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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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 Post subject: Re: New Tube Amp Ideas
PostPosted: 19 Jul 2019, 18:10 
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Joined: 23 Feb 2017, 02:02
Posts: 546
I do have multipe types of buckboost moduel my self but I always tend to use a 12 or 24v supply smps very cheap from china then just put a cheap chinese buck converter for the heaters.
The non heatsink ones will take up to 1.2A and 6.3V output running reliablity.


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 Post subject: Re: New Tube Amp Ideas
PostPosted: 19 Jul 2019, 22:20 
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Joined: 28 Dec 2010, 22:07
Posts: 349
gofar99 wrote:
Hi, I have used those modules. They are generally ok. I add a R-C-R-C filter after them with snubbers on the caps. The output of the module is not clean enough for me. A 1HY choke would not be a bad idea either. (everyone knows I really hate hum and noise).

Yes. I've noticed that these don't have a fuller duty cycle unless you draw close to the full power from them. So, even though their switching frequency is in the tens or hundreds of kHz, they provide power in bursts at an audio frequency. I found this out when testing a small boost converter designed for Nixie tubes. The output frequency is higher than 120 Hz, but it's still annoying and needs audio filtering (means large, heavy, expensive parts). As far as audio-freq chokes, I am VERY particular with how I spend my extra funds (as I don't have time to foul around). If I were to buy a choke of a few henries, I'd probably put it in an outboard power supply so it wasn't limited to one device.
gofar99 wrote:
Be aware that the output has a link to the 12 V negative input so if you need a heater lift circuit it can be a problem.

One of the modules I bought had a bipolar output. So, if it were to give me 300v, it was really giving me +/- 150v. That's a problem because if the current in the output transformer is asymmetrical, it will saturate the core and blow up the switching transistor. I modified it to put out a single-ended DC. I've also thought about designing a bench supply including a buck-boost converter that gives me a C supply of -1.5 to -50v (enough for anything from a 12AX7 to a 6DQ6). I'm an aspiring power engineer and I'm trying to learn to power everything from 12v, meaning I can run low-power tube ham radio gear from a solar panel and a lawn tractor battery. It starts with the vibrator (not something you should search for at work!)
gofar99 wrote:
Back to the original post...The phono section can be done it lots of ways. Some SE variations using 6DJ8s are pretty nice and simple. I prefer SRPP designs though. For the line stage I would think a 12AU7 would be about right. You really don't want a huge amount of gain.

From what I've read, the phono stage needs a gain of about 90, or just less than 40 dB. Then comes a volume control, then the line pre and power amp. I would argue that I DO need a lot of gain here. I'm going for 3 watts from an SE amplifier, but I'm also trying to extend the frequency response of my less-expensive transformers with about 20 dB of feedback. I already explained my logic but I'm also willing to deal with it not working as well as I'm hoping. I did some calculations today and figured that the 12AV5GA tubes should give me something like 28 gain. This is slightly higher than the transformer's voltage gain of 1/25 (5000:8). With enough gain, negative feedback should flatten out the frequency response and leave a modest gain of a little less than 20.
ILoveHiFi wrote:
Have you looked at aliexpress thats typically the place to look when your #China #Cheap
If your cheap shipping expect to recive items in 3 months time max at least 1 month

Meh. I've had good experience in the past with transformers from "Musical Power Supplies". Although he designs them as guitar output transformers, they sound pretty good. If I can extend the frequency range out to 50 - 15K, I'll consider that pretty good. I work in aviation so my hearing's not great in the first place. I'm also familiar with how long it takes things to get to me from China.
ILoveHiFi wrote:
I do have multipe types of buckboost moduel my self but I always tend to use a 12 or 24v supply smps very cheap from china then just put a cheap chinese buck converter for the heaters.

I think that's what mine is. It boasts an input range of 8-32v for 45-390v output. I'm going to set it to 320v with 20 of that eaten up in filter resistors. I'm also looking for a new aesthetic for this amp...something that doesn't look like an upturned cake pan with tubes and iron sticking out of the top. I'm also going to paint the inside black for obvious reasons :P.

Ed

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 Post subject: Re: New Tube Amp Ideas
PostPosted: 20 Jul 2019, 10:25 
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Joined: 04 Jun 2008, 20:59
Posts: 4024
Location: Arizona, USA
Hi, The modules I have do have both positive and negative outputs......however they are identical to the single output ones except the have a second rectifier, and filter. The boards are the same as are as far as I can tell all the components. They do go to the higher voltage if used single ended. I typically use about 200 v up to about 50 ma. Nothing is connected to the negative side and there is apparently no problem with that. Yes they do "cycle" at some odd frequency. If you don't filter it, it will introduce a pumping noise in the device you are powering. There is a heavier duty 70 watt type that seems less prone to the issues, it still does have a link between the 12 ground and HV ground.

The gain in the phono section is typically about 40 db mid band, then comes the line stage which typically will be in the 3 to 20 db range with most in the 3-6 range (I use fully passive at no gain) and then what ever gain you need in the power amp to drive it to full output. With most small pentodes in SE mode that would be in the 5-15 volt range. If the output of the line stage is around 1-3 volts (typical) then you need about a gain of 20 db to fully drive the output stage. Figure in the NFB and it will then determine how much above 20db you will need. I personally like to design clean stages without nfb and only use a small amount of frequency dependent nfb to insure stability above the audio band and to reduce the chance of emi interference. 3 db is about my max and starts at about 30KHZ.

Good listening
Bruce

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