2-ch hybryd ODS/ax84 P1/Superlite 18W

[adamG]

Hi Guys,

I think I need your help.
Recently, I've been building an amp...A kind of hybryd that is ODS 20 as I-ch. and ax84 P1/18W Superlite as II-ch.
The amp seems to be quite nice, but the output power is too high. Both channels are overdriven. In my project II-ch. needs to be overdriven, but I-ch. should be clean.
Power stage is acc. to 18_watt_Superlite.gift. Actually, its preamp stage is also adapted in my project as II-ch., but with Vol. and MV pots of 1MA.
I-ch. is acc. to Mini ODS 20. On PI there's 100R resistor to ground. Also, I've applied 39K resistor as NFB.
V1 plates voltages are approx. 155 VDC - I-ch. V2 plates voltages approx. 165 VDC. V3 plates approx. 190VDC.
Both channels have pots Vol, Treble, Bass, MV.
Maybe as the first mod I should change both Vol. pots and MV pots?
If necessary, I'll prepare the whole amp's schematic.
So, can you assist me how to reduce the amp's output power relating to both preamp channels ?
Thank you in advance

Regards,

Adam

[tubenit]

If necessary, I'll prepare the whole amp's schematic.

I view that as a non-negotiable essential!   2nd comment is EL84's break up pretty easily, so you may have some challenges with a clean channel.

With respect, Tubenit

[ac427v]

Yes please!  I would like to see an accurate schematic of your build.
"On the PI there's a 100r resistor to ground" This is a problem but I can't tell if it is the main problem. Is this resistor the tail of the feedback circuit? or the tail of the PI cathode? Either way it does not match the rest of the circuit.
An alternative to drawing a schematic would be to build the PI and power amp exactly the same as one of amps in your hybrid. And build each of the preamps exactly the same as on the original schematics. That allows a reader to analyze what is causing the problem.

[adamG]

Hi Guys,

Thank you very much for both replies.
Here's the schematic of the whole amp.
Any remarks highly appreciated.
Thank you in advance.

Regards,

Adam

[DummyLoad]

PDF files for those who don't have express SCH or EEschema.

don't believe that output stage will produce a "clean".


--pete

[adamG]

PDF files for those who don't have express SCH or EEschema.

Thank you Pete.
Upsss...I did not think that not everyone was equipped with express soft. Sorry

Any suggestions how to mod the circuit to get the clean, first?

But, the main concern is how to reduce the output power level. With both MVs on half or more, the amp's jumping up...

Present schematic with plates from my memory, only. I'll measure them soon. So there'll be the real figures.

Kind regars,

Adam

[tubenit]

I don't know IF you can accomplish your goal of a clean channel using EL84's?    However, you may be able to somewhat lessen the distortion on the channel 1?   

These are the things I would try.  Some may be useful, some may not? 

As a starting place, I would be inclined to switch to JJ 6V6's which have a reasonably clear "6L6-ish" tone?

The coupling caps between the LTPI and power tubes could also be .01 instead of .015 or .02.

The last thing I can think of is MusiCaps  capacitors have a VERY clear (not clean)  smooth tone.  I found I could use a larger size cap without getting a distorted tone using MusiCaps. Those caps are ridiculously expensive and I use them sparingly in my amps usually in an FX loop where I want as clear of a tone as I can get. You can try googling those to see if you can find any? You could try either a .02 or .01 after the LTPI.

With respect, Tubenit

[adamG]

I don't know IF you can accomplish your goal of a clean channel using EL84's?    However, you may be able to somewhat lessen the distortion on the channel 1?   

Thank you Jeff, very much!

When you previously mentioned about EL84's breaking up, I caught myself as I had just forgotten that issue...
So, what can I do now? I think, I'll extend the holes and swap power tubes. But, there's something important. My PT is dedicated to EL84s, which is 350 VDC maximum. Anyway, I need to check it out.
The PT is wired with a couple taps, because I prefere "regular" multi taps for graetz bridge. Then, I'm able to adjust the PT's HT output level   
The other mods you propposed, I'll try. I never thought that caps had such major impact on gain/output power.

