Tone shaping a Chieftain build

[Duncan]

A little while ago, I build myself a slightly modified Chieftain circuit (deleted the reverb, diode rectification) in a salvaged Traynor YBA-1, using the original transformers. The amp sounds great with the volume low (clean/cleanish) but once I start pushing it into overdrive, it becomes very shrill and harsh.

I understand that Class A amps sometimes have a more unhinged overdriven tone (whether from the preamp design or from the power amp doing its thing), but from all of the clips I've found of Chieftains online, I don't think my build should sound THIS ratty and different.

I've attached the schematic that I used to build the amp (updated to delete the reverb and showing FWB rectification). I've also flagged a few spots that I think would start getting me closer to what I want to hear.

Here is a zoom-in of what I was thinking:



Thanks in advance!

[shooter]

before I tweak, I roll speakers, try to get 4-5 different ones in groups of 1's n 2's, played long enough to decide "Is that better?"
2nd thing I do is knock the 25uF caps on the cathodes of the input stage to 10 -15uf "is that better?"
3rd thing I do is "maximize" the use of guitar knobs, minimizing the amp knobs


then, once the easy is out of the way, i plug in a solder pencil and change ONE thing, verify everyone works, THEN decide "is that better?"

[Duncan]

before I tweak, I roll speakers, try to get 4-5 different ones in groups of 1's n 2's, played long enough to decide "Is that better?"
2nd thing I do is knock the 25uF caps on the cathodes of the input stage to 10 -15uf "is that better?"
3rd thing I do is "maximize" the use of guitar knobs, minimizing the amp knobs


then, once the easy is out of the way, i plug in a solder pencil and change ONE thing, verify everyone works, THEN decide "is that better?"

Yeah I’ve tried the amp through several cabs (blackbacks, v30, Creambacks, some very dark sounding mackenzies) and the harsh remains.

I’ll try fiddling with the cathode caps and see how it responds.

Thank you!!

[bmccowan]

I like Shooter's approach.
I have experience with the Clubman and the Spitfire. They too, especially the Spitfire are bright/harsh.
I assume you have tried different tubes? I often find current production 12Ax7s to be brighter/harsher than I like. 5751s are a favorite to tame the beasts. I have a 5751 in the Spitfire and a 6SL7 (similar to a 12Ay7) in the Clubman. Could try an Au7 or Av7 for the PI.
I reduced the volume bright cap (C17 in your schematic) in my Spitfire. Also, many have encountered defective Mylar small caps - might be worth swapping others in.
Also - The Chieftain has the same MV as the Clubman. The V and MV pots do not play all that well together. I intend to change my MV to a PPIMV sometime in the future.

[Duncan]

I like Shooter's approach.
I have experience with the Clubman and the Spitfire. They too, especially the Spitfire are bright/harsh.
I assume you have tried different tubes? I often find current production 12Ax7s to be brighter/harsher than I like. 5751s are a favorite to tame the beasts. I have a 5751 in the Spitfire and a 6SL7 (similar to a 12Ay7) in the Clubman. Could try an Au7 or Av7 for the PI.
I reduced the volume bright cap (C17 in your schematic) in my Spitfire. Also, many have encountered defective Mylar small caps - might be worth swapping others in.
Also - The Chieftain has the same MV as the Clubman. The V and MV pots do not play all that well together. I intend to change my MV to a PPIMV sometime in the future.

Yeah, I’ve tried several different tubes with minimal success. I actually removed the bright cap on the volume pot too, which doesn’t appear to have made a significant difference

[Jonas]

I concur with shooter. The answer will be in speaker changes, minor component value adjustments.

I have a very similar circuit in a SE which sounds great, but some speakers did NOT work.

[ac427v]

Does your build include a cut or Brilliant control to reduce treble after the PI? It seems that both Chieftan and Clubman have a control for exactly your problem???

[Duncan]

Does your build include a cut or Brilliant control to reduce treble after the PI? It seems that both Chieftan and Clubman have a control for exactly your problem???

It’s got a brilliance control, but I have a feeling I’ve wired it incorrectly, as it doesn’t seem to change the tone all that much.

[shooter]

I used that on my "version", none of the guitarists used it or thought it did anything "useful"
hence my suggestion to "maximize" your guitar knobs and minimize the amp knobs.


since i'm not a guitarist, I look at amps as just a sub-system of an overall system.  I "weight" the player, guitar, pedals n speakers at 80% of overall "tone"

[bmccowan]

I'm kind of wondering if it's more than a matter of tone shaping. Removing the bright cap had little effect and adjusting the cut/brilliance control had little effect. It seems that the harshness you describes might be overriding the effect of those two tone shapers. Your amp should not be that shrill IMO. If you have not already, I would go over the amp looking for an error. And I would search for a faulty cap and/or resistor. Either one might not be hurting the low volume tone much, but could disrupt the tone as the power is cranked. I had a cracked resistor once that created havoc as it got warmer. These types of things can be tough to track down.
Although the amp has no NFB, you may want to try swapping the power tube to OT leads.
And, you express some doubts about your wiring. Close-up photos of sections of your circuit could help folks notice a flaw.

[Duncan]

So I took some of the feedback posted on here, and the amp is MUCH better now.

Here's all that I did:

Changed Treble pot to a 250KA
Changed Treble cap to .001uF
Changed the cathode bypass cap on V1 to an 8uF (was 25uF) electrolytic
Properly rewired MV and Brilliance controls

I may drop the cathode bypass cap on V2 as well to possibly 1uF, but for right now, everything is sounding awesome.

[stratomaster]

So I took some of the feedback posted on here, and the amp is MUCH better now.

Here's all that I did:
...

Changed the cathode bypass cap on V1 to an 8uF (was 25uF) electrolytic
...

I may drop the cathode bypass cap on V2 as well to possibly 1uF, but for right now, everything is sounding awesome.

Based on some experience with noise and hum I'd be inclined to at least double the V1 shared cathode to give your tone controls more to work with and minimize H-K hum. Then decrease the V2 cathode instead.  The 8μF is effectively 4μF per triode--which is a nice gentle rolling off of the lowest lows.  Something like at 4.7μF on the V2 will be a bit more aggressive with the low cut, but still leave the low mids largely untouched.  You may have to roll a few different tubes into/out of V2 to find one with the lowest hum.  You can put the fussier tube in V1 if you've put a good 100-220μF cap on that cathode. 

[darkbluemurder]

Late to the party but I have built a Chieftain clone. My 2cts:


- I find that the bass control is very useful with a wide range. If you cannot find a 1 meg. rev. audio pot, use a 1 meg audio and wire it in reverse - works great.
- The mid control is also very useful. For a finer control (the slight trade off is sacrificing some of the max mid settings) use a lower value mid control, e.g. 100k. Especially in amps where I used this control in later stages, 250k audio is simply too much IMHO.
- Personally I think the biggest weakness in the Chieftain design is the lack of a gain control between the first two gain stages. Without it I could not get a clean tone from the amp with humbuckers. I used a 1 meg audio pot with a 39pf bright cap. This also allows you to use a cathode bypass cap on the first stage for more gain on tap. But with the gain control there is a tap :) where there was none before.
- That treble control is useless and also redundant because there is the brilliance control which roughly does the same (IMHO) so I sacrificed it for the gain control.


Cheers Stephan