Help with Standel 25L15 Preamp Circuit

[cgpwannabe]

Hey Fella's,
I will start off by saying that I am a huge fan of the Standel 25L15 guitar amplifier. Almost everywhere you read, you will find that everyone says that the 25L15 has amazing cleans and unbelievable headroom. I have built 3 of these amps VERY similar to the hand drawn schematic that I attached to the thread. I am having problems in all of the my amps with a static-y sounding distortion that is very unpleasant when the amp is pushed passed about 5 for the JBL D130 speaker and about 3 for another speaker I have (University 6201). I would expect the headroom to be better than this, especially passed 3 for the University speaker!

Now taking a look at the schematic - the preamp design is VERY unique for a guitar amplifier. The 12AT7 tube is where I suspect problems. The first triode of the 12AT7 is the second gain stage and that outputs to the grid of the 12AU7 first triode. Off the plate of the first triode signal goes up through the bass and treble controls and back to the 12AT7 second triode - which is setup as a cathode follower. The cathodes of the 12AT7 are connected so the signal from the cathode follower is injected back to the cathode of the first triode - essentially negative feedback over 2 stages.

From a performance standpoint, the 12AT7 can be a very squirrly tube - it is susceptible to being very microphonic and also subject to strange oscillations. Some tubes work in it and other don't. It is definitely not setup like most cathode followers in guitar amps. I find it strange that both triodes are biased from the same cathode resistor when one side of the plate receives full B+ and the B+ of the other triode is dropped over a 47K plate resistor. Is this really an issue?

I have looked at the plate at the 12AT7 first triode with a scope and found a strange behavior ( I haven't seen it before, I hope some of you have). I have attached a picture of this behavior. Once the volume knob is rotated past about 3, these sharp peaks occur. They get very steep and then eventually get cut off. Does anyone know what is going on here? Is it because of the negative feedback?

Well I could go on forever, but I think I will leave it at this. I am relatively new to tube amp circuits - so if I am missing something obvious - I apologize. If anyone could offer any insight into what is going on with the 12AT7 from a design point of view - that would be wonderful and very helpful! Also if anyone knows of any other references that discuss tone controls over negative feedback with 2 stages (OTHER than Langford-smith page 646). Thanks everyone!

[Cameron]

Hey Fella's,
I will start off by saying that I am a huge fan of the Standel 25L15 guitar amplifier. Almost everywhere you read, you will find that everyone says that the 25L15 has amazing cleans and unbelievable headroom. I have built 3 of these amps VERY similar to the hand drawn schematic that I attached to the thread. I am having problems in all of the my amps with a static-y sounding distortion that is very unpleasant when the amp is pushed passed about 5 for the JBL D130 speaker and about 3 for another speaker I have (University 6201). I would expect the headroom to be better than this, especially passed 3 for the University speaker!

Now taking a look at the schematic - the preamp design is VERY unique for a guitar amplifier. The 12AT7 tube is where I suspect problems. The first triode of the 12AT7 is the second gain stage and that outputs to the grid of the 12AU7 first triode. Off the plate of the first triode signal goes up through the bass and treble controls and back to the 12AT7 second triode - which is setup as a cathode follower. The cathodes of the 12AT7 are connected so the signal from the cathode follower is injected back to the cathode of the first triode - essentially negative feedback over 2 stages.

From a performance standpoint, the 12AT7 can be a very squirrly tube - it is susceptible to being very microphonic and also subject to strange oscillations. Some tubes work in it and other don't. It is definitely not setup like most cathode followers in guitar amps. I find it strange that both triodes are biased from the same cathode resistor when one side of the plate receives full B+ and the B+ of the other triode is dropped over a 47K plate resistor. Is this really an issue?

I have looked at the plate at the 12AT7 first triode with a scope and found a strange behavior ( I haven't seen it before, I hope some of you have). I have attached a picture of this behavior. Once the volume knob is rotated past about 3, these sharp peaks occur. They get very steep and then eventually get cut off. Does anyone know what is going on here? Is it because of the negative feedback?

