Solid state power amp; impedance question

[Tom Phillips]

An option is to use an auto transformer as an impedance converter. The Weber WZC100 is advertized as a 100W unit selling for $48.
https://taweber.powweb.com/store/wzc100.jpg
The specifications do not list the frequency response so we don't know how it will perform for bass but you could contact them and inquire.
You would mount the auto transformer in an accessory box, connect the amp output between the 0 and 8 Ohm leads and the 4 ohm speaker between the 0 and the 4 Ohm leads.

There are commercial solid state amps that have built in auto transformers to allow them to deliver rated power into a variety of impedance loads. The appropriate tap is selected as is done with a tube amp OT with multiple secondaries.

[The Dude]

How many speakers in your 4 ohm cabinet and what is the impedance of each speaker? Can you just rewire the cabinet for maybe a 16 ohm load?

[loudthud]

I purchased two of those Weber WZC50 (the 50W version) about a year ago and recently got two WZC100. They were out of stock for a couple of months. I was not impressed with the size of the WZC100 and haven't put the project together yet, a 100W solid state amp on an 18W chassis. The core is barely bigger than the WZC50. The thing you have to watch out for in the Ampeg amp is the DC offset at the output. The DC resistance of the 8 Ohm tap is 0.264 Ohms at the end of rather long leads.

The 50W version works fine at a rail slamin 50W, 4 Ohms in to 8 or 2 Ohms out. I have no complaints.

[Bernardduur]

Short answer: although in theory mod is possible, it implies replacing/rewinding the power transformer by one putting out 0.7X the present voltage and 1.4X the present current; the power amp by one with transistors with same factors (same power, 0.7X the voltage, 1.4X the current, not forgetting a properly set short protection) and half the power supply (diodes would be probably enough, you'd have to double capacitance, overrated voltage doesn't hurt).

You sure yu want to go that way?

In a nutshell: an amp designer works backwards: starts with desired power and impedance, calculates voltage and current requirements, then which and how many transistors can safely supply that, then which power supply can feed them.

As you see, starting with : "hmmmmm, boss wants a 100W into 8 ohms amp" or "boss wants a 100W into 4 ohms amp" leads to different solutions.

FWIW I don't remember having seen that exact model in person, but distinctly remember its bigger brother from the late 70's or early 80's , the SVT240 (or was it SBT240?) , exact same preamp, same power amp but with twice the transistors (8 in total), for 240W RMS into 4 ohms ... driving 2 x 15" Altec 418 or 421 ... a BEAST of an amp.

Impossible to google its schematic because current brand owners abuse it by naming "SVT" anything they make, even if the only tube it ever saw was a toothpaste one.

Oh well.

No need to say that it was a big heavy head, had a huge transformer and lots of heatsinking.

Hey!!!! now you have the perfect excuse to buy yet another cabinet

A single 15", preferrably an EVM15L would be a perfect match.

My attic is flowing over in cabs. I just love different cabs for different purposes. I know I will have some nice fitting cabs for this amp

The Ampeg you were talking about is the V6-B; exact the same setup, just one preamp. The schematic is widely available on the net!

[Bernardduur]

An option is to use an auto transformer as an impedance converter. The Weber WZC100 is advertized as a 100W unit selling for $48.
https://taweber.powweb.com/store/wzc100.jpg
The specifications do not list the frequency response so we don't know how it will perform for bass but you could contact them and inquire.
You would mount the auto transformer in an accessory box, connect the amp output between the 0 and 8 Ohm leads and the 4 ohm speaker between the 0 and the 4 Ohm leads.

There are commercial solid state amps that have built in auto transformers to allow them to deliver rated power into a variety of impedance loads. The appropriate tap is selected as is done with a tube amp OT with multiple secondaries.

Hmmm, such a box is always handy I guess

[g1]

FWIW I don't remember having seen that exact model in person, but distinctly remember its bigger brother from the late 70's or early 80's , the SVT240 (or was it SBT240?) , exact same preamp, same power amp but with twice the transistors (8 in total), for 240W RMS into 4 ohms ... driving 2 x 15" Altec 418 or 421 ... a BEAST of an amp.

The Ampeg you were talking about is the V6-B; exact the same setup, just one preamp. The schematic is widely available on the net!

Ok, I was a bit confused when JM said SBT240, but it was the V6-B that had the the 240W poweramp. Preamp & poweramp in head unit.
The SST and SBT were preamps that went with powered cabs. The cabs were 120W each. The SBT had 2x15 and the SST had 4x12. You used 2 cabs with the preamp for a total of 240W. The SST was the guitar version with reverb & trem. It's preamp schematic seems to be extinct.

[J M Fahey]

Yes, that's the one.

I was quoting from faded memory , of course.

Main point is that preamp is basically the same, and to drive 4 ohms they use twice as many transistors, as in 8 total.

Of course, beefier transformer, supply and heatsinking.

Thanks for posting

[g1]

One more that I'll mention as it's related to topic.
The only solid state amp that I ever saw with an impedance switch was Traynor PS-600. It had switch position for 2 or 4 ohms.
What the switch did was change the primary tap of the power transformer.
I guess everything was designed for the higher current demand, then the switch just boosted the supply rails a bit so you didn't have as much power drop as you normally get when running into higher impedances.

[The Dude]

One more that I'll mention as it's related to topic.
The only solid state amp that I ever saw with an impedance switch was Traynor PS-600. It had switch position for 2 or 4 ohms.
What the switch did was change the primary tap of the power transformer.
I guess everything was designed for the higher current demand, then the switch just boosted the supply rails a bit so you didn't have as much power drop as you normally get when running into higher impedances.

I remember those! I used to have a pair of them and I seem to recall them being rated down to 1.3 ohms. Big heavy beasts built into their own Anvil style cases.

[g1]

Yep, when 2 ohms isn't low enough and you just gotta have another 4 ohm cab on there .

[Bernardduur]

haha, thanks all for the great replies! I've again learned a lot