Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: rake on July 08, 2018, 11:28:22 am
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Just a question (two questions actually) to pick the big brains here. (some of y'all just amaze me)
Okay, tonestacks, I see studying schematics that some stacks are right in the beginning
of the chain. (typically right after V1a) Others are way down the chain just before the PI.
1. Advantages / disadvantages of either?
2. has anyone ever tried doing both on the same chain?
Thanks in advance!
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I started like most with the TS up front, as I did more, I went mar_shelly an put at the end, liked it, ain't went back . I typically follow that with a DC coupled gain/CF to drive the PA
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. . . has anyone ever tried doing both on the same chain?
Statistically speaking no one puts more than 1 TS within an amp. "Exceptions" are a presence control or other variable NFB; but these are not TS's per se.
Additional EQ is common in a pedal chain and/or effects loop. However, these too aren't usually TS configurations per se. The main reason for these is to control overdrive tone. Too much bass > overdrive causes muddiness. Then, OD produces lots of harmonics (hi's). So it's desirable to cut mids & lows > overdrive; and to cut hi's afterwards.
Location of the TS in the preamp topology is significant tone-wise. See: Dave Funk's Tube Amp Workbook, Ch 13 The Preamp, p. 100. Ex's of gain stages, TS + any more gain stages:
VOX AC30 1+0
Fender 1+1
Marshall MV 4+0 (Allman Bro's)
Soldano SLO 100 5+0
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For a weird one, check out the Bass channel on the Blonde 6G6B Bassman
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Merlin on his preamp book explains that as TS drop the signal
the difference between V1a > V1b > TS versus V1a > TS > V1b
is that on the first configuration a higher level of signal drive the second triode with all the imaginable effects (V1b distorts earlier)
I hope I was able to explain this thing in an understandable way
Franco
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There's a difference before vs. after distortion. Frequency content changes with distortion, adding higher harmonics... preventing harmonics from being created has a different effect than trying to filter them out, once they overlap with other parts of the sound.
This is why high-gain amps reduce bass early with coupling and bypass caps, to prevent "farty" overdrive which is hard to fix afterwards. Usually done with bypass and coupling caps, not a tonestack (or anything else user-adjustable).
One benefit to having the tonestack later is the effects directly make it to the speaker, instead of being distorted. And working with the post-clipping frequency spectrum is just different and creates a unique set of possible sounds.
Technically, putting the tonestack towards the end of the preamp allows the most gain through the preamp and the possibility for lots of distortion. But most designs aren't maximizing available gain (especially in the earliest, clean stages) or if you're building your own, you can add another tube for more gain. No need to locate the tonestack just for that purpose. I think the reason high-gain amps put it later has more to do with the effect on the sound than gain or signal strength.
If this is a clean preamp, you won't notice as much of a difference. Though a DC-coupled cathode follower (which is often used to drive tonestacks) introduces its own type of mild distortion, typical of "clean" Fender designs for example.
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In my gigging days when I was using a home brew with a high gain preamp, I've put 2 tone stacks in the high gain channel.
It was built in my Fender 75 chassis, which was Fender's take on a MkIII Boogie.
I made it a lot more like the MkIII.
So the low gain and high gain preamp shared the regular Fender input stage - tone stack and vol controls (front panel) - 2nd stage.
The high gain circuit then took that signal via a gain control to a couple more cascaded stages, tone stack (back panel) and a channel master vol.
A relay were used to switch between the 2nd stage output and the high gain channel master vol.
This then fed an fx loop, reverb circuit, and power amp.
The second tone stack was simple, my own design, just facilitated variable amounts of boost to the treble and bass ranges, and was based on the treble and bass boost already in the 75; more aimed at fine tuning rather than a radical re-shaping.
I didn't want to use presence and resonance controls as I felt that they would mess with the regular Fender tone of the low gain channel.
To my mind, the 2nd tone stack was taking the place of the (optional) graphic eq of the Mk series, ie they allow the final tonality of the overdrive to be tweaked.
The regular 1st tone stack controls allowed the depth of overdrive in their frequency bands to be adjusted. So they eq'd the guitar to the overdrive, and the 2nd tone stack eq'd the overdrive to the fx (delay) / speaker / room.
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One advantage of having the TS after the first stage is that you get more tonal change when you adjust the controls. If you have it later after several gain stages and a CF, the controls aren't as effective as the gain levels are higher and they can't affect a change as easily. Notice the difference with a cranked BF Fender vs a cranked 5F6A Bassman. The BF amp can have a large tonal change even when cranked. If you design in a 1 + 2 or 1 + 3 arrangement you can get plenty of gain, and adjust the earlier stages for less bass reproduction so you don't fart out the tone overall. Many ways to design it to achieve similar goals.
Greg
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One advantage of having the TS after the first stage is that you get more tonal change when you adjust the controls. If you have it later after several gain stages and a CF, the controls aren't as effective as the gain levels are higher and they can't affect a change as easily. Notice the difference with a cranked BF Fender vs a cranked 5F6A Bassman. The BF amp can have a large tonal change even when cranked. If you design in a 1 + 2 or 1 + 3 arrangement you can get plenty of gain, and adjust the earlier stages for less bass reproduction so you don't fart out the tone overall. Many ways to design it to achieve similar goals.
Greg
This is my preference. Having a plate driven tonestack and I do like the 6G6B Normal Channel Bassman quite a bunch. Right after is 2 more stages that can be used. I think it has to do with the amp builders request. A lot of players simply prefer the Feel of a CF Stack. I know there is music I have played where it was necessary. Basically, Les Pauls into Marshall was very popular in the South with Allman Bros and Marshall Tucker seemed to crank them well. It is different for sure. Do these respond to eq like the AB763 Tone Stack. Not much does in the guitar world. Keeping in mind I really do not need that much Bass or Treble, I like it a lot.
Trainwrecks have a bit of gain and a very responsive stack.