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rick57 3rd September 2006 12:18 PM

45 into an 845 parafeed circuit
Hi guys

I’ve read just about all the posts on 845s here (and at AA). I’m interested in doing an 845 SE power amp, and just now have some funds and time. I'm after designs not too complex or too expensive.

I was going to say a two stage amp is preferable, but a three stage with not too much complexity is fine. I will probably use a TVC, but can add a good preamp if I need more gain.
I’m a way off being able to design a circuit, and with three kids and other audio projects too, I won’t have time for a lot of tweaking ..

I’d like to go for a 45 into an 845 parafeed. Who has a circuit for such an amp?

If using an IT, I’d lean towards not too much up or down; but I prefer to build cap-coupled first and maybe try IT later.


Gluca 3rd September 2006 01:14 PM

just an idea that works fine
tubelabs 45 into 845

Not a parafeed amp but you can mod the power stage. 845 needs a couple of stages to be driven properly.


quinnling 3rd September 2006 05:32 PM

Parafeed 845 into 845
Check out this amp out. Even though it is not a 45 into 845, you may get some ideas or mod it to use the 45.

rick57 4th September 2006 09:06 AM


I was for a while very keen on tubelabs’s approach, but I think the PCBs are not available yet, and I’m now likely to go P2P with an exact circuit.


il Monstro (282 pounds) is the maybe the King Kong of 845s . . alas adaptations are beyond me at this stage.
I will look further for a circuit that I’m after, or closer to it.



rick57 4th September 2006 02:58 PM

On further thought, I like the idea

IMO not attractive as laid out & shown (but easily changed). Apart from the extra power (which may or not be a benefit, depending on your speakers, dB needed), George’s claim:

‘the A2 amp does a much better job of recreating highly dynamic music which contains a lot of transients. Snare drums, rim shots, the sound of a cello being smacked sharply with the bow.”

Should really suit my primary musical taste (rock).

But "no free lunch" . . if dynamic music sounds better, I wonder what disadvantages might or do accrue??


Gluca 4th September 2006 06:18 PM

The driver capable of pushing the grid into A2 doesn't come for free ... tubelabs approach seems to be easy, cheap and strong enough to master an 845 in A2. A CF (or a step down IT) would probably not do the same job.


rick57 5th September 2006 02:33 PM

Gluca - your thoughts?
I am considering a small “reverse experiment” on tubelabs’s amp:

Have the two MOSFET stages switchable in or out, preferably seperately, to compare the sound (see if they sound preferable in or out) and have control over output for different speakers (and probably tone).

The two MOSFET stages (to my reading) join the existing circuit at seven points. So a switch that moves the MOSFETs in or out would need to join to all seven points - would this work? :confused:

Are switchable stages feasible?


Gluca 5th September 2006 04:34 PM

GAH!? I dont know. I dont like the idea of having switches hanging around in a 1KVish amp. Maybe George (tubelab) can pop in and say more.

Usually I build two different amps (left+right) and listen to them at the same time without switches. At the end I choose the one I like more (actually my left ch is always different from my right ch ... I keep on modding them -LOL-)


rick57 5th September 2006 09:29 PM

Yes, I intend to build two different amps (left & right) and listen to them at the same time,
one a tubelabs 45 into 845, and
one a “conventional” cap coupled 45 – 845, likely a 6P5GT input, 45 driver 845 v3.jpg here:

Both with a good 10 K OPT (that I have) and 1000V B+.

I was thinking of having a switch on the tubelab design, but you are right a switch on a 1 KV amp would require great care.


Tubelab_com 6th September 2006 01:26 AM


Maybe George (tubelab) can pop in and say more.
I finally have a few minutes (too many people hanging around my office at work to be on the forums)....

Yes, I built a breadboard version of my 845SE, but I was nowhere near it when the power was on. 1100 volts generally will not give you a second chance, and death is rather irreversible.....

I tend to set up the whole circuit, then listen and measure, then move on to the next circuit. This makes A-B comparisons hard. My workbench, and the Tubelab 3 breadboard does not have the space to accomodate two 845 sized channels simultaneously.

For simpler amps, I do build two (usually identical) channels and try any experiments on one of the channels. Use a switch (suitably grounded) that allows switching the two speakers. Differences become obvious when they swap sides. Sometimes it helps to use a mono input signal. This is absolutely the way to audition different OPT's, the "reference" transformer in one channel, and the "test" transformer in the other channel.

I have not had the time to tinker with my 845 amp in a while, but I did discover that the power supply is very important to the sound in this amp. This is true for all SE amps, but this one is seems to be more critical (or my last two power supplies were just lousy).

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