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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: 08 Mar 2020, 14:22 
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Minarai wrote:
But, I am posting because I realized I'm not certain I can use the output transformers I bought or if so, exactly how to wire them. They are Hammond 125J (datasheet attached).
Woodo is correct. The schematic you posted in single ended. The Hammond 125J is a push-pull output transformer. You need a sindgle ended transformer for you circuit. Either a Hammond 125BSE of 125CSE (http://www.hammondmfg.com/125SE.htm).

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PostPosted: 08 Mar 2020, 14:43 
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Got it. Thanks. I think it was the 10k:32ohm spec that threw me off. But that is the same ratio as 2.5k:8, so it does what we need?


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PostPosted: 08 Mar 2020, 18:03 
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Minarai wrote:
... so it does what we need?
Yes.

For a 2.5kΩ primary use the white wire with 8Ω speakers and the yellow wire with 4Ω speakers.

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PostPosted: 14 Apr 2020, 13:35 
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well folks, I have at last, finished it.
It has a little hum, but it actually sounds quite good! Exceeds expectations. Not a lot of volume, but perfectly enough for my small kitchen/dining.

I made every conceivable design mistake a person could shy of causing the project to fail. I would never build a chassis/ housing like this ever ever again. In my defense, it is cobbled together from things I've had lying around for years and wanted to use up. And i learned a lot.

I did not implement every suggestion made, (the signal ground is not isolated and there is no filter was used on the mains).

But I greatly appreciate all the input. I certainly couldn't have done it alone.

For my next project, I have a few other oddballs laying around... two VOM 8810 amps, for example. I got one for free and foolishly paid a little money for another thinking I could use them for monoblocks. A little research has revealed that these are generally not considered good enough quality for HiFi stereo applications but are reasonably well liked for guitar amps.

Perhaps I will start a thread about those next.


Thanks again,


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PostPosted: 14 Apr 2020, 18:03 
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Bringing something old back into service gives a good feeling. Are the voltages and current what you expected? Bruce’s suggestions may help eliminate hum you observe. There are some excellent grounding philosophies in the technical resources library on this site that have helped me on my project. How efficient are your speakers. This may have some bearing on the volume level if the amp is achieving reasonable output. When i changed from 85db @ 1 watt 1 metre to 89 dB speakers this made a discernible difference in volume although the former was loud enough when cranked up.

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PostPosted: 14 Nov 2020, 23:52 
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I've ended up revisiting this amp and thought I would give an update and ask a few questions.
I have been using this one on and off in my kitchen for about 6 months. It was my first amp and i barely knew what I was doing when I built it.
I've now built 6-7 more amplifier projects and I am gradually learning more about the technical side of tubes amps and their operation.
Even now, with much more to compare it to, I think it sounds great in my kitchen (with not especially efficient Celestion SL-6s).

The only issue is, I get hum if the volume nob is around half to 3/4. At full volume (where I usually keep it) it is very quiet. Is that indicative of anything in particular? I can't figure out why the hum would be in the middle of the volume pot (mostly just curious to learn).

Also, for what its worth, I decided to check some values, again for the sake of learning. I measured the plate voltage on the 50c5s at 96.5V and the plate dissipation at 3.26 watts. I guess that means I could theoretically bias them a little hotter, or raise the B+ a little? Would anyone recommend I try any of that, given I am happy with the sound and happy with the volume?

Cheers,


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PostPosted: 15 Nov 2020, 11:27 
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Volume control dependent hum level is always a grounding issue. Either you have a ground loop at your inputs (a common problem) or the inputs are not properly grounded in the first place. The hum is showing up across the volume control and when the impedance of the control peaks (at around 80% shaft rotation) then so does the hum.

Look at the discussion of control impedance here for a better understanding of this phenomenon. This page discusses the output impedance of the preamp but the same discussion applies to the control impedance presented to the grid of the first signal amplifier.

Do you have any pictures of your layout?

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PostPosted: 16 Nov 2020, 12:58 
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Thanks, thats helpful. I don't really understand the concepts in the link... how the effective stage output impedance (and volume?) does not track with the fractional impedance of the volume pot. Why doesn't the volume also drop after 85% on the volume control?
The answer to that its certainly that I lack any understanding of the fundamentals involved, so I'll take it upon myself to keep reading, and not expect you to attempt an explanation.

But it describes exactly what is going on with my amp and the cause is exactly what you suspect. This is the only amp I have where the input grounds are not isolated from the metal chassis. Its going to be hard to fix that now. The holes through the stainless steel chassis are 1/4". To safely drill them up to 3/8" I'll have to dismantle the whole thing. I'll start by detaching the RCAs and see if it fixes the problem.

Thanks for the reply


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PostPosted: 16 Nov 2020, 17:58 
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Actually, this makes me wonder... I had considered leaving the volume pots off of some future builds. In cases where the amp is always used with a pre, or a step attenuator volume control, it seems redundant to have a volume pot (and risk it coloring the sound). I had assumed that leaving out the volume pot wouldn't effect the circuit and it would simply operate as though the volume was at 100%.
Perhaps its more complicated?


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