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Hiwatt/Ampeg Clone
Hiwatt/Ampeg Clone
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Old 7th December 2017, 05:31 PM   #1
blackwhaleamp is offline blackwhaleamp  Belgium
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Default Hiwatt/Ampeg Clone

Hi,
I am building an sort of hybrid 200W Hiwatt amplifier including the midrange circuit from Ampeg SVT. The amp has two channels with Hi/Low. You can see the circuit attached
I just assembled the whole amplifier, and for some reason I have two issues
.- There is an oscillation, like something like a chopper sound, which appears when the volume/treble/master increases. Also, when I play with the midrange rocker switch, the same chopper sound appears
.- There is a dependency between the volume at each channel. I was expecting as I introduced a valve V3 to isolate both channel.
Apart from that, the sound is very clean and there is almost no hum.
Does anyone has an idea about how to solve this?
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Old 7th December 2017, 07:44 PM   #2
GeorgK is offline GeorgK  Austria
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I see that the wipers of your volume controls are connected directly to the grids of the 12AU7 which are floating above ground potential.
Suppose you run into trouble here, as the position of the volume pots have influence on the bias of the 12AU7s? Guess you would have to add decoupling capacitors.

Cheers
Georg

btw, in your schematic, the KT88s' control and suppressor grid connections are swapped. As the amp basically seems to work in reality this seems to be just a drawing error.
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Old 8th December 2017, 12:11 PM   #3
blackwhaleamp is offline blackwhaleamp  Belgium
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Thank for your comment. Indeed, I thought the same while I was doing the design. However, the original Hiwatt circuit connect the Master directly to the next pre-amp stage.

Click the image to open in full size.

Actually, C20 and C21 are the ones which are decoupling from HT to the 12AU7 grid.

I disconnected the circuit of the midrange, and the chopper-like oscillation is gone. I suppose there is something wrong there. Also, for some reason the Presence does not affect the audio signal at all

About the KT88, the schematic is not correct. I need to check that, but in the real circuit all is fine.

Do you have any other suggestion?
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Old 8th December 2017, 12:47 PM   #4
GeorgK is offline GeorgK  Austria
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Quote:
Originally Posted by blackwhaleamp View Post
Actually, C20 and C21 are the ones which are decoupling from HT to the 12AU7 grid.
Hi blackwhaleamp,

it is as I told you.
Unlike in the Hiwatt, the grid of your 12AU7 is floating above ground because auf the 13k cathode resistor. When turning the volume control down you pull it to ground. Got me?
Take a multimeter and check it by yourself. You need to decouple the grid(s).

Alternatively, you could decouple the whole tone stack including the volume control from ground, but I see no advantage in doing that.
You need to fix this mistake, chances are your filter section works as expected then.

and one more thing: The grids of your V3 have no grid resistors. (1M from grid to ground). Add them.

Cheers
Georg

Last edited by GeorgK; 8th December 2017 at 01:08 PM.
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Old 10th December 2017, 09:37 AM   #5
GeorgK is offline GeorgK  Austria
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Hi blackwhaleamp

How is it going on?
Have you given it a try?
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Old 13th December 2017, 05:29 PM   #6
blackwhaleamp is offline blackwhaleamp  Belgium
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No yet, I will work on it this week. In some other forum some guy told me that the power supply is not properly balance. As a consequence, there might be motorboating induced on the amp. If this is the case, the is the reason of the put-put-put sound.

I edited the circuit, this is the configuration that I will try
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Old 13th December 2017, 06:14 PM   #7
GeorgK is offline GeorgK  Austria
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Yes, looks better now regarding the input of the EQ stage. But the the schematic still lacks the grounding resistors on both grids of V3. I strongly advise you to put them there. With unterminated grids, V3 may be pushed into an undefined operating state.
Adding additional filter stages to the HV supply is certainly a good idea too, but check the plate voltages on the EQ and filter stages afterwards. If the sound suffers from too little headroom you can try to make the filter resistors smaller. Not sure if eg. R58 is necessary, you could try with and without an decide by the plate voltages you reach on the input and EQ driver tubes.

Cheers,
Georg

Last edited by GeorgK; 13th December 2017 at 06:22 PM.
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Old 14th December 2017, 09:19 PM   #8
blackwhaleamp is offline blackwhaleamp  Belgium
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I rearranged the circuit as the one attached. So, the motorboating (aka put-put-put) is gone . However, I still have some issues

1.- The amplifier is not as loud as it should. I am not sure if I am missing one additional pre-amp stage, or simple there is another thing wrong with the amp
2.- After the modifications, the pre-amp was not working. So, I connected the scope to C28 and C30 and suddenly it works . So now the midrange circuit works (with some noise)
3.- The PRESENCE is subtle, almost unnoticeable.

I will add the 1M resistors to the grid V3 tomorrow, and see the effect

Thank for the help
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Old 15th December 2017, 09:02 AM   #9
GeorgK is offline GeorgK  Austria
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Quote:
Originally Posted by blackwhaleamp View Post
So, I connected the scope to C28 and C30 and suddenly it works .
This really does not come as a surprise. By connecting the scope you discharge the grids of V2 - which have no grid leak resistors. Believe me, they are necessary.

Valves must not be operated with "open" grids. As you have connected them directly to the decoupling capacitors, they build up charge during operation. The data sheets specify maximum values for the resistors (usually 1M for preamp triodes, you will be fine with this. At the moment you are at infinite Ohms).

The gain structure of the whole amp may be something to think about, it might be wiser to drive the active EQ stage with higher level (you lose a lot in the passive tone stack before).

Last edited by GeorgK; 15th December 2017 at 09:05 AM.
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Old 16th December 2017, 09:57 AM   #10
blackwhaleamp is offline blackwhaleamp  Belgium
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Quote:
Originally Posted by GeorgK View Post
This really does not come as a surprise. By connecting the scope you discharge the grids of V2 - which have no grid leak resistors. Believe me, they are necessary.
You were 100% right. Now the whole pre-amp circuit works. I just need to fix this problem with the insufficient gain. The amp is not very loud
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