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Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
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Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
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I figured it may interest those on DIY to read of a recent mod to the DB110 monobloc.
I have a number of these I collected including a brand new one that came as a kit. I have tested quite a few.
Here,I changed out the 6V6GTA inc the excellent NOS Large anode (7591 style) for a cheap pentode which actually was produced by the 1000s from 1941 on.
Bogen ran the 6V6 well beyond the usual 6V6 limits with no ill effects at Va 365V.
The GTA version was an uprated version - quoting 3.5-5% THD at 14W*.
It is clear the DB110 was not able to produce anything close to the quoted figures above* cleanly.
The pentode I used has never ever been used in fixed bias AB, but turns out to be basically a miniature EL60. I also violate the manufacturer's limits and there was no sign at all of excessive heat or red plating.
The EL60 was the prototype version in Loctal of the ubiquitous EL34.
This particular valve was not intended for the kind of conditions imposed here, but the EL60/(34) works very well in fixed bias so I didn't worry.
This little pentode has much lower distortion than the 6V6 - quoted 1.8% at 13.2W out.
What little scarce info we have from Philips/Miniwatt quotes only cathode bias for this of 130 ohms compared with 300 for the 6V6 showing it as being far more sensitive - reducing drive requirements by more than 2/3.
The Bogen OPTs are excellent, ideally suited to 6V6, EL84 and now of course this.
The resultant conclusion was that (as usual) a combination of rectifier induced sag and weak PSU makes an otherwise excellent amp flawed to the point that it, like many 6V6 amps of the 60s and 70s only able to produce HALF the clean power it claims.
The tone controls used, also made distortion except when flat.
The resultant signal compression, chaotic speaker impedance matching, and distortion makes for the coloured sound which apparently many people appear to like, but clean it is NOT.
Here is a good explanation of the reasons why.
https://www.audiosciencereview.com/forum/index.php?threads/harmonic-and-intermodulation-distortion.25436/
Methods.
For testing I use "scope" program with a voltage divider from a dummy load fed into a professional studio audio card. This gives an instant output which is to some extent better than an oscilloscope.
The results are recorded for study offline with the amp off.
The audio recording program has FFT so that it is easy to see IMD levels showing on the spectra with STD SMPTE IMD tests.
60 Hz, 7 kHz and a 4:1 ratio (12 dB)
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Topic - 6V6 variants & old Bogen v Pentode mod - ivan_terrible 12:56:51 04/21/24 (2)
- RE: 6V6 variants & old Bogen v Pentode mod - ivan_terrible 05:10:11 04/29/24 (0)
- RE: 6V6 variants & old Bogen v Pentode mod - Spantou 12:32:17 04/26/24 (0)
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