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Old 10th February 2019, 08:02 PM   #1881
prazza1 is offline prazza1  France
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Nice ! What tube do you use ?
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Old 10th February 2019, 08:21 PM   #1882
e_fortier is offline e_fortier  Canada
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Amazing work Scott17 !

These Signal Transformers CH-4 look impressive.

Keep us posted w the overall sound

Eric
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Old 10th February 2019, 08:27 PM   #1883
MEPER is online now MEPER  Denmark
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How is the "coupling" between tube stage and input to MoFo made?
The perf board looks good…..often they look at bit "messy"....but not this one…..
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Old 10th February 2019, 09:13 PM   #1884
scott17 is offline scott17
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Thanks prazza1, the tubes are both 6N6P. I bought a few of these about 5 years ago always planning on a preamp of some kind one day. This seemed the perfect use. I read another poster here used 6N6P so I thought I'd try them. One tube is the driver section for both channels, the other tube is for the cathode follower section. Since the cathode follower cathode is at about 135V, I elevated the heater for that tube 100V. I have not put it on a scope yet, but the output of the tube section should be about 40VP-P with a 2VRMS signal.

Thanks e_fortier. When I get this hooked up to my Altec 604 cabinets I'll post my impressions. I also want to run an FFT analysis and see what the distortion levels look like.

MEPER, the tube stage uses a 1uF output capacitor, so that connects directly to the junction of R1, R2, & R3 on the MoFo. I deleted the 2.2uF C1 cap from the MoFo. I don't think the tube stage output cap needs to be greater than 1 uF in this case. I'll know more when I measure the frequency response.
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Old 11th February 2019, 01:00 AM   #1885
Eric is offline Eric  United States
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Scott, that's a sharp looking tube and solid state combo! There is something oddly alluring about tubes right next to heat sinks

Eric
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Old 13th February 2019, 09:12 PM   #1886
scott17 is offline scott17
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Default Test results

I took measurements of my build. These measurements are through the tube preamp section.
The MoFo power supply voltage is 22V. Amp biased at 2A.

With 2VRMS input @ 1KHz:
28VP-P output or 9.9VRMS right before clipping
9.9^2 = 98/7 ohms (dummy load) = 14W

The plots are from Rightmark Audio Analyzer.

I had a thump at power-on so I installed a speaker delay relay. Problem solved.
Listening on my Altec 604D cabinets, sounds good, I like it. Plenty of detail. Separation good. Listening is so subjective it's really pointless to try to describe. It's amazing to me that 2 heat sinks that I pulled out of the trash, with IRFP250 already mounted on them, can sound this good. Really the only thing I had to purchase to make this work were the 2 chokes.
Attached Images
File Type: png MoFo_freq-resp.png (2.8 KB, 210 views)
File Type: png MoFo_THD.png (7.4 KB, 214 views)
File Type: png MoFo_freq-THD.png (2.7 KB, 183 views)
File Type: jpg MoFo_under.jpg (733.0 KB, 186 views)
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Old 13th February 2019, 11:51 PM   #1887
GASCo is offline GASCo  United States
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Awesome implementation! Care to share a pic of the outside?
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Old 14th February 2019, 12:01 AM   #1888
scott17 is offline scott17
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Thanks. Post #1880 of this thread for the pic.
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Old 14th February 2019, 02:22 AM   #1889
Eric is offline Eric  United States
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Is it just me, or do others find there MoFo amps to have a bit more bass than other amps? This is definitely not a complaint as I like plenty of bass, I just continue to be surprised by what this amp produces. I'm wondering if this is the influence of the big, fat choke in the circuit?
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Old 14th February 2019, 02:35 AM   #1890
vdi_nenna is offline vdi_nenna  United States
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Low output impedance? I'm sure the extra (reserve?) power helps.

I personally don't think there's more bass, just better defined quality bass.
Good attack and definition.

Glad other keep finding good things about this amp.
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