Hybrid "Marshall" Amp

felipemuniz

Member
2012-03-23 11:07 pm
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?
 

felipemuniz

Member
2012-03-23 11:07 pm
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.
 
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.
 
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
 
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.
 
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.
 
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.
 

felipemuniz

Member
2012-03-23 11:07 pm
Well, today I made up the jcm800 preamp and tested here with this power amp from 3005, and it sounds very weird, sounds like very dirty distortion and with excessive noise. After that I connected this preamp directly on the V5 of my ac30 (inversor tube) and sounds pretty normal, more agressive that the jcm it self, but i guess that is because the inversor tube of the ac30 design works like a preamp tube too, not just inversor.

I'm now making tests on the power amp, with low gain preamps, using 12au7 tubes. Until now sounds good, but need more tests.
 
Does the 3005 power amp work/sound well if you apply guitar signal straight to it?

FWIW I have coupled many times a classic Tube preamp to an SS power amp successfully, but of course considering what Loudthud suggests.

Meaning:
1) attenuate the tube pre signal with, say, a 470K/47K divider
2) place across the SS amp input a couple series, reverse wired Zener diodes, say somewhere around 5V, so no peak higher than that can reach its input.

Although I suspect your SS amp may be unstable or oscillating or you have some ground trouble between preamp and power amp, otherwise you "should" have good sound at least at low settings of the Master Volume.
 

felipemuniz

Member
2012-03-23 11:07 pm
Does the 3005 power amp work/sound well if you apply guitar signal straight to it?

FWIW I have coupled many times a classic Tube preamp to an SS power amp successfully, but of course considering what Loudthud suggests.

Meaning:
1) attenuate the tube pre signal with, say, a 470K/47K divider
2) place across the SS amp input a couple series, reverse wired Zener diodes, say somewhere around 5V, so no peak higher than that can reach its input.

Although I suspect your SS amp may be unstable or oscillating or you have some ground trouble between preamp and power amp, otherwise you "should" have good sound at least at low settings of the Master Volume.

The guitar direct on the power amp sounds clean, without problems. Also, with a preamp tube using 2 12au7, until now is sounding good to me.

Well, I already see this that you said with zenners, but with LEDs, in the Vox 15r, I think... I'll try later with zenner and maybe LEDs too.
And about the divider, I used a 1M pot, but I can try a 470K pot too.

edit: the divider is after or before of the diodes?
 
Last edited:
The guitar direct on the power amp sounds clean, without problems. Also, with a preamp tube using 2 12au7, until now is sounding good to me.

Well, I already see this that you said with zenners, but with LEDs, in the Vox 15r, I think... I'll try later with zenner and maybe LEDs too.
And about the divider, I used a 1M pot, but I can try a 470K pot too.

edit: the divider is after or before of the diodes?
Ok, you are doing fine.
I didn't want to mention Leds, but it's the same idea, just consider them "1.9V Zeners" and it's fine :)
The 1M pot is fine, I mentioned 470K as an example.
In fact, I think best would be to use a , say, 470K resistor in series with a 50 to 100K Log pot, for a smoother regulation, but that's up to you.
Good luck.
 

JRGuitarGuy

Banned
2012-10-09 11:25 am
Just find a crate stealth and mod the **** out of it manxD I'm telling you its waaaaay too fun lol Testing New tubes/Caps - YouTube

Love the GEs of course xD

The mullards are some are almost too Holy grail to be in this amp rofl

Blackburn Mullards - YouTube

But it is technically kinda what you are talking about lots solid state junk more in preamp though, not power amp I kinda changed some of that though xD, It does what I wanted it to do with nice RCAs/GEs I have nice Fenderish type tones, get bored with that throw in the mullards then have a raging firestorm 6v6 plexi/jcm from hell etc lol
 
recently

hi to all, i just build a jcm800 2204 preamp and i modded it for my personal taste.i use SS 120 watt mosfet power amp with full range 100 watt speaker and yeah it works on me. from high impedance on the tube, i find some stuff like filter buffer ss op amp TL072, to filter high impdnce to lower and to matches with the SS power amp. mosfet amp is like tube amp for me :D i do have the pics stuff so u could see it and need some advice for it too. im a new valve enthusiast. maybe ill post it sooner,..thanks