Replace EL34 in the powersection with MOSFET?

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Hello,

I obtained an old amplifier, with transistor preamps and a tube invertor and power amp with EL34.

I am going to use the oputput trafo on a different project.

It seems wasteful to me, to waste a EL34 power amp on an old transistor preamp.

I was thinking would it be possible, to keep the power transformer, throw out the output transformer and build a mosfet power end?

Shcematic here

Thanks :)
 
Long time ago I made a class A+B output stage, bridged, for the BEAG guitar amp with damaged output transformer. The guy wanted it to be repaired overnight, but I did not have an OT, and did not have a chance to rewind one, so I installed instead of it a PS with 50W toroid, and made a SS output stage. It sounded nice, and musicians were impressed that a guitar amp now can play bass. B|

It is not class AB, it is class A+B, you can see what I mean on the graph to the right. Transistors never cut-off, and the amp goes from "triode" to "pentode" mode when overloaded, exactly like output tubes.
 

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I have been told many times things like codswallop, nonsense etc. Every time it was from someone who was not in a position to pass judgments.....

I have designed and built SS amplifiers that others have praised. They have SS outputs. SS devices can perform very well in the output stage from the perspective of sound quality, measured performance, longevity, stability, efficiency etc.

Perhaps I misunderstood your point altogether - my response was somewhat 'tongue in cheek banter'.
 
Hi,

That is not the issue. The point my valve lover was making
is your better off with a SS preamp than valves, generally.

rgds, sreten.

I've not made any pre-amps, but seems to me that if I did I'd want a buffer for a volume pot or a high gain low noise phono and I could envisage both being easier to achieve with SS. :)
 
I have designed and built SS amplifiers that others have praised. They have SS outputs. SS devices can perform very well in the output stage from the perspective of sound quality, measured performance, longevity, stability, efficiency etc.

Perhaps I misunderstood your point altogether - my response was somewhat 'tongue in cheek banter'.

Very well doesn't necessarily mean better.
However my point was about the preamplifier because the OP wrote: "It seems wasteful to me, to waste a EL34 power amp on an old transistor preamp." I disagree.
 
I have designed and built SS amplifiers that others have praised. They have SS outputs. SS devices can perform very well in the output stage from the perspective of sound quality, measured performance, longevity, stability, efficiency etc.

Perhaps I misunderstood your point altogether - my response was somewhat 'tongue in cheek banter'.

i am a tubehead and yet i agree....:cool:

Hello,

I obtained an old amplifier, with transistor preamps and a tube invertor and power amp with EL34.

I am going to use the oputput trafo on a different project.

It seems wasteful to me, to waste a EL34 power amp on an old transistor preamp.

I was thinking would it be possible, to keep the power transformer, throw out the output transformer and build a mosfet power end?

Shcematic here

Thanks :)

as with any vintage equipment, the first thing i would do is to verify that the irons in those gears are still good...

Very well doesn't necessarily mean better.
However my point was about the preamplifier because the OP wrote: "It seems wasteful to me, to waste a EL34 power amp on an old transistor preamp." I disagree.

yes, and perhaps a recap of all ecaps are all that is needed...
 
So you all suggest keeping the tube output stage with the transistor preamp?

Of course recap is important, I will check the irons, but they tend to be good in these amps.

Maybe I will change the output tubes to 6P3S, which have lower output power, but I have plenty of them, not like EL34.

Also I was thinking of throwing out the output stage and both transformers and putting in a simple 100W TDA7294 with a torroid.

Or is that a dumm idea?

Thanks and more suggestions are wellcome
 
Keep EL34, on the same power they sound cleaner than 6P3S. Also, voltages for 6P3S can be too high, and anode load resistance too low.

Buy Denon HT amp on ePay, if you want something solid state and more powerful. You can find some old model without fancy 4K 3-D support quite cheap, with Audyssey digital EQ.

Music - 70 has one of the best output stages, of a studio quality! There was also Music-130 that had 2 more EL34, more powerful, but also very good. Also there were Mono-70, great power amps, without pesky solid state inputs.
 
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Thank you for such a prompt reply. Ok I will do everything to keep the amp as is.

My idea was to turn it into an instrument (guitar,bass...) amplifier. The OT I wanted to use for a different all tube amp, so I was thinking what to do with the rest of this amp. Throwing out the OT would also make it lighter...

I am an incurable tinkerer... :)
 
As a person who doesn't like exploding speakers when I mess up (15" Black Widow are about $120 now) I wonder why people don't use FET's in output stages with existing 6000 ohm to 8 ohm output transformers. That is the transformers typically used for EL34 output stages, with 450 v power supplies. Thus a BJT input stage EL34 output stage amp would be perfect for such an experiment. Especially as the source which profits from most new EL34 sold is causing so much grief in Europe and the Middle East.
I have failed so far. My EF86 replacement class A with an NFET doesn't bias properly. Always on or off. EL34 push pull class AB is way beyond my design skill. But output transformers used for tubes do protect your expensive speakers from stupid wiring and design errors. A solder joint let go and blew up the fet gate, is where my EF86 replacement project stalled.
BTW the nfets I've bought come from Phillipines and Malaysia, which aren't causing major grief for anybody but revolutionaries right now. The four new EL34 I bought from Slovakia in 2014, well I think one is shorting out, waiting room on the coffee table for analysis. More On semi or Fairchild NFETs is my goal for future projects.
 
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