Bandmaster FB loop question

[Sir Cuitous]

I have been in a holding pattern as far as building anything new, so in the meantime I hope my "theory" questions aren't too mundane. The "wheels" are always turning and I may be tackling a couple of tweed amps soon.

My boss is interested in having me build an amp for him. It may be a few months off at this point, but he is interested in a tweed bandmaster. There is a feedback loop in the 5E7 that uses a 10M resistor. Can that loop be omitted? With a large resistor it can't be looping that much signal.

Can it be tweaked in anyway using a cap or pot? It seems to loop after the treble control - am I reading the schematic correctly?

[Enzo]

You haven't built the amp yet, why are you looking to eliminate that one part from the circuit?

Remember, signal is voltage like anything else, And across a resistor, voltage is dropped with current. You can't say that not much will happen because the resistor is large. You have to look at the circuit. From the output of that 12AX7 gain stage with CF, that 10M resistor sees somewhere as much as 600k or so or less to ground. SO by voltage division, depending upon volume control settings, you could see as much as 5% of your segnal fed back. That doesn;t sound l ike nothing to me.

Not my area of expertise to discuss the differences in sound and dynamics, but the with or without experiement can be done in seconds.

As far as tweaking, amps are amps, you do the same stuff to all of them to massage the tone. Add a brite cap here, add a rolloff cap there, alter the signal level here and there.

[booj]

Can that loop be omitted? With a large resistor it can't be looping that much signal.

Can it be tweaked in anyway using a cap or pot? It seems to loop after the treble control - am I reading the schematic correctly?

Sure, it can be omitted
No,it's not looping that much signal
Yes it can be tweaked. Don't know which cap or where, but a pot could bring it into play,.... maybe a 10 meg pot? or .. what was the question again?
It seems to loop BEFORE the treble control has anything to do with the signal.
The whole bass circuit is in between the feedback loop and the grid of the next stage
Is this a test?
I'm going to stop now.

[Gaz]

Is this a test?

B+ (pun intended)

[TD_Madden]

why not just use the narrow-panel Bandmaster/Super's 56K with a presence control?

[Chuck H]

To follow what Enzo was saying... If that 10M resistor IS passing 5% of the voltage from that point, no that doesn't sound like much. But this is a feedback loop. That amount being passed is relative to the voltage at the point where it terminates. So, for example, if there are 2 volts in front of the loop, and 20 volts at the end of the loop, thats 1 volt being fed back into 2 volts. Thats a substantial amount of feedback. This is just an illustration and not the actual numbers I'm sure. Also, the treble control is in the loop. So this is altering the 'fed back' portion of the signal. Whatever treble is omitted by the control is not being fed back. If the feedback loop ratio is substantial, as mentioned above, this could make for a very specific preamp voicing. So omitting that resistor could very notably change the tone of the amp. So if your guy wants a 5E7, build a 5E7. But as Enzo said, for experimental purposes you could easily try it both ways.

Chuck

[Bruce / Mission Amps]

I have been in a holding pattern as far as building anything new, so in the meantime I hope my "theory" questions aren't too mundane. The "wheels" are always turning and I may be tackling a couple of tweed amps soon.

My boss is interested in having me build an amp for him. It may be a few months off at this point, but he is interested in a tweed bandmaster. There is a feedback loop in the 5E7 that uses a 10M resistor. Can that loop be omitted? With a large resistor it can't be looping that much signal.

Can it be tweaked in anyway using a cap or pot? It seems to loop after the treble control - am I reading the schematic correctly?

If you look at all three of the narrow panel tweed amps you'll note that the local feedback resistor varies from 4m7 to 10m... as does the preamp tubes from 12AY7s to 12AX7s.
Stick to the stock set up first so you know what it is supposed to sound like as that resistor does make a difference in tone but can be tweaked to taste or eliminated.

[Sir Cuitous]

I must be getting somewhere with this stuff because these replies make a lot of sense. I will have to see for myself what happens - I guess it will be a test after all. 56k is not right for this FB loop. And this amp does already have the 56k loop from the OT secondary. If it sounds good without the 10M loop, I may try using a pot to divert some or all of the NFB to ground. Thanks for helping me understand this part of the circuit.