Big Muff TS looking for Mid-scoop to mid-hump control?

[Andy Le Blanc]

I remember noodling with that tone, equal values for R and C is a place to start
But ef86 and other pentodes are high impedance... I'd try putting a rather
small value cap before the tone , small like 500p to .005 max.
the high impedance allows this and give a similar response to a lager cap
at a lower impedance, other wise the low end will be boosted with that tone control.

[tubeswell]

FWIW these are the EF86 voltages I got from the earlier build with 100k Ra, 560k Rg2, and 680R Rk.

[Tone Lover]

Tubswell, Tubnit at hoffman added a cf stage after a 5879 tube that really made it sound nice i cant remember which post it was but that might be good reading from that perspective.
Bill

[tubeswell]

FWIW I finished the concept amp yesterday. Haven't had a chance to really play it a lot yet, but it sounds promising. The tone is quite good if I may say so, and there don't seem to be any troubleshooting issues (i.e.: no hums, buzzes, rattles). The supply voltage needs a tweak (5AR4 probably - and lowering first supply resistor to 4k7)

(Schematic of as-built version with voltages attached)

[Cliff Schecht]

So further to these helpful comments, here is a complete schematic idea for comment/suggestions

A few specifics I wondered about.

1) not sure whether to use a CF stage or not after the EF86. The obvious reason to use one would be impedance buffer for tone stack. This would be different to a Route 66 approach (which doesn't use a CF stage - and has a different stack). Any comments?

2) If I do use a CF stage not sure whether it should be a 12AU7 with 22k Rk or a 12AX7 with 100k Rk. The one drawn on the schematic is direct-coupled to the EF86 and a 12AU7 might steal a lot of current (from the EF86) but this could have a nice assymetry under overdrive conditions. (or not?) Any comments/ideas?

3) Tone stack. Shown a couple of variants (both using 1M pots). Which would be better for a hump/scoop control? Any bets? Or would I be better off with a typical Fender TB tonestack or one of those brown fender tonestacks? or...? (Reason I am thinking hump/scoop control is to better cope with the higher gain I would get with the EF86 morph control in 'pentode' mode. whilst retaining treble-bass balance of a single tone control for when in 'triode' mode - hence the (modified) big-muff concept TS in the actual circuit).

4) Tone stack recovery stage. (Borrowed from a 6G3 driver). Would this stage really be necessary in this amp? The reason I put it in is because when the EF86 morph control is in 'triode' mode, the amp ought to behave more like a 6G3 (methinks) (or not?).

5) I think I'll go for an LTP here. This is again borrowed from the 6G3. Alternative would be to not have the TS recovery stage (in 4) above) and have a cathodyne stage here. (Gaining a spare 1/2 a stage). The NFB insertion point would have to change. Methinks the LTP would give better drive for the output stage given the 'lowish' HT voltages expected.

6) Not sure whether to use a 12AX7 or a 12AT7 in the LTP. I have tried 12AT7 as LTP tubes before and they do have a nice mojo. Any comments?

7) the LFO stages are borrowed from a 5G9 and work well with fixed-bias 6V6s at those voltages. If I went with one triode and a mosfet follower (as I have done in other maps) I could get another spare triode. (And if I went with a cathodyne PI, then I could also probably to a reverb - because I'd have a whole spare pre-amp tube.)

I have put the standby between the 2 reservoir caps. I can't see the harm in doing that. Can anyone else?

TIA

Pete

1) I would try to go without the cathode follower at first. The thing I'm weary of with pentodes in the preamp is they tend to be very bright because of the extended frequency response. You can use the lossy tonestack to your advantage here to tame some of the highs before they get excessive later on in the preamp.

2) It seems like a lot of people talk smack about CF's but I love them when put in the right place. They aren't for every amp of course and can make a great amp into a shrill nightmare (treble is never rolled off enough). I think that with an EF86 in the preamp you have enough gain, even after losses, to leave out the CF though. I don't think I saw it stated above but what kind of gain are you getting out of that EF86?

