Neg Feedback Tweed Pro and Bandmaster

[buickwilson]

The tweed Pro has a 100K resistor in the NFB on the PI, and 5M on the 2nd gain block. The Bandamster has 56K on the PI and 10M on the 2nd gain block.

My 58 Bandmaster came with a 5M in the 2nd gain block and a 56K in the PI. I'm guessing if i change the the 5M to a 10M, it will give me a little more gain through the tone circuit, which I'm not sure I really need. Anyone play with these values?

[tlpruitt]

You are on the right track with those 2 resistors.

I just finished building a 5E5A pro and I initially used 100k for the NFB and 5M for the second stage feedback. I thought it needed a bit more headroom so I switched the NFB to 56k and second stage feedback to 10M and that did the trick. The result was more headroom, a tighter low end and it went into overdrive a little more gracefully.

I am thinking about putting a 50k linear pot in series with the 56k NFB so I can make the NFB adjustable from 56k to 106k.

BTW, I also did the "Paul C" mod to the phase inverter and that REALLY made a difference. I think the Paul C mod is hands down the best mod for amps with the type of phase inverter found in these amps (5E5A, 5E7, Princeton, etc.). It is super easy and it improves the tone and overdrive character of the amp tremendously. This thing can compete with my 5F6-A bassman now.

-Tim

[dukeh62]

BTW, I also did the "Paul C" mod to the phase inverter and that REALLY made a difference. I think the Paul C mod is hands down the best mod for amps with the type of phase inverter found in these amps (5E5A, 5E7, Princeton, etc.). It is super easy and it improves the tone and overdrive character of the amp tremendously. This thing can compete with my 5F6-A bassman now.

-Tim

Tim...sorry to get off-topic, but I just PM'd you with some Paul C mod questions...thanks!

[Wakarusa]

If memory serves the OT in the Pro and Bandmaster are different. The Pro expects a single 15", the Bandmaster 3x10". The different global negative feedback resistors (56K vs 100K) should actually provide about the same NFB levels given the different signal level present on the two OT secondaries.

As for the 5M vs 10M local NFB in the preamp you can test to taste (a decade box helps) or you can noodle the math and figure the right value for the closed loop gain you want out of the stage. I prefer something around 6M8 or 7M5.

[buickwilson]

Todd,

I think you're right... Guessing Leo did a little math to match the NFB in the PI to handle the slight mismatch of the speaker load (2.6 ohm) with a 4 ohm OT. And then he compensated the gain a little by reducing the NFB in the preamp.

[buickwilson]

follow-up..

So on the 58 Bandmaster 5E7 I tried both a 10M and 5M in the preamp and 56K and then 100K in PI section.

I think the best sound overall is keeping the stock configuration per the scheamtice of 10M in the preamp (btw, this amp came to me with 5M here which is what started this thread). amd 56k in the PI section.

I did like less NFB (ie using a 100K resistor) in the PI section to get a smoother creamer transition from low volume (vol pot backed off the strat) into distortion, but it comes at a cost of a little bit of mush, less defined bass and treble. The stock configuration is a little tighter which is a good thing with 40 year of loose P10Rs and a ~30 watt amp.

Thx all for your input...