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Old 3rd Oct 2017, 11:14 pm   #1
ITAM805
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Default Strange output stage?

I have this 1970's MusicMan 75W guitar amp in for repair. I'm rather puzzled by the output stage, the signal from the phase splitter apparently goes to the cathodes of the 6CA7's via a pair of transistors, rather than to the grid?

I've not seen this before, how does it work?

Many thanks
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Old 3rd Oct 2017, 11:22 pm   #2
ukcol
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Default Re: Strange output stage?

The output valves are run as grounded grid amplifiers. Grounded grid operation is quite common in RF amplifiers but I've not seen it in a commercial audio amplifier before.

I'm not sure why this is done here, perhaps it gives a better impedance match between the IC pre amplifier stage and the output valves? Someone here will know.

EDIT please note the grids are grounded at signal frequency but are above chassis with respect to DC.
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Old 4th Oct 2017, 1:05 pm   #3
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Default Re: Strange output stage?

Thanks Colin. Very strange configuration, certainly weird in a guitar amp!
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Old 4th Oct 2017, 1:24 pm   #4
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Default Re: Strange output stage?

It's a mostly solid state amplifier.

It has a push-pull output stage into an output transformer and each output device is a cascode combination of a valve on top and a transistor on the bottom.

You could view it as a common-emitter transistor stage driving a grounded-grid stage.

It's quite elegant in several ways. It ticks the 'has it got valves in it?' box. It ticks the 'are the valves really in the output stage and not in some do-nothing bit of the preamp?' box. It ticks the 'clipping effects controlled by valves' box too. but I'd expect it to be rather low on valve-sound when not driven to the limits.

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Old 4th Oct 2017, 2:01 pm   #5
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Default Re: Strange output stage?

Thanks David. It turns out that a certain Mr Fender had a hand in designing it. It's certainly styled like a 60's 'black-face' Fender. Not only that, it has a pair of GEC KT77 fitted, possibly worth a fair bit I suspect
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Old 5th Oct 2017, 9:05 pm   #6
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Default Re: Strange output stage?

Music Man did other amp's this style like the 2100-RD and the 2165-RD.
I suppose it's a convenient way of driving the output valves into grid1-current w/o wasting any grid-driving power.
All power that goes into this drive-circuit is in series with the anode-current and hence the output-transformer.
Much easier than doing a solid-state traditional grid-drive circuit imho.
Does it sound "genuine" ? Build one to check it out.
Since it's AC-coupled to the pre-stages ANY pre-stage will be OK, even hollow-state
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Old 5th Oct 2017, 9:50 pm   #7
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Default Re: Strange output stage?

Quote:
I suppose it's a convenient way of driving the output valves into grid1-current w/o wasting any grid-driving power.
Maybe not so simple as the grid-1 current obviously IS wasted.
Hmmm...
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Old 5th Oct 2017, 10:22 pm   #8
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Default Re: Strange output stage?

Yes, Quite right. With this principle it is far better to drive the grid as well as the cathode, at the same time. If Mr Fender did come up with this, he only 1/2 copied my brother Brett, who designed the attached circuit in the early '70's, though in this case being a lower power version of tube cathode drive, with a transistor, the grid current is very very small. But both the grid & cathode receive drive. The attached circuit is useful if you don't have quite enough overall gain for a magnetic phono cartridge, to easily add it to the front end of your valve amp. Other places you will see transistor cathode drive in tube circuits are in some scopes where they use this method as a x10 gain system.
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Old 5th Oct 2017, 10:30 pm   #9
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Default Re: Strange output stage?

The transistors in the output stage will have pose fairly low load impedance for whatever is driving them.

Whether or not it goes into grid current depends on the chosen bias and the condition of the output bottles. If the transistor current source says x milliamps, then the cathode willrise or fall to whatever Vgk it takes to do that current.


Because the grid screens the cathode area from the anode field, the valve is effectively the upper part of a cascode and there is no need to waste the current in g2. it might as well be used in triode-strapped form and the g2 current will go to the output transformer and help drive the load.

If it'd designed well, the valves will saturate before anything silicon hits its end-stops, so it ought to be a clean sounding amplifier until you start to clip it and then it ought to sound valve-y. Possibly too clean for some people. But at least the token valves are real ones and are doing the real work.

With the valves running to saturation there will definitely be some grid current.

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Old 5th Oct 2017, 10:35 pm   #10
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Default Re: Strange output stage?

The transistor under valve cascode has been around for a long time. I used it in the late sixtes using the valves to make the big voltage swings for driving the X plates of the VCR97 tube in a scope I built (and rebuilt and rebuilt). I must have seen it somewhere before.

Wireless world also had a circuit ideas page item with long tailled pair of valves with their 'ring of two reference' current source biasing the cathodes. I tried that with a pair of EF91s and it did the job well.

David
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Old 6th Oct 2017, 10:16 am   #11
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Default Re: Strange output stage?

I admit I'm still puzzled at the function despite reading up on 'grounded grid' amps? Checking the bias on this amp, via the emitter resistors as per Musicman technical bulletin, I note it's a tiny 6mA. Normally one would bias EL34 types at 20-40mA, pending the HT voltage of course, this has a massive 720 volts, even so would this o/p be considered a 'Class B' set up?
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Old 6th Oct 2017, 1:30 pm   #12
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Default Re: Strange output stage?

Looks very much like it! Or at least AB.

At 6mA per valve, you could expect to get a watt or so out before one of the valves cuts off. Anything more than that and it would be definitely out of the Class A mode.
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