3&4) My thoughts are the CF is again probably unnecessary. Having it so early in the amp won't get you much in the way of additional second order distortion because you aren't pushing it hard at all. As far as which tonestack, I thought the point of this amp was to use the BMP tonestack in an amp.. I hope you keep it in there and figure out how to tweak the values to your liking as opposed to "compromising" with a more traditional TMB tonestack. Also I bet that EF86 still doesn't sound like an AX7 in triode mode. Not that it matters much, again this tube is the first in the chain and won't develop any significant distortion itself, so your focus should be on the frequency response change that occurs between the two modes. Yeah the gain will change as well, but that's nothing the preamp volume knob can't fix.

5) Agreed. Cathodyne and paraphase would make this amp too easy to distort most likely, LTP will give you decent headroom before the amp starts singing.

6) With the amount of gain stages you have (or more specifically don't have), I'd go with a 12AX7 to try to incur maybe a bit more distortion from the PI/PA section. 12AT7 isn't much of a "tone" tube and will give you virtually no PI distortion, something I think you do want in this amp (especially with the BMP tonestack!).

7) I /hate/ wasting extra triodes in tubes. I feel like I'm literally wasting tubes when I put them in the V2 position in TW's because they don't use that second half. I'd at least look at making a switchable cathode follower from that second AX7 half. Like I said before though it's probably not necessary and that BMP tonestack isn't super lossy I don't think, but it might be a nice flavor to add. Or you could try to sneak in another tube and fit in reverb, but I don't ever use reverb on my amps..

No problems other than that it is hard on the switches. If you haven't already read Merlin's article on this: http://www.freewebs.com/valvewizard1/standby.html. The best place for that switch is on the HV CT ground. With that said, I build my vintagey Fender stuff with standby switches like this and it hasn't ruined even the cheap heavy duty Chinese switches I use. Not yet at least.. If it's an original design or anything other than a clone, I usually just leave the standby switch off..

[tubeswell]

Thanks for those comments Cliff.

I ended up using a 180k 1W across the stand by as per Merlin's suggestion. This allows a slow warm-up (I notice that the idle voltages throughout the amp climb up to a 'low peak' after the main power is switched on , and then up to full power with the standby switch flipped).

I went with the 12AU7 triode CF after V1 and it sounds sweet and thick/syrupy clean tone-wise. I have also subbed a 12AT7 in the LTP and it sounds even sweeter (For some reason, 12AT7s sound remarkably sweet in LTP where I have tried this - admittedly only in 2 amps so far), albeit with the expected loss in gain compared to a 12AX7 in there.

In triode mode it sounds very good also. I need to post some soundbytes once I have done those voltage tweaks I mentioned in my previous lost.

I decided to go with the 5G9 type trem (using 2 triodes) in the end as this is my favourite trem for fixed bias 6V6s in PP - got to have that awesome off-your-face slam

Cheers

Pete

[tubeswell]

how about this one from Merlin?

Cheers
Shane

I tried this tone stack today. very nice

[martin manning]

I decided to go with the 5G9 type trem (using 2 triodes) in the end as this is my favourite trem for fixed bias 6V6s in PP - got to have that awesome off-your-face slam

Looking good, there, Pete! Any issues with the trem re the high cathode-to-filament voltage on the follower? I thought the 5F6-A CF was pushing it at 180V... this thing is up to 260 on the 5G9 schematic!

[tubeswell]

I decided to go with the 5G9 type trem (using 2 triodes) in the end as this is my favourite trem for fixed bias 6V6s in PP - got to have that awesome off-your-face slam

Looking good, there, Pete! Any issues with the trem re the high cathode-to-filament voltage on the follower? I thought the 5F6-A CF was pushing it at 180V... this thing is up to 260 on the 5G9 schematic!

No I've never had a problem with this as long as you find a NOS 12AX7 to drive it. Doesn't matter if its a used or microphonic 12AX7, because its not in the signal path. (This is where I use up old 12AX7s)