Super reverb 4 ohm speaker load.

[Hella1hella]

Well, it looks like the fun is over for a bit! The amp has been giving me some shocks and one of the power tubes wont light up unless given a (very small) tap.... BUMMED! Just got new power tubes and grounded cable too... I am taking it in tomorrow and hopeing the tech gives me a discount since the amp was JUST in his shop a month or two ago to get everything I mentioned done.... He is known as the best tech in the area so I am hopeing its just something he overlooked and nothing too serious... Serious means $$ and $$ =new cabinet and speakers so I can A/B/Y this guy with my bassman!

[hasserl]

This thread is kind of dated now, but just for reference, you could replace the 6L6GC power tubes with JJ's 6V6S power tubes (with a rebias), which will match up impedance wise with the two 8 ohm speakers. It also will quiet the amp down a little making it more useable in most places. And you'll get a little more grind out of it as the 6V6's break up earlier than the 6L6's, if you like that sort of thing (I do!).

[Tbarshowdog]

I have a 65 Super and run 4 Celestion GOLD at 8ohm total. Then a Open Back Cab with 2 Celestion Vintage 30 at a 4ohm load connect to ext speaker jack. Runs my Amp with the 2-12 at total of 4ohm load and it Screams Big Time. Love the Gold and 2-12 Vintage 30 Fill bottom end and chime Top end from Gold 4-10. Does not hurt the Amp like that and sounds Amazing. Any one tried this like me ? Also tried the 2-12 Cab with G12M Greenbacks 8ohm Cab per 2-16ohm speakers wired to 8ihm load. and Also sounded amazing. Comments ?

[g1]

I have a 65 Super and run 4 Celestion GOLD at 8ohm total. Then a Open Back Cab with 2 Celestion Vintage 30 at a 4ohm load connect to ext speaker jack. Runs my Amp with the 2-12 at total of 4ohm load and it Screams Big Time.

If the internal speakers are wired for 8 ohm total, and you add a 4 ohm ext. cab, the total load becomes 2.67 ohms.

[Helmholtz]

Confused.

An original BF '65 SR (AB763) has a 2 Ohm output and its internal speakers are wired for 2 Ohm.

[SoulFetish]

You’re correct about the secondary Helmholtz

as Far as screen dissapation goes, don’t forget, when assessing screen dissapation under the higher load impedance, the amp will be most likely operating in the class B/cutoff portion of the duty cycle during heavy overdrive and the loadline shifts vertically in a dramatic way to 1/4th the P-P Impedance.

[Helmholtz]

You’re correct about the secondary Helmholtz

as Far as screen dissapation goes, don’t forget, when assessing screen dissapation under the higher load impedance, the amp will be most likely operating in the class B/cutoff portion of the duty cycle during heavy overdrive and the loadline shifts vertically in a dramatic way to 1/4th the P-P Impedance.

Not sure, I'm getting your point.

A fixed bias class AB amp runs in class B most of the time. That's why OT primary impedance is always chosen for class B.
A 6L6 in a SR wants to see a plate load of 1.1k. So the OT has to provide a total primary impedance Zaa of 4.4k (factor of 4 for class B).

The "class A" region, where each tube sees 2.2k is very small, maybe up to a couple of watts, depending on bias.

[SoulFetish]

Not sure, I'm getting your point.

Yeah, after reading how my post is written, it sounds like my comment about screen dissipation was directed at you, or in response to something you posted. It wasn't.
I simply mentioned at the beginning of my post to reiterate that you were right about the OT and speaker load, and I wasn't sure anyone provided a solid reasoning for pushing the output stage beyond it's designed operating area by purposefully mismatching the load impedance.

A fixed bias class AB amp runs in class B most of the time. That's why OT primary impedance is always chosen for class B.
A 6L6 in a SR wants to see a plate load of 1.1k. So the OT has to provide a total primary impedance Zaa of 4.4k (factor of 4 for class B).

