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Old 8th May 2009, 10:27 PM   #1
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Default Amplifier using a 833A

I feel its better to start a new thread since this amp isent very tubelab anymore.

Im now using a circuit by Electra-print, a TDA2030 driving the grid of the 833A by a interstage transformer, im using a 230/24V transformer for this, however electra-print makes a special transformer just for this but i dont have the 89 dollars it cost.

Here are a few pics:
Click the image to open in full size.
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Schematic:
Click the image to open in full size.

Heres a video of a startup, tho with a tubeamp as driver, when replacing it with the ss driver, hum level went down alittle.
http://www.youtube.com/watch?v=zA1-IkewNtQ
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Old 10th May 2009, 04:47 PM   #2
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Hi, i'm in the process of building an 833a amp. Your tube seems close to the other components, any issues with heat?
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Old 10th May 2009, 09:52 PM   #3
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My only problem is the autotransformer and the hv transformer getting very hot.
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Old 10th May 2009, 10:50 PM   #4
kimbal is offline kimbal  Australia
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Default At least he had a go !

I looked at the video, the pics and the circuit and have to say well done in getting the beast to work; BUT a Microwave oven transformer as a SE OP trany AAA..R ! I can't deal with it.
I know it was free and your on a budget like me - but even if you wind your own on an old core, that would have been far better !

I cringe at using these doorstoppers even for a power transformer - because of the high leakage inductance, welded laminations; etc.

But if it works, maybe as a bass amp - then it's a start. Why a Solid state driver ? I can understand a computer case, at least with a prototype and maybe a Microwave tranny for the HV main Power supply.

But all in all - well done for having a go.

Cheers - Kimbal !

PS: - Before the days of microwave oven transformers - when I was 16yo and without any money - I built an 807 push pull amp as a guitar amp and used a standard power transformer from an old 5 valve tube AM radio as the Output Transformer. It sounded cr*p ! No highs and a very muddy bass. Audible distortion was not "too bad" on the mids and upper bass, but I had no Oscilloscope to really know for sure. Estimated power out was about 15 watts @ 20% distortion, if I was lucky.

The 285-0-285 volt @ 85 ma HT winding was connected across to the two 807 plates wire to the two op tubes with its C/T to the 400v B+ line. The 6.3 volt AC secondary was the 4 /8 ohm output; with all other windings left floating. Never again !
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Old 10th May 2009, 11:39 PM   #5
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You may not believe it but this amp sound really good, and the freq response aint too bad either, and this is without any feedback what so ever, this rolloff is easily compensated for using an eq.

Click the image to open in full size.

I used the electra-print ss driver because i dident have parts for a tube based driver, also after reading the electra-print tech blog about the opamp driver i thought itd make the overall sound better since the TDA2030 driver will have a THD of 0.1% or less according to electra-print.
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Old 11th May 2009, 01:37 AM   #6
kimbal is offline kimbal  Australia
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I would like to know -

What sort of power output does this beast deliver ?
I assume the frequency response on the photobucket link was the green line ? What is you B+ rail and audio driver volts into the 833a ? A well designed SE 833 can deliever up to 150 watts RMS of audio into a speaker witha cherry red spot on the plate for normal operation. Does yours have any cherry red spot on the anode ?

It's interesting you say it sounds good, because some Australian tube audio hobbyists I emailed a couple of years ago said they were using a cheap $20 dollar line output audio transformer readily available in the local Altronic Electronics Shop; which gave a pretty good audible quality sound. I thought that was bad enough, but using a Microwave power transformer is really getting to the bottom of the bucket.

In the above project I quoted, the tubes driving the amp were 6CM5/ EL36. It may be of interest to some, but sorry it's not my way of doing things - but if it works as an alternative and you like the sound - OK go for it !

The full artical is as follows -

A Push-Pull Amplifier, featuring the 6CM5/EL36 and the Altronics-M1115 transformer.

The link is at >

http://home.alphalink.com.au/~cambie...ls_6CM5amp.htm
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Old 11th May 2009, 10:02 AM   #7
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I have a orange spot on the plate, B+ is around 1500V i think, havent measured anything. This i think is alittle too much plate current so im gonna fiddle w the bias resistor to reduce plate glow to barely visible red. Judging from the color of the plate, maybe 100-150W, it dissipates around 800W in heat, 250-400W from the tube and another 400W from the transformers.

The TDA2030 driver puts out around 10V into the 24V winding of a lil 15VA donut on full volume, out on the secondary comes around 120V, atleast in theory.

The freq response in the pic is the white line, the green is THD vs freq, yes the THD look alittle funny, propably because the opt is held together with scotch tape and the fact that theres no interleaving in sight.

The original intention w this amp was to use it as a bass amp but it sounded so good that im reconsidering.

Someday when i have the money i consider ordering a opt from electra-print as well as that opamp drive transformer, maybe even a power transformer.
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Old 11th May 2009, 03:25 PM   #8
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What Tekko doesn't tell you, is that the Swedish government has suddenly decided to open up a closed down nuclear power plant ...


Happy to see you spread your ideas worldwide and not only on Elektronikforumet.
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Old 11th May 2009, 08:23 PM   #9
kimbal is offline kimbal  Australia
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Quote:
Originally posted by Tekko
I have a orange spot on the plate, B+ is around 1500V i think, havent measured anything. This i think is alittle too much plate current so im gonna fiddle w the bias resistor to reduce plate glow to barely visible red. Judging from the color of the plate, maybe 100-150W, it dissipates around 800W in heat, 250-400W from the tube and another 400W from the transformers.

The TDA2030 driver puts out around 10V into the 24V winding of a lil 15VA donut on full volume, out on the secondary comes around 120V, atleast in theory.

The freq response in the pic is the white line, the green is THD vs freq, yes the THD look alittle funny, propably because the opt is held together with scotch tape and the fact that theres no interleaving in sight.

The original intention with this amp was to use it as a bass amp but it sounded so good that im reconsidering.

Someday when i have the money i consider ordering a opt from electra-print as well as that opamp drive transformer, maybe even a power transformer.
If what you say is right, then I am impressed with the frequency response outcome.

As for tweeking the bias to reduce the red glow, this is not the best way to reduce plate dissipation; as you may slide the tube into it's nonlinear region and make it run in class B or C.

From memory anything around 1000 to 1200 volts for the plate voltage is fine (but higher is better) and around 100-150 ma quiescent plate current - assuming you have a good emmission 833A tube. Measure the voltage across the filament bias resistor and divide it by the resistance to get the plate / bias current. Just don't push the plate dissipation (plate volts divided by plate current for a triode) over 300 watts unless you use the 833 C or D which can take 450 watts. These have carbon anodes. But don't take these adjustment figures as the final word - check the data curves.
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Old 13th May 2009, 10:16 AM   #10
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Yesterday i replaced the 230/24V donut with a 230/10V EI core transformer, due to the split bobbin design i lost everything above 1kHz so i tried the thing as a bass amd and oh my, this thing go so loud that the floor start vibrating, and the amp wasent even clipping yet.

In fact it worked so well that around 11pm two cops knocked on the door, but by then i had already turned it off.
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