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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 28 Jul 2020, 21:11 
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Location: Arizona, USA
Hi Likely an impedance issue. Try a smaller value resistor perhaps 10K.

Good listening
Bruce

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PostPosted: 30 Jul 2020, 05:34 
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Hi, this is my first post here, and also my first amplifier build.

I have now finalized the Forewatt, its sounds very fin, but I haven’t had time to listen closely (need to have the rest of the family away then, else the complains starts soon). I also added a balance control (Glassware) and tone-tilt control (Glassware tilt-2), so I had to attenuate input -4,5 dB to make them work (not a problem when running active). I don’t like the idea of relay for the passive / active switch, so I made an mechanical control with two K&G switches instead. In order to get the input selector shaft below the tubes, a small gearbox had to be constructed, works fine.

2 questions to the wise people here:
I noted that when I changed from ECC82 to ECC802S (both new JJ), the distortion slightly elevated, 2:and order went from -63 db to -59 db (0,5 v input 1 khz and +10 db gain), is this normal ? (level low so maybe nothing to be bothered about)

2: I use the TKD 2CP-2511 volume pot (the best I could find without going to stepped), when adjusting the volume manual (with the remote there is no noise), there is some minor crackling, someone noticed this from this pot?, it’s no big issue, but a bit irritating (the pot chassis is grounded), I could maybe change to a plastic knob, but haven’t found any nice one.

Br Svante / Sweden
FRA with tone control to bias higher frequencies:


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PostPosted: 30 Jul 2020, 21:35 
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Hi, All tubes of types that can be substituted for each other are by definition different. They will behave differently. Sometimes a little sometimes rather grossly. In your case I would consider the difference in harmonic performance insignificant. They are both in the roughly 1:1000 range when compared to the signal. Differences can be bad sounding or pleasing. Serious odd harmonic levels (say at the -30 to -40 dbv range) can be unpleasant. If the distortion is even harmonics it will sound nice and warm. This is why many folks like class A single ended triode amps in spite of the often fairly high distortion levels. What you have discovered is why tube rolling is so interesting. In an early Forewatt I tested a large number of different tubes. The distortion measured (on an HP distortion analyzer) ranged from about 0.3% to just over 1.5% at 1 volt output at several frequencies. The tubes were 12AU7s (several brands incl NOS), ECC802, ECC802S, ECC82, ECC82S standard pins and gold pins on all when available. I settled on the JJ ECC802S as something near the lower end that was: consistent, reasonably priced and I liked the sound of the long plate 83s over the short plate 82s. Naturally YMMV. This is part of the fun of diy, you can tailor it to your own tastes. BTW, tubes do "break in" over time and brand new tubes often behave, measure and sound differently after say 100 hour so use. Some a lot some not so much. Just another thing to consider.

The relay in the design is a carry over from the commercial kits. It allows for shorter signal paths in the bypass mode. Switches are fine for this if it suits the build better.

Good listening
Bruce

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PostPosted: 02 Aug 2020, 06:38 
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Hi,
yes, swapping the 100k series resistor with a 10k one did the trick. The influence between the two amplifiers is minimal and I was able to align the gain perfectly. Now the KT88 and the Gainclone are playing together wonderfully. The KT88 is used on the mid/high crossover and the Gainclone on the bass crossover of three way speakers.
BTW, I built this version of the Gainclone: https://www.circuitbasics.com/design-hi ... er-lm3886/
It sounds very good in relation to the price. Ordering pcbs was easy and with quality components I think it is as good as it gets. Cabinet and transformer being the most expensive parts. The only change I made to the design is I'm using the 1cm thick aluminium side walls of the mentioned cabinet as heatsinks (cabinet comes from Italy). Works fine.

Thanks for the help.


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PostPosted: 04 Aug 2020, 06:25 
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Will someone point me to the latest schematic. The thread has gotten pretty long.


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PostPosted: 04 Aug 2020, 08:13 
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Joined: 04 Jan 2020, 08:22
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Hi, i believe the latest complete is on page 49
br Svante


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PostPosted: 04 Aug 2020, 08:17 
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svante wrote:
Hi, i believe the latest complete is on page 49
br Svante


Thanks


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PostPosted: 09 Aug 2020, 11:02 
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Joined: 04 Jan 2020, 08:22
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Hi
I have now listen to the preamp, and its sound very good, compared to my NAD 162C I can separate details better, especially at higher frequencies much better, drum brushes sounds as brushes, not withe noise, The NAD also sounded much darker, so the tilt-tone control that I added I now only use at low listening volume to elevate bas (thought I would us it the other way around).

However I have an hiss noise, that originates from the b+, noise level in active mode is very god up to 11-12 khz, after that the flor elevates maybe 8 db. I can only her it when music is not playing from the listening position, but with the ear close to the speaker (98 db eff. Tekton Perfet set 15) its easy to hear, so its more irritating that a serious problem.
It doesn’t change with different tubes (hay tried 3 different), but the tube must be in, noise level almost same regardless of volume . It’s not airborne, using the amp turned on, but in passive mode, its dead quiet, can’t separate that noise flor from my scopes noise flor in FFT mode, se picture 1 (10-20 khz window). But in active mode is has some pigs going higher than -80 db above 12 khz, se picture 2.

I have tried different Caps on b+ close to the tube (the 1 uf), added parallel caps over the 4,7 uf on the b+, nothing changes much (I thought a 0,47 parallel with the 4,7 u improved looking at the scope, but couldn’t find it when listening, so will remove that)
Any ide to improve this? Maybe adding some choke in the b+ filtering?
Grateful for ideas to test here.
/Svante


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PostPosted: 09 Aug 2020, 11:11 
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Hi, What is the date of the schematic you used? There are a number of things that have evolved over time and most address the S/N levels. Newest versions use the LR8 regulators and a modified heater circuit. I'll see if I can find a clean schematic of those changes. Also I have found some tubes and tube brands to have higher noise levels than others. Which ones dis you try? Also did you build the version with the higher gain using the cathode bypass capacitor, that variation will sometimes have more noise as well. I find the standard gain (7) to be quite sufficient for normal systems.

Good listening
Bruce

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PostPosted: 09 Aug 2020, 11:35 
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Hi, Latest PS for the Forewatt.

Good listening
Bruce
Attachment:
2020.jpg


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