understanding the basic SSS circuit concept

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idnotbe
Posts: 24
Joined: Sat Mar 18, 2017 2:25 pm

understanding the basic SSS circuit concept

Post by idnotbe »

i am very new to the circuit world and seeing M Hartman's SSS schematics.
though i'm not sure that i am reading the circuit correctly, i have too much curiosity.

1. i find 2 triodes where the reverb signal and the main signal are mixed. (the yellow circil in the below schematics)
comparing to Fender Twin Reverb which has only one triode for the function, why there are 2 ??

2. Twin Reverb's signal goes to the reverb circuit after 1st gain stage -> tone stack -> 2nd gain stage.
but SSS goes to the reverb circuit before the 2nd gain stage. (1st gain -> tone stack -> split into the reverb & the main signal)
why??

3. from the player's perspective, reverb is just one of the time-based effects related to fx loop.
but in SSS, the fx loop is passive and does not use the send / return / mixing circuit. (the reverb exclusively uses the circuit)
why should the fx loop be treated independently??

4. i have the #002 layout (but not the schematics) and it has 5 pre-amp tubes.

- pre-amp 1 (12AX7)
- pre-amp 2 (12AX7)
- reverb 1 (12AX7)
- reverb 2 (12AX7)
- reverb tank driver (12AT7)
- phase inverter (12AX7) -> power amp
- power amp driver (12AT7) -> power amp

Hartman's seems to have 4 pre-amp tubes.

- pre-amp1 (1st and 2nd gain stage)
- reverb 1 (reverb send & return)
- reverb 2 (i don't know how can i call it. in between send & return)
- mixing stage

what's the difference? why one has 5 and the other has 4?
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mabruk
Posts: 16
Joined: Wed May 09, 2012 8:57 pm

Re: understanding the basic SSS circuit concept

Post by mabruk »

I will try to answer in the way I better can or know.

1. The fact of using 2 triodes or 4 in order to mix both dry and wet signal is just an option. Another option equally valid is by using just only one triode

2. The point where to take the signal sample from in order to trigger the reverb tank is also optional. I would do it as is done in the Twin Reverb circuit
in order to save one triode, since you can then go directly to the 12AT7 reverb driver from there.

3. There is no FXloop in the SSS nor in any Dumble amp. What you see is just a signal access but without a buffer+attenuator plus a recovery stage.
There are many simple ways to solve that problem.

4. There is no much difference, the original design is just a waste of resources.

In reality the major difference in a SSS amp in relation to a Dumble ODS is the reverb. For a 50w amp with two 6L6GC you don't really
need the Power CF drivers, neither in the case of four 6L6GC in a 100w amp. It could be convenient in the case of using 4 x 6550 valves
for 150w power amp.

The job done by the Hi and Low filters is unnecessary since there are many EQ options already in the Dumble ToneStack, so you can also
save the triode employed to just recover the losses introduce by those filters.
Last edited by mabruk on Fri Sep 29, 2017 8:49 am, edited 5 times in total.
talbany
Posts: 4679
Joined: Tue Mar 28, 2006 5:03 am
Location: Dumbleland

Re: understanding the basic SSS circuit concept

Post by talbany »

IMO..This is the best method of mixing in the reverb, sometimes referred to as a "Tweed mixer".
It does use 2 tubes for an extra gain stage and cathode follower to mix both dry and wet signals.
Since the signal is mixed at the follower end there is little to no loading at the PI entrance :D . Both gain stages before the follower assures that there is plenty of signal to drive the followers so the dwell option actually works pretty good..Since the send is off the follower (low impedance) this also helps to drive any outboard gear you put in the loop :D ..This is by far the best natural sounding and functioning reverb circuit I have used out of the 3 you mention and quite possibly the best amp w/spring reverb I have played through.

Hope this hepls

Tony
sss__002_revision_6_147.bmp
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" The psychics on my bench is the same as Dumble'"
mabruk
Posts: 16
Joined: Wed May 09, 2012 8:57 pm

Re: understanding the basic SSS circuit concept

Post by mabruk »

I have made one that works and sounds the same with the advantage that you save two preamp valves,
plus their circuit and componentes, and also have a proper FXLoop.
Last edited by mabruk on Fri Sep 29, 2017 7:47 am, edited 1 time in total.
tictac
Posts: 610
Joined: Fri Mar 31, 2006 4:42 am

Re: understanding the basic SSS circuit concept

Post by tictac »

do you have a complete schematic of this? :?:
mabruk
Posts: 16
Joined: Wed May 09, 2012 8:57 pm

Re: understanding the basic SSS circuit concept

Post by mabruk »

No. Not yet
Bear
Posts: 333
Joined: Tue Mar 29, 2005 4:38 am

Re: understanding the basic SSS circuit concept

Post by Bear »

talbany wrote: Fri Sep 22, 2017 10:26 am IMO..This is the best method of mixing in the reverb, sometimes referred to as a "Tweed mixer".
It does use 2 tubes for an extra gain stage and cathode follower to mix both dry and wet signals.
Since the signal is mixed at the follower end there is little to no loading at the PI entrance :D . Both gain stages before the follower assures that there is plenty of signal to drive the followers so the dwell option actually works pretty good..Since the send is off the follower (low impedance) this also helps to drive any outboard gear you put in the loop :D ..This is by far the best natural sounding and functioning reverb circuit I have used out of the 3 you mention and quite possibly the best amp w/spring reverb I have played through.

Hope this hepls
Tony, I had always understood the "tweed mixer"/anode mixer to rely on a shared plate resistor that basically auto-mixes the two stages. It seems the point of the SSS plate&CF stages is, as you say, about low impedance output. Or does it also have the effect of evening out the output level?
mabruk
Posts: 16
Joined: Wed May 09, 2012 8:57 pm

Re: understanding the basic SSS circuit concept

Post by mabruk »

talbany wrote: Fri Sep 22, 2017 10:26 am ..Since the send is off the follower (low impedance) this also helps to drive any outboard gear you put in the loop
I don't think you can take any advantage out of the followers low impedance to drive the loop since it is placed miles away from them.
You have a 220k resistor at least and then 1M pot in the middle. So all that resistance together with the parasitic capacitance found in
any shielded wire will make up a low-pass filter able to push down most high frequencies in the signal, no matter if the original source was high
or low impedance. That's a largely seen mistake in many Dumble amps, even using Dumbleators, since they would just help but the original
problem remains.

If you want to properly drive a loop you need to mix those signals from both 220k resistors in a small resistor of say 4k7 or 10k as a maximum in series with a big capacitor to keep low frequencies, otherwise you will also overcharge any device connected to the loop. Then you need to amplify the signal in the return with a valve or any other active device in order to restore the signal loss due to the voltage divider. Basically you need a very low resistance just at the send output which is the only way to avoid high frequency losses by the external cables through the loop.
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