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What do you think of this circuit
What do you think of this circuit
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Old 2nd December 2017, 07:56 AM   #1
Doppler9000 is offline Doppler9000  Canada
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Default What do you think of this circuit

Hello,

I came across this circuit and wondered what people thought before I dig in further on the design. It is a regulated 300B SE unit.

TIA
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Old 2nd December 2017, 07:19 PM   #2
6A3sUMMER is offline 6A3sUMMER  United States
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Fundamentally it should work.

A lot of resistor values are missing. If they are correct for the tubes that they will bias and load, OK.

The idea of negative feedback is tricky. Different output transformers, different layout,
etc. may require adjustment of the feedback circuit in order to make the amp stable into
a resistive load, and especially into the loudspeaker load that it will be used with.

I believe the only regulation is the 300B filaments, right?

I always use a series string of 2 fuses, i.e. a 2A slow blow, and a 3A fast blow
(don't get hung up on those example values).
The intent is to have a fast-blow fuse that is as small as possible, but that does not
blow during the inrush currents during turn on of the amp.
The slow-blow fuse is as small as possible but that will not blow during the warmed up
current of the amplifier.
Now the amp is double protected (against different types of amplifier faults).
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Old 2nd December 2017, 08:38 PM   #3
Ketje is offline Ketje  Belgium
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Quote:
Originally Posted by Doppler9000 View Post
Hello,

I came across this circuit and wondered what people thought before I dig in further on the design. It is a regulated 300B SE unit.

TIA
Missing alot ( values - connection VK3) Click the image to open in full size.
Mona
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Old 2nd December 2017, 09:19 PM   #4
petertub is online now petertub  Sweden
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unnedded Standby switch could be eleminated.
Filament for V1 and V2 should be middle grounded, not as shown at one side only.
R7 ( grid resistor for 300B) should be < 250k to avoid thermal runaway
Fuses should be added for B+ transformer and the filament circuits
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Old 2nd December 2017, 10:21 PM   #5
Globulator is offline Globulator  United Kingdom
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I note that the feedback loop includes the OPT which adds a laggy bandpass filter.

The bandpass effect means that even if you have a usable amount of feedback it will be less at the frequency extremes which is where you may need it. The lag affects stability as noted above.

I'd therefore recommend you add heavy feedback around the tube-only stages to give the OPT a nice clean low-impedance drive and predict this will sound much better. At least it did with the amp I tried it on, and the principles of why it did are solid.
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Old 5th December 2017, 09:22 PM   #6
Doppler9000 is offline Doppler9000  Canada
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Thanks, guys.

This is a commercial design from a less-experienced tube amp guy.

I have other schematics from him that I will post in another thread. He is pretty inexpensive but I don't want a nightmare on my hands, either.
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Old 5th December 2017, 09:30 PM   #7
kaputt is offline kaputt  Germany
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It's a pretty pedestrian circuit. If you get the feedback loop sorted, as mentioned, it should give you "warm and relaxed tube sound".
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Old 5th December 2017, 09:36 PM   #8
petertub is online now petertub  Sweden
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Quote:
Originally Posted by Doppler9000 View Post
Thanks, guys.

This is a commercial design from a less-experienced tube amp guy.

I have other schematics from him that I will post in another thread. He is pretty inexpensive but I don't want a nightmare on my hands, either.
Ignoring the tube's design limits might get you into trouble, this design
does ignore the 300A maximum g1 resistor. The absence of fuses will
make damages larger then necessary.
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Old 5th December 2017, 09:38 PM   #9
Doppler9000 is offline Doppler9000  Canada
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Is the circuit dangerous?
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Old 6th December 2017, 06:51 AM   #10
Kay Pirinha is offline Kay Pirinha  Germany
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No need for hum balancing resistors R8 and R9 in a DC heated DHT. Just connect the cathode combi C4||R10 to the negative heater terminal, i.e. H2.


Best regards!
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