Thank you and regards,

Adam

[ac427v]

I would not change the EL84 tubes first.Fix the negative feedback circuit. You have 39k/100 ohm now. That 100 ohm does not work with a 39k feedback resistor. Use 4.7k instead of the 100r.
Also, replace the two 470k resistors with 220k as Tubenit suggested.
How does it sound now?

[tubenit]

Try ac427v's suggestion.  IF that doesn't help much, I don't see any reason why you can't simply switch the power tubes with no other mod.  Nothing wrong with 350v on 6V6's, IMO.  They don't draw any more current then EL84's. 

With respect, Tubenit

[PRR]

> Both channels are overdriven.

You have _3_ gain-stages, high-gain EL84, with no NFB. Dimed, the input sensitivity is like 1mV; 20mV is more typical. The audio-taper gain and volume pots help; one or the other should be at "5" or the thing is a beast.

I see the "NFB" note but 39k:100r sets gain of 390, whereas V3V4V5 only has gain like 30 so the NFB "does nothing". Try changing 100r to 1k or 2k for "some NFB", 5k for a tame sound (and zero for loud and raw).

Probably typo, it would suck if true: V3 grid resistors can't be 470r, must be near 470k.

[DummyLoad]

not just the NFB loop (basically the lack thereof), the 6V6 and EL84 are not going to clip at the same input levels. the EL84 will drive into saturation much sooner than the 6V6 will, not to mention the gain difference. that is a very lopsided output stage IF it was built as depicted.


as already pointed out, there is a lot of gain in that preamp chain. try without a cathode bypass cap on the second stage of the "clean" channel. cathode bias will have less headroom than fixed bias.


 just my 2cents... 


--pete

[adamG]

> Both channels are overdriven.
Probably typo, it would suck if true: V3 grid resistors can't be 470r, must be near 470k.

Hi,

Upss...my mistake. Of course, the resistors are 470K not 470R. I know the "subtle" difference between them, but I need to check them out. It's possible I made such a mistake with mixing 470K and 470R ;)
I'll change the NFB resistor for the mentioned ones. Also, I'll play with different pots.

Thank you very much.

Kind regards,

Adam

[adamG]

Thank you Pete, very much.

Kind regards,

Adam

[adamG]

Dear Friends,

If I may, can you advice me how to play with the second channel to increase the gain/distortion?
What shall I change to get the hotter distortion?
I ask for that now, because the whole procedure to mod first channel together with power stage, would require to disconnect main board...

Thank you in advance.

Kind regards,

Adam
 

[ac427v]

Hey Adam, Your enthusiasm is contagious! Sadly, I have not had good tweaking results when making multiple changes at the same time. It is impossible to evaluate the effect of each change. Creating new circuits requires making many changes and listening to their effect. Easy accessibility to the circuit board is a vital part of the design and layout. If you can't do that then it is best to pick a known good circuit and copy it exactly.Anyway, I like the challenge of sharing ideas.
Could you clarify what distortion sound you seek? For example, I think of a tweed deluxe having a warm, soft distortion at mid volume. I think of a Marshall 800 having a harder, colder form of distortion. What sound do you want to create?

[tubenit]

If I may, can you advice me how to play with the second channel to increase the gain/distortion?

You could try a Dumblish approach of running the first channel into the second channel.  The Dumble HRM actually has a Marshallish tone stack in the OD channel.  So, you're headed that direction already. 

Having said that,  I would NOT do anything with the 2nd channel until you get the clean channel where you like it. AND if you
stick with EL84's, I think you stand a chance to continue with too much distortion,  IMO.

The "Option 2" schematic type gain stage with CF follower is VERY similar to the amp in this sound clip using 5881's OR 6V6's (I used both in this amp).  The rhythm is the clean channel and the lead is the OD channel engaged in this soundclip: 
https://soundclick.com/share.cfm?id=9575220

With respect, Tubenit

[adamG]

Hey Adam, Your enthusiasm is contagious!
Could you clarify what distortion sound you seek? For example, I think of a tweed deluxe having a warm, soft distortion at mid volume. I think of a Marshall 800 having a harder, colder form of distortion. What sound do you want to create?

