OT wire routing under board

[rp]

I think less is more.

Always!

[Alan0354]

From reading the discussions, all the wiring and shielding methods are to eliminate crosstalk between stages.....more notably from power amp section back to the preamp section or PI section. Is there any amp makers actually induce some positive feedback back to enhance the sound ( intentionally couple signal from later stages to the preceding stage)?

[andyfromdenver]

SteveM, et al. Thanks for the great info.

[Colossal]

From reading the discussions, all the wiring and shielding methods are to eliminate crosstalk between stages.....more notably from power amp section back to the preamp section or PI section. Is there any amp makers actually induce some positive feedback back to enhance the sound ( intentionally couple signal from later stages to the preceding stage)?

Yes, Dumble Steel String Singer. This is a hi-fi technique. Signal from cathodes and plates are fed back to previous stages.

[Alan0354]

From reading the discussions, all the wiring and shielding methods are to eliminate crosstalk between stages.....more notably from power amp section back to the preamp section or PI section. Is there any amp makers actually induce some positive feedback back to enhance the sound ( intentionally couple signal from later stages to the preceding stage)?

Yes, Dumble Steel String Singer. This is a hi-fi technique. Signal from cathodes and plates are fed back to previous stages.

Is this commonly done in OD amps to feedback signal from later stage to previous stage? I am not familiar with hi-fi technique.

I did some search and no luck in finding schematic of the Steel String Singer. But I did find the 5th gen ODS schematic and layout that is good!!!
What is the effect of this?

Thanks

[Colossal]

I did some search and no luck in finding schematic of the Steel String Singer. But I did find the 5th gen ODS schematic and layout that is good!!!
What is the effect of this?

Thanks

https://tubeamparchive.com/files/sss_00 ... 74_781.pdf

[Alan0354]

I did some search and no luck in finding schematic of the Steel String Singer. But I did find the 5th gen ODS schematic and layout that is good!!!
What is the effect of this?

Thanks

https://tubeamparchive.com/files/sss_00 ... 74_781.pdf

Thank you, you are of big help. I don't have much luck in doing searching here, kind of landing on whatever comes up!!!

Yes, the schematic actually showed the coupling. Any other amp particular the OD type have intentional feeding back from later stage to previous stages?

Thanks.

[Colossal]

Thank you, you are of big help. I don't have much luck in doing searching here, kind of landing on whatever comes up!!!

No problem. Sometimes it will take a few searches to find what you are looking for, but it's there. So much so that you will realize it's 2 in the morning and you have to work tomorrow. Lots of great stuff in the archives though.

The SSS is the only amp I've seen in the guitar world anyway that uses positive feedback in places. It is a very clear sound. YouTube Welagen SSS. Jelle Welagen is a member here and builds a mean D-clone. The SSS is a massive clean amp used by SRV. Very hi-fi but still extremely organic. The feedback gets some very special sustain and warmth going. Make you feel fuzzy all over and want to climb into the sound. Huge tone.

[Reeltarded]

Dont use site search

Google like this

(Your search string) site:ampgarage.com

[Alan0354]

Dont use site search

Google like this

(Your search string) site:ampgarage.com

Thanks. I tried a few time, just want to confirm it's like this as an example:

lead dressing site:ampgarage.com

[Alan0354]

No problem. Sometimes it will take a few searches to find what you are looking for, but it's there. So much so that you will realize it's 2 in the morning and you have to work tomorrow. Lots of great stuff in the archives though.

The SSS is the only amp I've seen in the guitar world anyway that uses positive feedback in places. It is a very clear sound. YouTube Welagen SSS. Jelle Welagen is a member here and builds a mean D-clone. The SSS is a massive clean amp used by SRV. Very hi-fi but still extremely organic. The feedback gets some very special sustain and warmth going. Make you feel fuzzy all over and want to climb into the sound. Huge tone.

Yes, I have been reading.

So it's safe to say in most case, we layout and wire to achieve minimum crosstalk between stages in order to let the circuit to make the sound, not from interaction between stages. This is important for me as the approach is completely different.

