Can anyone explain me this Loftin White amp?

[Giaime]

Hello everybody!

I've been struck by this circuit in which there are some things I can't understand. I must premit that I'm not absolutely going to build this amp nor prototype it, I'd just like to understand how it works.

Here it is:
http://tinpan.fortunecity.com/saints...mer/2a3-se.gif

The question is: What is the purpose of the 33k resistor between B+ and the cathode of the 2A3?
After some hours of math yesterday evening I concluded that IT DOES NOTHING, and that's not even a faliure-protection in case of the 6SL7 missing from the socket.

Can someone enlighten me? I've also seen circuits very similar to this where that resistor is not there.

Thanks in advance!

[SY]

I can't seem to get that link to work for me- I get a "denial" error message.

[Giaime]

Oh, I'm sorry. Please try this:
http://tinpan.fortunecity.com/saints/668/primer/

And click on "2A3 Loftin-White SE amp".

Sorry

[Nigel Goodwin]

Originally posted by Giaime

Can someone enlighten me? I've also seen circuits very similar to this where that resistor is not there.

I would suggest it's there to increase the positive bias on the cathode - like you say, it's probably not really required.

BTW, I got the same error message, so navigated to the page from further back on the site.

[SY]

I would suggest it's there to increase the positive bias on the cathode

Doubtful. It's got maybe 1.5mA flowing through it. No significant effect on the bias. I haven't scribbled around with this yet, but I'll bet that it either is part of a hum cancellation or DC feedback. Giame, did you draw out the Thevenin equivalents?

[poobah]

Hmm....

I bet that resistor is for "bootstrapping", it ensures that there is negative grid bias during warm-up?

Do DHT's take longer to warm up?

Giaime... I have seen your picture... you are young... you live in Venice... you should not be home, studying strange SE schematics on Friday night! Make the girls pay...

[Giaime]

I'm trying to understand, but:

1) Hum reduction shouldn't be possibile since 33k makes a too strong voltage divider, and obviously the cathode is bypassed to ground, so no injection of PSU hum at the cathode is possible.

2) DC feedback: when the amp is operating properly, Vak for the upper triode of the totem is fixed and independent of tube aging and other things (or not?), so Vg for the 2A3 is fixed. If we make the assumption that B+ doesn't vary, a decrease in cathode current of the 2A3 (tube aging) lead to the usual decrease of Vgk for the 2A3, like EVERY cathode biased triode of this planet and others. (the 390k grid resistor is TOO large to provide some variation to the Vg, that is mainly imposed by the totem).

3) protection: I've also thought about protection for the 2A3 when, accidentaly, the 6SL7 isn't in its socket. We will have at power up 0V at the 2A3 grid, but since there's the 33k in there we'll have something like 400V/85390ohm = 4.6mA (where the ohm value is the equivalent of the 2 parallel 100k in the PSU, the 33k resistor and the cathode resistors of the 2A3). Maybe more because at startup B+ could be much higher (if the PSU has a resistance of 200ohm, B+ could be somewhere between 420 and 430V).

So let's assume 5mA flowing in the 33k resistor: V = IR = 415, so cathode sits very very close to ground, like I guessed, just like the 33k resistor ISN'T there.

The 2A3 has now 420Vak with 0Vgk, it will start heavy conduction that will obviously counteracted by the cathode resistors, my guess is that even without the 6SL7 in the socket the voltages will be the same, since the grid is connected to the series of 820ohm resistors so it will have at 60mA something like 98V, just like if the 6SL7 was there.

I'm starting to think that the resistor is there to make a very light DC feedback to fix the voltages of the totem with those of the power stage, and the opposite (since the actual current in the 2A3 *could* influence totem voltages and so current in it).

But I'm not sure...

Edit: another thing crossed my mind. Maybe the resistor is just a leftover from a version of this amp that DIDN'T have a totem input stage (in which voltages are rougtly independent of the variations in tube parameters), but a standard grounded cathode stage (R loaded), where actual plate voltage is heavily influenced by the parameters of the input tube...

[poobah]

X-post...

OK... so it wasn't Saturday night... sorry...

[Netlist]

Just handing over these links… for what it’s worth.
http://home.earthlink.net/~jeremyepstein/freelunch.html
http://boozhoundlabs.com/monkey/

/Hugo

[sawreyrw]

The schematic is a little confusing the way its drawn; however, it is clear that the 33K resistor does effect the DC operating points of the circuit. It has no role in AC gain.