Another Fender Vibrasonic question/ohms

Dave Zirbel · Post by **Dave Zirbel** » 3 Aug 2002 3:07 pm

I know the amp is designed to take a 4 ohm load. There is an extension speaker jack on the back that says "parallel speaker jacks/100w/4 ohms total.

So if I have a 4 ohm speaker already in the amp, I can't use it in configuration with an extension speaker rated at 4 ohms, right? I need to run two 8 ohm speakers, or is there something I don't know. I would like to run a pair of 4 ohm JBLs. Will that hurt the amp? Will it cause the tubes to burn out faster?

Thanks, Dave Z

Ken Fox · Post by **Ken Fox** » 3 Aug 2002 4:56 pm

I checked the schematic for you, Gary. The extension jack is just in parallel with the other jack. The primary speaker jack does have a shorting contact that closes the output transformer directly to ground if no load is present. Never run the amp without a load at the main jack! Unfortunately it appears there will be an impedance mismatch with a parallel speaker, if in fact your main speaker is 4 ohms. On some of the old 135 watt Twin the second jack activated an 8 ohm tap and rewired the two jacks in series for a 4+4=8 ohm load. Your amp is not that way. An 8 ohm load would be the safest impedance match:

1/4 + 1/8 = 1/Resistance total.
R total = 2.67 ohms

A 4 ohm load at the extension jack would be a total load of 2 ohms.

Donny Hinson · Post by **Donny Hinson** » 3 Aug 2002 5:10 pm

Dave...are you sure your built-in speaker is 4-ohms? Most large single-speaker Fender amps used 8-ohm speakers, and used a different output transformer than the regular Twin Reverb. The output transformer is the one near the center of the chassis. Can you see the part number on it?

Dave Zirbel · Post by **Dave Zirbel** » 4 Aug 2002 12:07 am

I'm pretty sure the stock speaker is a 4 ohm 15' built by Eminence. This is one of the newer Fender amps built in the 1990s. It seems silly to have an extension speaker jack and not be able to use the stock speaker. I would need two 16 ohm speakers!?!?

Dave Zirbel · Post by **Dave Zirbel** » 4 Aug 2002 12:33 am

I just answered my own question. It takes a 4 ohm load whether its one or two speakers. It says in the owners manual.
http://www.google.com/search?hl=en&ie=ISO-8859-1&q=Fender+Vibrasonic+Custom
It doesn't say that it will hurt the amp.

DZ[This message was edited by Dave Zirbel on 04 August 2002 at 08:02 AM.]

Ken Fox · Post by **Ken Fox** » 4 Aug 2002 9:15 am

Here is a copy of some things about speaker mismatching from the Weber VST speaker company site:

When you use a load that is lower than the intended load, the output has to drive the load (speaker) with more current because it is a lower impedance than is expected. Two inherent problems associated with transformers are flux leakage and regulation. Flux leakage is also referred to as leakage inductance. It is related to the current in the secondary, and these problems increase as the current increases. As the current draw in the secondary increases, the primary has a more difficult time transferring the signal to the secondary, so the secondary signal to the load gets squashed, or 'soft-clipped'. This soft clipping is called regulation. While regulation is desireable in a power supply, it is undesireable in a transformer. In other words, in a power supply, if the input voltage or the output load current changes, we don't want the output voltage to change. In a transformer, we want the output voltage to follow the input voltage and not regulate at all. When you put a heavier load on the output than was intended, it will pull the output voltage down, hence regulation. The leakage inductance problem arises because the current from the heavier load causing the regulation to occur reduces the efficiency of the transformer by not allowing the output to follow the input. Transformer designers simulate or view this problem as having extra inductance in series with the primary. The extension of this idea then, is that with the heavier load, you could affect the efficiency of the transformer, alter the frequency response (due to the extra leakage inductance in series with the primary), and cause other distortions to occur. OK, on to mismatching the other way. A speaker is a current operated device in that it responds to the current through it to generate a magnetic field that works against the magnetic field of the speaker magnet to make the cone move in and out. Thinking in very short amounts of time, when the output charges up the voice coil with current, then the signal goes away or gets reduced, the cone system moves the voice coil back to its home or resting position. As it is moving back, it generates a voltage that is fed back up the line into the transformer and appears in the output circuit of the amp. This generated voltage is often referred to as flyback voltage, because we are charging up an inductor, then when we disconnect or stop charging the inductor, the magnetic field in the inductor collapses and induces this big voltage into itself. This big voltage then 'flies back' to the source of the charging current. There is a mathematical formula to determine how big the voltage is and it is related to the inductance of the voice coil, the amount of time it was fed current, and how much current it was charged with. The bottom line is that the voltage fed back to the output circuit is oftentimes much higher than the voltage that was used to drive or charge up the voice coil initially. This voltage gets transformed up by the turns ratio of the output transformer, and in many cases can be over 1,000 volts. What happens then is that arcing can occur between the pins on the output tube socket. Once this has occured, a carbon path forms on the tube socket between the pins. The carbon path allows a steady current to flow between the pins and eventually burns up the socket due to the heat that is generated. For example, it wouldn't be too uncommon to see a transformer turns ratio of 30:1. If we had a voltage fed back from the voice coil that was around 50 volts, 30 times 50 would be a 1,500 volt spike at the plate of the output tube. This is why you often see designers connect diodes in a string between the output tube plates and ground. They are trying to suppress these spikes and dissipate the energy in the diodes rather than allowing an arc to occur at the tube socket. So, when you use a higher impedance load on a lower impedance tap, the turns ratio is higher and resulting fed-back (flyback) voltage gets multiplied up higher than what it would have been with the correct impedance load.
It's just about impossible for me to answer how long an amp would last under these conditions. It all depends on how the designer took these potential problems into account in his or her design with regards to the quality of the tube sockets, the use of stringed diodes, the output circuit operating voltages, etc.

Stephen Gambrell · Post by **Stephen Gambrell** » 4 Aug 2002 4:21 pm

Ken, wouldn't the voice coil of the speaker be in parallel with the secondary of the output transformer, thus increasing the inductance of the entire circuit? I thought I knew a little about electronics, but I'd love to sit at your feet for a while! But I always thought that, as a rule, speaker impedance mattered less with tube amps than with solid state, since there is no output transformer in a S/S amp. I remember burning up stuff when I was a kid, tying speakers together, but them old tuck-and-roll Kustoms ain't really collector's items, are they? If they were, Mike Bagwell would have one!

Ken Fox · Post by **Ken Fox** » 4 Aug 2002 4:42 pm

Generally speaking, most tube amps can take an impedance match one way up or down. A transistor amp can take a higher impedance than rated with no problem, it even likes an open circuit (a tube amp does not). The problem with a tube amp is long term damage to the plates of the tubes and the sockets from what I gathered in the above article (that I copied from Weber's site).
The voice coil is merely the impedance/ load to the secondary of the transformer. It is not in close enough proximity to the primary winding to be part of the inductive/transformer action. Of course the impedance of the speaker load in why there is an inductive circuit action there and it's impedance directs what the primary impedance will be! [This message was edited by Ken Fox on 04 August 2002 at 05:45 PM.]