Thank you AC, very much.
What distortion sound? Hm...please, have a look here  https://perpetualinnovation.us/MyBBDir/showthread.php?tid=501563

Actually, that is why I took the topic name from, as well. Because, ax84's preamp is also based on the idea of TS conn. to plate.
Yes, It is problematic to combine three different blocks.
The thing is I made the pcb for the amp...Anyway, I disconnected it yesterday and applied a number of small eyelets to be able making mods.
We will see what I achieved 

Kind regards,

Adam
   

[adamG]

Having said that,  I would NOT do anything with the 2nd channel until you get the clean channel where you like it. AND if you
stick with EL84's, I think you stand a chance to continue with too much distortion,  IMO.

Jeff,
You're absolutely right. I won't touch OD channel until I am made with clean one. That's the priority.
I wouldn'd like to go with "standard" HRM or just cascade blocks idea as it is in ODS. I build both of them regularly, but this time I need to make something different.
I'd like to achieve two separate channels by two tubes, only.
What do you think about the att. idea of mosfet and how to mod this circuit to cooperate with PP power stage?

Thank you very much.

Kind regards,

Adam

[tubenit]

I'm not sure there will be much difference in the amount of overdrive between channel 1 and channel 2  IF each of them only has one tube?   And a cathode follower will not add gain.

However, this is how I have done the mosfet cathode follower on numerous amps with results I'm happy with.

With respect, Tubenit

[tubenit]

I think this would give you both clean & OD and would stay within your goals of 1 tube per channel and NOT sounding like a D-style ODS amp.  Just tossing it out for consideration & not saying you "should" do this. 

I always like the 5879 pentode for a "fat" tone.  The mosfet CF works well with the 5879 to allow the use of the tone stack.

with respect, Tubenit

[adamG]

Jeff,

Thank you very much!
Clean channel is almost done. Designing pcb I was that "smart" to consider splitting plate resistors V1 Pin 6 ;)
So now, I've swapped the single one for two.
Solution of your modifications within this channel got me possitive score. I think, I'll also reduce cathode cap V1 Pin 8. Then, I'll be successful, I think.  Thank you.
Mosfet idea is not that efficient, as I thought. No major tonal change. But, there might be another though... I attach the schematic what I think, at least to myself ;)
What do you think about the mosfet as CF and two pairs of diodes to charge more distortion?

I can play with this amp's circuit to modify it, as necessary.

Thank you in advance.

Kind regards,

Adam

[tubenit]

I have never used diodes to create distortion.  I really don't know anything about them?

Sorry, I am not able to help on this.

With respect, Tubenit

[glass54]

Hi Adam
Try the Marshall schematics in Doug's Collection particularly Marshall_jcm800_splitch_rev_50w_4210.pdf"
Some of these Marshall's were not that reliable, but this model uses pairs of diodes (back to back) driven by a Cathode follower.
Other more recent Marshall's have use LEDs but usually in Solid state format amplifiers 
You could experiment with this plus a possible series resistor before the diodes.
Kind regards
Mirek

https://el34world.com/charts/Schematics/files/Marshall/Marshall_jcm800_splitch_rev_50w_4210.pdf

[adamG]

I have never used diodes to create distortion.  I really don't know anything about them?

Jeff,

I also consider another tube within channel 2. If so, what circuit can you advise? Maybe D'Mars, non 5879?

Thank you in advance.

Kind regards,

Adam

[adamG]

Hi Adam
Try the Marshall schematics in Doug's Collection particularly Marshall_jcm800_splitch_rev_50w_4210.pdf"

Thank you Mirek, very much.
That idea seems to be suitable to my project.
I need to study it deeply.

Kind regards,

Adam

[glass54]

Hi Adam
Please find attached link to Pro Cat Distortion Pedal (SS but the guy has gone to a bit of trouble)
https://olivierjambois.com/some-modifications-on-the-pro-co-rat-distortion-pedal/
I would avoid the 2 diodes back to back unless it's what you're after ie a little boring, symmetrical clipping.
Red and Green LEDs back to back, for example, would possibly give you some even order distortion.
If you have an oscilloscope, it would be interesting to capture the wave forms and determine/compare the ones you prefer using a couple of different spot frequencies eg 200Hz and 1kHz.
Have fun 
Kind regards
Mirek

[adamG]

Hi Guys,

I decided to follow this path and add 4th tube.
That's D'Mars non 5879.
What do you think?
At last, channel I is ok ;) Now, I need to play with ch.II.

