy when discussing the circuit.
R1, C1, C2 etc

D) adding correct value names is good, for instance .0022 what? I know it is uf but saying uf is better or even better still IMO is 2n2, easier to read when you lose resolution. I dont like (.) in schematics, they are not needed, use 2K7 instead of 2.7K. K should be caps you can use u instead of uf, n (nano) and p (for pf) would be lowercase. Use u, n and p and you need no decimals.
for ohm use r like 350r , it leaves out the guess work that someone forgot a K or something (which by the way you should raise to at least a 1K)

Why does that first cap say C100/3 ? What value is it?
Normally you would say 22u/50V

If it is bassy and that is a 100u cap then change it to 1u

Seems like the program might be hard to draw in

Try this
http://www.capilano.com/html/dwwx.html
Or
www.cadsoftusa.com