Well I could go on forever, but I think I will leave it at this. I am relatively new to tube amp circuits - so if I am missing something obvious - I apologize. If anyone could offer any insight into what is going on with the 12AT7 from a design point of view - that would be wonderful and very helpful! Also if anyone knows of any other references that discuss tone controls over negative feedback with 2 stages (OTHER than Langford-smith page 646). Thanks everyone!

If I remember right ...put a 250pf cap to ground after the feedback .047 cap on v3a...the one going to the 270k resistor...
If you have the 25l15 schematic thats looks like its on a napkin..... thats the one I drew...I don't know how it got out.........it has this cap..and it should be what the reissue 25l15 circuit is. Bob Crooks was a genius .....way ahead of his time.

[rsi]

The scope seems to look a lot like what is shown on the second page of this article from way back:

http://ken-gilbert.com/images/pdf/fixbias.pdf

In the article it's positive feedback.

[Cameron]

The scope seems to look a lot like what is shown on the second page of this article from way back:

http://ken-gilbert.com/images/pdf/fixbias.pdf

In the article it's positive feedback.

Its a cathode biased amp.

[Structo]

Is this just a tube preamp or is this the preamp to a whole guitar amp?

[cgpwannabe]

Thanks for responding guys!

Cameron - you drew that schematic that I attached? Am I hearing you right? What was your affiliation with Bob Crooks, the way you speak of him it seems that you worked with him. If so - maybe you could tell me what is going on electrically with the 12AT7 - that tone stack is very strange. I think "rsi" is right - there is positive feedback going on. That would explain the shape on the scope. I also started at the 12AX7 and marked where the phase gets inverted (from gain stages) - all the way up to to the cathode of the the first triode of 12AT7. It seems that the signals are the same phase (indicating positive feedback) and NOT inverted like a normal negative feedback would be. Is this the way he designed it? What was the reasoning? How does this work?

Unfortunately I don't have any 250 pf caps on hand but I did throw a 0.001 uF cap right after the 0.047uF cap right to ground. I also tied 2 caps in parallel to get 0.0005 uF (500pf) and I didn't notice much. It didn't seem to help the nasty sounding distortion under very heavy signals (such as strumming). What exactly is the purpose of this cap? A friend of mine who reverse engineered an original 25L15 did not find that cap after the 0.047 uF cap - it just goes right to the 270K. I am not doubting you, I just want to know more!

Structo - this is the preamp to a whole guitar amp - all the way up to the cathodyne phase inverter. The power amp and power supply sections are on a different schematic.

[cgpwannabe]

One other piece of information that I thought I would throw in here is that I do have a reissue Stanel 25L15. It is one of the early ones with 6550 output tubes and also reverb and effects loop. Yes the circuit is quite different - but it still sounds really great. Now when I push the amp around 6 on the volume the distortion actually sounds really good! There is no static-y sounding distortion at all. So that leads me to believe there is something fishy going on in the amps that I built and with that hand drawn schematic that I attached.

I also added grid stopper resistors before each grid and that didn't help it - it may have smoothed it out a bit more, but it's still there.

[tictac]

The schematic you've attached is not the hand drawn schematic being refered to....

it looks like an old factory schematic....

TT

[Structo]

Yeah, the details on that schematic are hard to make out.

For instance, I think it shows a .01uF cap on the input.
This would be to stop RF from entering the amp's input.

I don't think so much it is a hand drawn schematic, but a poor copy that has been copied too much.

It is drawn a little bit unorthodox as far as schematics go but...

For instance, on the input to the amp, is that 625R on the cathodes with a 50uF cap bypass?

That is a strange ratio of the plate resistor (270K) to cathode, 625R?

There seems to be some areas of this design that are not very conventional.
But I don't pretend to know everything about tube amps.
Far from that, I am still learning and appreciate every effort to do so.

[cgpwannabe]

I do agree - the 270K plate resistor and the 620 ohm cathode resistor is strange and that puts the bias towards grid current limiting conditions. I did change that resistor, I think to a 2.7K to bias it more towards the center.

I think we are talking about different schematics. I have seen the "napkin" schematic and it is different than the schematic I posted. Cameron - how did you write that schematic - based on what information?