Thanks

[Leo_Gnardo]

Dont use site search

Google like this

(Your search string) site:ampgarage.com

I'm gonna have to try that, thanks Miles!

[tele_player]

Thank you, you are of big help. I don't have much luck in doing searching here, kind of landing on whatever comes up!!!

No problem. Sometimes it will take a few searches to find what you are looking for, but it's there. So much so that you will realize it's 2 in the morning and you have to work tomorrow. Lots of great stuff in the archives though.

The SSS is the only amp I've seen in the guitar world anyway that uses positive feedback in places. It is a very clear sound. YouTube Welagen SSS. Jelle Welagen is a member here and builds a mean D-clone. The SSS is a massive clean amp used by SRV. Very hi-fi but still extremely organic. The feedback gets some very special sustain and warmth going. Make you feel fuzzy all over and want to climb into the sound. Huge tone.

You're referring to the feedback from the plate of v1b to the cathode of v1a? Looks negative to me.

Robert

[gingertube]

Neither the 1M+0.1uF back to the input tube cathode or the feedback labelled LNFB on that SSS circuit are POSITIVE feedback. Both are NEGATIVE feedback.

When you apply a feedback signal to the cathode which has the same phase as the signal applied to the grid of that same tube, then you are reducing the grid to cathode voltage - that is NEGATIVE Feedback. In particular, it is PARALLEL derived and SERIES Applied, Negative VOLTAGE Feedback.
If I recall my feedabck theory properly, that means the the input impedance (at the stage where the feedabck is applied) is increased and the output impedance (at the stage where the feedabck is derived) is decreased.

POSITIVE feedback is very rarely used, bootstrapping anode loads is one place it is sometimes used but you have to be VERY careful to make sure of stability by either making the forward gain less than one, or by putting a negative feedback loop around the positive feedback loop such that is remains stable.

Examples of positive feedback I have seen are:
JADIS JA80 HiFI Power Amps (4 x KT88), POSITIVE Feedback is applied to the output tube cathodes via a dedicated windings on the output tranny. The amp is kept stable by a combination of Ultralinear Negative feedback and some shunt negative feedback from the output tube anodes back to the driver cathodes. This was probably done to lighten the load on the "wimpy" 12AX7 driver. This is not an example I would recommend to you, due to this, and some more serious problems such as shared 510K grid leaks for each pair of output tubes, these things blow up if you "look at them sidways".

The "WHITE POWERTRON" (look it up if interested) ECL86 (6GW8) Triode/Pentode drivers were used to drive a heap of KT88s. The pentode section were used as cathode followers coupled to the triode sections. The triode section anode loads were bootstrapped off the pentode cathode follower cathodes. This remains stable because the gain in the cathode follower (the forward path) is less than 1.

Many older SS Amps use this technique too, the Voltage Amplifier Stage (VAS) load is bootstrapped from the output. This is the secret of why some early SS amps sound quite tube like lovely. Certainly, replacing that bootstrapped load on the VAS with a current source, kills the tube like "magic" and turns it into another cold sterile and booring SS amp (I know this cause I tried it during my days in the Sandpit).

A big hint on why feedback is rarely used in guitar preamps is that all negative feedback relies on reducing the gain - when a circuit saturates there is no gain (more input will not produce more output) and ANY and ALL feedback function ceases. That is, it only works on the linear part of the transfer function up to the point where the tube saturates (or cuts off) and then works again when the tube comes out of saturation or cutoff. You will not generally not notice this on the initial attack but may notice it on the decay of a note as the signal dies away. My GUESS is that the SSS feedback circuits are attempting to control the harmonic structure on the note decay BUT that is just a guess. In any case the amount of feedack being applied is so insignificant that you might wonder why Howard bothered, and if it actually does anything at all.

Cheers,
Ian

[Colossal]

You're referring to the feedback from the plate of v1b to the cathode of v1a? Looks negative to me.

Robert

Hmm, no, I was thinking the later stages. Maybe the cathode follower after the filter recovery stage? I need to look at it. The LNFB are just that, negative and just flatten things out and reduce gain. I thought there was some positive FB in that amp, but maybe I'm mistaken.