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Hi-end GM70 amplifier with only 2 stages

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#81
audiodesign said:
I don't have ear the 813 Ribbed plates.

The GM70 if compared to the 813 RCA have more details but the sound is less pleasant and fatiguing.

Some my friends prefer the GM70 and others the 813 RCA.

The 845B cannot be compare with these other high voltage gain tubes because the 845B will be used with a different drive stage so many other parameters will change.

The sound quality of my 845 two stage amplifier as been compared to the my GM70 two stage amplifier in a recent session in my house with 4 persons with a lot of experience in sound reprodution and the result for all was good with both.
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In your first post I see last octave droop and 41dB dynamic range at full power ref to 50Hz hum, when 100Hz buzz is just -52dB. How can you evaluate the tone and detail of such an amp, especially on efficient speakers needed for SE low power, when having a strong hum issue and no sparkle? Apart from the nuisance, LF masks up to the mids in ear's cochlea. Check the strong 1kHz sidebands on your Clio FFT posted, LF modulates it. When cruising at 1W on 8 Ohm high sens speakers your 41dB squashes down to just 29dB ref to booming hum & to 40dB ref to grinding buzz. (11.43V/2.83V) log * 20 = 12dB closer to noise floor. I would not subjectively evaluate such a build before removing its issues to inaudible (at least) levels for 92-95dB SPL per W 8 Ohm transducers. Just my 2C.
 
#83
Ryssen said:
What is the optimum load for GM70?
I have a Lundahl trafo I can chose 6,5Kohm and 11kohm..
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Around here most people seem to be running 6K -7K primaries with the GM70 and 100 - 125mA of plate current at 1KV.

When I did the load lines it seemed like 6K was about right for 1KV/125mA quiescent operating point. I ultimately ended up at 7K trading off about 5Wrms for somewhat better (?) linearity. I have yet to build the amplifier, but have heard several GM70 amplifiers with 6K - 7K primary impedances, and it seemed that this was a good match.

No one I know locally is recommending anything over 8K primary Z for the GM70..

I designed my speakers to function properly with the source impedances typically encountered with (my) SE tube amplifiers rather than compromising amplifier HF extension in the quest for damping factor.
 
#87
Hey Audio, I have been following your build for some time now. I am just now getting ready to assemble my GM70. Here are some pix.

IMG_3870.jpg


IMG_3869.jpg
 
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#90
project questions

Hello Andrea,

I´m thinking to make your GM70 / 813 amp, I love your design but I have a few questions about the power supply.
Are the choke a LL1668/20H like the schematics or the LL1638/20H I see you use in the photos?
Another question is if I can use the same PS for a dual mono version?

Thanks for the answer and share your amazing projects.
 
#91
kevinkr said:
Looks like its going to be a cool amp project.. BTW what is that casting next to it??😕😀

I got my Lundahl LL1635 IT recently and will report on driver performance once I have some time to evaluate it.
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Hi Kevinkr, that cast piece is from a large fluid pump. It is from a shop a friend of mine owns. I have a whole box of them and trying to figure out how I can use them in the design.

I put the project aside while I built a pair of Open Baffel Line Array Econo Wave speakers but am back on it now. My friend Ross is designing the driver circuit, I have the PS all desiged, and now going to punch the top plate this week. The chassis needs to be finished as well. So maybe in two weeks I can start wiring it up.

I am using the 12HL7 with CCS for a driver, cap coupled.

I have a 37-0-37 PTX for the GM70 filly PS and also hope to have enough there for the bias supply as well.
 
#94
Good introduction Andrea!

This output transformer has about the 1/4 of the normal dc resistence of the competitors.
This means an increment of the 8% (about) of the output power.
The sound quality is great.


Bye

Walter Gentilucci
 
#99
waltube said:
Good introduction Andrea!

This output transformer has about the 1/4 of the normal dc resistence of the competitors.
This means an increment of the 8% (about) of the output power.
The sound quality is great.


Bye

Walter Gentilucci
Click to expand...

From those basic info it doesn't look a well balanced OPT. Yes you have 80 ohms DC resistance for the primary but your secondary resistance is 0.3 Ohms which means about 330 ohms if referred to the primary (0.3x33x33). So the overall efficiency of your OPT is about 95.5% at low frequency (or 0.2 dB power loss if you like). This is the same figure you get for the Lundahls in low loss connection.
Those specs make me think that you are using most of the available volume for the primary and if you are also using a larger than usual wire for the primary the OPT will have a substancially higher AC resistance already at 5 KHz (this also depends on the geometry).
If you look at Lundahls the primaries and secondaries use similar wires (all in series for the primary and series/parallel combinations for the secondaries depending on the turn ratio and they have costant or near costant power loss for the various 4/8/16 connections because you always use all the turns....). In these OPTs the primaries and the secondaries have similar DC resistances and similar AC resistances at all frequencies. AC resistances in particular are minimized also because they also use half space for the primaries and half for the secondaries. This is what gives minimum insertion losses....DC resistance is a meaningful parameter only at low frequency (below 1Khz).

45
 
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#100
I am trying to understand your post.
You wrote that the loss is similar to the Lundhal trafo; this is great!
Lundhal is a very good producers, if F.I.A.T. made a trafo with comparable characteristics it is a success!!
The main goal was to develop a stuff with a good ratio between cost and performance.

Regarding the dc on secondary, have you measured it on this trafo?
Regarding AC resistence, have you check it on this trafo for different frequencies?
Reagarding the importance of dc resistance on low frequency I agree with you.

Normally I accept the critics about project when someone bought the parts and make it.
This means that there is a true interest to find a better solutions.
This is not your case.

Bye

Walter Gentilucci


p.s-= 45 is a number, not a name!!
 
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