I can say with some confidence that the Standel company schematic is pretty darn close to the original Standel that my friend reverse engineered. There are a few strange things going on, but that is basically the structure.

Anybody have any insight on this negative feedback (or is it positive?) and tone stack?

[rsi]

The scope seems to look a lot like what is shown on the second page of this article from way back:

http://ken-gilbert.com/images/pdf/fixbias.pdf

In the article it's positive feedback.

Its a cathode biased amp.

Fig. 2 (The circuit if Fig. 2 does not
show biasing arrangements, because the
action depicted occurs whichever method
of bias is used.)

[Clyde]

Hot Blue Plates on the Hoffman board has the skinny on the Standel.

[M Fowler]

Not sure what is the correct transformers for this amp some show using 350-0-350 PT and 6K6 OT.

Another clone used 300-0-300 PT and 6k6 OT so could use TW Express transformers.

[cgpwannabe]

I used 350-0-350 PT and a 6K6 OT transformer - both Hammond. I am in the process of getting a hold of "Hot Blue Plates". I crossed paths with him on the Chetboard a while back - so we will see what happens. I still would like to find out more information about why the tone stack is setup the way it is, it seems like the cathode follower, conventionally, is placed BEFORE the tone stack, but this one is place after. But I don't know if it matters because of the negative feedback. I wish someone knew more about this...Thanks all!

[Tillydog]

I still would like to find out more information about why the tone stack is setup the way it is, it seems like the cathode follower, conventionally, is placed BEFORE the tone stack, but this one is place after. But I don't know if it matters because of the negative feedback.

I don't know the amp, but can take a stab at interpreting the schematic.

Let's number the valves/tubes to help me explain how I *think* it works: From left to right:

V1a top
V1b bottom
V2a
V2b
V3a
V3b

(V1a and b share a cathode resistor, so that 620R resistor would be 1.2k if one were comparing the circuit to one without a shared cathode resistor.)

The resistors and capacitors between V1A and V2A are a twin T filter that will scoop out the mids (provides about 18dB of scoop between ~400 and ~500 Hz).

V2a is a fairly straightforward gain stage, except that it shares a cathode resistor with V2b, so that any signal that goes through V2b will be *subtracted* from the signal into V2a.

V3a is a straightforward gain stage, but the signal form its plate splits two ways. Some passes to V3b which is wired as a phase inverter, but some passes back to V2b via the tone control network forming a negative feedback loop.

Remember that any signal going to V2b will be *subtracted* from the signal going through V2a.

The bass control is the 0.001u capacitor with a 1 meg pot across it. Treble signals can pass through the 0.001u capacitor and be dealt with by the treble control. Bass frequencies can only pass through the pot. With the wiper of the bass all the way to the left, the full signal is passed to V2b, so the maximum bass is *subtracted* from the signal. With the bass pot wiper all the way to the right, the bass signals to V2b are attenuated (by the voltage divider formed by the bass pot and the resistors around the treble control), so less bass is subtracted from the signal, effectively boosting the gain at bass frequencies.

The 0.01u cap on the treble pot wiper will shunt all high frequencies to ground. With the wiper of the treble pot at the top, the maximum amount of treble will be shunted to ground, then there will be no highs going to V2b to be *subtracted* from the signal, effectively boosting the overall gain at high frequencies. With the treble pot wiper at the bottom, no extra highs will be shunted to ground, so the maximum amount will be subtracted from the signal.


I think that's right, hopefully it's not too confusing. Your scope shots look like there is a clipping / grid current problem which is causing a non-linearity in the feedback loop.

ETA: I would check the biasing of V3a, in particular. Also, the bias resistor on V1a, V1b may have been chosen to deliberately limit the signal swing to stop later stages being overdriven. (The 'napkin' schematic has even lower values than the schematic above). If you have changed this, it could be part of the problem.

If you don't get anywhere, you could disconnect the nfb from the plate of V3A and see if/where you are getting clipping.

HTH

ETA#2

Have you included the 220k resistor to ground with an 'X' through it off V1a plate? (The one in parallel with the 0.005u cap). If not, I think you should.