Obviously, most of the power in a class AB amp is generated when operating in class B. That's why it's a thing. There are several factors that go into choosing the primary load impedance. The class B load line is clearly an important one. But I think you over state it. I can find no example in any tube data sheet listing common operating conditions, where the load impedance is listed according to it's class B operation (regardless of class of operation - class AB, or otherwise).
But my point was in reference to the relationship of the screen dissipation curve to the 0Vg1 plate curves. However, it was not intended to be directed at you, if that's how it appeared.

[Helmholtz]

I can find no example in any tube data sheet listing common operating conditions, where the load impedance is listed according to it's class B operation (regardless of class of operation - class AB, or otherwise).

All datasheet examples for class AB operation I've seen list the recommended Zaa for class B.

When you do a load line construction to find the optimal plate load Za for one tube in a class AB amp, there's some variation depending on operating conditions.
But Zaa is always found by multiplying Za by 4 and that assumes class B operation.

It's high output where correct plate load matters most.

Even the Vox AC30, which has a significant class A range, uses the class B primary impedance of 4k. The Zaa for pure class A would have to be 2k for 4xEL84.

[glebert]

FYI. You can get Jensen MOD speakers in 10" 4 ohm and they are cheap. Some people have tried them with classic Fender designs and like them.

[SoulFetish]

All datasheet examples for class AB operation I've seen list the recommended Zaa for class B.

When you do a load line construction to find the optimal plate load Za for one tube in a class AB amp, there's some variation depending on operating conditions.
But Zaa is always found by multiplying Za by 4 and that assumes class B operation.

It's high output where correct plate load matters most.

Even the Vox AC30, which has a significant class A range, uses the class B primary impedance of 4k. The Zaa for pure class A would have to be 2k for 4xEL84.

okay, yeah, I see what you mean. You are going to go for the knee of the curve. Good point. Even when I might use the class A load to set the bias and operating point, I'm assuming the class B Load to account for it.

[Helmholtz]

okay, yeah, I see what you mean. You are going to go for the knee of the curve. Good point. Even when I might use the class A load to set the bias and operating point, I'm assuming the class B Load to account for it.

Just take a look at the EL84 datasheet.

Recommended Raa for class B: 8k, recommended Raa for class AB: 8k, same plate and screen voltages.
Means that the optimal plate load per tube is 2k in both cases. And that's roughly what you get when doing a loadline construction.
Exact bias point has little influence on optimal load line and doesn't matter much at full power.

[SoulFetish]

Just take a look at the EL84 datasheet.

Recommended Raa for class B: 8k, recommended Raa for class AB: 8k, same plate and screen voltages.
Means that the optimal plate load per tube is 2k in both cases. And that's roughly what you get when doing a loadline construction.
Exact bias point has little influence on optimal load line and doesn't matter much at full power.

correct. plus,doesn't seem to have much affect on output power either. It does, however, have a significant influence on the operational characteristic, with regard to class of operation during signal drive, and the total tube dissipation.

for instance:
EL84 (push pull with p-p load of 10k) with plate voltage of 321, screen voltage of 319, and a grid bias of -13V operating in Class:AB1 should produce a power out of just over 17W
Max sine wave dissipation of just over 9W (per tube)*
while;
EL84 (push pull with p-p load of 10k) with plate voltage of 321, screen voltage of 319, and a grid bias of -9V operating in Class:AB1 should produce a power out of just under 17W
Max sine wave dissipation of just over 16W (per tube)*
- of course, this is assuming a full sine wave grid drive to the 0Vg limit

how this affects the perceived sound is subjective to the player/listener, I suppose

* calculation done via nickb's interactive datasheets @ bmamps.com:
http://bmamps.com/ivds.html

[Helmholtz]

It does, however, have a significant influence on the operational characteristic, with regard to class of operation during signal drive, and the total tube dissipation.

Now - as the discussion was about Zaa, did you also check dissipation for a full class A impedance of around 4k?

[Helmholtz]

EL84 (push pull with p-p load of 10k) with plate voltage of 321, screen voltage of 319, and a grid bias of -9V operating in Class:AB1 should produce a power out of just under 17W
Max sine wave dissipation of just over 16W (per tube)*

What made you choose an unrealistic grid bias of -9V?

That would result in an idle current of 75mA per tube or a plate dissipation of 24W (!) or 200% at idle acc.to Nick's tool.