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Old 2nd April 2004, 11:02 AM   #1
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Question High frecuency oscillation when moving volume pot


I'm having a low volume high pitch oscillation depending on the position of the volume pot. It is very funny, but ¿do you know where the Gremlin is hidding?. Here is the schematic.
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Old 2nd April 2004, 11:03 AM   #2
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Default About the OT...

There is something I don't understand about the Hammond connexion. According to the Hammond schematic, the primary should be connect like this:

RED to B+
BLUE to Plate

But if I do like that then the amp oscilates, so I have to reverse the connections of the primary RED to Plate and BLUE to B+.

¿Maybe I should connect the NFB resistor to the GRN/YEL+GRN 4 Ohm output, and not to the YEL output?
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Old 2nd April 2004, 01:09 PM   #3
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Default Positive Feedback?

the problem with oscillation is due to inversion of terminals of the secondaries. Instead of providing negative feedback, you´re providing positive feedback.

Swap the secondaries (earth turns "live", "live" turns earth) and you should be able to connect the primary according to the diagram.

I don´t know about the problem with volume setting. The ususal (1k) resistor at the input of the grid is intended to eliminate much higher (unhearable) frequencies.

Let me add that all this info is obtained from this forum. I´m just reposting it.
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Old 2nd April 2004, 02:28 PM   #4
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Erik, If I invert the terminals as you say, then the amp osclilates at very high volume with constant pitch (positive feeeback). What I am having now is a very low volume oscillation that only appears when the volume pot is at about 20-30% and then again when i'ts at 80-90%, changing the pitch while I move the pot.
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Old 2nd April 2004, 03:13 PM   #5
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It may be because of the layout of the amp rather than the circuit itself, possibly the layout is allowing some positive feedback to occur or allowing some RF pick-up.

Try to effect of a small cap, say 100-500pf in value from the grid of the KT88 to chassis or at the input, across the volume control. Alternatively, place a small cap across the feedback resistor. If these methods are unsuccessful try shielding the ECC83.
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Old 2nd April 2004, 04:01 PM   #6
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Default oscillation

Its possible something in your pre-amp maybe causing some dc to the i/p, changing the operating point of the first tube--Try a decoupling cap at the input socket, and make sure the socket isnt earthed to the chassis at this point--make a star-earth scenario to first tube, then back to main first filter cap in PSU--Best to star-earth the whole lot--Earth loops can be a pain in the xxxx!!--I have a home build 5.1 decoder that does this every now and again (Circle Surround based around AD chip), guess Ill repair it one day!

Another thing to check, are any of the high-current secondary connections from the o/p Tx, near any of the input stage components? Also check the placement of the o/p tube anode wiring for the same
A ferrite bead on the first grid may help,----You have got grid-stoppers havent you??? (150-220ohm direct to tube pin on socket, then use this as the tube grid connection ie, in series with the tube grid)

Hope this helps you, I know what these sort of faults can be like!!

Das Beste Oder Nichts
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Old 3rd April 2004, 04:43 PM   #7
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Smile I've got it !!

I have the solution I've connected the nfb resistor to the 4 Ohm speaker output, (not to the 16 Ohm) and there's no oscillation at all. Maybe that happens because Hammond is using combinations of two separate windings for the different output impedances, so there can be some positive feedback from the induction at that point.
I have also used a star grounding and there's absolutely no hum. (except a mechanical 50 Hz vibration from the transformer that can be heard when there's complete silence. I will try to find a better one)

This is the first time I can listen to a single ended amp, and I'm impressed. The sound is very "soft"? I don't know what adjectives to use. The high frequencies of the hi-hat or snare do not sound as crispy as I was used to hear, but gentle and soft.

If one of this days I am able to activate my web page, I will show the details of the construction and all the "don't do like this" that I've already made, so other beginners like me will be able to construct this amp with no pain. I recommend it!

My next step will be to plot the curves, but the voltages I read are about 100 V lower than the ones from the original schematic.

_______________Original_____ My amp
AC transf. out:___ 300+300V___ 312+301V
5U4G_out:_____ +380V_______+288V
Choke out:______+375V_______+282V
ECC83 plate:____+360V______ +253V
KT88 plate:_____ +370V______+267V
KT88 grid2:_____+372V______+272V
KT88 cathode:___+37,5V______+21,6V

I have a 300+300V / 300 mA transformer for a stereo amp and I am not using the polypropylene condensers in parallel with the supply electrolytic caps. Any idea about why I have this voltage dropout?
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Old 3rd April 2004, 04:56 PM   #8
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Here is the picture of the amp and the original schematic
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Old 3rd April 2004, 05:46 PM   #9
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Default what current?


Pardon my saying, but I think something is seriously not right here. If your transformer has the same output as the one in the original schematic and your voltages are off by nearly 25% something is amiss. Measure the DCR of your choke and then measure the DC voltage drop across it. Calculate the total current the amp is drawing. Per the original schematic is should be ~210mA with the original voltages. Make sure your power supply and cathode bypass caps are hooked up correctly and that they are in working order.

Maybe not, but my feeling,
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Old 3rd April 2004, 08:07 PM   #10
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Hi Michael, thanks for your reply

My transformer is not the suggested one. The original is a Tango U-808, but I believe the Hammond 1627SE have similar electrical characteristics; they both have a 2,5K primary impedance.

Hammond 1627-SE / Tango U-808

Tomorrow I will measure the current.
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