Issue with a Clone Hybrid Hiwatt/Ampeg

[blackwhaleamp]

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. If I remove the midrange circuit, the chopper is gone
.- There is a dependency between the volume at each channel. I was expecting as I introduced a valve V3 to isolate both channel.
.- The Presence does not affect the audio signal

I used this circuit for the midrange. I found it in an old thread from this forum (http://music-electronics-forum.com/t2029/)



Apart from that, the sound is very clean and there is almost no hum.
Does anyone has an idea about how to solve this?

[MarkusBass]

I don't know this version of the mid control. The one that I know use 3 triodes. There is always a problem with the inductor. Where did you get it from? Apart from this, on your schematic there is one connection missing between the pins of the switch (which makes that the inductor sometimes is not connected to the circuit). Is it just a mistake on the schematic, or you forgot about it also in the amp?

Mark

[Mike Sulzer]

I am wondering if the volume control interaction and the oscillation might be part of the same problem. If you keep the master low to avoid the oscillation, do you still have the volume interaction?

[g1]

I don't know this version of the mid control. The one that I know use 3 triodes. There is always a problem with the inductor. Where did you get it from? Apart from this, on your schematic there is one connection missing between the pins of the switch (which makes that the inductor sometimes is not connected to the circuit). Is it just a mistake on the schematic, or you forgot about it also in the amp?

Mark, there is a jumper across 2 pins of the switch so I don't think there is any error. I've attached the actual Ampeg schematic.

[Gingertube]

I see 2 problems from looking at the full schematic (2 of 2) on your link.

1) HT3 has WAY too many cascaded gain stages off it. You should NEVER run more than 2 cascaded gain stages from one power supply node and differential stages should have their own power supply node. So you need more power supply decoupling. This will cause a motorboating sort of instability which is what I suspect is your problem (from your description).

2) Most of the triode stages should have grid stops. I would put 10K grid stops on all of them including those 12AU7 triodes driving the mid circuits. The instability in this case would be high pitched squeals which does not seem to be what you are describing although if you have this high frequency instability it can trigger the motor boating problem described above.

Fix the power supply decoupling problem first and then check to see if you have other issues. Add decoupling between V4 and V5 and between V3 and V4.

Cheers,
Ian

[MarkusBass]

Mark, there is a jumper across 2 pins of the switch so I don't think there is any error. I've attached the actual Ampeg schematic.

Hmm, I was not refering to the actual Ampeg schematic. I was refering to the actual-actual schematic in the post #1 (attachment 2 of 2). I assume that this is the real schematic of the amp. And there the link is missing.

Mark

[pdf64]

To save duplication of effort, see responses to the same thing on another site https://www.thegearpage.net/board/in...ampeg.1890930/

[g1]

Hmm, I was not refering to the actual Ampeg schematic. I was refering to the actual-actual schematic in the post #1 (attachment 2 of 2). I assume that this is the real schematic of the amp. And there the link is missing.

Mark

Sorry Mark, I didn't even notice that second attachment in the first post.
Indeed he is missing the jumper on the switch.

[blackwhaleamp]

Thanks guys for the tips. I was busy during the weekend, but I'll have a look tomorrow

[blackwhaleamp]

I reworked on the circuit



I plan to make this changes and see how it goes.

[blackwhaleamp]

I made the inductors following this link Bass preamplifier in the Ampeg SVT tradition
Indeed, I didn't connect properly the rocker switch

[pdf64]

V3 still needs grid leaks.

[blackwhaleamp]

I rearranged the circuit like this:



The motorboating (aka put-put-put) is gone. The midrange circuit works (with some noise). 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.
3.- The PRESENCE is subtle, almost unnoticeable.

Someone recommended me to add a 1M resistors to V3 grid to ground. I will do that tomorrow, and see the effect

Thank for the help

[blackwhaleamp]

Hi,
I managed to solve almost every issue in the amp. However, there still one thing that I couldn't solve so far. When the amp is switch ON for at least 10min, there is an oscillation PLUK-PLUK-PLUK coming up. I changed the position of every knob to see if there is a correlation between that, and the oscillation. However, there is not.
I also add another filter stage at the power supply, just to check if that could help somehow. No luck so far.
I attached the latest version of the circuit to this post

I appreciate any help that you could get me
Thanks

[pdf64]

Just checking, is the LTP tail resistor R43 really 220k? Even if it's 22k, are you sure that the LTP can put out enough signal to fully drive the KT88 (ie to Vg1-k = 0, on peaks)?
And there's no global NFB loop around the power amp?
Does removing V4 stop the oscillation?
If not, then V3?