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felipemuniz 22nd March 2013 06:01 AM

Hybrid "Marshall" Amp
 
I was wondering if is possible to make a JCM800, changing the output stage, to solid state, using a design that also Marshall uses, using de MJ transistors.

http://www.drtube.com/schematics/marshall/3005.gif or
http://www.drtube.com/schematics/marshall/2200-pwr.gif

The preamp would be the JCM800, which uses two 12AX7, and the amplifier output, this would be the output stage, employed in 3005.

http://www.drtube.com/schematics/marshall/2203prem.gif

I think that would be the minimum necessary to adjust the bias of the stage trasistores, but unfortunately the diagram does not indicate where the bias is.

But is that possible? Can work? I can just connect this two stages, without any other changes? If is necessary changes, what it can be?

Nigel Goodwin 22nd March 2013 09:24 AM

I don't really understand what you're asking? - there's no problem connecting a transistor power amp to a valve preamp, nor is there any need to alter the bias at all.

JonSnell Electronic 22nd March 2013 10:43 AM

Fender and Ampeg use MosFet impedance matching connected directly between the ECC83 pre amp/driver and loudspeaker. Very successful as MosFets behave similar to valves when over driven.

felipemuniz 22nd March 2013 11:21 AM

Quote:

Originally Posted by Nigel Goodwin (Post 3422098)
I don't really understand what you're asking? - there's no problem connecting a transistor power amp to a valve preamp, nor is there any need to alter the bias at all.

Like Harleyjon said, I know that tubes have high impedance that, for example, a OP-amp, such a 1458. You can notice that if you look the pots on the equalizers, the pots in SS have a low value like 22k, and the pot on the tube preamp, is something like 250k, 500k or even 1M. This is because the impedance from the tube is a pretty high comparing with the op-amp.

Well, however, I can say that it wont will work. Maybe will, or maybe, need a little adaptation.

Harleyjon, you said about mosfet as impedance matching, but perhaps can you post here some schematic from this Fender amp that you mentioned, or maybe just a example or article from this design? Thank you by your reply.

JonSnell Electronic 22nd March 2013 11:43 AM

Unfortunately, I cannot post a copy from Fender as it is copyright but it is very simple. It consists of complimentary pair output stage with the sources connected together, matching the high impedance of the valve to the low impedance of the loudspeaker, the gates are biased slightly on to give around 10mA drain current at idle stopping any crossover distortion and is supplied from a + and - 65volt supply. The anode of the final valve stage is AC coupled and as the Fet gates are very high impedance, there is no loss of voltage/power from the valve. The Anode voltage of the final valve is variable from 80 to 250 volts. The design calls it Variable Plate Voltage. This gives variable harmonics and sounds. A sought after amplifier!
I trust you will find this an interesting design.

Enzo 22nd March 2013 12:01 PM

DOn't confuse the impedances around individual tubes with the impedance of a preamp overall.

Your preamp - regardless of what technology it uses - will produce a signal. When you have a PREAMP OUT jack on an amplifier, it matters not whther it was a tube or a transistor preamp. All that matters is that it is a line level signal. Likewise your power amp. The power amp has an INPUT jack, and it does not care where the signal came from - tube or transistor.

Harleyjon, what model Fender do you refer to? I am sure we have links to just about any Fender schematic available online. For example, Schematic heaven has a whole page of them:

Fender Amp Heaven Schematics - Tweed Blackface Brownface Silverface Bassman Champ Deluxe Princeton Super Vibroverb Reissue

JonSnell Electronic 22nd March 2013 01:10 PM

I cannot remember the Fender version but take a look at Ampeg SVT3pro, very similar.

Loudthud 22nd March 2013 10:25 PM

Many solid state power amps cannot withstand too much voltage on the input without damage. You'll fry the input transistors or worse yet the chip amp. Since many tube preamps can have 200V peak to peak on the output, the signal needs to be attenuated or clamped before you connect it to a solid state power amp. Impedance matching can also be an issue. Some tube preamps don't sound good when driving a power amp with a 22K input impedance. These problems have to be addressed on a case by case basis.

JonSnell Electronic 22nd March 2013 10:35 PM

Quote:

Originally Posted by Loudthud (Post 3423156)
Many solid state power amps cannot withstand too much voltage on the input without damage. You'll fry the input transistors or worse yet the chip amp. Since many tube preamps can have 200V peak to peak on the output, the signal needs to be attenuated or clamped before you connect it to a solid state power amp. Impedance matching can also be an issue. Some tube preamps don't sound good when driving a power amp with a 22K input impedance. These problems have to be addressed on a case by case basis.

The impedance matching that we are talking about is protected against overdriving if the gates are clamped to each respective rail.
The design has been very successful for a number of years and if there was a problem, don't you agree, it would have been addressed, some time ago.

Loudthud 22nd March 2013 11:17 PM

Clamping the input to the rails is a good start and on a MOSFET unity gain stage like the Ampeg, it's required.

I was speaking about a solid state power amp like the one in the Marshall 3005. When a solid state amp has a solid state preamp, the preamp is usually running on rails lower than the power amp so there is no problem. Another thing to watch out for is when the input of the power amp is driven so far that the input diff pair's current source is cut off. When that happends, the output suddenly slams the wrong rail.


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