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Old 11th Jul 2020, 9:40 am   #21
GrimJosef
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Default Re: Hope to start an audio amp with CV415/CV4046/TT15

One option for the bias supply would be to generate a negative as well as a positive output from the existing HT winding. It would just need a couple more diodes, connected the other way round. This gives you way too many volts of course, but compared with the 5V winding, which gave you way too few, it's not much more 'wrong'. You'd then just drop down to the voltage you want with a resistive divider. 300V at 5-10mA across the divider would only see you having to dissipate 1.5-3W. You wouldn't be working the mains transformer much harder if it's got a centre-tapped HT winding anyway. You'd ideally want a reservoir cap immediately after the diodes, to deal with the worst of the ripple, but it could be low value given the small current draw. You'd do most of the smoothing across the lower leg of the divider, where the voltage would be much lower.

Cheers,

GJ
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Old 11th Jul 2020, 10:19 am   #22
Radio Wrangler
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Default Re: Hope to start an audio amp with CV415/CV4046/TT15

For audio, the EF91 wasn't designed for low microphony, and you'll find the available gain somewhat less than an EF86.

They were designed as an RF/IF amplifier as a later generation replacement for the EF50. But in the 7-pin bottle it lacked multiple cathode pins to keep the inductance down for the best high frequency performance, so the EF80 took over and ran away with the TV IF market. The EF91 can be a bit unstable with the bandwidth of its gain and the barely adequate cathode connection. If one was in use at audio it would not be too hard to have RF oscillation going on and that could be noticed by the effects of moving things near it. As a youngster I bought a very big box full of ex-equipment EF91/6AM6/Z77 and used them for a few years playing with building oscilloscopes. I learned the hard way how to keep them stable. I also learned that lower impedance circuits were needed to go fast, and that If I still needed the same voltage swing, the current became higher. I wound up with 5763s driving deflection plates with lowish value big fat hot anode resistors. I hadn't made the break through to understanding active loads, still less distributed amplifiers at that time.

Anyway, the input stage of a power amplifier really shouldn't need anything as sensitive as an EF86. It's good practice to not have very small signals in places they don't have to be.

David
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Old 11th Jul 2020, 10:24 am   #23
AdrianH
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Default Re: Hope to start an audio amp with CV415/CV4046/TT15

GJ That is an idea, basically a bridge rectifier to a centre tap point on the transformer.

What I need to do is start loading the transformer and workout the regulation/load curve, it is an old transformer bought from this site.

But not now, I have just received orders I have to do something in the garden by SWMBO.

Play time will probably only return Monday.

Adrian.
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Old 13th Jul 2020, 7:35 pm   #24
AdrianH
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Default Re: Hope to start an audio amp with CV415/CV4046/TT15

Well no advance today, I was hoping to build a variable current load for the PSU just to map how it performed Volts against Current, I have a few devices such as BU508 or a S2000AF which should deal with up to 50 Watts on a decent heat-sink and cope with the HT voltages, but as the hfe of the devices are typically very low I need something else to drive them with that could also cope with several hundred volts and in that I seem to be lacking in the spares box. I continue to look.

So for now I have been having fun with my CV4014 or EF91 Valves for most of the day. I could not find any curves with a screen voltage less than 250 Volts so have been mapping them out myself. Then picking an operating point based on and selecting the anode load and grid values etc.

For small signal inputs to the grid, I have picked B+ 250 Volts, 47K Ohm anode load, 200 Volt on g2 and control grid around -2.3/2.4 Volts gives around 3mA Ia and 1mA Ig2. So a 600 Ohm Cathode resistor probably called for.

This would have been an excellent task for one of the valve tracer projects!

Anyway at the end with the settings I had selected I mapped out Va against Vg and plotted it in a spreadsheet to provide what I have below.

It is all a bit of fun for me as it was something to do and helped me understand things a bit better than just reading about Pentode curves. I get Vout/Vin of 175 but would imagine that would be loaded with the input resistance of any following stage and capacitance.

Adrian
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Old 14th Jul 2020, 1:20 pm   #25
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Default Re: Hope to start an audio amp with CV415/CV4046/TT15

This is a do not try this at home really, but I managed to add a variable load to the psu using a small Halogen lamp to dissipate some of the load and therefore heat. S200AF driven by a 2N3439 a Zener and a few resistors.
This give me a clue as to the output Voltage I can expect under what every load I use, looks like the full 300 Volts i require is out of my reach at 50mA+ draw but for a fun build it will do me for a single amp.

Time to crack on with another chassis.