Kind regards,

Adam

[tubenit]

I don't know how much gain you want??  As you have it drawn, I think you'll have ALOT of gain especially for EL84's?   IF you have too much gain for what you're wanting, maybe try some of these changes? 

The 220p cap across the LTPI 82k plate resistor is an "enhance cap" that helps smooth some of the higher treble frequencies.

With respect, Tubenit

[adamG]

I don't know how much gain you want??  As you have it drawn, I think you'll have ALOT of gain especially for EL84's?   

Thank you very much!
I do not expect a heavy metal machine, but vintage, kind of ODS amp or HRM amp.
I needed to prepare a separate pcb for the 4th tube with small eyelets through holes. So, I can mod as much as necessary;)
For this amp I'll keep EL84s, but if it comes a good amp, I'll try 6V6 for the next one.

Thank you and regards,

Adam

[1blueheron]

By no means an expert, but what about putting 7 pin sockets in the 9 pin holes, which with minimal effort could be adapted to fit without cutting your chassis or enlarging holes, (could always go back to EL84's if wanted)  and then using the 6V6's little brother 6AQ5.  6AQ5 should sound similar to the 6V6 as far as clean soundm not breaking up as early as the EL84, however when you push it hard, it will get a nice grindy sound to it.  I have them in a little PP silvertone amp and very much like the way they sound.  They have been known handle the plate voltages the EL84 does as long as you mitigate for the inability to dissapate heat as well as the larger glass surface of the 6V6 or EL84 so make sure they are well ventilated and get good airflow. 



 

[adamG]

...but what about putting 7 pin sockets in the 9 pin holes, which with minimal effort could be adapted to fit without cutting your chassis or enlarging holes, (could always go back to EL84's if wanted)  and then using the 6V6's little brother 6AQ5.  6AQ5 should sound similar to the 6V6 as far as clean soundm not breaking up as early as the EL84, however when you push it hard, it will get a nice grindy sound to it...

Thank you very much for above suggestion. By this, you've paid my attention to another possibility. I forgot, I have two pairs of nice and real NOS russian 6P1P.
As far as I know, Russians put 6V6 into smaller bottle dedicated to noval. They require just same noval socket, which is very well. So, first I'll try 6P1P. If there's no success with them, I'll go back to the idea of 6AQ5 or EL90.
Anyway, now I struggle with 2nd channel...

Thank you and regards,

Adam

[adamG]

Hi Guys,

Here's the brief report of the amp...
The amp sounds really good. Smooth, overdrive sound of 2-channel as it was supposed to be.
But, same issue within 2-ch.. It's huge amount of output volume... It must be reduced. It seems that the amp has its maximum on volume pot 1-2.
Plates voltages on tube 2 and 3 are approx. 160-170V. I'll measure them today, exactly.
So, any suggestions would be appreciated.

Thank you in advance.

Kind regards,

Adam.

[tubenit]

PPIMV as a master volume ………. and/or  change to 6V6 tubes.

[shooter]

It's huge amount of output volume

this one always gets me 
when I "come up with an idea" I typically try n find something commercial that I can listen to 1st.
and I'm always happier when it's LOUDER than I expect since there are usually 2-3 knobs between the pick n speaker for loudness control 

[adamG]

...change to 6V6 tubes.

It's done. No change with output power level on 2nd channel engaged, which was the purpose of swapping EL84s to 6V6s.
1st channel is really nice. 6V6s coused its lower level output, additionally.
But, what can I do with 2nd? How to reduce its output power?  It behaves as an amp of 50W, at least. Crazy. Overdrive is not natural.
Any ideas would be highly appreciated in case of quite a bit of funds and huge amount of efforts.

Kind regards,

Adam 
 

[shooter]

I cut it up, well deleted it 

you can just pull tube and add jumper then evaluate