FB question

[Chuck H]

I designed a2xel84 amp that I really like. So does a friend of mine. He asked me to build him one but with 4xel84's so he can get better clean volume for gigs.

The two tube model uses the 8 ohm tap for the FB loop. When I built the 4 tube model I wanted to keep all the parameters the same so I moved the FB loop to the 4 ohm tap. This is giving me the same FB voltage as the two tube model, all other FB loop part values being the same of course. But my question is...

After I did it, it occured to me that none of the vintage amps make this distinction for the FB loop. Why not? All the vintage amps use the same tap for both two and four tube models with all other part values remaining. This does mean that, for example, a 50 watt Marshall 1959 employs half the FB that a 100 watt 1959 does, right? Why did all the MFG's do it this way. Does this somehow make the smaller and larger amps sound and behave more similar than if the FB was the same?

The new amp I built is working and sounding good. I'm just wondering if there's something I've missed.

Thanks

Chuck

[daz]

Maybe i'm missing something here, because it makes no sense what so ever chuck that i could actually explain something to you !!!!!!!! And i'm sure i'm probably missing something and will make a fool of myself. but here goes....when they do that don't they use different values of feedback resistors? I mean, i can't say why they wouldn't just change taps, but maybe doing it by using the same tap and a different value resistor somehow keeps the tone of the loop more similar? Maybe it would require a different resistor even if they did change taps, so they just figure use a resistor thats different enough to make the result the same as the other model even when using the same tap.

Ok, now you can tell me why that was a stupid answer, but hey, whats new?

EDIT: ok, i answered my own question....no stupid, they DIDN'T use different resistors. DOH ! (had a look at a bunch of marshalls just now)

[BackwardsBoB]

I would expect them to use lower wattage transformers, and twice the Ra-a on the OT for the 50W model. It saves lots of money and prevents an easy field upgrade.

I see this on several Carvin models.

[Chuck H]

Yeh... Isn't that somthin'. Twice the power DOES mean twice the voltage. Since the FB loop is a voltage dependant circuit, if all other parts remain the same that means that there is twice the NFB employed in the four tube amps vs. the two tube equivalent models. And this seems to apply universally to most most vintage amps. Further, even with amps that use a different impedance (as is often seen in Fender amps) the MFG just uses the same FB loop and OT tap as for other impedances. So, for example, a Fender Super Reverb model AB763 is using one fourth the NFB as a Twin Reverb model AB763. The TR has twice the watts and twice the impedance of the SR but all the FB circuit values remain the same.?... Huh. Definitely something to pay attention to when trying to clone your favorite tone since, apearantly, the FB loop is widely variable in these vintage amps even though it appears the same on the macro view.

It hardly matters as I took the working chassis for the amp I built over to it's new owners place tonight and it just kills. Lot's of clean tone on tap compared to the two tube model and sounds just as good or better in every other respect. Now he's very anxious for me to finish the cab so he can own the amp. I must say that the OD tone is perfection with his blonde LP Special w/P90's. I built the thing and I don't have tone like this

Chuck

[MWJB]

"Further, even with amps that use a different impedance (as is often seen in Fender amps) the MFG just uses the same FB loop and OT tap as for other impedances". AB763 Fenders used the 9:1 loop on 6L6 amps with 2ohm & 4ohm outputs and 18:1 loops on single speaker 8 ohm 6L6 amps. On 2 output tube amps that equates to keeping the voltage fed back to somewhere around a volt and 1.5 to 2v on 4 output tube amps.

"a Fender Super Reverb model AB763 is using one fourth the NFB as a Twin Reverb model AB763." What, how?

The feedback ratio for both is 9.2:1, the Twin feeds back exactly twice the voltage of the SR, the twin makes twice the power & twice the voltage at the OT secondary, therefore proportionally they are the same.

There's no "right" value for a NFB loop, they function over a huge range (5.5:1 to 100:1 will "work" on a typical Fender) and are voiced by ear to taste. Big manufacturers won't tweak every loop for every combination of impedance, power, speaker configuration as this means ordering more lines of parts & less profit, when 2 or 3 options will cover all eventualities.

[Chuck H]

"a Fender Super Reverb model AB763 is using one fourth the NFB as a Twin Reverb model AB763." What, how?

Ok... It's a 1/2 ratio cut on the impedance and a 2x multiple on watts. This doesn't make it one fourth, true. The SR and TR both use the same PI and the same loop circuit values. The SR is 40 watts (roughly) at 2 ohms (about 9 volts) the TR is 80 watts (roughly) at 4 ohms (about 18 volts) At a 9:1 ratio that's one volt fed back for the SR and two volts fed back for the TR. I get it. I was just thinking too fast.

the twin makes twice the power & twice the voltage at the OT secondary, therefore proportionally they are the same.

To quote another "old timer", What, how?.. As I said before, the FB loop is a voltage dependant circuit. The preamp and PI don't know or care how many power tubes are in operation, with the exception of the FB loop.

There's no "right" value for a NFB loop, they function over a huge range (5.5:1 to 100:1 will "work" on a typical Fender) and are voiced by ear to taste.

Wow. I hope it's not unclear that I understand that.

Big manufacturers won't tweak every loop for every combination of impedance, power, speaker configuration

Why the hell not?

as this means ordering more lines of parts & less profit, when 2 or 3 options will cover all eventualities.

Hogwash. It's a single resistor value.

Chuck

[black_labb]

double the power isnt double the voltage, so its not halving and doubling the nfb.

40w into 4ohms is 12.65vrms
80w into 4ohms is 17.9vrms
more like 0.7 and 1.4x so its not as dramatic, but there is still a difference in the amount of nfb the circuit has.

they probably didnt tweak the nfb much as the amps were never designed to break up, and aslong as the amps weren't the nfb was doing its job. there wasnt much consideration put into making sure that the amps sounded right at the onset of clipping, which is where the nfb makes the most difference.

[MWJB]

"Originally Posted by MWJB
as this means ordering more lines of parts & less profit, when 2 or 3 options will cover all eventualities.

Hogwash. It's a single resistor value." - I wouldn't go as far as to say it was hogwash, Fender were pretty cost conscious, the 820ohm dropper was already an ordered part, as was the 100ohm & 47ohm resistors, buying more of these makes them cheaper. Tuning each loop may require values that weren't already on order, therefore making the new part more expensive, whilst eliminating possible saving on the current order part. Not significant for the small scale builder but if you're building tens of thousands of amps and have people examining your profit margins, then it becomes so.

Originally Posted by MWJB
the twin makes twice the power & twice the voltage at the OT secondary, therefore proportionally they are the same.

"To quote another "old timer", What, how?.. As I said before, the FB loop is a voltage dependant circuit. The preamp and PI don't know or care how many power tubes are in operation, with the exception of the FB loop" - All I'm saying is that the ratio is still 9:1 just output voltages & power are doubled on the 80W amp, so are proportionally the same. PI & preamp, number of tubes, are indeed irrelevant.

Black Labb - The SR is a 2ohm output & the Twin is 4ohms, so power & output voltage are doubled in this instance.

[black_labb]

Black Labb - The SR is a 2ohm output & the Twin is 4ohms, so power & output voltage are doubled in this instance.

ahh, thx.