Adrian
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Old 14th Jul 2020, 10:32 pm   #26
AdrianH
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Default Re: Hope to start an audio amp with CV415/CV4046/TT15

Here is hoping I have not done something daft?

Just ordered some push-pull output transformers suitable for the Mullard 5-10 EL84's etc. Decided not to wind my own transformers as I do not have two suitable identical cores, not much difference between 6K6 A-A and the 7K the spec sheet suggests. At least if performance is poor I can build a more standard amp.

Adrian
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Old 18th Jul 2020, 9:52 am   #27
AdrianH
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Default Re: Hope to start an audio amp with CV415/CV4046/TT15

I have been progressing slowly, made another small chassis to do one mono amp while I await transformer to arrive. One of the issues is to get the ground connection to the chassis, I can not cut and bend the solder tags out of it as in typical radio chassis, using 1.5mm steel makes this a no go for me and I would also need the punch, perhaps one day when I can get brass to make them!
So I am doing a dead bug style circuit using some copper clad, which is bolted to the chassis allowing me to solder to the board.

Anyway have wired the components to the EF and ECF valves, may not look very nice, as I am finding it difficult in the confined area. Have changed a few values from the original Radford circuit to suit my PSU.

The next stage is another piece of copper clad for connecting to the TT15 and transformer. I am hoping that if/when it all works I can sit down in Kicad do the circuit with final component values and complete a PCB layout, that is some time off in the future.

Initially I am going down the Cathode bias resistor route even though it will drop the B+ to around 220 Volts I really wanted closer to 300, but the mains transformer is not suitable, soI am on the hunt for a better unit.


Adrian
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Old 18th Jul 2020, 8:51 pm   #28
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Default Re: Hope to start an audio amp with CV415/CV4046/TT15

Looking good Adrian.

Any reason you've not opted to use tag strips to support the components and provide ground connections where needed ?

Using tag strips (single or double) can look very neat and makes it fairly easy to change values etc.

At the audio power levels you're aiming for I think cathode self bias is a good option.
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Old 18th Jul 2020, 9:41 pm   #29
AdrianH
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Default Re: Hope to start an audio amp with CV415/CV4046/TT15

Hi Tim;
At the moment things are in the air, I am using an EF91 which many say will be carp! This due to micro-phonics, noise, lack of gain etc. If it really is, (One has to consider my hearing over 8-9K is practically none existent) then I will have to go for a larger hole to fit a EF86 (9 pin rather than 7). But the end game will be to produce a PCB board if I get it working as mentioned in my previous post.

So for now it's a get it going project anyway I can, like my previous projects get it working on strip board and then use Kicad.

The transformers are on order from a Co. called Primary Windings. Again I have done a compromise as they are meant for the Mullard 5-10 so slightly lower impedance then required for the TT15, this gives me another option in switching for the standard EL84's. In which case it will require another chassis anyway.

Have fun be safe.

Adrian
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Old 18th Jul 2020, 10:17 pm   #30
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Default Re: Hope to start an audio amp with CV415/CV4046/TT15

I have a load of 2N3585 transistors if you're interested, FOC and I think were kept as spares for either HP or TEK scopes, present stock is over 50
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Old 18th Jul 2020, 10:32 pm   #31
AdrianH
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Default Re: Hope to start an audio amp with CV415/CV4046/TT15

Hello Top Cap, it took me a little while to figure out why the 2N3585 transistor at first, but I assume you are meaning for loading the power supply?

They are something to consider either for a proper build load or for a regulated HV power supply.

You have reminded me I need to ask a question about smoothing!

I will send you a pm.

Adrian
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Old 20th Jul 2020, 6:04 am   #32
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Default Re: Hope to start an audio amp with CV415/CV4046/TT15

Why not try a 6AU6? I've used em triode strapped, great little valves. I have a few if you want to give one a whirl. If you don't have enough gain you can use a bit of positive FB or bootstrapping to get a few more 10's of volt OP.

Andy.
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Old 20th Jul 2020, 9:06 am   #33
AdrianH
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Default Re: Hope to start an audio amp with CV415/CV4046/TT15

6AU6 or EF94, it's just I have the 91's to hand. I am not sure on gain, until I get the output transformer in place and test final voltage swing then it is difficult to say how I am doing. I have wired onto the grid resistors of the TT15 and with a 60mV p-p signal I am getting a 50 Volt p - p signal on the grids. But there will be some negative feedback added when the transformer is in place.

I am just playing try it and see but making a note of what others say as potential solutions.

Thanks for the offer.

Adrian
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