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Hiwatt/Ampeg Clone
Hiwatt/Ampeg Clone
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Old 16th December 2017, 02:50 PM   #11
GeorgK is offline GeorgK  Austria
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I meant "grids of V3" of course... :-)

I do not think the cathode follower V4b is necessary. It does not contribute to overall gain and there is no need for buffering.. You could transform it to a standard anode follower stage (probably with unbypassed cathode resistor), this should give you more than enough gain then. But I would possibly change the position of the master volume control to after that stage.
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Old 17th December 2017, 12:45 AM   #12
PRR is offline PRR  United States
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Quote:
Originally Posted by GeorgK View Post
...I do not think the cathode follower V4b is necessary.....
No; but it was done in some of the Originals so we "gotta do it too".

This amp has FIVE gain stages. Even counting the tone-stack and that nasty mixer, it should have GOBS of gain. Something is still not right.
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Old 17th December 2017, 07:09 AM   #13
GeorgK is offline GeorgK  Austria
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Not sure what the purpose of this cathode follower was in the original Hiwatt design. Obviously it what so important to Dave Reeves that he put up with adding a whole twin triode, even leaving one system unused. Mainly, it drives the PI with fixed bias. The Hiwatts of the 1970s even use it without signal going through it, just as a voltage source for the PI.
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Old 17th December 2017, 10:26 AM   #14
blackwhaleamp is offline blackwhaleamp  Belgium
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Quote:
Originally Posted by GeorgK View Post
I meant "grids of V3" of course... :-)

I do not think the cathode follower V4b is necessary. It does not contribute to overall gain and there is no need for buffering.. You could transform it to a standard anode follower stage (probably with unbypassed cathode resistor), this should give you more than enough gain then. But I would possibly change the position of the master volume control to after that stage.
Now it works properly. I remove the cathode follower, and I changed also the phase splitter for a Fender-like

Thank a lot for your help
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Old 17th December 2017, 12:01 PM   #15
GeorgK is offline GeorgK  Austria
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Quote:
Originally Posted by blackwhaleamp View Post
I remove the cathode follower, and I changed also the phase splitter for a Fender-like
Glad to hear that!
Just discovered:
The problem may have been that you used a 100k cathode resistor instead of 220k. As the PI is directly biased by this resistor it probably did not work properly.
Anyway, with a standard long-tailed pair you can't go wrong, although I would consider using a 12AT7 here.

I should do some research on what was the advantage of the Hiwatt design with the voltage source/cathode follower combination. I am curious. Maybe somebody knows?

Cheers,
Georg
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Old 17th December 2017, 04:01 PM   #16
PRR is offline PRR  United States
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Quote:
Originally Posted by GeorgK View Post
>.....a 100k cathode resistor instead of 220k. As the PI is directly biased by this resistor it probably did not work properly.
> ...what was the advantage of the Hiwatt design with the voltage source/cathode follower combination. ....
Do you mean his R37? 100K here works fine.

I checked it, because we know the Fender 5F6a CF runs on the edge of grid-current and its overload distortion is part of the "sound". But this scheme has the 1.5Meg+1Meg divider in front of the CF. CF grid is down near 100V, and it can easily pull 100K or 220K.

Why so many tubes? For one, I think a lot of guitar design is "try it and see". Some basic plan is adopted from other known-good amplifiers. Then various building-blocks, from other amps or seen in magazines, are tacked-in and tried. For another: sometimes it is about the "looks". The HiWatts are stunningly impressive builds, with their huge chassis and long row of tubes. They may have had a stock of chassis with a lot of tube-holes already punched. Might as well fill up all the holes, give the customer "a lot", even if a more economical design would work the same.

Why did Deusenberg build a V-12 engine? Why did Cadillac trump that with a V-16 (and then another)? Rolls-Royce had shown that a large Six can be smooth enough for any car. (Their later V-8 was more about lower hood than smoothness.) Bugatti Royale made-do with an Eight. But open the hood on a Twin-Six or a Caddy 16, you just say "WOW!" When you are trying to jack-up the selling price to make the most of/off your wealthiest customers, sometimes over-design is good design.
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Old 17th December 2017, 06:02 PM   #17
GeorgK is offline GeorgK  Austria
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Quote:
Originally Posted by PRR View Post
Do you mean his R37? 100K here works fine. I checked it, because we know the Fender 5F6a CF runs on the edge of grid-current and its overload distortion is part of the "sound". But this scheme has the 1.5Meg+1Meg divider in front of the CF. CF grid is down near 100V, and it can easily pull 100K or 220K.
Hi PRR,

you're right of course, it does not make much difference. The determining parts are the voltage dividing resistors at the input of the CF. My bad.

But I think the CF's DC output is way too high. About 100 Volts with 350V supply which should not be too far away from reality! No wonder the PI won't work properly. And not the best idea for the cathode isolation. Change the grid resistor to maybe 220k and things look different.

In the Hiwatt schematic both 12AX7s run on a rather low voltage (with a 100k filter resistor in their HV supply), on the clone probably on much more (especially V4a), with a direct effect on the DC output of the CF. Later designs of the Hiwatt pre dropped the separate filtering for both triodes and lowered the input voltage of the CF instead (which only serves as a voltage source there) by changing the grid resistor to 220k.

But it is certainly ok if it works now.

Last edited by GeorgK; 17th December 2017 at 06:08 PM.
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Old 17th December 2017, 11:43 PM   #18
PRR is offline PRR  United States
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Originally Posted by GeorgK View Post
....I think the CF's DC output is way too high.....
Ah! You are right. Taking 350V supply, 102V at CF output, 24K in longtail, and 100K for plates, there is only 33V across the one driver, 68V across the other. (They may self-equalize better than that; it is so jammed-up that I don't care to calculate exactly.)

Tail resistor could be 39K or so for better conditions. The OP could actually observe what is going on and ask for data-based comments.
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Old 18th December 2017, 12:19 AM   #19
GeorgK is offline GeorgK  Austria
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It would be interesting to know how much the DC-coupled phase inverter - or the fixed bias circuit in later revisions - contributes to some "Hiwatt sound".
I don't think Dave Reeves did it just for fun, the design can be found in his earlier Sound City amps too. Additional tubes were - and still are - a costly thing, especially in old Hiwatts with their finicky wiring. I don't think musicians could be impressed by an additional tube that adds no real features.
The advantage over a "standard" long tail inverter still is not really clear to me.
Maybe I can find something more on the web.
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Old 30th December 2017, 11:02 AM   #20
blackwhaleamp is offline blackwhaleamp  Belgium
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Hi,
I tested the amplifier with more details, and now there is an issue that still pop up. 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
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