*New Build Thread* 2nd Gen Hybrid 50w footswitchable FET

[amplifiednation]

Hi Taylor, I am in the middle of a 4x6V6 2nd gen hybrid build and was wondering why the gnfb resistor is shown as 3k3 when it is shown as 820R on Tonys 2nd gen layout. Did you ever try the lower value? BTW great build which has inspired me to have a go!

The NFB series resistor works together with the shunt resistor to ground. It's the ratio of those two resistors combined that gives you the total amount of feedback.

So in Tony's layout, the 2nd gen has the typical Fender 820R/100R network, resulting in a ratio of 8.2

In the layout I used the resistor values are 3300R/390R, resulting in a ratio of 8.4, so the two aren't really that different!

I do have some lower values I was going to try, but I am still settling in with the amp and haven't had time to start tweaking...although I do plan to work with it.

Tom - I have the OD relay wired up untraditionally with dual masters (one clean, one OD). I am liking the way it works right now, but might try the global master later.

[dobbhill]

Let's not oversimplify the Phase Inverter: the shunt resistor has a direct effect on the voltages all across the PI.

Of course, I could be wrong.....

D

[amplifiednation]

Let's not oversimplify the Phase Inverter: the shunt resistor has a direct effect on the voltages all across the PI.

Of course, I could be wrong.....

D

I could be too...but I know there is a relationship to the shunt resistor that impacts the NFB resistor and the total about of feedback sent back to the PI. That was the resistor in question.

I'm still slowly figuring all this out...whatever the case, it should be noted that not only is the NFB resistor different on the hybrid layout...the shunt resistor is as well, resulting in similar amounts of NFB being fed back into the PI.

I'm off to do some more PII (phase inverter investigating)

[amplifiednation]

From Aikens site...

The last resistor is usually a small value, such as 5K (Marshall) or 100 ohms (Fender), and is the shunt element of the feedback voltage divider for the global negative feedback loop (pot VR1 in the above schematic).  The feedback voltage applied to the phase inverter is the resultant divided-down version of the output voltage.  This resistor directly affects the amount of negative feedback, and thus, the overall gain of the output section, as well as the linearity, input range, and distortion.  The feedback divider ratio is the ratio between the series feedback resistor (R7) and the shunt feedback resistor (VR1).  The amount of feedback also controls the effective bootstrapped input impedance

[Dr d]

Thanks Taylor.....I am going to fire up my amp today and that was the one thing that foxed me. Cheers.

[amplifiednation]

Thanks Taylor.....I am going to fire up my amp today and that was the one thing that foxed me. Cheers.

Let us know how it goes!! Good luck!

T

[Dr d]

Thanks, Taylor.
Well I "fired her up" yesterday and apart from a few teething problems, she sounds wonderful! Teething problems include inactive presence and bass, and popping relays for OD and FET. My B+ is 440v but still struggled to get my voltages up for V1 and V2.....ended up with 22k to get 177/184 and 189/192 respectively using Electro Harmonix. Oh and mid boost is a bit subtle too. Even in its present state it sounds amazing. Particularly impressed with OD plus FET.......gorgeous! I'll sort out the niggles and report back and post some pictures for the first time!

[Structo]

The Presense control can be a bit subtle but when you crank up the amp you will hear it better.

Same goes for the Mid Boost, if you have it correct it fattens the tone a bit.

Play the amp for 10-12 hours before tweaking the circuits.

As for the relays, which ones did you use and do you have a diode across each coil?

[Dr d]

Thanks Tom.....i'm using 5V omrons and, yes, I have used diodes. Have managed to get the bass and presence working, though the mid control is quite subtle. Overall I am delighted with the tone of this amp....a keeper for sure!

[stevlech]

IIRC, The mid control in the 2nd generation tonestack is particularly subtle and it's caused by the large cap on the bass control. So it sounds as if that is working properly.

[Dr d]

Thanks for that! I was thinking about using this amp with a dumbleator. Is this common practice and if so presumably one would have to put a bright cap across both the clean master and the OD master?

[boldaslove6789]

Thanks for that! I was thinking about using this amp with a dumbleator. Is this common practice and if so presumably one would have to put a bright cap across both the clean master and the OD master?

You can use a 68pF bright cap on each Master (If there are seperate Master Vol) and then use a Shorting jack and a cap (shorting to Ground when) to roll the unwanted highs off when there is no D-lator in the loop.

AFAIK this what config the Ojai Bludo and (RF #102 ODS) has (but with 1 bright cap on the shared Master Vol as its a NON-HRM)

[Dr d]

Thanks for that Greg. Sorry if this has been discussed before but how does the shorting jack arrangement work and what value cap would you use?

[boldaslove6789]

Thanks for that Greg. Sorry if this has been discussed before but how does the shorting jack arrangement work and what value cap would you use?

Check out this thread/page, might point u in the right direction.

https://tubeamparchive.com/viewtopic.ph ... &start=105

You can also do this ("the RG-400 Simulation cap methodology")

Take a 68 pF bright cap, and put it across both Master Vol's, and then expiriemnt with a few cap values on the Effects Send "tip" to ground "Ring" (to roll off the unwwanted highs when not using a D-lator). I would start at .0022uF cap.

Then once you find the amount of high end you like rolled off, Take the cap and put it on the Send "tip" contact, and then the other side of the cap to the Ground of the FX send "Sleeve" using a Stereo Shorting jack. That way when the effects Send is plugged into the Lator it will disconnect the cap, and when there is nothing connected to the FX Send, it will shunt the cap to ground (rolling off the unwanted highs produced by the 68pF capacitor (I prefer a ceramic cap, as do others.)

A lot of us use some RG-400 cables from the Loop Send and Return to the Lator anyways, this will also have the same effect as the cable simulation cap (rolling off highs). Usually most RG-400 cable (can be found at Air Craft Surplus stores) will usually be around 30pF per foot, so you have to do the math and figure out hhow much highs you'll want to roll off by calculating how many Feet of RG-400 cable will equal the amount of your simulation capacitor.

hope that helps

[martin manning]

Usually most RG-400 cable (can be found at Air Craft Surplus stores) will usually be around 75ohm per foot, so you have to do the math and figure out hhow much highs you'll want to roll off by calculating how many Feet of RG-400 cable will equal the amount of your simulation capacitor.

RG400 cable is 50 ohm impedance regardless of its length, but that is not at all important for audio frequencies. It's capacitance is spec'd at 29.4 pF/ft.