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6J7 8th January 2013 12:58 PM

807pp + input phase splitter
 
2 Attachment(s)
Hello

Here is what I tried, it works but sound is unpleasant, dull, flat, no mid-range, no emphasis, etc. Too much treble because of "no mid-range" I guess.

Not so bad on electronic music (lot of bass and not much treble) but there is definitively a problem..

I tried local feedback with 300K resistors between 807 plate and EF80 plate: worse..

Sowter 8920 input phase splitter transformer.

Just for testing, I added a 1M pot at the output of the preamplifier to introduce a serious roll-off at high frequencies: too colored / mellow but it sounded much better like this!

Do you see something wrong ? thanks.

Palustris 8th January 2013 03:06 PM

How did you determine the value of the 10k resistor on the secondary of the input trans? Looking at the datasheet is says: A secondary Zobel network consisting of 47k Ohms in series with 150 pF across the two secondary windings in series may be use to adjust the resonance behaviour. This may need to be varied depending on the actual input capacitance of the driven stage.

According to the datasheet, the transformer works with a 20K+20K load.

djn 8th January 2013 04:17 PM

Did you try global feedback?

prairieboy 8th January 2013 04:55 PM

What's the voltage from the supply? Is the power tranny 240-0-240, or 120-0-120?

6J7 8th January 2013 04:57 PM

1 Attachment(s)
Quote:

How did you determine the value of the 10k resistor on the secondary of the input trans?
Frankly I don't know what I am doing :o
10K load at secondaries was recommended when using this transformer as isolating for my previous preamplifier.

This way (picture) ?

Quote:

Did you try global feedback?
Not yet.

6J7 8th January 2013 04:58 PM

Quote:

What's the voltage from the supply? Is the power tranny 240-0-240, or 120-0-120?
240-0-240

mwiebe 8th January 2013 06:59 PM

First double check things are going wrong upstream of your amp's input jack. If you have another amp plug it in and confirm everything sounds right with a working amp in place.

To to get a baseline that your amp works, as long as your input jacks are isolated from the chassis, split phase at the input jack. Just connect one grid to the pin and the other grid to the jack ground. Remember to add a grid resistor, 100K to 220K from each EF80 grid to ground (this can be done at the jack). I would also get rid of the 1K grid stopper, at least for a first listen. If the amp still sounds bad then your issue is not the input trans.

If the amp sounds good with the input trans out, you can try putting it back in, but you may not need to. Try it with unconnected secondaries--each leg connected directly to the grid, then with the 47K cap across the secondaries. You can also parallel the secondaries, dot to dot, and see if that helps. You are running all triodes so I don't think feedback will help with the issue you describe.
Matt

Palustris 8th January 2013 09:17 PM

This supply is probably 240-0-240 giving you 300V at the first cap. 300V is a good voltage for a triode connected 807 so should work fine. You could try lowering the cathode resistors to increase current through the 807s. You could try 510 ohm and 470 ohm. Measure the voltage drop across the resistor and use ohm's law to determine the current. Shoot for < 60mA.

Before that, I would listen to the amp with the zobel you have put on the schematic to see how that changes the sound. Or you could put 20k ohm resistors across each secondary of the input transformer to see how that changes the sound.

So, you have several simple things you can try. I think the overall topology of the amp is very good.

6J7 9th January 2013 12:44 PM

4 Attachment(s)
Thank you for your help.

This morning I tried with 20K+20K in series across the secondaries, it's better but still "harsh".

Time to learn how to use a scope and signal generator. I disconnected output stage and connected probes after the coupling capacitor (0.22uF).

Here are some picture and a video. It's awful, right ? what's going on :confused:

https://dl.dropbox.com/u/425350/QS2/MVI_1979.MOV

6J7 9th January 2013 04:08 PM

1 Attachment(s)
I replaced 150 ohms resistors between plate and screen with 1K.
4K3 grid stoppers instead of 1K. Same results.

So I disconnected the input transformer and tried like this but phase splitting doesn't work, lower EF80 doesn't split anything. Incorrect ground ?

Input socket is isolated from chassis, the only path to ground is through the lower 150K grid leak resistor I guess.


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